Measuring the strain induced in a shaft due to an applied torque has been the most common method used for torque measurement in recent years. Torque transducers based on strain measurement are normally made by applying strain gauges to a shaft to measure the shear strain caused by torsion.
The shear stress causes strains to appear at 45° to the longitudinal axis of the shaft. So, the strain gauges must be placed precisely at 45° to the shaft axis as shown in fig.
Otherwise, the arrangement is sensitive to bending and axial stresses in addition to those caused by torsion. The output is increased by using four gauges so that the adjacent arms have strains of opposite nature.
Also this arrangement provides complete thermal compensation. For taking signals in and out of the rotating shaft, slip rings and brushes are used. The arrangement of slip rings and brushes are shown on fig.
It is easier to measure bending strains rather than strains due to torque at 45° and so an arrangement using beams may be employed, in which the transmitted torque results in bending the beams. This arrangement is shown in fig.
A slip ring arrangement results in noise due to change in contact resistance also slip rings and brushes wear out and hence it needs to be renewed. A non-contacting type of arrangement as shown in fig is preferred.
This bridge supply and output signals are transmitted between the rotating and stationary member through transformers.
Through AC supply of the bridge, an amplitude modulated AC voltage proportional to the torque is obtained as the output of the bridge.
The AC voltage necessary for supply, the strain gauge bridge and the measurement signal can be transmitted via rotating transformer.