PID Controllers with a Constant Speed Actuator: Implementation Versions and Self-Tuning of Pulse Duration

Abstract

The article considers three implementation versions of controllers operating with constant speed actuators that are used in automation systems of a technological process: with a relay-pulse control unit (RPCU) with a lagging feedback and topping differentiator (a real differentiating element, RD), with calculating the output according to a PID algorithm and using the actuator model for its implementation, and with calculating the control output speed and using a pulse-width modulator. Formulas for calculating the parameters of the RPCU and RD primary components from the condition of approaching the equivalent PID algorithm are given. The system version with a PID controller with the actuator control proceeding from its model is considered in two options: with individual integrators for the controller’s integral component and the actuator model, and with a common integrator for the controller’s integral component and the actuator model. In the latter option, the signal from the actuator model is subtracted from the controller’s integral component, due to which the possibility of the integrator’s output to reach the integral component’s limit values is ruled out. The versions are compared by analyzing the automatic control system performance in the pulsed actuator’s operation mode based on simulation for the superheated steam temperature control section of a steam boiler equipped with a surface steam desuperheater. The analysis was carried out taking into account both the usual dynamic accuracy indicators and the number of actuators switched into operation during the transient. It is shown that the advantage of the version with the PID algorithm and common integrator for the integral component and the actuator model is that there is no need in this case to calculate the controlled variable second derivative; in addition, this version is free from the problem connected with accumulation of the difference between the signals from the actuator and its model. This version can be considered to be more preferred in implementing a PID algorithm in the software of programmed controllers. Recommendations for self-tuning of pulse duration and the control unit relay element parameters during the control process depending on the algorithm’s individual components are given. By using the proposed recommendations, it becomes possible to achieve an essentially lower actuator switching frequency during the control process.