Re: [CAD_CAM_EDM_DRO] Re: Digitizers, Renishaw

Hi,

I'm surprised that no one has yet suggested a capacitive probe. One of the
problems I see with the 6-balls type and other suggestions is corrosion,
oxidisation and the possible ingress of dirt, especially in a home-built
probe. However, if a capacitance type were made, there would be no contacts
or reflective surfaces to worry about and the probe should have a long life.
The kind of thing I am talking about (without knowing too much about the
subject and hoping for comments from those more knowledgeable) would have,
say, the top of the sensing 'stalk' surrounded by an upturned metal 'cup'
coated with a thin layer of epoxy or just paint internally to isolate it
electrically from the 'stalk'. Then a connection would be taken from the
stalk and the cup to a tuned oscillator circuit. Any movement of the stalk
would then change the capacitance between the connections and thus the
frequency of the oscillator and this change in frequency could be detected
and either used to flip a digital switch or used as an analogue change to
measure the actual deflection. I'm not clever enough to design the
electronics but I don't think it would be too difficult, the biggest problem
might be working out a suitable size for the sensor part to get a
sufficiently large capacitance change to swing the oscillator a large amount
for a very small movement of the stalk. If the stalk is supported on three
or four stretched horizontal springs, the 'cup' could be made of a tube and
separate plate top and so sense both lateral and vertical movements of the
tip separately. Comments anyone?

Ian
--
Ian W. Wright
Sheffield UK
www.iw63.freeserve.co.uk