Mechatronic design of a modular three-axis slider system for high-precision positioning applications
Author
Ulu, Erva
Advisor
Çakmakcı, Melih
Date
2012Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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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.
Keywords
Precision positioningencoder resolution
adaptive systems
measurement interpolation
quadrature encoder signals
modular design