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

Abstract

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.