### Author Topic: Motor selection question. Confused.  (Read 381 times)

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#### Croy9000

• Beginner
• Posts: 3
##### Motor selection question. Confused.
« on: September 07, 2011, 02:04:02 PM »
Ok, I am confusing myself on motor selection for a small bot. Hopefully someone can set me straight.

Here are the values I'm entering into the RMF calculator:

Mass: 3lbs
Desired Velocity: 3ft/s
Desired Acceleration: 1.5f/s2
Expected Efficiency: 65%
Incline Angle: 0
Wheel Diameter: .06m
# of Powered Wheels: 2

It tells me my RMF is 1.65, and my Rotation is 291rpm. Maybe I am using the calculator wrong, but in the Motor Characteristics section I enter 300 for RPM and play with the Torque value until I get close to my required RMF, which comes out to about .35lb-ft (67.2ozf-in according to this). I take this to mean how much torque I need to achieve my desired acceleration.

Now here is where I get confused (or maybe I was already confused above). I go to http://www.robotshop.com/dc-motor-selection.html and enter the same values. It tells me my torque is 2.0329ozf-in. I take this to mean how much torque I need to achieve my desired acceleration.

2.0329ozf-in is way less than 67.2ozf-in. Where am I going wrong?
« Last Edit: September 07, 2011, 05:12:19 PM by Croy9000 »

#### waltr

• Supreme Robot
• Posts: 1,905
##### Re: Motor selection question. Confused.
« Reply #1 on: September 07, 2011, 06:10:27 PM »
I put your numbers into both calculators. The one on the RobotShop gave me a torque of 4.06ozf-in, not the 2.03ozf-in you came up with.

This is still much different than the SoR calculator so I'll guess one or the other (or both) have errors in the calculations and/or make different assumptions. Both have a tutorial that goes through the calc's, maybe go through each to find the discrepancy.

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: Motor selection question. Confused.
« Reply #2 on: September 07, 2011, 07:04:24 PM »
Hi,

I put your numbers into both calculators. The one on the RobotShop gave me a torque of 4.06ozf-in, not the 2.03ozf-in you came up with.
The SoR calculator use wheel diameter, while the RobotShop calculator use wheel radius (which is why you got ~4ozf-in.
The rest of the differences is probably due to them not expressing the same thing.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

• Beginner
• Posts: 3