Current Loop 4-20 milliamp current loops are the signal workhorses in many processes. A DC milliamp current is transmitted through a pair of wires from a sensor to a controller or from a controller to its final control element. Current loops are used because of their immunity to noise and the distances that the signal can be transmitted. Since the signal being transmitted is current the voltage drop that occurs across conductors does not affect the signal, it just limits the length of the signal cable; and induced voltages do not affect the signal. Loop Scaling The level of the current in the loop is related to the value of the process variable or the controller output. How the current level and process value are related is the loop scaling. Output Scaling Scale outputs for a one to one correspondence. That is the controller output is configured for 0% to correspond to a 4mA signal and 100% to correspond to a 20mA signal. The final control element is calibrated so that 4mA corresponds to its 0% position or speed and 20mA corresponds to its 100% position or speed. Input Scaling Scale inputs for a one to one correspondence as well. If we were using a pressure transducer with a required operating range of 0 psig to 100 psig we would calibrate the instrument such that 0 psig would correspond to 4mA output and 100 psig would correspond to a 20mA output. At the controller we would configure the input such that 4mA would correspond to an internal value of 0 psig and 10mA would correspond to an internal value of 100 psig.
Posted on: Sat, 29 Mar 2014 15:40:39 +0000
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