| The ball bar is composed of two ultra-precision spheres of exactly the same diameter and a section of low thermal expansion coefficient rigid bar connecting the two spheres.The calibration value is the spherical center distance between the two spheres,and is the best one-dimensional standard for calibrating the indication error of the length measurement of various measuring instruments such as a coordinate measuring machine.It is also a standard for the calibration of spatial indication error of large-scale measurement instruments such as laser tracker,laser scanner and internal and external calibration of multi-camera.Standard ball bars have become one of the indispensable calibration tools in various measurement systems,but there are currently no measuring instruments for ball bar calibration in China.Therefore,the development of a length calibration instrument for ball bar center distance measurement has become an urgent problem to be solved.Based on the principle of laser interferometry and a trigonometric indirect measurement method of ball bar center distance,a new instrument for calibrating the length of ball bar is developed in this paper.The design can realize the overall structural scheme of completely overlapping the laser axis,the measuring axis and the movement axis,and eliminate the influence of Abbe error.The three-dimensional mechanical structure design of the whole machine is completed by using the mechanical design software Solidworks,including the selection of the measuring device and the design of the fixing mechanism,the design of the air-floating guide rail,the structural design of the fine-tuning platform,the three-point moving seat design and the vibration-damping leveling base.Several main error sources that affect the measurement results of the ball bar length calibration instrument are analyzed,and effective error elimination and error compensation methods are adopted to reduce the influence of the main error sources.Evaluate the measurement uncertainty of the length calibration instrument,analyze the uncertainty of all factors affecting the measurement results,and calculate the measurement uncertainty of each factor.The extended uncertainty of the length calibration instrument is about 1.724μm.According to the working principle of magnetic meter seat,a new type of ball bar is developed.A new type of rod head structure is designed to ensure the same precision every time the standard ball is absorbed.The composite carbon fiber is selected as the bar body to reduce the thermal expansion of the bar body and complete the design of the new type ball bar.Based on the relevant knowledge of material mechanics,the length deformation model of a new type of ball bar subjected to gravity is analyzed.It is obtained that the magnitude of force deformation is between 10 m,Meet the requirements of the use of the ball bar.Complete the assembly and check of the length calibration instrument.In order to measure the data conveniently,a small program for calculating the calibration value of the ball bar based on the windows system is designed.The measurement experiment of horizontal and vertical straightness error of sliding frame is carried out.The maximum linear deviation of horizontal and vertical direction is 3.27μm and 3.16μm,respectively,by means of mean least squares linearization fitting.The average deviations are 2.724μm and 2.571μm,respectively.Straightness error meets the requirements of measurement operation.The standard values of 650 mm,700mm,750 mm,800mm,850 mm were obtained from the measurement experiment of five groups of ball bars to be calibrated.Through the comparison with the German DAkks calibration certificate,the difference between the two standard values was 0.2 μm and the contrast coefficient value was less than 1.It indicates that the measurement uncertainty of the calibration ball bar of the length calibration instrument is within the range of measurement uncertainty assessed.Two experimental results indicate that the length calibration instrument meets the design expectations. |
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