From the physics of balance to motors and reducers, artificial intelligence, and the battery. Lifting the smooth exterior one layer at a time.
As we saw at the close of Part 2, the outward appearance of today's humanoids is, regardless of camp, growing steadily more alike; the broad outline of two legs, five fingers, and a camera on the head has effectively converged into one, and so the real competition takes place beneath that smooth skin, at the level of components and algorithms. In this part we open the shell and look, in turn, at the five essentials that actually make a humanoid move.
The order runs from the outside of the body inward. We start from the physical question of why "walking" is so hard, pass through the motors and reducers that actually produce that motion, arrive at the domain of artificial intelligence — what to see and how to judge — and finally come to a halt before the battery, which keeps all of it running for hours yet draws the least attention.
Part 1 mentioned Honda's ZMP in passing, but since the concept is the starting point of humanoid walking it is worth stating once more, precisely. Whether for a person or a robot, the area where the soles touch the ground is called the "support area," and as long as the zero moment point, or ZMP — the single point to which the forces and inertia acting on the whole body are reduced — stays within that support area, the machine will not fall.
The difficulty is that this condition must be held not while standing still but "while walking." The instant one foot lifts, the support area narrows to the area of the remaining foot and the body tends at every moment to topple forward, so the controller must recompute the angles and speeds of dozens of joints hundreds to a thousand times a second, continually drawing the ZMP back within the foot. The reason early humanoids walked so slowly and gingerly, and the reason recent machines can run and even jump, both come down to how quickly and precisely this computation is carried out.
When the command to keep balance comes down, what turns it into actual movement is the actuator seated in each joint, the "artificial muscle." As Part 1's hydraulic Atlas showed, the hydraulic approach that produces great force through fluid pressure was once the symbol of dynamism, but because of its weaknesses — heavy, expensive, and difficult to control precisely — almost every humanoid of the 2020s has turned to the electric motor.
At its core is a high-performance electric motor commonly called a BLDC. It produces more torque for the same weight, allows fine control, and is more favorable for managing heat and efficiency; yet the torque a motor produces directly is nowhere near enough to support a person's weight while walking. A motor is by nature good at turning "fast and weak," so a device to convert that fast rotation into "slow and strong" is indispensable — and that is the protagonist of the next chapter, the reducer.
The reducer, as its name says, slows the motor's fast rotation and in exchange produces proportionally greater torque, and it is the core that effectively determines the performance and precision of a humanoid's joints. The most widely used among them is the precision reducer called the harmonic drive, whose distinctive structure — a thin, flexible toothed cup pressed by an elliptical input shaft so that it meshes with a rigid outer ring at only two points — produces motion that is small and light yet nearly free of the looseness between teeth known as backlash.
This component matters not for its technical excellence alone. The harmonic drive is hard to make, so a handful of firms effectively hold an oligopoly over the market, and since a single humanoid contains dozens of such precision reducers, one per joint, the moment robots are mass-produced, who supplies this "costliest handful" becomes an enormous business in itself. It is precisely this point — where a question of technology turns directly into a question of money — that sits at the heart of the stock value chain we will take up in the next part.
However sturdy the legs and however precise the joints, without a "head" to judge what to see and how to move, a robot remains a machine that merely repeats fixed motions. The decisive difference between Part 1's ASIMO and today's humanoids lies exactly here: when cameras, an inertial measurement unit (IMU), and sensors that feel force and torque continually gather information about the surroundings and the body's posture, artificial intelligence interprets it and decides the next motion, and that command runs back down to the motors, a cycle that repeats hundreds of times a second.
The greatest recent change lies in the very way this "head" is built. In the past a person had to program every motion by hand, but the center of gravity has now shifted toward robots learning their motions on their own through vast quantities of video and simulation. In particular, as new artificial-intelligence models that learn vision, language and action as a single bundle have appeared, it is gradually becoming real for a robot to understand the single phrase "pick up the cup" and carry it out in an environment it has never seen before — and this is exactly why the companies we met in Part 2 are each staking everything on their own artificial intelligence.
The control that keeps balance, the motor that produces force, the reducer that refines that force, the artificial intelligence that handles judgment, and the battery that keeps it all running. A humanoid, as a single machine, is in the end a precise ensemble of these five layers, and if any one of them is weak the whole comes to a halt.
Yet if you look quietly at this list of parts, it is also a vast industrial map. The company that makes the motors, the company that holds an oligopoly on reducers, the company that sells the AI chips, and the company that supplies the batteries each stand guard over a chokepoint in this flow. In the next part we will turn this anatomical chart of the technology over as it is and follow the stock value chain, to see where the money actually flows in an age when humanoids are mass-produced.