september 2015 tate of technology reports sensors & …

SEPTEMBER 2015

STATE OF TECHNOLOGY REPORT

Sensors & Vision

Trends in Technology

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Table of ContentsSensing and vision ride the wave of robot and IIoT growth 5

Trends in TechnologyMachine Vision Technology Developments Let Industrial Robots “See” and Do More 6The rise of robots and the enabling sensing technology 8Which positioning sensor is right for my application? 13The impact of wireless technology on presence sensors with robots 16Sensors: Automation for nanopositioning systems 18Targeted detection with presence sensing 21

Back to Basics What does your sensing application require? 24Which Encoder Communication Protocol Is Right for Our Application? 28Two views of machine vision 31The role of presence sensing in collaborative robot applications 33How to hone your sensing applications 35

Technology in ActionHow sensors enable information-based decisions 383 types of inspection and measurement 42Presence sensors travel into new automation applications 45


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The rise and convergence of the Industrial Internet of Things (IIoT) and robot integration have meant continued growth for sensing and vision system

technologies. Information is king, and you can’t mea-sure what you can’t see or sense.

North American sales of machine vision systems and components grew by almost one-fourth in the first quarter of 2015. This was the market’s highest quar-ter in history, according to trade group AIA (www.visiononline.org). Machine vision systems include smart cameras and application-specific machine vi-sion (ASMV) systems. In Q1 2015, smart cameras ex-panded by 23%, while ASMV systems increased 24%.

Even machine vision components had a strong quarter with 11% growth over the first quarter of 2014. The leading product categories within ma-chine vision components in terms of growth were lighting (28%), cameras (11%) and software (8%). “Industry experts remain bullish on machine vision components for the next two quarters—less so for ma-chine vision systems however, where 55% of survey respondents believe the category will be flat, 25% ex-pect an increase and 20% expect a decline,” says Alex Shikany, AIA’s director of market analysis.”

Sensor sales in the United States are expected to climb at more than 6% through 2016 to $14.9 billion, according to a recent report from Freedonia Group (www.freedoniagroup.com). Process variable sen-sors will remain the largest category, while chemi-cal property sensors and proximity and positioning sensors will post the fastest growth. The Freedonia study analyzed the $11.1 billion U.S. sensors indus-try, presenting historical demand data for 2001, 2006

and 2011 and forecasting sales for 2016 and 2021 by sensor type.

The robotic renaissance has given us a renewed need for increased sensing technology and improved vision. Connectivity and collaboration have upped the ante to the point where a sensorless system might soon be considered a simple machine, comparable to a pulley or a wedge.

Robots soon will be as ubiquitous as the IIoT as-pires to be. Vision systems and sensors play a ma-jor role in the rise of each. Collaboration will bring humans and robots together, and connectivity will bring machines together, but none of this can occur without sensing technologies. The connected enter-prise will include equipment, robots and humans. Everywhere. All the time.

Sensors are becoming better, smaller, less expen-sive and embedded. From new technologies, such as LIDAR and leap-motion sensing, to established ones such as safety-rated sensors, all products are finding an equally impressive growth curve.

Machines and integrated robots are benefitting from mobile applications enabled by smart devices, as well. Industrial connectivity that includes sensor data can enable remote control and monitoring of equipment via access to big data and analytics over a network.

This State of Technology Report explores in greater detail these and other technology trends in the arena of sensors and machine vision. Drawn from the most recent articles published in the pages of Control De-sign, this special report includes articles on emerging trends and basic primers illustrating the latest tech-nology in action. We hope that you find it useful.

Sensing and vision ride the wave of robot and IIoT growthBy Mike Bacidore, editor in chief

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Machine vision technology developments let industrial robots ‘see’ and do moreToday’s robots have 20/20 vision

By Leslie Gordon

Machine vision systems have long played an im-portant role in inspecting, identifying and guid-ing parts. Current systems provide an increas-

ingly sophisticated integration of machine vision and robotics, boosting the number of automation options that can help find defects, sort products and complete other tasks faster and more efficiently.

Industrial cameras play a big part in this trend. For example, camera technology has continued to improve not only by providing higher resolutions but also by carrying IP67 protection ratings, says Jeremy Jones, marketing communications at Bau-mer. “Several years ago, 0.8 MP cameras were most commonly used, but they had low environmental protection ratings. Currently, 2 MP cameras are available that allow the development of more ad-vanced applications with integrated robotics thanks to their IP67 protection and capability to withstand higher shock and vibration.”

Applications that integrate robotics have demand-ing requirements for vision system robustness be-cause the systems typically work in harsh produc-tion environments. Some of these applications even involve moving parts where the camera is mounted

at the robot. The more stringent requirements for ro-bustness have been addressed by advanced specific camera design features, says Volker Zipprich-Rasch, head of marketing and product management, Vi-sion Competence Center, Baumer.

“For example, Gigabit Ethernet interface with power over Ethernet (PoE) allows single cable solu-tions, which are beneficial for applications where the camera is mounted at the robot and moved dy-namically,” says Zipprich-Rasch. “Advanced robotic applications benefit from cable length up to 100 m, and the system robustness is increased overall. Also, lightweight yet robust cameras are available so the robot needs less force to move the camera. A sophisticated mechanical design might include an IP65/67-rated metal housing to protect components against dust. Because the positioning repeatability of the robot is important, the camera sensor must detect position accurately, as well as work under a wide temperature range. These features support im-age acquisition with long-term stable reproducibil-ity for advanced robotic applications.”

Improved bead tools have also increasingly en-tered the picture. “For example, the heavy use of



adhesives and sealants in the automotive industry means the bead measuring tools available in vision systems support easy integration with robots that need to confirm presence, size and continuity of the dispensed material,” says Brent Evanger, senior ap-plication engineer, vision, at Banner Engineering.

Bead measuring tools detect and evaluate continu-ous flows of mechanically applied adhesives or sealants.

“In the automotive industry, a robot moves a part under an adhesive dispensing nozzle to lay down a track of glue in the specific pattern,” explains Evan-ger. “When the operation is complete, the robot moves the part to a vision inspection station where a camera checks out the applied material, deciding whether the glue is in the correct location and if there are any areas where too much or too little glue was applied. Results from the bead tool include the average width of the material along the length of the bead and the number and length of skips or gaps where material was missing. The vision sensor can then make a judgment call regarding the quality of the applied bead, passing the part on to the next sta-tion in the assembly process or rejecting the part for re-work.”

Although bead tools are not a brand new tech-nology, before they were developed, the primary method for ensuring the correct application of dis-pensed material was to closely monitor the tempera-ture and pressure of the material itself, hoping to detect the skips in application as irregularities in pressure, continues Evanger. “The problem was

that sometimes the pressure and temperature can be correct, meaning the dispensed material is flow-ing freely, but it may be shooting out sideways, not even hitting the part. Mis-directed flows of this sort might not be detected by simply monitoring the pressure in the supply line, for instance.”

Current vision systems also make wider use of in-dustrial communication protocols. “Years ago, there were only a handful of serial protocols such as RS-242, RS-422, and RS-485,” says Greg Raciti engineer-ing manager, Faber Industrial Technologies. “But Ethernet technology has changed the way vision sys-tems communicate with robots. For instance, many of the popular industrial Ethernet protocols have been incorporated into modern machine vision platforms, including Ethernet/IP, Modbus TCP and Profinet, to name a few. Because a single device can communi-cate with many various nodes on the network, the old peer-to-peer connection is no longer a limitation.”

Raciti adds that another advancement is easy-to-im-plement calibration routines. “Software tools allow the vision hardware’s coordinate system to be trans-formed into one that matches the robot system,” he says. “Most modern robots are programmed to move in millimeters and not pixels. Also, the robot has its own x-y-z origin. The capability to calibrate vi-sion into robot units means the robot controller is no longer responsible for converting units and offsets. Instead, the vision system can locate the centroid of a part to be picked and send the robot controller its precise location in the robot’s native units.”

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The rise of robots and the enabling sensing technologyThe increase in robotic integration may be due to affordability and easier programming,

but sensors collect the information they need to be productive

By Mike Bacidore, chief editor

Robots have their hands—er, arms—in more man-ufacturing and assembly processes than ever. Af-fordability and ease of integration has helped to

increase their popularity, but none of that would be practical without the use of sensors to let the robot’s con-trol system know what to do, based on the environment.

As these two technologies converge, several ques-tions arise, so we posed them to a panel of industry vet-erans. They include Chris Elston, senior controls engi-neer, Yamaha Robotics; Scott Mabie, general manager of Americas region, Universal Robots; Helge Hornis, manager intelligent systems group, Pepperl+Fuchs; Vic-tor Caneff, business development manager, assembly and robotics, Banner Engineering; and Balluff market-ing managers Wolfgang Kratzenberg, industrial iden-tification, Henry Menke, position sensing, and Shishir Rege, networking.

Presence sensors run the gamut—from ca-pacitive and inductive proximity sensors to ultrasonic and photoelectric sensors, not to mention safety devices. Explain the importance of sensing devices to robotic integration in machines and equipment.

ELSTON: There are three types of sensor categories that come to my mind that are added in conjunction to an off-the-shelf robot, not including the sensors that are already pre-designed into a robot such as encoder feed-back or, in the case of Yamaha Robotics, resolver feed-back devices. Basic tooling sensing is end-of-arm tool-ing (EOAT) sensing. Preventive sensing is for things you don’t want to see happen to a robot, like a crash. Safety sensing is to sense humans when they’re close to a robot in motion.

Each of these categories is important to ensure prob-lem-free, maintenance-free and injury-free operation of industrial robots.

EOAT sensing is important in regards that not ev-erything is manufactured perfectly. EOAT, or grip-per, sensors are typically proximity or photoelectric type that sense when an object is gripped into the robot tooling, which is wired into the robot’s logic process. If the gripper sensor is not active, for exam-ple, program structure will instruct the robot to take a different path or maybe stop operations to alert an op-erator nearby to look and see what the problem is and why the robot does not sense a part in the gripper.

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This type of sensing is typically twofold, where it’s important to sense part presence with a robot to en-sure trouble-free operations and it also instills a qual-ity standard when a part entering a work cell is veri-fied and detected by the robot EOAT before the robot attempts to advance the process on the given part. All robots should at a minimum have EOAT sensors as a standard when integrated into a work cell.

Preventive sensing typically is trying to go one step above and beyond basic EOAT sensing. Most of the time, preventive sensing is made up of a type of an-alog sensor such as a load cell. These are used when a robot picks up a part, but there is a misalignment in the process. The robot doesn’t typically have the ability to feel that the two parts will not fit together, so adding preventive sensing allows an analog mea-surement of the robot process. If the load cell detects a higher-than-normal threshold, the robot would be instructed to stop trying to insert or assemble the part in process.

It’s the same process as basic sensing but gives a ro-bot the ability to “feel” the part as it works. Some ro-bots have the ability to run in torque mode vs. posi-tion mode when these critical operations are required. Other times, preventive sensing might be breakaway tooling. If there’s a crash or a foreign object that tries to damage the EOAT, additional analog sensors can stop the robot from damaging the tooling with its powerful motors. One vendor that comes to mind is ATI Indus-trial Automation with its collision sensor. ATI makse several anti-crash tooling adapters that sense when a robot process is not going the way it was designed.

Safety sensing has come a long way over the years.

The most high-tech method out there today is Sick’s 3D scanners. Normally positioned and programmed at the floor level to detect when a human steps foot into the radius path of a larger robot. These types of sensing technologies keep us safe from robots that run automatically, as sometimes we might become distracted or unknowingly could step into the path of a moving robot.HORNIS: Sensors are the eyes, ears, and fingertips of any kind of automated system, including robots. While computational improvements have allowed robotics sys-tems to become more powerful, smaller, and cheaper, sensing devices have not kept up. As a result, research-ers and engineers involved in robotics have identified the need for better sensing technologies. Robotic inte-gration may also pay a significant role in Industry 4.0.

