# Homing and mastering

Stepper motors without encoders know nothing about where they are when the power comes on. Two procedures give
PAROL6 its position: homing, every time, and mastering, once per robot.

## Homing

Each joint drives to a switch or sensor whose position in the mechanics is known from the CAD model, then moves a
fixed number of steps from there to the after-homing pose. After that the firmware's step counters hold the joint
positions, and every move is counted from them. The process is the one 3D printers use. The sequence and what
can go wrong are in the Robot Manual's [Homing](https://docs.source-robotics.com/PAROL-docs/1.1/05-operation/homing/).

In the software: the commander's **Home** button, the API's `home()`. The API tracks whether the robot is
referenced and refuses planned motion before it (`MOTN_NOT_HOMED`); a `home()` on a referenced robot is a
planned joint move back to the after-homing pose, and `home(calibrate=True)` runs the firmware's sequence again.

## Mastering

The fixed number of steps from the switch to the after-homing pose is measured on the reference robot. Printers and
switches vary, so on another robot the homed pose is a fraction of a degree off per joint. Mastering measures the
error with the witness marks, pins through holes in neighbouring links that line up in the after-homing pose, and corrects
the number in the firmware ([Mastering](https://docs.source-robotics.com/PAROL-docs/1.1/05-operation/mastering/)).

![Witness marks on PAROL6](https://docs.source-robotics.com/PAROL-docs/1.1/assets/parolw.webp){ width="650" }

Mastering affects the **absolute** accuracy, whether the pose the software reports is the true pose. It does not
affect the **repeatability**, whether the robot returns to the same place every time, which comes from the
mechanics and the switches alone. A program taught on an unmastered robot runs correctly on that robot; it runs
offset on another.

## When the position is lost

A collision, a stall, a move by hand or a power failure breaks the count, and nothing in the robot can tell.
Home again. A program that must not run with a wrong position checks something external before it starts: a
sensor on an input, a camera, a touch.