Input from the measurement transmitter is compared with the set point voltage to produce a deviation signal. The deviation signal is combined with a characterized feed back signal to provide the input for the function generator amplifier. This amplifiers output is delivered to the feed back network, and to the final output which is a 10-50m.a. do signal for actuation of final operators.
Proportional action: It is a obtained by adjusting the magnitude of feed back signal. An increase in negative feed back means less effective gain and thus a broader proportional band.
Reset actions: It is obtained by charging the reset capacitor at a rate determined by the value of reset resister. The reset resister is variable, and constitutes reset adjustment.
Derivative action: The connection of a derivative capacitor across the feedback circuit delays feedback until the capacitor is charged to a value approaching amplifier output. This delay is controlled by value of derivative resister. This resister is variable and constitutes derivative adjustment.