-8 ROBOTS ON THE PAINT LINE - A TURNKEY SYSTEM SUPPLIER'S PERSPECTIVE 2 .fr ~

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Paul Mills Sales Manager

Nutro Machinery Corporation Strongsville, Ohio

Presented at: FINISHING '95 CONFERENCE AND EXPOSITION

September 18-21, 1995 Cincinnati, Ohio

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Robots on the Paint Line A Turnkey System Supplier’s Perspective

Paul Mills, Sales Manager Nutro Machinery Corporation Strongsville, Ohio

A r e you considering a robot for your paint line? This paper examines two questions; 1s a paint robot the right choice for your application, and if so, how does a robot impact the design and operation of your paint system.

There are three common choices for spray painting your part:

Manual. A human operator with a spray gun in hand.

Hard Automation. Spray guns affixed to stationary gun arms or relatively simple motion systems such as reciprocators, spinning guns and 2- or 3- axis positioners.

Robot. For the purpose of this discussion, a 6-axis programmable device capable of complex arm and wrist motion.

1. Robot ... or Not?

It’s surprising how many times a robot is suggested as the answer to a painting problem. A colleague of mine joked “would you hire a blind, one-armed operator for this paint job?”

While robots have become faster, more robust, more flexible and easier to program they are not the only, or best solution for every application. An ex-robot salesman confided that in nearly one quarter of the industrial applications where he sold robots - the robot wasn’t necessary.

Sometimes customers are so enamored with the “sex appeal” of a robot that they don’t stop to examine whether their parts can be painted more quickly and inexpensively with a less exotic technique. The decision to use a robot should be made only after weighing a number of factors:

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Part Shape Take a look at your parts. Complex parts with recesses, curved surfaces and picture frame-like qualities are great candidates for the flexibility of a robot. Others, like flat sheet panels or those having a simple geometry can be handled easily with less expensive mechanisms. In practice, if keeping the spray gun normal to the part surface requires a great deal of articulation then you will probably want to consider a robot.

Part Variety The number of different parts has a lot to do with how to paint them. A job shop where no two painted parts are the same would be a poor location to replace people with robots.

Robots are well suited to a range of dissimilar parts requiring high production rates. For example, current requirement by some auto makers to paint all plastic trim parts synchronously, as a “kit”, for better color matching is a good application for a robot.

Hard automation is a good solution for painting a smaller set of similar parts. For example, a small adjustment allows a few fixed-guns to spray both 15” and 1 6 diameter aluminum wheels. By adding upstream sensing, automatically handling a mixture of parts can be accommodated with hard automation at a relatively low cost.

Future requirements may tip the balance in favor of robots if new parts, or styling changes would be difficult to achieve by adding or repositioning guns.

Cycle Time The fact that a robot can move very fast can be misleading. Since a robot can move faster than a gun can apply paint, it’s the speed at which the part can be painted that’s of interest. While robots can be outfitted with only one or two spray guns, it‘s common to have many spray guns in a hard automation system allowing more paint to be applied in less time.

A good example of the difference in cycle time is the application of ceramic glaze to sanitaryware. Glazing robots using two spray guns require 70 seconds per piece because of the large amount of glaze that needs to be applied. With hard automation 180 pieces an hour, (or a 20-second cycle) is accomplished by using more than 20 spray guns on a variety of telescoping and fixed gun stands.

Line speed also plays an important role in the cycle time calculation since a robot can only paint while it follows or “tracks” the part as it passes through the work envelope.

Some finishes require a high film thickness for building depth of color or defect-hiding purposes. In such situations a robot can use a complex spray path, alternately spraying different areas of the part to build film thickness without causing runs or sagging in the paint.

A spray test is critical to determining the time required to achieve proper coverage and film build. You may find that a robot alone cannot paint fast enough. The solution may be a hybrid system using fixed or moving guns to lay down paint on some surfaces and a robot to handle more difficult areas.

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Capital and Operating Costs According to robot suppliers, the top reason for purchasing paint robots is to eliminate labor content. But concluding that replacing people with robots saves money is too simplistic. For single-shift operations with low volume parts requiring hundreds of programs in a low wage environment a robot may be too expensive to justify. Especially if the program is a short-lived one and the pay-back time requirement is very short. In such a scenario using manual labor is probably the lowest cost alternative.

Similarly, robots are often costly compared with hard automation solutions. A high quality reciprocator might cost $15,000 and a robot over $100,000. Many users stop short at comparing the robot cost to the cost of human spray painters without considering hard automation. Obviously a $1 00,000 robot that replaces a $30,000 worker per shift, three shifts a day quickly pays for itself. On the other hand a great deal of automation can be purchased for far less than $100,000.

