Guoyong Ye

Design of a precise and robust linearized converter for optical encoders using a ratiometric technique

A linearization method with improved robustness for determining the displacement from sine and cosine signals generated by optical encoders is presented. The proposed scheme is based on a ratiometric technique and a dedicated compensation method. The scheme converts the sinusoidal signals into a nearly perfectly linear output signal, from which the displacement is determined precisely using a simple linear equation. Under the condition of ideal input signals, the theoretical analysis shows that the converter enables a determination of the displacement with a non-linearity error below 0.0029 µm for a linear optical encoder with a period of 20 µm. The performance of the converter with non-ideal input signals is also evaluated by establishing the relationship between the positioning errors and the parameter deviations of the input signals. Due to the robustness of the converter against the signal amplitude imbalance, a signal processing circuit is developed to convert the signal phase-shift error into the signal amplitude imbalance error. A displacement measurement experiment was carried out by applying the converter to a linear optical encoder with a period of 20 µm. A positioning accuracy of 0.2 µm over a travel length of 80 mm was achieved under laboratory conditions. The feasibility of the proposed converter has been confirmed from the experimental results.

Design and development of an optical encoder with sub-micron accuracy using a multiple-tracks analyser grating

In this paper, an optimized optical encoder based on generalized grating imaging is presented. A multiple-tracks analyser grating is proposed to eliminate the second and third harmonic signals, and a photodiodes array with optimized cell width is used to suppress the fifth harmonic signal. The photodiodes array also guarantees the consistency and stability of the encoder signals benefiting from single-field photoelectric scanning. High-quality encoder signals are expected to be obtained from the above optimization, thereby ensuring high encoder accuracy. In the experiment, measured encoder signals with the approximately ideal Lissajous figure are obtained. FFT analysis of the encoder signals shows that the second, third, and fifth harmonic distortions are smaller than 0.3%, 0.5%, and 0.1%, respectively. The calibration results of the optical encoder show that the positioning error within one signal period is ±0.12 μm, and the positioning error over 150 mm measuring range is within ±0.2 μm.

Optimizing design of an optical encoder based on generalized grating imaging

The interpolation error of an optical encoder is largely determined by the imperfections in the electrical signals, such as amplitude error, phase-shift error, DC error and higher harmonic distortion. In this paper, an optimized optical encoder based on generalized grating imaging is presented. We first describe the basic principle and configuration of the optical encoder. To achieve four electrical signals with equal amplitudes, equal DC contents and accurate 90° electrically phase-shift, a photodiodes array is adopted as the photo-detector. The photodiodes array offers the advantage of single-field scanning. Then, to solve the remaining problem of higher harmonic distortion, a slit-width-modulated index grating is proposed to suppress the dominant third order and fifth order harmonics simultaneously, and the cells width of the photodiodes array is optimized to further reduce the fifth order harmonic. Experiments have been carried out to verify the theoretical results. The feasibility and effectiveness of the proposed optimizing methods have been confirmed from the experimental results.

Multiple harmonics suppression for optical encoders based on generalized grating imaging

Abstract In this work, we perform an investigation on the multiple harmonics suppression for optical encoders based on generalized grating imaging. We firstly analyse the formation of the harmonic distortion of the encoder signals and evaluate the interpolation errors caused by the higher harmonic signals. The result shows that the harmonic distortion of the encoder signals depends mainly on the higher harmonic components in the interference fringe. In addition, it shows that the higher harmonic signals influence the Lissajous figure of the encoder signals significantly and introduce a relatively large interpolation error. Then, conditions for eliminating the higher harmonic signals are studied, and optical filtering methods using specially designed index grating are proposed. We explain the filtering principle in detail and present three forms of index grating for multiple harmonics suppression. In particular, we show the patterns of the index grating for eliminating the dominant third and fifth harmonics, corroborating the results with numerical simulations. The results show that the amplitudes of the third and fifth harmonics are equal to zero theoretically, such that approximately ideal sinusoidal signals are obtained. Since the harmonic signals are considerably suppressed, the proposed methods will be useful for high-accuracy interpolation of encoder signals.

