The present invention relates to the measurement of distances, displacements and forces. In particular, the present invention relates to the measurement of displacements caused by mechanical forces, such as, for example, tractions or compressions and the relative forces producing said displacements. In more detail, the present invention relates to the measurement of displacements and forces by using optical transducers. Still in more detail, the present invention relates to the measurement of displacements and forces by using optical transducers and/or sensors comprising low cost optical fiber components. Finally, the present invention relates to a method, a device and a system for measuring distances, displacements and forces, with said device and system comprising low cost optical transducers and/or sensors.
Recently, much development work has been devoted to the development of devices adapted to measure and/or detect mechanical forces and displacements in a very reliable manner. Among the devices and systems developed and proposed, systems and devices based on very sophisticated electronic assemblies have became the most largely used devices and systems. This, in particular, was due to development in the field of integrated circuits and the corresponding reduction in size of circuits having very complicated functions, allowed the manufacture of very small electronic transducers, adapted to be used for different purposes and under very difficult conditions. For instance, electronic transducers are known, the size of which is kept less than a few cubic millimeters. Moreover, recent developments in the field of computing means, in particular, in the field of software used to process large quantities of data in a short time, allowed the data detected by the electronic transducers to be processed in an automatic and reliable manner. Finally, the decreasing costs of electronic systems, allowed for containing the costs for producing electronic transducers, thus allowing electronic transducers to be used for several purposes and applications.
However, in spite of all the advantages cited above offered by electronic transducers, electronic transducers are not free from drawbacks. The most relevant drawback affecting electronic transducers arises from the fact that electrical current is needed for operating the electronic transducers. It is appreciated that in the case of a force acting on an electronic transducer, the electrical current flowing through the transducer is influenced by the force acting on it, so that the variations in the current flow may be detected and used for obtaining an indication of the intensity of the force acting on the transducer. However, the electrical current flowing through the electronic transducers may also be influenced by the external environment, thus rendering electronic transducers less reliable for applications in critical environments, such as in structures exposed to electrostatic discharges during thunderstorms or in electromagnetically noisy industrial premises. Moreover, it may become difficult or risky to use electronic transducers in storage areas of highly flammable materials. Finally, some electronic transducers are also not suitable for biomedical applications because the risk of electrocution may arise.
Accordingly, in view of the problems explained above, it would be desirable to provide a technology or device that may solve or reduce these problems. In particular, it would be desirable to provide transducers suitable to be used in structures exposed to electrostatic discharges and/or in noisy industrial premises, or even in storage areas of highly flammable materials. In the same way, it would be desirable to provide transducers for measuring and/or detecting forces, suitable for use in biomedical applications. Furthermore, it would be desirable to provide transducers having low cost, light weight, reduced size and minimal invasiveness. It would also be desirable to provide transducers for the purpose of reliably measuring forces and displacements, in combination with being of low cost, simple and able to be used in well-known equipment.
Some attempts have been made recently for overcoming the drawbacks affecting electronic force measurement systems. In particular, some efforts have been devoted to the development of optical transducers to measure forces and the displacements they cause. Many of these optical transducers are based on the premise that forces may be measured and/or detected using evaluations of the effects on light transmitted through an optical path caused by forces acting, either directly or indirectly on said optical path. In particular, the working principle of many optical transducers is based on the variation in the photocurrent detected at the output of an optical path as a consequence of a variation in the link attenuation due to the force under test. In fact, it has been observed that a relationship may be established between the photocurrent detected at the output of an optical path with the mechanical stress acting on the mechanical path. Unfortunately, however, the known optical transducers are not free from drawbacks and some of them are also not as reliable as desired. Finally, assembling and manufacturing many of the known optical transducers is quite cumbersome and, therefore, quite expensive.
It would therefore be desirable to provide optical transducers overcoming the drawbacks effecting prior art optical transducers such as, for instance, reduced reliability, reduced application range, and high costs, while maintaining a satisfactory sensitivity.
