Low rpm, high power calculations help

[JCbullsallday]

Hi guys,

Im trying to build an electric wheelchair with mecanum wheels, and as you might know, each of its 4 wheels has to be independently powered for the chair to have holonomic motion.
With my desired speed to be around (brisk) walking speed only, say 1.15 m/s, and my mecanum wheels sized with a 4-inch radius, I know that I'm going to need a very low RPM (by my computations, ~110 rpm).
However, my computed power draw per wheel is around 300W, which is way large, that Im not sure if Im on the right track? My load capacity is somewhere around 136 kg, btw.

This brings me to my question: are my torque/rpm/power calculations correct??

per wheel:
r = wheel radius = 4 inches = 0.1016 m
circumference = 2pi(r) = 0.638 m
v = (max) running velocity = 1.25m/s
rpm = v(60/circumference) = 117.48 rpm

the chair:
u = coefficient of friction = ~0.7
N = normal force = mg
m = total mass (136 kg)
g = 9.81 m/(s^2)
vi = 0 (rest)
t = time to accelerate, say 2 s
a = acceleration = (v-vi)/t = 1.25/2 = 0.625 m/(s^2)

Finally:
Ftotal = uN + ma = m*(uN+a)
Fwheel = Ftotal/4 = 255. 248 N
Twheel = (Fwheel)(r) = 25.93 Nm

Pwheel = (Twheel)(rpm)(2pi/60) = 319 W

Is the power really supposed to be this large and my RPM that low???

Help/corrections would be very much appreciated!!

[asid61]

You put the u as 0.7, which seems really high for something that is rolling. That could be your problem.

[pilleya]

Using this calculator may make your calculations easier:

http://www.chiefdelphi.com/media/papers/3188?langid=2

A normal electric wheelchair, has one 200watt motor on each side and they usually have 10inch wheels and can go at least 5mph. If your wanting 1.15 m/s with 8inch diameter wheels you will only need about 110rpm. However your going to need a gearbox with a very large reduction to accomplish that in most cases, motor dependent.

1.15m/s is definitely not a brisk walking pace( thats only 4km/h) you might be better aiming for about 6-7km/h.

Have you thought about what motors you are going to use and how you are going to interface them to your wheels?