vEx PWM Motor Response

[yoyodyne]

Last night I measured the VEX motor speed vs. PWM setting to build a linearization look-up table. I basically got the non-linear response I expected but what I didn't expect was that the maximum motor output actually occurs at around 220 for "forward" and 15 for "reverse". Has anyone else noticed that the motor output drops off slightly at PWM settings of 255 or 0 relative to 220 and 15 for instance? The attached plot shows the test results. The right and left robot motors were each set to a PWM value. After 500ms, the number of VEX encoder ticks over the next second was recorded. The program walked the PWM setting from 127 to 255 and then back down to 0 so there are a double set of points on the scatter plot from 127 to 255. There was some load on the motors from the driving the wheels via a chain and the shaft encoders themselves via a second chain but not a lot of load. Any additional experience in the area of linearizing the PWM to motor speed/power would be appreciated.

Thanks,

Greg