What is internal model control (IMC) and how does it work?

IMC uses an explicit model of the plant in the controller structure. Controller C = Q/(1 - Q*Gm) where Q is IMC controller and Gm is plant model. When model is perfect (Gm = G), closed-loop TF equals Q - designer directly specifies desired response. Robustness is achieved by low-pass filtering Q: Q = Gm^(-1) * f(s) where f(s) is filter. Model mismatch causes error, but IMC structure provides disturbance rejection. IMC can be converted to equivalent PID for implementation. Foundation for model predictive control.