In general, the present invention is based on the consideration that forces, in particular, mechanical forces and/or actions such as tractions and/or compressions may be detected and/or measured using the variations of light through an optical path caused by said mechanical actions acting, either directly or indirectly, on the optical path. In particular, the working principle of the present invention is based on the consideration that the variation in length of an optical path due to a mechanical force acting on the optical path causes the phase of an optical signal transmitted through the optical path to be shifted and/or modified so that, if the optical signal, when exiting said optical path, is converted into an electrical signal, the variation in length of the optical path results in the phase of the electrical signal being also shifted and/or modified, thus differing from the phase the electrical signal would have had in the absence of any mechanical action acting on it. Accordingly, if a second path is used, say a reference path, with said second path being adapted to emit a reference electrical signal and not being subjected to the mechanical actions acting on the optical path (the sensing path), the electrical signal exiting the sensing optical path will have a phase differing from that of the electrical signal exiting the reference path, with said difference being related to the variation in length caused by a mechanical action or stress acting on the sensing optical path. Accordingly, by comparing the phases of the signals at the output of the sensing and reference paths, it is possible to determine the variation in the sensing path length and to relate this variation to the mechanical action acting on the sensing optical path. Although this detection approach may appear to be quite general in principle, it has been revealed to be very reliable for the purpose of detecting and/or measuring forces, in particular, mechanical forces, such as, for example, tractions or compressions or tension and compressive forces. Moreover, this detection approach allows the implementation of components suitable to improve the resolution, the sensitivity, the accuracy and reliability of the measurements. Moreover, when fibers, such as, for instance, polymer optical fibers (POF) are used for the purpose of realizing the sensing optical path, further advantages arise in terms of costs, besides the advantages common to the other types of optical fibers such as, for instance, lightweight, minimal invasiveness, immunity to electromagnetic interferences and impossibility to start a fire or an explosion. However, copper wire or coaxial cable may be used for the reference path. Finally, further advantages also arise due to the less demanding mechanical tolerances and the availability of low cost sources and photo detectors. A detecting and/or measuring approach according to the present invention can be used in the case of critical environments such as in electromagnetically noisy industrial premises, in storage areas of highly flammable materials, in structures exposed to electrostatic discharges during thunderstorms and in the monitoring of monuments or art pieces in general. The absence of electrical currents flowing through the sensing area of the sensor according to the present invention makes this transducer also ideal for biomedical applications avoiding the risk of electrocution. It is also possible to control several sensing points or areas, and the corresponding transducers, simultaneously, by means of complex yet quite inexpensive networks of sensors according to the present invention. Moreover, if suitable software is used, it is also possible to control the sensors via remotely using a network connection or the internet or web using standard protocols such as, for instance, the TCP/IP protocol.
It should also be appreciated that the detecting approach according to the present invention and, accordingly, the detecting transducers according to the present invention, contrary to some prior art optical transducers, does not require optical fibers to be interrupted, such as, for instance, in the case of optical transducers based on the reflection and/or absorbance of light, so that the whole transducers and detecting means and/or devices according to the present invention can be better isolated from dust, rain or the like, thus rendering the detecting transducers and/or devices according to the present invention particularly suitable for outdoor applications.
On the basis of the considerations as stated above, there is provided an optical transducer adapted to detect external mechanical actions acting on the transducer, the transducer comprising at least one sensing optical path adapted to transmit at least one sensing optical signal and to emit at least one sensing output electrical signal and at least one reference path adapted to emit at least one output electrical reference signal. Moreover, at least one portion of the at least one optical path is adapted to be exposed to external mechanical actions or forces, so that the transmission of the sensing optical signal through the sensing optical path is modified, resulting in a phase shift between the sensing electrical signal and the reference electrical signal.
