Humanoid Neck Joint

Tight space constraints and the head's high weight, paired with requirements for cheap mass manufacturability and aesthetic appeal, made the simple goal considerably more challenging. I cast a wide net, surveying ideas from the academic literature and initially attempting to adapt them to the existing joint. Torsion springs proved unwieldy. Cable-driven designs were overly complex. And the cam-follower design added too much height.

The solution? A ground-up rethink of the joint structure. I moved from a two-piece design to an organic unibody, creating a cavity in the middle. Here, a servo-driven stopper made of flexible TPU wedges between the tilt and nod joints, locking both in place with just one moving part. Stress hot spots in the servo cradle were identified with nonlinear contact FEA, and further validated with physical prototyping.

From here, I iterated on the stopper geometry, leveraging additive manufacturing to prototype. A data-driven test process backed each iteration, where I systematically measured each design's "breakaway torque" (i.e., the resistance to head topple). Aiming for maximum breakaway resistance while retaining enough compliance to be wedged into place with the available servo torque, I converged on a satisfactory design after four major design revisions and countless more incremental tweaks.

The result of all of that analysis and iteration? An elegant unibody joint morphology, firm enough to not just prevent head topple but to stay upright even when bumped. It's set-and-forget, too—once locked, all motors can be de-energized. And locking both joints is powered by just one discrete micro servo.

The compliance of the TPU has the added benefit of acting as a resettable mechanical fuse—if too much force is applied, the lock will give way before the joint or other expensive components are structurally harmed.