When considering the cost of a robot, remember to include not only the cost of a “robot- in-a-box”, but the additional expenses of integrating a robot into the paint system. This includes items such as teach handles, protective covers, documentation, training, tool holders, interface electronics and other items. One robot manufacturer estimates the cost of an integrated robot at 30-50% over the cost of a bare-bones robot.

Paint Savings A person can paint as accurately as a machine, but it is the repeatability of automated spraying that saves paint. Robots are remarkably consistent, and once a robot is programmed to paint a part, it can paint millions of them with little variation. Watch a human operator and it‘s surprising to see how many times they trigger a gun to “make sure” it works! Manual touch-up in one area inevitably results in over-spray to another

For specific operations, hard-automation can be even more efficient than a robot. Spray guns on a gun positioner can be triggered very precisely to apply paint in high speed applications with almost no overspray. Automatic masking allows even complex profiles to be painted inexpensively with hard automation systems.

Spraying less paint saves money in itself, but it also means less maintenance, clean- up, lower filter costs, VOC emissions, and other benefits. Estimates range from 25%- 30% paint savings for automated systems compared to human operators.

Technical Expertise Adding complexity to a paint line carries some risk. The standard maintenance toolbox may be of little help when a robot goes down. Troubleshooting and repairing robots requires special training and a degree of sophistication not found in many smaller plants. When evaluating the labor savings achieved by replacing hand-spray painters don’t forget to consider the training and technical support needed to keep the robot working properly.

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II. Integrating the Robot

Adding a paint robot to the finishing line involves a good bit more than setting the robot in front of a spray booth and plugging it in. The best systems are designed with a robot in mind from the start.

Selecting a Robot Type Painting robots are available in either hydraulic or electric designs:

Hydraulic robots. Hydraulic robots were once the dominant machines used in painting applications. Today they comprise a good part of the used robot business but are greatly outnumbered by their electronic counterparts. While they offer high load capability, this is less vital in paint applications where a couple of spray guns do not present serious load problems. Hydraulic systems do provide simpler solutions for meeting explosion proof standards since power cables to electronic servo motors can be eliminated. Today, their primary advantage is that they offer a cheaper alternative to electronic robots.

Electronic Robots Electronic paint robots use high speed, precision servo drives for their movement. They are now able to achieve faster movement and more precise positioning than a hydraulic unit. Without the need for hydraulic connections, they are more modular and require less mechanical maintenance.

Both robot types require attention to cleanliness and proper preventative maintenance. Both require a well trained, and technically knowledgeable support staff.

Testing Before purchasing any robot paint system it is important to test the robot's ability to paint your parts. Such a test can reveal a great deal including:

The expected cycle time The complexity of the programming task The precision required for workholder design

A good finishing systems house will have the ability to set up a test and demonstrate this capability for you.

Coating Material Compatibility Some limitations may be dictated by the material to be sprayed. Ceramic glaze and porcelain enamel for example are particularly destructive to the sensitive mechanisms found in many wrist designs. Special consideration for such abrasive materials include picking a robot suited to this rough environment along with additional shrouding for protection.

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Robots spraying solvent-borne material need to adhere to explosion-proof electrical requirements. This will necessitate the use of wiring suitable for hazardous locations for connections to the robot. Robots intended for liquid paint applications are usually designed with air purge designs to conform to these safety requirements. Some paint robot manufacturers have had difficulty meeting requirements for Factory Mutual (FM) insurance requirements. Always make certain that you discuss these types of compliance issues before you purchase a robot.

In powder applications it is sometimes necessary to add additional protection to prevent powder migration into critical robot components. Rubberized boots placed over servo motors and kept under positive air pressure will help prevent powder related failures.

Load Restrictions Make certain that the robot can hold everything it needs to. Robot payload specifications should reference a payload at a known offset.

Speed Paint robot speeds can vary quite a bit. It’s tempting to want the fastest robot available, but consider the maximum speed at which you can paint. Typically pattern distortion occurs around 5 feet/sec so a robot that can move faster has limited benefit for spraying (although speed is definitely an advantage when not spraying). Again, a spray test to determine cycle time for your part, coverage and film build is an important first step.

The Work Envelope The work envelope, dictated by the reach of the robot, (less the combination of gun and wrist distance when spraying inward), determines the maximum size part that can be painted. In continuous conveyor systems where the robot is tracking the conveyor the work envelope limits the maximum cycle time for each part.

Most manufacturers provide detailed information regarding the physical work envelope of their robots. In some cases the robot can reach below its own base. For a floor mounted robot this provides no benefit, but mounting the robot on a pedestal will extend the reach of the robot considerably providing greater flexibility.

