Re: [CAD_CAM_EDM_DRO] digitizing probe

Tom,
Although I should of course protest that you didn't buy a genuine Renishaw
probe (well they do pay my wages) congratulations on at least trying
digitizing.
I have not seen a Fowler probe and do not know what the contact materials
will be but assuming they have made a faithful copy, then you are on the
right lines. 12V is a good choice you need that much voltage to break
through any oxide film or muck on the contacts. Keep the capacitance across
the contacts as low as possible. In general use a screened cable and the
capacitance between the cores will be all you need. Your 100K sounds a
little high, we normally aim for around 0.5ma say 25K. This should allow a
million or more operations assuming the rest of the mechanism is up to it.
You will find with this higher current you can drop the comparator voltage.
This will reduce the pretravel before the probe triggers.

Have fun
Bill Phillips

>Gentlemen,
>
>I am not trying to get things going on the digitizing thread again, but I
>have a question for you electronics geniuses ( I say that respectfully,
>and gratefully):
>
>I am using a digitizing probe made by Fowler. It is a copy of a Renishaw
>cmm probe that the patents have now run out on. It has no debounce
>electronics or anything like that, just a set of contacts configured as
>the Renishaw probes are. My question is:
>
>I am applying + 12 volts through the probe contacts then through a 100
>kohm resistor then to ground to protect the contacts from too much arcing.
>Should I put a capacitor accross the contacts to further protect them from
>arcing? It seems as though a .1 ceramic might help.
>
>
>By the way, I am using 12 volts, because when I tried to use 5 volts, the
>contact resistance sometimes gave me erroneous indications. I compare the
>signal coming from the probe to a reference of 8 volts using an LM339 to
>avoid erroneous indications produced by contact resistance. I also use
>software to debounce the contacts.
>
>Thanks again for your help.
>
>Tom Eldredge