Patent ID: 6580964
Filing Date: 2003-06-17
Classification: G01B

Abstract:
A method of calibrating an analogue probe having a stylus with a workpiece-contacting tip of radius (r) comprising the steps of:a) mounting the probe and a calibration sphere of known radius (R) on a machine, b) causing relative movement between the probe and the sphere from a plurality of directions each of which is nominally normal to the surface of the sphere to bring the stylus tip into contact with said surface and deflect the stylus by a predetermined amount, c) reversing the relative movement and recording simultaneous values of the radial deflections of the stylus and of machine axis X,Y and Z positions at intervals at least until the stylus tip leaves the surface, d) extrapolating each of the recorded sets of readings to obtain values of the machine X,Y and Z axis positions when the stylus radial deflection is zero, e) calculating from the extrapolated machine axis positions the value (R+r) and the position of the centre of the sphere, f) at a pre-selected radial deflection of the stylus which is the same for each of the directions noting the a,b and c outputs of the probe, and, using a trial probe transformation matrix converting the probe a,b and c values into incremental X,Y and Z values of machine axis positions, g) using the calculated position of the centre of the sphere, determining the radius of the sphere (R+r) as measured in each of the directions, h) noting the differences in the radius measurements in each of the directions compared to the radius as determined in step (e) and, i) optimising the probe transformation matrix to minimise the differences in the calculated radius values.