Strain sensor

A strain sensor includes a quartz crystal element 9 having a resonant frequency which is dependent on strain imposed on it. The crystal element 9 is caused to vibrate at its resonant frequency by pulses of light transmitted along an optical fibre 5 from a remote location indicated generally at 1. A signal produced by the vibration at the resonant frequency is taken from electrodes 10 and 12 and amplified at 13 and 14 before being used to switch on and off a switch 15 controlling a light source 16. The output from the light source 16, consisting of flashes of light of a frequency indicative of the strain, is transmitted along a second optical fibre 19 and is detected by a photocell 20. The frequency of the output of photocell 20 is measured at 21 to give the desired measurement of strain and to provide feedback to the drive unit 3 which determines the frequency of the flashes of light produced at 2.

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 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, 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: 0090167. FIG. 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 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. 
The known 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 vibrating wire may result in a large 
energy loss since, as pointed out in European Specification No: 0090167, 
only a portion of the light wil 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 
wave, 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 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 FIG. 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 FIG. 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. 
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.

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 20 kHz 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 diode 2. 
Transistor VN66AF used as specified by the manufacturer is suitable. In 
some embodiments a phase locked loop is used which is not shown but could 
be part of the drive. This could be National Semiconductor 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 
photodetectors 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 cell 7, 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 40 V 
peak-to-peak across its secondary coil. This 40 V alternating voltage is 
applied across a quartz crystal device 9 by means of electrodes 10 and 11. 
It stimulates the quartz to oscillate at its resonant frequency which is 
nominally 20 kHz 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 piezoelectric, 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 +1 V which is applied to a voltage convertor 18 
which produces outputs of +1 V and -1 V 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 +1 V and -1 V 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 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 20 kHz. 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 piezoelectric 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.