Conditions for offset elimination in receding horizon controllers: a tutorial analysis

GONZÁLEZ, ALEJANDRO HERNÁN; ADAM, EDUARDO; MARCHETTI, JACINTO LUIS

CHEMICAL ENGINEERING AND PROCESSING

Elsevier

Año: 2008 vol. 47 p. 2184 - 2194

0255-2701

An offset free control is one that drives the controlled outputs to their desired targets at steady state. In the linear model predictive control (MPC) framework, the elimination of steady state offset may seem a little obscure, since the closed loop optimization tends to hide the integral action. Theoretically, implementing a well posed optimization problem and having unbiased steady state predictions are sufficient conditions to eliminate the output offset. However, these basic conditions are not always achieved in practical applications, especially when state space models are used to perform the output predictions. This paper presents a detailed practical analysis of the existing strategies to eliminate offset when using linear state space models with moderated uncertainties. The effectiveness of these strategies is demonstrated by simulating three different control problems: a linear SISO system where the effect of using the estimation of the control variable is highlighted; a continuous stirred tank reactor (CSTR) with nonlinear dynamics and the consequent model uncertainty and, a 2x2 system representing a distillation column that verifies the consistency of previous results and extends the conclusions to higher dimension systems.