For instance, material supply systems like KARIS Pro take advantage of robotics by enabling small and nimble automated units to act as individual AGV-like transport units that can, if required or advantageous, self-assemble into an intelligent conveyor system. KARIS Pro takes advantage of sensing devices from photoelectric scanners to RFID systems.MENKE: The primary mission of proximity sen-sors in robotic applications is to detect the presence or absence of work pieces and confirm that they are properly nested for work to commence. Especially on grippers and end effectors, sensors can confirm that the right component was picked up and was picked up correctly. This is accomplished with sen-sors located to detect the part itself, as well as the status of grippers and clamps. Such sensors can be simple discrete on-off types or more advanced types

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that provide continuous position or size informa-tion via analog or serial digital outputs. For exam-ple, an analog inductive proximity can measure the full travel of a robotic gripper to determine pre-cisely the degree of jaw opening or closing.

As work products and work cells are downsized, ro-botic work becomes more demanding, with a higher level of sensor precision required. As a result, minia-ture sensors are coming into wider use. In addition to obvious benefits like small size and low mass, minia-ture sensors are more exacting in their detection and operation. “Precision sensing” is a term that refers to the ability of miniature sensors to deliver more sta-ble detection points despite temperature fluctuations. They also offer more repeatable behavior from sensor to sensor, smaller windows of hysteresis—difference between on and off points—and better ability to detect very small targets that are invisible to larger sensors.CANEFF: Sensors give robots positional informa-tion to detect the presence or location of the material being processed, as well as personnel who may be ex-posed to hazards related to the robot system. Technol-ogies such as laser distance sensors and vision are the eyes of the robot that allow its end effector to properly pick up or process material and also avoid collisions with people or other objects in the area.

What is the most innovative use of pres-ence sensing in a robotic application you’ve been a part of?

HORNIS: Autonomous navigation is still one of the most challenging problems in robotics. While the au-tonomous car fleet built by the research division of a

well-known Internet company has logged many thou-sand miles in real-world traffic, the situation is quite different when the weather is not cooperating. Until such cars are able to drive in rain, snow and fog, the developers of sensors used to evaluate the environ-ment need to continue refining their solutions. We’ve invested significant resources in our pulse ranging technology (PRT), a time-of-flight method that offers significant advantages over a host of other attempts to provide reliable distance measurement information.

More specifically, we succeeded in implementing PRT in a 2D scanner so that we can offer a 360° field of view with virtually no wobble in the scan plane. This is an essential feature in robotic systems that must measure or detect objects close to refer-ence planes.

The second revolutionary development was the suc-cessful implementation of PRT using low-cost LEDs in-stead of laser light sources. These LEDs are then used to generate individual optical channels so that the resulting 2D scanner doesn’t have any moving parts. Having been able to solve the laser cost problem and constructing a 2D scanner without moving parts allows us to reduce the price point of both PRT and 2D scanners. Assuming that robot designers will continue to have many new ideas for low-cost robots, providing a reliable sensing and scanning technology at a lower price point is important.ELSTON: Preventive sensing has always intrigued me to try and write programs in robots that can react dynamically based on the force that’s sensed. It’s hard for a standard industrial robot to feel its way around a process. Most robots are programmed to a hard po-sition, and accurate placement of part and support

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tooling is critical to a robot’s success in a production environment. When one of those pieces changes un-expectedly, robots can’t think on their own to adjust the path or adjust pressure to manipulate tooling in a safe manner, unless you implement preventive sens-ing methods and write additional software code to handle those types of errors and handle the robot be-havior based on those logical changes.REGE: As robotic automation is evolving, robots are becoming more and more multi-purpose de-vices, rather than task-specific devices such as a de-cade ago. The need for quick tool change technol-ogy is on the rise. Inductive coupling technology for noncontact exchange of power and data over a small air gap is ideal for robotic tool changing. Inductive coupling offers several benefits over traditional pin-based coupling. Inductive coupling is noncontact, so there’s no mechanical wear and tear to worry about. Connections are instantaneous even with little axial offset between the base and the remote, so robots can identify the correct tool before engaging the tool changer.MABIE: Our robots can integrate presence sensing to implement dynamic collision avoidance with fix-tures and people. There are many ways to do this.

Contrast sensors can be used in conjunction with our robots to actually navigate the tool around, help-ing the robot locate a part and/or avoid unwanted collisions without using any conventional vision sys-tems at all. It’s an innovative, cost effective solution to a seemingly complicated problem.

A Microsoft Kinect sensor can be used to imple-ment motion control. Vision sensors are often used

with UR robots. One example, from Etalex in Mon-treal, Canada, included a Sick vision sensor that de-tected when the operator walked into the robotic cell, causing the robot to slow down.CANEFF: The use of adhesives for automobile as-sembly is becoming more common. However, veri-fying that glue is present and has been properly dis-pensed by the robot can be challenging with standard vision inspection tools. With a unique vision camera algorithm called “bead tool,” the adhesive can be in-spected for proper width, and, if any skips in the bead are detected, corrective action can be initiated.MENKE: Robots can follow pre-programmed paths, of course, but they can also be made smarter by pro-viding them with additional information about the status of objects in the physical world. The behav-ior of a robot can be altered in response to such en-hanced physical condition data. Presence sensors, especially those providing continuous analog or se-rial digital data, can be creatively applied to help the robot overcome less-than-ideal conditions in the pro-cess. One example is a robot removing and replac-ing components in a reusable transportation rack. The rack may be placed near the robot by a human worker or an AGV. In either case, the exact place-ment of the rack may vary in several directions. The robot can be programmed to approach the rack along a defined pathway. At a pre-determined point, laser distance sensors take over to help the robot locate key features on the rack and provide final real-time guidance into and out of the rack, even though it may be slightly out of the ideal position. This avoids damage to parts, tooling, and racks.

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Real Answers: Which positioning sensor is right for my application?How to make sense of the many options available for ensuring accuracy


Our metal stamping machines need to be able to run a variety of parts through the press. Any er-rors in the coil-change process can be costly. The

manual changeover itself is time-consuming, but a damaged press means expensive repairs and lost pro-duction. We need continuous position sensing, but our machines typically operate in harsh environments, and sometimes vibration and magnetic fields can be an issue. There are so many potential positioning-sen-sor solutions. What’s the difference? Any advice on where to start?MARK HERSUM: Would recommend using a po-sition sensor that is not based on a magnetostrictive technology for this application to due your concerns for shock and vibration tolerance as well as noise im-munity. Either potentiometric (track and wiper) or in-ductive technology sensors would be better choices.

One such sensor is Novotechnik’s LS1 Series. With-out knowing your requirements for stroke length and accuracy, these sensors have worked well in applica-tions similar to yours and have the following speci-fications: Programmable to optimize the sensor for your application, Stroke lengths up to 200 mm, lin-earity of < ±0.15%, life of >100 million movements, voltage or current output, sealed to IP 40, withstands shock up to 100 g and vibration up to 20 g. The LS1 Series has been tested to meet or exceed the follow-ing noise immunity standards: ESD EN 61000-4-2,

EN 61000-4-3, EN 61000-4-4 and EN 61000-4-6 for continuous, radiated, burst and disturbances in-duced by RF fields respectively. With this sensor se-ries you can typically measure within ±0.15 mm for a 100 mm stroke length. If you would like to discuss your specific application and/or you need a sensor with a longer stroke length, Novotechnik engineers would be glad to make further recommendations.

HENRY MENKE: Great topic - thank you. You’re right, there are a lot of position sensors out there and choosing one can be daunting. The reason is that there is an even bigger universe of applications and mounting conditions to consider, and there is no sin-gle universal sensor that will work in every situation.

Your statement about harsh environments is an im-portant one. A suitable sensor needs to be physically robust. Most industrial sensors anticipate shock, vibra-tion, temperature swings and the presence of water and/or oil. What many do not anticipate is the danger of physical impact. In that case, special brackets and guards are available to protect and “bunker” the sensor.

Regarding the best technology or physical sensing principle, it depends on the measuring range and whether or not the sensor needs to detect without any mechanical contact, i.e. at a distance. For short range (generally less than 12 mm or 0.5”) measurement of moving metal (ideally, steel) targets, analog inductive

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sensors are a great choice. Be wary of extremely strong magnetic fields, however; these can saturate the sen-sor coil core and cause it to lock its position. On the upside, inductive sensors are extremely hardy and are relatively inexpensive compared to alternatives.

For longer ranges (a few inches to a few meters), analog photoelectric distance sensors (e.g. infrared, visible red, laser) are a good option but keep in mind

they are subject to dirt, oil, mist, vapor, and unin-tended beam breaking caused by people, tools, equip-ment, or materials. This can cause unexpected and possibly dangerous machine reactions if not antici-pated by the programming.

Similar to photoelectric sensors but more robust physically and more hardened against dirty envi-ronments are the analog ultrasonic position sensors.

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They are still subject to unintended beam breaking and successful application can be a bit trickier. Usu-ally an ultrasonic is selected when a photoelectric solution will not work.

The position sensors discussed above may all work fine within their measuring performance envelopes, but for more precision position sensing jobs they may lack sufficient resolution (smallest detectable change) and accuracy (deviation from ideal mea-surement). This is where magnetostrictive linear po-sition sensors come into the picture. These devices are available for internal installation in a hydraulic cylinder, or as externally mounted versions for at-taching directly to the machine. Sensor lengths are dependent on the needed measuring range and are typically available between a couple of inches up to about 300 inches (more than 7 meters). The position resolution can be in the range of ± 1 μm with typ-ical accuracy to ± 30 μm. This type of sensor is in-herently an absolute positioning system, so that the sensor reports its position upon power-up with no homing. It is non-contact and wear-free, so service life expectancy is very long. This is contrasted with linear potentiometric or linear “pots”, which have a sliding contact element that is subject to wear and deterioration, particularly in dirty and/or high-vibra-tion environments. In particular, a linear pot can burn through its rated cycles in a stationary position due to machine vibration, resulting in a bad position reading at that spot. Linear pots can also become rather expensive in longer lengths relative to equiva-lent-stroke magnetostrictive sensors.

Magnetostrictive linear position sensors use a very strong permanent magnet as a position marker, and are largely unaffected by all but the strongest mag-netic fields. The magnet can be “captive” which

means it slides on a low-friction rail and is moved by an operating rod attached to the moving mem-ber of the machine, or the magnet can be “float-ing” above the sensor on a bracket that moves with the machine. Automatic gain control compensates for variation in the magnet-to-sensor distance. De-pending on the mechanical layout and restrictions in your situation, one or the other set up would be-come fairly obvious. Magnetostrictive linear posi-tion sensors have been around for a few decades and have been developed to a high degree of reliabil-ity and application flexibility. Shock tolerance is in the range of 150g and continuous vibration to 20g. Just about every conceivable electrical interface and housing variation is available.

If the machine “ways” or guide rails are fairly stable and precise, an emerging option is the mag-netic tape linear encoder. This consists of a preci-sion-coded magnetic tape that is glued to the ma-chine or installed in an aluminum track. A sensor head then floats over the tape or rides a sliding sled back and forth. The biggest caveat is the sensor-to-tape gap. Generally: the more precise the system, the tighter the gap tolerance. Some gaps are in the range of 0.3 mm, 2.0 mm, up to 6.0 mm or more. If possible, install the system so the sensor is stationary (no cable flex) and the tape moves with the machine. Otherwise, the flexing sensor cable must be man-aged (not impossible but easier to avoid it if you can). The position resolution for magnetic linear encoders can be in the range of 1 μm with typical accuracy to ± 20 μm for precision systems, and resolution of 10 μm with ± 0.4 mm accuracy for “big gap” systems. Magnetic linear encoders are generally incremental quadrature interface but there are absolute digital versions recently appearing in the market.