In particular, a first embodiment of the present invention relates to an optical transducer wherein said at least one reference path comprises phase shifting means adapted to maintain the phase shift between the at least one output sensing electrical signal and the at least one output reference electrical signal at a constant value in the absence of any mechanical action or force exerted on the at least one sensing optical path.
According to a further embodiment, the present invention relates to an optical transducer comprising means for collecting the at least one output electrical reference signal and to emit a further electrical reference signal, with the further electrical reference signal being shifted in phase with respect to the output reference electrical signal of about 90°.
According to still a further embodiment of the present invention, an optical transducer is provided wherein the optical path is adapted to transmit at least two optical sensing signals with corresponding different wavelengths, with only one signal of the two sensing optical signals entering the at least one sensing portion. Moreover, the optical path further comprises means adapted to receive the at least two sensing optical signals and to convert the at least two sensing optical signals into two corresponding output sensing electrical signals.
According to another embodiment of the present invention, an optical transducer is provided wherein the at least one portion of the at least one optical path has a predefined length adapted to be modified as a result of mechanical actions or forces acting on the at least one portion.
According to still another embodiment of the present invention, an optical transducer is provided wherein the at least one optical path comprises optical emitting means adapted to receive at least one input sensing electrical signal and to convert the at least one input sensing electrical signal into the at least one sensing optical signal.
According to a further embodiment of the present invention, an optical transducer is provided wherein the at least one reference path comprises a reference optical path adapted to transmit at least one reference optical signal and optical receiving means adapted to receive said at least one optical reference signal and to convert the at least one optical reference signal into the at least one reference electrical signal.
According to another embodiment of the present invention, an optical transducer is provided wherein the at least one portion of the at least one sensing optical path comprises at least two rectilinear portions disposed parallel to each other and joined by a curved portion.
According to a further embodiment, the present invention relates to an optical transducer is provided wherein the optical transducer comprises a plurality of sensing optical paths each adapted to transmit at least one corresponding sensing optical signal and to emit at least one corresponding sensing output electrical signal and each comprising at least one portion adapted to be exposed to external mechanical actions or forces. Moreover, the optical transducer comprises one reference path adapted to emit at least one electrical signal.
According to a further embodiment of the present invention, a measuring device is provided for measuring and/or detecting mechanical actions or forces comprising at least one optical transducer and measuring means adapted to measure the phase shift between the at least one sensing electrical signal and the at least one reference electrical signal.
According to still a further embodiment of the present invention, a measuring device is provided comprising first measuring means and second measuring means, the first measuring means being adapted to collect the at least one sensing electrical signal and the at least one reference electrical signal and to emit a first output electrical signal, the second measuring means being adapted to collect the at least one output sensing electrical signal and a reference electrical signal shifted in phase by 90° with respect to the reference electrical signal and to emit a second output electrical signal so that the phase shift between the at least one reference electrical signal and the at least one sensing electrical signal can be measured as a function of the amplitude of one or both of the output electrical signals.
According to a further embodiment, the present invention relates to a measuring device wherein the measuring means comprise mixing means adapted to mix the electrical signals and to emit electrical signals and wherein the measuring means comprise a low-pass filter adapted to receive the electrical signals, and to emit electrical signals.
Still according to the present invention, a measuring method is also provided. The measuring method for measuring mechanical actions or forces comprises providing an optical transducer so that at least one portion of the at least one optical path is exposed to said mechanical actions or forces, entering at least one sensing optical signal into the at least one portion of the at least one sensing optical path and converting the optical signal into an output sensing electrical signal. Moreover, the method comprises inducing the at least one reference path to emit the at least one output electrical reference signal, shifting the phase of the at least one output electrical signal so as to maintain the phase difference between the at least one output sensing electrical signal and the at least one output reference electrical signal at a constant value in absence of any mechanical action or force exerted on the at least one sensing optical path and measuring the phase shift between the at least one output sensing electrical signal and the at least one output electrical reference signal.
