Minimum settling-time PI control of pure delay processes under a hard non-overshoot constraint: exact boundary-contact characterization and the role of the MID point

Abstract

We solve, exactly, the problem of minimum settling-time PI control of a pure delay process K e^{-Ls} under the hard time-domain constraint of zero overshoot, y(t) <= 1 for all t. The closed loop is a neutral delay system whose step response is piecewise polynomial on the delay segments, with geometrically decaying jump discontinuities at the segment boundaries t = kL. The constrained optimum is characterized by an equioscillation-type contact structure whose active contacts sit at echo boundaries: kink maxima grazing the setpoint, jumps landing on the settling-band edge, and boundary troughs anchored to it. The number of contact equations equals the number of gains, so the optimum is exactly computable for every band delta. In a closed-form regime, delta in [(3-2 sqrt2)/4, (3-2 sqrt2)/2] approx [4.29%, 8.58%], the optimal gains are independent of delta: K Kp = 1 - sqrt2/2, K Ki L = sqrt2/2, and the optimal settling time is Ts*(delta) = (4 - sqrt2 - 2 sqrt(delta)) L. Outside this window the optimum solves an explicit two-equation polynomial system per regime, and Ts*(delta) is a staircase with exact flats at integer multiples of L from jump-landing pinning. As delta -> 0 the optimal gains converge to K Kp = e^{-2}, K Ki L = 4 e^{-2}, the generic multiplicity-induced-dominancy (GMID) point of the neutral quasipolynomial. The GMID response satisfies the hard constraint and uniquely maximizes the decay rate; yet at every finite delta the delta-adapted optimum strictly beats the fixed GMID tuning, by about 40% at delta = 2%. The MID point is thus the limit of the optimal gains without ever being the optimal tuning. A numerical extension to first-order-plus-time-delay plants quantifies the speed/robustness trade across Ms in [1.39, 1.76].