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The impact of wireless technology on presence sensors with robotsHas there been an impact? We posed the question to a panel of industry veterans.


What impact, if any, has wireless technology had on presence sensors with robots?

We posed this question to a panel of indus-try veterans. They include Chris Elston, senior con-trols engineer, Yamaha Robotics; Scott Mabie, general manager of Americas region, Universal Robots; Helge Hornis, manager intelligent systems group, Pepperl+-Fuchs; Victor Caneff, business development manager, assembly and robotics, Banner Engineering; and Bal-luff marketing managers Wolfgang Kratzenberg, in-dustrial identification, Henry Menke, position sens-ing, and Shishir Rege, networking.

Here’s what they had to say.KRATZENBERG: Ultimately, wireless technology has boosted the brain power of robots. Think RFID. In many applications RFID can be seen as an evo-lution from the simpler presence sensors. Now, instead of just identifying that something is pres-ent, we can identify with certainty what actually is present. Logic in the controls can then instruct the robot to perform a task based on the informa-

tion saved in the RFID tag’s memory. For instance, placing a tag on the end of arm tooling to identify the correct tool is being used for the correct job. In addition to a unique identifier specific to that tool, other information such as usage, maintenance and process data can be stored on the tag, as well. Ba-sically, the reader is placed on the end of the robot arm, and the robot is instructed to look for a spe-cific tool. When the reader identifies the correct tool the robot can couple with the tool. This en-sures the correct tool is being used and it has been properly maintained and it is fit to perform the task. This is all accomplished by sending data through the air wirelessly.

MABIE: Wireless technology is still considered not safe enough within the industry. It’s always eas-ier to see a broken wire than invisible electromag-netic signals. But the technology has been getting increasingly robust and will definitely be inf luenc-ing the industry in the near future.

HORNIS: We have not been involved in dedi-cated wireless technologies for robot control systems.

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Instead, we are working on SmartBridge, a wireless interfacing technology for a certain class of indus-trial sensors and, in addition to typical machine ap-plications, may be beneficial in robotics, as well.

In a nutshell, SmartBridge interfaces with IO-Link and other intelligent sensing devices and al-lows users to perform configuration and mainte-nance operations using smartphones and tablets.

In addition to this user-centric utilization, it is, of course, possible to intercept the wireless traffic using other devices or computers. In this scenario, SmartBridge acts as a parallel commu-nication channel between the sensor and, for in-stance, an automated maintenance system. While the process controller, for example, the PLC, is using the sensor data to run the process, Smart-Bridge sends other data such as temperature or operating hours to the maintenance system.

Imagine a sensor that monitors the rotational speed of an electric motor. The PLC is tasked with keeping the speed within certain tolerances. Combining this data with the current consump-tion of the motor, obtained from a second sensor, and the shaft temperature, from a third sensor, will allow the maintenance system to predict pos-

sible production interrupts due to bearing failure. Having enough data points will enable preventive maintenance to be scheduled without a loss in machine uptime and productivity.

CANEFF: With the necessity of deploying sen-sors on robot end effectors, cables that are being continuously f lexed can become unreliable and a maintenance burden due to frequent replace-ment. The use of wireless I/O to transmit analog and digital signals from sensors on the robot to the control system can improve the overall system reliability.

ELSTON: Wireless sensing is only handy to re-duce the wiring harness on a robotics arm. There is certainly always a concern when too many sen-sors, air lines or motor cables are dangling from a robotics arm that could be f lexed over time and cause failure. Or even snagging on another object in the work cells is typically the normal “death” of basic sensors on a robot EOAT. Honestly, I have seen too many wireless sensors on robots, but it seems a good place to start would be basic sensing of EOATs that could reduce some of that labor headache of looming wires from the base of the robot all the way to the tooling.

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Sensors: Automation for nanopositioning systemsEvolution of nanotechnology and nanomanufacturing impose demanding performance requirements

By Dr. Jingyan Dong, North Carolina State University

Multi-axis nanopositioning systems are widely used in most of modern nanotechnology and nanomanufacturing applications, such as

scanning probe microscopy, nanofabrication and na-no-patterning (e-beam lithography, for example), as well as various other scanned probe/tool-based me-trology and process equipment. In these applica-tions, the nanopositioning systems provide the dis-placement between the sample and the nanoscale manipulator, define the geometry of manufactured features with nanometer accuracy and implement high-precision alignment between different features.

Precision and accuracy are critical requirements for the nanopositioning systems used in nanotech-nology and nanomanufacturing. The nanoposition-ing stages are able to provide nanometer scale dis-placement with a total motion range around at least tens of microns. To achieve such high performance, the design of the positioning mechanisms, the selec-tion of the sensors and actuators and the implemen-tation of advanced controllers need to be systemat-ically investigated and integrated, which make the

nanopositioning systems very different from conven-tional scale motor-based motion systems.

Most nanopositioning systems adopt some flex-ure based mechanisms. When the actuation force acts on the mechanism, the flexure joints or flex-ure structures are deformed to produce the desired displacement. Compared with conventional scale motion slides, flexure mechanisms provide infinite motion resolution and preclude friction and back-lash found in sliding systems, thus the positioning resolution is not limited by friction or stiction. The trade-off for the resolution capability of the flexure mechanisms is their small range of displacement, since the flexure joints have to be operated at the elastic region of the flexure material. If a large mo-tion range is a critical requirement, some low fric-tion slides, such as air-bearing slides have to be se-lected to reduce the error from friction.

Many commercially available nanopositioner de-signs focused on single-degree-of-freedom axis mod-ules, and multi-axis systems are realized by stacking individual units together. From a kinematics point of

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view, these designs belong to the class of serial kine-matics systems in which the axes of motion are con-nected serially from a fixed base to the end effector to form an open kinematic chain. These designs are easy to implement and build from, and they have been used in many applications. The main prob-lem in these designs stems from the relatively large inertias and relatively low dynamic stiffness of the resulting systems, which implies relatively low posi-tioning bandwidths. In addition, these designs use different masses for motion in each axis that result in different natural frequencies and thereby per-formance capabilities for different axes, which in some instances are undesirable. Parallel kinemat-ics designs overcome the above problems by having multiple kinematic chains that connect the base and end effector in a parallel scheme. Compared with serial kinematics designs, parallel kinematics designs usually have high structural stiffness due to their truss-like structure and shorter kinematic chains. Control design for serial kinematics systems is relatively simple because each actuation maps to a single direction of motion, orthogonal to the oth-ers and are thus decoupled. For the parallel kine-matics systems, control design can be pretty com-plex; such decoupling is often not possible as the inf luence vectors of the actuators may not be or-thogonal.

Actuators affect virtually every performance at-

tribute (positioning resolution, repeatability, work space, bandwidth and size) of a nanopositioning system. Conventional actuators, such as motors, re-quire transmissions and bearings, and are generally difficult to miniaturize. Voice coil actuators can ideally provide infinitely high positioning resolu-tion. However, the coils for establishing magnetic fields cause heating, leading to large thermal de-formations. Piezoelectric actuators are widely used in nanopositioning systems due to their faster re-sponses, higher stiffness and larger force output.

Piezoelectric actuators are easy to control and need very low electric current in static or qua-si-static operation, which helps to alleviate heat-ing problems. Furthermore, piezoelectric actuators have very good energy conversion efficiency since electrical energy is directly transformed into me-chanical energy. One of the primary concerns when using piezoelectric actuators is their very small dis-placement output. For typical piezoelectric ceram-ics, the maximum strain capability is about 0.1%. To increase the displacement out, compound piezo-electric stack actuators need to be made by stacking multiple actuators together, and the lever transmis-sion mechanisms need to be used.

Other concerns regarding piezoelectric actuators in-clude high driving voltage, unidirectional load capa-bilities (large compressive loads but a very low tensile load), aging and vulnerability to high temperatures.

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Displacement sensors are essential to build a closed-loop nanopositioning system with improved accuracy. Their resolution and bandwidth have im-portant effects on the performance of the closed-loop system. Widely used high resolution sensors in nanopositioning include capacitive sensors, lin-ear variable differential transformers (LVDTs) and stain gauge sensors. All of these sensors can po-tentially provide nanometer scale resolution for nanopositioning systems with limited measure-ment range. Laser interferometers can potentially achieve sub-nanometer resolution and accuracy with measurement range in meter level. However, the equipment and its calibration are very expen-sive and complicated, and a well-controlled envi-ronment is generally required. The disturbances due to atmospheric temperature and humidity vari-ations, air turbulence and vibrations can degrade the measurement accuracy.

Closed-loop controllers are indispensable for a nanopositioning system to achieve nanometer pre-cision. Nonlinearities from the piezoelectric actua-tors, such as hysteresis and creep, as well as model uncertainties arising from f lexure-based mecha-nisms, need to be compensated by the closed-loop control system to provide high positioning perfor-mance. Due to the frictionless motion from f lex-ure structure, the f lexure joints act as pure springs, and the nanopositioning systems generally have ex-

tremely small damping effect, which make the con-trol design challenging for achieving a stable and high bandwidth system. Proportional-integral-de-rivative (PID) controllers don’t work well for such low damping nanopositioning systems, as it is dif-ficult to provide enough stability margins to sup-press the vibration. Proportional double integral (PII) controllers are the most common forms of controllers used for nanopositioning, since they can track ramp signals with zero steady-state error and are easy to implement. However, the best achiev-able bandwidth using PII controllers is very lim-ited, due to the simplicity of such controllers. Many advanced controllers, such as the robust controller and iterative learning controller, have been devel-oped recently to obtain high-bandwidth, high-res-olution nanopositioning. These controllers can sig-nificantly improve the positioning performance but require much more computational power from the control hardware.

The fast evolution of nanotechnology and nano-manufacturing impose demanding performance re-quirements for nanopositioning systems. More inno-vations in mechanism design, sensing and actuating techniques are being introduced, and advanced con-trol systems will continue to be developed to match the needs and offer new capabilities for emerging high-resolution semiconductor manufacturing and nanomanufacturing.

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To be sure you hit a target, you have to see or some-how sense it. That truth lies behind the theory and practice of presence sensors, which are used to de-

termine whether a box, bottle, piece of metal, or some-thing else is there or not. Armed with that knowledge, a machine can then send a package where needed, shape a piece of metal as desired or take other action.

Sometimes this function is referred to as proximity sensing or object detection.

“Typically what they mean by that is on-off type of sensors, sensors that turn on when a target is present and turn off when a target is removed,” says Craig Brockman, marketing manager of presence sensing for Rockwell Automation.

The oldest type of presence sensing is a limit switch. If contact with an object is OK, then a limit switch can be an acceptable solution, explains Brockman. Advantages are low cost and simple operation, thanks to rocker arms that move to open or close a circuit. But limit switches are mechanical devices. Hence, they’re prone to wear and misalignment, which can mean more maintenance.

What’s more, a limit switch requires contact. This can be a no-no, since mechanical contact can mar a surface and make some sort of cleanup or post-processing necessary.

For that reason, limit switches are not typically used for finished products, says Brockman. And there’s been a general movement away from limit switch presence sens-ing, he adds.

Other approaches exploit noncontact techniques, using light, sound or changes in magnetic fields to pick up the presence or absence of an object. All of these techniques have the advantage of not touching the surface of the target, and so they can be used on finished and unfinished prod-ucts alike. They also don’t depend upon mechanical mo-tion and so can be the basis for long-lived presence sensor.

In evaluating which of the different approaches is best, a key factor is the nature of the target. If it contains a ferrous metal, then an inductive presence sensor can work. In this method, a target entering a magnetic-field changes it, resulting in detection.

In contrast, a capacitive presence sensor can pick up a change in the electric field caused by any nearby object. It works in the same way some touchscreens detect when a finger is nearby but not in actual contact. In capacitive presence sensing, the target can be anything and doesn’t have to be metallic.

