# Motor sizing problem - Online calculators

Hi everyone,

I need two motors for my new robot and I used two online calculators to make the motor selection:

but I've got different results.

My input data:

• mass 7Kg
• 2 drive motors
• wheel radius 0.037 m
• velocity 0.38 m/s
• acceleration 0.2 m/s^2
• efficiency 65%
• maximum incline angle 20 degree

Results:

• Society of Robots: RMF 2.02 kg*m*rps, zero incline 0.214 kg*m*rps
• RobotShop.com: torque 3.87 kgf-cm

A motor like Pololu 67:1 Metal Gearmotor (150 RPM free running, 14kg-cm stall torque), according to Society of Robots is good only for zero incline angle and according to RobotShop.com is overkill even for 10 degree angle.

Which one should I trust ?

I looked at both

I looked at both calculators, plugged your data (5kg, 2 wheels, 0.74 dia., 1m/s vel., 0.1m/s^2 acc., 75% eff., 0 incline) and the first thing I notice is the fact that the SoR calculator does not give you exact motor data, but let’s you verify that your motor of choice would fit the required RMF. The RobotShop calculator includes battery data that I think is good for moving conditions only. At start, the motors will absorb a lot more current, since they need to break the static momentum. Anyway, the motors from Pololu are very good for your purpose, but I would suggest to get the ones with encoders. From practice, I discovered that home made encoders are not so reliable, but it all gets to your tools and materials at your disposal, plus your skills. People use the 29:1 motors for balancing robots because the quality of the motors is good and they have little backlash. Just make sure you get a high amperage motor driver and you’re good. Also, try to get a battery from a 12V cordless drill, that will at least halve your battery weight (and comes with a charger too).

Thank for the answer, I am

Thank for the answer, I am tented by the motors with encoder but a little worried by the 50 vs. 80 US\$ which translate in more then 30 euro with taxes. I will think about it.

22.01.2011

I agree with you, home build encoders may not worth. I think I will buy the motors with encoders however I am a little bit worried about the very high resolution of the encoders and the speed needed to read them.

I have a friend that is

I have a friend that is using an arduino compatible board for his AWD as a PID motor driver with the faster motors (350 RPM) and he said that using interrupts it’s working. Take a look here.

Thank you Ro-Bot-X for your

Thank you Ro-Bot-X for your advices regarding the motors. I have talked with an importer company to bring me the encoder version of the 67:1 Pololu motors - it will take like a month

I will come back with impressions about the motors.