System and method for detecting a displacement or a deformation of a building structure

Abstract

It is disclosed a system and method for detecting a displacement or deformation of a portion of a building structure. The system comprises an optical fiber, a first device connected to a fiber's first end and comprising a laser source emitting an infrared radiation, and a second device connected to the fiber's second end and comprising a fiber optic splitter and an infrared detector. The fiber optic splitter receives the infrared radiation from the optical fiber, divides it into an object beam and a reference beam, directs the object beam onto the building structure's portion and directs the reference beam onto the infrared detector, which detects a time sequence of holograms formed by reference beam interfering with object beam scattered by the building structure's portion. A data processing unit reconstructs the time evolution of the deformation or displacement of the building structure's portion by numerically processing the time sequence of holograms.

Description

[0001] SYSTEM AND METHOD FOR DETECTING A DISPLACEMENT OR A DEFORMATION OF A BUILDING STRUCTURE

[0002] Field of the invention

[0003] The present invention relates in general to the monitoring of building structures, for example in connection with assessment of seismic or hydro-geological risk, or in connection with the protection of fine arts. In particular, the present invention relates to a system and method for detecting a deformation or a displacement of a building structure or a portion thereof.

[0004] In the present description and the claims, the expression "building structure" will indicate a manufactured object or structure having dimensions of the order of magnitude of 1 m or more (up to several tens of metres) intended for any area in the civil engineering field (construction, geotechnical, infrastructural, hydraulic, electrical, structural or town planning areas) such as a civil or industrial building, a bridge or viaduct, a tunnel, a barricade, a dam, a dike, an aqueduct, a sewer, a canal, a pylon, a wind turbine or an electricity pylon.

[0005] Background of the invention

[0006] Detecting deformations or displacements of a building structure is useful in various areas, for example the assessment of seismic or hydro-geological risk, civil protection or the protection of fine arts. By detecting the displacements (for example oscillations, in terms of amplitude and frequency) of a building structure, for example, it is possible to assess the features of the building structure and any structural defects of the building structure, or to evaluate the response of the building structure to environmental factors (earthquakes, wind, rain and other natural phenomena) or anthropogenic factors (motor vehicle or railway traffic, underground works, etc.).

[0007] At present, detecting deformations or displacements of a building structure typically is performed by means of suitable sensors (seismometers) which are arranged in different points of the structure and / or on its surface.

[0008] EP 3 250 964 B1 in the name of the same Applicant discloses a system for monitoring a building structure which makes use of digital holography at infrared wavelengths. Specifically, the system of EP 3 250 964 B1 comprises a laser source emitting an infrared radiation and a lens-less off-axis interferometric arrangement that divides the infrared radiation into an object beam and a reference beam. The object beam irradiates a surface of the building structure, that scatters it. The reference beam interferes with the scattered object beam, so as to create a hologram. The system comprises an infrared detector which detects a time sequence of holograms and a data processing unit that reconstructs the evolution in time of deformations and / or displacements of irradiated surface of the building structure by numerically processing the time sequence of holograms. Differently from seismometers, the system of EP 3 250 964 B1 provides a monitoring of the building structure which is remote and which allows simultaneous monitoring each point of the irradiated surface, while allowing detecting both oscillations with an arbitrarily low frequency and deformations or displacements which are not periodic and which occur over time periods which may also be relatively long (hours or days).

[0009] Summary of the invention

[0010] The Applicant has perceived the need to improve the system disclosed by EP 3 250 964 B1.

[0011] The system of EP 3 250 964 B1 indeed shall also comprise a cooling system for the laser source. Typical cooling systems are water cooling systems (which make use of water to cool the laser source) or air cooling systems (which make use of an airflow generated by a fan to cool the laser source). Cooling systems typically vibrate, especially air cooling systems whose fan vibrates as it rotates. Such vibrations disturb operation of the interferometric arrangement of the system of EP 3 250 964 B1 , whose minimum detectable displacement or deformation is therefore disadvantageously limited by the vibrations of the cooling system.

[0012] In view of the above, the Applicant has faced the problem of providing a system and method for detecting a displacement or a deformation of at least a portion of a building structure, whose possible vibrations (for example due to the cooling system of the laser source) have a reduced impact on the detection capability of the system in terms of minimum detectable displacement or deformation.

[0013] According to embodiments of the present invention, this and other problems are solved by a system for detecting a displacement or a deformation of at least a portion of a building structure, comprising an optical fiber, a first device connected to a first end of the optical fiber and a second device connected to a second end of the optical fiber; the first device comprises a laser source configured to emit an infrared radiation into the first end of the optical fiber; the second device comprises a fiber optic splitter and an infrared detector; the fiber optic splitter is configured to receive the infrared radiation from the second end of the optical fiber and to divide the infrared radiation into an object beam and a reference beam, to direct the object beam onto at least a portion of the building structure and to direct the reference beam onto the infrared detector; the infrared detector is configured to detect a time sequence of holograms formed by the reference beam interfering with the object beam scattered by the at least a portion of the building structure; the system also comprises a data processing unit configured to detect a displacement or a deformation of the at least a portion of the building structure by numerically processing the time sequence of holograms.

[0014] Physically decoupling the interferometric arrangement (namely, the fiber optic splitter and the infrared detector) from the other components of the system (in particular, the laser source) in two distinct devices connected by an optical fiber advantageously results in a reduced impact of possible vibrations of the system (for example due to the cooling system of the laser source) on its detection capability in terms of minimum detectable displacement or deformation.

