Electronic Design

Industry-Aligning Standards Boost Sensor Networking

The IEEE 1451 Committee steps forward to join conflicting sensor user and manufacturer demands by adopting compatible interface standards.

Editor's Note: The words transducer and sensor are used interchangeably in this article. While a sensor can be a transducer, the reverse is not always true. A transducer is basically a device that converts energy from one domain into another. It can be either a sensor or an actuator.

The need for inexpensive sensor-to-host interfacing over wired and wireless networks is stronger than ever. Three factors have coalesced to advance this type of "smart" sensing: decreasing prices for sensors, embedded microcontrollers, microprocessors, and analog-to-digital and digital-to-analog converters (ADCs and DACs); the proliferation of networking and diagnostic software; and evolving interface standards that simplify sensor networking. As a result, industry, the government, the military, and businesses are looking to network sensors and digital communications for more efficient and flexible operation. Progress has been steady but slow, as potential users wait for the right standards to evolve to where sensors and actuators, no matter how diverse, can be used in a network in a plug-and-play fashion.

With conflicting opinions from sensor users and manufacturers about what constitutes a "smart" sensor, a pragmatic and modest approach to bring together these disparate views is coming via the adoption of interface standards, spearheaded by the IEEE 1451 Committee. In an attempt to bring order to the fragmented world of smart sensing, the Committee has focused on the most important element of sensor intelligence—sensor self-identification. Its latest proposal is a mixed-mode interface standard, the P1451.4, which promises plug-and-play capability for the mostly analog-sensor world in a simple, inexpensive, and totally compatible way.

Key to the 1451 standard is the definition of a transducer electronic data sheet (TEDS) template, which contains technical information to identify a networked sensor, its required analog interface, and the sensor's usage. Typical TEDS parameters include a measurement range, an electrical-output range, sensitivity, power requirements, and calibration information. Each parameter varies according to the type of sensor implemented and is defined by a specific TEDS subtemplate.

The TEDS definition is flexible. It encompasses subtemplates for every conceivable type of sensor and accommodates custom subtemplates for specialized parameters and requirements. Standard subtemplates include those for "Electronics for Piezoelectric" (IEPE) accelerometers and microphones, IEPE pressure sensors, Wheatstone-bridge sensors, strain gauges, load and force transducers, thermocouples, thermistors, resistance-temperature detectors (RTDs), linear-variable and resistance-variable differential transducers (LVDTs and RVDTs), resistive sensors, frequency output sensors, and any type of amplified sensor with a voltage output or a 40- to 20-mA current output.

The term "smart sensor," bandied about for over a decade, is generally applied to the most technologically advanced sensors: microelectromechanical-system (MEMS) sensors. Yet what constitutes a smart sensor is a matter of perspective. Sensor manufacturers would like to integrate more signal-conditioning functions on the same chip as the sensing element, which is no mean feat when you're trying to keep size and costs down in highly competitive markets. For the user, on-chip integration isn't necessarily the most important parameter as long as the sensor simply and cost-effectively does its job—sense, analyze, and act on signals. Because of the diversity of sensor applications, it has become clear that where each element of a sensor's network should reside in a network depends largely on the application involved.

Though highly integrated sensors are practical in some applications, they may not make much sense in others. Thermocouples typically operate from 300ºC to 500ºC, temperatures that silicon ICs cannot tolerate. Moreover, thermocouples consist of materials that are incompatible with the carefully controlled high-purity materials used in silicon ICs. Add to that the many system-interface issues a designer faces when trying to network a very wide variety of sensors cost-effectively, and it becomes obvious that the task is not easy.

It has become evident that the term "smart sensor" isn't the panacea suggested by early proponents. Instead, "smart sensor interfaces" may be the way to go. Smart sensor networks are becoming the approach of choice, particularly in the industrial world, where many legacy and what some would call "dumb" sensors predominate. Here, industry wants to exploit the operational cost and efficiency savings it can gain by networking its sensors and thus become more intelligent on a larger system scale. Smart sensing here means taking advantage of interface approaches using Web browsers, microcontrollers, and microprocessors.

Because inexpensive network-interface cards (NICs) are very common for networking PCs, why not use this approach for sensors? The problem here is that with NICs networking PCs, the PC's hardware and software capabilities are used to satisfy networking requirements. Sensors, on the other hand, require additional computational and signal-conditioning circuitry for NICs to be effective.

The IEEE P1451.4 proposed standard is part of a number of ratified and proposed standards transpiring from the Committee for over nearly a decade (see "The IEEE Family Of Transducer Interface Standards," p. 42). National Instruments leads by providing its Plug & Play Sensor Partner Program to build support for the IEEE P1451.4 proposed standard for a "virtual" Internet-enabled analog-sensor interface that could become a standard this year. With this virtual TEDS, users simply enter the sensor's serial number into the database, then download the TEDS so asset-management and application software programs can work with both legacy and TEDS-enhanced sensors. Several sensor companies have come on board, including Celesco, Endevco, Kistler, Lebow, Marco Sensors, Measurement Specialties, PCB Piezotronics, Sensotec, Transducer Techniques, Weed Instruments, and Wilcoxon.

