On 14/07/10 03:55, RussellMc wrote:
>> I'm designing a tool length measurement device for a mill.
>> I've settled on the mechanical design which is a tripod arrangement, but
>> what I'm after is some contacts to actually make and break the connections.
>> I know such beasts exist but I'm having difficulty finding sources for them.
>>
>> At the moment its looking like I'll have to settle with gold plated PCB
>> as the contact material.
>>
>> I'm looking for gold at the moment because I want to run the contacts at
>> a very low current/voltage to minimise arcing.
>>      
> Be careful with wetting current needs with contacts with very low currents.
> Something in hind brain suggests that even gold may not be optimum.
> Whether contact is point/area or wiping contact will affect longer
> term reliability.
>    
current plan is flat on flat, the wetting current was a concern hence 
the desire for gold.
> Arcing is less important than energy in arc (intrinsically safe
> black-hole beckons).
>    
The problem is the arc will keep the switch "on" even though it has 
opened and the goal is to detect the small movement to start.
Ideally we would like to detect it opening over .001mm or less.
> Normal lore is that anything under about 1 mA is unwise for wetting purposes.
> Potential used also relevant - 10V often suggested as useful minimum
> voltage for film breakdown but obviously very application dependant.
>
> Use of a combined capacitive/conductive method (shades of SawStop) may
> allow zero proximity / low conductivity operation.
>    
Probably not accurate enough is the problem (unless you have an idea for 
an implementation?), The only non-contact method I've worked out is 
laser interferometer, but that's "hard TM" ;->

> Contact sets from other equipment may be useful - eg direct mechanical
> operation of contacts from a relay specified for low contact current
> operation
>
> .
>                Russell
>    
I was hoping to be able to purchase the contacts off the shelf somewhere 
rather than destroying existing equipment and hacking stuff up.
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