Newsgroups: comp.robotics
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From: kilian@cray.com (Alan Kilian)
Subject: Re: Help - DC Motor Selection
Message-ID: <1994Aug29.124302.11869@driftwood.cray.com>
Lines: 72
Nntp-Posting-Host: poplar010
Organization: Cray Research, Inc.
Date: 29 Aug 94 12:43:02 CDT

> How do I compute the torque required to move my 50 pound robot?

Well, here goes nothin'

The things that determine this are:
1) The weight of the robot
2) How fast do you want it to accelerate?
3) How much of a hill does it need to climb?
4) What kind of surface will it be driving on?

I would think that the hardest thing your robot will need to do is
run uphill on grass, so we can neglect the acceleration part above.

Let's say you want it to be able to climb a 15 degree hill.

Pretend that your robot is driving on a level smooth tar surface.
Once it is moving, it will take approximately zero torque to keep
moving. 

Things that make zero the wrong number:
- There is friction everywhere.
- There is air. Wind resistance.
- Your level surface is not really level.

But, zero is just a fine number for right now.

Now first thing is to figure out what happens when you try to
climb a 15 degree hill with a 50 pound robot.

The force you need to generate is weight * sin(incline)
                                   50    * sin(15)
                                   50    * .3
                                       15 pounds of force

Now how do you get from pounds of force to torque?

You said that you were going to use 8 inch diameter wheels so that's
4 inches in radius.

if you have 15 pounds of force at a 4 inch radius, that's   4 * 15 inch-pounds
Which is 60 inch-pounds of torque which is 960 inch-ounces of torque

And that's a lot.

Now you say that you have a 3.4 : 1 reduction, so you can divide by 3.4
to get 280 inch-ounces of torque which is still a lot.

Now when I look in the #14N7 Edmund Scientific catalog, I see on Page 194
that they have a 12 inch-pound 95 RPM DC (36 volt) motor for $17.50
Which is 192 inch-ounces.

Now, you do have two motors, so you can divide the requirement by two,
BUT BUT BUT BUT we are still pretending that we are driving on smooth
tar with no friction, so I would over-rate the torque requirement by
at least two so you can't divide by two here and get a robot that will
go anywhere.

The moral of the story is that you need big macho motors, or many stages of
gear reduction to drive a 50 pound robot on grass up a 15 degree incline.


I hope that this gives you some ideas of what is required.

A note from experience: Take whatever motors you decide you need and buy
a motor that produces twice that torque. Manufacturers lie.

                     -Alan Kilian

-- 
 -Alan Kilian    kilian@cray.com 612.683.5499 (Work)
 "In 1969 We went to the moon and there was no air and only a bunch of
  dirt and WE LOVED IT!" -Tim Whitley talking about kids today and Apollo.
