force/torque sensor

A force/torque sensor is a small device, usually bolted into a robot's wrist between the arm and its hand, that measures how hard the robot is pushing on the world and how hard the world is pushing back. It is the robot's sense of touch through its grip — the same way you can feel, without looking, whether you are pressing a doorbell gently or leaning your whole weight on a wall.

The common kind is called six-axis because it reports six numbers at once: the push or pull along three directions (forward/back, left/right, up/down) and the twist around each of those three directions. Inside, a stiff metal body flexes by a tiny, invisible amount under load, and electrical strain gauges glued to it turn that flex into a changing voltage. The sensor's job is to untangle those voltages back into the six clean force and twist values.

Robots use this to do delicate, contact-rich jobs that pure position control cannot handle: pressing a plug into a socket, polishing a curved surface with steady pressure, or stopping the instant they bump something unexpected. Without it, a robot arm is strong but numb — it can hit an exact spot in the air, yet has no idea how firmly it is touching what is there.