Rest Angle behaviour in Hinge component

[virvine (Veronika)]

Hello,
I am a new user and I am trying to understand the behaviour of the Hinge component so that I can model a form finding project using ribbons.

I have created a simple example to understand how the hinge works (see attached).

BasicHinge.gh (12.1 KB)

What I don’t understand is what happens when the rest angle becomes greater than 0.75PI. If I drag the “Angle Factor” slider around, I can see the hinge open and close and it behaves well for all values between 0 and and up to (but not including) PI (mesh vanishes at PI). But when I set the RestAngle to 0.76PI and reset the solver, the mesh starts flopping around and never settles down. What is happening above 0.75PI?

Note: If I grab a tip of the hinge as it is flopping around and move it slightly, it settles down and behaves nicely again.

[DanielPiker]

Hi Virvine. This reply explains a bit the issue:

Overshooting problem and kangaroo 2 clarifications

So for the hinge example - one factor here is that this goal uses the signed angle in the plane perpendicular to the hinge axis. This is important so you can distinguish between mountain and valley folds in origami for example, or to distinguish between convex and concave configurations for a shell. Sometimes you need to control not just the angle the hinge ends up in, but which way around it goes to get there. It does mean though that in the special case where the rest state of the hinge has …

The issue is that the hinge measures the current angle in a plane (perpendicular to the fold edge), and it can be between -pi and +pi. This is sometimes useful for certain applications, but in other cases is a liability.
If the target angle is close to pi, the momentum can carry it slightly past pi, so it folds through itself and then has a new angle of almost -pi, so thinks it has to make a full revolution to get back to the rest state, causing it to start spinning.

I think I could make a modification to the hinge goal to avoid this, such as detecting when it jumps more than pi in one iteration, or just have a boolean option to always take the short way round.

[DanielPiker]

Okay, this fixes it. It’s a scripted goal, so you’ll need to set the assembly reference location by right clicking the C# component>Manage Assemblies, then locating your copy of KangarooSolver.dll (typically in C:\Program Files\Rhino 6\Plug-ins\Grasshopper\Components).
HingeFix.gh (11.5 KB)

[virvine (Veronika)]

Thank you! It works very nicely now.