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Calibrating Motors

Techbrick.com's Marco emailed me the following:

As we all know, one the endearing qualities of LEGO robotics is its inherent imprecision.

As good as the NXT motors are (and other components), they are, at their core, basic DC motors with gears. All of our motors (about 20 of them) all work slightly differently in terms of acceleration, power, braking, etc.

Yet it is essential that your motors (particularly paired driving motors) be as close as possible in performance specs.

So how do you do this without torque gauges, tachometers, etc.? Leu Beach, one of our new mentors, along with our programmers, Jonathan, Doug, Nate, and Cole, came up with the most amazing, simple, calibration system ever.

It took us about 15 minutes to test all our motors. The resultant matched set vastly improved the robots directional consistency and performance.

Read to be amazed? Read on....

http://www.techbrick.com/Lego/TechBrick/TechTips/NTXCalibration

Comments

Jetro said…
The 'explanation' says to put two motors on the same axle and "simply run both motors at the same rate. If one accelerates faster, runs faster, or breaks harder (on stop) it will kick up as shown. Pull off the second motor and try the next one."

It does not however say how to run them (with what programming language) and why or how this will make them calibrate so as to later turn at the same speed... Or did I not read carefully enough?
May 3, 2009, 7:38:00 AM
Chad said…
I read it to mean just try a different motor until you get two that match. I guess this would be possible in school settings where multiple kits are available. Maybe I didn't read it carefully enough either...
May 3, 2009, 1:20:00 PM
Find_Alan said…
Jetro - for their process you will need to create a program in NXT-G that's just one block long: a "move" block set for 360 degrees.

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A faster initial step to get started is to keep both motor outputs running constantly, attach a large gears to each motor, plug them in, and hold them together so the teeth mesh. You will actually feel if one is pulling the other... This is really fast to do, with our 12 motors we can get the first run of "matched" motors selected in 2-3 minutes.

Then you can stick them on a shaft... and use a standard "move" block to test for start and stop problems.
May 4, 2009, 3:36:00 AM
Jetro said…
basically you're not calibrating at all, just finding the most similar motors from a large pool of motors. Pity.. no good for the average user.
May 5, 2009, 3:22:00 AM
Dave Parker said…
Note that the Move block adjusts power to the motors individually to try to get them to match speeds, which may partially defeat the test. For determining a pair of matching motors, two Motor blocks would be better (both duration Unlimited, then put an empty Forever loop at the end of the program to keep the program from ending).
May 7, 2009, 4:02:00 AM
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