Add geometry file docs

en/SUMMARY.md

@@ -281,6 +281,7 @@
     * [Flight Modes](concept/flight_modes.md)
     * [Flight Tasks](concept/flight_tasks.md)
     * [Mixing and Actuators](concept/mixing.md)
+    * [Geometry Files](concept/geometry_files.md)
     * [PWM limit state machine](concept/pwm_limit.md)
     * [System Startup](concept/system_startup.md)
     * [SD Card Layout](concept/sd_card_layout.md)

en/concept/geometry_files.md

@@ -0,0 +1,80 @@
+# Geometry Files
+
+- Associated dev guide issue: https://github.com/PX4/PX4-Devguide/issues/349
+- Geometry files are found here: https://github.com/PX4/PX4-Autopilot/tree/master/src/lib/mixer/MultirotorMixer/geometries
+- They are TOML files
+
+@[youtube](https://youtu.be/orvng_11ngQ?t=1080) - Control Allocation - 18 minute in.
+
+[Slide 14 has info](https://static.sched.com/hosted_files/px4developersummitvirtual2020/1b/PX4%20Developer%20Summit%202020%20-%20Overview%20of%20multicopter%20control%20from%20sensors%20to%20motors.pdf)
+
+<!-- stuff just copied from the slide
+In a nutshell
+Desired forces
+and torques (m)
+?
+Actuator
+outputs (u)
+Control allocation
+matrix
+Actuator
+effectiveness
+matrix (B)
+Inverse
+Control
+allocation
+Matrix (P=B
++
+Moore-Penrose )
+-->
+
+
+## How to add a new geometry
+
+1. Create new TOML geometry file in [/src/lib/mixer/MultirotorMixer/geometries](https://github.com/PX4/PX4-Autopilot/tree/master/src/lib/mixer/MultirotorMixer/geometries).
+   For example "foo.toml".
+   with a new key (e.g.: key = "4fo") and add to **CMakeLists.txt** <!-- which one? -->
+
+2. In ROMFS/px4fmu_common/Create new mixer file **mixers/foo.main.mix** with a line containing the new key:
+   ```
+   R 4fo 10000 10000 10000 0  
+   ```
+   <!-- There was a comment that the numbers are not needed? Why are they needed in quad then -->
+3. Set the new mixer in **init.d/airframes/myconfig**
+   ```
+   set MIXER foo
+   ```
+
+
+## Fragment from some files
+
+Looking at the toml files they all have this first. So I am assuming file description follwed by info. 
+- Is key random?
+- Where is the description used?
+
+```
+# Generic Quadcopter in H configuration
+
+[info]
+key = "4h"
+description = "Generic Quadcopter in H configuration"
+```
+Then we have rotor default which always has this structure
+```
+[rotor_default]
+axis      = [0.0, 0.0, -1.0]
+Ct        = 1.0
+Cm        = 0.05
+
+direction = "CW" - except I Only see THIS line on Tri
+```
+Then we have one of these for each rotor - so 3 for a tri
+```
+[[rotors]]
+name      = "front_right_top"
+position  = [0.729, 0.684, 0.1]
+direction = "CW"
+```
+
+- How do I work out what each of these fields mean?
+- Are there geometries we can't capture? ie what if roto is on angle say?