The Study Of PID Parameter Variation Method For Automatic Control System

PID controller is the most commonly used controller in industrial process with features of clear physical meaning and convenient parameter setting. However, the time-varying nonlinear characteristic is prevailing in thermal process. What’s more, the industry application has put forward different requirements about the control quality under different conditions. In practical application, there are many examples of using variable parameter PID controller to meet the above requirements. To solve the above problem, two methods of PID controller with variable parameters are proposed.A nonlinear differentiation is constructed. The differentiation is formed by one minus first-order inertia. Replaced first-order inertia by rate limiter can get the describing function of a nonlinear differentiation. The response time and differential action of nonlinear differential are increased with the increase of amplitude of input. For coordinated control system of a typical 300 MW unit, the actual differentiation of boiler side controller and the front feed to load commands is replaced by this nonlinear differentiation. At the same time, the adjusting and simulation test is done. The experiment result shows that: for the nonlinear differential coordinated control system, the differential action becomes strong when AGC direction changes substantially and the amount of boiler fuel can change quickly to ensure the pressure stably. However, when AGC direction changes in small amplitude, the differential action becomes weak and the amount of fuel varies smaller to maintain the pressure before turbine in the allowable range. This coordinated control system is suitable for the thermal units with AGC direction changing in small amplitude and high frequency.In the inner loop of superheated steam temperature cascade control system, the model of reference adaptive control compensation is used to gain the dynamic compensation controlled in front zone. The simulation results show that the compensation is better based on the cascade control system of reference adaptive control by keeping the system in optimal working condition.To sum up, two kinds of variable parameter PID have achieved certain control effect in engineering application. The parametric variable PID has the advantages of good engineering application and simple parameter setting. It makes up for the defect of conventional PID controller.