Since the middle of the $20^{th}$ century, growth of information technologies with the invention of integrated circuit has led to other industries. Industries of semiconductor and large display need large working area and high precision equipments. Especially, requirement of their high-throughput and working range induces the development of ultra precision stages with high speed. The ultra precision stages are configured with high reproducible and low velocity ripple system - stepping and scanning system, and one servo and dual servo system. From the last of the $20^{th}$ century, a scanning system is regularized and compared with a stepping system in semiconductor industry for its high throughput. Since the last of 1980, a dual servo system, focused on step motions and high relative uncertainty motions, has been introduced. The dual system has some characteristics: good in-position stability, high sustaining force, and mechanical connection between its fine stage and coarse stage. Its fine stage has some troubles in being controlled due to uncertainties and disturbances transferred through its mechanical connection to the coarse stage. In addition, it may not have high frequency response due to high load and its connection’s stiffness. To solve this problem, a mechanical decoupled system was proposed in this paper.
A mechanically decoupled dual servo (MDDS) stage for ultra-precision scanning system is introduced in this paper. The proposed MDDS consists of a 6 axis fine stage for handling and carrying workpieces and a XY coarse stage with air bearing guide for its non-friction motion. Especially, the MDDS uses three moving magnet voice coil motors (MMVCM) as a planar actuation system of the fine stage to reduce the disturbances due to any mechanical connections with its coarse stage. VCMs are governed by Lorentz law. According to the law and its structure, there are no mechanical connections between coils and magnetic circuits. Moreover, the VCM doesn’t have force ripples due to imperfections of commutation components of linear motor systems - currents and flux densities. However, due to the VCM’s mechanical constraints the working range of the fine is about 5mm2. To break that hurdle, the coarse stage with Halbach magnet array linear motors is used for the fine stage to move about 200mm2. The linear motors are used in the coarse actuator due to its high ratio of force to volume. Anyway, because of its mechanically decoupled structure, the proposed MDDS can achieve higher precision scanning than other stages with only one servo.
Using MATLAB’s Sequential Quadratic Programming (SQP), the VCMs are optimally designed for the highest force under conditions and constraints such as thermal dissipations due to its coil, its size, and so on. For linear motors, Halbach magnet linear motor is also optimally designed in this paper. In addition, for their smooth movements without any frictions, guide systems of the MDDS are composed of air bearings. In addition, a leveling system is designed with three level actuators which are using a permanent and a changeable magnetic flux, and flexure circular plates as the precision guide. And then, precisely to get their positions, linear scales with 1um resolution are used for the coarse’s XY motions and plane mirror laser interferometers with 20nm for the fine’s XYθz. On scanning, the two stages have same trajectories and are controlled. The control algorithm is Parallel method. The embodied ultra-precision scanning system has position accuracies in 99% probability - $＼sigma_{x99%} = 0.02497＼mu$, $＼sigma_{y99%} = 0.03994＼mu$, $＼sigma_{z99%} = 0.04132＼mu$, $＼sigma_{th99%} = 0.0635arcsec$, $＼sigma_{thx99%} = 0.11989arcsec$, $＼sigma_{thy99%} = 0.088arcsec$. And it has smooth high precision velocity motion depending on an interface system of its controller and displacement measuring systems.
The first chapter presents motivation and the state of dual stages. The second chapter introduces the conceptual design of the mechanically decoupled dual system. The third and forth chapters describes its detail design - the fine and the coarse position system. The fifth chapter explains dual servo algorithms - Parallel type, reaction compensator, and perturbation observers. At last, some experiments results are shown to evaluate the proposed decoupled dual servo system.
