Laser auto-collimation technology is an important method of precision micro-angle measurement. It has the advantages of high measurement accuracy and simple optical structure. However, the position-sensitive detector (PSD) has nonlinearity, and the uniformity of laser spot is affected by the working distance of the auto-collimation system, which seriously affects the accuracy of angle measurement based on laser auto-collimation technology. To improve the accuracy of laser auto-collimation technology in long-distance angle measurement, PSD is calibrated by linear interpolation to solve the error caused by its nonlinearity. In addition, due to the non-uniformity of the light spot, the PSD zero point is not collinear at different distances, and the measurement error can be corrected by a laser interferometer. Experimental results showed that the interpolation calibration method effectively improved the angle measurement accuracy of PSD. After the zero deviation was corrected, when the PSD moved within 5m, the maximum standard deviation (SD) of the pitch error was 0.12 ", and the SD of the yaw error was 0.09 ". The method proposed in this paper can achieve high precision angle measurement based on PSD at long working distances.
Rotor-stator axial gap is a key design parameter that directly affects the efficiency and safety of large rotating machines. With optimum axial clearance value determined, the rotating machine can work with the highest efficiency. To realize active clearance control (ACC) of rotating machinery, high-precision non-contact online measurement of the axial gap must be carried out. However, traditional measurement methods are challenged by the extreme working environment, such as extreme temperature, high rotating speed, and narrow space conditions; no mature measurement method was reported to realize the high accuracy, non-contact, and online performance required by the axial gap measurement. In this paper, a rotor-stator axial gap measurement method is proposed based on frequency scanning interferometry, and the mathematical model of axial clearance measurement is established. The weak reflection signal of lens and time delay estimation algorithm is employed to undermine the impact of probe pigtail length drift caused by the temperature change under extreme conditions. The axial gap measurement prototype based on sweep frequency ranging was developed, and the measurement and system calibration was carried out. The experimental results verify the effectiveness and accuracy of the method. The drift of the system was less than 20μm in 30 minutes with millimeter level pigtail drift, and the measurement accuracy was better than 50μm.
Multi-degree-of-freedom measurement (MDFM) system is an effective way to measure multiple degrees of freedom errors simultaneously. In the MDFM system, the pitch angle and yaw angle are often measured based on the autocollimation measurement principle. Due to the lens fabrication and installation errors, the angle detector will deviate from the focal plane, causing defocus error. To address the problem, a defocus error model is established to analyze the influencing factors and solve the position of the focal plane. It is found that the straightness is the main factor of defocus error. When the defocus amount is present, the defocus error is larger with the increase of straightness. And the sensitivity of angle detector has a specific relationship with the intensity density of spot, which is related to the defocus amount. By using a precision displacement table, the position of the focal plane and the corresponding angle detector output can be accurately obtained. The experiment was carried out to verify the feasibility of this method. The angle detector and lens were installed precisely according to the angle detector output. When the vertical straightness is within ±400 μm, the influence of the defocus error on the pitch angle is less than 2 μrad. The results showed that the method can reduce the defocus error of auto-collimation measurement effectively, which can improve the accuracy of angle measurement in MDFM system.
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