## Monday, March 26, 2007

### Race Day!

How do we run a race with slowbots?

You have been so successful at designing turtlebots that a regular ready-set-go! type of race is not practical. So, we will think out of the box to come up with new kind of race- a theoretical race.

You will race by figuring out your robot's average speed and comparing that to the other robots to see who wins the race. We will use 10 minutes as the length of the race. The robot that moves the shortest distance would therefore win the slowbot race!

How do we figure out who goes the shortest distance?

The motor runs at 375 RPM. (Revolutions Per Minute, 375rev/1min)
Let's assume the motor runs at 90% capacity due to drag from the weight of the robot.
My gear ratio is 13824:1.

So the 375RPM's is reduced by 10% which equals 337.5 RPM.

If you think of a wheel on a desktop, when the wheel goes through one full revolution, it has moved along the desk the same distance as its circumference (pi * OD or 3.14 times the outside diameter). Then, you just need to convert rpm to miles per hour taking care to watch your units.

OD is a short form of saying 'outside diameter'. For a tire, it's the distance across the tire from one edge to the other edge (not the hole in the center where the tire is mounted on the rim). Let's say you had a tire/wheel with a 24" OD. One revolution of the tire would mean that the tire rolls along the pavement 24" * pi or about 75.4 inches (if you marked a spot on the tire and put that on the ground and rolled the tire one full revolution so that spot would be on the ground again, the distance between the two points would be 75.4 inches). Let's then say the tire is turning at 100 rpm. Therefore, the distance the tire is covering is 7,540 inches per minute (75.4 inches per revolution * 100 revolutions per minute). Finally, you change that into what units you want.

So my tire is 1.5 inches diameter, now multiply that by 3.14 which is 4.71 inches. The RPM is 337.5 x 1/13824 = .0244 rpm's because the motor output RPM is changed by the gear reduction to .0244 RPM's. Now I multiply rpm by distance by time which is .0244 x 4.71inches x 10 minutes = 1.15 inches . Got it? What is the distance for your robot?