In practice the gain will be adjusted once for a given set of operating conditions and will be left at that setting. An electronic PID controller works on the same principle as a mechanical or a pneumatic controller. SP - MV Controller output is a function of the instantaneous error signal and the coefficient in the PID algorithmn. By selectively setting the coefficient values to zero the PID controller can be made to act as P , I , D , PI , PD , or a full PID unit. There are several ways to implement a PID controller, but perhaps the parallel implementation is the simplest.
Error signal is fed to each input.
Outputs from all the three modes are algebraically summed together and fed to the final control element. This configuration allows for easy change in the mode of operation. By disconnecting the individual mode outputs, before the summation block, the controller can be made to operate either as P, PI, PD, or full PID controller. The schematic diagram shows the parallel implementation of PID controller using operational amplifiers. Op-amp 1 which is set as a difference amplifier acts as the summation block.