Servo idea: Windscreen-wiper motor w. homemade encoder

[Evilness]

Hi there

I just came up with an idea for a cheap and strong servo: Why not use a windscreen-wiper motor from a car?

Years ago, I once bought such a motor from a scrapyard, and I think it is strong enough to power the heaviest mills seen here. That is because these motors (well mine anyway) have a built-in reduction gear, thus reducing the rpm to about 50.

You'd have to combine this with an encoder, but I thought it should be possible to make that yourself, using a CD-sized transparent disc (you can find these on top and bottom of spindles with 100 CD's) and a transparent label with stripes on the side. Then all you'd need is a led and a phototransistor or something like that, to finish your encoder.

A little math: A CD has 6cm radius, so the edge is 2 * pi * 6 = 37.7cm long. If you want 200 stripes, they would be 37.7 / 400 (don't forget the spaces between them) = 0.94mm wide. This should be easily detectable for your sensor, and by using both the positive and the negative edge of your sensor's signal, you'd have a 400 pulses/rotatation encoder. Not bad for something homemade.

What do u guys think about these ideas?

[ESjaavik]

Originally posted by Evilness Hi there I just came up with an idea for a cheap and strong servo: Why not use a windscreen-wiper motor from a car?

Because it does not run well when asked to move very slow. And because it have different Amp/Torque characteristics running backwards than forwards. It is optimized for running forwards as most brushed DC motors are.

On the other hand it does a good job as auto-feed. Then the mentioned characteristics are not very important. If you can adjust the input voltage it does a much better job of cranking than your hands.

[ToyMaker]

One potential problem with wind screen wiper motors is that many of them are wired such that they *only* run 1 direction. Getting them to reverse can be quite a chore.
Another area for concern is backlash. The gear trains are not especially tight because they don't have to be. Depending on where you put the encoder, this could cause lots of motor seeking.

robotic regards,

Tom
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