ERROR DYNAMICS AND PERFECT MODEL FOLLOWING WITH APPLICATION TO FLIGHT CONTROL

The role of the system error dynamics in model-following control systems is presented in a unified theoretical framework. The approach clearly exposes the requirements for perfect model-following control laws to exist. The error dynamics selected are shown to determine if implicit or explicit following results, and the effects of these error dynamics and plant and model dynamics on the closed-loop stability, performance, and control system architecture are exposed. The importance of model and plant transmission zeros is also revealed. A new linear quadratic formulation is offered that can yield perfect model-following controllers, but guarantees internal system stability, unlike algebraic solutions for the control law. Further, the case of finite actuation bandwidth may be treated with the linear quadratic approach. The error dynamics are also shown to be key in this linear quadratic model-following formulation. The paper concludes with an example involving an unstable forward-swept-wing elastic aircraft, which demonstrates that high-fidelity model-following control laws can be synthesized for statically unstable aircraft with nonminimum phase transmission zeros.