[0015] Components of the system subjected to vibrations (such as the cooling system of the laser source) may indeed be housed in the first device. Since the optical fiber is not a rigid element, it does not propagate the vibrations of such components to the second device. Therefore, these vibrations advantageously do not disturb operation of the interferometric arrangement, which is housed in the second device. Since the minimum detectable displacement or deformation depends on the stability of the interferometric arrangement, it is advantageously independent of the vibrations of the cooling system or, in general, of any component housed in the first device.

[0016] Besides, implementing the interferometric arrangement by means of a fiber optic splitter advantageously results in a particularly light and compact second device. The second device may accordingly be mounted e.g. on a tripod placed nearby the building structure to be monitored, and may be further mechanically stabilized by any known stabilization system, for example a gimbal system. The possibly to use a stabilization system on the second device physically decoupled from the first device further reduces vibrations of the interferometric arrangement, which results in a particularly high sensitivity of the system. Specifically, the inventors have estimated a minimum detectable displacement or deformation of 0.015 pm may be reached. This makes the system according to embodiments of the present invention particularly suitable for detecting displacements or deformations in case the building structure is particularly rigid (e.g. it is a historical building, typically exhibiting oscillations with amplitude lower than 0.1 pm).

[0017] According to a first aspect, the present invention provides a system for detecting a displacement or a deformation of at least a portion of a building structure, the system comprising:

[0018] - an optical fiber having a first end and a second end;

[0019] - a first device connected to the first end of the optical fiber and comprising a laser source configured to emit an infrared radiation into the first end of the optical fiber;

[0020] - a second device connected to the second end of the optical fiber and comprising a fiber optic splitter and an infrared detector, the fiber optic splitter being configured to receive the infrared radiation from the second end of the optical fiber and to divide the infrared radiation into an object beam and a reference beam, to direct the object beam onto at least a portion of the building structure and to direct the reference beam onto the infrared detector, the infrared detector being configured to detect a time sequence of holograms formed by the reference beam interfering with the object beam scattered by the at least a portion of the building structure; and

[0021] - a data processing unit configured to reconstruct an evolution in time of the deformation or the displacement of the at least a portion of the building structure by numerically processing the time sequence of holograms.

[0022] The first device and the second device are distinct and physically decoupled devices.

[0023] Preferably, the laser source comprises a fiber coupled laser.

[0024] Preferably, the infrared radiation has a wavelength lower than 3 pm.

[0025] Preferably, the infrared radiation has a maximum output power of 10 W.

[0026] Preferably, the frequency of the laser source is stable within 60 kHz.

[0027] According to an embodiment, the optical fiber is suitable for being removably coupled with the fiber optic splitter comprised in the second device.

[0028] Preferably, the system further comprises a cooling system configured to cool the laser source during operation, the cooling system being housed in the first device.

[0029] Preferably, the data processing unit is positioned in the first device or in the second device.

[0030] Preferably, the system further comprises a display suitable for displaying data indicative of the deformation or the displacement of the at least a portion of the building structure as provided by the data processing unit, the display being positioned on the second device or in a third device other than the first device and second device.

[0031] Preferably, the second device is provided with a gimbal system.

[0032] Preferably, the second device further comprises at least one of:

[0033] - an optical system suitable for maximizing the optical power of the object beam scattered by the at least a portion of the building structure and directed to the infrared detector;

[0034] - a beam splitter located on the optical path of the reference beam and configured to combine the reference beam and the object beam scattered by the at least a portion of the building structure;

[0035] - an adjustable collimator located on the optical path of the object beam and suitable for adjusting the size of the object beam directed onto at least a portion of the object;

[0036] - a further adjustable collimator located on the optical path of the reference beam and suitable for adjusting the size of the reference beam; and

[0037] - a variable attenuator located on the optical path of the reference beam and suitable for adjusting the intensity of the reference beam such that the reference beam and the object beam scattered by the at least a portion of the building structure (9) are received at the infrared detector with comparable intensities.

[0038] Preferably, the data processing unit is configured to:

[0039] - provide a sequence of phase images of the at least a portion of the building structure by processing the time sequence of holograms; and

[0040] - calculate a displacement over time of at least one point of the at least a portion of the building structure represented by a certain pixel in the sequence of phase images of the at least a portion of the building structure, as a function of phase differences of the pixel between consecutive phase images of the sequence of phase images of the at least a portion of the building structure.

[0041] According to an embodiment, the data processing unit is configured to calculate the displacement over time of the at least one point of the at least a portion of the building structure along a direction N perpendicular to a surface of the at least a portion of the building structure and to divide the displacement along the direction N by a factor k which takes into account the inclination of a direction of incidence of the object beam on the at least a portion of the building structure relative to the direction N.

[0042] Preferably, the data processing unit is further configured to process the time sequence of holograms of the at least a portion of the building structure so as to provide a sequence of amplitude images of the at least a portion of the building structure.

[0043] According to a second aspect, the present invention provides a method for detecting a displacement or a deformation of at least a portion of a building structure, the method comprising: - by a laser source comprised in a first device connected to a first end of the optical fiber having said first end and a second end, emitting an infrared radiation into the first end of the optical fiber;

[0044] - by a fiber optic splitter comprised in a second device connected to the second end of the optical fiber, receiving the infrared radiation from the second end of the optical fiber, dividing the infrared radiation into an object beam and a reference beam, directing the object beam onto at least a portion of the building structure and directing the reference beam onto an infrared detector also comprised in the second device;

[0045] - by the infrared detector, detecting a time sequence of holograms formed by the reference beam interfering with the object beam scattered by the at least a portion of the building structure; and

[0046] - by a data processing unit, reconstructing an evolution in time of said deformation or said displacement of said at least a portion of said building structure by numerically processing said time sequence of holograms.