Calibration of non-contact incremental linear encoders using a macro–micro dual-drive high-precision comparator

The accuracy of a linear encoder is determined by encoder-specific errors, which consist of both long-range and cyclic errors. Generally, it is difficult to measure the two errors of a non-contact incremental linear encoder with a large measuring range and small signal period in one measurement because of the contradiction between long travel range and high resolution. To resolve this issue, a prototype high-precision interferometric comparator with a macro–micro dual-drive system is presented. The measurement and motion resolution of the comparator are 1 nm and 3 nm, respectively. A measuring range of 320 mm is realized and the theoretical maximum range of the comparator is 2 m. The comparator mainly includes a high-accuracy aerostatic linear-motion stage, a constant displacement ratio piezoelectric-driven stage, two laser interferometers, a 6-DOF grating pair position adjustment devices and a PC-based data processor. The measurable linear movement is afforded, respectively, by the long-stroke stage and the piezoelectric-driven stage for the long-range error and cyclic error measurement. The movement can be measured by the encoder and then be calibrated by the corresponding laser interferometer. In the experiment, the accuracy of a non-contact incremental linear encoder with a 20 μm-long signal period and 320 mm measuring range proposed by our team was calibrated after proper mounting. The long-range error is measured to be 3.123 μm, and the cyclic error is within ±0.159 μm, which matches well with the theoretical estimation given by ±0.145 μm. The measurement uncertainties are estimated and the results confirm the effectiveness and feasibility of the proposed scheme and instruments.

Transverse sensitivity suppression using multi-axis surface encoder with parasitic error compensation

Transverse sensitivity that is mainly resulted from parasitic error motions can introduce undesired motion components and remarkably lower the manipulation qualities of most inertial sensors. This problem becomes even more apparent for multi-axial sensors as additional demands for multi-degree-of-freedom detection become higher. In this letter, a method to minify the transverse sensitivity of an inertial sensor by multi-degree-of-freedom optical sensing and measurement has been reported and tested. A multi-axis-surface-encoder-based biaxial optical accelerometer is fabricated for scheme validation. The surface encoder adopts multi-reading-unit arrangement, and it can not only detect small changes in displacement to calculate the applied acceleration along X- and Y-axes but also quantify the parasitic error motion caused by Z-twist. A suitable compensation strategy is also developed to reveal the concerned outputs without parasitic errors. Experimental results show that the configuration combined with the ...

Fabrication of high edge-definition steel-tape gratings for optical encoders

High edge definition of a scale grating is the basic prerequisite for high measurement accuracy of optical encoders. This paper presents a novel fabrication method of steel tape gratings using graphene oxide nanoparticles as anti-reflective grating strips. Roll-to-roll nanoimprint lithography is adopted to manufacture the steel tape with hydrophobic and hydrophilic pattern arrays. Self-assembly technology is employed to obtain anti-reflective grating strips by depositing the graphene oxide nanoparticles on hydrophobic regions. A thin SiO2 coating is deposited on the grating to protect the grating strips. Experimental results confirm that the proposed fabrication process enables a higher edge definition in making steel-tape gratings, and the new steel tape gratings offer better performance than conventional gratings.

A novel optical rotary encoder with eccentricity self-detection ability

Eccentricity error is the main error source of optical rotary encoders. Real-time detection and compensation of the eccentricity error is an effective way of improving the accuracy of rotary optical encoders. In this paper, a novel rotary optical encoder is presented to realize eccentricity self-detection. The proposed encoder adopts a spider-web-patterned scale grating as a measuring standard which is scanned by a dual-head scanning unit. Two scanning heads of the dual-head scanning unit, which are arranged orthogonally, have the function of scanning the periodic pattern of the scale grating along the angular and radial directions, respectively. By this means, synchronous measurement of angular and radial displacements of the scale grating is realized. This paper gives the details of the operating principle of the rotary optical encoder, developing and testing work of a prototype. The eccentricity self-detection result agrees well with the result measured by an optical microscope. The experimental result preliminarily proves the feasibility and effectiveness of the proposed optical encoder.