Research On Key Technologies Of A Ball Bar Standard With Self-compensation Capability Of Thermal Errors

The ball bar standard is a frequently-used one-dimensional length standard providing a standard distance between the two spheres.The ball bar is widely used in measurements and calibrations for articulated arm coordinate measuring machines(AACMMs),laser trackers,multi-axis machine tools and industrial robots for its robustness,simplicity of use,portability and low cost.In this paper,a novel design method of the ball bar standard is proposed.The ball bar is capable of maintaining a near-zero thermal expansion of no more than 2?m in the temperature range of 10-30?.In this paper,design of the ball bar is described in detail.The thermal compensation principle of the ball bar is concretely analyzed.The structure of the ball bar is summarized to a common model and a formula to calculate the deformation.The 3D model of the ball bar is imported into the finite element analysis(FEA)software to engage thermal simulations.The ball bar is firstly measured by a coordinate measuring machine(CMM)to obtain the distances between the two standard spheres of the ball bar in different temperatures.The ball bar in different temperatures is then measured by the universal length measuring machine to obtain the distances between the vertices of the two spheres.The near-zero thermal expansion characteristic of the ball bar is verified by the experiments.After the verification of the ball bar's temperature independence,the thermal deformation of the ball bar is transformed into the micro displacement of the standard spheres.New measurement methods using laser interferometers,quadrant detectors and OMRON displacement sensor to measure the thermal deformation of the ball bar standard are discussed.The feasibility analysis and experimental verification of these measurement systems and methods are carried out.The ball bar standard can be developed to do in-line calibration for high-precision instruments such as AACMMs,laser trackers,multi-axis machine tools and industrial robots in the workshop,which can reduce the transportation cost of the off-line calibration and improve the precision of in-line measuring instruments simultaneously.The designed ball bar standard caters to the advancement of in-line calibration and the progress of intelligent manufacturing.