Robotics Glossary
Precise definitions of core humanoid robotics and actuator concepts
Coreless Motor
A coreless motor uses an ironless cup winding for low inertia, low cogging, and fast response in dexterous hands and other compact precision drives.
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.
Dexterous Hand
A dexterous hand is a multi-DOF robotic end-effector modeled on the human hand for grasping, tool use, and fine manipulation.
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.
Force Control (Impedance / Admittance Control)
Robot force control uses torque, impedance, or admittance control for compliant interaction. Calibrated QDD joints can estimate output torque from phase current.
Frameless Torque Motor
A frameless torque motor supplies a stator and rotor for direct integration into a robot joint, reducing redundant housing and drivetrain components.
Harmonic Drive (Strain Wave Gear)
A harmonic drive uses flexspline deformation for high reduction and low backlash. Robot arms commonly use it; dynamic legs may prioritize backdrivability.
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 QDD actuator pairs a high-torque motor with a low ratio of about 5–20, balancing torque density, backdrivability, and current-based torque estimation.
RV Reducer (Cycloidal Reducer)
An RV reducer combines planetary and cycloidal stages for high rigidity, load capacity, and shock tolerance in industrial robot base and shoulder joints.
Reflected Inertia
Reflected inertia is rotor inertia multiplied by gear ratio squared at the output. It directly affects joint backdrivability and impact response.
Robot Data Collection
Robot data collection records paired observations and actions, often through teleoperation, to train VLA models and imitation-learning policies.
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.
Six-Axis Force/Torque Sensor
A six-axis force/torque sensor measures three forces and three torques at one point for robot wrists, ankles, assembly, and contact control.
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.
Torque Density (N·m/kg)
Torque density measures actuator torque per unit mass. It helps compare joint weight, payload capacity, and dynamic performance.
VLA Model
A Vision-Language-Action model maps camera observations and language instructions to robot actions for manipulation and embodied-AI tasks.
World Model
A robotics world model predicts future environment states from the current state and an action, supporting planning, simulation, and policy training.