Error compensation for CMM touch trigger probes

We present the analysis of a simple mechanical model ofa common type of kinematic seat touch trigger probe widely used on modern coordinate measuring machines (CMMs). The model provides a quantitative description of the pretravel variation or ''probe-lobing'' characteristics that limit the use of such probes for high-accuracy dimensional measurements. We include the effects of stylus bending and the frictional interaction between the stylus ball and the part surface. The model is restricted to probes with simple straight styli, and we demonstrate significant error reduction both for vertically and horizontally oriented styli. In the latter case, gravitational forces are shown to play an important role in probe triggering and pretravel variation. Extensions to arbitrary orientations are discussed.