Angular position control of fluid-driven bi-directional motor

This paper describes the design of a rotational speed-control system and an angular position-control system for a fluid-driven bi-directional motor. The fluid-driven bi-directional motor has a driving principle similar to that of the fluid-driven spindle, which is designed for use in ultra-precision machine tools. The fluid-driven bi-directional motor was designed so that it is driven by low viscosity oil flow power. In this paper, the rotational speed controller for the motor is first discussed. In order to reduce the influence of external load torque on the rotational speed, a conventional disturbance observer is combined with the rotational speed-control system. The angular position-control system, which possesses the rotational speed feedback loop with the disturbance observer in the angular position feedback loop, is then discussed. The designed rotational speed and angular position-control systems are conventional I-P control and proportional control systems, respectively. The performance of the designed rotational speed-control system and the angular position-control system is studied via simulations and experiments. The performance of the designed control system is tested by the step response method as well as by the frequency response method, respectively. The simulation and experimental results show that the rotational speed and the angular position of the motor can be controlled by the rotational speed controller and angular position controller, respectively. In addition, the influence of the external load torque acting on the motor is successfully compensated for by means of the disturbance observer. The experimental result shows that the designed angular position-control system suppresses the steady-state positioning error to less than 0.02 degrees., even if external constant load torque acts on the motor.