Servomotor demo

Amy Conaway and Erick Haro built a brushless servomotor demonstrator

The upper disk is the rotor, carrying permanent magnet disks on its lower surface. Fixed stator coils drive the rotor.

The rotor also carries an optical encoder disk, which is read by LED/photodiode pairs. Two pairs read the A and B quadrature tracks, and one pair reads the once-per-revolution index track.

This photo shows the IR LED/photodiode pairs, and also a red and a green LED that indicate the polarity with which one of the stator coils is energized.
Electronics consists of four push-pull current drivers for the four stator coils
Click on the image to watch a quicktime movie of the motor. First the motor is rotated by hand until the index pulse is read. Then, you can see the varying polarity (red/green) and magnitude (brightness) of the current through each coil, as the motor rotates to a commanded 270 degrees and stops.
Here is another movie showing the signals from the A, B,and index tracks of the optical encoder. (A=green, B=yellow, I=red) as the encoder is turned slowly by hand. In one sense of rotation green follows yellow; in the other sense of rotation green precedes yellow.