The negative feedback is assumed. For the given value ofthe derivative (noise) filter order 1 ≤ n, and the given value ofthe set-point weighting 0 ≤ b ≤ 1, parameters q = {k, ki, kd, Tf}are proportional gain k, integral gain ki, derivative gain kd, andTf, time constant of the derivative (noise) filter of order 1 ≤ n. In(3) and (4) controlled variable y(t) and control variable u(t) rep-resent variations around their values in the nominal regime.For the PI controller kd= 0, and Tf= 0 is used here.According to (3) and (4), control signal u at the input toactuator AC in Fig. 1 is defined by