Nanometric Straightness Measurement of Machine Table Using Laser Beam Straight Datum

对激光束作为直线基准的实用性和应用进行了可行性研究。利用四象限光电二极管（QPD）和压电补偿器，设计了一种专门用于激光束能量中心的二维位置传感器/补偿器。位置传感器/补偿器的分辨率估计约为3nm，截止频率约为200hz。将激光束作为直线基准线与QPD位置传感器/补偿器相结合，在3 nm的精度范围内测量了静压气滑台和油滑台上某一点直线运动的二维误差。滑台直线误差运动大于50nm /100mm。从理论上讲，通过同时测量滑台上三个不同点的误差运动，就可以对滑台的直线运动进行完整的分析。这需要三个平行激光束作为一组基准，这些基准在要求的精度范围内对齐。用一组平行激光束进行的实验部分证实了该方法的有效性。影响该方法可靠性的最重要因素是光束路径的位置稳定性和角稳定性。在防止激光管热稳定性和光束路径气流影响的前提下，在200 mm的工作距离上，用窄氦氖激光束作为精度在3nm以内的直线度计量基准线，持续约1分钟。为了扩大所提出的方法的有效性，必须发展高稳定的激光束。

A feasibility study is made for the utility and the application of laser beam as a straight datum. By using a quadrant photodiode (QPD) and a piezo-compensator, the specially designed two-dimensional position sensor/compensator for the energy center of the laser beam is developed. The resolution of the position sensor/compensator is estimated to be about 3 nm with the cut-off frequency of about 200 Hz.Combining the laser beam as straight datum line with QPD position sensor/compensator, the two-dimensional error in straight motion of a certain point on hydrostatic air and oil slide tables are measured within the accuracy of 3 nm. The slide tables have straight error motion more than 50 nm /100mm. However, the repeatability of the error motion have been confirmed to be less than a few nm.Theoretically, complete analysis of straight motion of a slide table can be executed by measuring the error motion of three different points on the table simultaneously. This call for three parallel laser beams as a set of the datum which are aligned within a required accuracy. The usefulness of the method has been partly confirmed by the experiment using a set of two parallel laser beam.The most important factor affecting the reliability of the proposed method is the positional and angular stability of the laser beam path. Under the precautions against the thermal stability of the laser tube and the air flow in the beam path, a narrow He-Ne laser beam can be used as the metrological datum line of straightness within the accuracy of 3 nm over the working distance of 200 mm for about 1 minute. To extend the usefulness of the method proposed, the development of highly stable laser beam is essentially required.