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. Typically, after a 1 mm deflection, the probe will return to within 10 μm of its free state original position.
This characteristic of probe performance is called RETURN TO ZERO and is a feature of all analogue probes. It 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 rest after any displacement.
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 the machine motion, as the stylus is not necessarily in contact with a surface even though it is “deflected”. The CMM software should have a parameter for this minimum probe deflection and only deflections above this amount should be considered as the stylus being in contact with a surface.