joint controlled with AMO boards used as incremental encoder

[maggia80]

Description

We noticed it on the left pitch and roll shoulder in ergoCub1.1 S/N:001. The two joints are controlled in position and after a while even if there aren't any position commands the joint goes toward an HW limit. While moving the AMO reading seems to be blocked.
Sometimes the joint doesn't reach the HW limit, but it remains quite far from the previous position while reading a similar angular position.

Hypothesis

The AMO stops reading for some reason and the PID to reduce the error from the setpoint and the current position generates a PWM that moves the joint trying to reduce the error, but since the AMO is stuck to a fixed value, the error remains constant and the motor keeps moving.

Possible reasons:

1. the reading of the AMO is interpreted as a spike and the value is kept constant for a long period of time.
2. the AMO is not reading for some cable issues
3. the position of the AMO is too far or not well aligned with the magnetic target causing spikes.

[maggia80]

cc: @AntonioConsilvio, @ale-git @valegagge

[maggia80]

@AntonioConsilvio would be very helpful if you could log the encoder and motor position while this phenomenon occurs.