Joint Torque Sensor

A sensor built into a robot joint that directly measures the torque the joint is actually producing.

A joint torque sensor sits between a joint's reducer output and the link it drives, measuring torque only around that single joint's rotation axis — hence it's also called a one-axis torque sensor — and is mostly strain-gauge based, converting a tiny deformation of an elastic element into a torque reading. Motor current can also be used to estimate torque, but friction in the reducer makes that estimate inaccurate; measuring directly at the output lets the robot sense even very light external forces. KUKA's LBR iiwa carries one of these sensors on all seven axes, and the manufacturer states that this lets it immediately sense contact and reduce force and speed, allowing it to work alongside people without safety fencing; Franka's control interface likewise reports each joint's measured torque directly as tau_J. This is the basis for collision detection, torque control, impedance control, and kinesthetic teaching, at the cost of a higher price and a more complex joint design.

ExampleA person pushes on a moving KUKA iiwa arm by hand. The joint torque sensor's reading exceeds what the dynamics model predicts, and the controller judges that a collision has occurred, immediately stopping or yielding to the push.

Also called

Joint Torque Sensing, One-Axis Torque Sensor

Related

Six-Axis Force/Torque Sensor · Strain Gauge · Sensorless Force Estimation · Torque Control · Impedance Control · Collision Detection (Robot Safety)

Sources

KUKA LBR iiwa 产品页 (Chinese)
libfranka robot_state.h（tau_J: measured link-side joint torque sensor signals）