Tips for using TEDS sensors
Dan Romanchik, Technical Editor- August 1, 2004
Use of the IEEE P1451.4 standard (Ref. 1) to make sensors smarter is catching on among users, sensor manufacturers, and data-acquisition system manufacturers. The P1451.4 standard specifies a set of interfaces between the "smart sensor" and the data-acquisition system and between the sensor and the transducer electronic data sheet (TEDS).
Sensors that have a TEDS and a IEEE P1451.4 interface offer many benefits, including better accuracy and ease of use, but you'll garner these benefits only if you know how to use the sensors properly. To help you do this, I've gathered some tips from sensor and system manufacturers.
• Program the TEDS' detailed information and use it to improve sensor accuracy
The TEDS, which normally resides in an electrically erasable programmable memory (EEPROM) in the sensor, contains a wealth of information about the sensor. Various fields in the EEPROM describe the type of sensor, list the manufacturer and model number, explain how the sensor operates, and provide sensor calibration data (see Table 1 ). The TEDS actually has room for two sets of calibration data: standard calibration data that applies to the sensor model in general and extended calibration data that applies to a particular sensor.
Brian Betts, data-acquisition product manager for National Instruments, explains that the standard calibration data has tolerances wide enough to be applied to sensors from various production lots and vendors. You can, however, improve the accuracy of measurements taken with a particular sensor by a factor of 10 or more by using the extended calibration data.
Of course, taking advantage of this feature requires extra time and effort. First, you must characterize the individual sensor and program the TEDS EEPROM. Then, you must program the data-acquisition system to read this extended calibration data and scale measurements on that channel. Before doing all this, you should analyze whether the increased accuracy is worth the extra expense.
• Use TEDS sensors in existing data-acquisition systems.
Ron Denton, an applications engineer for Wilcoxon Research, says he is often asked whether TEDS sensors will work in existing data-acquisition systems. Denton says they should work just fine—IEEE 1451 specifies that TEDS sensors be backward compatible.
To achieve this compatibility, Wilcoxon's IEPE accelerometers use diodes to isolate the digital TEDS circuit from the standard analog circuit. When used with a standard data-acquisition system, the diodes prevent the digital circuitry from affecting the analog operation of the sensor.
This feature could be important in a situation where you must replace a standard sensor but only have TEDS sensors available. You might also want to keep this feature in mind when purchasing new sensors. If you plan to use TEDS sensors in the future, you may want to begin purchasing TEDS sensors now so you will have them available when you're ready to use them.
• Use a "virtual" TEDS with sensors that don't have one built in.
Most sensors today are not smart sensors and therefore do not have a built-in TEDS. No matter. NI's Betts explains that you can still gain many of the benefits that a TEDS provides by using a "virtual" TEDS. A virtual TEDS contains the same information as a built-in TEDS, but it is stored in a binary file in the test system rather than in an EE-PROM on the sensor.
Virtual TEDS come in two flavors: model-level TEDS and serial-level TEDS. Model-level TEDS contain basic calibration data for a particular model of sensor; serial-level models contain the extended calibration data for an individual sensor. As with built-in TEDS, you must make sure that your test program reads the TEDS and uses the calibration data to scale measurements.
Virtual TEDS files are available for thousands of sensors from many different vendors. To find a virtual TEDS for a sensor you currently use, you can contact the sensor manufacturer or use a searchable database on the NI site (sine.ni.com/vteds/VirtualTEDS ).
• Use TEDS data to have your test system tell you when a sensor needs calibration.
You must calibrate most sensors every year. The bad news is that many users fail to do this because they lose track of calibration requirements and schedules.
The good news is that a TEDS can (and should) include information about when the last calibration was performed, who performed the calibration, and when the next calibration is required. Your test program can then include routines that regularly read the sensor calibration information and notify you when a sensor needs to be sent to the cal lab.
Of course, you really want your program to inform you well in advance that a sensor needs calibration. If you give yourself a cushion, you can probably schedule sensor calibration without impacting test schedules.
