Mechatronic design of a modular three-axis slider system for high-precision positioning applications

Abstract

Following the recent improvements in precision engineering related technology, interest for micro/nano-engineering applications are increased and various micro/nano-scale operations and products are developed. For micro/nano-scale applications, high-precision equipment including micro/nano-positioning devices with high accuracy and precision are required. In this thesis, mechatronic design of a three axes micro/nano-positioning device is discussed in detail. In order to satisfy nanometer level precision, an adaptive method to increase the available measurement resolution of quadrature encoders is presented. Performance characteristics of micro/nano-positioning devices usually include positioning accuracy of their each individual axis, operation range, maximum velocity and maximum acceleration. For this reason, permanent magnet linear motors (PMLM) are chosen as actuators in the presented design due to their outstanding characteristics. Moreover, in order to provide high-flexibility in terms of applications and simplify the control of the system, modularity is one of the main concerns while designing the micro/nano-positioning system presented here. Building the modular single axis slider system, three axes positioning device is constructed by assembling three of them perpendicularly. In this design, linear optical encoders are used as feedback sensors. Movement range of the designed system is 120mm in each direction. Since the available linear optical encoders have measurement resolution of 1µm, resolution of them is to be improved in software for sub-micron level positioning applications. For this purpose, a new method to increase the available measurement resolution of quadrature encoders is presented in this thesis. This method features an adaptive signal correction phase and an interpolation phase. Imperfections in the encoder signals including amplitude differences, mean offsets and quadrature phase shift errors are corrected by using recursive least squares (RLS) with exponential forgetting and resetting. Interpolation of the corrected signals is accomplished by a quick access look-up table calculated offline to satisfy linear mapping from available sinusoidal signals to higher order ones. With the conversion of the high-order sinusoids to binary pulses, position information is derived. By using the presented method, 10nm measurement resolution is achieved with an encoder with 1µm off-the-shelf resolution. Experiment results demonstrating the effectiveness of the proposed method are presented. Validation of the method is accomplished for several cases including the best resolution obtained. Practical constraints limiting the maximum interpolation number are also discussed in detail.