According to a further embodiment of the present invention, a measuring method is also provided comprising fixing the opposed ends of the at least one portion of the at least one optical path to fixing means so that the mechanical actions or forces acting on the portion results in the length Ls of the portion being modified, thus generating a phase shift between the at least one output sensing electrical signal and the at least one output reference electrical signal.
Still a further embodiment of the present invention relates to a measuring method comprising measuring the phase shift between the output electrical sensing signal and the at least one output electrical reference signal in the absence of any mechanical action or force acting on the at least one portion of the at least one optical sensing path.
Further, additional embodiments of the present invention are defined in the appended claims.
Further advantages, objects and features as well as embodiments of the present invention are defined in the appended claims and will become more apparent with the following detailed description when taken with reference to the accompanying drawings, in which like features and/or component parts are identified by like reference numerals. In particular, in the drawings:
While the present invention is described with reference to the embodiments as illustrated in both the following detailed description and the drawings, it should be understood that the following detailed description, as well as the drawings, are not intended to limit the present invention to the particular illustrative embodiments disclosed, but rather the described illustrative embodiments merely exemplify the various aspects of the present invention, the scope of which is defined by the appended claims.
The present invention is understood to be of particular advantage when used for detecting and/or measuring mechanical actions, stresses or strains, such as, for example, tensions and/or compressions. For this reason, examples will be given in the following in which corresponding embodiments of the optical transducer and the measuring device according to the present invention are used for detecting both tractions or tensions and compressions as well as displacements caused by the tensile and/or compressive forces. However, it has to be noted that the use of the optical transducers according to the present invention is not limited to the detection and/or measurement of tractions or tensions and compressions and the resulting displacements; on the contrary, the optical transducer and the measuring device according to the present invention may also be used for the purpose of measuring and/or detecting different forces as well as distances. The present invention is, therefore, also useful for the measurement of all these forces, mechanical actions, distances and displacements, and the forces, displacement and distances described in the following are to represent any force, distance and displacement.
The first embodiment of an optical transducer and a measuring device according to the present invention will be described in the following with reference to
In
The working principle of the embodiment of the present invention depicted in
The signal generator means 1 produces a RF signal a at a frequency f, to be determined on the basis of general considerations such as, for instance, the trade-off between costs and performances. In general, the higher the frequency used, the better is the resolution of the deformation measured although ambiguity problems may arise with large displacements; accordingly, in the case of coarse or relevant displacements, lower frequencies may be preferred. Alternatively, it is also possible to use a frequency-variable generator, allowing measurements both at a frequency low enough to avoid ambiguities and at a frequency high enough to obtain the achieved resolution. The RF signal generated by the generator means 1 is divided into two sub signals b and c, not necessarily in an even way, by the divider means 2. One output of the divider means 2 feeds the light emitting means 3 while the other output is connected to the reference path 4 so that the signals b and c enter the optical emitting means 3 and the reference path 4, respectively. For instance, the light emitting means 3 may comprise an LED, a laser diode or the like. The signal b is converted by the light emitting means 3 into a corresponding optical signal b′ which enters the sensing optical path 5, in particular, the sensing portion 5′ of the sensing optical path 5. The optical signal b′ is transmitted through the sensing portion 5′ of the sensing optical path 5 and subsequently enters the optical receiving means 6 where the optical signal b′ is converted into a corresponding electrical signal d. At the outputs of both the optical sensing path 5 and the reference path 4, the corresponding signals d and e are collected by the measuring means 7 for measuring or determining the difference in phase between the signals d and e. To this end, the measuring means 7 are adapted to emit a signal f which is subsequently collected by computing means (not depicted in
Considering now a sinusoidal generator 1, the RF signal emitted by the sinusoidal generator 1 can be described by
a(t)=A cos(ωt) (1)
A being a constant.
