Rigging a Bending Robot Arm

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  06 June 2013
Rigging a Bending Robot Arm?

Hey, first post for the sake of solving this. I've been stuck on this one problem for ages and I still can't find a suitable solution. A fair bit of text ahead, so you can just click on the images if you want a quick version. You should still spot the problem.

So I've got this robot arm (imagine Bender's arm from Futurama) that I wanna rig. I want to be able to bend it in any direction comfortably and easily (and this includes backwards). Here's the arm. The question is: How would you rig this?

My first thought was to create a thorough joint chain, and attach that to an EP curve (2 Curve Degree - and I connected it with an IK spline handle). Once I did this, I'd turn the EP's into locators, giving me great control over the curve, and as a result, the joint and arm. Arm Rigged and the Curve.

This works fantastic (especially when I can offset the joints, so they retreat into the body). But there's one huge problem. Whenever I move the entire character and rig forward, the curve behaves weirdly. Here are images of the Arm Problem, and one just showing What the curve is doing.

I suspect it's doing that due to the relative position of the curve's origin point. And that's a big problem. I don't want it to do that, since it completely messes up the arms when I try to move the character forward (it's even worse when it rotates). It also happens only when I move the locators. If I move the EP's themselves (also assuming I never converted them to locators), then the only problem is when I rotate the points. Here's what happens when you rotate the EP's, if you're interested

The reason I'm not just using a basic arm rig is because it doesn't let me offset the arm into the body, and I get a bad 'slanting' on the arm, like this.. Plus I'm finding it difficult to skin it well. Getting a smooth curve across the arm is difficult (the image is the best I could do), and I don't want an obvious elbow (and I also don't want the root of the arm to move at all).

How would one rig/skin something like this? I haven't been able to find anything useful on the internet for a while. If you've found a tutorial on something like this, please let me know. The closest I could find was this tutorial on Bender, but it doesn't allow for backward-arm bending.

Last edited by stojkovski : 06 June 2013 at 03:28 AM.
  06 June 2013

Do you have the scene for this? Your pictures are excellent, but sometimes you just need to get in there.

Also, for this I would recommend a Ribbon setup. When set up properly you can bend and rotate those things into infinity, excellent fall of.

You can even find scripts that will make them for you. All it is a nurbs plane with follicles spaced out evenly with joints parented underneath that will follow alone when the plane is deformed.

Check it out.
  06 June 2013
A Ribbon set-up? I'll have to look into that.

I threw together the two experiments I tried (excuse the object names in the Outliner... copy/pasting is a nightmare for organisation). Let me know if I did anything wrong, since I'm still relatively new to all this.


  06 June 2013
Ok, so I looked at the scene, sorry for the delay.

I think it might be the leastSquaresModifier node that is not manipulating the curve properly.

The maya documentation states that it is not the best solution for curves with multiple points, or when you influence multiple points on the same curve.

So you could just use clusters and constraint the clusters to the locators and you should get the desired result. You could also skin it to a joint.

But again, the Ribbon setup is most ideal.

But clusters instead of the modifier you currently have, should get you going.
  06 June 2013
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