J Jpn. Soc. Precision Enginnering, Vol.69, No.2 (2003) pp.222-226.
This study has proposed a new coordinate measuring machine (CMM) based on a three degree-of-freedom parallel mechanism. This paper describes a kinematic calibration method using a double ball bar (DBB) system with fixed ball distance. The CMM with a touch trigger probe measures the moving ball of the DBB, and calculates position errors of the ball using kinematic parameters. The least square method using Jacobian matrix corrects gradually the kinematic parameters so that the position errors are minimized. The above calculations have been repeated in both numerical simulations and experiments. The position error approaches a zero level, but has never reached it. The calibration with 19 parameters decreased the position errors of the DBB from 168 Ám to 9 Ám in the experiments. Furthermore, accuracies of both length measurement with block gauges and profile measurement with an optical flat have been improved after the calibration. The measurement accuracy most improves when the repetitive operation is discontinued after three or five calculations.
Key words: coordinate measuring machine, parallel mechanism, calibration, kinematic parameter, double ball bar