Enhanced assessment of robustness for an aircraft's sliding mode controller

The design of a multivariable sliding mode controller based on the equations of motion of an unmanned aircraft or remotely piloted vehicle (RPV) is described and an emphasis on robust eigenstructure assignment is given. The linear and nonlinear model responses are compared when subjected to the same variable-structure control design. The ability of the controller to cope with uncertain parameter variations is investigated as a problem of robustness assessment. It is shown that after sliding has commenced, the response of the objective system is tracked by the corresponding nonlinear or perturbed system. A measure of sensitivity is defined as the proximity of the actual system modal structure and response to that of the designed objective. The nonlinear system with variable-structure controller is also shown to be robust in the sense that this objective response is always recovered after a disturbance or parameter variation.