Assuming an ideal behavior for the power divider means 2, the signals b and c emitted by said power divider means 2 can be described, respectively, by
b(t)=B cos(ωt) c(t)=C cos(ωt) (2)
where the constants B and C are related to A by the splitting ratio of the power divider means 2. Accordingly, the signals d and e entering the phase measuring means 7 can be described by
d(t)=D cos(ωt+φ) e(t)=E cos(ωt+θ) (3)
the constants D and E being related to B and C by the attenuations of the sensing path 5 and the reference path 4, respectively. The phase shifts φ and θ in equation (3) depend on the propagation velocity v of the signals in the sensing and in the reference paths 5 and 4 and on their lengths as well as on the phase delay introduced by any device along the path.
It appears, therefore, clearly from equation (3) that a variation in the length of the sensing path 5, in particular, in the length of the sensing portion 5′, due to an external mechanical action or force such as, for instance, a tensile or a compression force, will produce a variation of φ and, therefore, a variation of δ(φ−θ), and, in turn, a variation of the output signal f exiting the phase measuring means 7.
Accordingly, if the natural phase shift of the optical transducer and the measuring device depicted in
It appears clearly from the description given above that the resolution of the optical transducer and the corresponding measuring device depicted in
In the following, a further embodiment of an optical transducer and a measuring device according to the present invention and offering improved sensitivity will be described with reference to
The embodiment of the present invention depicted in
Moreover, the setup described in
In the following, an example of measuring means adapted to be used in combination with the optical transducer according of the present invention and adapted to be implemented in a measuring device according to the present invention will be described with reference to
In
The signals d and e depicted in
g(t)=D cos(ωt+φ)·E cos(ωt+θ)=½·D·E·[cos(2ωt+φ+θ)+cos(φ−θ)] (4)
so that, if the low-pass filter 10 has an ideal behavior, the signal f exiting the low-pass filter 10 may be described by
f(t)=½·D·E·cos(φ−θ)=k cos(δ) (5)
where δ is equal to φ−θ and k is proportional to the signal amplitudes.
This clearly shows that the signal exiting the measuring means 7 is proportional to the phase difference or shift δ which, in turn, depends on the lengths of the sensing and reference paths, although with a non-linear dependence.
At the beginning of the measurement session, and in absence of any mechanical action or force on the transducer, the value of f is recorded, say fz. Accordingly, if a mechanical action or force is applied to the transducer, for instance a tensil or compression action or force on the sensing portion 5′ of the optical path 5, the value of f changes, thus allowing the measurement of the displacement which can be then described by
f−fz∝ΔLs.
It results, therefore, that if the signal f is collected, the variations of the signal f may be put into relationship with the variation in length of the sensing portion 5′ of the optical path 5 and, in turn, with the mechanical action acting on the transducer. To this end, the signal f is sent to computing means, for instance to a personal computer, typically through a DAQ card, for the elaboration or processing of the signals and the recovery of the amplitude of the mechanical action or force from the phase variation information.
Since the phase variation is proportional also to ω, a resolution in the displacement in the order of a tenth of a micrometer requires the use of frequencies in the GHz range, while a resolution of a tenth of a millimeter is possible using a frequency of a few MHz.
Despite all the previously described advantages, the optical transducers and the measuring devices depicted in
An example of a further embodiment of an optical transducer and a measuring device adapted to keep the working point or range at which the transducer operates centered at approximately 90° will be described in the following with reference to
The optical transducer and the measuring device of
A further drawback affecting the embodiments of the present invention described above with reference to FIGS. 1 to 5 relates to the fact that, as apparent from equation (5), the output of the low-pass band filter 10 not only depends on the phase difference between the two signals d and e but also on the amplitude of these two signals. This implies that any variation in the received power, in the power of one or both of the two signals d and e, entering the phase meter or phase measuring means 7 and not due to a mechanical action or force acting on the transducer but for instance, to a variation of the optical attenuation in the optical path or of the electrical attenuation in the reference path 4 will be indistinguishable from a variation in the relative path lengths. In other words, it will not be possible to appreciate whether a variation in the output f exiting the phase measuring means 7 is really due to a variation in the phase difference between the signals d and e, resulting from a mechanical action acting on the transducer, or on a different reason generating a variation in the input power of one or both of the two signals d and e. Accordingly, a further embodiment of an optical transducer and a measuring device according to the present invention and adapted to overcome or at least to minimize this further drawback will be described in the following with reference to
In
The working principle of the embodiment depicted in
Supposing an ideal behavior for the phase shifter 14, the reference signal h exiting said phase shifter 14 may be described by
h(t)=E sin(ωt+θ) (6)
so that the output signal i of the mixer 12 may be described by
i(t)=D cos(ωt+θ)·E sin(ωt+θ)=½·D·E·[sin(2ωt+φ+θ)−sin(φ−θ)] (7)
After the ideal low-pass band filter 13 the output signal l may be described by
l= 1/2·D·E·|sin(φ−θ)|=k sin(δ) (8).
