# Society of Robots - Robot Forum

## Electronics => Electronics => Topic started by: Croy9000 on September 07, 2011, 02:04:02 PM

Title: Motor selection question. Confused.
Post by: Croy9000 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 (http://www.societyofrobots.com/RMF_calculator.shtml):

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 (http://www.unitconverters.net/torque-converter.html)). 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 (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?
Title: Re: Motor selection question. Confused.
Post by: waltr 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.
Title: Re: Motor selection question. Confused.
Post by: Soeren 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.
Title: Re: Motor selection question. Confused.
Post by: Croy9000 on September 07, 2011, 08:01:20 PM
The rest of the differences is probably due to them not expressing the same thing.

Yep. Although one seems to indicate I need an a significantly more powerful motor than the other. I am sure I am just missing something though.