Design of a magnetically levitated six degree of freedom positioning platform

Abstract

Integration of a measurement system directly in the production line can provide permanent information about quality of machined goods. This information is of great interest when aiming for higher product quality and production efficiency. Robotbased in-line surface metrology is a viable approach but faces the problem of random floor vibrations disturbing measurements at the sub-micrometer-scale in industrial environment. A vibration isolating metrology platform mounted to the robot arm and carrying a measurement device, is thus required to maintain a constant position to the sample. Following the idea of robot-based in-line metrology, a magnetically levitated six degree of freedom (DoF) positioning platform, which is able to maintain a desired position with respect to its supporting frame, is designed and developed in the course of this thesis. Eight conventional voice coil actuators together with custom-made current controllers are utilized to position the moving platform. Waiving a permanent magnet based gravity compensation and building a system able to levitate the full weight of the mover in every orientation, allows the successful operation in arbitrary direction of the supporting frame. In order to determine the actual position of the moving platform relative to the supporting frame in all axes, an integrated optical proximity sensor system is implemented. Based on a well decoupled mechanical design and thus a low level of crosstalk between the DoF, the applied control strategy includes six single-input single-output (SISO) proportional-integral-derivative (PID) controllers to stabilize the platform. The achieved positioning range of the 0.7 kg mover is about ±200 μm and ±0.11 in translational and rotational directions, respectively. Independent of the orientation of the supporting frame, a positioning control bandwidth higher than 100 Hz can be stated. By the validation with an external high-precision triangulation sensor, relative position accuracy of 99.6% is achieved and in the translational DoFs an rms-positioning resolution of 200nm is reached.