WO2019009740A1

WO2019009740A1 - Fibre optic distance sensor and method for distance measurement

Info

Publication number: WO2019009740A1
Application number: PCT/PL2018/050031
Authority: WO
Inventors: Tomasz NASIŁOWSKI; Marek NAPIERAŁA; Tadeusz Tenderenda; Tomasz STAŃCZYK; Łukasz SZOSTKIEWICZ; Anna Pytel; Piotr SZULEWSKI; Jarosław HERBST
Original assignee: Inphotech Sp. Z O. O.
Priority date: 2017-07-04
Filing date: 2018-07-02
Publication date: 2019-01-10
Also published as: PL422124A1; PL236080B1

Abstract

The fibre optic distance sensor which uses controlled introduction of bend losses to a optical fibre includes a fibre optic line connected to a light source and a detection system, containing at least one fibre optic sensor connected in at least one place with a winding module, and outside the attachment area or in this area - a section of the fibre optic line is led out of the winding module. The process of distance measurement using the distance sensor consisting in positioning in the area of connection of at least two elements a distance sensor between elements so that the distance sensor in accordance with the invention is subject to elongation and contraction, and then a light signal from the light source is fed to the fibre optic line, and then conducted in a transmission or reflectance set-up using at least one fibre optic line so that as a result of elongation or contraction of the sensor bend losses are introduced, and the optical signal strength downstream the winding module area is measured.

Description

FIBRE OPTIC DISTANCE SENSOR AND METHOD FOR DISTANCE MEASUREMENT

The invention pertains to a fibre optic distance sensor and a process of tracking distance between two elements. The device which is the invention subject operates on the basis of bend losses of an optical fibre where bend loss values enable to determine the distance between two points.

The invention has many possible applications in numerous technological fields where distance measurement, in particular due to security reasons, is important. The possible applications include engineering structures such as bridges, building elements, pipelines, and more. Due to its design based entirely on fibre optics, the device can be used in explosive atmospheres, since optical fibre-based measurement is passive and does not cause sparking. The principle of measurement using the device allows for a remote measurement and arranging the measurement station away from the measurement points, which enhances the safety of staff operating the device.

Optical methods are broadly applied in the field of distance measurement due to their accuracy and speed of measurement. Various devices or set-ups are used to measure distance, e.g. transmission or reflectance measurement techniques, phase shift, signal frequency or signal strength measurement, or measurement of light pulse passage time.

One of distance measurement devices that can serve as an example here is the rangefmderdescribed in U .S. Patent Number US4721384A. The device employs a high intensity laser diode and a receiver which is a position-sensitive detector. I n this case, the distance from a surface is determined based on time elapsed from the moment of emitting the light pulse to the moment of its reflection from the surface and then reaching the detector.

In another device, described in U.S. Patent Number US3619058A, a modulated light carrier signal is used. The signal, reflected from the surface defining the measured distance, is compared with a reference signal and the distance is determined on that basis. Such sensors are based on volume optics and most often, the light is propagated in these cases in a free space. For that reason, they are not used in environments where random pollution can occur or where a compact size of the device is what matters, especially in the measurement area.

There is also a method of making distance sensors using elements that show an increase in their resistance when elongated. In patent U.S. Patent Number US5Q15958A, the voltage change is measured, which corresponds to the elongation of a specific element. This solution employs an electric current and therefore cannot be used in explosive atmospheres. Additionally, in order to use this solution, direct access to the measured object is necessary - the sensor operator must perform every single measurement individually and personally, which results, among other things, in substantial strength losses of the signal transmitted through copper wires, when compared to fibre optic ones. This is particularly problematic where the sensors are located in utility or agricultural areas.

The bend loss phenomenon in optical fibre is commonly known, however considered undesired, which is evidenced by numerous methods of limiting it. There are solutions using an optical fibre wound onto a spool or spring, however such solutions are aimed at reducing bend losses and ensuring safe storage of longer optical fibre sections, and not sensor applications. For example patent U .S. Patent Number US 2007 036 506 concerns the application of a spring tape to wrap fibre optic cables. The invention is aimed at reducing the space necessary to store fibre optic cables. In the solution presented in the patent application, a condition is imposed that a certain minimum bend radius of optical fibre must be maintained, because bend losses are undesired.

