What Are Actuators? The Muscles of Humanoid Robots Explained
Actuators convert stored energy into physical movement at every joint of a humanoid robot. Without them, a robot cannot walk, lift, or grasp anything.
7 min read
Actuators convert stored energy into physical movement at every joint of a humanoid robot. Without them, a robot cannot walk, lift, or grasp anything.
An actuator converts stored energy into physical movement. Every joint in a humanoid robot, from shoulder to ankle, is driven by one.
An actuator is not a single component. It is an assembly of parts that work together: a motor that generates rotational force, a reducer that trades speed for torque, an encoder that tracks position, and a controller that coordinates everything in real time.
A typical humanoid robot carries between 20 and 56 actuators depending on its design and the number of degrees of freedom it supports. That range matters. A robot with 20 actuators is a capable industrial assistant. A robot with 56 actuators is reaching for the dexterity of a human hand.
The choice of actuator type defines the robot's personality in every measurable dimension: how fast it moves, how much it can lift, how long it runs on a charge, how loud it operates, how often it breaks down, and how much it costs to build and maintain. Two humanoid robots running identical AI software can produce wildly different real-world results based solely on their actuator architecture. This is why actuators deserve more attention than almost any other subsystem.
The three main actuator types are electric, hydraulic, and pneumatic. Each converts a different energy form into motion and carries distinct trade-offs.
Three fundamental approaches exist for powering humanoid robot joints, and the differences between them explain most of the competitive dynamics in the industry today.
Electric actuators use brushless DC (BLDC) motors to generate rotational force. That rotation passes through a reducer to increase torque, then converts to rotary or linear motion at the joint. A frameless BLDC motor consists of only a rotor and stator with no housing, shaft, or bearings, which eliminates weight and volume. Standalone motor efficiency reaches approximately 80%, but coupling the motor with a gearbox drops overall system efficiency to roughly 40%, according to Qviro's 2025 technical benchmarks.
Hydraulic actuators use pressurized fluid, typically oil at 3,000 to 5,000 PSI, routed through valves and cylinders. They operate on Pascal's Law: a small force applied over a small area generates a much larger force over a larger area. The raw power density is extraordinary. The original Boston Dynamics Atlas used hydraulics to perform backflips and parkour that no electric robot could match at the time.
Pneumatic actuators use compressed air and are inherently compliant, making them attractive for applications requiring safe human contact. McKibben pneumatic actuators can simulate human muscle behavior. However, they are harder to control precisely, noisier, less energy-efficient, and offer lower force per unit volume than either electric or hydraulic alternatives.
Electric actuators won because of economics, controllability, and maintenance costs. Hydraulics are powerful but commercially unviable at humanoid robot scale.
The shift from hydraulic to electric is not a trend. It is an engineering inevitability driven by three compounding forces.
Economics: A BLDC motor coupled to a planetary roller screw costs a fraction of an equivalent hydraulic system. Tesla's target price for Optimus is $20,000 to $30,000 per unit, a price point that is structurally impossible with hydraulic architecture. The component count is lower, the supply chain is simpler, and the manufacturing processes align with existing automotive production lines.
Controllability: Electric actuator motion control operates in a different class from hydraulic systems. Position, velocity, acceleration profiles, and output force can all be controlled with sub-millisecond precision. The peak torque of modern servo motors reaches approximately three times their nominal torque for short bursts, enabling rapid corrective responses when a robot steps into an unseen hole or catches a falling object.
Maintenance and reliability: Hydraulics carry fundamental operational problems. The pump runs continuously whether the joint is moving or not, dumping waste heat into the system. A hydraulic humanoid dissipates roughly 5 to 8 kilowatts of thermal energy that cannot be recovered. Annual maintenance costs for a hydraulic humanoid exceed $50,000, compared to approximately $5,000 for an equivalent electric system. Every component in the hydraulic chain, including pumps, hoses, valves, cylinders, and seals, is a failure point.
In April 2024, Boston Dynamics retired the hydraulic Atlas and introduced an all-electric replacement. CEO Robert Playter was explicit: commercialization required an architecture that was quieter, cleaner, more reliable, and cheaper to manufacture and maintain.
A modern electric actuator contains four core sub-components: a frameless BLDC motor, a reducer, an encoder, and a controller that ties them together in real time.
Understanding what is inside the actuator is essential for anyone evaluating build-or-buy decisions in humanoid robotics.
The motor is a frameless BLDC torque motor. Frameless means the motor ships as a bare rotor and stator, designed to be embedded directly into the robot's joint structure. This eliminates the weight and volume of a separate housing. Key specifications include torque density (torque per unit weight) and the peak-to-nominal torque ratio, which is typically 3:1 for high-end motors. Major suppliers include CubeMars, Maxon Motors, TQ Motors, and Mosrac.