[0047] Brief description of the drawings

[0048] The present invention will become clearer from the following detailed description, given by way of example and not of limitation, to be read with reference to the accompanying drawings, wherein:

[0049] - Figure 1 schematically shows a system for detecting a displacement or a deformation of at least a portion of a building structure according to an embodiment of the present invention;

[0050] - Figure 1a schematically shows the building structure having a surface irradiated by the object beam emitted by the system of Figure 1 ;

[0051] - Figure 2 is a flow chart of the operation of the system of Figure 1 ; and

[0052] - Figure 3 schematically shows a system for detecting a displacement or a deformation of at least a portion of a building structure according to another embodiment of the present invention;

[0053] The Figures are not in scale.

[0054] Figures 1 , 1a and 3 are schematic representations, not to scale, which do not reproduce the preferred distance and angle values shown in them. Quantitative evaluations of distances and angles of the system schematically shown in Figures 1 , 1a and 3 are indicated in the continuation of the present description.

[0055] Detailed description of embodiments of the invention

[0056] Figure 1 shows a system 1 for detecting a displacement or a deformation of at least a portion of a building structure 9 (a building, by way of a non-limiting example) according to an embodiment of the present invention.

[0057] The system 1 preferably comprises a first device 1a and a second device 1b. The first device 1a and the second device 1 b are distinct and physically decoupled, meaning that no rigid connection is present between the first device 1 a and the second device 1 b.

[0058] The system 1 also comprises an optical fiber 2out connecting the first device 1a and the second device 1 b. The optical fiber 2out may have a length for example comprised between 1 m and 5 m.

[0059] The first device 1a preferably comprises a laser source 2. The second device 1 b preferably comprises a fiber optic splitter 3 and an infrared (IR) detector 7.

[0060] The laser source 2 is preferably suitable for emitting a radiation in the infrared region. In particular, the emission wavelength of the laser source 2 is preferably lower than 3 pm. More preferably, the emission wavelength of the laser source 2 is comprised between 1.4 pm and 2.95 pm. For example, the emission wavelength of the laser source 2 is equal to 1.55 pm. The laser source 2 is preferably a continuous source. Alternatively, a pulsed laser source may be used.

[0061] The laser source 2 preferably is a fiber coupled laser, whose output fiber coincides with the optical fiber 2out. According to a particularly advantageous embodiment, the laser source 2 is a fiber optic laser. Use of a fiber optical laser contributes reducing weight and size of the first device 1a, which may accordingly be more easily transported near to the building 9, for example in a handbag.

[0062] Preferably, the maximum output power of the laser source 2 is 10 W, more preferably 5 W, even more preferably 1 W. The preferred range for the output power of the laser source 2 mainly depends on the distance d between the surface of the building 9 to be irradiated and the surface of the IR detector 7 (also termed herein below “reconstruction distance”), on the size D of the surface of the building 9 to be irradiated and on the sensitivity of the IR detector 7. Assuming to irradiate a surface of the building 9 in a substantially uniform way (and disregarding air absorption), the inventors have estimated that, if the reconstruction distance d is increased by a factor n, the output power of the laser source 2 shall be increased by a factor n2. The inventors have estimated that, for reconstructing the image of a surface having a size D of 1 m placed at a reconstruction distance d of about 10 m, an output power higher than 1 W is enough. Besides, the output power of the laser source 2 should be kept as low as possible in order not to damage the irradiated surface of the building 9.

[0063] Further, the infrared radiation emitted by the laser source 2 is preferably linearly polarized.

[0064] Further, preferably, the infrared radiation emitted by the laser source 2 has a coherence length which allows working within a desired reconstruction distance d, with no need to balance the lengths of the optical paths of the object and reference beams. A higher reconstruction distance d requires an increased coherence length, hence a narrower frequency bandwidth of the laser source 2. For example, if the desired reconstruction distance d is 10 m, the frequency bandwidth of the laser source 2 is preferably equal to or lower than 5 MHz. If instead the desired reconstruction distance d is 50 m, the frequency bandwidth of the laser source 2 is preferably equal to or lower than 1 MHz. Furthermore, in order not to introduce artifact displacements in the measurement, the frequency of the laser source 2 is preferably stable within 60 kHz between two subsequent acquisitions.

[0065] The first device 1a also preferably houses a cooling system 2a configured to cool the laser source 2 during operation. The cooling system 2a is preferably an air cooling system comprising a fan. This type of cooling system is advantageously lighter and more compact than other types of cooling systems, such as water cooling systems, thereby contributing to reduce weight and size of the first device 1a.

[0066] The first device 1a also preferably houses a battery suitable for powering the laser source 2 (not depicted in Figure 1 ).

[0067] The laser source 2 is preferably coupled to a first end of the optical fiber 2out. Besides, the fiber optic splitter 3 is coupled to a second end of the optical fiber 2out, so that the infrared radiation emitted by the laser source 2 is coupled into the fiber splitter 3 via the optical fiber 2out.

[0068] More specifically, the fiber optic splitter 3 is preferably provided with an input optical fiber Sin and two output optical fibers 3out1 , 3out2. The second end of the optical fiber 2out is preferably suitable for being removably connected to the input optical fiber Sin of the fiber optic splitter 3. The removable connection between second end of the optical fiber 2out and input optical fiber Sin of the fiber optic splitter 3 is preferably implemented by means of an optical connector 4, such as a PC (Physical Contact) optical connector or an APC (Angled Physical Contact) optical connector. The fiber optic splitter 3 is suitable for dividing the infrared radiation received from the laser source 2 through the input optical fiber Sin into a first beam (also termed herein after "object beam 0") and a second beam (also termed herein after "reference beam R"), for directing the object beam 0 onto a surface of the building 9 and for directing the reference beam R onto the IR detector 7. Each one of the object beam 0 and reference beam R is preferably output through a respective output optical fiber 3out1 , 3out2 of the fiber optic splitter 3. More specifically, the output optical fiber 3out1 is directed towards the building 9 so that, thanks to its divergence, a surface of the building 9 is efficiently irradiated. Similarly, the output optical fiber 3out2 is directly directed towards the IR detector 7 so that, thanks to its divergence and to its distance from the IR detector 7, substantially the entire surface of the IR detector 7 is almost uniformly irradiated. The object beam 0 then irradiates a surface of the building 9, that scatters it. At the IR detector 7, the reference beam R interferes with the scattered object beam 0 thereby creating an interference pattern, namely a hologram.

