WO2014181014A1

WO2014181014A1 - Optical sensor, system and method for detecting the presence of ice on surfaces

Info

Publication number: WO2014181014A1
Application number: PCT/ES2014/070226
Authority: WO
Inventors: Francesc DÍAZ GONZÁLEZ; Airán RÓDENAS SEGUÍ; Miquel TRAVERIA BONDI; Javier MARTÍNEZ GARCÍA
Original assignee: Universitat Rovira I Virgili
Priority date: 2013-05-07
Filing date: 2014-03-26
Publication date: 2014-11-13
Also published as: ES2530885B1; ES2530885A1

Abstract

The invention relates to an optical sensor for detecting the presence of ice on surfaces, characterised in that it comprises: a waveguide having an optical coating with a refractive index that fulfills the relation n_water ≤ n_material < n_ice at a wavelength λ_T for which n_water < n_ice, and a core contained in the coating and suitable for propagating an optical signal of λ_T, the waveguide having a sensor segment that is installed on a surface with one face of the coating exposed; means for injecting an optical signal of λ_T into the waveguide; and means for detecting the optical signal transmitted by the waveguide downstream of the sensor segment. The sensor detects the formation of ice, differentiating same from the presence of water. The sensor is resistant to erosion and can be mounted without projecting from the surface.

Description

OPTICAL SENSOR, SYSTEM, AND PROCEDURE FOR

DETECT THE PRESENCE OF ICE ON SURFACES

The present invention relates to an optical sensor for detecting the presence of ice on surfaces.

It also refers to a system for detecting the presence of ice comprising at least one of said sensors, and a method for detecting the presence of ice on surfaces.

STATE OF THE PREVIOUS TECHNIQUE

There are several applications in which it is necessary to detect the formation of ice or the presence of ice on a surface.

For example, the presence of ice in any aerodynamic element of an aircraft or a helicopter alters the shape, weight and mass distribution of the same: when the formation of ice begins, usually at an edge of attack, a distortion occurs of laminar flow and turbulence are generated, so that the resistance is increased and the lift decreases. The formation of ice is therefore a risk to security, and in fact has played an important role in aviation accidents, and also affects the control of the aircraft and increases fuel consumption.

Another field in which it is convenient to control the formation of ice on a surface is for example that of wind turbines, since the formation of ice on the blades reduces performance, can cause excessive loads, etc. Also in ships, naval structures and other underwater applications in which it may be convenient to detect the formation of ice. It may also be useful to detect the presence or absence of ice in refrigerators, cold rooms and the like, or in water pipes.

To address this problem, different types of optical sensors have been proposed, for example sensors that comprise an optical fiber along which an electromagnetic wave propagates, configured and mounted so that the wave propagation is affected to some extent. , by phenomena of reflection, refraction and scattering of light (scattering), by the presence of water or ice.

However, known optical sensors have different limitations that make their application not entirely satisfactory and reliable: some known sensors in general are not able to discriminate between the presence of ice and the presence of water, so in practice to know if ice is formed, additional temperature or other sensors are required; Some sensors require elements that protrude from the surface, thereby affecting the aerodynamics of the surface, or can be installed only in certain positions and therefore do not measure directly in the area of interest; in other cases the characteristics of the sensors make them not resistant to erosion.

The present invention provides a sensor that at least partially resolves the limitations of known sensors.

EXPLANATION OF THE INVENTION

According to a first aspect, the present invention provides an optical sensor for detecting the presence of ice on surfaces, characterized in that it comprises:

- a waveguide with an optical coating material whose refractive index n _m of ter¡ai satisfies the relationship n n _m _agu a≤ ter¡ai <

at a working wavelength λ _τ for which n _a gua <n _h ¡ _e io, and with a core zone contained in the coating material and suitable for the propagation of an optical signal having the wavelength of work, presenting the waveguide an appropriate sensor section to be installed on a surface on which the presence of ice must be detected so that one face of the lining material of the sensor section is exposed;

- means for injecting into the waveguide an optical signal with a working wavelength λ _τ for which n _water <n _h ¡ _e io; Y

- means for detecting the optical signal transmitted by the waveguide running down said sensor section.

