[Rock-dev] Finding a common robot body coordinate system

[Jakob Schwendner]

On 02/14/2011 03:38 PM, Christopher Gaudig wrote:
> Hi!
>
> As there seem to be at least three different coordinate systems in use
> by people I talked to, I would like to start a discussion which CS we
> should declare as a "common standard" for rock. The ones used so far
> seem to be:
>
> a) x rightward, y forward, z upward (used in at least one DFKI project
> and at my alma mater)
>
> b) x forward, y rightward, z downward (ISO 1151 / DIN LN 9300 standard
> for aircraft, SNAME 1950 standard for marine vehicles, used in Fossen:
> Marine Control Systems)
>
> c) x forward, y leftward, z upward (used in at least one DFKI project
> and for automotive applications)
>
> I would like to find out what you guys think about these or if you have
> additional suggestions. Does any of the three seem more (or less)
> intuitive to you?
For imoby we have used a) for the robot coordinate system, and analogous 
to it ENU (East North Up) as the world frame. Reasoning for it:
- Right handed is more common than left handed, and has some other 
advantages.
- I like my head to have a more positive value than my toes unless I am 
doing a handstand. Up positive is the most intuitive for me.
- The forward movement I also like to be positive.
- Left/right I guess could be argued, but also I find right positive 
more intuitive.

-> The above constraints lead to a right-front-up frame for a robotic 
system. And using East North Up as the world frame makes sense then.

> I know that choosing a common CS will mean some work for some people,
> but will later save lots of debugging time I guess.
I agree. Also, it's not so hard to convert between frames, in order to 
interchange with other people that use other standards. I just makes 
life easier not having to do that.
> In addition I'd like to find out what you think about the use of the terms
>
> surge (fwd/back)
> sway (left/right)
> heave (up/down) and
>
> roll (rotate about surge axis)
> pitch (about sway axis)
> yaw (about heave axis)
>
> to describe robot movements without having to remember where x points to.
There are a number of names for these axis. I personally would not like 
to have any of them in the base classes, since I think they are domain 
specific. This also goes for the "euler" angles. It's very easy to get 
stuff wrong with them, and they are imho not a suitable means for 
representing orientations (for various reasons). For display that's 
another matter. Euler angles in degrees is actually the most intuitive 
textual representation I think.

The following representations are most suitable for interchange (and 
hence use in the base classes):
* translational parts (e.g. position, velocity, acceleration) is clear, 
I guess - 3-Vector
* orientation - quaternion
* small change in orientation - scaled axis of rotation
* uncertainty covariance of orientation - cov. matrix over scaled axis 
of rotation
* rotation speed - scaled axis of rotation
* rotation acceleration - scaled axis of rotation

Once we agreed on something here, we should probably have documentation 
page on the issue. We should define both the frames and the 
representations.

cheers,

Jakob