Elsevier

Precision Engineering

Volume 35, Issue 2, April 2011, Pages 302-308
Precision Engineering

An evaluation of a modulated laser encoder

https://doi.org/10.1016/j.precisioneng.2010.11.008Get rights and content

Abstract

This paper introduces a new method of interpolation for sub-nanometer-resolution linear encoders. This method, called SPPE (scanning position probe encoder), uses high-order harmonics information obtained by a sinusoidal scanning pickup located on a periodic grating surface. The proposed encoder uses a current-modulated laser diode with diffractive grating optics. Since the electrical current changes the laser-diode wavelength, the interference light intensity is modulated as a sinusoidal scanning pickup on the scale grating. Phase-detection circuits can decode the position information in the pickup signal by using phase-locked loop techniques. The decoder achieves an interpolation rate of over 1/40,000 with interpolation errors of less than ±1 nm. A new interpolation-error measuring system was developed for the encoder. Finally, the evaluation results reveal that the presented encoder shows both high resolution and strong robustness.

Introduction

Demands for sub-nanometer-resolution linear encoders are constantly increasing in the precision industry field. Precision encoders usually employ laser-interference optical principles with fine pitch gratings; therefore, they are often called grating interferometers. There are two major interpolation methods for the sensors: homodyne [1], [2], [3] and heterodyne [4], [5]. Fig. 1 shows diagrams of the two systems. Both of the two methods have advantages and disadvantages. Homodyne systems are simple and inexpensive for many applications. However, their accuracy can suffer from imbalance in their two-phase signals. Homodyne systems constantly require fine tuning procedures for amplifier gain and offset to keep sufficient interpolation accuracy. While heterodyne methods have remarkable advantages in signal robustness, their maximum measurable velocities are limited by their heterodyne frequencies. Furthermore, the most serious difficulty of heterodyne systems is the cost of light sources such as Zeeman laser tubes or EOM (electric optical modulator) devices, which are several times more expensive than average industrial encoders.

In the late 1990s, Ohara introduced a third method of interpolation. This technique, the scanning probe position encoder (SPPE) [6], [7], [8], [9], uses an oscillated pickup probe with a sinusoidal grating, shown in Fig. 2. The pickup signal becomes periodic with high-order harmonics. The information of scale displacement is transferred into the phase of the harmonics, which can be decoded by a phase detection circuit using a phase-locked loop (PLL) technique. This SPPE technology can overcome the velocity limitation of the frequency shift by using multiple harmonic frequencies. Since the pickup signal is single and analog, imbalance between two phases is not an issue. The original SPPE sensor used a laser spot scanner similar to a CD/DVD pickup. From the industrial point of view, a large diameter beam is desirable because of the robustness against the grating surface defects. This study provides new concept interference optics using a current-modulated laser diode that can make an SPPE signal with large-diameter laser beams. The combination of the presented interference optics and the SPPE decoding technology can simultaneously realize both better resolution and increased system robustness.

Section snippets

Optical concept

Fig. 3 shows the concept for the modulated interference optics of the SPPE. A sinusoidal-modulated current drives the laser diode. The laser wavelength changes with the current due to thermal effects in the chip [10]. The laser beam from the diode is collimated by a collimator lens, and then divided into two portions, E1 and E2. E1 goes to the fixed grating directly, and E2 passes through a rectangular-solid-shaped glass piece before coming into the fixed grating. The glass has a larger

Prototype design

Fig. 4 shows the configuration of the encoder optics for our prototype model. We combined the two modulated encoder optics into one to cancel the laser wavelength drifts. The laser diode wavelength is susceptible to drift because of atmospheric temperature fluctuation. In our current modulation concept, shown in Fig. 3, the center of the modulated wavelength drifts from the temperature changes. If the center of modulated wavelength is changed, the encoder position output will also change.

In

Stability

Fig. 10 shows the position output noise and the stability of our prototype. Fig. 10(a) is a ten-second-long time-trace with 10 kHz sampling. The standard deviation (STD) of the whole data set is 14.3 pm. Fig. 10(b) is the Allan standard deviation, which shows the signal deviation versus averaging time. The Allan standard deviation (or Allan variance) is widely used for stability evaluations of gyroscopes and frequency oscillators. A low-frequency peak in the graph causes environmental

Conclusion

This paper introduces a modulated laser encoder with SPPE interpolation logic. The simple-configuration optical head with a current-modulated laser diode provides a high-order harmonics signal with a grating scale. The SPPE decoder achieved a 1/40,000 interpolation rate with the signal. Our prototype encoder's position output stability was 14.3 pm (STD) with a 4-μm-pitch grating scale. The interpolation error measured by a new concept using a tuning fork system was within ±1 nm across the wide

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