Plug-and-Play Sensors

Most sensors put out some sort of analog signal -- a voltage, current, or resistance, that varies in a fixed relationship to a physical parameter. Instrumentation that converts the signal to engineering units must know a few things about what parameters to expect from the sensor. That's because full-scale range, sensitivity, connection schemes, and other factors all vary greatly from one kind of sensor to another. The end result is that it takes some effort to match up sensors with monitoring electronics. This configuration process may be no big deal when there are only a few sensors involved. But it can be problematic when gangs of sensors get wired into multichannel measurement systems.

Plug-and-play sensors address the labor involved in connection and configuration. Based on open industry standards, plug-and-play sensors incorporate ways of automatically identifying themselves. Benefits include quicker, more automated system setup; better diagnostics; less downtime for sensor repair and replacement; and an easier time keeping track of sensors themselves as well as the data they generate.

Two factors have promoted widespread adoption of plug-and-play sensors: the IEEE P1451.4 Smart Transducer Interface draft standard and the Internet. IEEE P1451.4 is a proposed standard for self-describing analog sensors using standardized Transducer Electronic Data Sheets (TEDS). The Internet can bring the plug-and-play concept to legacy sensors and systems via distribution of so-called virtual TEDS. The next generation of measurement and automation systems will use these concepts to become even more automated, robust, and smarter.

The 1451 standard itself has been around since the early 90s. The IEEE P1451.2 standard was issued in 1997, and that's when the term TEDS first started to circulate. The first three versions of the standard set the signal conditioning and data-acquisition structure by embedding these in the sensor itself. The P1451.4 standard differs in that it calls for a sensor with an embedded EEPROM, with the data acquisition and signal conditioning separate from the sensors.

The 1451.4 standard calls for a mixed-mode interface. This is essentially the digital and analog signals being transferred back and forth between the signal conditioning hardware and the sensor, making it more compatible with the legacy sensors in place. The first three versions called for sending only digital data back to the computer. The next version of the standard being drafted now is P1451.5, which adds wireless capability to sensors. Right now, the P1451.4 is still a proposed standard which is expected to be ratified and issued by mid-year.

TEDS for sensors

The P1451.4 Mixed-Mode Interface defines a way of adding self-identification technology to traditional analog-mode sensors and actuators. It defines a transducer dubbed mixed-mode because it supplies both an analog and digital interface. The analog part provides a signal representing the physical phenomenon (temperature, pressure, force, etc.) The sensor also provides a serial digital interface for communicating with an embedded memory device. The memory contains the binary TEDS information that identifies and describes the sensor. This description includes data such as manufacturer, sensor model number, serial number, measurement range, sensitivity, and calibration information. In general, TEDS parameters give enough information to convert the electrical data to engineering units.

The heart of the IEEE P1451.4 standard is the TEDS information structure. The TEDS typically resides in an EEPROM embedded in the sensor and is accessed by the measurement system through an inexpensive serial interface.

The TEDS structure is compact yet flexible enough to handle a wide range of sensor types and requirements. The standard also specifies a collection of templates to handle different sensor types. The collection of IEEE standard templates include IEPE (integrated electronics for piezoelectric transducers) accelerometers and microphones, IEPE pressure sensors, Wheatstone bridge sensors, strain gages, load and force transducers, thermocouples, RTDs, thermistors, LVDT/RVDT, resistive sensors, and amplified sensors with voltage or current outputs. The standard also lets manufacturers define custom subtemplates that can be used instead of, or in addition to, the standard templates to accommodate specialized parameters and requirements. Finally, TEDS stores custom data the manufacturer defines such as sensor location and additional maintenance information.

The standard defines two types of mixed-mode interfaces; Class 1 and 2. Class-1 interfaces primarily handle constant-current powered, typically 4 to 20 mA, piezoelectric transducers such as accelerometers and microphones. They define a scheme for sequentially switching between analog mode and digital TEDS mode on a single pair of wires. Constant-current, IEPE transducers work from current sourced by the measurement system on the signal wires. Class 1 transducers implement TEDS by switching the direction of the current. Reversing the direction of current switches the sensor into digital TEDS mode.

Most sensor types implement a form of the Class-2 interface, which separates the digital TEDS interface from the analog sensor output. The analog input/output of the transducer is left unmodified. The two-wire TEDS interface is in parallel with the analog interface. This approach brings plug-and-play to many types of sensors or actuators whether they are amplified or not. Thermocouples, RTDs, thermistors, bridge sensors, electrolytic chemical cells, and 4 to 20-mA current-loop sensors are all examples of devices in this class. In fact, it's easy to retrofit these capabilities onto sensors having various packaging options.

The digital portions of both Class 1 and 2 interfaces are identical, based on the 1-Wire protocol from Maxim/Dallas Semiconductor, Sunnyvale, Calif. (www.maxim-ic.com). This scheme is simple and economical. It is a master-slave, serial scheme that provides both power and I/O on two wires. And there are 1-Wire EEPROMs commercially available that make it easy to add TEDS to sensors.

The P1451.4 standard can describe any basic sensor regardless of whether the TEDS is physically located in the sensor or not. For example, a test and measurement system with access to a TEDS file, or virtual TEDS, that describes a specific sensor could use the TEDS information to automatically configure the system. A legacy sensor with a virtual TEDS (one not resident in EEPROM) doesn't provide autodetection, although it extends the value of TEDS standardization to the large installed base of legacy, analog sensors. Virtual TEDS are also valuable in applications where sensor operating conditions prevent the use of electronics in the sensor.

Plug-and-play sensors are deployed in a variety of test and measurement applications. For example, IEPE piezoelectric accelerometers have been available for years with P1451.4 compatible TEDS. But for plug-and-play sensors to have a broad impact on the measurement and automation industry, more sensor and instrumentation companies will have to implement the IEEE P1451.4 specification. Data-acquisition instrumentation will have to have TEDS- enabled analog input channels, and measurement and automation software must be TEDS compatible as well.

Kit brings TEDS to sensors

The fundamental component of any plug-and-play system is the TEDS processing software, referred to in the IEEE P1451.4 standard as the Transducer Block, or T-Block. National Instruments has developed a TEDS LabView library of virtual instruments, or VIs. They implement basic reading and writing functions for decoding TEDS, as well as editing and recompiling independent of the binary source data, such as model number, manufacturers ID, etc.

Implemented as a library of VIs, TEDS can be embedded into LabView applications to develop plug-and-play systems or to customize system configuration management. The library can also be patched into a sensor manufacturing or calibration system to automate sensor EEPROM programming.

The Plug-and-Play Sensor Development Kit is a complete system for evaluating, using, and developing sensors compliant with the IEEE P1451.4 standard. Intended primarily for sensor manufacturers, developers, and system integrators, it includes information on interfacing, reading, and managing TEDS data, even creating and reprogramming sensors.

The broad adoption of plug-and-play sensors requires the active participation of suppliers of instrumentation, software, and sensors. To that end, National Instruments is working closely with leading sensor companies such as Lebow, Sensotec, Endevco, and Transducer Techniques to implement the P1451.4 standard in sensors. Endevco and Sensotec have TEDS-enabled sensors available now. These suppliers represent a wide range of measurement technologies, including accelerometer and vibration sensing, load and force transducers, pressure transducers, position and displacement transducers, and temperature sensors.