Abstract
We are developing a clean robot which operates in an ultra high vacuum and clean environment without a mechanical contact and moves 3-dimensional motion, namely vertical direction, rotational motion round the vertical axis and radial direction perpendicular to the vertical direction. A linear actuator for the radial motion is levitated by linear magnetic bearings and propelled by a linear stepping motor and have an end effecter (hand) to hold a silicon wafer. A rotary actuator takes in charge of vertical and rotational motions and is levitated by outer type active radial magnetic bearings and a passive thrust magnetic bearing. In this paper, we propose a new controller for the rotary actuator using the H-infinity method. Previously PID and LQG controller was designed for rotary actuator [1] [2]. And rotor's contact-free levitation was achieved using these controllers. But there were some problems. The problem of prime importance was that the sensitivity from disturbance input to error output indicated high gain. So we aim to achieve lower sensitivity using newly designed H_∞ controller. Firstly, a dynamic model of the rotary actuator is presented considering 4 degrees of freedom in the rotor, 4 degrees of freedom in the stator and the flexibility of the stator. Secondly, a H_∞ controller is designed for the radial magnetic bearing. Thirdly, a series of experiments are carried out for estimating a performance of the H_∞ controlled system. We have confirmed from the experiment result that the changes of relative displacement between the rotor and the stator of the radial bearing are less than the radial clearance and the control currents in the magnetizing coils are also under the current capacity of the power amplifiers. And we have confirmed that H_∞ controller's performance achieves lower sensitivity than that of PID and LQG controller.
