Robust modified partial internal model control for stable, unstable and integrating processes

For open-loop stable processes, the internal model control (IMC) provides a simple and effective parameterisation of stabilising controllers. However, for unstable processes, it cannot be directly used for control system implementation. In this paper, a new unified approach for the IMC design is proposed for stable, unstable and integrating processes, using the modified IMC (m-IMC) structure in which an additional controller K(s) is introduced along with the basic IMC. Also, the partial IMC (PIMC) concept is utilised, wherein we write the plant as the summation of stable and unstable parts and consider only the stable part as the internal model. The combined approach is thereby referred to as modified partial IMC (m-PIMC). This paper presents a graphical method to obtain controller parameters for the proposed m-PIMC to satisfy the given gain margin (Am) and phase margin (ϕm) specifications. Further, stability boundary and robust boundary for the controller parameters are obtained which respectively give a set of all controller parameters stabilising the close loop and a set of all controller parameters ensuring robust stability. Seven different well-known standard stable, unstable and integrating process models are considered for the proposed m-PIMC design, and simulation examples are given for four selected plant models.