Double Differential Claw

Why a differential

The standard way to build a two-axis wrist is a serial stack: one servo rotates the whole assembly for roll, and a second servo mounted on top of it handles pitch. It is simple to reason about, but the second servo then rides at the end of the arm as dead weight, and each axis only ever has one motor behind it. A differential couples the two servos instead — turning them the same direction produces one motion, turning them opposite produces the other. Both motors contribute to both axes, which roughly doubles the torque available at the wrist while moving one servo's worth of mass back toward the shoulder.

The design

The wrist is a gear differential: the two servo inputs feed a shared output, so their sum drives pitch and their difference drives roll. Because neither servo carries the other, the load path stays symmetric and the moving mass sits close to the arm rather than hanging off the tip. Upstream of the wrist, the arm carries five degrees of freedom in total, out to the gripper at the end.