Torsion meters are used for the measurement of power transferred through a propulsion shaft.
Principle
A torque of value T is applied to a shaft of fixed length L and radius r. An angle of twist θ is generated and is dependent of the modulus of torsional rigidity G and given by
T/r = Gθ/L
The modulus of rigidity, the raidus and the length of the shaft are all fixed thus the torque on the shaft is proportional to the angle of twist Typical system
Two AC generators are mounted so that they are driven by the main shaft and area at set distance apart L. A sinusoidal waveform is produced. One of the generators is adjusted so that at minimum torque the generated waveforms are 180' out of phase. The outputs from the two generators are then added and the resultant voltage is used as the measurement of torque
As the torque is applied to the shaft so the twist causes the waveforms to shift in phase. When the two waveforms are now added an output ac current is produced which may be amplified and rectified to give an ouput voltage proportional to the torque appled to the shaft.
Another method of achieving this is to replace the generators wit sensors and toothed ring.