Also the solution disclosed in U.S. Patent Number US 2008 118 2017 is used as a wrapping and storage method of fu nctional, longer fibre optic sections. In this solution, the fibre is would onto spools and attached with resins. The solution itself is aimed at reducing the volume of long fibre optic cable sections without producing any bend losses which are deemed unfavourable for this application.

However, it is also possible to use bend losses occurring in optical fibres to build microbend sensors.

U.S. Patent Number US 4891511 describes a microbend sensor enabling measurement of displacement. In the case of this invention, bend losses on the optical fibre correspond to displacement. Such a sensor makes it possible to measure displacements, but to a very small degree only, in the micrometre range.

In solution described in U.S. Patent Number US 5164605, an element is presented which is a coil with an optical fibre positioned therein. The solution allows for the measurement of displacement of movable elements by introducing bend losses when decreasing the coil pitch.

U.S. Patent Number US 5900556 discloses the design of a fibre strain sensor based on a flexible polymer optical fibre in the form of a series of loops (a spring). The spring is attached to an element which, when shortened, introduces bend losses in the spring, which influences the optical signal propagating in it.

Known distance measurement solutions using volume optics are not useful in many industrial applications where a very compact size of the device in the measurement area is necessary. In turn, sensors containing electrical or electronic components, known in the state of the art, must be additionally secured to be used in a flammable or explosive conditions. Sensors based on electronic components use electric signal, which is limited in terms of information transmission distance. In the case of the optical signal, however, the measurement can be read far from the measurement site. Therefore it was advisable to develop a fibre optic distance sensor and a method which would be reliable and safe given such a simple and inexpensive design, in particular in applications consisting in determining distance between two elements of a structure. This has been achieved with the sensor according to the hereby invention.

The fibre optic distance sensor, preferably installed in the area of connection of at least two structural elements, uses controlled introduction of bend losses in an optical fibre and contains at least one fibre optic line, connected to a light source and a detection system, placed partially on a winding module.

The fibre optic line contains at least one section of the sensor optical fibre introducing bend losses, and preferably at least one section of optical fibre of another type. In another preferred embodiment, the fibre optic line contains at least two sections of sensor optical fibres, preferably connected by splicing, and is installed in such a way so that it is connected at least in one spot with the winding module, and a fibre optic line section is led out of the winding module. Preferably the fibre optic line section can be additionally covered with a protective layer. Preferably the optical fibre section can be covered with a metal layer, and then attached to the winding module, preferably by electrolysis. Preferably the fibre optic line section is covered with a shielding material, in particular in the form of a tape, to prevent twisting and damage of the fibre optic line during winding.

Whereby the preferable solution to led the sensor optical fibre out of the winding module is that in the attachment area the fibre optic line should be bent, and the input and output of the fibre optic line are located at one end of the winding module. In another embodiment, the sensor optical fibre is led out of the area of the winding module in such a way so that in the area of attachment, the sensor optical fibre goes out from the winding module through an opening in its casing, axially to the rotation of the winding module mechanisms within which an anti-twisting mechanism could be installed, as a preferable option. In the preferred embodiment, optical signal is fed on the one side of the fibre optic line, and the other end of the line ends in a layer reflecting the signal. In such a case, the measurement takes place in a reflective set-up. Preferably the reflective layer has the form of an optical fibre front face cut at right angle, of a Bragg grating, or a coated mirror.

In particular, the winding module has the form of a flexible element to whose surface the fibre optic line is attached indirectly or directly, at least at one spot and at least partially firmly.

In particular, the winding module has the form of a spiral spring which introduces linear movement. The fixed end of the spring is preferably attached to bearings, and the fibre optic line is attached to a spring in any way, for example with glue or tape. The bearings and the free end are attached to the elements moving in relation to one another in the direction in which the spring extends. During the retracting movement, the free end comes close to the fixed end with bearings, and the spring with the optical fibre attached is wound onto the bearings. Winding the optical fibre on the bearings results in bend losses being introduced on the optical fibre, which results in turn in a decrease of power measured using a power meter. Preferably the sensor optical fibre is attached to the spiral spring along the whole length of the fibre, preferably by gluing. In another preferred embodiment, the winding module is shaped as a cylinder positioned coaxially with the rotation axis, and is connected with the rotation axis by the spiral spring which introduces rotating movement, with the free end of the spring attached to the internal surface of the cylinder. The other end of the spiral spring is attached to the immovable element placed on the rotation axis of the cylinder. The sensor optical fibre being a fibre optic line section is attached in at least one spot to the external surface of the cylinder and can be wound onto it. The process of distance measurement described in the invention consists in placing a fibre optic sensor according to the present invention in the structure subject to elongation or contraction, and then introducing the light into the fibre optic line with a light signal from a light source, and then, in a transmission or reflectance set-up, the strength of the signal conducted through at least one fibre optic line is measured so that as a result of elongation or contraction of the sensor bend losses are introduced, and changes in the optical signal strength downstream the winding module are measured.

