Physical AI is a hardware supply chain wearing a software brand. These are the segments where the real constraints live.
The joint as a product: frameless motors, integrated drives, thermal limits, cycle life, and cost-down roadmaps. The single largest line item in a humanoid bill of materials and the segment everything else on this page routes back to.
Harmonic, cycloidal, and planetary reducers, bearings, and the backlash-versus-stiffness tradeoffs that decide how precisely a limb can be placed. The hardest layer to manufacture at volume and the most common source of lead-time pain.
Neodymium, praseodymium, dysprosium, and the sintered NdFeB magnets that make high-torque-density motors possible. Heavily concentrated supply, active export policy, and a domestic-capacity buildout that is as much industrial policy as it is business.
Encoders, torque and force sensors, IMUs, and tactile skin. The difference between a machine that holds a position and a machine that can safely work next to a person is entirely in this layer.
Vision-language-action models, imitation learning, sim-to-real transfer, and the whole-body controllers that turn a policy output into forty coordinated joint commands. The part everyone talks about — and the part least likely to be the bottleneck.
What it takes to go from hand-built prototypes to thousands of units: supplier qualification, tolerance stack-up, automated assembly, and the unglamorous industrial engineering that decides whether a robot costs $200k or $20k.
Funding rounds, public-market exposure, vertical integration moves, and where value accrues along the chain. Includes the uncomfortable questions about how much robotics revenue is actually in anyone's numbers today.
Export controls on critical minerals, domestic content requirements, workplace safety standards for collaborative machines, and the certification regimes humanoids will need before they work anywhere regulated.
Why the gearbox choice constrains what a joint can do.
| Type | Strength | Weakness | Typical humanoid use |
|---|---|---|---|
| Harmonic / strain wave | Near-zero backlash, high ratio in a compact package | Expensive, torsional flex, limited shock tolerance | Arm and wrist joints where precision dominates |
| Cycloidal | High shock tolerance, high stiffness, long life | Heavier, more complex to manufacture | Hips, knees, and other high-load lower-body joints |
| Planetary | Cheap, efficient, widely available | Meaningful backlash unless heavily preloaded | Lower-precision joints and cost-sensitive designs |
| Direct drive / quasi-direct | No gear backlash, excellent force transparency | Needs a large, heavy, magnet-hungry motor | Legged locomotion where compliance beats precision |