Advanced multivariable control law design for future ACT rotorcraft

• Main Author: Foster, Neale Prescott
• Published: University of Leicester 1995
• Subjects: 620
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.674391

This thesis considers the use of variable structure and H-infinity control system designs for the improvement of the handling qualities of the Lynx helicopter, with full flight envelope operation and enhanced turbulence rejection capabilities. A model-following variable structure controller is designed, where the 'ideal model' was designed in four different ways using the following methods: eigenstructure assignment, Hinfinity minimum entropy, Hinfinity one degree-of-freedom loop shaping, and Hinfinity two degrees-of-freedom loop shaping. The combination of the ideal model and the loop shaping is seen to allow the time and frequency domain objectives to be specified during the variable structure control system design. The decomposition of the Hinfinity one degree-of-freedom and two degrees-of-freedom control laws into state-feedback and observer form allows the use of these observers in the implementation of the variable structure controllers. Extensive testing and evaluating of handling qualities, throughout the operating flight envelope, is performed on the non-linear Rationalised Helicopter Model. Successful Advanced Flight Simulator trials performed at the Defence Research Agency are described, which signify the first real-time 'flight' of a helicopter controlled by a variable structure control law design. A variable structure observer is designed to complete the variable structure observer/controller framework, and shows good handling qualities and time response characteristics throughout the flight envelope. A novel method of combining established Hinfinity control system design procedures with modelling of the turbulence entering the linearised system equations, enables generic Hinfinity control law designs with significantly enhanced turbulence rejection qualities to be successfully demonstrated. The turbulence effects are modelled as perturbing the helicopter's velocity equations, and are included as an extra input to the Hinfinity Standard Compensation Configuration. Therefore, no statistical knowledge of the turbulence was required. The variable structure control law designs are tested for their turbulence rejection capabilities, and exhibit high levels of handling qualities without having been designed explicitly for turbulence rejection in the design procedure.