The On-line Validation of Modulating Sensors

The use of distributed sensor networks for process measurement and control applications is becoming more common as sensors become cheaper, and progress is made towards a standard protocol for a shared digital communication network. As the use of distributed sensor networks increases, then so does the need for reliable methods of sensor validation. The validation of sensor performance is usually achieved off-line, by removing the sensor from the process plant and testing it using a known reference. Such methods are inconvenient, and sometimes just not possible in some process applications. This paper discusses strategies which may be used to validate a sensor's performance on-line. One strategy is based on the reciprocal relationship that exists between the input and output energy of modulating sensors. In the case of a sensor with a physical input and electrical output, the electrical response of the sensor will change as the physical characteristics of the sensor changes. However, in addition, in some cases, the physical characteristics of the sensor can also be altered electrically. One way of doing this is by changing the current or voltage used as the signal carrier for the sensor. A case study of a platinum resistance thermometer using the self heating characteristic for on-line validation of the static and dynamic performance of the sensor is presented. The sensor system in question is connected to a digital communications bus, and all validation routines are controlled from a central processor.