A novel method for laser beam straightness error measurements based on common-path compensation is proposed. The basic principle of the method is described and the math-model is established. Furthermore, the influence of laser beam drift on straightness error measurements is analyzed. The experimental results show that the measurement accuracy can be greatly improved by using the common-path compensation for laser beam drift.Laser beam is normally used to measure the geometrical errors of machine tools [1] . However, the measurement accuracy is not high enough because of the laser beam drift that includes parallel drift and angular drift [2][3][4][5] . A straightness error measurement system with common-path compensation for angular drift of beam is proposed. In this system, the influence of the beam angular drift can be decreased. Usually, the angular drift of laser beam is caused by the inner temperature change in the laser or the mechanical rotation of measurement head [6] .The principle of measuring straightness errors is shown in Fig.1. These two beams are of common-path and the angular drift can be monitored on real-time scale. Straightness errors in two directions can be compensated by certain arithmetical methods, and in this way, the accuracy of measuring straightness errors can be improved. Fig.1 The measurement principle of laser beam straightness errorsThe influence of laser beam angular drift produced by the inner temperature change in the laser on straightness errors is shown in Fig.2. When the R-R moves along the linear stage, the straightness errors in both Y and X directions can be expressed as:where y and x are straightness errors in Y and X directions respectively, Y and X are the position changes of laser spot on the QD in Y and X directions respectively, l is the distance between R-R and the laser, l is the distance between R-R and the QD, and are the beam angular drift in Y and X directions respectively, and they can be obtained by
In this paper, a newly developed system for simultaneously measuring five-degree-of-freedom errors of machine tools is introduced. A single-mode fiber-coupled laser module is used to eliminate the beam drift of the laser source. Only a cube corner retro-reflector and a beam splitter are adopted in the moving target in order to sense the straightness errors and angle errors (pitch and yaw) respectively and simultaneously. A special prism (including a beam-splitter facet and an all-reflecting facet) was adopted to measure the roll. A simple common-path compensation method for laser beam drift is put forward, and a 2D position-sensitive detector and a lens are used to monitor angular drift of the laser beam. It is convenient to measure the parameters of the numerical control machine in workshop due to non-cable connecting between moving target and measurement head. The error analysis and theory analysis of roll which provide potent theoretical guidance in the future are given in detail.Keywords: five degree-of-freedom, beam drift, quadrant photodiode detector, position sensitive detector .INTRODUCTIONAs we know the precision measurement of numerical control (NC) machine tool and processing center play the important role in machine tool industry and machine processing industry. There are 21 errors which need to be measured for a classical 3 axis NC machining facility. The whole measurement period is long and efficiency is low as only one error is measured after setting and adjusting one time. At present, many methods of multi-degree-of-freedom measurement simultaneously are used such as geometrical optical measurement method [1] ,diffraction measurement method [2] ,laser tracking method [3] ,laser interference with collimation method [4] ,three parallel beams method [5] , holographic lens prismatic method [6] and double-CCD method [7] . However, complex configurations, high costs and low measurement precisions are the shortcomings of these methods. In this paper, a newly five-degree-of-freedom (5DOF) measuring method is put forward. Only one retro-reflector(R-R),one reflector,one special prism are used to measure straightness errors, angular errors(yaw and pitch), roll error respectively. It is convenient used in industry due to non-wires in the moving part, small size and simple structure.
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