Re: Motor / gearbox Design for 100 lb AGV
- From: bry.grinnell@xxxxxxxxx
- Date: Sat, 3 May 2008 10:47:34 -0700 (PDT)
On Apr 22, 12:42 am, c...@xxxxxxxx (Curt Welch) wrote:
bry.grinn...@xxxxxxxxx wrote:
On Apr 21, 5:45 pm, Gordon McComb <NOSPAMg...@xxxxxxxxxxxxxxxxxx>
wrote:
bry.grinn...@xxxxxxxxx wrote:
we are CURRENTLY using 4 of these motors...
http://www.robotmarketplace.com/products/0-BHG62.html
Your web site documents say you went with this motor:
http://www.robotmarketplace.com/products/0-BHG20.html
This looks like a fine motor for a much lighter robot.
Note also the web says they are good for robots in the 10 to 15 lb range.
That should have clued you that you were in the wrong range for what you
were trying to do.
and this is our motor controller (the AX3500)
http://www.roboteq.com/ax3500-folder.html
No comment on this because you're using the wrong motors anyway.
2 8 in rubber wheels for the back, and 2 8 in omni wheels for the
front (allow to be slid both ways...)
First off, you lose torque with wheels this large. You get faster
speed, but you need to balance this with the needs of adequate torque.
Also, the omni wheels are frightfully inefficient. You're losing a lot
right there. Rethink these. They are seldom a practical solution.
2 12V small car batteries, and a 6 channel R\C controller\reciever
Lead acid batteries are extremely inefficient and heavy for their
capacity. See if you can upgrade to at least nickel-metal hydride.
Li-po batteries would be even better, but very expensive.
The motors we bought were
crappy and the gear connecting the motor to the gearbox was falling
off the shaft\friction fit...
Friction fit? The motor shafts appear to use a flatted-D, and so should
be secured with a metal set screw. However, for the weight you're
talking about, even with lighter batteries, I'd look for something with
a key, a hex shaft, or some other configuration that eliminates slip.
-- Gordon
here is our team website, check out the pics
Very cool.
http://www.egr.msu.edu/classes/ece480/goodman/spring08/group04/index.htm
and for the motors, the gear running from the actual motor to the
attached gearbox comes off, not the actual output shaft. it is a tiny
gear with no screw on it at all...
We only got those omni wheels because to hopefully make the front
wheels slide smoother, we could use orbital wheels maybe? or just get
smaller wheels to bring the required torque down...and suggestions on
which motors to buy? that would make the 2 wheel drive system work?
Looking at the nice write up on motor selection, I see a few issues.
You calculated you needed an RPM of 420, and a torque of 4 ft-lbs (total
for all 4 motors combined).
The motor you selected has a no-load RPM of 510 and a stall torque of 1.85
ft-lbs.
Here's the catch which no one told you....
Motors have a torque of 0 at the no load speed, and a speed of 0 at the
stall torque rating. These two points can be plotted on a speed vs torque
graph and you can draw a straight line between these two points to get a
good estimate of what type of torque the motor can produce at different
speeds. At half the no-load speed, it will only be producing half the
stall-torque. So at 210 rpm, those motors can only produce .925 ft-lbs.
That's about half the power you calculated you needed which means the unit
will be able to go only half as fast as you wanted - if your torque
calculation was correct.
We can use a little math and calculate the torque at your desired 420 RPM
and we find this:
1.85 * ((510-420) / 510) = .33 ft-lbs.
So we see at your desired RPM, the motor you bought could only produce .33
ft-lbs of torque or 1.32 total for all 4 which is _way_ short of what you
calculated was needed at 4 ft-lbs.
In addition, I wouldn't trust the procedure you used to calculate required
torque. You made the calculation based on the rolling friction at the
desired speed. However, with hard wheels on a hard surface like concrete
or a basketball court, the rolling friction will be insignificant. The
power loss due to the transmission between the wheels and the motors
(gears? Chain?) and the power loss in the wheel bearings will be
determining factors of the torque at speed, not the rolling friction.
