View Full Version : Mechanical Leg Rig

Trig Fuller
04 April 2008, 11:26 AM

wondering if anyone can help with a problem that's been driving me nuts all morning.

I'm building a robot and there are joints(I don't mean C4D joints) for independent rotation.(ie one joint for X rotation, one for Y etc)

The upper body is going to be animated with FK, but I've been trying to rig the legs with IK(purely to make it easier to do walking). Problem is every thing I've tried doesn't work. I even tried adding joints(the c4d kind) setting up an ik rig for them(which is easy) and tying them with expresso to the rotation of my joints(not the c4d kind - getting cofusing sorry).

Anyone got any suggestions. You can see a simplified version of the leg in the attached image. (blue rotates around Y, Red rotates around X, Green Rotates around Z and white doesnt at all).



04 April 2008, 12:29 PM
Check out this thread here (
might give you some pointers. rwebbart gives an example ik leg that's based on parenting that might be useful to you. Might still need xpresso to take care of the individual planes of rotation you're looking to achieve, but overall the advice in that thread helped me out quite a bit.

I'd elaborate more, but I have class in 30min.

good luck,

Trig Fuller
04 April 2008, 03:09 PM

I made some progress - but not much.

I added a null object to the tip of the foot, then set up IK chain from the Hip Y object to the tip object. Now when I move the hip down the tip stays in place, but the other joints all rotate to accomodate this.

So I added constraints on the rotation of each object - basically constraining each to the object above it in the heirachy, with the axis I want them to move not selected in the constraint tag. But now whenever I move the top thing down the foot tip moves away from the position it was previously locked into. And the constraints down seem to work in the way I would expect(ie the way my understanding of what a constraint is).

The heirachy is this

Hip Null
>Hip Y, Rotation Constrained to Hip Null - all axis except Y
>>Hip X, Rotation Constrained to Hip Y - all axis except X
>>>Thigh, Rotation Constrained to Hip X - all axis
>>>>Knee Y, Rotation Constrained to Thigh - all axis Y
>>>>>Knee X, Rotation Constrained to Knee Y - all axis X
>>>>>>Shin, Rotation Constrained to Knee X - all axis
>>>>>>>Ankle Y, Rotation Constrained to Shin - all axis except Y
>>>>>>>>Ankle X, Rotation Constrained to Ankle Y - all axis except X
>>>>>>>>>Foot Z, Rotation Constrained to Ankle X - all axis except Z

This doesnt stop all unwanted rotations either, for instance when I move the Hip null down the hip Y rotates around the X axis, when the constraint says it shouldn't.

Regarding the ankle X position, what I seem to be missing is the Root Goal from previous versions.

Sorry to go on, but I've spent a lot of today faffing about with no success.


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04 April 2008, 03:09 PM
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