Straightness measurement of a long guide way
A comparison of dual-beam laser technique and optical
collimator

Gianmarco Liotto and Charles Wang
Optodyne, Inc.,
Compton, California, USA

Reported here is a technique to determine the guide way straightness by measuring the slopes at each point along the travel using a dual-beam Laser Doppler Displacement Meter (LDDM). The straightness is obtained by integrating the local slopes at each measuring point over the total travel. The slope or the angular change of the dual-retroreflector is equal to the difference of the two linear displacement changes divided by the separation of the 2 retroreflectors. Since both displacement and angle can be measured simultaneously, the straightness can be measured in one continuous stroke without stopping. Hence it is more accurate and also saves time. The theoretical basis of the technique, the accuracy of the measurement, and a comparison with other techniques will be discussed.

To verify the performance, data were taken on a long guide way by both dual-beam laser system and by a wire collimator. Repeated measurements were performed for a length of 15 m, 10m, and 5 m. The test setup, data collection and analysis, and test results will be reported.

I.      Introduction
For large machines with long guide ways, it is important to align the guide ways both in the vertical direction and horizontal direction. Conventional methods are: using a piano wire, using an autocollimator, using electronic level, using a laser beam and a quadratic detector and using a laser interferometer and Wollaston prism. For the piano wire method, the resolution is low and time consuming. For the electronic level method, the accuracy is not enough and also in the vertical direction only. For the autocollimator method, the resolution and accuracy is relatively low. For the laser interferometer and Wollaston prism method, the alignment and setup are very difficult due to the complex optics. For the laser beam and quadratic detector method, because of the air circulation or turbulence the accuracy is rather limited for long guide ways.

Described here is a technique to determine the straightness by measuring the slopes at each point along the travel using a dual-beam Laser Doppler Displacement Meter (LDDM)[1]. The straightness is obtained by integrating the local slopes at each measuring point over the total travel [2]. The theoretical basis of the technique, the accuracy of the measurement, and a comparison with other techniques, is discussed.

II.      Straightness definition and measurement methods
A straightness error is defined as the deviation perpendicular to the direction of travel as shown in Fig. 1. Assuming the ideal travel path is a straight line, any deviation from the straight line in the horizontal plane is called horizontal straightness and in the vertical plane is called vertical straightness.

The laser interferometer and Wollaston prism method is based on the optical path difference between two laser beams at a divergent angle and the target. The optical path difference is proportional to the vertical displacement of the target of the straightness. Hence it is a measure of the straightness in the direction perpendicular to the travel in the plane of the two beams. This method requires very complex and expensive optics, the alignment and operation are very difficult and time consuming.