Mechanically inclined, not electrically engineer inclined needing to
power a 100 lb t shirt shooting vehicle (AGV). Team already has 2 12
volt (small car battery) power sources and 8 in wheels. Notion is to
have 4 motors, one for each wheel. Design top speed about 6 mph (want
to be able to keep up with it with a slow jog).
Issue is how to decide among the many motors? Want beefy enough motor
to accelerate and but also want to keep it light and cost effective.
Suggestions welcomed. Most robot parts dont seem adequate to power a
100lb vehicle.
Look at the parts sold for the combat bots. They are around the size and
power you are looking at. Here's a store that sells to that market with an
example high power motor:
http://www.robotmarketplace.com/products/NPC-T64.html
I don't know how to estimate how much power you need to push your machine.
Is it going to be running on grass or a hard surface? It will need more
power to run and turn on grass. Motors and power controllers for these
size motors will not be cheap if you want real control over the machine
(like R/C control with variable speed and steering). The above store might
be able to help you pick parts to meet your needs.
You will probably save money by using 2 motors and 2 speed controllers
instead of 4 smaller motors.
1. What's a "shirt shooting vehicle"?
2. How does this thing steer? With four wheels, differential
drive won't work very well.
3. See "http://www.roboteq.com " for motor controllers in the
indicated range.
John Nagle
Animats
It is a vehicle with an air powered cannon to shoot T-shirts into the
crowd of a sports game (basketball) for example. Application is smooth
floor. You are correct, manuverability will be important to help aim
the shirt. The cannon mount is fixed, and does not spin, the vehicle
will need to turn to aim the shirt. Thank you.
OK. Go with two powered wheels, two casters, an
AX2550 Roboteq controller, an R/C receiver, and some surplus DC
gearmotors like these: "http://www.73.com/a/0184.shtml ".
John Nagle
I'm also working on a large robot. I'm using two 24VDC wheelchair
gearmotors (ebay) and two rubber-wheeled swivel casters (Harbor
Freight). I'm not sure why you're using 8 wheels for an application
on a smooth floor. This will only serve to reduce maneuverability.
Robots with more than 4 wheels are usually used for very rough
terrain.
BRW
This is similar in weight and function of a "battle bot," and there are
plenty of plans and descriptions around the Web you can follow if you
don't want to get into the formulas and equations. "Cost effective" is
relative, but the beefier motors and drive controllers aren't exactly
cheap. Reduce cost and weight by using two motors and two support
casters. If it's good enough to cart grandpa around in his Hover-round,
it's good enough for a robot.
-- Gordon
OK, heres the scoop, this is my dads post, and my project....I have a
little more info...
we are CURRENTLY using 4 of these motors...
http://www.robotmarketplace.com/products/0-BHG62.html
and this is our motor controller (the AX3500)
http://www.roboteq.com/ax3500-folder.html
2 8 in rubber wheels for the back, and 2 8 in omni wheels for the
front (allow to be slid both ways...)
http://andymark.biz/am-0098.html
2 12V small car batteries, and a 6 channel R\C controller\reciever
The tank shoots t-shirts at MSU sporting events during half time\other
events, and hopefully lions\pistons games...
We need this thing to turn\manuver better...The motors we bought were
crappy and the gear connecting the motor to the gearbox was falling
off the shaft\friction fit...I am not good with electronics, are these
motors strong enough and just poorly made? or do we need to look at
all new motors\controller?? Any input would be awesome!
This looks like a fine motor for a much lighter robot.
No comment on this because you're using the wrong motors anyway.
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.
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.
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
On edit I see you're talking about the internal construction of the
motors. Anyway, if this happened to more than one, then I'd suspect the
motors simply cannot handle the weight of the robot, and you're
demanding too much from them.
I'd do this: get 2 wheelchair motors and two matching wheels. Cost from
the typical Web source will be about $300 each set. Sorry, that's just
the way it is. Robotmarketplace sells these types of motors, from NPC (a
wheelchair parts company). Here is their direct URL:
http://www.npcrobotics.com /.
Supplement these with two swivel casters. You can get heavy-duty ones
from the same source as above, and other outlets (Harbor Freight has
some nice ones that are larger, and not too expensive).
NiMH battery packs, rated for the voltage/current of the motors you are
using. Be sure to get a suitable charger. Remember that many wheelchair
motors can be operated at 12-24V, and speed is increased with the higher
voltage. Therefore, if you need the speed, double-up the packs to get
24V.
A suitable controller for the motors.
-- Gordon
The best motor you can afford! At $150-300+ each, I imagine the
selection will be mainly what kind of money you have to spend.
Chat up the folks at the site and ask their recommendation. They're more
familiar with the products, and they may be able to help you select the
best motor for the least cost.
As for the other message, I don't see that you need powered omniwheels.
Personally I think you're wasting battery power here. A traditional
differentially-steered robot with swivel casters (either two on one
side, or a set both front and back, depending on design) is
tried-and-true.
-- Gordon
As an added note, the NPC site also shows "package deals" that at the
least indicates what they think works together. They do show sealed
lead-acid batteries, which for the size of these motors may be what you
need, but do remember the application they are looking at tends to be
combat robots, where the thing needs a HUGE amount of current for a
short period of time (usually just minutes).
-- Gordon
here is our team website, check out the pics
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?
Your web site documents say you went with this motor:
http://www.robotmarketplace.com/products/0-BHG20.html
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.
Very cool.
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....
On Apr 22, 12:42 am, snipped-for-privacy@kcwc.com (Curt Welch) wrote:
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...
Wow, the pictures make all the difference in the world. I see several
aspects of your design that make life difficult for your drive wheels.
It looks like you have a set of tank treads on the outside which are
mearly cosmetic. However, it also looks like you plan on them working
by being dragged along. Consider lightening the load by making the
tank treads non-moving and depict them with carved foam and paint.
The width of the wheel base (the real, functional wheels) is fairly
narrow when compared to the length of the wheel base. (However, this
is only an issue when turning) Consider both the possibility of
moving the wheels away from the front and back, and further out
towards the sides.
Since you have 4 wheels, you need to realize that you will often be in
situations (such as door thresholds and transitioning ramps) where one
wheel is up in the air. So, you can end up with one of the wheels
carrying half of the weight for a time. You might have been tempted
to think that each wheel only carries 1/4 the weight.
You have some hefty pillow blocks there. Perhaps if you went to
another wheel support method, you could lighten the load even more.
Another source for affordable wheels and motors would be those made
for electric scooters (the toy scooters, not for handicapped) They
seem to be under $50 for a motor and wheel. Look at Pep-Boy Autoparts
for a store chain that carries them.
Here is one source for electric scooter parts;
http://www.partsforscooters.com/Vehicle-Type/Razor-Scooter-Parts
Another solution may be a hub-motor wheel. Here is one link to a
chinese manufacturer. They have some calculations at the bottom of
the page to help you choose. http://www.goldenmotor.com /
Joe Dunfee
I think the tshirt shooting tank is more like the scooter than the
combat bot. If the tank can only do 4 mph and it takes a while to
achieve that speed, i dont think its a problem. Maneuverability is
more important, like the scooter. Since aiming the tshirt cannon will
be done largely by turning the tank, it needs to have a tight turning
radius. I like the scooter powertrain analogy. And...isnt everything
explained by F=ma? :)
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