Inductive sensors are inexpensive and widely used. They share a characteristic with capacitive sensors, one that helps determine whether or not they’re suitable.

“How far away is the target from the sensor face?” asks Brockman. “If that’s a relatively short range, less than 50 mm, then everything is still on the table. If it’s longer than 50 mm, you can basically take off inductive or ca-pacitive from that list.”

For ranges beyond that, say from 2 cm out to 50 m, pres-ence sensing comes down to light and sound. Both types of sensors generate a beam that interacts with an object in ways that can be detected, either by the beam being blocked or reflected back. Which type of presence sensor is best for a particular situation is determined by the tar-get’s attributes and such factors as its rate of movement.

Transparent liquids present problems for light-based pres-ence sensors, since the probing beam easily travels through them. With little light absorbed or reflected, there’s almost no change in the signal between the liquid being present or absent. Hence, for liquids, ultrasonic presence sensors that use high-frequency sound may be the best choice, although there are tube-mounted liquid-level-detection fiberoptic sensors, says Kristen Chonowski, product mar-keting manager at Omron Automation and Safety.

Targeted detection with presence sensingProximity sensors enable object detection to determine what is where

By Hank Hogan

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As for light-based presence sensors, they come in a va-riety of configurations, she adds. For instance, the light can arrive at and depart from the sensing area via fiber-optics. This offers the advantage of making the sensor head remote, thereby reducing the size of the presence sensor and potentially making it more robust in the face of harsh environmental conditions.

Another basic configuration choice has to do with location of the emitter and receiver with respect to the target. Light-based presence sensors can be through-beam, retro-reflective or diffuse. In the first and second cases, an object is present when a beam is broken, with a through-beam approach having the emitter on one side of the target and the detector on the other. In a retro-re-flector, a reflector sends the beam back to the detector, which sits on the same side of the object as the emitter.

In contrast, a diffuse presence sensor detects the light coming back from the target itself. Consequently, this approach can be impacted by the orientation of the ob-ject, its speed of travel, and surface differences, with the latter a characteristic that can be exploited.

“Sometimes the difference between something being right-side-up or upside-down could be its color shade,” says Chonowski. “If one side is much darker than the other, then, rather than recommending a color sensor, a standard diffuse reflective photo eye can be used.”

Light-based techniques can work over long ranges, offering detection of objects that are meters or tens of meters away. Key to achieving the longest possible dis-tance is the brightness of the source, with a more intense source yielding reliable presence sensing of more remote objects. That reach comes at a price, however.

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“Lasers will give you the longer range, but they usually don’t have the temperature range that LED versions have,” says Tom Corbett, product manager of photoelectric sensing at Pepperl+Fuchs.

He adds that, no matter the light source, in general diffuse sensors have the most limited reach, with ret-ro-reflectors offering more range. Through-beam techniques offer the longest range.

It’s not only the source but also the environment that determines the range, particularly over time. Corbett notes that accumulating dust and dirt will diminish a light-based signal, with one possible eventual result being that the difference washes out between a target being present or not. Such a drop in the signal can be detected and the sensors then cleaned. If this happens too often or such routine

maintenance is too difficult due to where the ob-ject detector is mounted on a machine, then a light-based presence sensor might not be the right choice. In these circumstances, an ultrasonic or inductive technique may be the answer.

Beyond the target and environment, it’s vital to remember that a presence sensor is not there sim-ply to detect an object. It also has to fit within what-ever space is available and notify the larger system, a requirement that determines the final aspects of the configuration. Examples of what must be consid-ered could involve how the presence sensor is wired, which could be to a PLC or to trigger a solenoid.

Summing up this aspect of a presence sensor, Cor-bett asks, “How is it going to interface with the rest of the machine?”

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Many sensing applications re-quire both accuracy and preci-sion. The trick is to know when

one or the other takes precedence.“Accuracy is the closeness of

the measured quantity to the true answer,” explains Wade Mattar, flow product manager, Schnei-der Electric. “And precision is the closeness of repeated mea-surements to each other (Fig-ure 1). Custody transfer, filling operation and batch operations are a few examples that require a combination of both accuracy and precision, or reliability.” But many applications are out there.

For example, although a robot may have a very good repeatabil-ity at +/-0.020 mm, it has difficulty replacing a CNC machine due to the robot’s poor accuracy in this space, says David Perkon, vice president of advanced technology, AeroSpec. “A multi-axis CNC ma-chine starting at a datum, or zero point, and moving 600 mm in any

direction will move that exact dis-tance within a tight tolerance,” he says. “A robot or multi-axis gantry on the other hand, moving 600 mm in any direction will have much higher variation in the final position due to poorer accuracy, but it will precisely move to the same inaccurate position.”

Combining accuracy and preci-sion around a taught robot posi-tion or within a limited range of motion is common and is a good practice if accuracy is suspect, Perkon explains. “Where poor ac-curacy starts to show is in large spaces where the desired posi-tions are calculated from a single starting position,” he says. “The error adds up. However, teaching multiple points and taking advan-tage of the robot’s or motions sys-tem’s repeatability often helps.”

Applications that require both accuracy and precision include part dimension and tolerance measures, part positioning and

What does your sensing application require?Because accuracy and precision come at a price, you need to understand just how accu-

rate and how precise a system needs to be.


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ACCURACY AND PRECISIONFigure 1: Accuracy is the closeness of the measured quantity to the true answer, and precision is the closeness of repeated measurements to each other.

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other metrology tasks, explains Ben Dawson, di-rector of strategic development, Teledyne Dalsa. “Machine vision tasks that use edge-based mea-surements lend themselves to this combination be-cause we can measure edge position to a fraction of a pixel and have calibration methods that reliably translate pixels into standard measures,” he says. “On the other hand, some types of applications, such as verifying that a part is present or detecting surface defects, generally do not need high accu-racy and precision; you just want to know if the part is there or undamaged.”

If an application needed to move a robot arm to the same place repeatedly for an assembly process, for instance, high precision may be desirable, says Matt Hankinson, senior technical marketing man-ager, MTS Sensors. “Typically, the linear placement is adjusted after the mechanical setup of the system to correct any offsets, so accuracy, or true distance traveled, isn’t required,” he explains. “If an appli-cation is not going through an initial adjustment and it’s critical to travel a known distance without any offset, then high accuracy may also be required from the sensor. There is also an ISO 5725 standard with all the details.”

Imagine using a linear scale inscribed on a metal bar to measure a length, offers Peter Thorne, di-rector of the research analyst and consulting group at Cambashi. “Perhaps repeated readings give re-sults all within 0.1%; this is a measure of the preci-sion,” he explains. “If the scale had suffered an im-pact that compressed the metal bar, every reading might be 1% different from the true value. When using sophisticated measurement devices, there can be many possible sources of these systematic errors.”

The development of a production process can define

calibration procedures to achieve required accuracy, as well as tooling or sensor specifications, setup and test-ing to achieve required precision, explains Thorne.

“Statistical methods then provide an effective way of handling the variations found during production,” he says. “It may be possible to identify trends in read-ings and predict when they will fall outside of speci-fication, enabling some preventive action before this happens. The pattern of readings may be enough to identify likely causes. For example, electrical current consumption on start-up can identify wear and tear of motor bearings. There is a trade-off between ex-tensive measurement—for best prediction and de-termination of problems—and the time and cost of measurements—fewer sensors and readings generally make a process step faster and lower cost. Good spec-ifications help handle these trade-offs.”

Almost all applications need to deliver both pre-cision and accuracy to defined requirements, says Thorne. “Precision, even without accuracy, demon-strates control of a process; it delivers consistent re-sults,” he explains. “The hunt for and elimination of systematic errors will guide the process to deliver the required, accurate results.”

Both accuracy and precision come at a price, warns Colin Macqueen, director of technology, Trelleborg Sealing Solutions. “It’s important to understand just how accurate and how precise a system needs to be,” he explains. “Let’s say, for example, that you’re design-ing an hydraulic cylinder that will position a compo-nent during a manufacturing process and will need to hold that component in a specific position. You would then need to look at your system and ask how import-ant it is that the cylinder stops at a specific location—accuracy—and how important it is that the cylinder stops in the same location every time—precision.”

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A prime example of an application requiring both accuracy and precision, or repeatability, would be a custody-transfer application, where value of a com-modity such as petroleum fuels is being measured, or in a safety system such as cooling water for a nu-clear reactor, offers John Accardo, senior applica-tions engineer, Siemens.

“In a flow-related scenario, this would mean that the meter to be proven accurate would need some form of proving system or traceability to a calibration laboratory that was close to the real value that it is try-ing to measure,” says Jason Laidlaw, oil and gas con-sultant, Flow Group, Emerson Process Management.

“If this was mass, then the ultimate standard is 1 kg in Paris and accuracy referenced to this is achieved

by a sequence of unbroken measurements to get to the metering point, which in the United States would pass through NIST. You then need a meter that has good repeatability so you can determine how close you are to the reference, so one with precision of +/- 10 kg is not going to give very good results in deter-mining how close you are to 1 kg. If this was time, then measuring to 1 s can be done on pretty much any watch; how accurate it was would depend on how close you could compare to an atomic reference clock. If you then wanted to measure 1 ns, a normal watch won’t do this, so you need a timer counter that gives more precision, and generally costs more than an average watch. However, the accuracy is still then related to how close this value can compare to an atomic reference clock.” Laidlaw offers two visual aids as explanation (Figure 2).

“Accuracy is of paramount importance when there is a requirement to measure something to an actual standard, for example, a cut-to-length application,” explains Henry Menke, marketing manager, posi-tion sensing, Balluff. “The measurement error will be directly reflected in cut-length errors. Another application is precision CNC movement during part manufacturing. Again, any measurement deviation will appear as additional dimensional tolerances in the final part, beyond that caused by other sources. Typically, applications demanding accuracy also de-mand precision, because precision is a necessary but not sufficient condition to achieving accuracy.”

Precision is important in motion-control appli-cations, continues Menke. “A measurement system lacking in one or both elements of precision—reso-lution and repeatability—will not perform well,” he says. “Lack of sufficient resolution, for example, works against velocity control because velocity is the mathe-

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ACCURACY AND PRECISIONFigure 2: Think of the relationship between accuracy and precision as a graph or as the way marks group on a bull’s-eye.

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matical derivative of position. Poor position resolution results in even poorer velocity resolution, leading to un-stable velocity control as the system tries to react to lim-ited real-time data regarding instantaneous position. In extreme cases, this can result in bang-bang operation of the prime mover as it tries to go full-on first one way and then the other. This leads the system tuner to dial back the gain, slowing down response time.”

A motion control system with sufficient resolution but lacking in repeatability may operate smoothly and with good response time, but it will produce in-consistent results because the controller “trusts” that the position feedback is the same time and again, ex-plains Menke. “As far as the controller is concerned, it has closed the loop and made the measured vari-able match the command value,” he says. “How-ever, if the measurement is not very repeatable, the motion control system has no way to compensate. Measurement sensors with high precision and accu-racy tend to be those that employ a calibrated scale as part of the operating principle. The two primary technologies are optical scales and magnetic scales. The scales are manufactured to exacting tolerances by transferring the position encoding from a master reference onto it. The scale in combination with a reader head is called a linear encoder.”

Optical and magnetic encoders, for example, are targeted toward different classes of accuracy and pre-cision. “The optical encoders typically offer the ab-solute highest levels of accuracy and precision, but come at higher cost,” explains Menke. “They also come with some application considerations related to tolerance for shock, vibration and contamination by dirt particles and fluids. Magnetic encoders find application in areas where the ultimate level of accu-racy and precision is not required, but high accuracy

and precision are still required. Magnetic encoders are typically less expensive than optical, especially at longer measured lengths. Magnetic encoders are also more tolerant of adverse application conditions such as shock, vibration and contamination.”

Accuracy is used in roll diameter, thickness mea-surement and loop control applications, offers Dar-ryl Harrell, senior application engineer, Banner Engineering. “Repeatability is used in positioning applications and to determine if a part has fallen out of its specified tolerance.”

