Evil Robotics

2/22/03  Do the Math

I found the motor specifications in the Enigma Industries website.  Based on their information I will calculate the rpm at 12 VDC from 9.6 VDC. 

Question 1:  What does this do to the typical specifications supplied by Enigma Industries? 

To get typical no load angular velocity given angular-velocity constant is 55.6 rpm/volt.

Since velocity is squared the new peak power can be calculated as a ratio

Now on to the estimated stall current,

Note this does not mean the motor will handle the heat. That will need to be tested.  I will need to set up a temperature meter on a motor, run 12 volts and check to see if the heat generated exceeds the wire insulation rating.  I'd like to keep it under 200° F.

Question 2:  If a 3” diameter wheel is rotated 667.2 rpm assuming no wheel slippage, what is the top end speed?

Question 3: How many Hp will it take to move a 12 lb mass from 0 to 5.95 mph (8.73 ft/s) in 0.25 seconds?

mass = m = 12 lb

velocity = v = 8.73 ft/s

time = t = 0.25 s

This makes a big assumption that the motor gear ratios are set properly for the maximum velocity.  Lower gear ratios will increase the torque to the wheels (better in a pushing match) and shorten the time to reach the maximum velocity, but the maximum velocity will be lower.  Higher gear ratios will increase the top end velocity but sacrifice acceleration time and pushing power.

My gut feeling is that this tells me that a drive system with two motors will be enough to push my robot and the opponent if the opponents wheels are off the ground.  If I used four drive motors then I might even be able to push some opponents in a straight up pushing match.

The top end speed of 5.95 miles per hour is a bit slow.  Some ways to change this would be to get a different gearbox, or increase the wheel diameter.  Getting a different gear box would be too difficult or too expensive.  Going to a four inch diameter wheel would increase the weight of the wheels.  Since I am not willing to sacrifice either of these design compromises, I will have to live with the current design speed.