[0069] In order to allow the object beam 0 as emitted by the output optical fiber 3out1 irradiating the surface of the building 9 and the object beam 0 as scattered by the irradiated surface of the building 9 reaching the IR detector 7, the second device 1 b is preferably provided with an outer casing having at least one opening (not depicted in Figure 1 ). The opening may be provided with an optically transparent window capable of allowing passage of the infrared radiation in both directions (namely, from inside the second device 1 b towards the outside and vice versa).

[0070] The fiber optic splitter 3 is preferably configured such that the reference beam R and the object beam 0 scattered by the irradiated surface of the building 9 are received at the IR detector 7 with comparable intensities. The fiber optic splitter 3 is also preferably configured such that substantially the full dynamic range of the IR detector 7 is used to record the interferometric pattern.

[0071] However, when the intensity of the object beam 0 is so low that the dynamic range of the IR detector 7 cannot be efficiently used, it is profitable to increase the intensity of the reference beam R (paying attention not to saturate the IR detector 7) in order to maximise the signal-to-noise ratio (SNR). To this purpose, it is preferable to use a fiber optic splitter 3 with a splitting ratio suitable for providing a sufficiently high intensity of the reference beam R and, when needed, to attenuate it. However, considered that the intensity of the object beam 0 is strongly reduced by the scattering on the surface of the building 9 and that it attenuates significantly as the reconstruction distance d increases, most of the infrared radiation emitted by the laser source 2 is preferably directed to the object beam 0. Hence, preferably, the fiber optic splitter 3 is configured such that the optical power of the object beam 0 is higher than 80% of the total optical power of the infrared radiation received by the fiber optic splitter 3, more preferably higher than 90%, even more preferably higher than 99%. The inventors have made positive tests using a fiber optic splitter PN1550R1A1 (155nm 1x2 Polarization-Maintaining Fiber Optic Coupler) by Thorlabs Inc. based in Newton (New Jersey, United States), configured such that the optical power of the object beam O is 99% of the total optical power of the infrared radiation received by the fiber optic splitter 3, and the optical power of the reference beam R is accordingly 1 % of the total optical power of the infrared radiation received by the fiber optic splitter 3.

[0072] The IR detector 7 is configured to detect a time sequence of interference patterns between object beam O and reference beam R, namely a time sequence of holograms. The IR detector 7 preferably is a camera comprising a 2D array of N x M detector elements (or pixels). The IR detector 7 preferably is an InGaAs detector. The inventors have made positive tests using a detector WiDy SenS 640 manufactured by New Imaging Technologies (NIT) based in Verrieres le Buisson (France), with 640x512 pixels, a pixel size of 15 pm x 15 pm, a frame rate up to 230 frame / s, an exposure time from 10 ps to 1 s and spectral response in the range 0.9 pm - 1.7 pm.

[0073] The fiber optic splitter 3 and IR detector 7 therefore act as an off- axis holographic arrangement, namely an interferometric arrangement wherein the two interfering beams (namely, object beam O and reference beam R) are not focused on the surface of the IR detector 7 (lensless) and are reciprocally tilted by a non-null angle as they impinge on the surface of the IR detector 7 (off-axis).

[0074] The system 1 also preferably comprises a data processing unit 8 cooperating with the IR detector 7. The data processing unit 8 is preferably configured to receive from the IR detector 7 the detected time sequence of holograms in a discretised form, to store it and to process it, as it will be discussed in detail herein after. The data processing unit 8 is also preferably provided with a display 8a suitable for displaying data indicative of a deformation or a displacement of the irradiated surface of the building 9 as obtained by processing the time sequence of holograms.

[0075] The data processing unit 8 may be in the form of an FPGA unit. The data processing unit 8 may be positioned either in the first device 1a or, as depicted in Figure 1 , in the second device 1 b. As to the display 8a, it is preferably positioned such that the operator may easily see it as he uses the system 1 . For example, the display 8a may be positioned on the outer casing of the second device 1 b, as schematically depicted in Figure 1. Alternatively, the display 8a may be incorporated in a third device (not depicted in Figure 1 ) physically separated from the first device 1a and second device 1 b, for example a handheld tablet.

[0076] Physically decoupling the interferometric arrangement (namely, the fiber optic splitter 3 and the IR detector 7) from the other components of the system 1 (in particular, the laser source 2 and cooling system 2a) in two distinct devices 1a, 1 b connected by the optical fiber 2out advantageously results in a reduced impact of possible vibrations of the system 1 (for example due to the cooling system 2a) on its detection capability in terms of minimum detectable displacement or deformation.

[0077] The cooling system 2a is indeed housed in the first device 1a. Also other components of the system 1 subjected to vibrations may be housed in the first device 1a too. Since the optical fiber 2out is not a rigid element, it does not propagate the vibrations of such components to the second device 1 b. Therefore, these vibrations advantageously do not disturb operation of the interferometric arrangement, which is housed in the second device 1 b. Since the minimum detectable displacement or deformation depends on the stability of the interferometric arrangement, it is advantageously independent of the vibrations of the cooling system 2a and, in general, of any component housed in the first device 1a.