Kevin Kaufenberg, product manager at Heiden-hain says coordinate measuring machines, laser trackers and length gauges are some measurement applications that must encompass accuracy, while pick-and-place machines and robot arms are appli-cations requiring precision.

“Accuracy is degree of conformance to a specifica-tion or set of specifications,” says Phillip Warwick, product specialist, Eagle Signal & Veeder Root. For example, a product document may state ‘accurate to within +/-10% of set point.’ Precision is a scale or degree of accuracy, such as in the case of a position indicator; a reading or set point of 2.531 is more pre-cise than a reading of 2.53.

“Many applications for timers require a combina-tion of precision and accuracy,” explains Warwick. “Digital timers offer best set-ability and also may be set to provide timing precision of 1 ms. A typical ap-plication where timing accuracy and precision are important would be in a chemical injection pump, where a pump or solenoid is energized for a specific amount of time, for the purpose of ensuring proper ratios of compounds or mixtures. A high degree of accuracy is attainable due to the precision of the set points.”

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Question:

We’ve been using resolvers to track motor-shaft position in the CNC machines we build for years. And no one has asked, so we’ve never

considered changing that technology. But now a cus-tomer has requested we also utilize encoders. While the variety of encoder types—capacitive, magnetic, op-tical—is pretty straightforward, we’re having some dis-cussions as we try to understand the communication protocols available and which direction to go.

Can I replace a resolver with a sin/cos encoder? What about Hiperface, EnDat, SSI? If the application dictates the encoder or resolver, does the device dictate the communication protocol?

What readers had to say:VIKRAM KUMAR: Encoders these days typically

have what is called a standard ET7272 output line driver which can be configured as NPN or PNP type outputs. Furthermore, encoders can be specified to have 5V Out-put voltage (TTL compatible). What you will want to specify for your CNC machine is whether the encoder needs to be absolute or incremental. In terms of commu-nication protocols, you can now find encoders that have SSI, ProfiBus, DeviceNet and even Ethernet I/P commu-nications, so it all depends on your application.

HOWARD SALT: Quick answer: Yes, you can replace a resolver with an encoder, however it’s the subsequent electronics that will need changing too. You mention a CNC machine and Hiperface, EnDat and Ssi (protocols). A lot depends on the controller you are using for your CNC. If it’s a Fanuc, it will “often” be a serial encoder “talking” Fanuc. A Heidenhain controller talks EnDad,

Siemens, Siemens, etc. There is the ability with some of these controllers to take analog or quadrature encoder in-puts, too. A multitude of controllers and a multitude of encoder inputs. In short, make your controller choice then choose the encoder “protocol” to suit your controller and then choose the encoder (with the correct protocol) to suit your performance requirements.

RICHARD HALSTEAD: In general resolvers are more robust than encoders. They tolerate shock, vibration, temperatures better than most encoders. So you may want to have your customer take some responsibility for the reli-ability of the encoders on the machine. That said; encod-ers offer more features, higher resolutions greater accuracy so there are good reasons to use them. Given the machine environment you may want to consider some of the opti-cal interfaces to keep the noise problems to a minimum.

JEFFREY NAZZARO: There are a few things to look at when switching to a new feedback device. When switching from a resolver to an encoder you need to make sure that the encoder can survive in the applica-tions environment. Resolvers are much more robust rel-ative to high temp and shock. Next, I would look at re-quired resolution. Resolvers are typically 12 bit. If you are switching to an encoder with higher resolution than this make sure that the input frequency of your controls can digest all of the pulses/rev relative to the applications rpm. Another factor to consider is cost. The protocols you called out typically are associated with absolute en-coders vs. incremental encoders. Absolute encoders can be much more expensive especially if quantities being ordered are less than 1,000. Finally, what protocol you choose to go with can be primarily dependent on the motor and drive that is being offered. Different drive

Real Answers: Which encoder com-munication protocol is right for our application?If the application dictates the device, does the device dictate the communication?

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manufacturers may choose Hiperface over EnDat, or may offer both as an option. You need to make sure that the protocol you choose for the motor feedback will be a match for your selected controls.

CHRISTIAN FELL: The basic question to answer first is whether you want to use incremental or absolute angular information. Incremental is more common for speed control, while absolute is favorable for positioning. A sin/cos unit would be preferred if you need very high reso-lution (>16 bit steps; better 0.005 deg), because the output signals can be interpolated by the controller, but that elec-tronics will add to costs on controller end. Because you have used a resolver so far, most likely the resolution of any type of A-quad-B pulse output encoder will do the job. TTL, push-pull or differential line-driver outputs are most common in these days and it depends on the counter card of the controller available plus EMC considerations what to choose. For absolute encoders there are some different considerations. While all of the above mentioned inter-faces (Hiperface, EnDat and SSI) can do the job, two of the three leave you with single sources for the encoders due to the patent situation.

SSI is a very good option, because it’s easy to imple-ment, even with simple control systems, it’s sufficiently fast (can run with up to 2MHz clock) and offers basic er-ror checking features (multiple transmission, line break/short detection, parity or CRC features). You will also find hybrid units that provide SSI and incremental outputs to combine dynamic speed control with absolute position-ing. BISS would be another option, but will leave you with a limited number of encoder suppliers as well. In terms of costs and robustness a magnetic sensing system is rec-ommended, the modern systems don’t have issues with long cycle times and low accuracy any more. Depending on your current resolver specs/target specs you might even end up with lower overall costs.

DAYMON THOMPSON: Protocol isn’t as import-ant. Resolvers are more passive and can be thought of as analog in this scenario. The analog signal is converted into a digital resolution (for example, 16 bit). Resolvers are

also known for having higher vibration resistance in ap-plications. Resolvers used in applications can be replaced with a sin/cos encoder, as long as the resolution is taken into account or the number of counts per encoder revolu-tion is considered. The more resolution there is within the encoder, the more feedback information about position and/or speed you can evaluate in the control of the drive.

Most vendors of motors and drives have preferred com-munications protocols. Some of these encoder commu-nications protocols are open, while others are proprietary. This means that not all motors are compatible with all drives. There are a few drives available that encompass more protocols and allow much more flexibility in mo-tor selection. For example, the AX5000 EtherCAT servo drive from Beckhoff offers encoder options with BiSS, EnDat, and Hiperface in the same drive. This drive can also act as a VFD or handle vector control. Ultimately, the application dictates if incremental, single-turn absolute or multi-turn absolute encoders are needed. The encoder communication protocol is less important than well-sized motors and encoder selection that is appropriate for the application. Leveraging a more flexible motion system as a whole can make resolver and encoder replacement chal-lenges much easier to solve.

MATT TELLIER: The answer to your question is not straightforward. First, what protocols can your CNC con-troller accept as motor feedback? There are many data protocols that will work for your application. Multi-turn encoders are available for all of the interface types you listed but there are also networked encoders – Devicenet, Profibus, Ethernet/IP and Profinet just to name a few. What resolution does your system require? Multi-turn en-coders can provide 12 bit resolution per turn, 13 bits per turn, 16 bits per turn; you get the idea. All of these encod-ers will provide you with absolute position feedback. The choice will be determined by factors that you will need to examine further – resolution requirement, update rate of position and available interfaces for your controller.

Be aware that the most durable and reliable position feed-back is the resolver you are already using. So when you se-

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lect an encoder make sure the encoder you select can meet the mechanical, electrical, and environmental require-ments of the system. All encoders are not created equal.

TONY UDELHOVEN: The short answer to the dis-cussion question is that resolvers, like encoders or rotary inductive sensors, can be purchased with various output protocols. What is available will vary from manufacturer to manufacturer. Over the years, encoder technology has become the dominant design for rotary feedback devices. As a result, a manufacturer’s encoder product lines typi-cally offer a wider variety of output protocols than a resolv-er’s design. Selection of what rotary feedback technology is used comes down to price, performance and applica-tion requirements. Resolvers have a reputation for being able to offer high shock and vibration resistance. Rotary Inductive Sensors made by TURCK use a contactless de-sign that is extremely robust and offers a variety of output protocols. TURCK also offers optical encoders utilizing a double-bearing design for robustness. These feature the precision associated with optical encoders, including a va-riety of standard protocols for incremental, single-turn ab-solute and multi-turn absolute applications.

SCOTT ORLOSKY: I’ve read through the responses and see there are a lot of good answers already. Let me add to and reinforce some of the points. Resolver outputs are inherently absolute, however you may or may not require an absolute output. As mentioned by one of the contribu-tors, it is common practice to use an incremental encoder for speed (say for spindle speed control) and an absolute encoder for position (tool position would be an example). Resolution and precision for optical encoders exceeds that of resolvers, so your mechanics need to be pretty solid otherwise the encoders will reveal the errors (backlash or other non-linearity in your system).

Regarding protocols, for incremental encoders A and B in quadrature with complements and an optional index are very standard. This gives you a choice of a wide vari-ety of standard controllers. Most industrial systems have moved to 24 volts as the standard supply voltage and it is recommended to use this voltage and to take advantage of

a differential output as that combination gives you the best noise immunity. For absolute encoders I agree that SSI is the most universal interface and again gives you the wid-est selection of controllers to work with. I would avoid pro-prietary protocols only because it locks you into specific suppliers and reduces your flexibility.

BOB SETBACKEN: What type of machine is this? How big a project are you planning? Is this a total redesign or do you hope to just swap an encoder for a resolver? Most Sin/Cos encoders are not strictly absolute. They will have 512, 1024 or 2048 cycles per revolution, so they are not the same as a resolver with one cycle per revolution. An en-coder would be more like a two-speed resolver. One set of outputs would be one cycle per revolution and another set would be 512/1024/etc. So, you have some work to do even if this most basic conversion is planned.

The other comments have covered the ideas in rotary solutions, but you might also consider just leaving the re-solvers in place for speed control, and wrapping a position loop around them using a set of linear encoders on the ways. Not knowing the application, this of course would not work for a rotary table or spindle speed/tapping control.

One other comment is that inductive encoders are more or less like a printed circuit resolver. They are very robust, and provide pretty much any output protocol you might need. They come in large and small bore sizes and are suitable for hash environments.

GREG BOVA: There is no perfect conversion from resolver output to Sin/Cos encoder output. There are plusses and minuses to every change in feedback which has to be designed-in properly for a finely-tuned system. The device used does not dictate the communication pro-tocol. The engineering team manufacturing the machine decides on the overall communication structure required for the most efficient operation desired and monitoring of the machine. We would need a lot more information on the actual machine requirements to help further in this discussion. Thank you for your excellent question on feed-back devices. I wish you well in your new program replac-ing resolvers.

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For machine vision, basic differences are blurring and on the verge of disappearing. As a result, a long-run-ning battle may soon end, and machine builders

will then have one less choice to worry about.Fundamentally, machine vision cameras can be clas-

sified into two camps. On one hand are charge cou-pled devices (CCDs). The alternative vision sensors are built using complementary metal-oxide-semiconductor (CMOS), the same technology that powers computer chips. The two have long been locked in a struggle, which looks to be ending.

“Just in the past year, it’s become pretty clear that CMOS is going to be the dominant one,” says Vineet Aggarwal, when discussing the future of the two tech-nologies in industrial machine vision applications. Ag-garwal is senior group manager for embedded systems products at National Instruments. The company works with many different vendors of machine vision cameras.

Both CCD and CMOS sensors are silicon-based and both convert incoming photons into electrons. Of the two, CCD is the older technology. It was the only game in town from the mid-1970s until the mid-1990s, which is when the first commercial CMOS sensors appeared. One result of this technological headstart is that CCD sensors were traditionally considered higher quality, which was defined by two key attributes.

