At the moment of a touch, before the contact, the probe is at rest with no output signals from the focus sensors. The trigger point is determined as the time when the sensors detect the tilt motion of the floating plate. The distance between the touch and the trigger points is called the pre-travel distance, and the distance after the trigger point is called the overtravel. When the probing system sends out a trigger signal and the computer takes the current position as the reference zero point. In Fig. 10, the probing system has a sensitivity of 0.48482 pF/m and 0.12072 pF/m, while nonlinearity of 0.078% and 0.36% during axial approach and radial approach, respectively. It can be seen from residual of fitting curve, the nonlinearity of the probing system is proved, but very small during the scanning range.