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.
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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.
The three main actuator types are electric, hydraulic, and pneumatic. Each converts a different energy form into motion and carries distinct trade-offs.
Electric actuators won because of economics, controllability, and maintenance costs. Hydraulics are powerful but commercially unviable at humanoid robot scale.
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.
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.
Three open challenges define the actuator research frontier: thermal management under sustained load, backdrivability, and cost reduction at manufacturing scale.
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.