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2851 |
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A Study of Modulated Interference Laser Encoder – New Approach for a High Resolution and High Stability Position Measurement System |
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Optics and Interferometry |
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| Organization |
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| Content |
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Fields in Nanotechnology including the semiconductor industry require highly-reliable machines with high-resolution positioning. Nowadays laser interferometers, capacitance gauges and interference laser encoders achieved sub-nanometer resolution. However the laser interferometer operated is often affected by small air temperature changes. The capacitance gauge is limited by its small measurement length. For these reasons, optical encoders are widely used in precision positioning machines today. However those resolutions are limited by the optical grating pitch. High resolution encoders have to use high-spatial-frequency gratings such as a hologram scale. It pushes up the sensor cost and also requires meticulous installation and signal adjustment procedure. Without proper adjustments, the encoder may produce unacceptable interpolation errors and consequently loses its precision. These are the biggest disadvantages for high resolution optical encoders.
We propose a new concept laser interference encoder. It combines a small optical scanner and conventional interference encoder optics. High frequency scanning of interference laser on moving scale creates harmonic signals which contain position information.
This paper shows 1: Concept of scanning interference encoder, 2: Prototype detail design includes the optical scanner, and 3: Evaluation results.
In the first chapter we explain formulas of interference intensity in conventional laser encoders. From those formulas we describe effects of laser beam scanning on an index grating. Thus we get the formulas of the scanning encoder output light intensity. This results show the output is identical with the output of SPPE (scan position probe encoder) which developed by Dr. Ohara in researches of SPM technology. Original SPPE uses signals from a tunnel current of SPM or spotted laser beam on reflective gratings. Proposed scanning laser interference optics use practical large area i.e. 1mm diameter or lager, on gratings. Therefore it will become quite stable against damages of gratings and also it will remove difficulties about install procedures. Moreover we can apply the SPPE phase detection logic using PLL (Phase-Locked Loop) to decode the signal. SPPE logic can interpolate 1:20000 ratios from origin special frequency. It means the signals to produce position information up to 7.6pm LSB from 4 micro meter pitch grating scale. The entire signal processing logic is implemented on one FPGA chip, which also keeps the cost of the system low.
In the second chapter, we describe detail of the prototype design. The key component of the prototyping is an optical scanner. We developed a tiny tuning-fork quartz oscillator. Using twist vibration mode of the fork, we can realize precious and stable optical scan in very low cost range. We will describe detail of the scanner and related optics.
In the final chapter, we show the evaluation results. Tolerance angle error of prototype is over 1.5’ (0.4 mrad) in each angles, and tolerance gap error is over 0.2mm. Total drift in one month is within 10nm. The above prototype uses relatively long grating pitch 4 micron meter. It realizes great robustness and easy installation of sub-nano meter sensor in industry. |
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