good news for sensor users: ieee p1451.4 meets plug and...

Document: Plug and Play BasicAuthor: Martin Armson, Marketing Director,

Sensotec, Inc., Columbus, OhioDate: October 2002

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Good News for Sensor Users: IEEE P1451.4 Meets

Plug and Play

Have you ever misplaced a paper calibration sheet, struggled to connect hundreds of remotesensors to a signal conditioner, or longed for some way to simplify sensor inventory? If so, you'llbe glad to know that sensor manufacturers are moving rapidly ahead with a simple-to-usesolution: plug and play. When implemented according to IEEE 1451.4, this data acquisitiontechnology promises to significantly simplify the tasks of configuring and using automaticmeasurement systems.

Getting up to speed on plug and play and its governing standard will let you be among the first toreap the benefits of this rapidly developing technology.

IEEE P1451.4: Newest Member of the Family

Standard P1451.4 is the latest addition to IEEE's family of Smart Transducer Interface Standards,which was established to address some of the obstacles affecting the development of smartsensor systems while achieving as many of the potential benefits as possible. In particular, theintent was to develop a comprehensive set of sensor and software protocols that would allowconsistency and interoperability between the components of smart sensor systems—even thosefrom different manufacturers.

The first in the family, IEEE 1451.2 was never adopted for any practical applications, but itcontained useful concepts that were carried over into IEEE 1451.3. The migration to IEEEP1451.4 reflected a need to simplify the P1451.3 protocols and to add real value withoutincurring excessive costs.



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Fig 1 Digital TEDS data available on an analog sensorThe great advantage of IEEE P1451.4 is that it gets around the complexities of other smart sensorconcepts by focusing on the sensor. Essentially, P1451.4 provides a relatively simple mechanismfor building plug and play technology into traditional analog sensors. The standard specifies theformat and content of self-identifying parameters that are stored on an EEPROM located on thesensor in the form of a transducer electronic data sheet, or TEDS. The standard also defines twomixed-mode (analog and digital) interfaces that will enable various types of automatic sensingsystems to access the sensor's self-identification data.

Transducer Electronic Data SheetTEDS

Basic TEDS Manufacturing ID Sensotec

Model Number 41

Serial Number 462992

Version Letter 53e

Standard andExtended TEDS

Calibration Date April 22, 2002

Measurement 200KN

Response Time 0.0005

Sensitivity 1.998 mV/V

Bridge Impedence in Ohms 350

Excitation Nominal V 10

Excitation Maximum V 15

Excitation Minimum V 3

User Area Sensor Location 23 right dyno

Calibration Due Date April 21, 2003

Templates Special Calibration Data 12.3 + 0.175 + 0.00563x

Wiring Code Wiring code #15

Fig 2 The transducer electronic datasheet (preliminary) for a load cell



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Although not yet published (the P designates its preliminary status), IEEE P1451.4 has drawnconsiderable attention from sensor manufacturers. Led by National Instruments in collaborationwith 12 major sensor manufacturers, including Sensotec, a significant portion of the industry hasdecided not to wait for the final standard, which will probably differ only slightly. Thesecompanies are already producing or designing signal conditioners and sensors that comply withthe preliminary version.

Plug and Play Basics

Plug and play data acquisition technology has been around for about 8 years but is only nowbecoming widely available. By automatically supplying a signal conditioner with a sensor'sunique identification data, the technology considerably simplifies system configuration andenhances system integrity by reducing the need for human data entry.

Typically, setting up a data acquisition system is a two-step process. First, you manually entersensor data (range, sensitivity, scale factors, etc.) from a paper calibration sheet into your signalconditioner. Next, you set up the signal conditioner to support that sensor by using pull-downmenus (in a software-based system) or pushbutton controls on a discrete signal conditioning unit.

A basic plug and play system cuts that work in half by eliminating the need to enter data from thepaper calibration sheet. You simply plug your load cell, for example, into an appropriate signalconditioner. The signal conditioner "interrogates" the load cell by reading the self-identificationdata from its onboard TEDS and presents those data to you. You then take your usual steps toadjust your signal conditioner.

What Are Sensor Manufacturers Doing Now?

Several manufacturers have been putting P1451.4-compliant TEDS on their sensors for the pastyear or so. Sensor data are thus electronically available for interrogation, and some signalconditioners are already adapted to do that interrogation.



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Fig 3 A load cell showing theEEPROM that is part of theintegrated sensor electronics.The TEDS data resides on theEEPROM

Amongst the systems that already have interrogation capabilities, most implement plug and playpassively: After the signal conditioner reads and presents the data, you must take over to adjustthe system electronics—set up the excitation voltage, display resolution, etc.

For a simple analogy that illustrates a passive implementation, consider your computer system. When you want to add a new peripheral, such as a new printer, you plug the peripheral into acompatible CPU (one based on the same operating system as the peripheral). The system readsthe self-identifying information on the printer, but you're not quite ready to click and print. Youmust first set the system to accommodate the device by responding to a few system prompts,usually with the help of a software wizard.

