Good News for Sensor Users: IEEE P Meets Plug and Play

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1 Good News for Sensor Users: IEEE P Meets Plug and Play Have you ever misplaced a paper calibration sheet, struggled to connect hundreds of remote sensors to a signal conditioner, or longed for some way to simplify sensor inventory? If so, you'll be glad to know that sensor manufacturers are moving rapidly ahead with a simple-to-use solution: plug and play. When implemented according to IEEE , this data acquisition technology promises to significantly simplify the tasks of configuring and using automatic measurement systems. Getting up to speed on plug and play and its governing standard will let you be among the first to reap the benefits of this rapidly developing technology. IEEE P1451.4: Newest Member of the Family Standard P 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 smart sensor systems while achieving as many of the potential benefits as possible. In particular, the intent was to develop a comprehensive set of sensor and software protocols that would allow consistency and interoperability between the components of smart sensor systems even those from different manufacturers. The first in the family, IEEE was never adopted for any practical applications, but it contained useful concepts that were carried over into IEEE The migration to IEEE P reflected a need to simplify the P protocols and to add real value without incurring excessive costs. 1

2 Fig 1 Digital TEDS data available on an analog sensor The great advantage of IEEE P is that it gets around the complexities of other smart sensor concepts by focusing on the sensor. Essentially, P provides a relatively simple mechanism for building plug and play technology into traditional analog sensors. The standard specifies the format and content of self-identifying parameters that are stored on an EEPROM located on the sensor in the form of a transducer electronic data sheet, or TEDS. The standard also defines two mixed-mode (analog and digital) interfaces that will enable various types of automatic sensing systems to access the sensor's self-identification data. Transducer Electronic Data Sheet TEDS Basic TEDS Manufacturing ID Sensotec Model Number 41 Serial Number Version Letter 53e Standard and Extended TEDS Calibration Date April 22, 2002 Measurement 200KN Response Time Sensitivity 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 x Wiring Code Wiring code #15 Fig 2 The transducer electronic datasheet (preliminary) for a load cell 2

3 Although not yet published (the P designates its preliminary status), IEEE P has drawn considerable attention from sensor manufacturers. Led by National Instruments in collaboration with 12 major sensor manufacturers, including Sensotec, a significant portion of the industry has decided not to wait for the final standard, which will probably differ only slightly. These companies are already producing or designing signal conditioners and sensors that comply with the preliminary version. Plug and Play Basics Plug and play data acquisition technology has been around for about 8 years but is only now becoming widely available. By automatically supplying a signal conditioner with a sensor's unique identification data, the technology considerably simplifies system configuration and enhances 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 enter sensor data (range, sensitivity, scale factors, etc.) from a paper calibration sheet into your signal conditioner. Next, you set up the signal conditioner to support that sensor by using pull-down menus (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 the paper calibration sheet. You simply plug your load cell, for example, into an appropriate signal conditioner. The signal conditioner "interrogates" the load cell by reading the self-identification data from its onboard TEDS and presents those data to you. You then take your usual steps to adjust your signal conditioner. What Are Sensor Manufacturers Doing Now? Several manufacturers have been putting P compliant TEDS on their sensors for the past year or so. Sensor data are thus electronically available for interrogation, and some signal conditioners are already adapted to do that interrogation. 3

4 Fig 3 A load cell showing the EEPROM that is part of the integrated sensor electronics. The TEDS data resides on the EEPROM Amongst the systems that already have interrogation capabilities, most implement plug and play passively: After the signal conditioner reads and presents the data, you must take over to adjust the 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 a compatible CPU (one based on the same operating system as the peripheral). The system reads the self-identifying information on the printer, but you're not quite ready to click and print. You must 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 complete system 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 PC would recognize the printer and without any user intervention would install the right driver and print a test page Figure 1 illustrates the difference between passive and active implementations. 4

5 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 the TEDS principle through its SIG CAL technology. Before the release of P1451.4, the TEDS data used by SIG CAL were generated by a Sensotec protocol. The company has since changed its EEPROMS to match the protocol dictated by the standard. By focusing only on the sensor's self-identification data, P allows for development of both active and passive versions of plug and play. Manufacturers are free to decide which version makes the most sense for their equipment and their customers' needs. Consider the Possibilities Plug and play technology implemented according to P will substantially simplify the use of 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 and enter data from a paper calibration sheet. You don't have to endure the hassle of having the sheet filed in one location while the sensor is used in another. Conversely, if you like to keep your calibration sheets with their sensors, you needn't worry that someone will move the sensor and leave 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 5

6 to their signal conditioner. Say, for example, that you have attached multiple sensors to an aircraft structure and then run your sensor cables to a signal conditioner in another part of your plant. Once you get to the remote location, you need to know which sensors are on the engine assembly, which are on the wing, etc. Plug and play technology introduces the potential for enabling the signal conditioner to read not only a sensor's type and calibration information but also its location. When installing your sensor, you could use a TEDS burner (already available from some manufacturers) to inscribe location information in a user-definable field on the TEDS. Plug that sensor in and the signal conditioner tells you exactly where on your test stand it's located. Having location information available on the sensor for signal conditioner interrogation will be a tremendous advantage for applications using hundreds or even thousands of sensors. The potential is less dramatic but still significant for systems with small channel counts: Why spend any time figuring out connections? Swapping Made Easy. Even a rugged sensor can be damaged in an industrial testing situation. When that happens, you want to change sensors and get your test back up and running as soon as possible. Swapping sensors wouldn't present much of a problem if sensor output were completely consistent from sensor to sensor. But the real-world variation in sensor sensitivity means that with a traditional measurement system, a swap also requires some system readjustment. With a TEDS sensor that automatically provides calibration data to an active signal conditioner, even a technician unfamiliar with calibration procedurescan swap sensors quickly without jeopardizing the integrity of system operations. The signal conditioner reads the data from the new 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 selected sensitivities, 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 also help you inventory your sensors which can be a difficult task if you have multiple sensors scattered over a large installation, placed remotely in the field (especially in a hazardous environment), or frequently moved from place to place. You can spend much more time refining your measurement system and much less looking for your sensors. Mix and Match. Wouldn't it be convenient if you could plug sensors from one manufacturer into signal conditioners from another? Plug and play implemented according to P makes 6

7 that mixing and matching possible. All sensors manufactured according to the standard will carry (at a minimum) the same basic self-identification information on TEDS formatted in exactly the same way. Therefore, once manufacturers adapt their signal conditioners to read these universal TEDS (as many are already doing), you will indeed be able to plug a Sensotec sensor into a signal conditioner from another manufacturer for example, currently you can plug a Sensotec 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 the same ease of use already achieved with plug-in peripherals and other hardware in the computer world. Summing Up Plug and play implemented according to IEEE P holds enormous potential for simplifying the use of electronic measurement systems. In addition to automating configuration tasks and thereby reducing human error, plug and play eliminates the headaches of managing paper calibration sheets. It can also ease cabling problems, simplify inventory control, and enable you to mix equipment across manufacturers. Learning a bit about plug and play and its governing standard now will help ensure that you're ready to take full advantage of this rapidly developing technology. For more information on IEEEP P plug and play technology, see "IEEE Standard on the Way for Smart Plug and Play Systems," a National Instruments white paper. To help get your plug and play implementation off to a smooth start, see the Sensotec white paper, "Implementing Plug and Play: Some Things to Consider." 7