The Conveyor The robot expects a part to be presented in a well-defined position. In order to do this the robot needs to know where the part is, and how fast it’s moving.

A limit switch or photo-eye that is tripped by the workholder or hanger can be used to trigger a “start” signal to the robot controller. Placing this switch outside the spray booth is a common practice in order to allow the robot to begin painting the part as it enters the spray booth.

Conveyors typically fall into two categories; indexing or continuous running conveyors.

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On indexing systems the conveyor moves a part into position then stops for several seconds while the part is sprayed before indexing again. Indexing conveyors must be programmed to achieve the required cycle time and also to provide smooth acceleration and deceleration of the conveyor to avoid “jerking” the conveyor and parts. However an indexing system provides the simplest means to paint the part since it is not a moving target. During the dwell time, the part can also be rotated in place if necessary.

Continuous running systems require hardware and software to inform the robot about the part position as it moves. A device commonly used for this purpose is an encoder. Relative, or incremental encoders are used on conveyor drive shafts to calculate the conveyor position from a known “home” position. This digital timing information is sent to the robot controller.

Once the timing information is received by the robot controller, the software uses this data to calculate the appropriate translation of the robot’s coordinates. The robot can then “track” the part by using these changing coordinates.

Part Fixtures Our eyes, brain and hands work together to correct for the unexpected. If a part is slightly cock-eyed, a spray painter can quickly and unconsciously compensate for the problem. A robot does not share this ability. Since the robot is “blind it assumes that the part is oriented in a specific way consistent with its stored painting program. An improperly positioned part will not be properly painted.

Therefore, the workholder should be designed to present parts in a uniform, and precise manner. Bent spindles, swaying hangers, slippage, or sloppily racked parts will undoubtedly cause quality problems.

On overhead systems a good deal of attention should be given to selecting both the proper attachments to the conveyor, and to rack design.

Attachments should provide stability to limit swaying of the rack and limit rotational movement. Where rack rotation is required, positive engagement of a sprocket or star- indexer is important to assure the rack turns smoothly and predictably. For example an accurate chain driven rotator with variable speed motor can provide smoother and more reliable rotation than simple belt driven systems.

A locating pin that selves as an electronic “flag” attached to the sprocket can be detected with a photoelectric sensor or proximity switch. The spinner turns the part until the pins’ presence is sensed. The spinner is then stopped in the desired orientation.

The rack should be designed in a way that rigidly fixes the orientation of the part along all axes. If the part position cannot be located precisely the robot program will produce poor quality parts.

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Early in the system design phase you should consider using a conveyor that provides more accuracy. A chain-on-edge conveyor provides more precise part movement and orientation. A high quality chain-on-edge design uses a precise machined chain driven by accurate, variable frequency drives. The spindles should be designed to spin freely and accurately and should be engineered and constructed to avoid spindle bending.

To avoid costly tooling, a universal spindle can be oulfitted with removable workholders that provide a secure fit to each part.

Communications In robot paint systems, bi-directional communication needs to be established between the robot controller and the system PLC.

Usually a “start” command is passed from the system PLC to the robot to instruct the paint program to begin. While the start command could be delivered directly from the sensors to the robot controller, often other events are triggered from the same signal such as the triggering of pneumatic solenoids and similar functions.

On systems where different parts are being painted, a part ID is also provided to the robot controller so that the right paint program is executed. The part ID is usually entered by operators into a keypad terminal as parts are loaded onto they system, and in more automated systems by bar-code readers or vision systems. The system PLC can keep track of each part and it’s associated ID through shift registers as it travels around the system.

The robot controller also communicates back to the PLC with status information on the robot and it’s purge system, as well as fault conditions such as overtravel limits and other problems.

Collision Avoidance Several steps are customary to ensure that the robot will not be damaged by striking the booth, conveyor or other structures.

The robot manufacturer usually provides several built-in means of avoiding collisions. In some robots, an internal rubber bumper is placed at the intended extremes of motion. The robot will “bounce” softly when the bumper limits are reached. A second level of manufacturer protection is a software limitation. The robot controller is programmed with a horizon that becomes a boundary for the painting program. This is intended to prohibit programming the robot to an unsafe position. Finally, a limit switch in the robot provides a mechanism to disable the robot entirely if the other mechanisms are defeated or fail.

The end-user may want to introduce additional collision avoidance specific to the painting environment. For example, if the robot is reaching into a booth opening, software limits should be programmed to prevent the arm from striking any edge of the booth opening.

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A sensor might be added to prevent the robot from striking the conveyor or another structure in the paint booth.

Spray Booth Design The first consideration of the spray booth design is whether the robot can fit into the booth. Robots need a good bit of clearance to paint within the designated work envelope to accommodate retraction of the arm. Often the booth needs to be a good bit wider and deeper than a standard automatic booth to accommodate the required workspace.

