The physical representation of a steel strip-rolling mill is a damped-spring system [8]. The output thickness sensor is located a negligible distance from the output of the mill, and the objective is to keep the thickness as close to a reference value as possible. Any change of the input strip thickness is regarded as a disturbance. The system is a non-unity feedback system, as shown in Figure DP9.6. Depending on the maintenance of the mill, the parameter varies as 80 ≤ b.
Determine the phase margin and gain margin for the two extreme values of b when the normal value of the gain is K - 170. Recommend a reduced value for K so that the phase margin is greater than 40° and the gain margin is greater than 8 dB for the range of b.
