Design and robustness evaluation of an H_∞ loop shaping controller for a 2DOF stabilized platform

Motion controlled stabilized platform is a 2DOF parallel manipulator which has been developed to reject angular disturbances and keep its surface horizontal. It is used for the stabilization of heavy loads such as antennas, surgery tables, etc in ocean going crafts. Use of variety of payloads causes significant changes in the dynamic properties of the plant. The controller has to be robust against the uncertainties caused by the change of payload. Two facets of this paper are the robust controller design and robustness evaluation. Traditionally such a platform is modeled through time varying nonlinear model, thus providing the rationale for a nonlinear or adaptive controller. In this paper the authors have proposed that linear identification methods may be used to establish a linear model for a particular load condition. Secondly a robust controller is designed using H_∞ based method [6], so that the designed controller may be robust against other load variations. The method facilitates modeling of the plant with complex loads. In this work, first, a model has been identified by input/output measurements, after that a control design procedure based on H_∞ optimization is used, which explicitly trade between nominal performance and robust stability. The performance and robustness is evaluated at different loads. The designed controller is compared with an existing phase lead controller and shows obvious improvements for load variations. It is shown by experiments, both in simulation and on actual rig that design through H_∞ is suitable controller design method for robustness as well as performance. The design choices made are validated by simulation results and rigorous testing on the actual stabilized platform.