It’s wise to provide sufficient room around the robot for access to components that will require periodic maintenance and repair. Whenever possible it’s a good idea to provide observation windows so that the robot operation can be monitored. Some customers have even installed a video camera to provide monitoring of the robot’s performance.

While humans require a breathable level of VOCs to work, robots can operate in an environment heavier in contaminants. A recirculating ventilation system allows the booth air to be used longer before turn-over, sending less air, or air more concentrated in VOCs to the abatement system. This will lower abatement costs and present fewer regulatory problems.

Recirculating booths also require less air make-up. In environmentally controlled booths this means less expense for heating and air-conditioning of make-up air.

Path Programming Most electric painting robots offer several means of programming the spray path.

Point-to-Point Programming As the name implies, movements are programmed by entering the coordinates of each successive point. Beginning from a home or origin position, the robot is told which direction and how far to move to the next point. Along with the move the program instructs the spray gun to spray or turn off. This type of programming is very slow and tedious for complex paths requiring many interim steps.

Ordinarily, this type of programming is performed using a teach pendant. The teach pendant is a small hand-held electronic console that can be brought into the spray booth for programming. The teach pendant may be disconnected from the robot in operation, and a single teach pendant can be used for a number of robots. The teach pendant provides a keypad for entering instructions, a readout to display position, speed and programming instructions, and a joystick or similar device for easy control.

Lead-Through Programming. Many robots allow the programmer to physically move the robot arm through a path while the robot controller “memorizes” the movement. This is a quick way to program the path. The program can then be refined with additional point-to-point instructions to fine-tune the path.

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A teach handle is required to accomplish lead-through programming. The teach handle is outfitted with buttons to toggle the recording off and on and to make movement of the robot arm easier.

Off-Line Programming Some robot suppliers offer packages that allows painting to be simulated off-line with graphic 3-dimensional operator interfaces. The path can be executed, evaluated and fine-tuned off-line so that system downtime is minimized. The ability to fine tune the path leads to greater transfer efficiency and a high quality finish. Much development is currently taking place to enhance these programs so that a CAD drawing of the part can be imported, and a self-optimizing routine will create the most efficient spray path.

Along with path information, many systems permit programming various spray parameters such as triggering the gun on or off, color changes, and entering analog values for atomization air, fan air, and flow rates. This detailed control can be used to produce the highest quality finish.

Gun Mounting Spray guns should be mounted in a fashion which facilitates easy removal for repair and maintenance. The mounting should also be designed so that the gun is held rigidly in a preset position since the paint program is based on a known spray gun orientation. Care should be taken to avoid damage to fluid delivery and air hoses by suspending them from the booth ceiling, or taking care not to wrap them around the wrist inadvertently.

Gun Cleaning Most robots are integrated into systems using a variety of colors to paint. A popular programming feature is to program the robot to clean the spray gun for either periodic maintenance or color changes by spraying the gun into a purge bucket. Programs have even been written to have the robot rub the spray gun tip against a cleaning pad or brush.

Robot Safety Robots, driven by powerful electric motors, have been known to cause injuries and even deaths.

Spray booth doors should be safety interlocked so that the robot is stopped entirely whenever the booth door is opened. This will prevent the robot from striking someone entering the booth.

Particular caution should be exercised when programming the robot. In one instance, an operator was pinned against a spray booth wall and crushed to death by a robot because he had defeated the safety switch on the teach pendant. This switch, found on pendants and teach handles is designed to place the robot into a limp, or relaxed mode when the switch is released.

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Always think twice when programming movement of the robot, especially if you are programming in “tool coordinates”. Since tool coordinates are referenced from the spray gun tip (which constantly changes with respect to absolute coordinates in the spray booth), programmers have experienced unexpected results when programming in tool coordinates because of the difficulty in keeping track of changing reference frames.

111. Conclusion The choice of technology for painting parts depends upon a number of technical and economic factors. A broad range of solutions from manual spray stations to multi-robot installations are available.

Using sophisticated interfaces and fast, precise motion systems, hard automation and robotics provide significant advantages for many spray paint jobs. Sometimes the best solution is a combination of technologies.

When robots are to be used, careful thought should be given to the overall system design to accommodate, and maximize the benefits of the robot. Part fixturing, conveyor design, system PLC selection, spray booth design, programming tools and safety systems should all be engineered with the robot in mind.

Finally, there is no substitute for putting together a knowledgeable team from the start This team should include your paint supplier, application equipment supplier and a turnkey systems house familiar with all aspects of integrating a painting robot into a complete finishing system.

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