Determining/measuring sub-micron displacement

Hi John,

Yes, the probe is soldered to the disc - with a flat 'foot' so the disc must give..

Over travel is a problem on this setup. more than 1mm in horizontal will pop the disc from its tri-point holder. - open up, refit the disc, and reset runout!

Over travel of 1mm in Z will probably break the piezo disc. Can be avoided in a CNC machine if probing is auto and sensible probing speeds are used, but crashes are inevitable.

I am looking to spring mount the disc tri-point support ring to the runout setting mechanism so that it can 'spring' away- supported on three pointed pins in three mating holes with the cones on the hole periphery - hard to describe, but will self centre and have a 'hard' location, with the three pins on a ring that moves laterally to set runout. Photo's when I have done it...

Could be worth looking at the various Renishaw patents to see how they solved this problem. (I assume that you don't intend to make and sell these probes!)

I'm inspired by your work to evaluate my own electrical contact-sensing probes.

The advantage of using a mirror to show the deflection is that a light beam has no easily measurable weight or inertia. And can be made as long as is practicable, to maximise resolution..

Hence use in mirror galvanometers

Howard

I hesitate to add more ramblings to this thread, and lower its signal-to-noise ratio. I tend to post late in the evening, after good food and drink - not the best recipe for clear thought. I'm afraid I've muddied the waters by my last post - it's largely based on a false assumption about how this probe works.

Joe, I'll PM you.

I have 'improved' the probe tip allowed displacement before damage. A ring that is used to centre the probe tip is now fitted with three hard pins, ground conically, the cone resting in three bevelled holes in the tri-support disc holding the piezo element.

Two of the pins have extensions into the mating hole so that large ball displasements do not knock the ring of the pins.

The pin ring fitted into the head, held by the three runout grubscrews.

The spring assembly that makes it all work..

View of the motion when the ball is displaced

The setup seems to work well - with it set up in the mill with the magnifier arm I still get the same results as before for contact detection displacement ( very, very small..) and when I jiggle the ball around , apply excessive displacement ( 4mm approx) the ball seems to return to the same place, within the small dimensions measured by my magnifier arm - 'resetability' seems to be very good.

Now what to do with the thing...