Haoyu Yu

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.

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-resolution reflective scale grating for an optical encoder using a patterned self-assembly process

Steel tape scale grating of a reflective incremental linear encoder has a key impact on the measurement accuracy of the optical encoder. However, it is difficult for conventional manufacturing processes to fabricate scale grating with high-resolution grating strips, due to process and material problems. In this paper, self-assembly technology was employed to fabricate high-resolution steel tape scale grating for a reflective incremental linear encoder. Graphene oxide nanoparticles were adopted to form anti-reflective grating strips of steel tape scale grating. They were deposited in the tape, which had a hydrophobic and hydrophilic grating pattern when the dispersion of the nanoparticles evaporated. A standard lift-off process was employed to fabricate the hydrophobic grating strips on the steel tape. Simultaneously, the steel tape itself presents a hydrophilic property. The hydrophobic and hydrophilic grating pattern was thus obtained. In this study, octafluorocyclobutane was used to prepare the hydrophobic grating strips, due to its hydrophobic property. High-resolution graphene oxide steel tape scale grating with a pitch of 20 μm was obtained through the self-assembly process. The photoelectric signals of the optical encoder containing the graphene oxide scale grating and conventional scale grating were tested under the same conditions. Comparison test results showed that the graphene oxide scale grating has a better performance in its amplitude and harmonic components than that of the conventional steel tape scale. A comparison experiment of position errors was also conducted, demonstrating an improvement in the positioning error of the graphene oxide scale grating. The comparison results demonstrated the applicability of the proposed self-assembly process to fabricate high-resolution graphene oxide scale grating for a reflective incremental linear encoder.