However, you used an ungodly high number for the rolling friction
coefficient (.1 instead of a more reasonable .01) so who knows if that
might produce a reasonable answer or not.
However, even if you had good numbers for power loss in the transmission
and bearings, that's not the big load. The big load is what happens when
you try to accelerate a bot that heavy from a stand still. The F=MA force
is what the torque of the motors have to overcome to accelerate the bot to
the desired speed in something less than an hour. :) When a machine that
heavy starts from a dead start, or switches from reverse to forward
quickly, you will be drawing the full stall current on those motors for a
short period.
Then we add to this the fact you are using skid steering to make the bot
turn. That will require a ton of extra torque when you try to make it turn
which is a complex function of the mass of bot and the friction of those
omni-wheels you are using. In addition, only two motors (the non-omni
wheels) are required to supply most the torque to make it turn since the
motors on the omni-wheels only push it forward and don't help it turn.
All in all, I wouldn't trust your torque calculation. But you might have
gotten them in the right ball park anyhow by you extremely conservative
guess at the rolling friction coefficient.
However, if you have the bot running with those motors, you can use that
get a good estimate of what you really need. How fast does it go when
running straight? Can it turn? How fast? How long does it take to
accelerate? Using those numbers along with the specs of the motors you
have, you can estimate how much more power you need. Tell me how fast it
runs with those motors and I'll take a stab a making a guess on what you
need.
If it runs at half the speed you want, (and assuming your RPM calculations
are correct), we can assume the motor is running at about 210 RPM. Those
motors should be putting out about
1.85 * ((510-210) / 510) = 1.05 ft-lbs.
of torque at that speed. The HP produced at that operating point
translates to:
hp = torque (in ft-lbs) * rpm / 5252
1.05 * 210 / 5252 = .042 HP per motor
or
.17 HP total for all four motors.
In this case, if you want it to go twice as fast, you need around .3 HP
total. Or, if you switch to the 2 motor design, .15 hp per motor.
Checking the web site again... This starter kit seems to imply it would
work for you sized bot (60 to 120 lb bots):
http://www.robotmarketplace.com/products/package01.html
which uses these motors:
http://www.robotmarketplace.com/products/NPC-41250.html
Checking the 24 V chart, we see it puts out .16 HP at 165 RPM and 60.1
in-lbs or 5 ft-lbs. That makes it about 4 times as powerful as your
current motors.
So two of these motors operating at that range, should push your bot to
around the 10 MPH rating (assuming your current machine is running at about
half the speed you want). But it's running at 165 RPM, instead of the 420
you need, so you need to add a 2.54:1 gearing or drive on that to get it
into the right speed/torque ratio. Or just use 20" wheels.
But if the current motors can't get your machine running half the speed you
want, you will need something larger, or maybe 4 of the NPC-41250s instead
of 2.
Or go to an even larger motor like this:
http://www.robotmarketplace.com/products/NPC-T64.html
But you will need higher power controllers for that as well. And you are
now up to the $330 range instead of the $50 range you started at for each
motor. I hope you have a good budget for this project. :)
BTW, don't trust anything I write without double checking it. I have _NO_
experience building these types of bots so I might be completely wrong
about something important here. I just enjoy the learning experience of
trying to figure out someone else's problems....
--
Curt Welch http://CurtWelch.Com/
c...@xxxxxxxx http://NewsReader.Com/
I think we figured it out...got some more money, so we can spend some
cash on some nice motors... This is what we are thinking....
2 of these motors for the rear wheels
http://npcrobotics.com/products/viewprod.asp?prod=42&cat=20&mode=gfx
Changing to 10" pneumatic flatproof tires in the back, and a 10" heavy
duty swivel caster wheel in the front...
Going with the same motor controller, pretty sure it will be adequet
to power the motors, do you think they will fry it? Thanks for your
input, check out the video of it shooting...
http://youtube.com/watch?v=F4VNLXnWJTA
.
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