Torque Sensors

A Torque Sensor is a transducer that converts a torsional mechanical input into an electrical output signal. 
Here are 2 types of Torque Sensors: 
a reaction that measures static torque, 
and a rotary that measures dynamic torque. 
Metal foil Strain Gauges are bonded onto a flexure of a structural member that deforms when a torsional or twisting motion is applied.
In most cases, 4 metal foil strain gauges are used to obtain maximum sensitivity and temperature compensation. 2 of the gauges are usually in tension, and 2 in compression, all 4 strain gauges are wired in a Wheatstone bridge configuration with compensation adjustments that correct for temperature errors.