03-04-2010, 07:25 PM
Instrumentation that converts the signal to engineering units must know a few things about what parameters to expect from the sensor. That's because full-scale range, sensitivity, connection schemes, and other factors all vary greatly from one kind of sensor to another. The end result is that it takes some effort to match up sensors with monitoring electronics. This configuration process may be no big deal when there are only a few sensors involved. But it can be problematic when gangs of sensors get wired into multichannel measurement systems.Most sensors put out some sort of analog signal a voltage, current, or resistance that varies in a fixed relationship to a physical parameter.
1.1 Sensor
A sensor is a device that measures a physical attribute or a physical event. It outputs a functional reading of that measurement as an electrical, optical or digital signal. That signal is data that can be transformed by other devices into information. The information can be used by either intelligent devices or monitoring individuals to make intelligent decisions and maintain or change a course of action.
1.2 Smart Sensors
A smart sensor is simply one that handles its own acquisition and conversion of data into a calibrated result in the units of the physical attribute being measured. For example, a traditional thermocouple simply provided an analog voltage output. The voltmeter was responsible for taking this voltage and transforming it into a meaningful temperature measurement through a set of fairly complex algorithms as well as an analog to digital acquisition. A smart sensor would do all that internally and simply provide a temperature number as data. Smart sensors do not make judgments on the data collected unless that data goes out of range for the sensor.
IEEE 1451.4 defines a relatively simple, straightforward mechanism for adding smart, plug and play capabilities to traditional analog sensors. Without adding any new hardware to the system, these plug and play sensors can bring real, immediate benefits in ease of use and productivity to any measurement and automation system that uses sensors. Additionally, IEEE 1451.4 defines a standard framework for sensor description, embodied specifically in the TEDS, which can scale from today's traditional analog sensors to tomorrow's smart networked sensors.
Two factors have promoted widespread adoption of plug-and-play sensors: the IEEE P1451.4 Smart Transducer Interface draft standard and the Internet. IEEE P1451.4 is a proposed standard for self-describing analog sensors using standardized Transducer Electronic Data Sheets (TEDS). The Internet can bring the plug-and-play concept to legacy sensors and systems via distribution of so-called virtual TEDS. The next generation of measurement and automation systems will use these concepts to become even more automated, robust, and smarter.
The 1451.4 standard calls for a mixed-mode interface. This is essentially the digital and analog signals being transferred back and forth between the signal conditioning hardware and the sensor, making it more compatible with the legacy sensors in place. The first three versions called for sending only digital data back to the computer. The next version of the standard being drafted now is P1451.5, which adds wireless capability to sensors. Right now, the P1451.4 is still a proposed standard which is expected to be ratified and issued by soon.
Plug-and-play sensors address the labor involved in connection and configuration. Based on open industry standards, plug-and-play sensors incorporate ways of automatically identifying themselves. Benefits include quicker, more automated system setup; better diagnostics; less downtime for sensor repair and replacement; and an easier time keeping track of sensors themselves as well as the data they generate.
The IEEE P1451.4 plug-and-play sensor concept appears to be one of those rare technologies whose strength and value come from its simplicity and focus. Although it doesn't fit many of the typical definitions of a smart sensor, it does provide real, tangible benefits in a way that builds on, not replaces, existing systems and technologies
