Absolute measurement has always been one of the important development directions of precision measurement, there are problems that the diffraction and mask working parameters are not considered in the positioning pulse analysis of the absolute code mask at present. Therefore, in order to solve the coupling optimal performance problem of absolute positioning code in actual work, an absolute code should be designed for working in the best parameter-model. In this paper, a multi-parameter model of absolute code working status is established, and the influence of working parameters on its positioning performance is analyzed respectively. The analysis shows that the distance and the angle between the mask and the grating, and the width of the unit code will affect the positioning accuracy. The three parameters restrict each other, and there is a coupling optimal solution. The optimal working state can be obtained through parameter analysis, so as to provide the design and installation parameter guidance of mask. The proposed research can help the practical application of absolute positioning measurement.
Absolute testing for metrology has always been one of the important development directions of precision measurement. The mask with binary code is an important structure for forming absolute positioning pulses in grating encoders. The increase in the number of codes is beneficial to the resolution of positioning, but design of codes has always existed the problem that the optimal design cannot be obtained when the number of codes increases. This paper proposes a design method of binary code based on the genetic algorithm, which can get the required binary code more quickly when the number of codes is greater than 150 or even higher. The specific method can randomly generate binary codes with their fitness factors, and the binary codes enter the algorithm as the parents based on the mutation, crossover, and selection. Then the reproduce binary codes will have higher and higher fitness factor. This method can quickly generate satisfactory binary codes with specified performance, thus providing high resolution at the nanometer level for absolute positioning measurement. This work provides help and reference for future absolute positioning measurements.
For expanding the measurement range and improvement of accuracy of multi-axes grating encoder, a mathematical model of measurement angle and diffraction spot with QPD was established. We proposed a light spot position calculation method with consideration of both the optimized composite algorithm of laser beam feature of Gaussian distribution and the QPD diagonal algorithm. In this method, we use the piecewise polynomial fitting method to fit and solved the parameters of the traditional Infinite integral algorithm and the Boltzmann function fitting algorithm. Meanwhile, we introduce a weight factor and use the Composite algorithm to compensate the spot position error. Based on the given QPD model and the basic parameters of the laser beam, simulation works are carried out and results show that the maximum error of the spot position can reduce to be an order of 10-6 mm within the 2 mm measurement range using piecewise cubic polynomial fitting, around 10% of the traditional methods.
For solving the problem of sub-mirror installation and posture monitoring and compensation, an absolute four-degree-of-freedom (DOF) grating encoder that is able to monitor four degrees of freedom's absolute position and pose in the θx, θy, θz, and z-direction is proposed. In this grating encoder, a grating reflector and three quadrant photodetectors (QPD) are employed and an optical path is configured based on the laser autocollimation principle. A model for the solution of the four-DOF motions from outputs of the three QPDs is established. A calibration method for the identification of the relationships between the absolute positions and QPDs outputs is proposed. A prototype four-DOF grating encoder is constructed for verification of this proposal. Test results demonstrated that the method proposed in this research can achieve absolute position distinguishing with a sub-arcsecond and sub-micrometer accuracy in rotation angles and z-direction, respectively.
Heterodyne grating interferometer is widely used in precision positioning due to its high precision and robustness. However, the polarization states of two frequency components in a dual-frequency laser are easy to overlap with each other because of the non-ideality of optical components. It will cause nonlinear error, which limits the measurement precision of the grating interferometer. To improve the frequency aliasing of heterodyne grating interferometer, a polarization adjustment module is proposed to adjust the polarization angle of the dual-frequency laser. The dual frequency system outputs two laser beams with different frequencies separately. The module realizes the polarization adjustment of the two frequency components through two groups of the polarizers and the half-wave plates (HWP). Finally, the polarization directions of two frequency components are orthogonal and combined by the beam splitter (BS). Thus, the nonlinear error caused by frequency aliasing is removed. The polarization adjustment module has the advantages of not changing the direction of the laser propagation and simple structure, which makes it easy to realize integration. It can provide a reference for the solution of frequency aliasing of heterodyne grating interferometer.
Laser interferometers and grating interferometers are typical optical measurement devices. The measurement resolution and range of the two devices are generally nanometers and meters, so they can meet the needs of high-resolution, largerange measurement. Whether it is a laser interferometer or a grating interferometer, it can be implemented based on a technical route based on the principle of homodyne interference or heterodyne interference. Heterodyne interference is not sensitive to changes in signal amplitude and DC offset, and can effectively avoid measurement errors. To design a highprecision displacement measuring device based on the principle of heterodyne interferometry, the key is whether it can accurately measure the phase change of the measured signal relative to the reference signal. The accuracy of the phase measurement determines the accuracy of the displacement measurement. Phase measurement methods can be divided into two categories: analog method and digital method. In this paper, a high-precision phase measurement system based on FPGA is designed based on the automatic digital phase detection method. The hardware part of the system includes FPGA, high-speed ADC module, signal conditioning circuit, the phase detection algorithm selects the automatic phase detection algorithm, and finally realizes the output display of the phase measurement results. The experimental results show that the deviation between the experimental data of all measurement points and the true value does not exceed 0.1°. Therefore, the accuracy of the phase measurement system designed in this paper is 0.2° and 0.018° resolution.
