In recent years, multi-loop proportional-integral/proportionalintegral- derivative (PI/PID) controllers are one of the most widely used industrial controllers. The reason for this popularity is due to the industrial perspective, efficiency, improved productivity, and product quality goals since multi-loop control systems satisfy all of those demands for more effective operational strategies and allow more advanced tools to be implemented. In addition, they are also easily understood by their sound theoretical foundation, required fewer parameters to tune than multivariable control systems that help maximize the economic benefits, and easily obtained the loop failure tolerance that is very important for practical applications. Therefore, many design methods of multi-loop PI/PID controllers have been suggested in the process of control literature over the years. Among them, the wellknown IMC-PID approach is often utilized because of its powerful framework and effectiveness for the design and implementation. It attracts the control engineers and academics because the tradeoff between the robustness and performance are directly related to one design parameter as the time constant of IMC filter or the closed-loop time constant, equivalently. This is a very useful characteristic of the IMCPID approach, which guarantees multi-loop control systems to be tuned satisfying the robust stability condition. Furthermore, the direct synthesis for the multi-loop PI/PID control system that has the similar features of the IMC-PID approach is also considered by many authors.

In this textbook, the novel analytical methods based on the generalized multi-loop IMC-PID approach and direct synthesis are proposed different states of the robust design of multi-loop proportionalintegral/ proportional-integral-derivative (PI/PID) controllers. The proposed methods are aimed at achieving the desired closed-loop responses for multivariable processes with multiple time delays. It is known that the interactions between input/output variables are very common phenomena encountering in the multi-loop feedback control loops, which can be a serious obstacle to achieve a good performance of the multi-loop control system. To overcome this impediment, several techniques are suggested for taking the interaction effects fully into account such as weighted sum Mp criterion, multi-loop Ms criterion, etc.