“One of these was better signal-to-noise ratio, and the other was a lower number of dead pixels, or pixels that don’t respond to light. But CMOS has improved greatly in the past few years and has closed the gap to

the point where the two are almost interchangeable for most applications,” says Rick Roszkowski, senior direc-tor of marketing for the vision products business unit of Cognex, which offers both CCD and CMOS sensors in its products.

An indication that CMOS vision sensors have gained parity is the fact that Japan’s Sony, which has tradition-ally only produced CCD sensors, released its first global shutter CMOS sensor in early 2014. That development is particularly important to the machine vision market, according to Michael Gibbons, director of sales and mar-keting for Point Grey Research. The company makes products with both CCD and CMOS vision sensors.

CMOS has traditionally used a rolling shutter that sequentially exposes each line of pixels in the sensor. Thus, not all parts of a scene would be captured at the same instant in time, and so objects moving fast enough could be blurred. With a global shutter, on the other hand, the entire sensor is exposed at the same time, eliminating a source of image distortion. Hence, the growing availability of global-shutter CMOS sensors means the technology is better suited for a wider range of machine vision applications, Gibbons explains.

Also important to the growing use of the newer sensor technology is that most digital-grade consumer cameras use CMOS sensors. As a result, CMOS is the target of the bulk of research and development spending.

That R&D work exploits a key characteristic of CMOS sensors—the light-to-electron converting sili-con sits adjacent to circuitry. This means that individ-

Two views of machine visionAlso important to the growing use of the newer sensor technology is that most digi-

tal-grade consumer cameras use CMOS sensors.

By Hank Hogan

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ual pixels can be read out in a largely parallel fashion, which makes the vision sensor capable of a faster frame rate. Another consequence is that analog-to-digital con-vertors can be built in, allowing features like integrated gain, offset and dark level adjustment. This makes it less expensive to integrate the sensor into a vision system and potentially reduces overall cost, according to Ro-szkowski.

Because CMOS sensor technology is both newer and evolving more rapidly, machine vision applications can benefit from such features as high dynamic range, vari-able trigger modes, light control output, windowing and on-chip image scaling, as well as the ability to exclude everything outside of multiple regions of interest.

However, it’s not quite time just yet to abandon CCD in all machine vision applications. For one thing, the vision technology found in consumer cameras is close to but not exactly the same as that in industrial applica-tions, which means machine vision cameras are not fol-lowing precisely the consumer device cost curve. One reason is that consumer cameras squeeze a lot of pixels into a tiny chip, which means each pixel is small, per-haps 1.5 microns (µm) across. A machine vision camera has much larger pixels that are typically 4.5 µm in size. The advantage of bigger pixels is they collect more light, but the downside is the sensor chips are larger and there-fore more costly for an equivalent number of pixels.

What’s more, CCD still offers advantages in certain applications. Because every pixel on the chip is read by the same electronics per tap or output channel, no pix-el-to-pixel variations are introduced by the readout cir-cuitry itself. Also, since there’s no circuitry in the way, nearly the entire surface of the sensor can collect light.

The result is generally greater sensitivity, and that leads to the only circumstances where the older technology might still be preferred over the newer.

“There’s only one reason to go CCD over CMOS—low light levels. In all other situations, CMOS offers greater speed and equivalent sensitivity, while benefit-ing from advances in consumer camera technology,” says Joachim Linkemann, senior product manager at Basler. Like the other camera manufacturers, Basler uses both CCD and CMOS sensors in its products.

Thanks to this greater sensitivity, CCD sensors are still preferred for scientific applications, such as those in astronomy and the life sciences. Both typically involve a photon-poor source, and so need to get the most out of any light that arrives. While industrial users typically don’t face the same issues, there can be cases where light levels are low and CCD sensors are a better solution.

On the other hand, good lighting is almost always critical to machine vision success. Hence, the in-stances in an industrial setting where light levels are low enough to make CCD sensors strongly preferred over their CMOS counterparts could be rare.

However, even this low-light advantage is in jeopardy. Roszkowski reports changes are underway that prom-ise to improve CMOS sensors in this area. “CMOS is moving to back-illuminated designs, which would allow them to be much more sensitive than current CMOS devices in the near future,” he says.

This approach puts the light sensing material on the back side of the chip while the circuitry stays on the front. Consequently, there is no decrease of incoming light arising from shadows cast by metal traces, transis-tors or other circuit components.

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What should we expect from presence sensing with the increase in collaborative robot applications—where humans and robots work in the same space?

We asked for advice from a panel of industry veter-ans. They include Chris Elston, senior controls engi-neer, Yamaha Robotics; Scott Mabie, general manager of Americas region, Universal Robots; Helge Hornis, manager intelligent systems group, Pepperl+Fuchs; Vic-tor Caneff, business development manager, assembly and robotics, Banner Engineering; and Balluff market-ing managers Wolfgang Kratzenberg, industrial iden-tification, Henry Menke, position sensing, and Shishir Rege, networking.

Here’s what they had to say. MABIE: Presence sensing will play a huge role in col-laborative robot applications. It is better to be more aware than less. If we can improve the robot’s environ-ment awareness to achieve more dynamic planning and collision avoidance, then why not? Of course, the pres-ence-sensing technology needs to be cost-effective, eas-ily accessible and readily integratable.

One of the key value drivers of the UR robot is the ease of integration with peripherals such as sensors. We’re making our software intuitive and GUI-based, as opposed to conventional command- or language-based programming, which is also available for advanced pro-grammers who want that level of access. Our Polyscope

interface is now more open and accessible to end users, enabling them to build their own interfaces and wiz-ards, even implementing their own custom communi-cation protocol.

HORNIS: Safety will continue to play an important role. But there is a second level of safety that will be of greater importance in automation and robotics in the fu-ture. The biggest issues with safety systems as we know and use them today are cost and complexity. We expect assistance systems, not unlike what we see in modern cars, to be much more important in the future. Assis-tance systems will not provide functional safety but help operators and robots in such collaborative situations. As-sistance systems will be used in situations where cur-rently nothing is used due to cost and complexity. It is too early to tell what kind of assistance systems will be accepted, but, again looking at the automotive market, it seems clear that the inclusion of a large number of low-cost technologies—ABS, ESP, blind zone assistant, distance radar, backup sensors—is ultimately much bet-ter than having just one safe-rated solution at the same price point, perhaps a backup sensor that utilizes an ex-pensive safe-rated scanner.

CANEFF: Although any robot has the potential for use in collaborative operation, new-generation robots on the market that are smaller, have power- and force-lim-iting functions and speed monitoring are commonly as-sociated as a collaborative robot. These functions alone

The role of presence sensing in collaborative robot applicationsWhat should we expect from presence sensing with the increase in collaborative robot

applications—where humans and robots work in the same space?


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do not necessarily address all safety requirements, and monitoring the separation between operators and the robot may be needed to comply with applicable stan-dards such as ANSI/RIA 15.06-2012, Safety Require-ments for Industrial Robots and Robot Systems. Current safeguarding technologies such as safety laser scanners and light curtains are still applicable; however, advance-ments in vision and LIDAR technology for 3D-safe-ty-rated monitoring of the space around a robot’s work envelope would open up more potential applications for collaborative operation.

ELSTON: Collaborative robots seem to be in an arena of their own when it comes to industrial robots. They are used when programming ease is desired and reducing safety costs is desired. End users typically favor collaborative robots because of the ease of integration. Because collaborative robots must work on the princi-ples of torque to sense when a human is near, they tend to already have preventive sensing built in along with safety sensing. However, the trade-off with this type of torque sensing is speed, and precision is given up. Col-laborative robots have an advantage with this type of technology already onboard, whereas the same “touchy-feely” response would need to be added to an industrial robot as an extra. The only difference is we don’t want to give up the speed and precision we already are familiar with when using an industrial robot.

REGE: The age of robots started out as replacing hu-mans in areas where operations are repetitive, hazard-ous or tedious. Then the concept of productivity and efficiency drove robotics automation to new heights, not only elevating robots’ operation speeds, but also increasing their payload capacity. This led to tremen-dous growth of robots in most sectors of manufactur-ing. Presence sensing related to operations, for exam-ple, picking correct object or number of objects, and presence sensing related to safety, for personnel inside of the robot work zones, started to become more and

more important. This development led to the growth of photoelectric and ultrasonic sensors on the end-of-arm tools on the robots, in addition to safety gates and interlocks, zone monitoring and other safety devices around the robot installation.

Now that robotics automation has come full cir-cle—working alongside humans instead of replacing them—the role of presence sensing has evolved to include smarter sensing technology, not only on the end-of-arm tools, but also around the robot itself. The new era of cage-free robots or collaborative robots brings new innovative applications to life, and with them come challenges of how to ensure the safety of people working around the robot while maintaining the efficiency or throughput for which the robots are employed in first place.

Software-based safety, which now comes standard with most robots, relies heavily on smart sensing technologies such as safe zone monitoring sensors, vision systems, and force-torque sensing. But visual-ization of the zones are still caged inside of the ma-chine HMIs. Programmable tower lights can play a significant role in the visualization of a robot’s work-ing conditions. Traditional stack lights were only ca-pable of showing current robot status of operation, but new programmable tower lights can be used to map directly to show the robot’s working condition.

For example, as someone enters a robot operating zone, it can show the robot slowing down visually using level mode of operation. The tower light can also show if any maintenance is required and the type of mainte-nance, or it can show the performance condition of the robot, its efficiency. With this on-demand and on-site visualization, people working around robots can get di-rect indication of how their behaviors are affecting the operation’s efficiency, which is very similar to a digital gauge in your car showing the impact on mileage of your car based on your driving behavior.

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In sensing applications, performance can be improved by combining accuracy and precision, but what’s the difference between them?Consider accuracy and precision in both two-di-

mensional and three-dimensional spaces, advises David Perkon, vice president of advanced technol-ogy, AeroSpec. “In its simplest form, accuracy is measured everywhere in space, and it is measured

to national standards,” he says. “It is exact every-where in the 3D space around the equipment work envelope, which is important in many applications. Precision, or repeatability, may vary depending on where the tool or part is positioned. A noted differ-ence between accuracy and precision is that, in the 3D space, repeatability is more consistent in some positions than others.”

In machine vision, accuracy means how close a measurement is to its true value relative to a stan-dard, explains Ben Dawson, director of strategic de-velopment, Teledyne Dalsa. “Precision is the fineness to which the measure can be made, often limited by measurement noise,” he says. “For example, when a machine vision system measures a dimension, the number of valid—noise-free—digits in the measure is the precision, and the accuracy is how close the mea-sure is to a reference dimension in, say, millimeters.”

You can imagine measurement as shooting ar-rows at a bull’s-eye target, suggests Dawson (Figure 1). “The center of the target is the desired true value of the measurement,” he explains. “Then accuracy is how close to the center your measurement arrows are, and precision is how closely your arrows are clus-tered, assuming that arrows have a finite diameter.”

How to hone your sensing applicationsFind out to hit the bull’s-eye with accuracy and precision


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BULL’S-EYEFigure 1: If an application isn’t going through an initial adjustment and it’s critical to travel a known distance without any offset, then high accuracy may also be required from the sensor.

Precision is a measure of the sensor’s ability to re-peatedly report the same value when moving to the same position, says Matt Hankinson, senior technical marketing manager, MTS Sensors. “It’s a statistical measure of the random spread or variability of values at the same point on the sensor scale,” he explains.

“Accuracy is a measure of the closeness to the true value across the full measuring range. In many ap-plications, it is important to have good repeatability, or precision, from a control perspective, so the move-ment is consistent, but it isn’t necessarily important to have absolute accuracy with the true value. The term accuracy is sometimes used to mean trueness, which is technically a different parameter. Trueness is the closeness to the true value over an average of samples and represents a systematic bias in the measurement. It is possible to have a large spread of measurements—poor precision—that average out to the true value, which would have a high degree of trueness. Accuracy is technically a combination of the multiple errors source for the overall closeness to the true value for each measurement. Resolution, not to be confused with precision, is the underlying smallest change that can be measured and doesn’t factor the repeatability of precision.”

