Re: [CAD_CAM_EDM_DRO] re: probe design

I've got a design idea for that probe that any of you could make. I'll try
to get it sketched up and posted this weekend.

I'm not into electronics much any more but I'd like to know why you'd need
to worry about large currents in the probe? Seems to me a very small
current would work just as well. Just run the signal thru some kind of
amplifier if necessary. Then you can build the entire assembly from copper
and plastic. Use your CNC machines to make parts sized for your machine.

I also seem to remember that, even tho' gold has the highest rating for
current conduction, next choices would be (I think) aluminum, copper or
brass. Steel would be rather far down the list.

Smoke

> >Stainless is suggested for all the contacts. Wouldn't gold be better?
>
> Is it a good idea to use a soft metal for contacts whose position is very
> critical? If there were much current flowing a soft metal would make

sense,

> but if everything is pretty high impedance a hard metal should work fine.
>
> How about a hard, conductive material like tungsten carbide? Balls and

rods

> are available.
>
> This could make a pretty good probe, but for really high sensitivity

sensors

> they use a laser reflection or a capacitance sensor. It would be easier

to

> make a capacitance sensor, it is easy to imagine a bridge circuit working
> with a few plates built into the probe.
>
> Capacitance sensors are used at the sub nanometer level, this should be

good

> enough for this application. Noise limits are on the order of
> picometer/root Hertz.
>
> They have the additional advantage of being a linear sensor, not a switch.
> You'll get a nulled output, but if anything moves the output will go
> positive or negative.
>
> It is possible to make them insensitive to temperature.
>
> There is voluminous information on capacitance sensors, a good place to
> start is Foundations of Ultraprecision Mechanism Design, by Smith and
> Chetwynd. There are plenty of references there that will lead you
> elsewhere. Probably there are web references too.
>
> The book referenced above also has an interesting section on the elastic
> contact of a ball with a plane. The simplified analysis was done by Hertz,
> and it give lots of very useful design ideas.
>
> Elliot Burke
>
>
>
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