How do I determine what size drive motor I need to run a robot I want to build??

Question

ok.... help please!! thanks =)

Answer 1

start with the high end of the weight class you are building for. Then consider the arena and how quickly you want to get across. just for an example consider a 100lb robot operating in a 50ft square, and say you want to cross it in 4 seconds. if we assume a low end motor that doesn't get the robot up to speed in 4 seconds, rather it accelerates the whole time, then we can figure out accel and force. It's easier to work in metric. 100lbs = about 40kg, 50ft = about 15.25m
relating distance to acceleration from 0 is d=1/2 at^2 or 15.25=1/2 16 a. a = 1.9m/sec. We'll use 2m/s for easy math from now on..
since we need to accelerate 40kg at this rate, by f=ma, f=40*2 = 80 kgm/s^2 = 80N
now consider the wheels. the radius of the wheels affects how the torque of the motor is transferred . Assume the wheels are about 8inches across, the radius is 4inches or 10cm. multiply the required force by the radius to get the required torque. in the example 80N x .10m = 8Nm. that is the power rating of the motor you want.
having done battlebots, I would consider this the minimum, and look at twice that for the maximum. that gives the range of motor power 8Nm to 16Nm. you might also see motor ratings in ounce-inches (oz-in). 1Nm = about 142oz-in.
your plan is will give you different values. and if you are running 2 motors (1 each side) then you can drop the torque rating in half for each motor as both motors will help accel the bot across the ring.