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6 wheels, math check please

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Mackattack:
Hello,
 
So here's the deal, I need to drive 6 wheels (12.8" tall each, bot foot print about 3' x 4'), bot weighing 300 pounds, at 10 miles per hour over grass/mud/snow and occasionally up a 45 degree incline. I calculate 378Lb-in torque each motor at 232 rpm!!!!!!!! This sounds high. Not due to the math, just more than I expected. This thing needs to be RC controlled too (and then FPV).
 
Really, 378in-lbs? per wheel?

I'd like to have brushless motors and drives with gearheads (inline style). So any advice for inline planetary motors and gearheads would be appreciated. And, any advice for motor controls (that could be Arduino'd and/or RC controlled to would be fantastic.
 
Jim

jwatte:
Yes. the math works out that way.
Radius of the wheels is 6.4 inches.
Weight per wheel is 75 pounds.
75*6.4 = 480 pound-inches, or 40 foot-pounds. But, as you will only climb a 45 degree hill, 378 pound-inches may allow you to just barely crawl up that -- certainly not at 10 mph.
Now, to have some room to actually move upwards, while overcoming friction, and also assuming you slip on at least one wheel, you'd want more than that. I'd go for 50 foot-pounds per motor to have at least some margin.

Good sources for these kinds of motors may be golf carts, for example.

Can't really recommend a motor driver until you know the voltage and current draw of the motor. You should specify the peak voltage and current of your controller to be at 2x the battery voltage and stall current of your motor, to make sure that you will never exceed it. A motor may draw 2x rated current while reversing direction, and inductive kickback can be estimated at voltage, plus the drive voltage, so 2x voltage total.

What kind of batteries are you looking at? And battery management system?

Mackattack:
Poop, I was hoping I was wrong...

OK, I'm off to search golf cart motors, or any other reco's like wheel chair motors maybe?

Duane Degn:
You were a little wrong.

The torque determines you maximum acceleration. As long as you can accelerate you should be able to reach your 10mph speed given enough time.

The 45 degree slope is the killer here. You need enough torque to overcome the force exerted by your robot's weight under the influence of gravity. This force is the weight of your robot times the sine of the slope of the incline.

The sine of 45 degrees is about 0.71, so you'll need your motors to provide 71% of the torque previously calculated.

480 lb*in * 0.71 = 340.8 lb*in

If your robot can take a the slopes with a running start, you could get by with less powerful motors. Using smaller wheels would also reduce the torque required at the cost of reduced speed.

The above calculations don't include friction and assume all six wheels will have good traction.

Do you really need to climb a 45 degree incline? That's awfully steep. You'd only need half the torque if you limited yourself to 20 degree inclines which is still steeper than most roads. Wheel chair ramps generally have less than a 5 degree slope.

jwatte:

--- Quote ---As long as you can accelerate you should be able to reach your 10mph speed given enough time.

--- End quote ---

Meanwhile, in reality, there are losses both in the system (motor, transmission, axles, wheels) and vehicle/conditions (road, air drag/wind, etc.)
Not to mention that components may wear, may not meet nominal specifications except under ideal conditions, etc. And you certainly don't want to run an electric motor at its stall current for any significant amount of time; you'll burn the motor out!

Specifying "exactly on the border" numerics is how cheap consumer junk stays cheap, and also keeps disappointing with its inability to actually achieve stated specifications. You need the margins for anything robust.

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