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The humanoid body
A humanoid robot weighs roughly the same as an adult human, typically around 58 kilograms. Yet inside that familiar form lies a serious engineering challenge:...
A humanoid robot weighs roughly the same as an adult human, typically around 58 kilograms. Yet inside that familiar form lies a serious engineering challenge: building a machine that can move through a world designed entirely for biology.
The engineering choices embedded in that body will determine whether these machines remain expensive curiosities or become ubiquitous tools.
The Frame: Building with Air#
When Tesla unveiled its second-generation Optimus robot in late 2023, the headlines focused on its improved dexterity and smoother movements. But buried in the technical specifications was a more consequential achievement: the robot had shed 10 kilograms without sacrificing strength or performance.
The secret was a material called PEEK, Polyether Ether Ketone, a high-performance thermoplastic polymer that weighs half as much as aluminum while maintaining comparable strength. PEEK offers exceptional heat resistance, wear resistance, and chemical resistance while being derived from petroleum-based sources through a chemical process called step-growth polymerization.
Weight matters in humanoid robots for the same reason it matters in humans: every kilogram you carry costs energy. A lighter robot walks farther on the same battery, responds faster to unexpected obstacles, and requires less powerful (and less expensive) motors to achieve the same movements. Assuming that joints and gears use approximately 1kg of pure PEEK material and limbs use about 8kg of carbon fiber-reinforced PEEK composite, a single humanoid robot consumes roughly 6.6 kilograms of PEEK.
But PEEK represents just one material in an expanding palette. Modern humanoids incorporate polycarbonate blends, carbon fiber composites, thermoplastic elastomers for synthetic skin, and specialized polymers like polyphenylene sulfide for gears and bearings. Each serves a specific purpose: carbon fiber for skeletal strength, thermoplastic elastomers for touch-sensitive skin, specialized polymers for components that must withstand millions of cycles without degradation.
The frame is a hierarchy of materials balancing competing demands: light enough to walk efficiently, strong enough to carry loads, resilient enough to survive falls, and manufacturable at scale.
The Muscles: Actuators and the Price of Movement#
Actuators typically account for more than 30% of the overall bill of materials cost in a high-configuration humanoid robot. These are the devices that convert electrical energy into physical motion, the artificial muscles that enable a robot to walk, grasp, and manipulate objects.
Every actuator contains four essential components: a motor, a gearbox (or reducer) to multiply torque, an encoder to measure position, and control electronics to coordinate movement. The prevailing practice involves using a specific actuator in each joint, consisting of a gearbox, torque motor, encoder, and motor control.
Current humanoids under development feature between 16 and 60 degrees of freedom. Tesla's Optimus utilizes 50 degrees of freedom powered by 28 actuators: 14 linear actuators for movements like knee extension and 14 rotary actuators for movements like shoulder rotation.
Every degree of freedom adds capability but also complexity and cost. A six-degree-of-freedom hand can perform perhaps 60-70% of human hand functions. Increase that to 11 or 12 degrees of freedom, as Tesla and Fourier Intelligence have done, and you approach human-like dexterity. But each additional degree of freedom requires another actuator, another point of potential failure, and more sophisticated control algorithms.
Five critical factors determine motor selection. First, precision: even minimal deviations in individual joints accumulate into significant misalignments when multiple joints move together. Second, torque density: how much force can be generated relative to the motor's weight and size. A humanoid's legs must support the entire robot's weight and generate enough force for walking, running, or climbing stairs. Its arms must be strong enough to lift objects yet light enough that they don't throw the robot off balance.
Third, reaction time and dynamics. For humanoids to respond flexibly to environmental changes, they require excellent dynamic control and fast response times. This demands motors capable of generating peak torques approximately three times higher than their nominal continuous torque.
For decades, hydraulic actuators were the only technology capable of generating the necessary forces. Boston Dynamics' original Atlas robot relied on hydraulics, using fluid pressure to move limbs with explosive power. But in April 2024, Boston Dynamics announced a transition to electric actuators for its next-generation Atlas, a shift driven by commercialization requirements.
Electric actuators offer what hydraulics cannot: precise control over position, velocity, and force at every instant. They are quieter, cleaner, more efficient, and far easier to maintain. The cost is power density. Electric motors must work harder to match the brute force of hydraulics, which is why the race to develop high-torque, lightweight electric actuators has become central to the entire industry.
