Emerson Rosemount 56, Инструкция по эксплуатации

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Instruction Manual Section 7: PID Control
LIQ-MAN-56 April 2017
PID Control 57
7.1.3 Proportional Bias
Most processes require that the measured variable be held at the set point. The proportional
mode alone will not automatically do this, if an output greater than 0% is needed to keep the
PV at setpoint. At setpoint, the control output is 0%, and if a non-zero control output is
needed to keep the PV at the setpoint, proportional alone will only stabilize the measured
variable at some offset (deviation) from the desired setpoint.
Bias is used to provide a constant control output at the setpoint to maintain PV at the
setpoint. The effect of Bias is expressed as follows:
Proportional Output (%) = [Gain * (PV – SP) * 100 / (URV – LRV)] + BIAS
Fig. 7-4 Example of Reverse Acting Control
Fig. 7-5 Direct Acting Control with Bias