Force-torque sensor

The measurement is a wrench at a frame#

A six-axis force-torque sensor reports linear forces along three axes and moments about those axes. Together those values form a wrench. The numbers are defined at the sensor frame, so moving the reference point changes the reported moment even when the physical load is unchanged.

Robots commonly place a sensor between an arm and its end effector. Sensors can also sit at feet, joints, tools, or test fixtures. A wrist sensor measures the net load transmitted through that interface, including contact forces, tool weight, and inertial effects.

Contact feedback needs calibration and compensation#

Force control can compare a measured wrench with a desired one during polishing, insertion, or hand-guiding. Humanoid foot sensors can help estimate ground contact loads, while wrist sensors can detect contact during manipulation.

Raw readings are not automatically external contact forces. Bias, the attached tool's mass, its centre of gravity, gravity direction, and the transform between sensor and robot frames must be handled. Traversaro and colleagues present calibration methods for these quantities using sensor reorientation. Fanello and colleagues describe in-situ six-axis calibration validated on force-torque sensors in the iCub humanoid.

One sensor cannot explain every contact#

The reading combines all loads transmitted through the mounting interface. It does not by itself reveal where several contacts occurred or how force is distributed across a fingertip. Tactile sensing can provide more local contact information, while model-based estimation can infer loads at locations without a dedicated transducer.

Sensor range, noise, cross-axis coupling, overload capacity, drift, temperature, mounting stiffness, and sampling rate all affect use. Saturation or an incorrect bias can mislead a feedback controller. A force-torque sensor supplies a measurement; it does not on its own make contact safe or controlled.