Sensor Smarts

A new standard for analog sensors, the most common kind in use today, should make the problems of miscalibration a thing of the past.

Like most standards, the new standard goes by an unlovely name - in this case, IEEE 1451.4. But IEEE 1451.4 marks a huge advance in sensor technology and is already being applied in research and industrial laboratories. This new standard marries the tried-and-true robustness and cost effectiveness of analog sensors with the intelligence of digital equipment. Now, what does that mean in practice? A lot of things - in the long term, one of the most important aspects of IEEE 1451.4 is that it offers a standard interface and protocol by which a sensor can describe itself over a network. With the advent and adoption of intelligent networked and wireless sensors, the notion of self-identifying devices may seem fairly elementary, but it has taken over eight years to make it happen with analog sensors. Most commercially available sensor networks today are based on proprietary communications protocols, limiting their usefulness and hampering their adoption and IEEE 1451.4 could change all that.

We'll return to the long-term promise of IEEE 1451.4, but for now let's stay with the immediate problem of calibration. IEEE 1451.4 will eradicate one of the most common sources of sensor errors today: incorrectly transcribed calibration information from sensor data sheets. To understand how these errors arise and why they're such a big problem, look at how sensors are traditionally used. Analog sensors typically output a voltage that is proportional to the magnitude of whatever it is they've been designed to measure - be it temperature, pressure or something more exotic.

Analog sensors persist in a digital world because they are cheap, extremely reliable and rugged. Simply put, they can take a beating that would damage or destroy a digital sensor, which can output a number describing the measured quantity directly.

Although analog sensors are easy to hook up to a data acquisition system and monitor output voltage, converting that voltage back into Celsius, Pascals or whatever else is trickier. In other words, when a temperature sensor registers 2.5 V, we need to know how to translate that voltage to the actual temperature, be it 100, 50 or 2500°C.

Until now, the only way to know was by looking up the sensor's data sheet, a short (usually) manufacturer-supplied document that details how to calibrate the sensor correctly. Someone has to enter this information from this sheet into the data acquisition system, which is usually based on a personal computer. A single moment of human error here can make a sensor - and all the data it gathers - worse than worthless.

This was the impetus for the IEEE 1451.4 standard. Developed by the IEEE in conjunction with companies such as National Instruments Corp, Aeptec Microsystems Inc, Crossbow Technology Inc, as well as the National Institute of Standards and Technology and the US Air Force, among others, IEEE 1451.4 is built around the concept of the Transducer Electronic Data Sheet, or TEDS.

A TEDS describes the sensor's electrical interface requirements - how the data acquisition hardware needs to be configured to read the sensor properly - and tells the acquisition system how to scale analog output voltages properly into the units that correspond to the physical property being measured, such as degrees Celsius. By doing this automatically, IEEE 1451.4 eliminates the possibility of human error in transcribing the data sheet.

The heart of IEEE 1451.4 is the use of a digital read-only memory (ROM) chip embedded in the analog sensor that stores the sensor's electronic data sheet, as well as information identifying the sensor: namely its type, manufacturer and a serial number. When hooked up to an IEEE 1451.4-enabled data acquisition system, the ROM chip transmits the TEDS to the system, similar to the way a USB mouse or printer identifies itself to a PC after it is plugged in.

Very importantly, IEEE 1451.4 does not dictate where the relatively delicate ROM chip should be placed in relation to the tougher analog sensing element: the chip can be added inside the sensor housing, in the sensor connector that attaches to the data acquisition equipment or even inside the sensor cable. This allows the analog sensor itself to still be placed in harsh environments, with the ROM chip located in the usually more benign environment at the other end of the sensor cable.

Even if you can't install new sensors, IEEE 1451.4 may still be able to help. The standard also establishes the concept of 'virtual TEDS'. It allows an IEEE 1451.4-enabled data acquisition system to download correct calibration information for the billions of legacy analog sensors already in place that don't have a built-in TEDS chip (just as long as someone has created a virtual TEDS for that sensor). An entire database of TEDS files for tens to millions of sensors can be stored on the disk or on a server accessible over the internet. A unique ID number, sorted by vendor and model or serial number, identifies each TEDS.

IEEE 1451.4 could have a vast number of uses. The digital smarts given to analog sensors by IEEE 1451.4 will allow them to be easily integrated into networks. Networked sensors today cannot normally communicate with other devices or systems outside of their proprietary networks. But coupled with related IEEE standards that describe how data should be transmitted over wired and wireless networks, it should be possible to easily monitor IEEE 1451.4-enabled sensors over almost any network. This would encompass a wide variety of operating platforms for data acquisition, storage and visualisation, including, for example, devices such as PDAs.

Back in the present day, a consortium of more than two dozen sensor, instrumentation and software vendors has been created to promote the implementation of IEEE 1451.4 into their products. Already, some sensor types are now more common with TEDS than without, and several hardware and software platforms including National Instruments LabVIEW are available to read and write TEDS data. And as new digital and communication technologies are introduced, they will incorporate TEDS, making possible the kind of ubiquitous smart sensor networks of tomorrow that technological visionaries have predicted and the kind of ubiquitous calibration errors that plague us now would become a thing of the past.