As a result of change in the measured distance, the winding module of the sensor according to the invention will unwind the sensor optical fibre and reduce bend losses, which will result in a change in power indication on the detector. When the distance between elements is reduced, the optical fibre will be retracted up and bend losses will increase, which will cause the power indication on the detector to change.

The distance measurement process in accordance with the invention includes the design of the sensor system, where preferably using at least one source and at least one detector, at least two distance sensor are connected in any manner; preferably the distance sensor are connected in parallel or in series. Preferably, many wave lengths propagated via one fibre optic line can be employed in the measurement. Filters, circulators, switches, multiplexers can be preferably used to connect a number of distance sensors. Preferably the distance measurement could be distributed, preferably using a time or frequency-based measurement method. Preferably the system of connected distance sensors allows for a simultaneous distance measurement on all sensors included in the system. Preferably the system of connected distance sensors allows for a measurement on at least one selected sensor in the system. It is also possible to use a number of sensor in accordance with the invention, connected in parallel or in series, and introduced only by the light source in any existing monitoring station. In that way, the need for more expensive and damage-prone electronic sensors is eliminated, as well as the need for using mobile or stationary generators. The absence of power supply eliminates the risk of explosion. The invention has the advantage consisting in a possibility of fast measurement read-out even several kilometres from the place of detection. The prototype tests have demonstrated that the fibre optic sensor in accordance with the invention is efficient both in transmission as well as reflectance set-ups. At the same time, it is possible to execute a system of sensors based on the distance sensors in accordance with the present invention to monitor size changes within a specified area. Preferably the sensor in the unwound position introduces 0 dB of losses and preferably not more than 10 dB of losses in the rolled position. Preferably the initial position in which the sensor is installed is the position in which half of the length of the fibre optic line is rolled up.

In the preferred embodiment, the sensor in accordance with the invention is multiplied and installed in series in subsequent [places] located along the areas of distance measurement.

The fibre optic sensor in accordance with the invention is presented in Fig. 1, which shows a diagram of connections of the sensor elements according to embodiment 1, while Fig. 2 shows the mechanical design of the sensor in embodiment 1, Fig. 3 shows the course of the fibre optic line in embodiment 1, Fig. 4 shows the optical power dependence on the measured distance in the monitored area, Fig. 5 shows the course of the fibre optic line in embodiment 2, where measurement is implemented in a reflective set-up, Fig. 6, which shows a diagram of connections of the sensor elements according to embodiment 2, Fig. 7 shows the course of the fibre optic line in embodiment 3 with double fibre optic line, Fig. 8 shows the optical power dependence on the measured distance in embodiment 3 with double fibre optic line, Fig. 9 shows the course of the fibre optic line when the line is attached to the side of a spiral spring connected with bearings in accordance to embodiment 4, Fig. 10 presents the sensor in accordance with the invention in a transmission set-up, with multiplied sensors installed subsequently in a series in accordance with embodiment 5, Fig. 11 presents the dependence of optical power on the measured distance in embodiment 5 with multiplied sensor, Fig. 12 shows the sensor with a fibre optic line installed on the surface of the cylinder according to embodiment 6, Fig. 13 shows the cross-section of the sensor with a fibre optic line wound onto the surface of the rotating cylinder in line with embodiment 6. Embodiment 1.

The fibre optic distance sensor, using controlled introduction of bend losses on an optical fibre includes a fibre optic line connected to a light source and a detection system, the line containing a fibre optic sensor connected with a winding module, and outside the attachment area or in this area - a section of the fibre optic line is led out of the winding module.

