Joints and Actuators

Market research · Joints & actuators

Joints and actuators: what makes a humanoid move

Explainers on the parts that decide what a humanoid robot can lift, how long it runs and what it costs to build: reducers, motors, sensors, actuator types, joint layouts and reliability. Every article cites its sources and shows the mechanism with diagrams and charts.

Reducers & transmissions

How torque gets from a fast motor to a slow, strong limb.

01Harmonic vs planetary vs cycloidal reducersStrain-wave, planetary and cycloidal reducers compared on ratio, backlash, efficiency, shock, life and suppliers.Read the explainer → 02Gear ratio in humanoid jointsHow one number trades torque, speed and N² reflected inertia, with published ratios from 6:1 to 160:1.Read the explainer → 03Backlash and torsional stiffnessWhy a zero-backlash joint still twists: reducer wind-up, hand position error and dual-encoder compensation.Read the explainer → 04Linear vs rotary actuatorsRoller-screw linear actuators versus rotary reducers: cost, efficiency, back-drivability and which robots use which.Read the explainer → 05Belts, cables and linkagesWhy humanoids mount knee motors near the hip and drive the joint through belts, cables or linkages.Read the explainer → 06The bearings inside humanoid jointsCrossed rollers, four-point rings, the strain-wave flexible bearing and hand micro-bearings, and who makes them.Read the explainer →

Motors, electronics & sensing

The electromagnetic and electronic layer inside every joint.

01Frameless torque and coreless motorsHow the two motor types inside humanoid joints work, where each is used, and how torque scales with stack length.Read the explainer → 02Rare-earth magnets in humanoid jointsWhy every joint motor runs on NdFeB magnets, how heat drives dysprosium demand, and who controls supply.Read the explainer → 03Joint servo drivesThe 48 V electronics in every joint: field-oriented control, kHz current loops, silicon versus GaN, EtherCAT.Read the explainer → 04Joint control loopsHow a 50 Hz policy, the joint PD law, MIT-mode commands and kHz current loops work together.Read the explainer → 05Joint encoders and torque sensingHow a joint knows where it is and how hard it pushes: dual encoders and joint torque sensors.Read the explainer → 06Hollow-shaft jointsHow power and data get through every joint: bore sizes, four-wire buses, cable torsion limits and slip rings.Read the explainer →

Actuator types & performance

Architectures, and the numbers that decide what a joint can do.

01Quasi-direct-drive actuatorsThe Mini Cheetah’s 6:1 recipe, reflected inertia’s N² tax, and the heat cost of low ratios.Read the explainer → 02Back-drivabilityWhy some joints yield when pushed: reflected inertia, MIT’s proprioceptive actuator, QDD versus harmonic drives.Read the explainer → 03Series elastic actuatorsWhy Apollo keeps a measured spring in the joint, from NASA’s Valkyrie to Apptronik, and what it costs.Read the explainer → 04Variable stiffness actuatorsJoints that retune their own spring with a second motor, what DLR’s designs achieve, and why production robots skip them.Read the explainer → 05Hydraulic vs electric actuationWhy Atlas went electric, how each system works, and who still runs on fluid.Read the explainer → 06Artificial musclesMcKibben, Myofiber, HASEL and twisted-polymer muscles compared with human muscle and the electric joint.Read the explainer → 07Torque densityWhat N·m per kg really means: module versus motor figures, peak versus continuous, and viability thresholds.Read the explainer → 08Peak vs continuous torqueThe thermal wall: why joints hold peak torque only briefly, the heat path out of the stator, and cooling options.Read the explainer → 09Energy efficiency in humanoid jointsWhere the battery goes: copper loss rising with torque squared, gearbox friction, regeneration and cost of transport.Read the explainer → 10Quiet jointsWhere joint noise comes from, published dB figures for gearboxes and robots, and the levers that make joints quieter.Read the explainer → 11How many actuator types does a humanoid need?Why robots use far fewer actuator designs than joints: Optimus, Berkeley Humanoid Lite, Unitree and RobStride.Read the explainer →

Joints by body region

From the hip to the fingertip: how humanoids lay out their joints.

01Degrees of freedomWhat the spec sheet’s favourite number counts, and why 23 and 88 DoF aren’t comparable.Read the explainer → 02Range of motionPublished hip, knee, ankle, waist and wrist ranges for Unitree, Fourier and Booster against human norms.Read the explainer → 03Hip, knee, ankleHow humanoids budget leg joints: the 3+1+2 layout, published torque figures and the move to parallel ankles.Read the explainer → 04Parallel ankle mechanismsWhy most new humanoids drive the ankle through two shin-mounted actuators, and what it costs in control.Read the explainer → 05Toe jointsActive, passive or flat feet: what research humanoids gained from toes, and why commercial robots skip them.Read the explainer → 06Shoulder, elbow, wristHow humanoids allocate arm DoF, with per-arm counts for Optimus, Figure, Eva, Apollo and G1.Read the explainer → 07Waist and neck jointsThe mid-body DoF budget, from one lumbar yaw joint to a 3-axis waist and 2-axis neck.Read the explainer → 08Tendon-driven handsHow Optimus, NEO and Shadow move 20+ finger joints with cables from forearm actuators.Read the explainer → 09Linkage-driven fingersThe pen-sized micro linear actuators that push the linkages curling humanoid fingers.Read the explainer →

Reliability, safety & industry

Lifetime, protection, testing, cost and the supply chain.

01How long does a joint last?Strain-wave L10 ratings, the cube law linking load to life, documented failure modes and field reliability.Read the explainer → 02Overload protectionHow joints survive falls: reducer torque limits, why high ratios amplify impacts, slip clutches and fall-aware control.Read the explainer → 03Joint brakesWhat holds the robot when power goes: spring-applied versus permanent-magnet brakes, power draw and stop categories.Read the explainer → 04Sealed joints and IP ratingsKeeping dust and water out: IEC 60529 ratings, IP67 Atlas, lip seals and their friction cost.Read the explainer → 05Grease in humanoid jointsHow lubrication sets efficiency, cold starts and service life, and what to ask suppliers.Read the explainer → 06How joints are testedDynamometer benches, eight core measurements, end-of-line checks and the standards still in draft.Read the explainer → 07What does a joint cost?Inside the actuator bill of materials, from motor and strain-wave reducer to encoders and drive.Read the explainer → 08Who builds the joints?The 2026 supplier landscape: Tesla’s chain, harmonic reducer makers, roller-screw challengers and Schaeffler.Read the explainer →

Go deeper: The Humanoid Actuation Report

249 pages on motors, reducers, screws, bearings, encoders and drives, the vendors behind every major platform, and the cost curves that decide adoption to 2036.

The Humanoid Actuation Report All humanoid.guide reports