I created this character for a small JRPG project in a retro style. The custom rig was difficult but rewarding to make. I created targets for the eye and cannon to point to, IK controls for the legs and cannon arm, limited the rotation of the robot's lid/helmet, and linked the cannon's scale to the lid rotation so it would automatically pop up if you opened the lid while animating. You can see the cube-shaped control (the cannon's scale) change size when the lid rotates.
Final animations