The fibre optic line contains a sensor optical fibre section (7) and is attached in such a way so that the sensor optical fibre (7) is firmly fixed on the winding module. The winding module has the form of a spiral spring (6) which introduces linear movement, and its internal end is attached to bearings (5). Whereby the sensor optical fibre is led out of the winding module in such a way so that in the attachment area, the fibre optic line is bent and the input and output of the fibre optic line are located at the free end of the spring (10).

Light from the source (1) is introduced into to the sensor element (2) using the optical fibre (4). The sensor element includes a spiral spring (6) to which the optical fibre (7) is attached. The input and output of the optical fibre (7) positioned on the spring (6) are located at the free end of the spring (10). After passing through the sensor element (2), light is conducted through the optical fibre (4) to the power meter (3).

The fixed end of the spring (9) is attached to bearings (5), and the bearings (5) similarly as the free end (10) are attached to the elements moving in relation to one another in the direction in which the spring extends. During the retracting movement, the free end (10) comes close to the fixed end (9) with the bearings (5), and the spring (6) with the attached optical fibre (7) is wound onto the bearings (5). Winding of the optical fibre on the bearings (5) results in bend losses being introduced in the optical fibre, which results in turn in a decrease of power measured using the power meter (3); this effect is presented in Fig. 4, where the characteristics of dependence of power measured at the meter (3) on the measured distance (6) is shown. The diagram (Fig. 4) clearly shows that with decrease of the measured distance, the power measured on the meter (3) decreases.

The spiral spring (6) is made of a material shaped and strained in such a way so as to form a tight coil. The dimensions of the spring are selected in such a way so that when the degree of its retraction changes, bend losses introduced in the optical fibre attached to the spring have non-zero values. The optical fibre (7) is a standard optical fibre of SMF type and was selected in such a way so that the size of the spring introduce measurable bend losses, detectable by the power meter (3). The light source (1) is a DFB laser emitting light wave of 1550 nm wavelength. The losses introduced by the sensor element (2) are between 1 dB and 1.9 dB and depend on the measured distance.

The process of distance measurement between elements described in the invention consists in placing a fibre optic sensor (2) according to the present invention in a structure subject to elongation and contraction, and then introducing the light into the fibre optic line with a light signal from a light source (1), after which the signal is conducted in a transmission set-up via the fibre optic line so that as a result of elongation or contraction of the sensor bend losses are introduced, and variations in the optical signal strength downstream the winding module (2) are measured.

When the measured distance is increased, the winding module of the sensor according to the invention will unwind the sensor optical fibre (7) and reduce bend losses, which will result in a change in power indication on the detector (3). When the distance between elements is reduced, the optical fibre will be retracted and bend losses will increase, which will cause the power indication on the detector (3) to change.

The embodiment presents the measurement process in the transmission set-up.

Embodiment 2. The fibre optic distance sensor, using controlled introduction of bend losses on an optical fibre includes a fibre optic line connected to a light source and a detection system, the line containing a fibre optic sensor connected with a winding module, and outside the attachment area or in this area - a section of the fibre optic line is led out of the winding module. In this embodiment, the distance sensor and the measurement process using the distance sensor are presented, with measurement carried out in a reflective set-up.

The fibre optic line contains a sensor optical fibre section (7) and is attached in such a way so that it is firmly fixed in the winding module. The winding module has the form of a spiral spring (6) which introduces linear movement and its internal end is attached to bearings.

Light from the source (1) which is a superluminescent diode is introduced into to the optical fibre (4) in which it is then propagated to the circulator (8), and from there - to the sensor element (2). The sensor element includes a spiral spring (6) to which the optical fibre (7) is attached. Light is introduced into to the optical fibre (7) positioned on the spring (6) at the free end of the spring (10), while at the other end of the optical fibre, attached at the fixed end of the spring (9), there is a Bragg grating reflecting light. After being reflected at the end of the sensor element (2) and propagated outside its area, light is conducted through the optical fibre to the circulator (8), and then to the power meter (3). This solution makes it possible to use a shorter optical fibre section, as it does not need to run twice along the full length of the spring.