With these sensor characteristics, the attenuation of the optical signal along the stretch of the waveguide that is close to the exposed face of the body is significantly different if there is presence of ice on the surface, and therefore on the exposed face of the body, or if there is only presence of air or water. The following explains in more detail how the sensor can detect ice reliably, and more

particularly differentiate between the presence of ice and the presence of water.

In the propagation of the optical signal along the waveguide, there are one or more guided electromagnetic modes.

On the face of the sensor section that is exposed, that is, on which water or ice can be deposited, there is a jump in the index of refraction: if the index of refraction of the external environment is greater than that of the guide, it can arise an evanescent coupling between the guided electromagnetic modes and the external environment, which translates into a loss of power of the guided modes.

Because the waveguide is suitable for working in the spectral region where the condition n n _m _agu a≤ ter¡ai <n _h _and io is met, in the presence of ice will produce evanescent coupling and signal loss optical, while in the presence of water or air will not, since only in the presence of ice there is an increase in the index of refraction between the material and the outside.

In embodiments of the invention, these phenomena are used for the univocal detection of ice against water, efficiently and without the need for further parameters. In addition, the sensor can be mounted without protruding from the surface, so it is particularly appropriate in applications where it is important not to affect the aerodynamics of the surfaces, and the core area of the waveguide is protected from erosion. because it is not exposed directly.

Preferably, the index of refraction of the material meets the relation n _a gua <nmateriai <n _h ¡ _e io to the working wavelength λ _τ .

In some embodiments, especially when surface aerodynamics is important, the sensor section is appropriate to be installed on the surface so that the face of the section lining material sensor is exposed through an opening in the surface; in this way the sensor can be mounted level with the surface, and does not affect its aerodynamic characteristics.

The waveguide may comprise an optical fiber, from which the outer coating has been removed at least in the area of the sensor section intended to be exposed during use, and optionally a part of the thickness of the optical coating has also been removed.

Alternatively, at least in the sensor section of the waveguide the core zone may be manufactured within a body of appropriate material, so that the refractive index varies in this area; that is, the rest of the waveguide on both sides of the sensor section can be configured in another way.

For example, optical fibers may be coupled to an input end and an output end of the waveguide that includes the sensor section. This allows the transmitter and the detector to be located at any distance from the sensor section, and following any path, thanks to the flexibility of the optical fiber, which can be particularly useful in applications in which it is convenient to minimize the space occupied by the system in the surface, such as the wing of an airplane, for example.

In some specific embodiments, the working wavelength is between 3000 and 5000 nm, preferably between 3200 and 4000 nm; and the material can be selected from crystalline quartz, vitreous quartz, indium fluoride, or compounds thereof.

In embodiments of the invention the thickness between the core zone and the face of the coating material intended to be exposed is less than 250 pm, preferably less than 50 pm; These ranges allow sensors to have small dimensions.

The sensor may further comprise an anti-erosion coating applied on the face of the coating material of the sensor section intended to be exposed. This coating allows to extend the life of the sensor, and It will be suitable especially in conditions where the surface is subject to high friction or particularly aggressive environments.

According to a second aspect, the invention also relates to a system for detecting the presence of ice on surfaces comprising at least one optical sensor as described, and means for comparing the intensity of a detected optical signal with at least one threshold value, and generate an alarm in case the optical signal is lower than a threshold value.

The system may comprise calibration means for determining at least one threshold value, which may be appropriate for determining at least one threshold value once the system is installed on a surface.

In some embodiments, the system comprises a plurality of said optical sensors, intended to be installed in a predetermined area, at a distance from each other, and a common control unit connected to said plurality of sensors, said control unit comprising means for providing information on the distribution of ice in an area

predetermined, depending on the intensities of the optical signals detected at the outputs of the plurality of sensors.

According to another aspect, the invention relates to a method for detecting the presence of ice on surfaces, characterized in that

understands:

- select a working wavelength λ _τ for which n _agu a <n _h ¡ _e io

- providing a waveguide comprising a material of

optical coating whose index of refraction n _ma ter¡ai meets the relation n _ag ua≤ n _ma ter¡ai <n _h ¡ _e io to the working wavelength λ _τ and a core area contained in the coating material and appropriate for the propagation of an optical signal that has the working wavelength;

- install the waveguide on a surface where the presence of ice must be detected, so that in one section of the guide a face of the lining material is exposed, defining a sensor section of the guide;

- transmit an optical signal with a working wavelength λ _τ along the waveguide, and - detect the optical signal down the sensor section.

