Motor Repeatability Test - Lego Mindstorms NXT
Channel: Matt Bernhardt
Date: 26/12/2013

As I've worked with the Lego Mindstorms platform, both the printer design published by David J. Perdue and Laurens Valk (seen in http://www.youtube.com/watch?v=H6p9-e9wgLk&list=PLA74F9ABD50B3392D ) and the standard car design (seen in http://www.youtube.com/watch?v=-Or0xmJpCWk&list=PLA74F9ABD50B3392D ), I've become interested in the repeatability of the motors.

To help explore this, I put together a simple swing-arm system to measure repeated action. I wanted to see where the arm stopped each time, and how that stopping position changed as motor power varied.

While the overall stopping position was relatively close to the reference line at each power level, the stopping position on any given swing during the program was significantly beyond the reference line. This has ramifications for operations that require consistent positioning, like the beginning and ending of printing operations when the pen begins or ends a drawing operation.

As shown in these trials, one strategy to compensate for this over-swing is to lower the power to the motor. Other strategies could be to add gears to require greater motor rotation for the same overall motion.

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