RESUMO
The appearance of limit cycle oscillations in control systems with fixed threshold based samplers degrades the performance of the control loop, accelerates the wear out of actuators, and introduces an unnecessary communication overhead in distributed control systems. In this paper, the role of the input signals to the control loop is taken into account when analyzing the existence of limit cycles induced by fixed threshold samplers. With this analysis, a methodology to re-tune PID controllers while running to avoid limit cycle oscillations generated by ramp-like excitation signals is presented. Implementation guidelines and several examples to illustrate the usefulness of the method are provided.
RESUMO
In this paper we present a new method for tuning PI controllers with symmetric send-on-delta (SSOD) sampling strategy. First we analyze the conditions that produce oscillations in event based systems considering SSOD sampling strategy. The Describing Function is the tool used to address the problem. Once the conditions for oscillations are established, a new robustness to oscillation performance measure is introduced which entails with the concept of phase margin, one of the most traditional measures of relative stability in closed-loop control systems. Therefore, the application of the proposed robustness measure is easy and intuitive. The method is tested by both simulations and experiments. Additionally, a Java application has been developed to aid in the design according to the results presented in the paper.