LQR approach to robust stabilization of state space systems with matched uncertainties.

This note shows an elegant relationship between the quadratic optimal control and robust stabilization for linear time-invariant (LTI) systems, where the former control can robustly stabilize the latter system, provided that the matched uncertainty is bounded. Through reviewing the relevant literature, some common mistakes in regard to this relationship are found. The correct results are obtained and proved in both frequency and time domains. The results are applicable to both single- and multi-input cases. They are significant as the simple LQR design for the nominal system can be utilized to directly solve-with no further effort-the complex robust stabilization problem for a class of linear uncertain systems.

Active disturbance rejection control for non-minimum phase systems under plant reconstruction.

Active disturbance rejection control (ADRC) for non-minimum phase (NMP) systems is a challenging problem due to the conflict between stability and feedback tuning. The key point on this issue is to avoid heavy feedback tuning for robustness enhancement. We perform plant reconstruction to obtain an explicit expression of internal disturbance, such that it can be reduced by cascade compensation. Then, a new ADRC scheme is developed based on plant reconstruction and the internal stability criterion for ADRC system is derived. The stability conditions provide guidelines on the design of the cascade compensator and disturbance observer. It also indicates that the cascade compensation contributes to the robust stability of NMP systems. Simulation results of two typical NMP systems are provided to show the efficacy of the proposed ADRC scheme. Physical realizability is also demonstrated through experiments on a motion NMP system.

Design, analysis and application of a new disturbance rejection PID for uncertain systems.

In this paper, a new disturbance rejection proportional-integral-derivative (DR-PID) scheme is proposed for a class of minimum phase plants with low relative order. The essential active disturbance rejection (ADR) mechanism that is otherwise hidden in PID control structure has been illuminated and clarified in this paper for the first time.The proposed DR-PID scheme is derived on the basis of a modified disturbance observer to embed the active disturbance rejection mechanism seamlessly in the classical PID structure. Such a DR-PID scheme is implemented in a typical two-degree-of-freedom control structure that contains a standard PID controller and a pre-filter. The internal stability condition is established by investigating the closed-loop poles according to Rouche's theorem. The ensuing internal stability condition provides effective guidelines for DR-PID design that has infinite gain margin with minimum plant information. Five numerical comparisons are performed to illustrate the effectiveness of the new DR-PID scheme. The physical realizability of the proposed DR-PID scheme is also demonstrated by experiments on a magnetic levitation system.

Combined gain and phase margins.

In this note, the limitations of conventional or separate gain or phase margin are shown. Then, the combined gain and phase margins are introduced to overcome the limitations and suit both stable and unstable systems. A simple method for their computation is presented and their applications in stabilization is demonstrated.