I am new to Blender and am rigging a simple ball joint, a sphere inside a socket so it can move up to 30 degrees away from center in any direction. I can add pictures on request.
How do I add constraints that stop it from clipping through the joint? I found the “limit rotation” constraint works pretty well, but if both the x rotation and y rotation are 30 then they combine to make it move more like 45 degrees into a corner and clip through.
Is there any way I can say absolute value of y plus absolute value of x cannot exceed 30?
You can do it with an IK chain. The problem is that when you rotate an object 30 degrees in both X and Y, the resultant rotation is confined to a square of 1 relative unit - the diagonal measurement is then SquareRoot 2 and the angle of the object is then arcTan SquareRoot 2 or 45 degrees as you say. The issue now is that you must confine the movement within a circle, not a square, so it never travels beyond 30 degrees in any direction. My first thought was to use a Limit Distance constraint, but that is way too complicated to implement for a rotating object. So I chose IK:
I made the armature in its “bent” position as above, I also rotated the top link 30 degrees to fix my bone chain, then added the IK constraint to the “limit” bone with “stop” as the target. I also limited movement of “stop” so it can only move along its own Y axis and cannot move down - this ensures “link-2” cannot go beyond 30 degrees. To straighten or bend the mechanism you simply move “stop” along its local Y axis (currently also Global Z). To rotate the mechanism, you rotate “link-1” about its local Y axis - there is a keyframed animation in there for you to watch.
If you make the armature in its “straight” position, it will not work as the bones are dead in line and the IK resolver does not know which way to go, so nothing moves if you try to move the “stop” bone downwards. Normally with Ik chains you always build in a slight kink to allow the resolver to work, in this situation I built in the full movement for ease of construction and operation, since I knew the maximum angle was 30 degrees. You can straighten an Ik chain to dead straight, but you cannot bend a dead straight one.
Now - if you want to have more control over this so you would only be moving one bone, that will be do-able, but means a little more effort. Often with rigs, you would put the constraints on the operating bone, not the “link-1” bone and that will keep the rig within limits. So if you let me know what the overall objective is, there may be a better way to do this, but this answers your question for now…
On this one you just move the “limit” bone around…
The only thing here is that you may not be able to control the “limit” bone so well, like if you parent it to another bone, the Limit Distance constraint gets overruled.
Cheers, Clock.
EDIT: you can use a Copy Location constraint to position the “limit” bone PROVIDED the Limit Distance constraint is BELOW the Copy Location one in the Constraints list, other wise it gets overruled by the Copy Location constraint and is therefore ineffective.
Oh that’s brilliant! The 2nd example is exactly the motion I was looking for, thank you
To be clear, what you essentially did was lock a bone at the center position, make a 2nd bone that cannot exceed a certain distance from 1st bone, and then make the joint point at the 2nd bone, right?
Also, what are the advantages of using bones rather than parented empties? I have not worked with bones much.
Yes you are correct, the “stop” bone is the anchor bone and the IK target bone “limit” on my rig has the Limit Distance so that it can only move within a theoretical sphere. I then used this as the target for the IK constraint on the link-2 bone. Targets need not be connected to the IK chain all the time, the IK chain will still point towards it via the shortest route. I prefer this to using a Track To constraint - its more predictable and flexible.
The main advantage with bones to rig with, for anything other than really simple stuff, is that all the actions are contained in one object so Blender runs more efficiently and they are far more capable than any other method I have ever tried. You cannot use IK in anything other than an Armature, also you can have many and varied types of bone parenting that are difficult to achieve without bones. I have been down the constraints/object parenting route and much prefer Armatures - although they mean a little more learning. The classic mechanical model ideal for IK is a digger - two booms and a bucket. If you use an IK chain and move the bucket, the booms follow as you want them - this I cannot do easily with anything other than bones. IK means Inverse Kinematics - so it works backwards to real life - you move the booms in a real digger and they position the bucket - in Blender you move the bucket to where/how you want it and all the rest, including hydraulics, follows.