[0078] Besides, implementing the interferometric arrangement by means of the fiber optic splitter 3 advantageously results in a particularly light and compact second device 1 b. The second device 1 b may accordingly be mounted e.g. on a tripod placed nearby the building 9, and may be further mechanically stabilized by any known stabilization system, for example a gimbal system (reference number 5 in Figure 1 ). The possibly to use the stabilization system 5 on the second device 1 b physically decoupled from the first device 1a further reduces vibrations of the interferometric arrangement, which results in a particularly high sensitivity of the system 1 . Specifically, the inventors have estimated a minimum detectable displacement or deformation of 0.01 pm may be reached. This makes the system 1 particularly suitable for detecting displacements or deformations in case the building 9 is particularly rigid (e.g. it is a historical building, typically exhibiting oscillations with amplitude lower than 0.1 pm).

[0079] Besides, the system 1 may be advantageously implemented to be easily transported and installed in proximity of the building 9. In particular, if the laser source 2 is implemented as a fiber optic laser 2 and the cooling system 2a is implemented as an air cooling system, the first device 1a is relatively light and compact, and is then transportable for example in a handbag. Besides, as discussed above the second device 1 b comprising the interferometric arrangement is very light and compact. Field operators may then easily carry the system 1 with them to monitor the building 9 and work safely in any environmental conditions (accessibility of the ground surrounding the building 9, safety measurements if the building 9 is unstable, etc.).

[0080] Further, use of an infrared radiation in the range lower than 3 pm (preferably between 1.4 pm and 2.95 pm, for example 1.55 pm) allows using a very sensitive IR detector 7, such as for example an InGaAs (Indium Gallium Arsenide) detector. The increased sensitivity of the IR detector 7 allows reducing the output power of the laser source 2 (1-10 W), thereby significantly increasing the safety of the whole system 1 .

[0081] The operation of the system 1 according to an embodiment of the present invention will be now described with reference to the flow chart of Figure 2.

[0082] The operation of system 1 is preferably divided into two separate steps: an optical acquisition step 20 and a numerical processing step 21.

[0083] During the optical acquisition step 20, the system 1 is brought into proximity of the building 9 to be monitored. For this purpose, as mentioned above, the second portable device 1 b may be mounted on a tripod to irradiate a surface of the building 9. The reconstruction distance d between the surface of the IR detector 7 and the surface of the building 9 to be irradiated depends on the environmental conditions (accessibility of the ground surrounding the building 9), on any safety measurements (if the building 9 is unstable, access beyond a certain point will not be allowed) and on the size of the surface of the building 9 to be irradiated (the linear visual field of the system 1 increases with an increase in the reconstruction distance d).

[0084] The second device 1 b is then manually oriented by the field operator so that the surface of the building 9 to be irradiated falls within the visual field of the system 1. The direction of the object beam 0 (called below also "irradiation direction") therefore forms, relative to a direction N perpendicular to the surface of the building 9 to be irradiated, an angle (see Figure 1a).

[0085] Then, the laser source 2 is switched on and starts to emit infrared radiation. The infrared radiation is divided into an object beam 0 and a reference beam R by the fiber optic splitter 3.

[0086] The object beam 0 then irradiates a surface of the building 9. The object beam 0 is then scattered by the irradiated surface of the building 9 and then reaches the IR detector 7.

[0087] On the other hand, the reference beam R is directly directed onto the IR detector 7. Therefore, the object beam 0 scattered by the irradiated surface of the building 9 and the reference beam R interfere with each other on the surface of the IR detector 7, thus creating an interference pattern or hologram of the irradiated surface of the building 9, which is detected by the IR detector 7.

[0088] Preferably, during step 20, the IR detector 7 acquires a time sequence of holograms. The time sequence of holograms is preferably acquired at the acquisition frequency of the IR detector 7. If, for example, the acquisition frequency of the IR detector 7 is 25 photograms / s, 25 holograms per second are acquired. Each acquired hologram exhibits interference fringes with a certain fringe spacing. Each hologram may be described in terms of the bidimensional intensity distribution according to the following equation:

[0089] H(x,y)=|R|2+|O|2+R* O+R O* [1] where x and y are the two orthogonal coordinates of the surface of the IR detector 7, while R* and 0* are the complex conjugates of the reference beam R and the object beam 0, respectively.

[0090] Since the IR detector 7 is only sensitive to infrared radiations, the component of the artificial light or sunlight in the visible range do not disrupt operation of the system 1 at step 20. Besides, the infrared component of the artificial light and sunlight does not impair the operation of system 1 , because it is incoherent with object beam 0 and reference beam R, and accordingly merely represents a background noise. In any case, an optical band-pass spectral filter (not depicted in Figure 1 ) centered around the emission wavelength of the laser source 2 may be optionally provided in front of the IR detector 7, in order to filter out the infrared component of artificial light and sunlight.

[0091] The time sequence of holograms acquired by the IR detector 7 during step 20 is then stored by the data processing unit 8.

[0092] The data processing unit 8 then preferably carries out a numerical processing step 21 on the acquired time sequence of holograms. The numerical processing step 21 may be carried out at the end of step 20, namely at the end of acquisition of the entire time sequence of holograms. Alternatively, since each hologram in the time sequence undergoes individual numerical processing, step 21 may start immediately after acquisition of the first hologram, and then continue in parallel with acquisition of the successive holograms.

