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.

 

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 ...

 

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-...al_Components_Supplies_ET&hash=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.

 

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.

 

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?

 

Hi thatoneguy,

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.

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.

 

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).

 

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.

 

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.

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

 

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

 

http://www.youtube.com/watch?v=LeLyPv724c4

That uses standard R/C Servos, nowhere near the torque of a wiper motor.

50 to 100 oz-in (0.25 - 0.5 ft lb)

High Torque Versions are available that are capable of even higher torque, if you don't mind full size over standard/mini.

I'd suggest the metal gear type, far smaller and direct coupled, as shown in the video.

 

Hi, thanks for the replies.

I was thinking, can in any way a gas strut help the motor in lifting the load??

 

Hi, thanks for the replies.

I was thinking, can in any way a gas strut help the motor in lifting the load??

Yes, but the energy expended lifting a load will be less than the energy required to compress it again. If the purpose was only to lower heavy objects to the ground, gravity would help compress the strut again, if it is to lower while compressing the strut without a load, the power of the needed servos would be greatly increased.

 

Another question,

If I use PWM to control the DC motor speed, will its torque decrease when set to a low speed??

 

Yes, but not as severe, that is an advantage of PWM compared to sending a lower voltage. It does reduce as the motor averages out the pulses to DC, but the peaks help, but not detrimental to the extremes you would get by say, sending it 1V instead.