Knowing the precision and accuracy of readings is key to effective measurement and sensing, ad-vises Peter Thorne, director of the research analyst and consulting group at Cambashi. “Take repeated readings, and, even when the item being measured is not changing, you will get a range of values,” he warns. “The differences are caused by random rea-

sons like electrical noise, or perhaps vibration of the equipment you are using. The precision of the mea-surement quantifies variations between repeated readings. Accuracy is different. It quantifies the gap between the true value and the measurement. Accuracy is often more difficult to determine than precision is, because systematic errors can impact the accuracy without changing the precision.”

If you’re carrying out a set of repeated measure-ments, precision describes the ability of your pro-cess to get the same result each time for multiple measurements of the same item, but it doesn’t take into account whether the results are close to the true value, explains Colin Macqueen, director of technology, Trelleborg Sealing Solutions. “Accu-racy describes how close your results are to the true value, but it doesn’t take into account the consis-tency of the results,” he says, “so precision tells you how far your individual results are from the mean, whereas accuracy tells you how far your mean result is from the true value.”

Precision is the degree to which the correctness of a quantity is expressed, says John Accardo, se-nior applications engineer, Siemens. “It is essen-tially how accurate your accuracy is,” he explains. “A better term, and one more commonly used, would be ‘repeatability.’ Then we could apply these answers. Accuracy is to be within a certain percent-age of a defined goal, such as +/- 0.5% of actual f low. Repeatability would be within a certain per-centage of a certain reading every time the same circumstances occur, such as +/- 0.5% at 100 gal/

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min. A measurement can be repeatable without be-ing accurate, but it cannot be accurate without be-ing repeatable.”

Accuracy is the closeness of agreement between a measured quantity value and a true quantity value of a measurand, explains Jason Laidlaw, oil and gas consultant, Flow Group, Emerson Process Management. Accuracy is how close a measured value is to a traceable reference source. Precision is the closeness of agreement between indications or measured quantity values obtained by replicate measurements on the same or similar objects under specified conditions—how well you can determine the scale of your measurement, how grouped they may be, which leads to how repeatable you can de-termine the result, he says.

“Accuracy is an indication of the degree of cor-rectness of a measurement,” says Henry Menke, marketing manager, position sensing, Balluff. “A measurement is said to be accurate when the indi-cated value closely corresponds to the state of the actual variable in the real, physical world. All prac-tical industrial measurement systems exhibit some degree of inaccuracy. The specification of accuracy is normally given using terms such as error, devia-tion or nonlinearity. All things being equal, smaller is better when it comes to this specification.”

Precision is an indication of the ability of a mea-surement system to detect small changes in the measured variable and closely repeat a measure-ment time and again under the same conditions, says Menke. “So, precision is actually defined by

two specifications: The ability to detect small changes in the measured variable is typically called resolution,” he says. “Again, smaller is better. The second specification of precision is normally given using terms like repeatability or repeat accuracy—approaching the same measured value from the same direction—and hysteresis or bi-directional re-peatability—approaching the same measured value from opposite directions. In both cases, the more precise system will yield a smaller figure.”

Every sensor has a measure of variation from the ideal output, explains Darryl Harrell, senior appli-cation engineer, Banner Engineering. “Accuracy is a measurement of a sensor’s actual output vs. the ideal output,” he says. “Precision is what we call re-peatability. This is a sensor’s ability to consistently detect a target at the same range.”

The definition advanced by ISO associates true-ness with systematic errors and precision with ran-dom errors, and it defines accuracy as the combi-nation of both trueness and precision, says Kevin Kaufenberg, product manager, Heidenhain. “We would add that the measure of accuracy is truly defined in terms of measurement by national stan-dards,” he explains. “The National Institute of Standards and Technology (NIST), which is part of the U.S. Department of Commerce, sets the bench-marks at which measurement science and standards in terms of accuracy must abide. Precision is the ability to perform accurate measurements repeat-edly and reproducibly under different environ-ments and users.”

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Information is power. And it can come from data gathered with any number of sensing devices.

These three applications demonstrate the vari-ety of sensors and the equally broad palette of uses for them. From replacing a linear variable differen-tial transformer (LVDT) with a laser distance sen-sor in a nut-welding, error-proofing application to swapping above-the-conveyor reflector sytems for integrated light-bar sensors, as well as applying mag-netic sensors to verify pneumatic-cylinder stroke length, these uses of sensors are a clear indication that machine-building innovation is alive and well.

Laser distance sensorsPaslin, a system integrator based in Warren, Michi-gan, designs and builds automated welding systems, gauges, tooling and fixtures. To support body con-struction operations in the automotive industry, Paslin builds a line of nut-welder machines. This particular machine is responsible for welding the nuts onto the front rail of the automobile frame.

During the operation, a flanged nut is placed into position at the position to be welded. A welding gun ram is then lowered onto the nut and the position is checked just prior to welding to ensure the correct orientation. On occasion, the nuts would get placed upside down, with the flange facing upward (Figure 1). Historically the method that had been used to verify the nut orientation was to measure the stroke distance of the ram when it lowered onto the nut. If the nut was inverted with the flange up, the distance traveled would be reduced by approximately ¼ in to 3/8 in, and therefore the control system would

know to reject and replace that particular nut prior to welding.

“The machine as a whole uses a variety of sen-sors: proximity sensors for detecting the base part, air pressure and water flow sensors for validating the correct conditions prior to welding, as well as prox-imity for detecting the nut feeding cylinder and the status of the bowl feeder,” says Ron Pomaville, direc-tor of controls for Paslin. “The typical failure modes for the automatic nut feeder would be a missing nut, an upside down nut, a double nut, or a nut out of po-sition. Using the stroke distance, the control system can determine if the gun is in the correct calibrated depth.”

Historically, an LVDT had been attached to a fixed position on the machine frame and onto the

How sensors enable informa-tion-based decisionsOptimize production, eliminate manual intervention and incorporate quality checks

By Gary Highley, contributing editor

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IN POSITIONFigure 1: Paslin wanted to make it easier for both assembly workers and maintenance personnel to work on the machine, which meant transitioning to a different method for sensing the ram position and therefore the orientation of the nut.

PASL

IN

ram of the machine in order to measure the stroke length. An LVDT is an electromechanical device used to convert the rectilinear motion of an object to which it is coupled mechanically into a correspond-ing electrical signal. LVDT devices typically use a nickel-iron core surrounded by primary and second-ary windings to enable measurements as small as a few millionths of an inch.

While LVDT devices can achieve very high ac-curacy of measurement, they are mechanical de-vices and require a connection to both the fixed and moving portions of the machine. Paslin wanted to reduce the maintenance required on its machines due to environmental contamination and make it easier for both assembly workers and maintenance personnel to work on the machine. Obtaining this improvement, in part, meant transitioning to a dif-ferent method for sensing the ram position and therefore the orientation of the nut.

Several alternatives were considered. Paslin de-cided to eliminate the electromechanical LVDT de-vice and replace it with a Balluff BOD 21M laser distance sensor (Figure 2).

“The new application of the sensor is for using an analog laser sensor for detecting the distance of travel the weld gun makes prior to the weld,” ex-plains Pomaville. “It is also helpful in non-operator applications to signal the robot that the gun is clear and the robot can re-position the part. This sensor was chosen over previous versions with LVDTs. The sensor was cheaper, easier to mount, easier to set up and easier to maintain, and it could still protect against all failure modes.”

Advantages of the laser sensor over the previously used LVDT device was that it only needed to be mounted on one fixed point of the machine—mechan-ical adjustment was no longer necessary—and it was resistant to any environmental dust and contamination that may occur. These sensors are wear-free and are available with sensing resolutions down to 30 µm.

Interfacing the laser sensor required little modi-fication of the control system, as they are available with a variety of analog output options to include the common 1-10 Vdc and 4-20 mA standards. The immediate advantages were in the initial setup of the machines.

“The technical interface to the system is very sim-ilar—analog input 24 Vdc, for example,” says Po-maville. “The major difference was in the setup re-quired. Since the switch was easier to mount and could be tested by placing an object in front of the sensor, it could be checked for linearity more easily than removing the LVDT mounting arms or having to measure approximate stroke.”

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MOUNTED TO MEASUREFigure 2: Paslin decided to eliminate the electromechanical LVDT device and replace it with a laser distance sensor mounted on the machine.PA

SLIN

Light bar sensors

TGW Systems in Spring Lake, Michigan, is a man-ufacturer of automated conveyor equipment used to transport a wide variety of products. Their conveyor and sorting systems are used in food and beverage, manufacturing and production, and order fulfill-ment and distribution centers. Companies who use material handling conveyors are constantly looking for ways to improve the reliability and performance of their conveying systems. They typically are or-ganizations that specialize in highly automated lo-gistic or material handling solutions that demand adherence to tight shipping schedules and high throughput volume. Unscheduled downtime caused by equipment failures can be both costly and disrup-tive to the business operations.

Package sensing for conveyor systems has tradi-tionally been accomplished with above-rail, over-the-conveyor photo eye and reflector systems that can sense a package or product moving down the conveyor line when it breaks the light beam. Photo-electric sensors mounted above the conveyors have been the standard for many years. It’s a proven sys-tem but has limitations on where it can be used.

TGW’s distributors and end users were looking for a way to increase the reliability of the conveyor sens-ing systems and reduce the need for maintenance. Above-rail sensors are typically not suitable for use with telescopic type conveyor systems. Telescopic conveyor sensors are typically embedded inside the railing system to provide the clearance needed to enable telescopic operations. There may not be enough clearance for the sensor bodies and cable to be mounted inside the side railing. Additionally the above-rail sensors have a difficult time sensing thin film items such as envelopes that may not present a large enough profile and so they go undetected.

Photoelectric sensors and reflectors mounted above the conveyor are prone to damage from the product

being moved. The sensors and reflectors can be phys-ically damaged by items such as loose garments com-ing down the conveyor and other material handling resources in the area. Other downtime events can be caused by sharp impacts to the sensor housing by ob-jects being used to support the day-to-day warehouse activity. Conveyor vibration over time can also loosen mounting hardware, causing misalignment issues that add to downtime costs. The goal was to find a system to reduce all of these failure modes.

TGW decided to use a Telemecanique OsiSense XUY Roller Conveyor Sensor, which is mounted be-tween the rollers of the motorized roller conveyor sys-tem, thus eliminating the above-rail system (Figure 3). “Being installed between the rollers makes it less subject to damage and misalignment, and it makes for a clean-er-looking installation,” explains Greg Braden, senior development controls engineer at TGW. “The system is very easy to install or retrofit, with no reflector needed.”

Having the sensor embedded in the conveyor be-tween the rails eliminates the need for above-rail mounting hardware. This in turn eliminates dam-age due to product hitting the mounting systems and sensors and improves reliability. The XUY roller conveyor sensor can be configured with a variable number of photo eyes for full-width detection and does not require a reflector.

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BENEATH THE BARSFigure 3: TGW decided to mount the photo-eye sensor between the rollers of the motorized roller conveyor system, thus eliminating the above-rail system.

TGW

“Since the sensor is buried between two adjacent conveyor rolls, it virtually eliminates the physical damage that is common to above-the-rail-mounted photoelectric sensors,” says Allan Hottovy, business development manager at Telemecanique Sensors. “In addition, this sensor does not typically require mounting brackets, a reflector or any other special-ized installation tool. Since the conveyor side rails are used as the mounting brackets, unscheduled down-time caused by loose or damaged mounting hardware is drastically reduced. Damage to the sensor compo-nents is reduced and life expectancy is extended.”

The application of the sensor allows for a clean-looking and reliable solution within any type of conveyor frame type, explains Tony Britton, tech-nical support supervisor for TGW. “Whether it is TGW’s CRUZchannel or C6 frame, the Teleme-canique sensor works seamlessly within TGW’s plug-and-play CRUZ-Logic controls framework.”

