Vibrating Beam Sensors, an attractive
option to replace strain gauges

Vibrating steel forks, mounted on weldable platforms, look to be
the current most attractive option to replace strain gauges. The
output frequencies allow measurement of strain to extreme
accuracies, while consuming only about a microwatt. They can
be powered remotely, requiring no wires or batteries, and can
be made immune to external interfering vibrations.

First test applications are in pressure sensors but they could
soon be quite ubiquitous, potentially cheaper than surface
acoustic wave sensors in electric power steering systems and
robust enough to go onto jet engine compressor blade roots.

The devices were first conceived six years ago by Professor
Barry Jones, director of the Brunel Centre for Manufacturing
Metrology, part of Brunel University at Uxbridge in West London.
However, given the usual speed at which new research ideas
are funded in the UK, their further development has only recently
attracted further funding and is being supported under an
INTErSECT (INTElligent SEnsors for Control Technology) EPSRC
Faraday Partnership. The project has been dubbed REMISE,
which stands for REsonant MlcroSEnsor modules for measurement
of physical quantities, although it has unofficially been named,
the singing strain gauge.

Development is proceeding under the auspices of research fellow
Dr Tinghu Yan, in conjunction with a parallel study at Southampton

The basic idea is to take a piece of thin sheet and etch out three
narrow beams, joined to the original sheet at each end. Brunel is
making its devices out of steel, while Southampton is investigating
even smaller designs etched out of silicon. A thick-film area of
piezoelectric material is deposited on each end and the beams
are driven in such a way that when the centre of the middle beam
is rising, the centres of the outer beams are falling and vice versa.

The output from the thick film piezoelectric transducer at one end
is passed to an amplifier to provide slight positive feedback to the
driving piezoelectric transducer at the other end, so that the device
oscillates at its resonant frequency. The exact frequency of
resonance depends on the tension applied externally to the beams,
as is the case with violin and guitar strings.

Frequency can be measured very accurately using digital techniques.
This makes strain measurements more accurate than analogue
changes in conventional strain gauge resistance and consequential

The Q factor, which is the measure of the quality of a particular
resonance, of the first devices made in air has been found to be
about 1,000. Because the beams need only vibrate with an
amplitude of about 1 micron, power consumption, even in air, is
only about a microwatt. In initial experiments, the first devices
- about 12mm long and with a natural frequency of 6KHz -
could be powered remotely by an inductive loop.

Professor Jones says that it should easily be possible, in future
designs, to reduce the average power consumption further, by
making the devices smaller and running them only when required,
with standby power available from a capacitor. The capacitor
could be kept charged by a GHz electro-magnetic signal from
a source up to 3m away, while still keeping well within the limits
imposed by licence-free, low-power radio regulations.

The next intended designs are to be 6 to 8mm long and are
expected to resonate at 15 to 20kHz. Finite element analysis is
used to determine resonant frequencies that will be well away
from any interfering frequencies expected from any particular
application. "We have never found external vibration to be a
core issue," explained Professor Jones, when Eureka enquired
about the state of play at this year's mtec show.

The beauty of the steel products is that they have the potential
to be made, even in small quantities, at prices which should
compete favourably with conventional strain gauge devices.
They can, as in the prototype designs, be spot welded onto
supporting plates, which can in turn be welded onto the devices
to be monitored.

The silicon devices being investigated at Southampton are around
3mm long, and are expected to have even smaller power

Their advanvantage is that driving and amplifying electronics can,
in theory, be incorporated into the same piece of silicon required
to perform sensing. Their disadvantages are that they would
have to be mounted in evacuated and sealed chambers, and initial
tooling and set-up costs are likely to be higher.

Either type of device will probably have to be mounted in protective
containment if used for monitoring deflections on jet engine fan
blade roots, but automotive applications might not require such a
high degree of protection. The intention behind mounting the
sensors on fan blades is to continuously monitor compressor torque,
so as to optimise performance and drive components harder. If
loading is inferred, a margin of safety has to be left in the design
to allow for uncertainties in what might actually happen. If loading
is actually measured, a component can safely be driven right up
to just below the load at which it is expected to start to fail or
exhibit an unacceptable fatigue life.

'Singing'strain gauges could one day
be used to monitor deflections in
jet engine compressor blade roots
as in the Rolls-Royce Trent 500
and 600 engines.

Applied to electric power steering systems, reliable torque measurement
prevents unintentional overloading on wheel bearings or other steering
system components. It is thus highly desirable, if not essential, in such
systems. However, present designs, based on surface acoustic wave
sensors, such as that first revealed in Eureka's September 1993 cover
story, are more expensive than automotive customers would like.

The research is expected to continue for another three years, but
should anyone have a more immediate application, Professor Jones
and his colleagues would like to hear about it. Since the technology
has been successfully demonstrated and tested in prototype form,
it could doubtless be implemented quite quickly if required.


For more information, contact :-

The Brunel Centre for Manufacturing Metrology


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