[chbot] Cheap Rack and pinion servos?

[Yani Dubin]

Hi Brahm,


On Tue, Apr 21, 2015 at 9:16 PM, Brahm Boelee <brahmboelee84 at hotmail.co.uk>
wrote:

> That linearmotion site is super interesting
>

I have salvaged some of the hospital bed type actuators (which feature on
the homepage of this site - not sure if these are the ones you are
interested in).

However not sure the ones I have had more than 30cm of travel. But I
thought I'd mention a few things to be aware of with these, from my brief
experimentation, if you are considering them.

The two brands of unit I have pulled apart were based on a 24V brushed DC
motor design (one was German, one Taiwanese from memory).


The insulation rating in each case means you only get a 10 percent duty
cycle (the name place specifies something like 2 mins on 18 mins off).
Therefore they are designed for brief spurts at maximum power, and if you
exceed this, you risk damaging the insulation and reducing the life of the
motor.

One (or both) of them had a ~40:1 reduction gearing ratio between the motor
worm and the toothed actuator arm (orthogonal coupling), meaning it took
quite some time to actuate (at a very rough guess, 15 seconds for 30cm -
about what you might expect to gently raise/lower a patient). So not
suitable for rapid movement.

I evaluated the speed of the motor by cobbling together a DIY tachometer.
The Dewert motor was achieving 2000rpm with the 15.7V my bench supply could
provide, and sucking 200mA. So maybe 3500 rpm at rated load, and I think I
saw a 50W mentioned somewhere.



No idea if this is of interest, but if you want to know more about these or
see one in motion, let me know and will see whether I can reassemble one :)

Regards,
Yani.
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