KEYWORDS: Field programmable gate arrays, Computer programming, Signal processing, Data conversion, Optical filters, Filtering (signal processing), Digital signal processing, Interfaces, Diffraction gratings, Logic
Compared with the four-phase optical structure, the grating encoder based on two-phase optical structure reduces the number of optical devices used in the system and makes the system more compact. Due to the high requirements for realtime and parallel processing of algorithm solution, the powerful parallel computing ability of Field Programmable Gate Array (FPGA) and customized hardware acceleration algorithm are needed to improve the real-time performance. In the previous research, the displacement signal generated by the grating encoder can be input into the FPGA through analog to digital converter (ADC) sampling, and then complete self-designed filter filtering, phase correction and displacement solution. In this paper, further, the ADC sampling rate adjustable interface is added to the FPGA, the global signal and the dc offset remove algorithm is added, and the displacement solution results in the form of fixed-point number are output to the host computer through the MicroBlaze (MB) soft core. MB core can realize process control and interface conversion on FPGA, and use a small amount of logic resources to replace the functions of MCU and DSP of traditional embedded measurement system, so as to further improve the integration of the instrument. A series of experiments are carried out on the two-phase FPGA platform. ADC sampling rate is 200ksps, 8-Channel synchronous parallel sampling, FPGA system clock frequency is 200MHz. The linear displacement table is set to drive the measurement grating at different displacement speeds, and the total stroke is set to 10mm. The FPGA real-time displacement solution platform is tested. The experimental results show that FPGA obtains accurate displacement solution results under different speed tests. In the test of 2 mm/s, the maximum cumulative displacement measurement error is 5um, which shows the real-time performance and accurate displacement solution performance of FPGA platform.
Grating interferometer has great application potential in precision measurement. In view of its miniaturization, a compact installation based on ridge prism is adopted in this paper, and the interference signal with 90° phase is obtained by numerical calculation. The specific scheme is to divide the laser beam through the splitting prism to make the two beams diffract at the reference two-dimensional grating and the measurement two-dimensional grating respectively. The four beams of diffracted light are collimated through the ridge prism. The collimated two groups of beams produce interference at the splitting prism, and the interference signal is received by the photodetector. Differing from the conventional method, the second group of 90° phase interference light data is obtained by derivation. Through the solution of the interference signal, the displacement in X and Z directions of the measurement grating can be deduced. Experiments were carried out to verify the measurement accuracy of the grating interferometer. The moving speed of scale grating is 1μm/s ,distance is 0.5 mm and the sampling frequency is 100 Hz. Repeatability and measurement accuracy of the grating interferometer are obtained. The repeatability measurement error is less than 0.5%. Specially, this method benefits for simple installation, high precision, high stability and low energy loss. It has high practical value in industrial production. Precision measurement of two degrees of freedom of short-distance displacement is realized and error of the measurement results is analyzed.
KEYWORDS: Field programmable gate arrays, Signal processing, Computer programming, Digital signal processing, Optical filters, Data processing, Filtering (signal processing), Sensors, Phase measurement, Optical design
Six-degree-of-freedom (6-DOF) grating encoders have a wide prospect of application. Aiming at the requirement of real-time detection of 6-DOF grating encoders, this study designs and builds a real-time calculation system platform based on field-programmable gate array (FPGA). We realized a real-time parallel calculation of 16-path displacement signal and 24-path angular displacement signal, respectively. Specifically, the optical interference signals, generated by the translation and rotation of the motion stages, are firstly shaped by the front-end analog circuit. We further sampled the front-end analog circuit into an FPGA through a analog-to-digital convertor (ADC) for the realization of the digital filtering, amplitude normalization, phase correction, and phase-information calculation. Thus, the calculated signals on the 6-DOF motions can be displayed in real time. The established system was evaluated with the experimental parameters in terms of the translation with a 50 μm/s moving speed and an 18 mm stroke and the rotation with a frequency of 0.5 Hz, a step length of 100 micro-rad, and within a reciprocating rotation of 24 s. Finally, a linear-displacement error of <1 nm and an angle displacement error of <0.9 micro-rad were achieved, respectively. Furthermore, the system delay of <15 ms is obtained, exhibiting a high performance for the real-time measurement and high integration in the practical application.
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