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Post by b***@gmail.com
Post by Gordon McComb
Post by b***@gmail.comwe 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
Post by b***@gmail.com
Post by Gordon McCombThis 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 youwere trying to do.
Post by b***@gmail.comSee AlsoUpcoming Fortnite Collabs 2025: Predictions & Wishes
Post by Gordon McComb
Post by b***@gmail.comand 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.
Post by b***@gmail.com2 8 in rubber wheels for the back, and 2 8 in omni wheels for thefront (allow to be slid both ways...)
First off, you lose torque with wheels this large. You get fasterspeed, but you need to balance this with the needs of adequate torque.Also, the omni wheels are frightfully inefficient. You're losing a lotright there. Rethink these. They are seldom a practical solution.
Post by b***@gmail.com2 12V small car batteries, and a 6 channel R\C controller\reciever
Lead acid batteries are extremely inefficient and heavy for theircapacity. See if you can upgrade to at least nickel-metal hydride.Li-po batteries would be even better, but very expensive.
Post by b***@gmail.comThe motors we bought werecrappy and the gear connecting the motor to the gearbox was fallingoff the shaft\friction fit...
Friction fit? The motor shafts appear to use a flatted-D, and so shouldbe secured with a metal set screw. However, for the weight you'retalking about, even with lighter batteries, I'd look for something witha key, a hex shaft, or some other configuration that eliminates slip.-- Gordon
here is our team website, check out the pics
Very cool.
Post by b***@gmail.comhttp://www.egr.msu.edu/classes/ece480/goodman/spring08/group04/index.htmand for the motors, the gear running from the actual motor to theattached gearbox comes off, not the actual output shaft. it is a tinygear with no screw on it at all...We only got those omni wheels because to hopefully make the frontwheels slide smoother, we could use orbital wheels maybe? or just getsmaller wheels to bring the required torque down...and suggestions onwhich 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 (totalfor all 4 motors combined).
The motor you selected has a no-load RPM of 510 and a stall torque of 1.85ft-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 thestall torque rating. These two points can be plotted on a speed vs torquegraph and you can draw a straight line between these two points to get agood estimate of what type of torque the motor can produce at differentspeeds. At half the no-load speed, it will only be producing half thestall-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 unitwill be able to go only half as fast as you wanted - if your torquecalculation was correct.
We can use a little math and calculate the torque at your desired 420 RPMand 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 .33ft-lbs of torque or 1.32 total for all 4 which is _way_ short of what youcalculated was needed at 4 ft-lbs.
In addition, I wouldn't trust the procedure you used to calculate requiredtorque. You made the calculation based on the rolling friction at thedesired speed. However, with hard wheels on a hard surface like concreteor a basketball court, the rolling friction will be insignificant. Thepower loss due to the transmission between the wheels and the motors(gears? Chain?) and the power loss in the wheel bearings will bedetermining factors of the torque at speed, not the rolling friction.
However, you used an ungodly high number for the rolling frictioncoefficient (.1 instead of a more reasonable .01) so who knows if thatmight produce a reasonable answer or not.
However, even if you had good numbers for power loss in the transmissionand bearings, that's not the big load. The big load is what happens whenyou try to accelerate a bot that heavy from a stand still. The F=MA forceis what the torque of the motors have to overcome to accelerate the bot tothe desired speed in something less than an hour. :) When a machine thatheavy starts from a dead start, or switches from reverse to forwardquickly, you will be drawing the full stall current on those motors for ashort period.
Then we add to this the fact you are using skid steering to make the botturn. That will require a ton of extra torque when you try to make it turnwhich is a complex function of the mass of bot and the friction of thoseomni-wheels you are using. In addition, only two motors (the non-omniwheels) are required to supply most the torque to make it turn since themotors 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 havegotten them in the right ball park anyhow by you extremely conservativeguess at the rolling friction coefficient.
However, if you have the bot running with those motors, you can use thatget a good estimate of what you really need. How fast does it go whenrunning straight? Can it turn? How fast? How long does it take toaccelerate? Using those numbers along with the specs of the motors youhave, you can estimate how much more power you need. Tell me how fast itruns with those motors and I'll take a stab a making a guess on what youneed.
If it runs at half the speed you want, (and assuming your RPM calculationsare correct), we can assume the motor is running at about 210 RPM. Thosemotors 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 pointtranslates 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 HPtotal. 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 wouldwork 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 60.1in-lbs or 5 ft-lbs. That makes it about 4 times as powerful as yourcurrent motors.
So two of these motors operating at that range, should push your bot toaround the 10 MPH rating (assuming your current machine is running at abouthalf the speed you want). But it's running at 165 RPM, instead of the 420you need, so you need to add a 2.54:1 gearing or drive on that to get itinto the right speed/torque ratio. Or just use 20" wheels.
But if the current motors can't get your machine running half the speed youwant, you will need something larger, or maybe 4 of the NPC-41250s insteadof 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 arenow up to the $330 range instead of the $50 range you started at for eachmotor. 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 wrongabout something important here. I just enjoy the learning experience oftrying to figure out someone else's problems....
--Curt Welch http://CurtWelch.Com/***@kcwc.com http://NewsReader.Com/