[0093] The numerical processing of each acquired hologram is preferably as that described in EP 3 250 964 B1 . Specifically, during a first sub-step 210, each acquired hologram is preferably filtered, so as to cancel the DC term or zerothdiffraction order, namely the term |R|2+|O|2of equation [1], Since the system 1 has an off-axis configuration (namely the reference beam R and object 0 impinge on the IR detector 7 with different angles), such DC term |R|2+|O|2is advantageously spatially non superimposed to the other terms R* O+R 0*, and accordingly may be filtered out in the spatial frequency domain.

[0094] Then, during a second sub-step 211 , a zero padding operation is preferably applied to each acquired hologram, namely the array of N x M pixels of the filtered, discretized hologram is extended by introducing a number of additional fictitious pixels, the intensity of which is set to zero. Preferably, the zero padding operation is that described in EP 1 654 596, in the name of the same Applicant.

[0095] Indeed, as known in digital holography, for reconstructing an image of an object starting from the acquired hologram, a mathematical algorithm derived from the diffraction theory is executed, in particular a mathematical algorithm implementing the known Rayleigh-Sommerfeld formula. Such formula basically contains a double integration of the digitalized hologram multiplied by a numerical copy of the reference beam R and other terms. Such double integration, in principle, involves considerable calculating effort. However, its numerical implementation may be simplified by converting the integrals in Fourier transforms. Indeed, since the hologram is discretized, the Fourier transforms actually are discrete Fourier transforms, which may be easily calculated by means of known FFT (Fast Fourier Transform) algorithms. To operate the transformation, in particular, different methods are known, including: the angular spectrum method, the convolution method and the Fresnel method. The Fresnel method is advantageous over the convolution method, in that it involves one single Discrete Fourier Transform (DFT) which can be easily implemented by means of the FFT algorithm. The spatial resolution of the reconstructed image is quantified by the so-called "reconstruction pixel", whose sizes along the directions x and y are given by the following equations: where N and M are the number of pixels of the acquired discretized hologram along the directions x and y, A is the emission wavelength of the laser source 2, d is the reconstruction distance (namely, the distance between irradiated surface of the building 9 and IR detector 7 and Ax and Ay are the pixel sizes of the IR detector 7 along the directions x and y. From the above equation [2], it is apparent that A^ and Aq are proportional to the wavelength A and the reconstruction distance d, whereas they decrease with the number of pixels N x M and the pixel physical size. Consequently, spatial resolution of the reconstructed image may be worse than the physical one imposed by the sampling theorem, depending on the values of the parameters of equations [2],

[0096] The zero padding operations advantageously allows enhancing the spatial resolution of the reconstructed image. More specifically, by adding fictitious pixels with null intensity to the N x M array of the acquired hologram, A^ and Aq are reduced and the spatial resolution is increased. Preferably, the fictitious pixels are added as contour of the acquired hologram, that is without interleaving them among the effective pixels. This assures that no spurious frequencies arise in the reconstructed image as a consequence of the discontinuities that such interleaving would introduce. The number of fictitious pixels depends on the desired resolution in the reconstruction of the image. The maximum resolution which may be obtained is equal to the physical boundary established by the sampling theorem. Although the advantages of the zero padding operation have been discussed above with reference to the Fresnel method only (where zero padding basically allows compensating the resolution reduction entailed by use of a longer wavelength, see above equations [2]), the zero padding operation may be used in combination with other methods, such as angular spectrum method or convolution method.

[0097] Then, at a third sub-step 212, each acquired discretized hologram (filtered at sub-step 210 and possibly "enlarged" at sub-step 211 ) is processed for reconstructing a phase image of the irradiated surface of the building 9.

[0098] Such sub-step 212 in particular comprises applying to the acquired discretized hologram (filtered and possibly "enlarged") a mathematical algorithm implementing the above mentioned known Rayleigh-Sommerfeld formula which basically emulates the diffraction effects of the propagation of a numerical copy of the reference beam R across the hologram and gives, as a result, the object wavefront reconstruction, focussed at the distance d. Preferably, the algorithm is based on the above mentioned Fresnel method, which is particularly easy and fast in comparison to other known methods. However, according to other variants, other numerical focusing methods can be used, for instance the angular spectrum method or the convolution method.

[0099] The execution of the numerical focussing of sub-step 212 provides a complex reconstructed wavefield, namely a matrix wherein each element or pixel of the matrix is a complex number. Sub-step 212 preferably further comprises calculating the phase of the complex reconstructed wavefield by calculating the phase of each pixel of the matrix separately, namely by calculating the phase of each complex number of the matrix. The resulting matrix is the phase of the complex reconstructed wavefield, namely the reconstructed phase image of the irradiated surface of the building 9 at a given moment. Sub-step 212 also preferably comprises filtering the complex conjugate of the complex reconstructed wavefield, which is provided by the numerical focussing of the hologram.

[0100] Further, sub-step 212 may comprise other numerical processing operations on each hologram and / or each complex reconstructed wavefield, e.g. for improving the signal-to-noise ratio of each reconstructed phase image.

[0101] As mentioned above, the numerical processing 21 is applied to each hologram of the time sequence of holograms acquired at step 20. The execution of the numerical processing described above on each hologram of the time sequence therefore produces a time sequence of phase images of the irradiated surface of the building 9.

[0102] Then, during a subsequent step 22, the data processing unit 8 preferably uses the time sequence of phase images of the irradiated surface of the building 9 for reconstructing the evolution in time of any displacement or deformation of the portion of the building 9 corresponding to the irradiated surface. For this purpose, during step 22, the data processing unit 8 preferably uses all the phase images of the sequence.

