Robotics Glossary
Precise definitions of core humanoid robotics and actuator concepts
Crossed Roller Bearing
A crossed roller bearing alternates cylindrical rollers at 90° in a V-shaped raceway, letting a single bearing carry radial, axial, and moment loads simultaneously — giving robot joints high stiffness and rotational precision.
Degrees of Freedom (DoF)
Degrees of freedom count the independently controllable joint axes of a robot and directly determine its motion capability: a 6-DoF arm reaches arbitrary poses, while humanoid robots typically need 30+ DoF for whole-body coordination.
Dual Absolute Encoder
A dual absolute encoder setup places one absolute encoder at the joint motor input and another at the output, directly measuring true post-reduction joint angle for precise closed-loop control and zero-calibration startup.
Embodied AI
Embodied AI is the paradigm where an agent perceives, decides, and acts through a physical body interacting with the real world; humanoid robots are its primary platform, closing the perception–decision–action loop in reality.
Hollow Shaft Motor
A hollow shaft motor has a through-bore along its output axis, letting cables, tubing, and sensor harnesses pass through the joint center — dramatically simplifying robot wiring and expanding joint range of motion.
Joint Motor
A joint motor is an integrated robot actuator combining a frameless motor, gearbox, encoders, and driver in one unit, mounted directly at a robot joint — the core power component of humanoid robots and robotic arms.
MIT Protocol (CAN Motor Control)
The MIT protocol is a CAN-bus motor control scheme from the MIT Mini Cheetah open-source project, sending position, velocity, feed-forward torque, and gains in a single frame for hybrid force-position control — a de facto standard for robot joint motors.
Planetary Gearbox
A planetary gearbox uses a sun gear, planet gears, and a ring gear with multiple simultaneous tooth contacts and coaxial input/output — delivering high torque density, compactness, and efficiency for robot joint motors.
Quasi-Direct Drive (QDD) Actuator
A quasi-direct drive actuator pairs a high-torque motor with a low reduction ratio (~5–20), balancing torque density with backdrivability and enabling force estimation from motor current — the dominant actuator scheme for dynamic legged robots.
Sim-to-Real
Sim-to-Real is the approach of training robot control policies at scale in physics simulation and then transferring them to real robots — the dominant training paradigm for humanoid locomotion today.
Teleoperation
Teleoperation is real-time remote control of a robot by a human operator — both a practical deployment mode for humanoid robots today and the core method for collecting real-robot demonstration data to train embodied-AI models.