Improve rotational coupling

[tobolar]

I don't understand the meaning of the component Modelica.Mechanics.Rotational.Components.Coupling introduced with #3786. Even reading the documentation does not help.

Considering the Coupling icon, I would expect similar (or even the same) behaviour like Modelica.Mechanics.Rotational.Components.Disc, since it makes an impression of a gear shift (formschüßige Kupplung). Regarding behaviour (equations), it behaves exactly like Modelica.Mechanics.Rotational.Components.IdealGear with ratio=-1.

The example Modelica.Electrical.Machines.Examples.DCMachines.DCPM_Drive even works fine with Rotational.Components.IdealGear idealGear(ratio=-1).

[AHaumer]

Consider coupling two electric machines at the shaft: one of them rotates clockwise, the other one counter clockwise.
Yes an IdealGear with ratio=-1 does the same job, but it's not intuitive especially for beginners.

[tobolar]

I wonder how the real mechanism looks like. "two electric machines at the shaft" - this is what confuse me, because one shaft could hardly rotate in different directions.
The icon reminds me of a jaw coupling, but the behaviour of the component seems be different.

[AHaumer]

Jaw coupling is the right idea:

For electric machines, positive direction (right) is defined clockwise when looking at the front of the shaft end.
Therefore one machine rotates cw (right), the coupled machine ccw (left).

[beutlich]

I am with @tobolar here: If Coupling simply is a specific paramerization of IdealGear, it should be either removed again or - if really requiered - implemented as extension (e.g. short class definition) to make this dependency transparent.

[AHaumer]

Well a coupling primarily is not an IdealGear, so extending (including inheriting the icon) is not a good idea.
Could use the IdealGear inside the coupling model with its own icon.

[AHaumer]

The misunderstanding stems from the following:
Looking at the system, both coupled machines turn in the same direction.
Looking at each of the machines separately, one machine turns left (clockwise), the other one turns right (counterclockwise).
This affects the induced voltage as well as control of the machine.
Control of one machine doesn't take into account the other machine (the load).

[tobolar]

@AHaumer I can follow your explanation.
Considering again the example Modelica.Electrical.Machines.Examples.DCMachines.DCPM_Drive is also helpful: when connecting dcpm1directly to dcpm2, it would result in the same rotation of both machines.

So a solution is needed to "induce" an opposite rotation.

I see several possibilities.

1. To introduce a flag in the machine(s) to enable changing the sign between the mechanical and electrical torque/rotation. Then, the machines could be connected directly.
2. Since Coupling is a particular solution for a particular example, it could be moved e.g. into Modelica.Electrical.Machines.Examples.Utilities. As a mechanical component Modelica.Electrical.Machines.Losses.Friction also does.
3. To use IdealGear with ratio=-1 instead of Coupling. The model is mechanically wrong but works as expected. Btw. this is the case of Coupling in the current implementation as well.
4. Rename and redesign icon (jaw coupling-like is misleading) of Coupling and enhance the documentation to be clear what is the component intended for.

[AHaumer]

@tobolar you're right with your introductory explanation.

1. I would like to avoid the flag - in a real machine there's no such flag, and many users won't understand the meaning of it.
2. Coupling is not specific for electric machines, rather for all machines where direction of rotation influences the behaviour.
3. The Coupling is not an IdelaGear.
4. Why do you dislike the icon? I can imagine to enhance documentation and icon, and instead of writing equations, I could connect the IdealGear internally (not extend from!).
   I'd prefer No. 4 ...

[AHaumer]

Addressed by #4019 -> closed.

[tobolar]

I also tend to 4.

Why do you dislike the icon?

Well, the icon make an impression the component behaves like a jaw coupling, but it, in fact, behaves quite differently. This is simply misleading to users.

[AHaumer]

Well, actually the device acts lika a jaw coupling, indeed.
It's only a matter of how cw and ccw rotation is defined for the coupled machines.
For me it's completely clear - maybe electric machines are the only ones for which this is defined.
But I can't imagine that this is not defined clearly for other machines, e.g. pumps, fans, etc.
If a pump is only allowed to be rotated cw, you have to operate the electric machine correctly.
No I'll keep the jaw coupling icon.