Quaternion-based H∞ attitude tracking control of rigid bodies with time-varying delay in attitude measurements

The problem of attitude and angular velocity tracking in the presence of exogenous disturbances and where feedback measurements are subjected to unknown time-varying delays is addressed. Sufficient conditions which guarantee stability and disturbance attenuation performance in the H-infinity sense are provided. Results are presented in the form of LMIs, which allow the conditions to be simply and efficiently computed. Using a simple quaternion-based linear state feedback controller and a feedforward term to compensate the nonlinearities of the system dynamics, simulation results illustrate that the control law is able to effectively track desired trajectories and reject disturbances even in the presence of large time-varying delays.