Research On Key Technologies Of Absolute Distance Interference Measurements By Frequency-sweeping

With the development of the aerospace, shipbuilding industry, large equipment manufacturing industry and other industries, the parts size of large machinery and equipment reach hundreds of millimeters to several meters. Positioning precision for part assembly reaches tens of micron. In assembly application, laser tracker is the most common equipment used to measure. Absolute distance measurement technology is a key technology in laser tracker. In this paper, both advantages and disadvantages of different types of absolute distance measurement methods are discussed. And research the absolute distance measurement technology by frequency sweeping interferometry, whose theory and technology details is analyzed. The main contents are as follows:1) An absolute distance measurement system by frequency sweeping interferometry is designed. A reference interferometer is used for measurement basis to avoid the problem, that the range of laser frequency in frequency sweeping interferometry is difficult to measure directly. The main factors influencing measure precision are analyzed in this paper. A temperature control system and an interference signal processing unit are designed base on the analysis. And propose a distance decoding method basing on interference signal phase least-square fitting and build an experiment platform to verify the system performance.2) Design an interference signal processing unit. The asynchronous measurement cause a measure error, so the interference signal of reference and measure interferometer need to be collected, processed and output synchronously. Achieve a 20 MHz high-speed synchronous collection and high resolution solution for interference phase signal.3) Propose a Distance decoding method based on interference signal phase leastsquare fitting. The measure precision based on the distance computational formula that choose phase change in two moments to calculate measure distance is low and the data availability is poor. According to the phase linearity between the reference interferometer and measure interferometer, a phase solution based on interference signal phase leastsquare fitting is proposed. This phase solution is effective to decrease the influence of phase measure error.4) Design a temperature control system for reference interferometer to restrain the influence of Material thermal expansion, which is related to the stability of reference length. All parts of reference interferometer is fixed on metal baseplate, so the thermal expansion influenced by temperature will cause a degradation for stability of reference length. Then design a temperature control system based on MSP430 and thermoelectric cooler(TEC).Build an absolute distance interference measure platform based on frequency sweeping. We testify the key technology unit and some system characteristic parameters and compare the result with UOI-S500 homodyne interferometer. The results show that: within the range from 200 mm to 1000 mm, the measurement repeatability Up(k=3) of measure system is less than 50μm.