RPM measurement

I wanted to check motor speed (revolutions-per-minute or RPM) independently of the built-in speedometer.  For this, the Firefly can be laid horizontally on a bench with the wheel free to move.  A video camera records the movement and white tape around half of the wheel eases analysis.

Video processing (via Octave) yields a graph of the speed:

The raw signal is the average pixel brightness in a rectangular region covering the tire, for each video frame (60 Hz frame rate).  This signal is shifted to have near-zero mean (blue curve), and the zero-crossings (red dots) are found.  Two zero crossings correspond to exactly one wheel rotation, and their time-difference determines motor speed (RPM). For 36.0 V the no-load speed is 383.7 rpm.

During this test, the battery produced 36.0 V, and the display showed 21.8 km/h.  This implies a wheel circumference of 21800/383.7/60 = 0.947 m, which is only one percent less than my measurement of 0.958 m.  Conclusion: The built-in speedometer is accurate.

Analysis code

The motor (probably a high-RPM, 12" wheel version of the Q100 at BMS battery) contains 16 poles (8 pairs) and has a reduction gear ratio of 8.2.  Confirming these numbers is that parameter P1 = 131 in the LCD setup (8.2 x 16 = 131.2). Therefore, the commutation speed or eRPM for a fully charged battery is 8*8.2*383.7*41.8/36=29200.