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Add work coordinate system documentation (G54,...)

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hzeller committed Jul 6, 2018
1 parent fa902ce commit 3e0fe99f7d39266fa233b5ae6dc182ed52dd5483
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* `X- Y-` - end point of spline (absolute or relative depending on current mode)
* `I- J-` - relative offset from start point to control point

### Coordinate Systems

#### Machine Origin
The Machine Origin is defined to be where the end-switches are located, i.e.
where the `home-pos` is configured for that particular axis in the
configuration file (see [sample.config](sample.config)). Together with the
range with each axis, this defines the available machine cube within the
tool can move.

The `machine-control` program will always make sure that the machine is never
allowed to execute moves that escape this machine cube.

Often, the home position is not at the bottom left corner of the machine cube
as one would expect in a simple coordinate system with only positive
coordinates. In particular CNC machines for instance have the Z-axis origin
at the very top (as homing at the very bottom will certainly crash the spindle
somewhere). So relative to the _machine origin_, only negative Z coordinates
are valid in that case.

Let's assume a machine that has the X axis homed on the left, but the Y axis
on the back of the machine cube and the Z axis on the top; in the
[gcode2ps]-visualization, this shows a little right-handed coordinate system
marker, colored in Red (X-Axis), Green (Y) and Blue (Z) at the left/back/top of
the machine cube:

<img src="img/machine-origin.png" width="50%"/>

#### Work Coordinate Systems

BeagleG supports 9 work coordinate system slots, G54-G59 and G59.1-G59.3.

The position of these coordinate system can be set using the `G10` command
according to the G-Code standard with `P1` to `P9` referencing the work
coordinate systems `G54`, `G55`, etc. When setting the
coordinate system, the X/Y/Z values passed to G10 always are relative to
the _machine origin_. Let's set the G55 coordinate
system (`P2`). We want it to be at the bottom of the machine cube, 100mm in X/Y
direction from the left bottom corner. Given the homing position of our
sample machine, this is partially in negative coordinates:

G10 L2 P2 X100 Y-100 Z-150 (100mm to the right, 100mm to the front and 150mm down)

<img src="img/set-g55.png" width="50%"/>

The coordinate was given in this case in the context of common absolute
coordinates (`G90`).
If coordinates are chosen to be relative with `G91`, we are _updating_ the
coordinates _relative_ to the previous location of the work coordinate system.
So with a

G91 G10 L2 P2 X50 (50mm to the right)

We are moving G55 work coordinate system 50mm to the right relative to its
previous location:

<img src="img/set-g55-update.png" width="50%"/>

#### Default work coordinate system

If you have specified a parameter file (with the `--param` option to
`machine-control`), the last used coordinate system including its configured
position is chosen.
Otherwise `G54` is the default coordinate system originating at the machine

So in particular if you are having a machine origin at an inconvenient spot,
it is a good idea to supply a parameter file to persistently configure your
coordinate systems.

### M Codes

Command | Callback | Description
@@ -235,3 +308,4 @@ for the user to handle (see description in API).
[RepRap Wiki]:
[Intro GCode]:
[gcode2ps]: ./
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