Accordingly, when the sensitivity of one of the two measuring means 7′ and 7″ is at its minimum, the other is at the maximum and vice versa. In other words, the working point or operating range of at least one of the two measuring means 7′ and 7″ is always maintained in the region of maximum sensitivity depicted in
Moreover, the outputs f and l can be used also to avoid the dependence of the measurement from the received power since the power can be estimated considering that
√{square root over (f+l2)}=D·E·√{square root over (cos2 δ+sin2 δ)}=D·E (8′)
There is, however, a further drawback affecting the embodiments described above with reference to
The most important difference between the embodiment depicted in
The purpose of the embodiment depicted in
The first generator means 1 generates a signal a at a frequency f1, whilst the second generator means 116 generates a signal a′ at a frequency f2 that differs from the frequency f1 by a small quantity f0. The second generator means 116 is locked to the first generator 1 to ensure that the frequency difference f0 is kept substantially constant. Accordingly, either
f2=f1+f0 or f2=f1−f0 with f0<<f1 (10).
The signal a exiting the oscillator or first generator means 1 is split by the power divider means 2 into two signals b and c, wherein the signal b is coupled to the optical emitting means 3, while the signal c acts as the reference signal and is connected directly to the measuring means 7″; in particular, as depicted in
Considering sinusoidal RF signals, the signals entering the mixer means 9 can be written as
d(t)=D cos(ω1t+φ)
m(t)=M cos(ω2t) (11)
where D and M are constants that take into account the attenuations along the respective paths and φ the relative phase shift of the signal d(t) with aspect to the signal m(t). The signal g(t) exiting the first mixer means 9 enters a band-pass filter 17 centered at a frequency f0. In particular, it is important to note that, in the embodiments described above with reference to previous figures, a low-pass band filter was used, whilst in the present case, a band-pass filter centered at a frequency f0 is used.
Assuming an ideal behavior of the components, the signal f exiting the band-pass filter 17 may be described by
f(t)=F cos(ω0t+φ) (12)
that is, a signal having the same phase shift as the sensing signal d but at an angular frequency ω0, corresponding to the frequency f0.
The same considerations as stated above also apply to the reference path 4; in fact, in this case, the signals e and n entering the second mixer means 12 can be described by
e(t)=E cos(ω1t+θ)
n(t)=N cos(ω2t) (13)
where E and N are constants that take into account the attenuation along the paths and θ the phase shift of the signal e(t) with respect to the signal m(t) or the signal n(t).
Again assuming an ideal behavior, at the output of the band-pass filter 18 centered at the frequency f0, the signal 1 exiting the band-pass filter 18 may be described by
l(t)=L cos(ω0t+θ) (14)
That is, a signal having the same phase shift as the reference signal e but at angular frequency ω0.
