308 CHAPTER 8 Industrial process controllers Proportional Algorithm Setpoint + - Error + Integral Algorithm Derivative Algorithm Controller + + Controller Output Plant Process Variable Feedback Variable FIGURE 8.1 A schematic of a feedback control loop with a PID controller. the rate of change of the process variable and makes changes to the controller output to accommodate unusual changes. Each of the three control functions is governed by a user-defined parameter, which vary immensely