An experimental investigation of the effects of the compliant joint method on feedback compensation of pre-sliding/pre-rolling friction

Mechanical bearings (i.e., sliding and rolling bearings) are widely used for motion guidance in precision positioning stages due to their low cost, large motion range and high off-axis stiffness. They are also finding increasing use in ultra-precision positioning, e.g., for low-cost and long-range nanopositioning in vacuum environments. However, mechanical-bearing-guided motion stages suffer from nonlinear pre-motion (i.e., pre-sliding/pre-rolling) friction which adversely affects their precision and motion speed in both tracking and point-to-point positioning applications. A compliant joint method has recently been proposed for simple, accurate and robust feedforward compensation of pre-motion friction in tracking motions, with excellent results. This paper experimentally investigates the influence of the compliant joint method on feedback compensation of pre-motion friction, which is critical to achieving fast settling in point-to-point positioning. It shows using a model-free (PID) controller that, for the same feedback gains, the mechanical-bearing-guided motion stage equipped with compliant joints exhibits much more linear closed loop dynamics and higher bandwidth compared to the traditional motion stage without compliant joints. The compliant-joint-equipped stage also has much faster settling time in point-to-point positioning experiments for most step motions tested, except for one particular step size where it settles slower than the traditional mechanical-bearing-guided motion stage due to the compliant joint dynamics. With the addition of an inverse-model-based disturbance observer to the PID controller, the settling time of the stage with compliant joints becomes uniformly much faster than the traditional mechanical-bearing-guided motion stage; its robustness and stability margins are also shown to be superior to those of the traditional mechanical-bearing-guided motion stage.