Re: [CAD_CAM_EDM_DRO] digitizing probe

Thanks, Bill,

I am going to stick to the 12 volts, based on what I see on the scope. I
will drop the resistance some, and use your current reccomendations of .5 ma
as a goal in the future.

Thanks again.

Tom Eldredge

----- Original Message -----
From: Bill Phillips <cad@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, June 13, 2001 4:09 PM
Subject: 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
>
>
>
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