Regbot encoder analysis

Regbot encoder issues

The Regbot motors use a magnetic encoder at the motor axle, as shown below.

The encoder has 12 magnets and two HAL sensors. The sensors have a separation and position that should give symmetric A and B signals, as shown. If all magnet dipoles have exactly the same size and strength, then velocity can be estimated using the time between any two flanks. But this is not the reality.

A more stable velocity is obtained using the time between the same flank on the same signal, e.g. the rising flank on the A signal. This gives a velocity estimate at every flank, but the velocity is estimated over a longer time.

The average delay is half the time between two equal flanks on the same signal plus the time since the flank was detected.

The following plot tests this. A fast PI velocity controller is used to obtain stable velocities between 0.05 and 1.3 m/s. The motor gear is 9.6:1, and the wheel radius is 0.03m.

The magnetic encoder is used to calculate motor (and robot) velocity, which is estimated based on the time between encoder ticks.

At a robot velocity of 5 cm/sec, the average delay is often more than 30 ms. The velocity controller is not well designed for this delay.

The velocity is more stable at 10 cm/sec, and the average delay is less than 10 ms.

At 0.5 m/sec and faster, a repeating pattern is visible. This pattern is related to the velocity and is repeated for every motor rotation. This indicates that the magnets in the encoder are not totally equal.