A two degree of freedom level control

A two degree of freedom control is proposed for liquid level control. The l oad estimation is achieved in an indirect manner. Only one design parameter is required for the load estimation. As for the feedback loop, a P-only fe edback action is employed. It is equivalent to a P control at disturbance f ree condition and becomes a PI controller under load disturbance. Steady-st ate characteristics, dynamic properties and robustness issues are explored. Design procedures are also proposed. A surge tank example is used to illus trate the performance of the proposed control. The results show that the tw o-degree of freedom control gives good nominal performance and is robust wi th respect to modeling errors. Improved performance are also observed for s ystems with time delay or right-half-plane zero. (C) 2001 Elsevier Science Ltd. All rights reserved.