ABSTRACT
This chapter provides an introduction to a feedback process control loop and a description of the action provided by Proportional (P), Integral (I), and Derivative (D). The controller output is determined by the three controller actions P, I, and D in response to the error and how it changes with time. However, if there is a change to increase the temperature setpoint or a change to increase the load (process water flow rate), the proportional action by itself cannot eliminate the error. However, many industrial analog controllers and microprocessor distributed control system controllers use a capacitance lag (filter) of about 0.05 to 0.10 in series with the process variable signal to reduce the effect of derivative action from setpoint changes and from short time constant noise described earlier. The chapter presents a flow diagram of a PID feedback control loop with the process streams and instrument lines labeled.