Preferably a waveguide is provided comprising an optical coating material whose refractive index n _ma teri ai fulfills the relationship

n _agu a <n _ma ter¡ai <

at the working wavelength λτ.

The waveguide can be installed on the surface so that the face of the lining material of the sensor section is exposed through an opening in the surface, and preferably so that the face of the exposed lining material is level with the surface .

In some embodiments, the method further comprises:

- compare the intensity of the detected signal with at least one threshold value, and

- determine that there is ice on the sensor section of the waveguide if the intensity of the detected signal is lower than said threshold value.

The method may further comprise determining at least one threshold value; In this case, the determination of at least one threshold value can be performed once the waveguide is installed on a surface.

In embodiments useful for monitoring an area of a surface, the method may comprise:

- install a plurality of waveguides in a predetermined area, at a distance from each other,

- providing a common control unit connected to said plurality of waveguides, and

- provide information through the control unit on the

distribution of ice in the predetermined area, depending on the

intensities of the optical signals detected.

Other objects, advantages and features of embodiments of the invention will be apparent to the person skilled in the art from the description, or can be learned with the practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Particular embodiments of the present invention will now be described by way of non-limiting example, with reference to the accompanying drawings, in which:

Figure 1 is a graph showing refractive indices of water and ice as a function of wavelength; Figure 2 schematically shows the structure of a sensor according to a first embodiment of the invention; Figures 3 to 5 schematically show three possible embodiments of the sensor; and Figure 6 is a graph that qualitatively illustrates the attenuation of a signal transmitted along the waveguide of the sensor during an icing process.

DETAILED EXHIBITION OF MODES OF EMBODIMENT

In accordance with embodiments of the present invention, an optical sensor for detecting the presence of ice on a surface takes advantage of the phenomenon of evanescent coupling of electromagnetic modes guided to detect ice, and in particular to be able to discriminate between the presence of ice and the presence of Water.

For this, an optical signal is transmitted along a waveguide of the optical sensor; The sensor is designed so that if there is ice on the sensor, an evanescent coupling of at least one guided mode of the signal with the ice is produced, and that this coupling does not occur in the presence of water: since the evanescent coupling has the effect an important attenuation of the signal transmitted by the waveguide, in the presence of ice there is a significantly greater attenuation than in the presence of water, which allows to differentiate the two situations.

The specific characteristics of a sensor according to some non-limiting examples of embodiment are described below. A characteristic of the sensor is that it is designed to work at a working wavelength λ _τ for which the refractive indices of water and ice meet the relationship

forge ^ ice

Figure 1 is a graph showing the variation of the refractive index of water and ice as a function of wavelength, in an area of the spectrum between 2 pm and 100 pm (logarithmic scale).

As can be seen in the graph, although in general the refractive indices of water and ice are similar, in the area of the spectrum between approximately 3 pm and 90 pm (middle infrared and part of the far infrared) there are seven IR1 spectral bands, IR2, ... IR7, where the index of ice is higher than that of water. These bands have been indicated in the graph with an approximate average value, for illustrative and non-limiting purposes only, and correspond to areas around approximately 3500 nm (IR1), approximately 4640 nm (IR2), approximately 5860 nm (IR3), approximately 8000 nm (IR4), approximately 14500 nm (IR5), approximately 54000 nm (IR6), and approximately 73500 nm (IR7), each band having a different characteristic spectral bandwidth.

The sensor is therefore designed to work with a working wavelength λ _τ that is in one of these bands IR1 to IR7.

