Modelling and control of a hydraulically actuated two-degree-of-freedom inertial platform

This paper investigates modelling and control issues associated with a two-degree-of-freedom inertial platform for naval applications. In the modelling part, the dynamics of the system are physically characterized and then experimentally identified. It is found that due to the inverted-pendulum structure and the use of hydraulic actuators, the system is open-loop unstable and exhibits different frequency responses when the amplitude of the input signal changes. Moreover, the identification experiment reveals that the inclinometer used has a major resonant peak that will limit the control system performance. Therefore, a complimentary filter scheme is proposed to condition the sensor signals so as to produce a more acceptable absolute-angle measurement. In the control part, two proportional-integral-derivative (PID) controllers, whose control parameters are computed using a non-linear optimization scheme to achieve optimal disturbance rejection with reasonable robustness and noise sensitivity properties, are respectively designed for the pitch and the roll subsystems. Experimental results indicate that when the platform's base frame encounters a biaxial sea-wave motion, the resulted control system can attenuate the vibration to within 10 per cent.