The fixed end of the spring (9) is attached to bearings (5), and the bearings (5) similarly as the free end (10) are attached to the elements moving in relation to one another in the direction in which the spring extends. During the retracting movement, the free end (10) comes close to the fixed end (9) with the bearings (5), and the spring (6) with the attached optical fibre (7) is wound onto the bearings (5). Winding of the optical fibre on the bearings (5) results in bend losses being introduced on the optical fibre, which results in turn in a decrease of power measured using the power meter (3); this effect is presented in Fig. 4, where the characteristics of dependence of power measured at the meter (3) on the measured distance (6) is shown. The diagram (Fig. 4) clearly shows that with decrease of the measured distance, the power measured on the meter (3) decreases.

The spiral spring (6) is made of a material shaped and strained in such a way so as to form a tight coil. The dimensions of the spring are selected in such a way so that when the degree of its retraction changes, bend losses introduced in the optical fibre attached to the spring have non-zero values. The light source (1) is a superluminescent diode emitting light wave of 1310 nm wavelength. The optical fibre (7) is a 1310 nm wavelength multimodal optical fibre and was selected in such a way so that losses introduced to it range from 0.5 dB to 3.7 dB and depend on the measured distance.

The process of distance measurement described in the invention consists in placing a fibre optic sensor (2) according to the present invention in a structure subject to elongation and contraction, and then introducing the light into the fibre optic line with a light signal from the light source (1), after which the signal is reflected at the end of the sensor optical fibre and propagated through the circulator to the detector (3) so that as a result of elongation or contraction of the sensor bend losses are introduced, and the optical signal strength downstream the winding module (2) is measured.

Embodiment 3. The fibre optic distance sensor, using controlled introduction of bend losses on an optical fibre includes a fibre optic line connected to a light source and a detection system, the line containing a fibre optic sensor connected with a winding module, and outside the attachment area or in this area - a section of the fibre optic line is led out of the winding module. In this embodiment, the distance sensor and the measurement process using the distance sensor are presented, where two fibre optic lines are attached to a single winding module.

On the winding module, there are two fibre optic lines, each of them including a sensor optical fibre section (7) attached so that it is firmly fixed in the winding module. The winding module has the form of a spiral spring (6) which introduces linear movement, and its internal end is attached to bearings (5). Whereby the sensor optical fibres(7) are led out of the winding module in such a way so that in the attachment area, the fibre optic lines are bent and the input and output of the measurement fibres (7.A and 7.B) on the spring (6) are placed at the free end of the spring (10). The bent section of the longer measurement fibre (7.A) is positioned in the area of the fixed end of the spring (9), and the bent section of the shorter measurement fibre (7.B) is positioned in the area located at half of the length of the spring (6).

Light from the source (1) which is a superluminescent diode is introduced into the optical fibre (4) in which it is then propagated to the input switch (13) which guides it further, depending on the position of 7.A or 7.B optical fibre in which it is further propagated to the sensor element (2). The sensor element includes a winding module formed as a spiral spring (6) to which the optical fibres (7.A i 7.B) are attached. The way the optical fibres are positioned on the spring in the extended position is shown in Fig. 8. After passing through the sensor element (2), light is further conducted through the optical fibre to the output switch (14) and then to the power meter (3). The solution proposed in Fig. 8. allows for obtaining a higher measurement resolution, and therefore a greater accuracy of distance measurement (6).

The fixed end of the spring (9) is attached to bearings (5), and the bearings (5) similarly as the free end (10) are attached to the elements moving in relation to one another in the direction in which the spring extends. During the retracting movement, the free end (10) comes close to the fixed end (9) with bearings (5), and the spring (6) with the attached optical fibres (7) is wound onto the bearings (5). Winding of the optical fibres on the bearings (5) results in bend losses being introduced in the optical fibres, which results in turn in a decrease of power measured using the power meter (3); this effect is presented in Fig. 9, where the characteristics of dependence of power measured at the meter (3) on the distance is shown. The diagram (Fig. 9) clearly shows that with decrease of the measured distance, the power measured on the meter (3) transmitted through optical fibre 7. A decreases. At the first stage of spring retraction (6), losses are introduced only in optical fibre A (7.A) and after the spring is retracted to half the measured distance, losses in fibre A (7.A) cause the transmission to fade; then switches (13) and (14) are activated and turned to B position. Further retracting of the spring (6) increases losses in optical fibre B (7.B), which results in a decrease of power measured on the power meter (3).

