How to Select Robot Joint Modules for Arms, Torso, Head, and Legs in AGV and Humanoid Robots

Robot AGV platforms, lifting legs, folding legs, torso, head, and dual arms each have different functional requirements. Therefore, different types of harmonic gear joint modules and planetary joint modules should be selected accordingly.

This article explains how to choose the most suitable joint modules for each robot subsystem.

What Functional Requirements Do Different Robot Subsystems Have?

1) Dual Arms (Shoulder, Elbow, Wrist)

Key requirements:

Low backlash, low friction (for force control and teaching), high torque density, lightweight design, cable-routing capability (hollow shaft or side exit), and low noise.

Critical risks:

Insufficient rated torque may cause overheating

Backlash and friction lead to instability in force control

Low repeatability and insufficient rigidity may cause end-effector vibration or positioning drift

2) Torso (Waist Yaw / Pitch / Roll, Chest Rotation)

Key requirements:

High axial and radial load capacity, structural rigidity, thermal stability, and strong impact resistance.

Critical risks:

The waist joint carries the combined load and moment of the upper body and dual arms, requiring extremely high structural strength and robust output bearing systems.

3) Head (Gimbal / Sensor Rotation Unit)

Key requirements:

Miniaturization, low noise, low vibration, and smooth low-speed motion for precise control.

Critical risks:

Motor cogging torque and encoder noise may cause micro-vibration

Electromagnetic or cable interference may affect precision sensors such as cameras and IMUs

4) Lower Limbs

A. Lifting Legs (Extension, Support, Posture Adjustment)

Typical implementations:

Electric actuators such as ball screw systems, belt-driven lifting systems, rack-and-pinion mechanisms, or scissor lift structures. Rotational joint modules are mainly used for auxiliary actuation or posture adjustment.

Key requirements:

Reliable self-locking/braking, impact resistance, long service life, high transmission efficiency, IP protection, high low-speed torque, and long-term thermal stability.

B. Folding Legs (Hip, Knee, Ankle Joints)

Key requirements:

High peak torque, excellent impact resistance, high structural rigidity, low backlash, reliable braking/holding mechanisms, and overload protection.

Module requirements:

These joints are close to primary load-bearing joints and require strong focus on structural strength, bearing system design, and output interface robustness.

2,How to Select Joint Modules for Different Robot Subsystems?

Dual Arms (Shoulder / Elbow / Wrist)

Arm Joints (Shoulder & Elbow)

Arm joints require high torque density, precise motion control, and reliable force-control capability. They also need lightweight construction and support for human–robot interaction through integrated sensing.

High torque density enables precise motion control

Integrated torque sensor supports force control and human–robot interaction

Lightweight and compact design is essential for robotic arm optimization

Recommended solution:

The TCHL harmonic gear motor is recommended for shoulder and elbow joints.

Torso (Waist Joint)

The torso (waist joint) requires high structural rigidity, high torque output, and strong load-bearing capability, as it supports the entire upper body and dual arms. Cable routing and system integration are also critical.

Large hollow structure enables internal cable routing and integration

High torque output ensures stable upper-body support

High structural rigidity improves overall system stability

Recommended solution:

The HPJM harmonic joint motor is preferred for the robot torso (waist).

Head Module

The head joint requires miniaturization, low vibration, and smooth motion control, especially for vision and sensor stability.

Low noise operation ensures sensor stability

Minimal vibration improves imaging and perception accuracy

Smooth low-speed motion enables precise positioning

Recommended solution:

Compact TCHL harmonic joint module

Lifting Legs

Lifting legs require high load capacity, reliable vertical motion, long service life, and strong impact resistance for posture adjustment and support.

High load reliability under continuous operation

Strong impact resistance for dynamic movement

High efficiency and long service life

Actuation systems may include linear or rotary solutions depending on design.

Recommended solutions:

Electric cylinders

Ball screw linear actuators

HPJM harmonic joint motor

Upcoming HPJM planetary joint motor

Folding Legs (Hip & Knee Joints)

Hip and knee joints require extremely high peak torque, high structural stiffness, and strong impact resistance, as they are the primary load-bearing joints in locomotion.

High peak torque for dynamic motion

High structural rigidity for load support

Reliable braking or holding capability

Strong impact resistance for walking and running conditions

Recommended solution:

The HPJM harmonic joint motor is the preferred choice for high-load leg joints.

Summary of Robot Joint Selection Principles

Robot joint module selection should be based on the following key engineering factors:Torque requirements,Precision requirements,Impact resistance,Control strategy (force control vs position control),Structural space constraints,Cost optimization

For detailed joint module selection guidance and customized robot solutions, please contact us.

HONPINE provides one-to-one technical consultation for robot joint module selection, system design, and integration support.