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Author Topic: Pulling a rope up and down with small weight  (Read 644 times)
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Your calculation of the wheel circumference is rubbish.

Never let plain facts get in the way of a good story.   smiley-lol
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"In this house, we obey the Laws of Thermodynamics" Homer J. Simpson
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Steppers okay but you cannot reliably use the step count as a position indicator

I disagree. If the stepper is properly sized for the load then step counting provides accurate reproducible positioning. As well as avoiding the need for position encoders it will also hold the load when static (though it will be consuming power) negating the need for a braking system.
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How about something like this-
http://www.allelectronics.com/make-a-store/item/DCM-351/24-VDC-GEAR-MOTOR-W/TURNTABLE/1.html

You could mount some screws equally spaced(around 10-12 just for a guess) around the outside  perpendicular to the disk. 

 Something like this                    __I________I__

Mount a microswitch  to sense each screw as it passes thus giving some feedback.

A simple 2 relay setup could control forwards and backwards and off.
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dc or stepper is dependent of what you have in the box
stepper needs only driver. when disabled driver the weight should go down thus giving a startposition
then take 1200 steps or something to hoist the weight. and 1200 to unwind again
when weight is down disable driver to safe energy, on topposition have current about 25% to keep the heat down.

dc needs a feedback just like old mouse with a wheel inside
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I 2nd the idea of using an automotive motor, especially a windshield wiper motor, since they have worm gears that will hold a load without power, and are rated to run continuously. I see them priced around $15 on the surplus web sites.

In regards to the power rating, you can multiply the max current times the voltage to know its wattage capability.   You can also convert the torque with the RPMs needed on your wench to determine wattage needed.  Here is a web site that will do that, http://www.magtrol.com/support/motorpower_calc.html

For position sensing, there are several possibilities.  But, I think the easiest to use is a multi-turn potentiometers.  For example, $4.60 is this one, http://www.robotshop.com/en/dfrobot-rotation-sensor-v2.html.  It turns a maximum of 10-turns like most multi-turn pots. So, you have size your drum to keep it within 10 turns. If it needs more than 10 turns, you have to gear down your connection to the multi-turn pot.

Thinking about your torque needs, that decision will be driven by staying within the 10 turns because it will drive the decision about the drum diameter.

Note that a potentiometer is not the most durable device.  So you don't generally want it on a robot wheel. But, you are not continuously moving it, and it should hold up for quite a while.  I would make sure you have code to prevent it from oscillating if part of the pot starts to go bad and generates a varying signal.  Just stop the motor after getting close to the target is enough.

-Joe Dunfee
« Last Edit: November 03, 2013, 01:51:13 pm by cadcoke5 » Logged

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