The spiral spring (6) is made of a material shaped and strained in such a way so as to form a tight coil. The dimensions of the spring are selected in such a way so that when the degree of its retraction changes, bend losses introduced in the optical fibre attached to the spring have non-zero values. Optical fibres 7.A and 7.B are microstructural. The light source (1) is a superluminescent diode emitting light wave of 1310 nm wavelength. The losses introduced by the sensor element (2) are between 1.5 dB and 7.6 dB and depend on the measured distance. The process of distance monitoring described in the invention consists in placing a fibre optic sensor (2) according to the present invention in the structure subject to elongation and contraction, and then introducing the light into the fibre optic line with a light signal from the light source (1), after which the signal is conducted in a transmission set-up so that as a result of elongation or contraction of the sensor bend losses are introduced, and variations in the optical signal strength downstream the winding module {2} are measured.

Embodiment 4.

In this embodiment, a distance sensor and distance measurement process concept is presented in which it is possible to attach a fibre optic line to the winding module in such a way so that the durability of the fibre optic line is increased due to the reduction of forces affecting it.

The fibre optic line contains a sensor optical fibre section (7) and is attached in such a way so that the sensor optical fibre (7) is firmly fixed on the winding module. The winding module has the form of a spiral spring (6) which introduces linear movement, and its internal end is attached to bearings (5). Whereby the sensor optical fibre (7) is led out of the winding module area in such a way so that in the attachment area, the fibre optic line is bent and the input and output of the fibre optic line positioned on the spring (6) are located at the free end of the spring (10).

Light from the laser source (1) is introduced into the optical fibre (4) in which it is then propagated to the sensor element (2). The sensor element includes a spiral spring (6) to which the optical fibre (7) is attached. The way the optical fibre (7) is positioned on the spring (6) in the extended position is shown in Fig. 10. It is assumed that the optical fibre (7) is attached to the edge of the tape forming the spring (6). After passing through the sensor element (2), light is conducted further through the optical fibre (4) to the power meter (3). In the solution presented in Fig. 10, the optical fibre after winding is not placed directly between the spring coils, but on the side.

The fixed end of the spring (9) is attached to bearings (5), and the bearings (5) similarly as the free end (10) are attached to the elements moving in relation to one another in the direction in which the spring moves. During the retracting movement, the free end (10) comes close to the fixed end (9) with the bearings (5), and the spring (6) with the attached optical fibre (7) is wound onto the bearings (5). Winding of the optical fibre (7) onto the bearings (5) results in bend losses being introduced on the optical fibre (7), which results in turn in a decrease of power measured using the power meter (3); this effect is presented in Fig. 4, where the characteristics of dependence of power measured at the meter (3) on the distance is shown. The diagram (Fig. 4) clearly shows that with decrease of the measured distance, the power measured on the meter (3) decreases.

The spiral spring (6) is made of a material shaped and strained in such a way so as to form a tight coil. The dimensions of the spring are selected in such a way so that when the degree of its retraction changes, bend losses introduced in the optical fibre attached to the spring have non-zero values. The optical fibre (7) is a standard optical fibre of SMF type and was selected in such a way so that the size of the spring could introduce measurable bend losses, detectable by the power meter (3). The light source (2) is a DFB laser emitting light wave of 1550 nm wavelength. The losses introduced by the sensor element (2) are between 3 dB and 5.3 dB and depend on the measured distance.

The process of distance measurement described in the invention consists in placing a fibre optic sensor (2) according to the present invention in a structure subject to elongation and contraction, and then introducing the light into the fibre optic line with a light signal from a light source (1), after which the signal is conducted in a transmission set-up via the fibre optic line so that as a result of elongation or contraction of the sensor bend losses are introduced, and variations in the optical signal strength downstream the winding module (2) are measured.

When the measured distance is increased, the winding module of the sensor according to the invention will unwind the sensor optical fibre (7) and reduce bend losses, which will result in a change in power indication on the detector (3). When the distance between elements is reduced, the optical fibre will be retracted and bend losses will increase, which will cause the power indication on the detector (3) to change. Embodiment 5.

