Dynamic analysis and robust motion control of frictionless positioning device suspended by cone-shaped active magnetic bearings

Abstract

Active magnetic bearings(AMBs) have nowadays been widely adopted in machines and robots, which are operated under the clean room and vacuum chamber environment. Particularly in semiconductor industry, the environment must be ultra-clean to prevent samples from contamination during handling and processing. AMBs can apply a force without mechanical contact and simultaneously do not introduce any mechanical constrains on the controlled object. In addition, they have advantages such as active position control of the suspended part, control of force and stiffness of the handling point, and load sensing capability. The cone-shaped AMBs are very useful as a positioning device because it has a simple structure compared with the conventional AMB and it can achieve the five d.o.f. motion control of a moving part using only four cone-shaped magnet pairs. Separation of the controllers for stabilizing and tracking is very effective for an inherently unstable system like AMBs. The stabilizing controller is to ensure the stability of the closed loop system and the incorporated tracking controller enables the exact tracking of the command input despite of the uncertainties. An outer tracking loop is used to track the command input so that the system performance is not seriously affected. The stabilizing and tracking controllers of AMBs usually use the optimal controller with integral action. However, since such an AMB system often has unknown, nonlinear, or time-varying parameters during operation, a good tracking performance is not likely to be achieved when we rely on a simple linear controller. In order to attain a desired performance for the system with some uncertainty, a linear controller normally requires re-tuning of the controller gains, otherwise a robust control law should be adopted. In this work, a frictionless positioning device suspended by cone-shaped AMBs is developed in the laboratory, which is driven by a brushless DC motor equipped with a resolver. The de...