The reducer converts high-speed, low-torque motor output into low-speed, high-torque joint motion. Three types dominate: harmonic drives for compact zero-backlash applications like wrists and elbows; planetary roller screws for linear motion in knees and ankles, offering longer lifespans and higher efficiency than ball screws; and cycloid reducers for high-shock joints like hips and shoulders.
The encoder tracks exact joint position and velocity. High-resolution encoders enable the sub-degree positioning accuracy required for dexterous manipulation. Without precise encoder feedback, the control loop cannot compensate for disturbances or maintain stable locomotion.
The controller executes the control loop at update rates of 1 kHz or higher, reading encoder data, computing required motor current, and driving the motor at the correct torque and speed. This update rate is not a technical nicety. It is the foundation of bipedal balance.
Fewer than ten suppliers globally can manufacture high-precision humanoid actuators at scale. This concentration makes the supply chain the primary constraint on industry growth.
Actuators account for the largest share of humanoid robot hardware costs. Hardware represents roughly 70% of total humanoid robot costs, and actuators are the most expensive hardware sub-system within that. Scaling from thousands of units per year to the millions required for mass adoption demands enormous new manufacturing capacity that does not yet exist.
The supply chain is concentrated in ways that create real strategic risk. Fewer than ten suppliers globally can manufacture high-precision, high-torque actuators suitable for humanoid robots. The companies that control this supply chain will ultimately control the pace of humanoid robot adoption.
The leading players are responding differently. Hyundai Mobis will supply actuators for the production version of Boston Dynamics' electric Atlas, leveraging Hyundai's automotive manufacturing infrastructure. Tesla is building actuator manufacturing in-house at its Fremont and Austin facilities. Chinese manufacturers like AgiBot and Unitree benefit from mature domestic supply chains for motors and electronic components, which gives them a structural cost advantage in the near term.
Software can be copied. AI models can be trained on commodity hardware. But a reliable, high-performance actuator manufactured at scale requires years of engineering iteration and billions in capital investment. This asymmetry makes the actuator supply chain one of the most defensible moats in the entire Physical AI stack.
Three open challenges define the actuator research frontier: thermal management under sustained load, backdrivability, and cost reduction at manufacturing scale.
The actuator is not a solved problem, and the gap between current performance and what humanoid robots need is wide enough to drive significant R&D investment for the next decade.
Thermal management is the primary operational constraint. A robot may be rated to lift 50 kilograms, but thermal limits mean it can sustain that load for only seconds before the actuator overheats. Solving this requires advances in motor winding design, heat dissipation materials, and active cooling integration, none of which have reached commercial maturity.
Backdrivability is essential for safe human interaction and energy-efficient locomotion. Backdrivability means external forces can move a joint freely, which is how humans absorb impact when walking and how robots should interact safely with people. Highly geared systems resist backdrive. Direct-drive systems sacrifice torque. Finding the right balance for each specific joint is an active area of research, and no manufacturer has published a convincing answer for all joint types simultaneously.
Cost reduction is the market access problem. The humanoid robot market needs actuators priced in the hundreds of dollars per unit, not thousands. Achieving this requires both manufacturing innovation and standardization across the industry. That standardization has not happened yet, and without it, every manufacturer is solving the same engineering problem independently, which slows everyone down.
A typical humanoid robot has between 20 and 56 actuators, depending on its design and degrees of freedom. A robot at the lower end handles basic locomotion and manipulation. A robot at the higher end approaches human-level dexterity, including articulated hands and fingers.
Boston Dynamics retired its hydraulic Atlas in April 2024 because hydraulics are commercially unviable at scale. The company's CEO Robert Playter cited the need for a quieter, cleaner, more reliable, and cheaper architecture. Hydraulic systems require over $50,000 per year in maintenance costs versus approximately $5,000 for equivalent electric systems.
A frameless brushless DC (BLDC) motor consists only of a rotor and stator with no housing, shaft, or bearings. This compact design is embedded directly into the robot's joint structure, eliminating weight and volume. It is the standard motor architecture in modern humanoid robots because of its high torque density and integration efficiency.
Backdrivability is the ability of external forces to move a robot's joint freely. It matters because it enables safe human interaction, impact absorption during walking, and energy recovery during motion. Highly geared actuators resist backdrive, while direct-drive systems sacrifice torque. Finding the right balance is one of the industry's primary open engineering challenges.
Key actuator suppliers include CubeMars, Maxon Motors, TQ Motors, and Mosrac for motors. Hyundai Mobis supplies actuators for Boston Dynamics' electric Atlas. Tesla manufactures actuators in-house at its Fremont and Austin facilities. Chinese manufacturers like Unitree and AgiBot leverage domestic supply chains for cost advantages.
Actuators are essentially the muscles of humanoid robots, converting stored energy into movement at every joint. As manufacturers race to scale production, which actuator technology do you think will win out: electric motors, hydraulics, or something else entirely? I would love to hear what you are seeing in the market or reading about.