In promoting its program, National Instruments announced development tools for the proposed 1451.4 standards at the Sensors Expo Conference last September in Boston. These include the TEDS library for LabView, which implements basic TEDS management functions, and the Sensors Development Kit to support legacy sensors.

The most immediate impact of plug-and-play sensors is a faster, more automated system setup. This is critical, because without it, setting up and configuring a measurement system means manually entering multiple sensor measurement parameters for each data-acquisition channel. Such a manual procedure is prohibitively expensive and time consuming when dealing with hundreds or thousands of sensors. Also, manual data entry (i.e., sensors connected to the right channels and correct data entries) leads to incorrect test data.

Due to the IEEE 1451 Committee's efforts, a number of IC manufacturers are trying to support sensor interfacing with devices that conform to the standard. Many have developed analog-to-digital converters and digital-to-analog converters (ADCs and DACs), DSPs, memories, Web browsers, and network-monitoring products that conform to the standard.

One of the earliest interface products was the Bfoot network-capable application processor (NCAP) from Agilent Technologies. The NCAP, as spelled out in the IEEE 1451.1 Standard, defines a common object model for the components of a smart transducer and the networks that connect the transducers to the outside world. The Bfoot thin Web server made of custom ASICs provided total plug-and-play Internet connectivity using standard interfaces. However, perhaps because it was a product ahead of its time, it didn't attract much market interest. Agilent then discontinued it.

Dallas Semiconductor has developed a one-wire protocol that allows the use of simple, low-cost EEPROMs with just two leads for interfacing sensors and TEDS templates with a data-acquisition system. As specified in the proposed IEEE P1451.4 mixed-mode interface standard, the interface handles both Class 1 interfaces with analog outputs and digital TEDS signals over the same wire pair, as well as Class 2 interfaces where analog and digital signals are treated separately on different wire pairs. Class 1 interfaces are intended primarily for constant-current-powered IEPE transducers (like accelerometers and microphones) and define a method for sequentially switching between the analog mode and the digital TEDS mode on a single pair of wires. Switching is controlled by the direction of the constant-current source. Class 2 interfaces separate the digital from the analog signals using a dual interface (Fig. 1).

Many single-wire EEPROMs are commercially available with capacities ranging from 256 bits up to 4 kbits for storing sensor TEDS information. James Truchard, president, CEO, and cofounder of National Instruments, notes that "many legacy sensors being used in industry already have the data-conversion and signal-conditioning electronics, and all they need is a relatively inexpensive and simple EEPROM to interface to the IEEE 1451 standard."

Even ADC IC manufacturers now make products with the TEDS in mind. Analog Devices Inc. has on-chip flash EEPROM storage on its ADUC8XX line of 12- to 24-bit MicroConverter ADCs. "The whole concept of the MicroConverter line was driven by the development of the 1451 standard," says Brian O'Mara, senior applications manager at Analog Devices. "It provides the precision and flexibility called for in the standard."

Some MEMS sensor manufacturers like Silicon Microstructures are offering sensors that are customized to the user's needs (Fig. 2). Such a sensor can have not only the pressure-sensitive element on the chip, but also the signal-conditioning electronic circuitry. The output signal can be a signal-conditioned, calibrated analog voltage, or it can be mask-tailored to select from a variety of digital communications protocols.

Given the support that the IEEE 1451 standard has received from the industry so far, it's only a matter of time before interfacing sensors becomes a routine task. As more standards definitions evolve, greater productivities will become possible thanks to the advantages inherent in networking sensors of all types, no matter where they're located. As an industry demonstration for a machinery-monitoring system showed not too long ago, interoperable plug-and-play sensors from many different companies can indeed work seamlessly together, given the right hardware and software products (Fig. 3). This was explained in a presentation given by National Instrument's David Potter, Director of Emerging Markets, Product Strategy Group, entitled "The Impact of IEEE 1451.4 on Measurement and Automation," presented at last September's Sensors Expo Conference.

Agilent Technologies Inc. (650) 752-5000 www.agilent.com
Analog Devices Inc. (800) 262-5643 www.analog.com
Celesco Transducer Products Inc. (818) 701-2750 www.celesco.com
Dallas Semiconductor Corp. (972) 371-4000 www.dalsemi.co
Endevco Corp. (800) 982-6732 www.endevco.com
Kistler Instrument Corp. (888) 547-8537 www.kistler.com
Lebow Products Inc. (248) 643-0220 www.lebow.com
Marco Systemanalyze und Entwicklung GmbH +490 0 8131 5161 0 www.marco.de
Measurement Specialties Inc. (800) 236-6746 www.msiusa.com
National Instruments Inc. (512) 683-0100 www.ni.com
PCB Piezotronics Inc. (716) 864-0001 www.pcb.com
Sensor Synergy Inc. (847) 353-8200 www.sensorsynergy.com
Sensotec Inc. (800) 298-9228 www.sensotec.com
Silicon Microstructures Inc. (408) 577-0100 www.si-micro.com
Transducer Techniques Inc. (800) 344-3965 www.transducertechniques.com
Weed Instruments Co. (800) 880-9333 www.weedinstrument.com
Wilcoxon Research Inc. (800) 945-2696 www.wilcoxon.com
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