Adaptive output-feedback tracking: the case of MIMO plants with unknown, time-varying state delay

In this paper, we develop a simple model reference adaptive control (MRAC) scheme for a class of multi-input-multi-output (MIMO) linear dynamic systems with unknown state delay which is also robust with respect to an external disturbance with unknown bound. A suitable Lyapunov–Krasovskii type functional with “virtual” gain is introduced to design the adaptation algorithms and to prove stability