### Author Topic: Motor Sizing using RMF Tool ... What am I doing wrong ?  (Read 1878 times)

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#### alphaCentaur • Beginner
• • Posts: 2 ##### Motor Sizing using RMF Tool ... What am I doing wrong ?
« on: January 21, 2014, 08:19:03 AM »
Hi everyone !

This is my first post here and I would appreciate your help.

I am trying to find suitable motors for a four wheeled robot and doing the following calculations:-

Robot Characteristics
==============

weight(w)=50 kg
wheel diameter(d)=8 inch=0.2032 m ==> wheel radius (r) = 0.1016 m
required velocity (v) = 1 m/s
required acceleration (a) = 0.1 m/s^2
inline angle=0 deg
no of wheels = 4
expected efficiency = 0.85 ( 85 % )

Calculations
=======

required force (F) = m x a = 5 N
required Torque (T) = F x r = 0.508 Nm
required Torque per wheel (Tw) = T/4 = 0.127 Nm
required torque for 0.85 efficiency Tw=0.149 Nm ==> Tw_kgf= 0.0152 kgf-m
required rpm = (V*60)/(pi*D) = 93.989 rpm ==> rps= 1.567 rps

RMF = Torque * rps = Tw_kgf * rps = 0.0239

However, the RMF calculator shows an RMF of 0.768 kg*m*rps. (screen shot attached)

What am I doing wrong here ?

Thanks.
« Last Edit: January 21, 2014, 08:30:50 AM by alphaCentaur »

#### jwatte

• Supreme Robot
•     • Posts: 1,345 ##### Re: Motor Sizing using RMF Tool ... What am I doing wrong ?
« Reply #1 on: January 21, 2014, 04:09:36 PM »
Note that the RMF calculated is for a single motor, and you divide the values in four for four separate driving wheels.

#### alphaCentaur • Beginner
• • Posts: 2 ##### Re: Motor Sizing using RMF Tool ... What am I doing wrong ?
« Reply #2 on: January 22, 2014, 12:22:56 AM »
@jwatte

thanks for the reply, but still there is a large difference between values:-

RMF (from RMF Tool)/4=0.192

RMF (from manual calculations)/4 = 0.005

there seems to be something wrong with my calculations maybe, but I am unable to trace it.

#### jwatte

• Supreme Robot
•     • Posts: 1,345 