In large-scale metrology, when multiple instruments are used, the measuring field space can be large and the targets can be scattered. However, fixed-pointing distance measurement can limit flexibility. Therefore, there is an urgent need to improve automation and efficiency. This paper proposes a beam-guided laser detection module for absolute ranging that combines camera recognition and position-sensitive detection. The module uses a dual-axis fast steering mirror (FSM) driven by a compact field-programmable gate array (FPGA) to change the beam direction. The angular range for beam guiding is larger than ±20°. The mechanical structure and software are detailed introduced in the paper. This module can be directly combined with an absolute ranging system, such as frequency scanning interferometry or dual-comb ranging. It is expected that these systems will find broader applications in the industrial and geodetic survey fields.
KEYWORDS: Repetition frequency, Ranging, Distance measurement, Switching, Interferometers, Signal processing, Metrology, Frequency combs, Signal detection, Scientific research
Absolute distance measurement based on the dual-comb system is a promising tool for the large-scale measurement in the industrial field. There are two combs with slightly different repetition rates in the dual-comb system, and the distance information could be obtained by the asynchronous optical sampling (ASOPS). However, the measured range could be limited within the pulse-to-pulse interval range by the non-ambiguity range (NAR) without the extension. This paper proposes a method to extend the unambiguous range by changing the repetition range. Firstly, the method of dual-comb ranging and the principle of extending the non-ambiguity range were introduced. Secondly, the two implementations of changing single repetition frequency and switching repetition frequency were compared through simulation. Then, according to the simulated results, the effective conditions and theoretical maximum range of extended ranging were clarified. Finally, a long absolute distance measurement was designed and the ranging results of switching repetition frequency were compared with the conventional incremental interferometer, which proves that this method is reliable for extending the non-ambiguous range.
A multilateration system is a 3-dimensional coordinate measurement system, through measuring multiple distances between a target point and laser interferometry measuring instruments, with high precision and large measuring scare, which consists of three laser trackers or tracers at least. A quantifiable analysis method is proposed, for the effect of stations’ mutual position relationship on the measurement accuracy of the multilateration system. The method is based on a mathematical model which linearizes the relationship between the coordinate measurement error of the system and the distance measurement error of laser trackers. Some methods, optimizing the mutual position relationship of stations, are derived from the model, which can improve the measurement accuracy. The rationality of the methods is verified by simulations and experiments. The results show that the max error of the multilateration system improved is only 0.020mm, which is better than a laser tracker. The multilateration system, with the optimized layout of stations, has higher precision.
High-precision and long-range absolute distance measurement is a vitally important topic in large-scale metrology, and it has broadened applications in the manufacturing industry. Especially, the dual-comb time-of-flight distance measurement is a promising method combining with the advantages of fast speed and high accuracy. The measurand would be determined by asynchronous optical sampling (ASOPS) methods with a slight difference in repetition rates. Here, the home-built optical platform of the Er-fiber femtosecond frequency combs would be introduced with a repetition rate of around 200 MHz and carrier-envelope offset frequency of 20 MHz. Besides, the strategies of precision enhancement for dual-comb time-of-flight distance measurement with nonlinear intensity detection are discussed by the numerical simulation. The sampling interval could be optimized by choosing a proper range of repetition rate and difference of repetition rates. A fine curve fitting method is also proposed for further precision improvement. The results of numerical simulation would provide a valuable reference for the experimental process.
Field radiometric calibration is an important technical means to ensure the observation accuracy of satellite remote sensors on-orbit operation. Accurately obtaining the spectral radiometric characteristics of the site is of great significance to improve the accuracy of field radiometric calibration. In order to better meet the field calibration requirements of thermal infrared band satellite remote sensors, a Multi-channel Self-calibration Infrared Radiation Thermometer (MSIRT) with automatic observation capabilities has been developed, which is used to measure the atmospheric downward radiance and ground surface radiance in field. In this paper, the structure, working principle and laboratory calibration of the MSIRT was introduced. In optical design, the automatic switching of four spectral channels in the range of 8 ~ 14μm was realized by using the filter wheel, and the instrument was equipped with an ambient temperature blackbody and a heated blackbody, respectively. The real-time calibration of the detector can be realized through the alternate measurement of high and low temperature blackbody, which ensures the high precision measurement requirements. The developed instrument also has the ability of automatic observation and remote data transmission. In the laboratory calibration of MSIRT, the liquid bath blackbody was used as the standard radiation source, and the calibration uncertainty was 0.5 %, corresponding uncertainty of the equivalent radiance temperature was 0.3 K (@ 303 K, 11 μm). The experimental results show that the MSIRT can meet the measurement requirements of high precision, and has important applications in the field calibration of thermal infrared satellite remote sensor.
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