Figure 2 partially and schematically shows the structure of a sensor according to an embodiment of the invention. The sensor 100 of Figure 2 comprises a waveguide 101 including a material of optical coating (cladding) 102 whose refractive index n _m of ter¡ai satisfies the relationship n n _m _agu a≤ ter¡ai <n _h _e io at the working wavelength λ _τ , and preferably the ratio n _acute to <n _mate riai <

For example, if a working wavelength λ _τ of 3394 nm is chosen, which is within the range IR1 of Figure 1, it can be seen that at this wavelength n _water is approximately 1.45 yn _h ¡ _e io it is approximately 1.56, so the material chosen for the optical coating of the sensor should have a refractive index at 3394 nm that is between these two values: for example, crystalline quartz meets this condition, since at this wavelength it has a refractive index close to 1.5.

Within the optical coating material 102 there is a core zone {core) 103, suitable for propagation of an optical signal at the working wavelength. The structure of the guide with a core and an optical coating can be formed by any known technique: for example, the core area can be formed by laser writing in a block of appropriate optical coating material, so that in this area the index of refraction vary to the extent necessary to constitute an appropriate core.

Since the difference between the refractive index of the core and that of the coating is generally very small, in general both the core and the coating can have refractive indexes that are between that of water and ice, at the wavelength of work.

In order for a signal to propagate, the material must have a certain degree of transparency at the working wavelength (very high transparency is not necessary in this application, since the distances are short and the objective of the waveguide it is not that the signal propagates with the minimum losses), and the core must have a refractive index

slightly higher than that of the optical coating material, although the difference may be very small: for example, the core may have a 0.1% higher index than the coating, or even only 0.01% higher, as is known for An expert in the field. Figure 2 shows schematically and only by way of illustration an electromagnetic guided mode MG of an optical signal, which propagates along the guide 101.

The waveguide 101 is configured so that it has a section 104, which will be referred to below as a sensor section, suitable for being installed on a surface S, in contact with an external means M, in which it wants to detect the presence of ice. The sensor is installed in such a way that a face of the coating material 102 of the sensor section 104 is exposed to an external means M, which can be for example air, water or ice, in the same way as the surface S. The shape of the sensor may be suitable for the sensor to be installed within the surface S and the sensor section 104 of the waveguide 101 is exposed to the outside environment M through an opening in the surface S, and with the face of the lining mat 102 level with the surface S itself, as shown in figure 2.

In other alternatives, especially in applications where aerodynamics is not critical, the sensor can be placed on the surface, or protruding through an opening, with the exposed sensor section.

If the sensor must be used in demanding environmental conditions, it may be convenient to select the optical coating mat also taking the hardness into account, so that it can resist chafing.

The sensor 100 also comprises in figure 2 an emitter 105 capable of injecting into the waveguide 101 an optical signal with the working wavelength λ _τ for which

n _ag ua <n _h ¡ _e io, and a detector 106 capable of detecting the signal transmitted along the waveguide 101. The emitter can be, for example, a laser.

Thanks to the condition that has been chosen for the index of refraction of the material of the optical coating 102, when ice is formed on the sensor section 104, there will be a jump in the refractive index between the sensor section and the ice. it increases in an outward direction (n _ma terÃai <nmeio), and consequently there will be an evanescent coupling of at least one way guided with the ice along the sensor section 104 of the waveguide, so that the Optical signal will preferably be confined to ice. As a consequence of this coupling, the intensity of the signal transmitted by the waveguide and detected by the detector 106 will be significantly lower than that emitted by the transmitter 105.

On the other hand, when there is air on the sensor section (n _a ¡re = 1 <n _ma ter¡ai), or water

(n _agu a≤ ter¡ai _ma n), there will be an increased refractive index in the interphase between the sensor section and the external environment, but in any case reduced, and therefore no evanescent coupling with the outside environment and no signal intensity will be lost throughout the guidance by this phenomenon. Sensor design factors, such as the guide material, the type of guide used, the distance between the core 103 and the external medium M (i.e., the minimum thickness e of the optical coating in the sensor section 104), the Selected specific working wavelength, etc., determine the degree of effectiveness of the sensor, and can be adjusted according to the specific requirements of each case: for example, depending on the type of application and environment in which it is implemented, the Sensor can be designed to meet different requirements for size, environmental resistance, detection efficiency, etc.