It appears clearly from equations 11 and 13 and 12 and 14, respectively, that the signals f and l exiting the first band-pass filter 17 and the second band-pass filter 18, respectively, are sinusoidal signals that maintain the same phase shifts as the sensing signal d and the reference signal e, respectively; however, their frequency is changed from f1 to f0. Accordingly, the phase can now be easily measured with good accuracy because the operating frequency is low. Moreover, since the signals f and l are now AC signals, whilst in the previous embodiment the signals exiting the measuring means were DC signals, the signals f and l are no longer sensitive to any offset introduced by the various stages composing the setup. In other words, the embodiment of
Considering that the phases can also be written in terms of time delays, the phase difference 6 can also be described by
Equation (15) implies that the same phase difference 6 at the frequency f0 results in a time delay corresponding to f1/f0 x the time delay at the frequency f1. In other words, the same variation in the length of the sensing path 5, in the length of the sensing portion 5′, produces a much stronger effect on the signal f(t) than on the signal d(t). For example, considering standard POF both for the sensing and reference paths 5 and 4, f1=20 MHz and f0=1 kHz, a length variation of 1 cm produces a phase difference δ circa 0.36°, corresponding to a time delay of only 50 ps at f1 but of one microsecond at f0.
According to the circumstances, the embodiment of
The measurement of the phase between signals f(t) and l(t) can be performed in several ways because the involved signals are low frequency signals. For example, this can be done by means of a PC connected to the circuit through a digitizing acquisition board. Then a user-friendly program can also be used to control the whole measurement procedure and compute the displacement. In this case the program should perform the operations described in the following steps.
First, acquiring the signals f(t) and l(t).
Second, optionally, acquiring the reference signal at f0 from the third generator.
Third, reconstructing ideal noise-free signals from the acquired ones. If the third generator is used this may be done by recovering the signal parameters using a digital lock-in technique (i.e., a sort of synchronous detection) or other narrow-band filtering techniques, otherwise a three-parameter or four-parameter sine-fit may be used.
Fourth, measuring the time delay between the reconstructed noise-free signals and estimating the variation in length of the sensing fiber with respect to a previously stored measure taken at the zero value.
Fifth, repeating the whole procedure hundreds of times and the average value and standard deviation are computed to give the user an estimation of the confidence of the measurement process.
In all the embodiments described above with reference to FIGS. 1 to 7, the sensing portion 5′ of the optical sensing path has been described as that portion of the optical path included between the two locks or fixed means 8. However, it has been revealed to be difficult in all these embodiments to ensure that the rest of the sensing path, that portion of the sensing path outside the two blocks or fixed means 8, is not influenced by a mechanical action or force acting on the transducer. In practice, also the transmission of the optical and/or electrical signal through portions of the sensing path other than the sensing portion 5′ is influenced and/or modified by a mechanical action or force acting on the transducer; in other words, in all the embodiments described above, the entire length of the sensing path may somehow affect the output of the transducer, even if the part outside the sensing portion 5′ is kept loose and within a protective housing.
This unwanted effect can be overcome considering a two wavelength scheme as depicted in
As apparent from
The most important difference between the embodiment of
In the embodiment of
Furthermore, it is also possible to recover not only the variation in the length Ls of the sensing portion 5′ but also its absolute value without the necessity of a prior calibration; in particular, this is possible by combining the two wavelength technique with a frequency variable generator or a two frequency generator in order to make measurements both at a frequency low enough not to have ambiguities in the fiber length and a frequency high enough to obtain the desired resolution. A variation of the scheme shown in
All the embodiments described above with reference to FIGS. 1 to 8 allows the detection and/or measurement of mechanical actions, forces, or displacements acting on the transducer; accordingly, if a plurality of transducers is provided, a measuring device can be obtained allowing to detect a plurality of mechanical actions, forces, displacements, stresses or the like simultaneously. An example of such a measuring device will be described in the following with reference to
In the embodiment of
As apparent from
The measuring device depicted in
Summarizing, it arises from the above disclosure that the optical transducers and the measuring devices according to the present invention overcome or at least minimize the drawbacks affecting the prior transducers known in the art; in particular, the optical transducers and the measuring devices according to the present invention allow the reliably detection of mechanical actions or forces such as, for instance, tensile forces and/or compressive forces, as well as stresses, displacements, shocks or the like. Moreover, the optical transducers and the measuring devices according to the present invention allow detection of both mechanical actions, stresses, displacements or the like acting on or arising in a single region and multiple mechanical actions, forces stresses, displacements or the like, acting on or arising in corresponding multiple regions.