[0103] In particular, during step 22, the data processing unit 8 preferably calculates a displacement of each point of the irradiated surface, represented by a given pixel of the phase image, along the direction N perpendicular to the irradiated surface of the building 9. The displacement of each point is preferably calculated as a function of the difference between the phase of that pixel in a certain phase image and the phase of the same pixel in the preceding phase image in the sequence. By carrying out phase differences of a certain pixel between successive phase images, it is possible to reconstruct the displacement in time, along the direction N, of the point of the irradiated surface represented by that pixel, according to the equation: ASN = (1 / k) (A Acp) / (47cn) = Sm / (2k), [3] where ASN is the displacement of the point corresponding to the pixel along the direction N between two successive phase images, Sm is the variation in the source / building / IR detector optical path, Acp is the phase difference of the pixel between two successive phase images, A is the wavelength, n is the refractive index of air, k is a projection factor which depends on the inclination of the irradiation direction (namely the direction of the object beam 0 as impinging on the irradiated surface) relative to the direction N and is equal to cos\| / , where is the angle between the irradiation direction and the direction N. For displacements of 1 pm and for angles less than 8°, the correction by the factor k is less than the sensitivity of the interferometric technique (estimated at 0.01 pm). Equation [3] is valid only in the approximation where the irradiation direction substantially coincides with the direction of the line joining the IR detector 7 to the point of the irradiated surface corresponding to the pixel being examined.

[0104] By repeating the operation on all the pixels of the phase image, it is therefore possible to reconstruct the displacement along the direction N of each point of the irradiated surface of the building 9 represented by the phase image, and hence to detect any deformation or displacement of the irradiated portion of the building 9 as a whole.

[0105] Optionally, for at least one of the holograms of the acquired time sequence of holograms, the data processing unit 8 also calculates the amplitude of the complex reconstructed wavefield, by calculating separately the amplitude of the complex value of each pixel of the matrix which represents the complex reconstructed wavefield. This allows the reconstruction of an amplitude image (namely a proper image) of the irradiated surface of the building 9. This amplitude image may be advantageously used during step 20 for selecting the surface of the building 9 to be monitored. Once the pixels which represent the surface to be monitored on the amplitude image have been selected, the data processing unit 8 preferably selects the corresponding pixels in the phase images and reconstructs the displacement along the direction N of those pixels only, on the basis of their values in the various phase images of the acquired time sequence.

[0106] Optionally, step 22 may also comprise a Fourier analysis of the progression over time of the variation of the optical path Sm for one or more points of the irradiated surface of the building 9 represented by the corresponding pixels of the phase image. Optionally, step 22 may also provide for a frequency filtering (passband filter) operation, in order to exclude any frequency components which can be attributed to vibrations of the system 1 itself. Optionally, step 22 may also provide for a frequency filtering (passband filter) operation, in order to select a single frequency of interest and calculate the displacement at the selected frequency of interest.

[0107] The results obtained at step 22 are then stored and may be displayed on the screen 8a of the processing unit 8, for example in graphical form.

[0108] The system 1 therefore uses digital holography in order to detect deformations and displacements of the building 9 or portions thereof. The use of digital holography offers several advantages.

[0109] Firstly, digital holography is an optical approach which allows remote monitoring of the building 9. The system 1 is indeed positioned at a certain distance from the building 9 which, as mentioned above, may be adapted depending on the accessibility and the conditions of the area surrounding the building 9. It is not required to use sensors or devices which need to be positioned inside the building 9. The system 1 therefore allows easy and safe monitoring of building structures which have limited access for safety reasons or because they are buildings of artistic or historical interest.

[0110] Moreover, owing to the use of digital holography, the system 1 may monitor the building 9 in a substantially continuous manner from both the spatial and time point of view.

[0111] From a spatial point of view, with digital holography it is indeed possible to detect displacements of any point of the surface of the building 9, provided that it is irradiated by the object beam 0. The spatial resolution of detection (namely the number and the density of the points of the irradiated surface of the building 9 which can be monitored separately) is determined substantially by the resolution of the phase images reconstructed by the system 1 (namely by the size of the reconstructed pixels, as discussed above).

[0112] From a time point of view, the system 1 allows detection of the displacements of each pixel-point of the irradiated surface of the building 9 in a substantially continuous manner. The temporal resolution of detection (namely the time interval occurring between two consecutive detections of the phase of a same pixel) is substantially determined by the acquisition frequency of the IT detector 7.

[0113] Moreover, the system 1 is capable of monitoring the building 9 in real time. The numerical processing carried out by the data processing unit 8 is indeed relatively simple and may be performed for each hologram of the acquired time sequence substantially during the time interval occurring between the acquisition of two successive holograms. The displacement of each point may therefore be reconstructed substantially in real time, namely while the deformation or displacement to be detected is occurring.

[0114] The use of digital holography, and in particular digital holography in the infrared range, also offers other advantages.

[0115] Laser sources suitable for emitting IR wavelengths lower than 3 pm (e.g. fiber coupled lasers) are broadly available with good spatial and temporal coherence properties, so it is possible to expand the object beam 0 to irradiate large scenes and to obtain high visibility interference fringes even with a very high imbalance between the object beam path and the reference beam path.

[0116] As to the minimum amplitude of deformations and displacements which can be detected, the inventors have estimated that, by optimizing the visibility of the fringes and the noise signal ratio, the minimum displacement of each pixel-point which can be detected by the system 1 is substantially equal to one hundredth of the wavelength. Using wavelengths in the short infrared range (for example 1.55 pm), the minimum displacement which can be detected is therefore of the order of 0.015 pm.

[0117] As regards instead the maximum frequency of periodic deformations or displacements which can be detected, it is mainly limited by the sampling theorem and depends essentially on the acquisition frequency of the IR detector 7. For example, in case the IR detector 7 has an acquisition frequency of 25 photograms / s, the maximum frequency of periodic deformations or oscillations which can be detected is about 10 Hz. There is instead no bottom limit to the frequency, such that the system 1 is able to detect periodic deformations or displacements having a frequency which is arbitrarily low and also deformations and displacements which are not periodic.