It should be noted that the sensor can be mounted, as in Figure 2, level with the surface, and that the layer of optical coating material 102 present in the sensor section 104 protects the core 103 from the environment, and from abrasion, without damage the evanescent coupling on which the detection is based; All this is very useful for example in applications in aircraft fuselages and the like.

The length that the sensor section 104 must have in order for evanescent coupling to occur in the presence of ice can be very small, for example of the order of 1 mm or a few millimeters, so the sensor hardly affects the surface on which it is install

In general, the greater the difference between the ice index and the index of the optical coating material, the smaller the distance necessary for a significant proportion of energy from the guide to be transferred to the outside environment by evanescent coupling, so The length of the sensor section will also depend on the material chosen.

The thickness e of the optical coating material 102 in the sensor section 104 may be a few microns, but it may also be considerably greater, depending on the specific working wavelength, the length of the sensor section 104, etc.

The core 103 of the waveguide 101 can have any section, for example round or rectangular, and can also be designed as a zone with multiple cores. The coating material 102 may also have any shape that is appropriate to contain the core area 103 and so that the sensor can be placed in the working position; although it can also be provided that the waveguide is partially surrounded by a sheath or that it comprises another outer protective material, which serves to mount the sensor in the working position.

In Figs. 3, 4 and 5, three possible variations of an optical sensor according to the invention are schematically shown.

In figure 3 the sensor comprises a body of optical coating material 102, which contains the core area 103 and which is mounted in an opening A of the surface S, so that one face of the coating 102 of the sensor section 104 is exposed . In the figures, the opening A has been exaggerated to facilitate its identification, but usually the sensor will be mounted so that there is no clearance between the surface S and the sensor.

The core zone 103 extends from one wall to another of the body of optical coating material 102, following a path that makes it possible for the sensor section 104 to be close to the surface S and at the same time provides sufficient space for the connections in the ends of the core zone. In this embodiment, the emitter 105 and the detector 106 may be mounted directly adjacent to the material body of

coating 102, as the figure shows.

Figure 4 shows a sensor similar to that of Figure 3, but in which the emitter 105 and the detector 106 are connected to the waveguide 101 through auxiliary optical fibers, 107 and 108, which can have any suitable length to mount the emitter and the detector in positions far from the surface S in which the presence of ice must be detected.

A slightly different embodiment of the sensor is shown in Figure 5: in this case, the sensor waveguide is an optical fiber 201. The optical fiber 201 comprises, as is known, a core zone 203 {core), a

optical coating 202 (cladding), and an outer coating 209 (coating or buffer). Fiber 201 is selected so that optical coating 202 meets the aforementioned condition of

nagua≤ n _mat eriai <n _h ¡ _e io at the working wavelength λ _τ ,

and preferably n _AGU <n _mat eryAI <io n _hie. The optical fiber 201 comprises a sensor section 204, in which the outer covering 209 has been removed, so that a face of the coating 202 is exposed on the surface S. In the sensor section 204, a part of the optical coating can also be removed 202, depending on the thickness e that is to be left between the core 203 and the outside environment.

The emitter 105 and the detector 106 are located at the ends of the optical fiber 201, which will have the length that is most suitable for each application.

In this case, the optical fiber 201 encapsulated in a block or support body 210 of any material, such as a resin or an adhesive, which is not involved in the operation of the sensor and simply has the function of allowing the assembly of the fiber optic 201 and in particular the sensor section 204 in a suitable manner on the surface S.

Studies have shown that the sensor is capable of detecting the presence of ice since the nucleation of the crystals occurs, without the need for a complete layer to form or for the ice to accumulate to a certain thickness; therefore, the sensor allows a very early detection.

In all the embodiments of the sensor that have been described it can also be provided that there is an anti-erosion coating applied at least on the sensor section, that is to say on the face of the optical coating that is exposed: it is enough to design this coating with the appropriate thickness for Do not prevent significant evanescent coupling in the sensor section. This thickness depends, among other things, on the coating material and the working wavelength chosen, the level of effectiveness required, and also the length of the sensor section; In some cases, the thickness should be substantially less than the working wavelength, especially if the sensor section is to have a reduced length.