To make use of this TEDS feature, you will have to ensure that your calibration process will support TEDS sensors. In addition to recording calibration data, the process must also be able to write calibration data to the TEDS in the sensor.
• Use TEDS to verify that sensors are wired to the right channels.
Keeping track of which sensors connect to which inputs is always a challenge, especially when the system has many channels. TEDS sensors, however, can identify themselves, so you don't need to label connectors or cables.
Instead, you can have your test program read the basic TEDS information and automatically configure the test system before you run a test. If you do this, you will probably want to include a description of the sensor location or parameter being measured in the TEDS' user area. A TEDS sensor could, for example, tell the test system that it is installed in the "left oven, production line number 4."
Researchers at Ohio State University used this feature in a recent project. When collecting data from embedded roadway sensors, they discovered it was easy to connect a sensor to the wrong input. They tried a number of different labeling schemes, but weathering caused labels to fall off and caused color-coded connectors to fade.
Moving to TEDS sensors eliminated this problem. The researchers now simply connect their sensors to the data-acquisition system in any order and the test program figures out which sensor is connected where. They can't make a mistake.
• Simplify asset management using TEDS sensors
Betts also explained that the most common support requests that sensor manufacturers get are for sensor calibration data sheets that users have misplaced. And without a data sheet, a sensor is unusable. Because TEDS sensors store calibration data internally, this is no longer a problem. The data sheet always travels with the sensor—it can't be lost or misplaced.
• Use a remote converter to use standard sensors in a TEDS environment.
By using a TEDS remote converter, you can use both high-temperature charge accelerometers and low-impedance IEPE accelerometers at the same time, says James Matthews, product manager for Endevco. Being able to use both types of sensors in a test system makes the system much more flexible, he notes. The remote converter is an electronic device that converts high-impedance charge signals into low-impedance voltage signals. In addition, the converter contains the TEDS EEPROM.
• Hot swap TEDS sensors to minimize downtime.
Matthews also notes that you can hot swap sensors should a sensor fail or should your measurement requirements change. This capability allows you to change sensors without taking down a test system, thus minimizing downtime. Remember that if you do this, you will have to get the test system to read the TEDS data for the new sensor.
• Buy products with the Plug&Play logo to ensure interoperability.
Finally, when looking for sensors, systems, and software, look for products with the Sensors Plug&Play logo (Figure 1). Products marked with the logo use a common TEDS software platform that ensures interoperability with other IEEE P1451.4 devices.
Table 1. Sample TEDS data.
|TEDS structure||Example Sensor: IEPE Accelerometer|
|Source: National Instruments
|Basic TEDS||Manufacturer ID||43|
|Standard and Extended TEDS (fields will vary according to transducer type)||Calibration date||January 29, 2003|
|Sensitivity at ref.||1.094E+03 mV/g|
|Reference frequency||100.0 Hz|
|Measurement range||±50 g|
|Electrical output||±5 V|
|User area||Sensor location||Strut 3A-p2|
|Calibration due date||April 15, 2004|
|Manufacturers of TEDS-related products|
|Here is a sampling of companies that make TEDS sensors or the systems and software that support them.
|Endevco, www.endevco.com sensors, systems||Honeywell Sensotec, www.sensotec.com pressure sensors, torque sensors, load cells, accelerometers, and LVDTs||Kistler, www.kistler.com accelerometers|
|Lebow, www.lebow.com force sensors, torque sensors||National Instruments, www.ni.com data-acquisition systems, software||PCB Piezotronics, www.pcb.com pressure sensors, microphones|
|RDP Group, www.rdpe.com LVDT displacement sensors, load cells||Watlow Electric Manufacturing Co.www.watlow.com thermocouples, RTD sensors||Weed Instrumentwww.weedinstrument.com thermocouples, RTD sensors|
|Wilcoxon Research, www.wilcoxon.com accelerometers, vibration sensors|