Error Detection And Calibration Of Parallel Double-Joint CMM

Parallel Double-Joint CMM(Coordinate Measuring Machine)studied in this paper is a new kind of CMMs and it belongs to the area of non-orthogonal coordinate measuring equipment. It has a lighter weight to be carried, less rotary joint, simply physical construction, easy to operate. Compared with multi-articulated arm CMM, this measuring machine has lower cost and higher measurement accuracy, meanwhile it has advantages as follows: agile in measurement, large measuring range, fit to site survey etc. Because of the advantages the Parallel Double-Joint CMM has been developed rapidly in recent years.In order to obtain higher measuring accuracy, we did the works includes the following aspects in this paper:In this paper, analyzes and calculates the random error of the accuracy of circle grating, runout of shaft, kinematic error of straight joints, and deflection error of the larger connecting rod and machine base, then calculates the influence to measurement accuracy of the system. Based on the structural characteristic of measure machine, indicated the accuracy influence caused by machine weight and the error of circle grating bias.Combined with the structure of the Parallel Double-Joint CMM, building mathematical model with errors by 4 parameters D-H model and proving the correctness of the model.Analyzes every error of the Parallel Double-Joint CMM, and designs relevant calibration scheme. Based on parametric measurement method and rollback method, uses error separated method to calibrate the depth of parallelism between the joints, the length of the both connecting rod, the bias error and zero-error of the circle gratings, and the position error of straight joints.Adopted Gauss-Newton method and combined the characteristic of the machine's measuring head, designed standard rod method for next calibration to achieve a higher measuring accuracy of the CMMs.