A new implementation method of low stiffness for magnetic levitation gravity compensator

Magnetic levitation is one of the solutions for high-precision positioning systems due to its non-contact characteristic. Magnetic bearings have the potential of replacing the air bearings that are used in the semiconductor industry in which the vacuum environment is increasingly required. For the traditional z-actuators in the multi-DOF fine stage, a continuous power loss is inevitable because the z-actuators are required to support the moving mass. Therefore, the heat form the coils will cause the temperature rise and structural distortion, which both deteriorate the positioning accuracy. A good solution is to support the moving mass by passive magnetic force. However, the major difficulty in using passive magnetic force is the highly position-dependent characteristic. From the aspects of positioning accuracy and vibration isolation, the stiffness that are defined as the position dependency of the magnetic force should be kept as low as possible. To date, the main reasons of the stiffness error between theoretical analysis and experiment results are ascribed to assembling error and machining tolerance. However, it is not entirely true for the low-stiffness applications. In this paper, the essential reason causing the stiffness error is analyzed and a new method to realize low stiffness for passive magnetic levitation is proposed.