# The encoder magnet

The encoder reads a diametrically magnetised magnet on the rear end of the motor shaft. The magnet must sit
centred on the shaft's axis, parallel to the board, about **1 mm** above the encoder chip, and it must not move.

![About 1 mm between the magnet and the encoder chip in the middle of the board.](https://docs.source-robotics.com/STEPFOC-docs/108/assets/STEPFOC_MAGNET.png){ width="700" }

## The magnet

A neodymium disc, **diametrically** magnetised (north and south across the diameter, not through the thickness).
The shop's [6 × 2.5 mm diametrical magnet](https://source-robotics.com/products/6x2-5mm-diametrical-neodymium-magnet)
is the size the printed spacers are made for. An axially magnetised magnet looks the same and does not work: the
magnet test may even pass, and the position is wrong.

!!! note "The size is not fixed"
    6 × 2.5 mm is the stock size, not a requirement. Any diametrically magnetised disc of about 6 mm diameter
    works, and the thickness is a way to set the gap: a thicker magnet brings its face closer to the encoder, a
    thinner one farther, so with a shaft or a spacer that leaves the wrong distance you can pick the thickness
    that gives 1 mm instead of changing the spacer. What matters is the 1 mm from the magnet's face to the chip,
    the magnet centred on the shaft, and the diametrical magnetisation.

## Fitting it

1. The motor needs a rear shaft. Most NEMA 17 motors have one; a motor without it needs a magnet holder on the
   front shaft and a different mechanical layout.
2. Press the magnet onto the shaft centred. The [magnet tool](https://github.com/Source-Robotics/STEPFOC-stepper-controller/tree/main/STEP%20files/Tools)
   in the firmware repository is a printed jig that centres it; a 3D-printed magnet spacer from
   [Stepper motor accessories](https://github.com/Source-Robotics/STEPFOC-stepper-controller/tree/main/STEP%20files/Stepper%20motor%20accessories)
   fits between the shaft and the magnet when the shaft is too short.
3. Fix it with a drop of glue. A magnet that can turn on the shaft is the classic cause of a calibration that
   reports success and a motor that then runs badly: the magnet slipped between calibration and use.
4. Check the height after the board is mounted: with the spacer for your motor length, the gap is 1 mm. A magnet
   that touches the chip can destroy the microcontroller; a magnet 2 mm away or more gives a weak signal.

## Checking it

With the driver powered and the serial terminal open, `#Magnet` answers `Magnet Good` when the chip sees enough
field, `Magnet Bad` when it does not. The calibration routine runs the same test first. 

!!! warning "What the magnet test checks"
    Only that a magnet is close enough to the encoder. It passes with a magnet that is not glued, sits off-centre or
    can slip on the shaft, and fails only when there is no magnet or it is too far away. The 1 mm gap, the centring
    and the glue are yours to check.