Torque Fundamentals

Torque Fundamentals

Robots move by rotating joints. Torque is the rotational equivalent of force, and understanding it is essential for actuator sizing.

Definition

Torque is the cross product of a position vector and a force:

τ = r × F = r · F · sin(θ)

Where:

• r = moment arm length (m)
• F = applied force (N)
• θ = angle between r and F

Units: Newton-meters (N·m)

Mechanical Advantage

A longer moment arm produces more torque from the same force. This is why wrenches are long and why robot link geometry matters.

Quasi-Direct Drive

The CatBot actuators use a quasi-direct drive (QDD) architecture: a low-reduction-ratio gearbox (typically 6:1 to 9:1) between the motor and output. This means:

• High backdrivability — the joint can be pushed back by external forces
• Low reflected inertia — feels light and responsive
• Torque transparency — controller gets accurate force feedback

The tradeoff: you need a motor with high peak torque, which means managing heat carefully.