An external instrument provides an AC
excitation voltage to the strain gage
bridge via the excitation transformer. The
sensors strain gage bridge then drives
a second rotary transformer coil in order
to get the torque signal off the rotating
sensor. By eliminating the brushes and
rings of the slip ring, the issue of wear is
gone, making the rotary transformer
system suitable for long term testing
applications. The parasitic drag torque
caused by the brushes in a slip ring
assembly is also eliminated. However,
the need for bearings and the fragility of
the transformer cores still limits the
maximum rpm to levels only slightly
better than the slip ring. The system is
also susceptible to noise and errors
induced by the alignment of the
transformer primary-to-secondary coils.
Because of the special requirements
imposed by the rotary transformers,
specialized signal conditioning is also
required in order to produce a signal
acceptable for most data acquisition
systems, further adding to the systems
cost that is already higher than a typical
slip ring assembly