Output variable structure control of the longitudinal motion of an aircraft

A variable structure control system (VSCS) essentially tries to keep the dynamic behaviour of a system constant by dynamically varying the feedback gains of the system. Two compensator blocks contain gains that the VSCS switches dynamically in order to maintain the performance of the closed loop system. The switching is controlled by reference to a performance measure or switching function that is constructed from either the state vector or the output vector of the system. This performance measure is used in a switching strategy to minimise the deviation of the system performance away from the desired performance. The output from the performance measure is zero when the system is performing correctly, and the switching strategy is designed to maintain the performance measure at zero at all times. As an example of VSCS, the control of the longitudinal motion of an aircraft is considered