When performing a measurement over a large distance with a sufficiently high resolution, it is possible to connect the sensors (2) in series. Then, the signal from the optical reflectometer (16) is conducted through the optical fibre (4) to the sensor elements (2) connected in series. Each sensor element (2) includes a winding module formed as a spiral spring (6) to which the optical fibre (7) is attached. The input and output of the optical fibre (7) positioned on the spring (6) are located at the free end of the spring (10). The sensor elements are connected in series, which means that the output of the first element is connected to the input of the second one, etc. up to the last element, where the output is not connected and forms the end of the measurement line.

In each of the sensor elements (2) the fixed end of the spring (9) is attached to bearings (5), and the bearings (5) similarly as the free end (10) are attached to the elements moving in relation to one another in the direction in which the spring extends. During the retracting movement, the free end (10) comes close to the fixed end with bearings (9), and the spring (6) with the attached optical fibre (7) is wound onto the bearings (5). Winding of the optical fibre onto the bearings (5) results in bend losses being introduced in the optical fibre. The measurement of distance between subsequent movable elements is implemented using an optical reflectometer (16). From the reflectometer (16), the signal is sent through the optical fibre to sensors (2), and the measurement is executed through the analysis of the signal returning to the reflectometer (16). Such a measurement type makes it possible to determine losses occurring at each sensor element, and determining the distance between the elements on that basis. The main advantage of the presented solution is that the measurement on many sensor elements can be carried out simultaneously, without the need for using optical switches. Fig. 13 presents a characteristics of loss measurement along the optical fibre to which the sensor elements are attached (2). The point drops of the measured signal indicate the position of a sensor element (2), which enables its clear identification. In turn, the level of change in signal strength in individual drop points makes it possible to determine distance in individual measurement areas.

The spiral springs (6) are made of a material shaped and strained in such a way so as to form a tight coil. The dimensions of the springs are selected in such a way so that when the degree of their retraction changes, bend losses introduced in the optical fibre attached to the springs have non-zero values. The optical fibre (7) is a standard optical fibre of SMF type and was selected in such a way so that the size of the spring could introduce measurable bend losses, detectable by the power meter (3). The losses introduced by the sensor element (2) are between 3 dB and 5.3 dB and depend on the measured distance.

The process of distance measurement described in the invention consists in placing a fibre optic sensor {2} according to the present invention in a structure subject to elongation and contraction, and then introducing the light into the fibre optic line with a light signal from the light source (1), after which the signal is conducted in a transmission set-up so that as a result of elongation or contraction of the sensor bend losses are introduced, and variations in the optical signal strength downstream the winding module {2} are measured.

Embodiment 6. The fibre optic distance sensor, using controlled introduction of bend losses on an optical fibre includes a fibre optic line connected to a light source and a detection system, the line containing a fibre optic sensor connected with a winding module, and outside the attachment area or in this area - a section of the fibre optic line is led out of the winding module.

In this embodiment, a concept of a distance sensor and a distance measurement process is presented which use a winding module including a spiral spring which introduces rotating movement, and a cylinder.

The fibre optic line includes two sensor optical fibre sections (7) and a bent optical fibre section (11) between two sensor optical fibre sections. The fibre optic line is attached in such a way so that the bent section (11) is firmly fixed in the winding module. The winding module contains a cylinder (19), a pin situated coaxially (18) attached to one of the structural elements the distance between which is measured, and a spiral spring which introduces rotating movement and whose one end (9) is attached to the pin (18) and the other end (10) is attached to the internal surface of the cylinder (19). The sensor optical fibre (7) being a fibre optic line section is attached to the external surface of the cylinder (19) and would around it. Whereby the sensor optical fibre (7) is led out of the winding module area in such a way so that in the attachment area, the fibre optic line is bent, and the input and output of the fibre optic line are attached to one of the structural elements the distance between which is measured. The bent section (11) is attached to the external side of the cylinder (19). Light from the source (1) which is a superluminescent diode is introduced into the optical fibre (4) through which it is then propagated to the sensor element (2). The sensor element includes a winding module formed as a spiral spring (6) placed inside a rotating cylinder (19). One end of the spring (6) is attached to the immovable pin (18) placed in the central area of the cylinder (19), and the other one is attached to the internal wall of the movable cylinder (19). The spring (6) is situated in such a way so that when the cylinder (19) rotates around the pin (18), the strain in the spring (6) increases. At the moment of measurement of the distance by the sensor, the spring (6) inside the cylinder (19) forces its rotation. When the measured distances decreases, the cylinder (19) driven by the spring (6) starts to rotate, which results in the optical fibres (7) being wound onto it. Winding of the optical fibre (7) on the cylinder (19) results in bend losses being introduced on the optical fibre (7), which results in turn in a decrease of power measured using the power meter (3).