In one embodiment, by way of a concrete and non-limiting example, the sensor can have the following characteristics: - optical coating material: crystalline quartz

- working wavelength: λ _τ = 3,394 m

- thickness of the coating on the core: e = 10 pm

- circular section core, radius r = 10 pm

- length of the sensor section: 10 mm

In such a sensor, the transmitted signal would not suffer attenuation in the presence of outside air, while it would suffer a significant attenuation of approximately 6 dB (therefore there would only be 25% transmission) in the presence of ice.

So far it has been discussed how the sensor is able to detect the presence of ice, and discriminate against the presence of water. However, embodiments of the sensor according to the invention can also detect the presence of water (differentiated from the presence of air), to provide more complete information of the situation on the surface, at each instant or over time.

Indeed, when there is water on the sensor section there is also a certain attenuation of the signal transmitted by the waveguide, but in this case the loss of intensity is due to absorption phenomena (there is no evanescent coupling), and the loss It is significantly smaller than in the presence of ice.

In short, the sensor can identify three different situations, each of which corresponds to a certain attenuation or loss of signal strength:

- when the external medium M (see figure 2) is air, the signal will practically not suffer attenuation, or in any case it will have a known (maximum) level, which depends on the characteristics of the sensor and the line between the transmitter

105 and detector 106;

- if after a while water is deposited on the surface S (that is, the external medium M becomes water), a certain loss due to absorption in the signal will occur; in some cases the signal strength may be reduced for example to an approximate level of 80% (attenuation of approximately 1 dB);

- if ice crystals are subsequently formed in the deposited water, so that the external medium M becomes ice, evanescent coupling of at least one electromagnetic mode guided by the signal with the ice crystals will occur, and the loss of intensity in the sensor section it will be much higher than in the case of water; In some cases, the intensity detected in the presence of ice can be, for example, around 20% (attenuation of approximately 7 dB).

Figure 6 illustrates qualitatively the evolution of the signal intensity along these three stages in time, showing on the left of the figure an axis with the signal intensity, and on the right an axis with the corresponding attenuation . The graph is purely illustrative, since the concrete values may vary depending on the materials and the geometry of the sensor, the working wavelength, the conditions of the external environment, etc.

In a practical application, by means of a sensor calibration one or more suitable threshold levels can be established to provide warning signs of the formation or presence of ice, and even of the presence of water. The thresholds will be established for each application at the most appropriate levels depending on how you want to monitor the situation, the security required, etc.

In addition, since the level of evanescent coupling will also depend on the characteristics of the ice layer, by properly calibrating the sensor it is possible to evaluate the thickness of the ice layer.

Using one or more optical sensors according to embodiments of the present invention, a system capable of detecting ice at a point on a surface can be constructed, but also a system capable of monitoring and providing information about the external environment (air, water or ice) that there is at every moment on the surface, and even on the thickness of the ice. Depending on the information provided by the system, a user can make the decisions deemed appropriate: for example, if the system is installed in the fuselage of an aircraft, the pilot may decide to activate means to remove the ice or to prevent its accumulation. An automatic control of defrosting systems or the like can also be provided, depending on the signals provided by the detection system.

With several sensors installed remotely from each other and connected to a common control unit it is also possible to perform multipoint sensing, and artificial intelligence or fuzzy logic strategies can be used to extract additional information regarding the spatial distribution of ice on the surface or the type of ice.

In a system with several sensors an independent transmitter and detector can be used for each of the sensors; In some applications, it may also be useful to have sensors with different characteristics from each other and that work with different wavelengths, for example if any point on the surface to be controlled is more critical, it requires a higher level of accuracy, etc.

Alternatively, a system with several sensors can use a common emitter and a single working wavelength.

In all cases, the calibration can be individual for each sensor, so that each sensor has different thresholds associated; the differences between the thresholds may be due to the fact that the specific characteristics of the sensors are different, but also to the willingness to monitor some points of the surface with respect to others.

Calibration can be performed at the factory or before installing the sensors, but it may be more accurate once the sensors are installed on a surface, and even periodically re-calibrate the sensor. For this, the system control unit may be equipped with the necessary functionalities.

A "dry" calibration can be performed, that is, with air as an external medium, to set the normal level or zero level of the signal strength, and assign default percentages of this level as thresholds; or specific calibrations can also be performed in the presence of ice and / or water, to set appropriate attenuation thresholds.

