I can't figure out how to fix this problem.
Hello jeewon , Besides Dr. Wayne Dixon's digital chiropractic services, I believe your answer will involve a Pole Target for your IK setup, an object that will give the arm a preferred direction to bend. Place an Empty object just behind the right elbow joint, for example, and give it a good name (ie: PT_arm.R) Then, in the IK setup for that arm add the name of the Pole Target. Adjust the Roll of the arm bone if it rotates. Parent the Pole Target to your Root bone or the Center of Gravity bone so that it will follow your character during walk cycles or fight scenes. This method works great for the knees too.
I hope your rigging goes well!
Hi Jeewon,
It looks like you are using Rigify, so there are 2 ways to control the IK pole vector.
If you are NOT using a pole vector, you need to animate to the rotation on the first segment of the IK arm. Maybe you hide the control in your rig but it's the stupidly massive double arrow shape. Animate the Y rot on that control and it will control the direction of the elbow.
If you ARE using the pole vector, you need to animate the pole vector control after clicking the button in the UI to display and enable it.
If it is a problem with the actual generated rig, go to your metarig and change the limb from "Automatic Rotation" to either X or Z (depending on which way you need it to bend.) Then regenerate.
This should force it to bend as intended, although you need to make sure you actually have a bend in the elbow in edit mode.
Hope this helps.