What Is a Joint Motor?

A joint motor (also called a robot joint module or integrated actuator) packs a frameless torque motor, gearbox, encoders, driver, and bearings into one compact unit. Mounted directly at a robot's joints, it outputs controlled torque and rotation — the "muscle" of humanoid robots, quadrupeds, and robotic arms.

Core Components

• Frameless torque motor: generates raw torque and sets the power ceiling.
• Gearbox: trades speed for torque; common choices are the planetary gearbox and harmonic drive.
• Encoders: measure joint angle for closed-loop control; high-end designs use dual absolute encoders.
• Bearing: carries loads while preserving rotational precision; load-bearing joints typically use a crossed roller bearing.
• Driver: runs current/velocity/position loops and receives commands over CAN/CANFD buses (see MIT protocol).

Key Specifications

The parameters that matter most are rated torque (continuous output), peak torque (short-term ceiling), weight, envelope dimensions, reduction ratio, and communication interface. A humanoid robot typically needs 20–40 joint motors across several torque tiers — the BXI Elf 3, for example, runs 31 85/70/50-series joint motors spanning 25–150 N·m peak torque.

For a sizing methodology, see the joint motor selection guide.