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Joint-Level Architecture for a Full Humanoid

Duration: 55 min · Level: Intermediate · Module: 2. Actuator Architecture · Focus: joint-design, torque, kinematics, systems

Learning objectives

By the end of this lesson you will be able to explain and apply:

  • Ankle joint
  • Knee joint
  • Hip joints (3 DOF each)
  • Shoulder (3-4 DOF)
  • Finger joints

You will then consolidate these ideas in the hands-on lab below.

Why this matters​

A complete humanoid has 40-50 joints, each with different torque, speed, and compliance requirements.

Overview​

A complete humanoid has 40-50 joints, each with different torque, speed, and compliance requirements. Mapping the right actuator topology to each joint class — ankles, knees, hips, spine, shoulders, elbows, wrists, fingers — is one of the most consequential design decisions for G1.

Key concepts​

Key idea

Ankle joint: highest impact loads (3-5× bodyweight at heel strike); needs stiff, high-torque actuator; QDD with peak 200+ Nm recommended

  • Knee joint: highest continuous torque; often uses the largest motor in the robot; 150-250 Nm continuous, 400+ Nm peak
  • Hip joints (3 DOF each): abduction/adduction + flexion/extension + rotation; combined torque budget ~300 Nm; ball joint or 3-actuator universal joint
  • Shoulder (3-4 DOF): human shoulder is the most complex joint; redundant DOF for reach; target 80 Nm to support 5kg arm raise
  • Finger joints: micro-actuators or tendon-driven with proximal motors; 1-5 Nm per finger joint; cable routing is the key engineering challenge
  • Spine: often underspecified; at least 2-3 DOF (pitch + roll + yaw); critical for whole-body motion; often uses low-ratio harmonic drives
Hands-on lab

Spec out the complete actuator selection for G1's 40-joint system: fill a table with joint name, DOF, peak torque requirement (estimated from dynamics), continuous torque, speed requirement, recommended actuator type, and approximate mass budget per joint.

Check your understanding​

Try to recall each answer before expanding it.

Q1. What do you know about Ankle joint?

highest impact loads (3-5× bodyweight at heel strike); needs stiff, high-torque actuator; QDD with peak 200+ Nm recommended

Q2. What do you know about Knee joint?

highest continuous torque; often uses the largest motor in the robot; 150-250 Nm continuous, 400+ Nm peak

Q3. What do you know about Hip joints (3 DOF each)?

abduction/adduction + flexion/extension + rotation; combined torque budget ~300 Nm; ball joint or 3-actuator universal joint

Q4. What do you know about Shoulder (3-4 DOF)?

human shoulder is the most complex joint; redundant DOF for reach; target 80 Nm to support 5kg arm raise

Q5. What do you know about Finger joints?

micro-actuators or tendon-driven with proximal motors; 1-5 Nm per finger joint; cable routing is the key engineering challenge

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Part of Module 2: Actuator Architecture.