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  #1  
Old 02-02-2013, 11:30 AM
Dritech Dritech is offline
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Default Wiper motor torque

Hi,

Does anyone know the approximate torque value (kg-cm) of the common 12V car wiper motor?? I am asking this question so to have a rough idea of the torque required by the servos to build a robotic arm.
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Old 02-02-2013, 01:59 PM
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PackratKing PackratKing is offline
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How much weight do you intend to manipulate with the robot...?
A wiper motor commonly has a gearbox or torque multiplier attached, and a device to change motor rotation to lateral with an eccentric to run wiper linkage...
they are quite strong.
I would guesstimate 2 Kg
Tho' give it time, others here know more than I ...
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Old 02-02-2013, 02:37 PM
Dritech Dritech is offline
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Hi PackratKing, thanks for the reply

I intend to lift at least 1Kg. I was considering using a motor like the one in the link below -200RMP and 15kg-cm. (although I have to check about the quality of the motor first. I don't want it to break apart after 1 week):

http://www.ebay.co.uk/itm/DC-24V-1-8...item2c6a5ece1d

I was also considering using studs as shown in the attached diagrams, but i wonder if it will help increase the lifting capacity.

Any help would be appreciated.
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Old 02-02-2013, 03:06 PM
SPQR SPQR is offline
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If you are using screws attached to the motors, you'll have a huge mechanical advantage, depending on thread type -
as noted by Archimedes a couple of years ago.
So lifing a kilo shouldn't be a big issue.


Edit: If the levers are like you note in the drawing, you'll lose some mechanical advantage, but most likely it will be made up in the screw.

Last edited by SPQR; 02-02-2013 at 04:32 PM.
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Old 02-02-2013, 04:52 PM
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With the worm gear on the motor, sites selling them state anywhere from 30ft-lb to 50 ft-lb of torque at roughly 60-70 RPM.

They will be rather large, going from size of motors for scale, your arm looks to be the size of a human arm or kid's leg.

The 3rd class lever moving the first section around the anchor is a disadvantage as noted earlier, it also blocks a lot of possible movement area near the arm. Have you thought about placing it on the other side of the base?
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Old 02-02-2013, 05:42 PM
Dritech Dritech is offline
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Hi thatoneguy,

Quote:
They will be rather large, going from size of motors for scale, your arm looks to be the size of a human arm or kid's leg.
No I'm not going to use a car wiper motor for this project. I gust want to know so that I can compare its torque with DC geared motors available on ebay.

Quote:
The 3rd class lever moving the first section around the anchor is a disadvantage as noted earlier
What are the disadvantages please?? so is using 1st class lever more efficient??

Which positions are the best to place motor 1 and 2 to get the best from these motors and therefore increasing the load capacity??

And am I right in saying that if the motors are places at the joints/axis (like in the arm attached diagram), it will not provide its full power?? If yes, why professional robotic arms (like ABB and KUKA) have there motors positioned at each axis ??

Thanks for the replies and sorry for my poor English.
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Old 02-02-2013, 05:50 PM
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Wiki explains Lever classes well

The way yours is operating, more power is needed, but distance is multiplied (small movement near pivot results in large movement at end).
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Old 02-02-2013, 06:09 PM
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Ok, let's start with the easy part.

How much weight do you want to lift? (1 kg - we have that answer)
How far do you want to lift it?
How fast do you want to lift it?

Once we have that, you can determine the best way to accomplish the task with the smallest motor.
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Old 02-02-2013, 06:40 PM
Dritech Dritech is offline
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Quote:
How far do you want to lift it?
I want the dimensions to be approx 30cm for the first section and 20cm for the second section (please see attached diagram), therefore the load can be lifted at about 50cm far from the base.

Quote:
How fast do you want to lift it?
I was planning to use PWM to very the speed. Ideally the maximum speed will be that similar to the video in the link below.

http://www.youtube.com/watch?v=LeLyPv724c4
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  #10  
Old 02-02-2013, 10:51 PM
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Ok, so you want to copy that arm yourself, but with different dimensions, like your first drawing.

Let's call the left motor on your drawing "A" and the right motor "B".
Let's call the platform level "platform" and lets call the arm below it the "support"

The support and motor "A" are clearly not a problem because the fulcrum is actually shifted toward the weight, so you need less than 1kg of force to lift it (not exactly correct because you need to consider the mass of the support and the platform.

The platform, and motor "B" is not a problem, because the fulcrum of the lever for supporting/lifting the weight is in the middle of the whole system, so you just need 1 kg of force to move it up and down.

So you have a huge mechanical advantage from the screw rod, and a moderate mechanical advantage on the support and no advantage on the platform.

I have a stepper motor with 127 oz-inches of torque, and when it is going, I cannot stop it with pliers.
That is 0.9 Newton-meters.

So I've looked around the finding a DC motor with that torque should be pretty straight forward.

You might just build you system, and try a motor or two
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