Other companies are offering systems that actively use a sensors interrogation data to completesystem configuration: their electronics self-calibrate, set up the scaling and engineering units,and begin taking measurements within a few seconds. To use the preceeding analogy; the PCwould recognize the printer and without any user intervention would install the right driver andprint a test page Figure 1 illustrates the difference between passive and active implementations.



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Fig 4 Active and passive implementation of plug and play

For over 8 years, Sensotec has been supplying active plug and play implementations based on theTEDS principle through its SIG CAL technology. Before the release of P1451.4, the TEDS dataused by SIG CAL were generated by a Sensotec protocol. The company has since changed itsEEPROMS to match the protocol dictated by the standard.

By focusing only on the sensor's self-identification data, P1451.4 allows for development of bothactive and passive versions of plug and play. Manufacturers are free to decide which versionmakes the most sense for their equipment and their customers' needs.

Consider the Possibilities

Plug and play technology implemented according to P1451.4 will substantially simplify the useof automated measuring systems. Here are some of the most significant potential benefits.

No More Paper. As mentioned earlier, plug and play eliminates the need for you to read andenter data from a paper calibration sheet. You don't have to endure the hassle of having the sheetfiled in one location while the sensor is used in another. Conversely, if you like to keep yourcalibration sheets with their sensors, you needn't worry that someone will move the sensor andleave the sheet behind or worst of all, that the calibration sheet will get misplaced or lost.

Labeling and Cabling Made Easy. Sensor users often find themselves with a bundle of cables,trying to figure out which cable goes with which sensor so they can make the proper connections



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to their signal conditioner. Say, for example, that you have attached multiple sensors to anaircraft structure and then run your sensor cables to a signal conditioner in another part of yourplant. Once you get to the remote location, you need to know which sensors are on the engineassembly, which are on the wing, etc.

Plug and play technology introduces the potential for enabling the signal conditioner to read notonly a sensor's type and calibration information but also its location. When installing yoursensor, you could use a TEDS burner (already available from some manufacturers) to inscribelocation information in a user-definable field on the TEDS. Plug that sensor in and the signalconditioner tells you exactly where on your test stand it's located.

Having location information available on the sensor for signal conditioner interrogation will be atremendous advantage for applications using hundreds or even thousands of sensors. Thepotential is less dramatic but still significant for systems with small channel counts: Why spendany time figuring out connections?

Swapping Made Easy. Even a rugged sensor can be damaged in an industrial testingsituation. When that happens, you want to change sensors and get your test back up and runningas soon as possible. Swapping sensors wouldn't present much of a problem if sensor output werecompletely consistent from sensor to sensor. But the real-world variation in sensor sensitivitymeans that with a traditional measurement system, a swap also requires some systemreadjustment.

With a TEDS sensor that automatically provides calibration data to an active signal conditioner,even a technician unfamiliar with calibration procedurescan swap sensors quickly withoutjeopardizing the integrity of system operations. The signal conditioner reads the data from thenew sensor's TEDS, adjusts its electronics, and you're testing again almost immediately.

If you've previously overcome the swap problem by paying extra for sensors with selectedsensitivities, the TEDS solution will save you not only time but also money.

Plug and Play Inventory Control. Burning location data onto each sensor's TEDS will alsohelp you inventory your sensors—which can be a difficult task if you have multiple sensorsscattered over a large installation, placed remotely in the field (especially in a hazardousenvironment), or frequently moved from place to place. You can spend much more time refiningyour measurement system and much less looking for your sensors.

Mix and Match. Wouldn't it be convenient if you could plug sensors from one manufacturerinto signal conditioners from another? Plug and play implemented according to P1451.4 makes



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that mixing and matching possible. All sensors manufactured according to the standard willcarry (at a minimum) the same basic self-identification information on TEDS formatted inexactly the same way. Therefore, once manufacturers adapt their signal conditioners to read theseuniversal TEDS (as many are already doing), you will indeed be able to plug a Sensotec sensorinto a signal conditioner from another manufacturer for example, currently you can plug aSensotec sensor into a National Instruments signal conditioner of an appropriate type (that is, pressure sensor to a strain gage signal conditioner). Sensing technology will then enjoy thesame ease of use already achieved with plug-in peripherals and other hardware in the computerworld.

Summing Up

Plug and play implemented according to IEEE P1451.4 holds enormous potential for simplifyingthe use of electronic measurement systems. In addition to automating configuration tasks andthereby reducing human error, plug and play eliminates the headaches of managing papercalibration sheets. It can also ease cabling problems, simplify inventory control, and enable youto mix equipment across manufacturers.

Learning a bit about plug and play and its governing standard now will help ensure that you'reready to take full advantage of this rapidly developing technology.

For more information on IEEEP P1451.4 plug and play technology, see "IEEE Standard on theWay for Smart Plug and Play Systems," a National Instruments white paper. To help get yourplug and play implementation off to a smooth start, see the Sensotec white paper,"Implementing Plug and Play: Some Things to Consider."

good news for sensor users: ieee p1451.4 meets plug and...

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