Tony Smith wrote:
>>To get a "force" just put a spring on the end of your 
>>solenoid something like a ball point pen spring would do. 
>>Compress the spring by X amount = Y force. Should be close 
>>enough for government work.
> 
> Doesn't work as well as you'd hope.  It can 'soften the blow' that solenoids
> often have, but putting the spring on the other side, so that the solenoid
> pulls against it works a bit better.
> 
> It depends on what the client defines as a keypress, being hard, soft, slow,
> fast, rapid presses, multiple presses, etc.  And when you've finished
> explaining the options, the answer is 'yes, all of that, thanks'.  (Gosh,
> that sounded a bit cynical.)  As the piano-playing robot folk found out,
> it's not that easy.  Even with solenoids, you really don't get that much
> control, just different flavours of 'thump'.  Voicecoils, that's the ticket.

As long as they have position feedback.
We use one from an ancient disk drive (4" diameter 6" long) to pull muscles in
a research lab. 2kg of force with 1kHz response (for stiffness measurements).

Speaker horn drivers work reasonably well too, but their throw range is about 3mm max.

You can add position sensing to a solenoid with IR/LED pair reflecting off the back
side of the core.

There are even commercial IR units with digital output that have cm range,
in the $10 range. Roboticists use them all the time in their summo combat
robots.


Robert


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