The Design of PI + Approximation 2 Degree of Freedom Cascade Controller for Vertical Coupled Liquid Tank System

This paper presents the proportional-integral (PI) plus approximate 2 degrees of freedom (A2DOF) controller, which is compared to cascade PI+PI controller and PI+PID controller, applying to the vertical coupled liquid tank system to observe their performances. The cascade control has an inner loop for the liquid level control of tank# l that can make a faster response. There also is an outer loop for controlling the liquid level of tank# 2 that can make the slower response. The performances of PI+PI, PI+PID, and PI+A2DOF controllers were experimentally compared. It can be noticed that the PI+A2DOF controller gives more improved performance than PI+PI and PI+PID controllers. The PI+A2DOF has an overshoot of about 0.8% of the input with a settling time of about 20sec, which is better than two time of a dual controller. In the sudden setpoint change, the PI+A2DOF controller spent time in the new setpoint about 15sec that, faster than two times of both compared controllers without overshoot. The fact and experimental results demonstrate the usefulness and practicality of our method.