Note: Descriptions are shown in the official language in which they were submitted.
lZ31851
I
A STRAIN SENSOR
This invention relates to a "strain sensor" which
term is used in this specification both to refer to
apparatus which produces an indication of strain and to
5 apparatus which, by sensing strain, is able to give an
indication of some physical property such as temperature
or pressure which is related to strain. The invention
arose in the design of a strain sensor for use in
environments where it is not convenient or not desirable,
10 e.g., because of a fire risk or because of risk of
electrical interference, to transmit electrical energy
to and from the point where the strain is to be measured.
A known strain sensor for use in such environments
is described in European Patent Specification No:
15 0090167. Figure 1 of that Specification shows a sensor
element in the form of a wire which vibrates at a
frequency dependent on the tension applied to it. The
wire is at a first location to which light is transmitted
from two separate light sources in a control room at
20 a second location. Light pulses from one of these
sources is used to produce electric current which induces
vibration of the wire. Light transmitted continuously
from the other source is reflected off the wire when it
passes through a given position during each cycle of its
vibration thereby producing pulses of light at the
frequency of vibration, which is indicative of strain.
These light pulses, reflected from the wire, are transmitted
to the control room at the second location where their
frequency is measured and used as an indication of
strain.
heknown system referred to above suffers from a
number of disadvantages. Firstly, it requires the use
of at least one beam splitter which necessarily
introduces extra expense, bulk and energy loss.
Secondly, the technique of reflecting light from a
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vibrating wire may result in a large energy 105s since,
as pointed out in European Specification No: 0090167,
only a portion of the light will be reflected back along
the path from whence it came. For this reason,
expensive microlenses are needed, as described on page
5 lines 26 to 30, to make use of all the available energy.
Thirdly, the modulation depth achieved by such an
arrangement is limited i.e., the light is not modulated
by 100% of its original intensity by the vibrating wire.
Fourthly, the shape of the pulses of light is likely to
be more in the form of a sine wave than the required
square wire, necessitating the use of a pulse shaping circuit
before the frequency of the pulses can be detected.
Fifthly, the system described in the European Specification
No: 0090167 relies for its operation on a large amplitude
of vibration, see line 21 page 9. From the point of view
of mechanical performance it is better to have smaller
vibrations.
Finally, the vibrating wire sensor of the aforementioned
European Specification, requires a special hole to be
drilled through the field coil for the optical output
fibre and is capable of producing an optical output
only. This sensor is therefore restricted in its
applications to optical systems, unlike the sensor used
in the embodiment of the invention to be described in
which a piezoelectric sensor gives an electrical
output and can therefore be used in other environments,
thereby reducing the cost per sensor by virtue of bulk
production.
This invention aims to reduce the problems described
above and provides a strain sensor comprising: at a
first location optical to electrical transducing means,
a sensor element of the type of which has a resonant frequency
dependent on strain imposed on it, and means for using
energy from the transducing means to stimulate
vibration of the element, to amplify a signal produced
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as a result of the vibration and to generate light
modulated in accordance with that signal and therefore
carrying information indicative of the strain; means
for transmitting the modulated light carrying this
information to a second location; at the second location
means for receiving this modulated light carrying information
indicative of strain, and a light source; and means for
transmitting light from the light source at the second
location to the transducing means at the first location.
By using some of the electrical energy from the
transducing means to amplify the signal produced as a
result of the vibration it is possible to ensure that
the amplified signal is in exactly the required form,
preferably in the form of a series of square shaped
pulses which are then converted into well defined
optical pulses of 100% modulation occurring at the same
frequency as the frequency of vibration of the element.
The electronic amplification of the signal does however
also lend itself to an alternative procedure of generating
pulses of light constituting a binary code defining the
frequency of vibration, but these pulses not appearing
at that frequency. It is notable that, because there is
no need to transmit light both to and from the sensor
element of the invention, as in the arrangement shown in
Figure 1 of the aforementioned European Patent Specification,
there is no need for a beam splitter equivalent to that
shown in that Figure, or the three beam splitters shown
in Figure 2.
The sensor element is preferably a piezoelectric
element such as a quartz crystal provided with suitable
electrodes. The frequency of vibration of such a crystal
varies with varying strain on it and, it can conveniently
be induced to vibrate by applying electric pulses to
it: and will generate electric signals as a result of
and at the frequency of vibration.
