Implications of Internal Model Control for PID Controllers

For a large number of single input-single output (SISO) typically used in the process industries the Internal Model Control (IMC) design procedure is shown to lead to PID controllers, occasionally augmented with a first order lag. These PID controllers have as their only tuning parameter the closed loop time constant or equivalently, the closed loop bandwidth. On-line adjustments are therefore much simpler than for general PID controllers. As a special case, PID tuning rules for systems modeled by a first order lag with deadtime are derived analytically. The superiority of these rules in terms of both closed loop performance and robustness is demonstrated.