Plug-and-Play Comes to Sensors

The use of sensors that convert physical phenomena such as pressure and temperature into measurable electrical signals continues to increase and evolve in test and measurement, and in factory automation applications.

The dominant standard interface, or output, for these sensors is an analog signal—typically voltage, current, or resistance—which varies in proportion to the physical parameter. Instrumentation hardware and software condition and digitize the analog signal, accurately converting the resulting data into engineering units. In order for the measurement system to properly condition, convert, and interpret the sensor signal, the system must be precisely configured with the operating parameters of the sensor. In multi-channel systems, proper connection and configuration of every sensor channel is a critical and often time-consuming operation.

A new standard for sensors, proposed by the 1451.4 working group of the Institute of Electrical and Electronics Engineers (IEEE), reduces the time and challenges associated with sensor configuration. The standard establishes a universally accepted method of making sensors plug-and-play—similar to the way a Universal Serial Bus (USB) mouse is plug-and-play with a computer—and defines a mechanism for adding self-describing behavior to sensors with an analog signal interface.

The IEEE 1451.4 Mixed-Mode Interface for Smart Transducers standard defines a mechanism for adding self-identification technology to traditional analog-mode sensors. In addition to providing traditional analog outputs, an IEEE 1451.4 smart sensor also provides a digital interface for communicating with an embedded memory device within the sensor. This memory contains the binary Transducer Electronic Data Sheet (TEDS) information that identifies and describes the sensor. The TEDS contains information such as manufacturer, sensor model number, serial number, measurement range, sensitivity and calibration information. TEDS templates are specific to the sensor type they are associated with, and are specified for a wide range of sensor types.

Embedding the sensor data sheet on a chip in the sensor provides several unique advantages. It speeds sensor setup time by eliminating manual data entry from a paper data sheet, which also eliminates the potential for user data entry error. There is no chance of the data sheet being misplaced, as the information is stored digitally on the sensor itself. The process of tracking and labeling sensor cables is no longer necessary because the sensor identifies itself and its location to the data acquisition system.

IEEE 1451.4 also improves compatibility between sensors and data acquisition systems from various vendors. All sensors manufactured according to the standard will share, at a minimum, the same basic information in the same format. In fact, an industry consortium of more than 20 sensor, instrumentation, and software vendors is working to adopt the technology and improve interoperability. A Sensors Plug&Play logo has been adopted by this consortium, which, when applied to a device or software, ensures that the product conforms to the IEEE 1451.4 guidelines.

There are two types of mixed-mode interfaces defined in the standard—Class 1 and Class 2 interfaces. Class 1 interfaces are primarily intended for constant-current powered piezoelectric transducers, such as accelerometers or microphones, for example, and define a scheme for sequentially switching between analog mode and digital TEDS mode on a single pair of transducer wires. Constant-current powered transducers, generally referred to as integrated electronics for piezoelectric (IEPE) transducers, incorporate internal signal conditioning powered by a constant current that is sourced by the measurement system on the signal wires. Class 1 transducers take advantage of this de facto analog standard by adding the TEDS with a switch that is controlled by the direction of the current source. By reversing the direction of the current, the instrumentation system switches the sensor into digital TEDS mode.

Most sensor types implement a form of the Class 2 interface, which requires additional wires for digital TEDS communication. The analog input/output of the transducer is left unmodified, and the two-wire TEDS interface is added in parallel to the analog interface. Users can then implement TEDS on virtually any type of amplified or unamplified sensor, including thermocouples, Resistance Temperature Devices, thermistors, bridge sensors and 4-20 mA current loop sensors. In fact, with the add-on approach of Class 2, it is very easy to retrofit existing sensors, with a variety of packaging options.

Brian Betts, brian.betts@ni.com, is signal conditioning product manager at National Instruments.

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