Grasshopper

algorithmic modeling for Rhino

# Tube Bending and Stretching in Kangaroo

Hello guys,

Can anyone know how to bend a tube/cylinder on Kangaroo? What I'm trying to do is to bend the tube and at the same time tube can be able to stretch. I have a look in many tutorials and example videos but most of them is how to bend line, curvers etc.

Thanks

George

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### Replies to This Discussion

Hello George,

Can you explain a bit more about what you are trying to do?

Are you trying to simulate some real material?

Hello Daniel,

Thank you for your reply. I'm trying to simulate something like Festo's Bionic Handling Assistant as the image url below:

In my case, I have a cylinder were it bends and lengthens when air flows. I'm trying to do the bending/Stretching part using Kangaroo but I found information only about curve bending etc. Please let me know if there is any source to help me understand how it works.

Thanks

George

Hi George,

It looks like that arm is made up of a series of rigid triangles, joined by many linear actuator elements.

Here is a definition which simulates that, with graph-mappers for controlling the various lengths of the elements along each of the 3 muscle strands.

Coming up with an intelligent way of controlling these actuators is a difficult challenge, as unlike a typical 6-axis industrial robot arm, this is an example of a highly redundant manipulator requiring a completely different approach to the inverse kinematics (for a given target pose, there are infinite kinematic solutions instead of a small finite number).

Attachments:

the learning method is striking

goal babbling

Hi Daniel,

Thank you very much for your help. Apologize for the simple question but I tried to change the triangle shape from top view of your example and just create a rectangle shape from top view. I think I have used the same technique as yours but it doesn't work. I'm missing something and I can't make it work. Could you please advice me how to change it?

Actually I like to use 4 muscle stands instead of 3.

Thanks

George

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Hi Daniel,

I've tried to use something like the arm you send with no luck. In the beginning I was trying to use square and 4 points instead of a triangle but it didn't work and then I try to do yours from the beginning and even that it doesn't work. I've baked all parts and I saw your connections and I tried to do the same. Does it matters in which sequence we implement each part?

This is pretty cool!! Is this based on the logic of Daniels code above?

George,

Bending is a difficult discussion topic with Kangaroo. The plugin works best with axial forces. I know some people have been working with bending, though I don't know if it's yet been really well applied to bending forces.

I guess we should ask: the photo you posted, that's not really a tube but a collection of small pieces connected end to end in a spine-like fashion. Is that what you're going for? Lots of smaller pieces connected? Or are you trying to do something more like bending a solid steel pipe?

Hi Michael,

While what you write was indeed true a few years ago, Kangaroo can now simulate bending of rods and shells, through the bend and hinge components. (Some examples here:

http://www.grasshopper3d.com/video/kangaroo-bending-validation

However, I think you are right that in this particular case it maybe makes more sense to treat it as built up of axial elements so there might be no need to use bending forces in this case.

Daniel,

I've seen some of the Vimeo examples and am looking at the feltshell.gh example right now. They look great and clearly visually replicate bending behavior. However the reason I was hesitant to reference these was that I've yet to see any studies of Kangaroo's bending analysis compared to an accepted shell analysis program like SAP, ETABS, or SAFE. Has anyone run this? Visual replication always makes me nervous because stiffness/strength factors can be chosen at random and it still 'looks' correct.

As part of my masters thesis, I verified Kangaroo's ability to act as the primary engine and correctly replicate axial forces and deflections in truss members including p-delta effects. I wasn't able to do the same, however, for bending forces in shells/beams. Has someone else?

As a side note, I was actually even more intrigued by this example (http://vimeo.com/90045868). Can you share the definition? It looks like an attempt at beam behavior? Is that correct?

by June Lee

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