Hi,

if I understand correctly the moment a probe event is detected the advancing axis is decelerated to a stop. If it were crash stopped

there is the potential for to lose steps and therefore the probing would be inaccurate and the machine would have lost reference.

For a given probing speed V_{p} and a de-acceleration (given by the motor tuning), a, then the distance traveled, d, from the probe event (t=0)

until stopped at t_{s}:

d=1/2 * a* t_{s}^{2}

where t_{s} is solved from V_{p}=a*t _{s}

or:

t_{s}= V_{p}/a

(Gallieos equations for constant acceleration)

So you can calculate the overrun and effect on accuracy, or even make allowance for the overrun in the calculation of the probe point.

I probe PCB material for the purpose of applying a leveling correction. It accommodates any warp, twist or bow in the material. I normally probe at 50mm/min.

My steppers are tuned to an acceleration of 375mm/s^{2}. I have tried probing at 10mm/min and noticed no difference in the probe file that results.

I conclude therefore that the axis acceleration is such that the overrun is so small that it is masked by the lost motion (flexure + backlash) which is less than 4 um.

There is one circumstance where a significant probe inaccuracy can occur. When a probe makes contact the motion controller, be it a parallel port or an external

controller, how quickly does the controller apply the de-acceleration? In the case of my ESS the best I have been able to measure (using my storage scope)

is 15us between the probe event and the step pulse stream changing to max de-acceleration. Note that probing is supported at hardware level by the ESS.

If I waited for the probe event to be signaled to Mach4 before the de-acceleration was applied then the delay would be about 20 ms, about 10ms each way comm

delay. It would be worse, much worse, in Mach3 given the longer comm delays.

My contention is that: IF the motion controller supports probing at the hardware level resulting in near instantaneous application of de-acceleration AND the maximum

acceleration of the de-accelerating axis is reasonable THEN excepting extreme probe approach velocity the probe results will be accurate to within the flexure/backlash

of the machine.

IF however probing is programmed as opposed to realtime implementation THEN even moderate approach velocities produce significant overrun and therefore

inaccuracy.

Craig