The Senses: Perception Through Layers#
A human body contains millions of sensory receptors: photoreceptors in the eyes, mechanoreceptors in the skin, proprioceptors in muscles and joints, vestibular sensors in the inner ear. A humanoid robot must approximate this perceptual richness with far fewer sensors, carefully positioned and integrated.
A typical humanoid robot is equipped with four six-dimensional force sensors (located at the wrists and ankles) and 28 joint torque sensors. These tactile components account for approximately 15% of the total bill of materials cost.
Vision systems provide the robot's primary window into the world. Most humanoids in 2025 use a combination of cameras and LiDAR for navigation, collision avoidance, and object detection. Cameras capture color, texture, and fine detail. LiDAR measures distance with precision, building three-dimensional maps of the environment even in darkness or poor visibility. Tesla's Optimus represents a notable exception, relying solely on cameras, following the same vision-only approach the company uses for its vehicles.
Tactile sensors provide immediate and accurate feedback about contact forces and states between robots and objects, enabling real-time adjustments to grasping force, posture, and hand trajectories. This becomes essential when handling soft, fragile, or irregularly shaped items. Advanced tactile sensors can detect not only pressure but also texture, slip, temperature, and material properties.
Inertial measurement units combine accelerometers and gyroscopes to track the robot's orientation and acceleration, critical for maintaining balance during walking or recovering from disturbances.
The challenge is not building individual sensors but integrating them into a coherent perceptual system. Humans achieve sensorimotor integration effortlessly, combining touch and vision to guide movement. Robots can approximate this only under controlled conditions.
The Power: The Battery Problem#
Most humanoids today operate for only about two hours on a single charge.
A humanoid that must conserve every watt-hour compromises on weight, strength, and speed.
Lithium-ion batteries have become the standard choice for humanoid robots due to their high energy density and longer lifespan compared to traditional batteries. Advanced semi-solid NMC batteries achieve energy densities up to 350 Wh/kg, providing a balance between high energy capacity and lightweight design.
But energy density tells only part of the story. A humanoid needs both sustained power for walking and sudden bursts of power for lifting, climbing, or recovering balance. This has led to interest in supercapacitors, which store electrical charge and can discharge it extremely rapidly, complementing batteries for tasks requiring instant power.
Solid-state batteries represent the next horizon. These batteries use solid electrolytes instead of liquid ones, eliminating the risk of leakage and thermal runaway while offering increased energy density and faster charging times. According to industry analysis, 2025 is a turning point for battery technology in humanoid robots, with companies like Funeng Technology and Changhong Power developing specialized battery packs specifically designed for robotic applications.
Most humanoid robots operate on battery voltages between 48-58V with capacities ranging from 2-8 kWh. For context, a small electric vehicle might use a 50-60 kWh battery. A humanoid must accomplish complex manipulation and navigation tasks with one-tenth the energy storage, all while keeping the battery light enough that carrying it doesn't consume most of the available power.
The battery shapes the humanoid's working day. A two-hour runtime means the robot must return to charge before completing many useful tasks. A four-hour runtime changes the equation entirely, opening applications previously impossible. An eight-hour runtime, matching a human work shift, would transform the economics of deployment.
Every engineering choice in the body of a humanoid robot traces back to this constraint: how do you build a machine capable of human-like movement and manipulation within the energy budget of a small battery?
The Economics of the Body#
The distribution of costs in a humanoid robot reflects the hierarchy of challenges: actuators consume more than 30% of the total bill of materials, sensors account for approximately 15%, and power systems represent a smaller but crucial fraction.
Actuators dominate costs because generating human-like motion efficiently and affordably remains unsolved. Sensors take a smaller share because perception technology, driven by decades of smartphone and automotive development, has matured faster.
Using estimates for PEEK consumption and current material prices, the value of structural materials in a single humanoid robot amounts to approximately 2,300 RMB (about $320), representing roughly 1.65% of a robot priced at $20,000.
Every component must be light enough to preserve battery life, strong enough to perform useful work, precise enough to manipulate delicate objects, durable enough to survive years of operation, and cheap enough to manufacture at scale. No single material or technology solves all of that.
But before we examine how these machines think, there is the most challenging component of all: the hand.
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