Measuring Lego Motors with Wedo

If you've read through Lego's intro to wedo material (and the related simple mechanical machines) you'll find its an excellent introduction to gearing and ratios. Using a big and a small cog you get taught how to speed up, slow down and reverse the direction of rotation from either a hand crank or a motor.

But that section of the material doesn't actually use the Wedo... perhaps we could use it to put some numbers on those observations! How fast does a lego motor actually turn, and do the gear ratio's do what they're supposed to?

Here's the experimental setup:

The motor (driven by the wedo) drives the propeller blades which pass in front of the distance sensor (motor on connector 2, sensor on 1).  The first thing to do is check that the setup is viable:

make wedo device

make wedoConnector number

make wedoValue number

when start

.set wedoConnector to 2

.set wedoValue to 0

.tell wedo to "setMotorSpeed"

.forever

..set wedoConnector to 1

..forever

...tell wedo to "readDistance"

...say [wedoValue]

...wait 0.5 secs

This just turns the motor off, and prints out the reading from the sensor. Turning the blades slowly by hand shows that the value reported by the sensor does change. While we don't really care about the exact value, we can detect that the blade passes in front of the sensor when the value drops below 0.1.

when start

.set wedoConnector to 2

.set wedoValue to 0.3

.tell wedo to "setMotorSpeed"

.forever

..set wedoConnector to 1

..repeat until wedoValue >0.1

...tell wedo to "readDistance"

..say "ping"

..repeat until wedoValue <0.1

...tell wedo to "readDistance"

..say "pong"

Now we can check to see if there's a blade present - wait until there isn't (print ping), wait until there isn't (print pong). As the motor turns we're not tracking its rotation.

when start

.set wedoConnector to 2

.set wedoValue to 0.3

.tell wedo to "setMotorSpeed"

.forever

..reset timer

..set wedoConnector to 1

..repeat until wedoValue >0.1

...tell wedo to "readDistance"

..repeat until wedoValue <0.1

...tell wedo to "readDistance"

..say [1/(timer*2)]

Now we just measure the time it takes us to go around the loop. As there are two blades to the propeller, each iteration represents half a turn. A full turn takes twice that. We could print that out as time per revolution, but if we take the reciprocal that gives us revolutions per second.

For my motor (regular Wedo M-size) 0.3 is about as slow as it can go (in fact it takes a nudge to get started at this speed). We can measure speed for a few power settings:

0.3 Power = 0.85rps
0.5 Power = 1.55rps
0.75 Power =2.5rps
1(full) power=3.5rps

Theres a bit of variation, as the wedo probably only updates the reported value on a periodic basis, but the results are fairly consistent and representative.

Now lets mod the setup, and add an extra gear. This makes the fan turn faster (according to the lego introduction to cogs!)

0.3 Power = 2.5
0.5 Power = 4.6
0.75 Power = 7rps
1(full) power=10.4rps

In each case the new speed is almost exactly three times the previous one... The exception to that is the 0.75 which is a little low... However looking at the raw data for this particular measurement, it seemed to present the most variation 6.94 was the most common reading, but it was also the lowest, with occasional readings of 7.6 and 7.9 - no other reading had any where near such a wide variation, so it looks like this speed doesn't mesh well with the underlying sampling rate - lets just put it down to experimental error.


Now the moment of truth... Lets count the number of teeth on the cogs: The small one has 8, while the big one has 24... a factor of THREE!!!! One turn of the big cog turns the little cog three times, so the blade spins three times faster.
Awesome!