Diametric Magnet
A diametric magnet is a small cylindrical magnet whose poles are arranged across the diameter rather than along the axis; this geometry produces the field direction that a magnetic angle sensor measures. It is bonded to the very end of the shaft, centred on the axis of rotation, and faces the sensor across a fixed gap of a few millimetres.
In magnetic angle measurement the sensor reads not the magnitude of the field but its direction. For this reason the magnet's pole axis must rotate smoothly over the sensor surface as the shaft turns. An ordinary axially magnetised magnet does not do this job.
- Centring: if the magnet is off-axis, an angle error arises and the error varies over one revolution; position repeatability is degraded.
- Gap: mounting too close or too far takes the field strength outside the sensor's working window.
- Iron parts nearby distort the field; this is taken into account when choosing the bracket.
On the GS740-3P the magnet read by the AS5600 encoder is of this type and comes aligned from the factory. It is not a field job; if the servo body is opened and the magnet is moved, position feedback is corrupted and calibration must be redone.
Context in which this term is used: Electronic speed and throttle control on a diesel engine