RAYLEIGH WAVE ROTARY TORQUE TRANSDUCERS
TORQSENSE Transducers make use of the little known Rayleigh Wave
or Surface Acoustic Wave (SAW) phenomenon. This patented method
has allowed the development of small transducers, which are not only
exceptionally accurate, but require no physical contact between the
revolving shaft and its housing.
In a SAW type sensor, the surface waves are produced by passing
an alternating voltage across the terminals of two interleaved
comb-shaped arrays that are layed onto one end of piezoelectric
substrate. A similar ‘receiving’ array, at the other end of the transducer,
converts the wave back in to an electric signal.
The frequency of the wave generated is dependant on the spacing
of the ‘teeth’ in the comb array and as the direction of the wave
propagation is at right angles to the teeth, any change in its length
alters the spacing of the teeth and hence the operating frequency.
Tension in the transducer reduces the operating frequency while
compression increases it.
To measure torque in a shaft, two SAW sensors are bonded rigidly
to a shaft at 45 degrees to the axis and connected in a “half bridge”
configuration. When the shaft is subjected to a torque, outputs can
be combined to produce either ‘difference’ or ‘sum’ signals leading
respectively to torque or temperature signals.
The SAW transducers are self-contained and require no physical
connection to the shafts housing. Instead, the signals are exchanged
via a capacitive coupling device comprising two discs, one of which
rotates with the shaft while the other is stationary.
The frequency of the exciting oscillation used is typically 200 MHz,
with the frequency shift produced varying by up to 1MHz. On this basis,
the torque can be measured to a resolution of one part in a million.
Over a typical bandwidth of 1MHz, the accuracy does not vary by
more than 0.1 percent.
Measurement of torque requires the transducer to be placed in line
between the load and the drive source. This transducer, being a
finite size, will influence the dynamics of the system. The most
important mechanical parameters affected will be inertia and
compliance. Keeping them low will result in a higher natural resonant
frequency, which will enable rapid torque changes to be observed.
The SAW technique has a sensitivity of two orders of magnitude
over resistive strain gauges. This enables a stiff, non-compliant
structure to be utilised.
The electronic interface is based on frequency change, not voltage level.
This improves the rejection of noise, which is further improved by the
operating frequencies (200MHz), and use of wide bandwidth
transmission line connection techniques, which can respond to
sub microsecond changes.
The type of electronic discrimination used sets the available bandwidth,
which can be from a low 5KHz to more than 1MHz, and would far
exceed any mechanical system variation.
The main advantages of torqsense...
· Exceptional sensitivity
· Simple RF coupling communicates with sensors in a non-contact way
· Improved EMC performance
· High bandwidth
· High signal to noise
· Frequency output for ease of signal processing
· Stiff robust assembly with low compliance
· Relatively low cost
· Differential system means errors due to temperature can be greatly reduced
· Fail safe operation
· Impervious to contamination from surroundings
· Self auditing
Article submitted by Sensor Technology Ltd.
Email: info@sensors.co.uk Web site: www.sensors.co.uk
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