Double Differential Claw

Why a differential

The standard way to build a two-axis wrist is a serial stack: one servo rotates the entire assembly for roll, while a second servo mounted on top provides pitch. The arrangement is straightforward to design and control, but the pitch servo becomes dead weight at the end of the arm, and each axis relies on only a single motor for torque. A differential couples the two servos instead. When both servos turn in the same direction, they generate one axis of motion; when they turn in opposite directions, they generate the other. As a result, both motors contribute to both axes, roughly doubling the torque available at the wrist while moving the mass of one servo closer to the shoulder, where it has less impact on arm inertia.

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.