Plug-and-Play Sensor Sanity

Envisioning a variety of analog sensors compatibly operating on a network can sour the stomach of the most optimistic system integrator.

Similarly, contemplating exactly how to make those original, IEEE 1451.2-standard smart (aren't these supposed to be plug-and-play?) sensors work properly left many a control system designer gasping at their overzealous complexity.

IEEE 1451 was approved in 1997, and went over with a thud. It never fulfilled its promise of simplifying the integration process for sensor signal processing. It defined digital outputs, thereby adding an extra step to the predominance of analog sensors in play. There were very few industrial applications of the standard, despite a clear need for a system to more sanely connect industrial sensors for a common application.

There's hope on the horizon. Last fall, a group of sensor manufacturers, following the lead of National Instruments (www.ni.com) made a big push for the approval of IEEE P1451.4, which looks to immensely simplify configuration. The suppliers have said they'll deliver compliant products for virtually all types of standard analog sensors. The ultimate goal is to make transducers plug-and-play devices on a network with measurement instruments, control systems, and DAQ systems.

This standard doesn't solve the network problem, but it's a beginning toward practical plug-and-play operation.

IEEE P1451.4-compliant sensors actually aren't that smart, lacking much of the self-diagnostics and calibration muscle of the earlier versions. But they'll more than make up for that with a promised (no kidding, this time) plug-and-play capability that works.

There could be instant credibility, as well, since a number of vendors have been making 1451.4-compliant sensors for a year or more. Integrated electronic piezoelectric accelerometers and compatible signal conditioners are currently available, and load cells, pressure and temperature sensors, and displacement transducer suppliers are on course to manufacture compliant sensors.

The focus of IEEE 1451.4 is on the basic sensor itself, so there's a simpler mechanism for constructing plug-and-play capability compared to the previous standard.

IEEE P1541.4 says compliant sensors will have an embedded, low-cost EEPROM memory chip that specifies the format and content of transducer electronic data sheets (TEDS) that store important information and parameters for self-identification and description. With TEDS, you'll no longer manually input this data during configuration. Good-bye to human error entering the data during fast-moving, high-stress system commissioning. So, swapping out a sensor, knowing that there's always some measure of sensitivity adjustment needed, can be handled automatically. The TEDS tells the system how to adjust.

By having a signal conditioner along with a sensor's ID, setup and changes should be significantly improved. To minimize the TEDS memory overhead, the manufacturer sub-templates that carry the formatting details for each device type will reside in the software or the instrument, not in the sensor itself.

At the moment, most of the systems are passive. Once the device has announced itself to the system via its TEDS, and the signal conditioner reads and relays the sensor's data, some minimal manual intervention is needed via the software. Some more recent "active" systems complete the configuration themselves.

The standard proposes a simple, low-cost serial communications protocol with power and data on one wire and a reference ground on a second. The protocol, Dallas Semiconductor's 1-Wire, allows use of commercially available 256-bit chips, with 4 KB of storage, in small packages.

National Instruments and its Plug & Play Program partners are also examining how to apply the capability to legacy sensors via an online database that users can access to download virtual TEDS to their systems. Since the standard maintains the analog output of the sensor, there should be high-level compatibility with legacy sensors and systems. The standard dictates a complete compatibility with non-plug-and-play devices.

Members of the Plug & Play Sensors Program

National Instruments

Celesco

Endevco

Kistler

Lebrow

Macro Sensors

Measurement Specialties

PCB Piezotronics

Sensotec

Transducer Techniques

Watlow