## Re: measuring torque (was: Best Deal on Controller/Driver for 10, 24vdc steppers?)

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• This Excel drive calculator might help with selecting the motor you need.
Message 1 of 4 , Aug 7, 2013

This Excel drive calculator might help with selecting the motor you need.

• It isn t rocket science, just simple sums, plain physics, and fancy electronic gizmos or CAD models or whatever are not needed. Remember Blackbird, the X-15,
Message 2 of 4 , Aug 7, 2013
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It isn't rocket science, just simple sums, plain physics, and fancy electronic gizmos or CAD models or whatever are not needed. Remember Blackbird, the X-15, B-58 Hustler, were designed using slide rules.

We still don't know what sort of robot was 5'8" tall and made from welded aluminium but 10 motors is a lot if it doesn't have some sort of arm and gripper so we will go with that.
How long is it from your shoulder to your hand? About 2 feet. Call it 25 inches.
With 1000oz.in torque that is a holding load of 40 oz (1000/25=40) at the hand with the arm horizontal providing the arm itself weighs nothing.
How heavy is an arm and hand? Lets look at a human arm - remember steel and aluminium tend to be heavier than flesh. Lets approximate the volume to 3"x3"x25" = 9x25 cu inch =225 cu inch. (just measure your own arm to check).
Lets suppose the arm is all water and no bone etc.
225cu inch is near enough 231 cu inch and there are 231 cu inch in a US gallon and a US gallon of water weighs 10lb.
Lets say the arm and hand is equal thickness from shoulder to elbow then the centre of mass will be 12.5 inches from the shoulder and the torque necessary to hold the arm out horizontal will be 10lb at 12.5 inches = 160 oz at 12.5 inches =2000 oz.inch.
So a hulking 1000oz.in NEMA34 won't even work for a human arm even if the hand is holding nothing.

From my experience for something like a robot figure I reckon on needing 5 times the holding torque to allow for gearbox inefficiency, inertial loads etc. (forget life expectancy - if you are designing a hobby robot rather than an industrial robot then it is not going to wear out ever). Peter reckoned on doubling twice and so got four times, about the same.
And that is when using DC motors. Using stepper motors and relying on counting steps to know the position I would be unhappy using less than a figure of ten times - just a gut feeling since I avoid stepper motors for anything other than simple light duty use such as 3D printers.

So to move a human arm about at any reasonable speed with a 1000oz.in NEMA34 at the shoulder you would need 20 to 1 gearing!!!!

DAvid

----- Original Message -----
Sent: Tuesday, August 06, 2013 6:15 AM
Subject: [SeattleRobotics] measuring torque (was: Best Deal on Controller/Driver for 10, 24vdc steppers?)

You guys out there with experience can probably spitball a 1000oz/in torque figure from experience, but in general is there a an inexpensive DIY way of measuring torque for an application like this? Say you've got some isolated portion of the robot built and now you need to select motors.

Then once you've got the actual torque on the drivetrain, approximately how much spare capacity should you allocate in order to maximize life and minimize hiccups?

Best,
James

On Mon, Aug 5, 2013 at 8:01 PM, David Buckley wrote:

Without knowing anything about your robot other than 5'8" and welded aluminium I'd say that if the motors are going to be big enough they are not going to be low cost to drive unless you build the circuitry yourself.
Ten motors and 5'8" sounds like you are thinking of arms etc rather than whirling round bits of decoration, in that case you need to be looking at the very least at 1000oz.in joint torque for arms that are not really going to be able to do anything.
Roughly that's a 5" long NEMA34 with maybe 2A/phase from 24v, and probably some gearing as well.
Anything you put in-between the Pi and the motors needs to be intelligent enough to drive the motors because the Pi won't be able to do it.
Maybe I am completely off in the wrong direction??
In any case stepper motors on a robot? That is so 1960s and rather like 'rules? In a knife fight?'.
DAvid

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