Magnetic sensorsCompass Automation in Elgin, Illinois, designs and builds custom automation systems. It works in man-ufacturing processes across all industries focusing mainly on assembly automation and inspection au-tomation systems.

It applies a magnetic sensor from IFM efector to pneumatic cylinders to measure stroke length. Com-pass also uses hydraulics and servos, but not with this sensor. Pneumatics are included on its product assembly machines, and the MK5328 magnetic sen-sors are used to sense the cylinder position as fully extended or retracted (Figure 4). They’re interfaced to an Allen-Bradley PLC.

“In order to build a successfully functioning ma-chine, it is important to know the state of every actu-ator, and IFM’s sensors allow us to do this efficiently,” explains Bill Angsten, co-founder and executive vice president of Compass. “The mechanical engineering team equips every pneumatic actuator in our machines

with at least two cylinder switches to provide the con-trols engineering team with all of the signals they need to write a properly functioning code.”

Angsten also appreciates the technical assistance that comes with the magnetic sensor. “One of the greatest advantages IFM has over its competitors is the accessibility to technical help,” he explains. “IFM has a wide database of almost every pneumatic actu-ator that we put on our machines, and they’re able to tell us which cylinder switches to use for each ac-tuator. By having a set list of sensors to choose from, the mechanical engineer saves time in engineering the cylinder switch solution. This improves overall efficiency, which the customers will see in engineer-ing cost. This efficiency is seen in every aspect of the machine, from the mechanical designing down to machine build and electrical wiring. The IFM prod-ucts are also easy to install and adjust while on the machine, and their wide selection of cabling allows greater flexibility to our electricians in order to wire the machine as cleanly as possible.”

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EXTENDED OR RETRACTEDFigure 4: Magnetic sensors are used to sense the cylinder position as fully extended or retracted.

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At the only NATO-approved munitions manufac-turing location in the United States, production is hitting its stride. But a potential slump was

avoided, thanks to some updated automation and an organized installation plan.

“When the global war on terrorism began, muni-tions supply was a huge issue of concern,” says Rod Emery, P.E., VP of operations at RedViking, designer, builder and integrator of manufacturing solutions and dynamic test systems in Plymouth, Michigan. “They were worried they wouldn’t get the munitions supply they needed. The current systems were unable to keep up with new levels of production.”

The facility’s original five lines of production equip-ment were built in the late 1960s and early 1970s. “The instrumentation, controls and measurement were an-tiquated, and the parts and technology were no lon-ger available,” says Emery. “The machinery was con-stantly failing, or the measurement was calling parts bad that weren’t bad. They had too much downtime and too much scrap. The mechanics and the config-uration of the machines were solid though. They just needed to be maintained. The bearings, shafts and gears were still available.”

The five production lines are responsible for produc-ing 1.4 billion rounds of ammunition per year. When RedViking was asked to create a solution, it decided the best course was to build one new system from scratch with all new technologies and instrumentation and then prove the system before replacing the existing five lines.

Who’s on first?The new machine created a need for high-speed, non-contact inspection technology for test and mea-surement. “Every 50 ms, you’re measuring a part,” says Emery. “We’re using high-speed cameras for non-contact measurement of the dimensions of the case. There was another location where we were us-ing eddy current to look for tears or holes in the cas-ing. We were using laser scanners to look for abra-sions on the product (Figure 1).”

The first system was completed in 2010, and Red-Viking proved it out at its own facility. It utilized modern technology that includes Rockwell Automa-tion ControlLogix PLCs, RSView SCADA and Pow-erFlex drives with SERCOS communication, along with off-line measurement instruments developed using the National Instruments LabVIEW platform.

3 types of inspection and measurementMunitions manufacturer gets controls and automation overhaul without interrupting production


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QUICK DRAWFigure 1: The new machine created a need for high-speed, non-contact inspection technology for test and measurement using cameras, eddy current and laser scanners.

“We also included a Rockwell Automation servo system for the sample insert wheel,” says Emery. “The sample wheel is used to validate the equipment, and it’s done twice per shift. They put known samples into the sam-ple wheel to make sure they matched. The wheel goes from zero speed to matching up to 1,200/min.”

To prove out the first system, RedViking ran tens of thousands of rounds, and defects were included to prove the system would eject defective parts. “The customer was responsible for creating the defect, so they hired a contractor,” explains Emery. “We went through the runoff. They put 20 bad parts among the tens of thousands of good parts, and we ran it and we got to the end of the line and we had one less defect than we were supposed to. They’d put perma-nent dye on the defects to identify all 20, and some-how we missed one of them.”

The engineers sifted through the tens of thou-sands of cases by hand to find the undetected one with the red dye. “It was hours,” says Emery, “but we finally found the needle in the haystack.”

However, the case had no defect. “That particular one was supposed to have a very small hole,” explains Emery. “But the people who were supposed to put the defect in it didn’t. They missed it.”

Advance the runnerWhen the first newly designed machine went in, it was important to keep the lines up at all time. “We installed and did the factory-acceptance test at their location,” says Emery. “Then we began tearing down

the equipment that it replaced. The inspections on the old machines were done by probes, and they weren’t able to do all of the inspections we were do-ing. They had eddy current, but it wasn’t covering the entire case. They were using X-ray. As each turret passed, it performed the inspection.”

The first machine was all hardwired. “There were two separate control panels—one that contained the drives and PLC and the other contained the com-puters and data acquisition system,” explains Em-ery. “When we looked at trying to streamline the in-stallation process, we converted to a connectorized solution. We did that on the first one while it was at our facility to make the installation process quicker.

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MAKE AND MEASUREFigure 2: Continuous production and continuous inspection ensures the equipment won’t be shutting down.

It was a combination of industrial Ethernet and fi-beroptic. For the discrete stuff, it was a Harding 40-pin connector.”

The first system was completed in about 40 weeks. “We built a lot of the panels and instrumentation ahead of time on the other machines, but it was about 12 weeks per machine for the refurbishment,” says Emery. “It was important for us to make sure there was not a line down. The whole time they went through this, there was never a line down. By the end, we had it down to three days for putting in the new machine and pulling out the old one.”

Once the brand-new system was installed and four of the production lines were replaced with newly equipped systems, the facility had five lines in production, which allowed for the final existing machine to be refurbished and then installed sepa-rate from the rest of the line.

“On this equipment, there are three different ways a piece will move through the line,” explains Emery. “The primary way is through the production line—continuous production and continuous inspection. Because they have had a lot of challenges to keeping the equipment running, they wouldn’t shut down production (Figure 2). They would insert a wedge into the line, so every case gets ejected into a gon-dola, which can be wheeled away and inserted into the equipment in a separate location. The second way is through a hopper. It puts it out and inserts it into another chain that ties into the inspection. The third method is that servo-driven sample wheel. The sixth system was not connected to a manufacturing

line. It would only run in that hopper mode. These are separate pieces of equipment that are separate from the manufacturing line.”

Manufacturing runsThe manufacturing line used three large mo-tors, 150-200 hp, that were driving the main drive turret with only a chain connecting them. “The speed-matching was critical,” explains Emery. “If any motor was running at a different speed from the others, it would rip the chain apart. We used a SERCOS network for that kind of speed, with one motor as master. The most exciting part of that was the validation. We’d hit an e-stop to make sure it ramps down to zero speed safely and synchronously and be sure parts weren’t flying.” When RedViking modernized all of the controls and hardware, there were a lot of the same PLC and HMI riddles to track every part through the production line. “It was im-portant to fire the eject chute quickly enough so it ejects only one part and it’s the right part,” says Em-ery. “The SCADA system gives all of the production awareness for how many parts and how much scrap there is.”

The final machine was installed in 2013. “The customer was able to meet its requirements and the line never went down,” says Emery. “This project helped to drive activity with our LabVIEW develop-ments. It further defined our load-share capabilities, and some of what we learned about controls archi-tecture and drives technology and project planning is being used for future machine development.”

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Sometimes well-known technologies like pres-ence sensors don’t change and gain new inno-vations as much as the world changes around

them, wakes up to capabilities they already have and drags them into new applications.

“With so much economic globalization and new markets emerging, there’s increasing demand for automation,” says Kristen Chenowski, sensors and industrial products marketing manager at Omron Automation and Safety. “This means more need for worldwide safety standards and certifications, such as UL, CSA and CE, which means more pres-ence sensors in mats, light curtains and other de-vices to protect people.”

Just under a year ago, Chenowski adds, Omron re-leased a series of fiber-optic presence sensors that have a maximum through-beam range of 6 meters, which is a big gain over the previous 4-meter maximum.

“An improved microprocessor, better fiber-optic connections to its amplifier and greater percent-age of emitter light returned to the amplifier give it more stable internal detection, which allows it to help with more challenging targets,” adds Che-nowski. “Also, for food and beverage applications, we just released a photoelectric sensor family in June and our stainless-steel sensor, which is ep-oxy-sealed, washdown-rated and tested for EcoLab/Diversey chemicals and cleaning agents.”

Tony Udelhoven, sensor division director at Turck Inc., reports that the overall microprocessor rev-olution during the past 10 years is also enhanc-ing many sensor designs and capabilities, such as Turck’s Q-Track line, and giving users more control over their sensors’ measurement spans and other pa-rameters. Q-Track also added the IO-Link protocol in 2011.

Presence sensors travel into new automation applicationsEven though new markets and applications are emerging, Will Healy, strategic marketing

manager at Balluff, reports that new and existing users still share something in common—

they all want lower-cost sensors with more features in smaller sizes

By Jim Montague

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“Previously, a user might buy a sensor for an en-coder with 1,024 pulses per revolution, but if they later needed 584 or 3,400 pulses, then they had to buy another encoder,” Udelhoven explains. Now us-ers can adjust pulse counts in the sensor. Linear dis-tance transducers produce analog values over their sensing ranges, such as 0 to 10 V over 200 mm, but some users might want to adjust the span by start-ing at 110 mm and going to 190 mm, and now they can do it. They also can change outputs from 0 to 10 V to 4-20 mA, and even adjust the slope of their sensors. It’s all done in the software.”

Over the five years since they were introduced, Turck’s inductive linear and rotary sensors have progressed from linear distance transducers to 12-bit rotary transducers to 16-bit, internal-resolution sensors with a wide variety output configurations, explains Udelhoven.

Even though new markets and applications are emerging, Will Healy, strategic marketing manager at Balluff, reports that new and existing users still share something in common—they all want low-er-cost sensors with more features in smaller sizes. “Many machines worldwide are getting smaller, faster and using more robots, so users want more sensors onboard in less space,” says Healy. “So in January, we launched a photoelectric sensor with a 2-mm lens and a family of miniature precision

sensors. Smaller sensors are also light, and this lets robots move faster. IO-Link also gives us better di-agnostics and answers more sophisticated ques-tions, such as is a device there, is it powered, and is it running marginally? With inductive proximity sensors, IO-Link can help deliver data on assured and rated functions, including whether a target is at or beyond its assured range, or if it’s sensing the right type of metal at the right distance and with the right shape.”

To help presence sensors communicate with their new applications and users, Kevin Zomchek, pres-ence sensing marketing manager at Rockwell Au-tomation, agrees the biggest enabler is IO-Link, which is standardized, point-to-point and complies with the IEC 61131-9 standard.

“IO-Link eases commissioning, allows better troubleshooting and is more f lexible,” Zomchek says. “So instead of just getting simple, discrete on/off inputs from sensors, we can use IO-Link to communicate with sensors, see their operating margins and check more condition-based infor-mation, such as whether a lens might be dirty. We can even use IO-Link to change sensors’ setups on the f ly by accessing their profiles and parameters through the HMI, and use IO-Link’s automatic de-vice configuration (ADC) function to download new parameters.”

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september 2015 tate of technology reports sensors & …

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