It will be understood from this description that, in a specific embodiment, a method for detecting the presence or formation of ice may comprise, for example:

- select a working wavelength for which the refractive index of the water and the refractive index of the ice meet the forge condition ^ ^ ice,

- providing a waveguide, which can be an optical fiber or a guide of another type, which has a core area and an optical coating, of a material whose refractive index, at the working wavelength, is between the of ice and water;

- install this waveguide on the surface in such a way that in a section of the guide, which will constitute the sensor section of the guide, the optical coating is placed with a face exposed to the medium outside the surface;

- transmit an optical signal with the expected working wavelength, along the waveguide,

- detect the optical signal down the sensor section, and

- compare the intensity of the detected signal with one or more thresholds, to determine depending on the attenuation that has occurred, if ice has formed on the sensor.

In all cases, the transmitted optical signal can be continuous or pulsed, and detection can be done at appropriate time intervals.

The procedure can be applied with a system of several waveguides installed remotely from each other, and can also comprise calibration steps, as described above. It can also be used to issue not only warnings or signals related to training or presence of ice, but also with the presence of water on the surface, properly establishing the characteristics of waveguides, working wavelength, and thresholds.

Although only some particular embodiments and examples of the invention have been described herein, the person skilled in the art will understand that other alternative embodiments and / or uses of the invention are possible, as well as obvious modifications and equivalent elements. In addition, the present invention encompasses all possible combinations of the specific embodiments that have been described. The numerical signs relating to the drawings and placed in parentheses in a claim are only intended to increase the understanding of the claim, and should not be construed as limiting the scope of the claim's protection. The scope of the present invention should not be limited to specific embodiments, but should be determined only by an appropriate reading of the

attached claims.

For example, the uses that can be given to the sensor are multiple, although explicit reference has only been made to the detection of ice and / or water in

aircraft Also in wind turbine blades the sensor may be very appropriate, since these surfaces have problems similar to those of the plane wings in terms of aerodynamics.

Another application may be the detection of ice on surfaces that are submerged, since the sensor reliably discriminates ice from water; and it can also be used to detect ice in refrigerators or in water pipes, and in general in any circumstance where you want to detect the formation of ice quickly and reliably.

Claims

one . Optical sensor (100) to detect the presence of ice on surfaces, characterized in that it comprises:

- a waveguide (101; 201) with an optical coating material (102; 202) whose refractive index n _m of ter¡ai satisfies the relationship n n _m _agu a≤ ter¡ai <I rícelo a wavelength work λ _τ for which n _a gua <n _h ¡ _e io, and with a core zone (103; 203) contained in the cladding material (102; 202) and appropriate for the propagation of an optical signal having the working wavelength, the waveguide (101; 201) presenting a sensor section (104; 204) suitable to be installed on a surface (S) in which the presence of ice must be detected so that a face of the lining material (102; 202) of the sensor section (104; 204) is exposed;

- means (105) for injecting into the waveguide (101; 201) an optical signal with a working wavelength λ _τ for which n _water <n _h ¡ _e io; Y

- means (106) for detecting the optical signal transmitted by the waveguide (101; 201) downstream of said sensor section (104; 204).

2. Optical sensor according to claim 1, characterized in that the refractive index of the material meets the relation n _water <n _mate riai <n _h ¡ _e io to the working wavelength λ _τ .

3. Optical sensor according to any one of claims 1 or 2, characterized in that the sensor section (104; 204) is suitable for being installed on the surface (S) so that the face of the material of

coating (102; 202) of the sensor section (104; 204) is exposed through an opening (A) in the surface (S).

4. Optical sensor according to any one of claims 1 to 3, characterized in that the waveguide comprises an optical fiber (201), from which the outer covering (209) has been removed at least in the area of the sensor section (204) ) intended to be exposed during use.

5. Optical sensor according to claim 4, characterized in that a part of the thickness of the optical coating (202) has also been removed in the sensor section (204) of the optical fiber (201).

6. Optical sensor according to any one of claims 1 to 3, characterized in that at least the sensor section (104) of the waveguide

(101) the core zone (103) is manufactured within a body of material

(102) appropriate, so that the refractive index varies in this area.

