# Rotational Constraint for a Bone

Figure 2 shows the arm I want to rig, which includes an accordion-like elbow hinge. In this figure, the armature has been removed and displaced to the left along the green arrows so you can see it better.

Figure 1 shows the [magnified] bones of the same arm. Bone1 and bone2 are the primary bones of the arm, while hollow1 and hollow2 should control the position of the two bands of verts that populate the inside of the hinge. Hollow3 merely helps for parenting and controls no verts.

How do I control the rotation of hollow1 and hollow2, so the two divide the elbow angle into three even angles? I want to have just one bone to rotate and let these two bands of verts go about their business without my help.

Thanks.

### Attachments

Action constraints can trigger aribitrary motion in one bone based on the rotation of another.

Have you used them before?

Mike

Nope, but I’d love to know how.

I was sure this would come down to constraints, IK tricks, or both but I don’t know how to use constraints. I found them in the UI and researched them but failed to create a rotational constraint between one bone and another from Pose Mode.

Some grueling, painful step-by-step instructions would be appreciated.

Here is a vid tut that shows the basics (for a finger rig, but you should be able to adapt the principles)

http://www.zingg-pereira.ch/blender/NewbieTutFinal.htm

The basic idea is :

• you animate your “hollow” bones the way you want them to move. That will create an Action as usual, call it say “HollowAct”

(If you don’t know about animating armatures / Pose Mode, you’ll have to learn that first / check the Wiki Manual). You should also be comfortable with the IPO editor / adjusting curves etc.

You then for each hollow bone :

For the :

Ob: field it’s the name of the Armature

Bo: is the bone you want to drive the action, in your case Bone2

AC: The name of the action (“HollowAct”)

Under AC: is a popup where you can select the axis of rotation of the driving bone (Bone2) that you want to trigger the action.

Start, End are the starting and ending frames of the Action

Min, Max are the ranges of the driving bone’s rotation you want to affect the action.

With all that filled in, rotating your control bone should trigger the action.

Here’s an example file :
http://www.4shared.com/file/5730153/c6c99520/actionconstraintexample2.html