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### Author Topic: Robot motor factor clarification  (Read 1067 times)

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#### TSPCO

• Jr. Member
• Posts: 18
##### Robot motor factor clarification
« on: March 01, 2009, 08:16:02 AM »
Ok I have used the RMF calculator on this site, according to it (as I understand it)
I would need a 400 lb-ft motor to move my 300 pound wheelbot at 10 mph(14.7ft-sec) on a 13inch tire
please note I stated pound-foot not pound-inch, now the engine in my my wifes Yukon is spec'd at about 400 foot pounds, I don't think she would appreciate her engine being put in my wheelbot.
Conversely I know a few larger folks that weigh  250 pounds and ride around on mobility scooters (PMV's) that
weigh about 200 pounds, so we have a 450 pound load moving at 10 (9) mph that use less than a 200 in-lb transaxle, yes I wrote in-lb.
What is wrong here?

• Supreme Robot
• Posts: 11,632
##### Re: Robot motor factor clarification
« Reply #1 on: March 02, 2009, 11:26:53 AM »
Well, that depends a lot on your desired acceleration, efficiency, and incline, which will make large differences in results.

Lets say you set 100% efficiency (meaning the motor is spec'd for the output after the gears, not before). Set incline to 0 degrees. And acceleration to .3 ft/s^2.

RMF will end up being 105 lb ft rps. (1260 lb in rps)

Now doing a quick search on motors, this guy can handle that:
http://www.robotmarketplace.com/products/NPC-T74.html

It won't be able to match your rpm however, so you'd either have to reduce your required speed, or find another motor . . .

#### TSPCO

• Jr. Member
• Posts: 18
##### Re: Robot motor factor clarification
« Reply #2 on: March 04, 2009, 06:28:08 AM »
I am still not sure I just ran a 250 lb machine at 14ft-s with 1 foot diameter tires the rmf came out at 1700, so I grabbed a spec from a 20 horsepower two cylinder engine  just for fun the rmf for the engine came out as 1300. I don't know about you but I don't see a problem with a 20 hp gas engine moving 250 lbs at 14ft-s.
So I guess I just don't get it.
the data in fed into the calculator is as follows
Mass 250
Velocity 14
Desired acceleration .3
Expected efficiency 95
Incline angle 10
Wheel diameter 1.08
# of powered wheels 2

Torque (from the engine spec sheet)  32.6 ft-lbs
speed after gear reduction 248 rpm.
dazed and confused, Ryan

• Supreme Robot
• Posts: 11,632
##### Re: Robot motor factor clarification
« Reply #3 on: March 06, 2009, 08:54:21 PM »
I think what is confusing you is the ability of the motor to move the vehicle in a situation vs. being able to move at the minimum specification you defined.

For example, a much smaller motor can still move it up a 10 degree hill incline, but don't expect the same max velocity and maximum acceleration. Unless of course you define it in the RMF calculator. Also, at maximum expected velocity, the acceleration is zero (although acceleration is needed to reach that velocity). In your previous calculations you were giving it an ability to accelerate at its maximum velocity.

I looked up a wheel chair myself to find typical specs . . .
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=200108642009

It has a spec of:
maximum 136kg
1.16 inch wheels
maximum speed 5.8666 (4mph)

Now, that means it'll do 4mph with perhaps a child on it, but not some 130kg guy on it. Also, since it's max speed, acceleration has gone to zero and incline must be 0. In theory, holding a velocity requires zero torque, but some small amount will be needed to counter friction.

I got an RMF of ~15 lb ft rps. This would be maximum RMF (power) the motor could do, no matter the velocity/accel/torque you defined.

If I showed you a 3D graph representing acceleration, torque, and velocity, it'd all make more sense. At the maximum spec of a motor, as you increase one number, the others respectively must be decreased.