The direct-error-compensation method of measuring the error of a six-freedom-degree parallel mechanism CMM

A six-freedom-degree parallel mechanism CMM is designed. It has some characteristic, such as good system rigidity, rapid motion velocity, not-accumulative error, and a flexible end position stance. The errors influencing the probe position stance are analysed: they are original error of the machine, the process error for measuring, and other stochastic errors. Amongst them, the original error of the machine is the main factor causing probe error. Probe error influences directly the measuring precision of the six-freedom-degree parallel mechanism CMM. A coordinate system for analyzing the position is set-up and its position reverse solution model is obtained. Based on the position reverse solution model and according to matrix complete differential-coefficient theory, the relationship between the original machine error and the probe position stance error is deducted, and the error model is built. Creating a program in the Matlab language to calculate the error model, the position stance error of the probe is obtained under the condition of determinate machine error. Using this result to correct the probe position stance, namely the direct-error-compensation method, can clearly improve the measurement precision. The theory and flow chart of this method are introduced in the paper.