Structured Adaptive Model Inversion control with fault tolerance and actuator position limits

Abstract

Structured Adaptive Model Inversion (SAMI) is a model reference adaptive control formulation that takes advantage of the dynamical structure of the state-space descriptions of a large class of systems. Most dynamic systems can be broken into an exactly known kinematic level part, and a momentum level part with uncertain system parameters. The SAMI formulation enables the imposition of exact kinematic differential equations, and restricts the adaptation process that compensates for model errors to the acceleration level. Dynamic inversion is used to solve for the control The dynamic inversion is approximate, as the system parameters are not modeled accurately. Hence an adaptive control structure is wrapped around the dynamic inverter to account for uncertainties. In high performance dynamic systems the total number of actuators used may be greater than the number of states to be closely controlled or tracked. In the case of redundant actuation, it is still possible to closely track the desired states even if some of the actuators fail, as long as the number of active actuators is more than or equal to the number of states to be tracked. Therefore, if it is possible to reconfigure the control after failure, the stability and performance of the system can be maintained. The research presented in this thesis is the result of an attempt to provide fault tolerance, without the need of a fault detection algorithm, by extending the SAMI methodology. Traditional adaptive control lacks an adequate theoretical treatment for control in the presence of actuator saturation limits. The research presented in this thesis attempts to extend the adaptive control methodology to facilitate correct adaptation in the presence of actuator saturation. The central idea is to modify the reference trajectory on saturation, in such a way that the modified trajectory approximates the original reference as close as possible, and can be tracked within saturation limits. Results presented in this thesis for an inverted pendulum on a cart and nonlinear six degree of freedom simulation of an aircraft show that the synthesized control laws are able to handle parametric uncertainties, initial error conditions, actuator failures and actuator position constraints.