PID controllers
Digital controller implementation

Digital controller implementation in PLC requires use of analogue outputs. Such configuration is usually more expensive, than basic one with binary only outputs. Much more expensive and difficult is to amplify analogue control variable to appropriate power level, needed to drive actuators or plant, respectively. It is the reason, why discontinuous controllers are often applied in practice. The parameter adjustment is simplified, and binary only outputs can be used. The actuators are switched using relays, solenoids, electro-pneumatic transducers, nonlinear elements etc.

The performance by means of output value behavior is usually a little bit worse for discontinuous controllers, than for continuous ones, but it is usually satisfactory for most applications. Unwanted effect of discontinuous control can be the oscillatory response and frequent actuator changes. The union of both principles, a multi-step discontinuous and digital PID controller, is a digital PID controller, where the computed control variable is led through a multi-step element. The multi-step output value is encoded as a combination on binary output lines. Continuous behavior can be approximated by use of fast switching of actuated value with variable duty cycle, i.e. pulse width modulation, or by pulses of constant width and variable spacing, i.e. Frequency modulation.