7. Optical sensor according to claim 6, characterized in that optical fibers (107, 108) are coupled to an input end and an output end of said waveguide (101) which includes the sensor section (104).

8. Optical sensor according to any one of the preceding claims, characterized in that the working wavelength λ _τ is between 3000 and 5000 nm, preferably between 3200 and 4000 nm.

9. Optical sensor according to any one of the preceding claims, characterized in that the material is selected from crystalline quartz, vitreous quartz, indium fluoride, or compounds thereof.

10. Optical sensor according to any one of the preceding claims, characterized in that the thickness (e) between the core area (103; 203) and the face of the coating material (102; 202) intended to be exposed is less than 250 pm , preferably less than 50 pm.

eleven . Optical sensor according to any one of the preceding claims, characterized in that it further comprises an anti-erosion coating applied on the face of the coating material (102; 202) of the sensor section (104; 204) intended to be exposed.

12. System for detecting the presence of ice on surfaces, characterized in that it comprises at least one optical sensor (100) according to any one of claims 1 to 9, and means for comparing the intensity of a detected optical signal with at least one threshold value , and generate an alarm in case the optical signal is lower than a threshold value.

13. System according to claim 12, characterized in that it comprises calibration means for determining at least one threshold value.

14. System according to claim 13, characterized in that the calibration means are suitable for determining at least one threshold value once the system is installed on a surface (S).

15. System according to any one of claims 12 to 14,

characterized in that it comprises a plurality of said optical sensors (100), intended to be installed in a predetermined area, at a distance from each other, and a common control unit connected to said plurality of sensors (100), said control unit comprising means to provide information on the distribution of ice in an area

predetermined, depending on the intensities of the optical signals detected at the outputs of the plurality of sensors (100).

16. Procedure for detecting the presence of ice on surfaces, characterized in that it comprises:

- select a working wavelength λ _τ for which n _agu a <n _h ¡ _e io

- providing a waveguide (101; 201) comprising an optical coating matenal (102; 202) whose refractive index n _m satisfies the relation n ter¡ai _agu a≤ n _ma ter¡ai <n _h _e io at the working wavelength λ _τ and a core zone (103; 203) contained in the cladding mat and suitable for the propagation of an optical signal having the working wavelength;

- install the waveguide (101; 201) on a surface (S) on which the presence of ice must be detected, such that a face of the lining material (102; 202) remains on a section of the guide exposed, defining a sensor section (104; 204) of the guide;

- transmit an optical signal with a working wavelength λ _τ along the waveguide (101; 201), and

- detect the optical signal downstream of the sensor section (104; 204).

17. Method according to claim 16, characterized in that a waveguide (101; 201) is provided comprising an optical coating material (102; 202) whose refractive index n _ma terìai fulfills the relation n _a gua <n _ma ter¡ai <n _h ¡ _e io at the working wavelength λ _τ .

18. Method according to any one of claims 16 or 17, characterized in that the waveguide (101; 201) is installed on the surface (S) so that the face of the coating material (102; 202) of the sensor section ( 104; 204) is exposed through an opening (A) on the surface (S).

19. Method according to claim 18, characterized in that the sensor section (104; 204) of the waveguide (101; 201) is installed on the surface (S) so that the face of the coating material (102; 202) exposed is level with the surface (S).

20. Method according to any one of claims 16 to 19, characterized in that it further comprises:

- determine that there is ice on the sensor section (104; 204) of the waveguide (101; 201) if the intensity of the detected signal is lower than said threshold value.

twenty-one . Method according to claim 20, characterized in that

It further comprises determining at least one threshold value.

22. Method according to claim 21, characterized in that the determination of at least one threshold value is performed once the waveguide (101; 201) is installed on a surface (S).

23. Method according to any one of claims 16 to 22, characterized in that it comprises: - install a plurality of waveguides (101; 201) in a predetermined area, at a distance from each other,

- providing a common control unit connected to said plurality of waveguides (101; 201), and

- provide information through the control unit on the

distribution of ice in the predetermined area, depending on the

intensities of the optical signals detected.