State interface with automatic coordinate transformations

[flixr]

A general state interface that holds all the most important vehicle states like position, speed, attitude, etc. for fixedwings and rotorcrafts alike. So any subsystem/module that needs to know any of these states should not get it directly from the ahrs, gps, airspeed sensors or whatever and just read it from the state interface. It will also do the appropriate coordinate and fixed/floating point conversions automatically and only when needed. This should decouple e.g. estimation and control more and also the "consumers" of vehicle states don't need to worry about where the data is actually coming from and in what format it was estimated or measured.

Current status:

• Most of the state interface and needed conversions are done
• You can set e.g. the position in any coordinate system you wish: stateSetPositionNed_i(pos_ned_i) to set the position in fixed-point NED coordinates
• If you need to read the position somewhere else in a different representation, simply call: stateGetPositionLla_f() and only then the LLA float position representation is calculated and returned on the fly. It's also only calculated once, until a new position is set.

There are however some open questions that need addressing before we go further:

• How to deal with the problem that a given representation can't be calculated or is not valid, e.g.
• something needed for conversion not initialized (e.g. LTP origin not initialized)
• simply no measurement of the state available (e.g. no airspeed sensor)
• state not valid (e.g. no 3d GPS fix, hence pos/speed not valid)
• Add extra functions to set multiple representations of the same state at once, or allow to set a state without marking the other representations as uncalculated?
  • E.g. when the GPS already does the work and calculates several representations, why not use that instead of only setting one and doing the conversions again ourselves...

See feature request #101

[gautierhattenberger]

How to deal with the problem that a given representation can't be calculated or is not valid

Providing test function (e.g. isPosValid(),...) should be enough for the critical parts to test themselves if they should start or go to some critical/safety mode

• ex: if the ahrs is not yet aligned, the attitude state is not valid and the control cannot run (or the motors can't start at all ?)

Add extra functions to set multiple representations of the same state at once, or allow to set a state without marking the other representations as uncalculated?

add a set(*format1, *format2, ...) function that can set all formats and only set the one that are not pointers to NULL ?

• ex: setPos(&ltp, &ecef, NULL, NULL) set the ltp and ecef representation but not utm or lla.

I think it would be good to merge this as soon as possible. It looks that the major parts needed to use it on rotorcraft are done and must be tested by as much people as possible (eg: merge into master)

[flixr]

Since this is supposed to become a very stable API, we should make sure we choose the name wisely.
Any comments on the names of the get/set functions? CamelCasing ok?
Maybe rename speed to velocity where appropriate?
So stateGetVelocityNed_i() instead of stateGetSpeedNed_i()
Since speed normally refers to the scalar quantity and velocity to the vector.

[gautierhattenberger]

I'm fine with replacing speed by velocity and use speed only for the norm of the speed vector.

I think we can keep the current casing. Maybe the question is do we keep state in front of all functions. It could make the code lighter but more ambiguous.

[flixr]

I think I would keep the state prefix for clarity at the expense of slightly longer names.
Feel free to change stuff (like speed to velocity, the valid functions, set functions for multiple ones, etc), as I don't have time to work on this right now....