Correction of the on-off control system using PLC

The paper deals with an analysis of features of on-off controllers. It was assumed that the control quality can be described by three distinctive indexes: the period, the amplitude and the mean value of the controlled signal in the steady state. The on-off controller with positive hysteresis was a basic form of the controller that was analyzed. To eliminate the disadvantages of this basic form the controller structure was augmented by inertia corrector. Its impact on mentioned above indicators was shown and described. To compensate the negative influence of inertia corrector on the mean value of the controlled signal, an additional proportional corrector was proposed. Its influence on control quality was shown and discussed. To show how easy it is to build the proposed structure of the controller, its exemplary implementation in the ladder logic language of the S7-1200 PLC was described.