Robust PI/PID controllers for load disturbance based on direct synthesis.

It is recognized that disturbance rejection is much more important than set-point tracking for many process control applications, leading set-point tracking to a secondary level of interest. In this paper a proposal for robust tuning of PI/PID controllers designed under the direct synthesis for load disturbance (DS-d) approach is presented. As with the IMC-like approaches, the resulting DS-d tunings are expressed in terms of a unique parameter that determines the desired speed of response of the regulatory behavior. Even at first sight it may seem quite simple, there is no known guide on how to select such parameter in order to achieve some desired robustness. As it will be shown, for some process dynamics, this selection is not as simple as it may seem. Tuning expressions for the most common types of process models are provided such that the closed loop time constant is the fastest one that allows to reach the desired robustness. Simulation examples show the application of the suggested tuning.

IMC based feedforward controller framework for disturbance attenuation on uncertain systems.

This paper presents a generalization of the Internal Model Control (IMC) approach to feedforward control action generation. As is well known, one of the distinctive features of the IMC formulation is the possibility to distinguish between the nominal and uncertain situations. However the consideration of the presence of uncertainty for the feedforward part of the controller is not as simple as that for the feedback one. In this sense, what is proposed in this paper provides a more systematic way of addressing a robust feedforward design. In addition, being one of the major drawbacks of the Internal Model Control to have to deal with stable systems, the generalization presented here allows for unstable plants as well as unstable disturbance models.