Two-degree-of-freedom control scheme for cascade control systems

A new two-degree-of-freedom control structure is proposed for cascade control systems. Analytical method of controller design is derived for the general process model. Its dominant merits are that the primary deficiency of tuning trouble occurring in a conventional cascade control system is essentially overcome. With this structure, the set-point tracking and disturbance rejection property of primary and secondary loop are no longer coupled. Meanwhile, the approximation for the closed-loop transfer function of the secondary loop is no longer necessary in the design of primary loop controller, so the system control performance is improved. The simulation results demonstrate that the proposed method is superior to the existing methods.