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The strain may be imposed by a force acting
externally on the sensor or may be generated internally
e.g., by temperature changes or gradients within the
element in which latter case the indication of strain
can be used as an indication of temperature.
One way in which the invention may be performed
will now be described by way of example with reference
to the accompanying schematic drawing of a strain sensor
constructed in accordance with the invention.
lo At a first location, indicated generally by the
reference numeral 1 on the drawing, is positioned a
source of light 2 which, in this particular embodiment,
is a laser diode. The laser diode 2 is driven by a
drive circuit 3 so as to produce pulses of light as
shown graphically at 4 having relatively long pulse periods
compared with the interpulse periods. The drive circuit
3 has a nominal frequency of 20kHz but this can be
controlled to a limited extent by a control signal on a
line 22. The choice of drive circuit 3 will be appropriate
to the laser 2. Transistor VN66AF used as specified by
the manufacturer is suitable. In some embodments a
phase locked loop is used which is not shown but could
be part of the drive. This could be National Semi-
conductor NE565.
Pulses of light from the laser diode 2 are transmitted
along an optical fibre line 5 to a remote location
indicated generally at 6. The light issuing from the
optical fibre line 5 is incident on a bank of photo-
detectors or solar cells of which, for simplicity of
illustration, only three are shown in the drawing. A
minority of these detectors, as indicated by the single
cell7, are connected to the primary of a step up
transformer 8. Because of the intermittent nature of the
light pulses the transformer 8 generates an increased
a.c. voltage of approximately 40V peak-to-peak across
its secondary coil. This 40V alternating voltage is
applied across a quartz crystal device 9 by means of
electrodes 10 and 11.
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It stimulates the quartz to oscillate at its resonant
frequency which is nominally 20kHz i.e., the same
nominal frequency as the drive circuit 3, but which
varies about this frequency depending on the strain
imposed on the quartz crystal 9. The oscillations of
the crystal 9 cause the crystal, because it is piezo-
electric, to generate a voltage across electrodes 10
and 12 at a frequency equal to the frequency of
vibration. This piezoelectric voltage causes current
flow through an amplifier 13 acting as a current-to-
voltage convertor. The output voltage of the current-
to-voltage convertor 13 is amplified at 14 and is used
to switch on and off a switch 15 in time with the pulses
generated between electrodes 10 and 12.
The majority 17 of the photocells receiving light
from the fibre optic line 5 produce an output of +lV
which is applied to a voltage convertor 18 which produces
outputs of +lV and -lV used to drive the devices 13 and
14. Any suitable voltage convertor can be used at 18
but Intersil*ICL7660PC is an example of a suitable
readily available circuit.
The switch 15 is connected across the +lV and
-lV outputs of the voltage convertor 18 and in series
with the light emitting diode 16 so that~the latter is
switched on and off in time with the oscillations of the
crystal 9. The resulting light is transmitted along a
fibre optic line 19 to the first mentioned location 1
where the light is received by a photocell 20. The
frequency of the output of the photcell 20 is measured
in a receiving circuit 21 which a suitable circuit is
National Semi-Conductor LH0082 Fibre Optic Receiver. This
produces an output on line 22 indicative of the frequency
of vibration of the crystal 9 and therefore indicative
of the strain imposed on it. The signal on line 22
is used both as an output of the device giving a measure
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of the strain and as a feedback signal to the drive
circuit 3 as previously described. This feedback
arrangement facilitates resonation of the crystal at
the particular frequency dependent on the load.
In an alternative form of the invention the drive
3 for the light source 2 could be designed to
repetitively sweep the frequency through a range
including the nominal frequency of 20kHz. In such an
arrangement the receiver 21 would be replaced by a
device for detecting the frequency at which the light
received by the detector 20 reached a maximum
intensity. In this embodiment of the invention there
would be no need for a feedback from the receiver 21 to
the drive 3.
Other modifications are of course possible. For
example the transformer shown at 8 could be replaced by
any one of many other possible voltage increasing devices.
Alternatively the components 8 and 18 could be replaced
by a single transformer having two secondaries designed
to produce a high voltage for stimulating the piezo-
electric element, and a low voltage for the amplifier.
In another arrangement the illustrated amplifier
could be replaced by an amplifier/encoder designed to
produce pulses of light at 16 in the form of an
encoded message representing the frequency of vibration.
A suitable decoder would of course then be needed at the
location 1.
