Enhancing thermocouple temperature measurement accuracy with local RTDs to compensate terminal temperature difference

摘要

An I/O circuit for measuring temperatures uses multiple cold-junction compensation sensors permanently affixed near the terminals of the terminal block in order to compensate significant temperature variation across the terminals of the I/O module (up to 3° C.) that can substantially affect the accuracy of thermocouple measurements. The use of these multiple sensors is enabled by a compensation system that corrects for the distance between the built-in sensors and the terminals, a multiplexer that accommodates the additional signal burden produced by the sensors, and a compensation system that allows low-cost sensors to be used and calibrated to as little as a single high accuracy sensor. In one embodiment, a third temperature sensor with relatively higher accuracy is used to compensate for lower accuracy of permanently affixed sensors.

主权项

1. An I/O circuit for use with an industrial control system comprising:
a terminal block having at least one first terminal for releasably receiving leads of at least one first temperature sensor; a multiplicity of second temperature sensors permanently affixed in thermal communication with each of the at least one first terminals of the terminal block; at least one second terminal for connecting at least one third temperature sensor having a higher accuracy than the multiplicity of second temperature sensors: and input circuitry electrically connected to each of the at least one first terminals, the second temperature sensors, and each of the at least one second terminals for receiving temperature signals from each of the at least one first temperature sensors, the second temperature sensors, and each of the at least one third temperature sensors, wherein the input circuitry is operable to compensate the temperature signal received from each of the at least one first temperature sensors using temperature signals from the second temperature sensors and from each of the at least one third temperature sensors.