A two degree of freedom level control

A two degree of freedom control is proposed for liquid level control. The load 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 feedback action is employed. It is equivalent to a P control at disturbance free condition and becomes a PI controller under load disturbance. Steady-state characteristics, dynamic properties and robustness issues are explored. Design procedures are also proposed. A surge tank example is used to illustrate the performance of the proposed control. The results show that the two-degree of freedom control gives good nominal performance and is robust with respect to modeling errors. Improved performance are also observed for systems with time delay or right-half-plane zero.