Your Practical Guide to Plug and Play SensorsAugust 1, 2005 By: James Wiczer, Sensor Synergy Sensors
Most plug-and-play (P&P) sensors are also—but not always—smart. Many smart sensors are also P&P. And neither smart nor P&P sensors are necessarily network ready. It might sound confusing at first, but it won't be after you read this article.
The term "Plug-and-Play Sensor" has a reassuring resonance, implying an easy-to-install—and use—smart sensor component you simply plug into a special socket. No struggling with printed data sheets, instruction manuals, and keyboarding; the newly matched pair of high-tech objects does all the work. The sensor and your favorite data acquisition (DA) system are now successfully exchanging information.
Figure 1. The basic functional components of a plug-and-play smart sensor are defined by the IEEE 1451 standards.
The plug-and-play (P&P) concept has been popularized by the PC industry to refer to the interconnection of computers and peripherals (printer, scanner, fax, etc.) with little or no effort on the part of the user. Despite some initial glitches, the computer P&P features embodied in open standards have become accepted and, to a great extent, even required. Its accessibility and ease of use for both the experts and the technically challenged are also appealing to the sensor industry, where the benefits are lower costs for installation and maintenance, and a longer life cycle.
Just What Is a P&P Sensor?
In the sensor industry, P&P denotes an enhanced sensor component with machine-readable information that enables the automated setup of a sensor interface. The setup process typically entails connecting a sensor to a DA system, a control system, a data network, or some other form of higher level system that requires sensor data. Collecting and interpreting these signals requires knowledge of certain key sensor properties and application details.
Are P&P Sensors Smart?
For the simplest case, these details include the sensor calibration factor, which converts measured electrical signals into the desired measured quantity, and the physical units of the measured value. This simple case assumes an ideal linear sensor and may be a useful approximation for many applications. Others might require more sensor details such as operational dynamic range, offset signal magnitudes generated when no measurand is present, frequency response, corrections for nonlinearities, compensation for temperature effects, and correction for aging.
This collected set of information is often called the Transducer Electronic Data Sheet (TEDS). Although each protocol has its own definition of fields found in the TEDS, the IEEE 1451 standards identify several types of required and optional TEDS that can offer more. With minimal user involvement, P&P sensors communicate some or all of these factors automatically when the sensor is physically connected to the higher level system (see Figure 1).
The IEEE 1451 Family of Smart Sensor Standards
P&P sensors are responsible for transferring sensor-specific properties to higher level systems, and therefore need a set of standard rules or protocols that tell each side of the communication channel what to expect from and send to the other. These protocols have been the object of much discussion and investigation. One set of standards is the IEEE 1451 , established by the Institute of Electrical and Electronics Engineers and described in the sidebar, "The IEEE 1451 Family of Smart Sensor Standards". Other organizations and application-specific industry segments have developed their own protocols. There is no "best" solution; the requirements of each application will help determine the best P&P approach.
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