# choosing the suitable DC motors

I'm working on a 4x4 robot that is supposed to carry stuff total_mass=100kg (including the robot). the robot should have average speed 20km/h
assume that inclination_angle=0 acceleration=0.5m/s^2 wheel_diameter=0.25m direct drive (no gears) 4 motors 4 wheels

What kind of DC motors should i use (brushed or brushless or other)?
what are their properties? (torque, rpm ... combination)

I'm not so good at math and conversions.

• You're going to need to get better at math and conversions :-) It's not about speed and mass, without any resisting forces the tiniest motor can get any mass to any speed. What accelerations are you looking for? How steep of an incline is the robot expected to traverse? You need to know a lot more about the application before you pick motors. I would say regardless of all this brushless is almost certainly the technology of choice if we're talking electrical. – Guy Sirton Feb 15 '13 at 0:02
• When you face something like this and don't know where to start, find something similar - ie, other people's documented projects, or look at outdoor-mobility electric wheelchairs. That will give you a good starting point / sanity check for your conclusions. – Chris Stratton Feb 15 '13 at 0:09
• thanks @GuySirton, i'm trying to find the minimum requirements then i'll go up from there, so we can assume that inclination_angle=0 and acceleration=0.5m/s^2 – mabdrabo Feb 15 '13 at 0:10
• @ChrisStratton that's what i did exactly, i found electric wheelchairs the closest to my case, the thing is wheelchairs are 2WD, in my case it's 4WD. and i don't really know how is force and torque distributed, and couldn't find the right terms, so googling didn't help – mabdrabo Feb 15 '13 at 0:14
• @mabdrabo: OK. So we know F=ma right? So 0.5*100kg = 50N. 4x4, are you going to have a motor per wheel? What is the diameter of your wheels? Are you planning to direct drive or use a gear box? Torque is given in Newton-Meter so that would be the force one meter away... – Guy Sirton Feb 15 '13 at 0:49

Let's work out the basic physics of the situation (note this assumes the rotating mass is zero, a reasonable first approximation for this system):

F = ma

F = 0.5m/s^2 * 100Kg = 50N

50N / 4 wheels * r = 50 / 4 * 0.125 = 1.5625N-m (Thanks for checking my math Dave)

The circumference of the wheel is pi * 0.25 ~= 0.785m

20km/hr ~= 25478rph ~= 424rpm (shouldn't be a problem for a brushless motor)

So four brushless motors that have this torque in this speed range should nominally meet the requirements. You would normally take a fairly large margin (e.g. x2). If you want to drive uphill or over uneven terrain you need to consider gravity.

A good reference for the physics with handy calculators is: http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html

• @Guy What is 'r' here? – Umar Jul 2 '15 at 7:59
• @Umar r is for radius (1/2 the diameter) – Guy Sirton Jul 5 '15 at 20:41