A fail-safe torque transducer, and automotive power steering system, utilizing
a two-stage series coupled torsion bar system. A first stage torsion bar provides
tuning of the steering system, and by reducing its diameter relative to the second
stage, provides a lower torsional spring rate and higher operational stress level
than that of the second stage. The second stage torsion bar provides the stress/strain
indicia referenced by a torque-measuring device, and is stressed at a lower level
than the first stage and therefore has a higher cycle life than that of the first
stage bar. Hence, if stress related failure ever occurs, it will only occur in
the first stage torsion bar, thereby preventing any torque from reaching the second
stage, thereby causing a zero strain signal and thus rendering the system fail-safe
and not subject to instability of vehicle steering if the torsion bar breaks.
