SP25M technical terms

Return to zero

The probe has a nominal absolute centre position where the functions of stylus configuration and probe orientation cause it to rest. Because of small amounts of internal friction, when the probe is displaced from this zero point, the stylus will not return to exactly the same point on the scale and the axis deflection readings will show a different value.

This characteristic of probe performance is called RETURN TO ZERO and is a feature of all analogue probes and is not a source of error as the scale system continues to monitor position. Rather, it is merely a factor which must be taken into account when designing control software for using the probe. It can be given a value which represents the diameter of a sphere around the nominal zero position within which the probe will return to reset after any displacement.

SP25M has a return to zero value of less than 5 microns after a deflection of 0.5 mm (typically 1 µm). It is important to take this into account as it affects the minimum amount of deflection necessary before the stylus is considered to be in contact with the surface. Because the stylus can return to a value other than the nominal zero, the CMM must recognise the fact that the range of rest positions of the stylus must not cause machine motion, as the stylus is not necessarily in contact with a surface even though it is ‘deflected'.

Minimum probe deflection

The CMM controller should set a parameter for minimum probe deflection, above the return to zero value. The stylus should only be considered to be in contact with a surface while the deflection exceeds this threshold.

Maximum probe deflection

A spherical operating range of ±0.5 mm (±0.02 in) deflection in all direction s in all oreintations is guaranteed provided that the stylus carrying recommendations are adhered to. The mechanical travel of the scanning module is greater than the transducer operating range. If this range is exceed the axis signals (P, Q, R) become invalid.

Probe over range signal

This signal is asserted by the probe to indicate that the transducer operating range has been exceeded. The CMM controller must then take appropriate recovery action.