The optical transducers and the measuring devices according to the present invention can be applied to the inspection of damage to structures, and especially to composite structures. They can be applied, for example, to the checking of constructive work and to the measurement of strains. The checking of damage to and/or displacements in structures can be done by inserting one or more of the optical transducers according to the present invention into the structure to be checked, for instance by fixing the sensing optical paths to the regions to be inspected while leaving the reference path unexposed to any mechanical actions or forces. In particular, the optical transducers according to the present invention have been revealed to be useful for the measurement of movement or forces on iron beams used to improve the structural stability of walls, especially in ancient buildings or after earthquakes. Measurement of the formation of landslides is also possible, for instance by fixing the transducer between poles in the ground. Furthermore, the optical transducers according to the present invention allow the measurement of deformation in composite materials by incorporating the sensing optical path into the structure. In the same way, measurement of distances and displacements in hostile environments, such as highly flammable, explosive or electro-magnetically noisy environments, are also possible, as well as measurement of forces in bioengineering.
Useful implementations of the optical transducers according to the present invention can be obtained by connecting the transducers to a PC or computer equipped with a digitizing card (DAQ) through a low noise multiple channel amplifier; of course, the PC or computer can control several transducers simultaneously, so that it is possible to devise complex yet quite inexpensive network of sensors. Moreover, using suitable software, it is also possible to control the sensors via a remote network such as the internet or web using standard protocols such as TCP/IP.
The optical transducers according to the present invention are characterized by quite extended operation range, from 10 μm up to several centimeters, while keeping a good resolution. Moreover, no interruption of the optical path, of the optical fiber, is necessary, thus resulting in better isolation from dust, rain or the like, making these transducers particularly suitable for outdoor applications.
The transducers according to the present invention have been tested using standard commercial POF having the objective to keep the costs as low as possible. Then, considering the modulation bandwidth of the typically available light emitting diodes or LEDs, a frequency f1 was used, corresponding to 20 MHz, with f2=f1+1 kHz, as well as commercial signal generators. However, other solutions based on DDS chips and microcontrollers may also be implemented.
It has finally to be noted that, in the optical transducers according to the present invention, both glass or polymer optical fibers may be implemented or used, depending on the circumstances. In particular, if the resolution requirements are in the order of millimeters and the operation temperature allows it, polymer optical fibers can be used for the sensing path. This implies very low costs in terms of sources, detectors and connectors, besides the advantages common to all the types of optical fiber sensors such as light weight, minimal invasiveness, immunity to electromagnetic interferences and impossibility to start a fire or an explosion. The latter two properties are particularly interesting because they allow the transducers to be used in critical environments such as in electromagnetically noisy industrial premises, in storage areas of high flammable materials, in structures exposed to electrostatic discharges during thunderstorms and in the monitoring of monuments or art pieces in general. The absence of electrical currents flowing through the transducer makes this transducer also ideal for biomedical applications so as to avoid a risk of electrocution. Moreover, thanks to their higher deformability, polymer optical fibers allow longer displacements to be measured with respect to commercial silicate fibers.
It has also to be noted that the principle of working of the optical transducer according to the present invention is based on the relative phase shift of electrical signals and not on the interference of optical signals.
While the present invention has been described with reference to particular embodiments, it has to be understood that the present invention is not limited to the particular embodiment described but rather that various modifications may be introduced into the embodiments described without departing from the scope of the present invention, which is defined by the appended claims.
Number | Date | Country | Kind |
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06008340.9 | Apr 2006 | EP | regional |