To reduce or eliminate the overshoot appearing for 1DOF PI control, the structure can be modified by adding a feedforward controller Fr(s) for the reference, as shown in Fig. 4. The feedback of ωm is unchanged related to the 1DOF PI controller, so the same parameter selections (13) and (18) can be used. It means that the load-torque rejection and the measurement noise amplification remain the same as in the case of the 1DOF
control. One common modification is to use the so-called setpoint (i.e., reference) weighting [30], given in a feedforward form as Fr(s) = Kp(b − 1), b < 1. (IP control [1], [4], [7] is a special case when b = 0.) This makes the controller less “aggressive” for steps in the reference, which reduces the overshoot but also increases the rise time. We here propose a slightly more elaborate scheme.
In the 2DOF control structure, the closed-loop transfer function Gc(s) from ωref to ωm becomes