The spiral spring (6) is made from a thin steel tape strained with a rotational torque resulting in bending strain in the tape. The dimensions of the spring (6) are selected to enable the measurement of distance within the range of 0-800 mm through the introduction of the rotating movement of the cylinder (19) wherein the spring is placed. The spring parameters allow it to make up to 20 revolutions, which is definitely sufficient to wind the measurement optical fibre (7) onto the cylinder (19). The optical fibre (7) is a standard optical fibre of SMF type and was selected in such a way so that the size of the spring could introduce measurable bend losses, detectable by the power meter (3). The optical fibre (7) is partially coated with copper to allow attaching it to the spring by electrolysis. The light source (1) is a superluminescent diode emitting light wave of 1550 nm wavelength. The losses introduced by the sensor element (2) presented in Fig. 12 and 13 are between 0.1 dB and 2.5 dB and depend on the measured distance. The process of distance measurement described in the invention consists in placing a fibre optic sensor (2) according to the present invention in a structure subject to elongation and contraction, and then introducing the light into the fibre optic line with a light signal from a light source (1), after which the signal is conducted in a transmission set-up via the fibre optic line so that as a result of elongation or contraction of the sensor bend losses are introduced, and variations in the optical signal strength downstream the winding module (2) are measured. When the measured distance is increased, the winding module of the sensor according to the invention will unwind the sensor optical fibre (7) and reduce bend losses, which will result in a change in power indication on the detector (3). When the distance between elements is reduced, the optical fibre will be retracted and bend losses will increase, which will cause the power indication on the detector (3) to change.

Claims

Patent claims

1. The fibre optic distance sensor, characterised in that a controlled introduction of bend losses to a optical fibre is used, includes a fibre optic line connected to a light source and a detection system, containing at least one fibre optic sensor connected in at least one place with a winding module, and outside the attachment area or in this area - a section of the fibre optic line is led out of the winding module.

2. The sensor recited in claim 1 characterised in that said optical fibre line consists of at least two fibre optic sections.

3. The sensor recited in claim 1 or 2 characterised in that said optical fibre line consists of at least two different types of optical fibres.

4. The sensor recited in claim 1 or 2 or 3 characterised in that said sensor optical fibre is led out of said winding module in a way so that in the attachment area, said fibre optic line is bent, and the input and output of the line are located in the same area.

5. The sensor recited in claim 1 or 2 or 3 or 4 characterised in that said winding module includes a flexible element, in particular a spiral spring.

6. The sensor recited in claim 1 or 2 or 3 or 4 or 5 characterised in that in particular said winding module has the form of a spiral spring which introduces linear movement, the internal end of said spring being attached indirectly or directly to bearings, and said fibre optic line is attached to the spring in any manner. The sensor recited in claim 6 characterised in that the entire length of said fibre optic line attached to said winding module is attached firmly by electrolysis and/or by gluing the fibre optic line to the spiral spring. The sensor recited in claim 1 or 2 or 3 or 4 or 5, characterised in that said winding module contains a cylinder, a pin situated coaxially attached to one of the structural elements the distance between which is measured, and a spiral spring which introduces rotating movement and whose one end is attached to the pin and the other end is attached to the internal surface of the cylinder, and the sensor optical fibre being a section of the fibre optic line is attached in at least one place to the external surface of the cylinder and can be wound onto it.

The sensor recited in any of the above claims characterised in that it is connected in series with other sensors in accordance with the invention.

The process of distance measurement using the sensor recited in any of the above claims characterised in that in the area of connection of at least two elements there is a distance sensor positioned so that the distance sensor in accordance with the invention is subject to elongation and contraction, and then a light signal from the light source is introduced into the fibre optic line, and then conducted in a transmission or reflectance set-up using at least one fibre optic line so that as a result of elongation or contraction of the sensor bend losses are introduced, and the optical signal strength downstream the winding module area is measured.