Torque

The turning effect of a force on an object, equal to force times lever arm, measured in newton-meters.

Torque describes the effect a force has in rotating an object about a point or axis; it's also called the moment of force. It's defined as τ = r × F, where r is the vector from the rotation axis to the point where the force is applied and F is the force; its magnitude is τ = rF·sinθ, where θ is the angle between r and F, and r·sinθ is the lever arm — the perpendicular distance from the axis to the force's line of action — measured in newton-meters (N·m). The longer the lever arm, the more torque the same force produces. The rotational counterpart of Newton's second law is τ = Iα, where I is the moment of inertia and α is angular acceleration. In a robot, every rotary joint's motor outputs torque around its own axis: the torque needed to overcome gravity, inertia, and contact forces at each joint drives the choice of motor and reducer, joint-module datasheets list rated and peak torque, and torque control means sending a torque command directly to each joint.

ExampleAn arm extended horizontally, holding a 1 kg object 0.5 m from the shoulder joint, requires the shoulder to produce roughly 1 × 9.8 × 0.5 ≈ 4.9 N·m of torque from that object alone, not counting the arm's own weight.

Also called

Moment of Force

Related

Wrench · Torque Control · Joint Torque Sensor · Peak Torque · Gravity Compensation · Moment of Inertia

Sources

Wikipedia: Torque
Modern Robotics (Lynch & Park, 2017)