[0118] Figure 3 shows a system T according to another embodiment of the present invention.

[0119] The system T comprises some additional components with respect to the system 1 shown in Figure 1. The additional components shown in Figure 3 are optional and may be included in the system singularly or jointly in any combination thereof.

[0120] Specifically, in addition to the components depicted in Figure 1 , the second device 1 b of the system T optionally comprises an optical system 11 suitable for maximizing the optical power of the object beam 0 collected and directed to the IR detector 7. The optical system 11 for example may comprise a high numerical aperture telescope. For example, the inventors have made a test where the reconstruction distance d was equal to 3 m and the optical system 11 comprised a lens with a diameter of 50 mm and a focal length of 70 mm. The lens formed a scaled down image of the irradiated surface of the building 9 at a distance of about 150 mm from the surface of the IR detector 7. This way, the reconstructed image of the irradiated surface of the building 9 extended substantially through the entire field of the view of the system 1 . The end of the output optical fiber 3out2 emitting the reference beam R was also placed at a distance of about 150 mm from the surface of the IR detector 7, thereby creating an appropriate angle between object beam O and reference beam R, without the output optical fiber 3out2 intercepting the object beam O.

[0121] In addition, or alternatively, the second device 1 b of the system T optionally comprises a beam splitter 12 arranged to combine the reference beam R emitted by the output optical fiber 3out2 of the fiber optic splitter 3 and the object beam O scattered by the irradiated surface of the building 9. The beam splitter 12 is preferably located on the optical path of the reference beam R, between the end of the output optical fiber 3out2 emitting the reference beam R and the IR detector 7, so as to deflect the optical path of the reference beam R by a desired angle. By way of non-limiting example, in Figure 3 the angle is substantially of 90°. The splitting ratio of the beam splitter 12 is preferably chosen, together with the splitting ratio of the fiber optic splitter 3, in order to obtain the desired intensities of the object beam O and reference beam R on the surface of the IR detector 7. Use of the beam splitter 12 allows providing a more compact interferometric arrangement, and hence a more compact second device 1 b.

[0122] In addition or alternatively, the second device 1 b of the system T optionally comprises an adjustable collimator 13 located at the end of the output optical fiber 3out1 emitting the object beam 0. The adjustable collimator 13 may comprise one or more lenses and it may be used for increasing or decreasing the size of the object beam 0 as output by the output optical fiber 3out1 of the fiber optic splitter 3.

[0123] In addition or alternatively, the second device 1 b of the system T optionally comprises an adjustable collimator 14 located at the end of the output optical fiber 3out2 emitting the reference beam R. The collimator 14 may comprise one or more lenses and it may be used for increasing or decreasing the size of the reference beam R as output by the output optical fiber 3out2 of the fiber optic splitter 3.

[0124] In addition or alternatively, the second device 1 b of the system T optionally comprises a variable attenuator 15 located on the optical path of the reference beam R, between the end of the output optical fiber 3out2 emitting the reference beam R and the IR detector 7. The variable attenuator 15 is adjustable such that the reference beam R and the object beam 0 scattered by the irradiated surface of the building 9 are received at the IR detector 7 with comparable intensities, when the two beams 0, R have sufficiently high intensities, or such that the reference beam R is received with a higher intensity with respect to the object beam 0, when the latter has a low intensity value. This allows optimizing the interference pattern of reference beam R and object beam 0, namely the visibility of the interference fringes of the hologram. The variable attenuator 15 for example may comprise a polarizer 15a and a half-wave plate 15b. Alternatively, the variable attenuator 15 may comprise a neutral density filter.

[0125] The operation of the system T is preferably according to the flow chart of Figure 2 described above.

Claims

CLAIMS1. A system (1 , T) for detecting a displacement or a deformation of at least a portion of a building structure (9), said system (1 ) comprising:- an optical fiber (2out) having a first end and a second end;- a first device (1a) connected to the first end of the optical fiber (2out) and comprising a laser source (2) configured to emit an infrared radiation into the first end of the optical fiber (2out);- a second device (1 b) connected to the second end of the optical fiber (2out) and comprising a fiber optic splitter (3) and an infrared detector (7), the fiber optic splitter (3) being configured to receive the infrared radiation from the second end of the optical fiber (2out) and to divide the infrared radiation into an object beam (0) and a reference beam (R), to direct the object beam (0) onto at least a portion of the building structure (9) and to direct the reference beam (R) onto the infrared detector (7), the infrared detector (7) being configured to detect a time sequence of holograms formed by the reference beam (R) interfering with the object beam (0) scattered by the at least a portion of the building structure (9); and- a data processing unit (8) configured to reconstruct an evolution in time of said deformation or said displacement of said at least a portion of said building structure (9) by numerically processing said time sequence of holograms.

2. The system (1 , T) according to claim 1 , wherein the first device (1a) and the second device (1 b) are distinct and physically decoupled devices.

3. The system (1 , T) according to claim 1 or 2, wherein the laser source (2) comprises a fiber coupled laser.

4. The system (1 , T) according to any of the preceding claims, wherein the infrared radiation has a wavelength lower than 3 pm.

5. The system (1 , T) according to any of the preceding claims, wherein the infrared radiation has a maximum output power of 10 W and / or wherein the frequency of the laser source (2) is stable within 60 kHz.

6. The system (1 , T) according to any of the preceding claims, wherein the optical fiber (2out) is suitable for being removably coupled with the fiber optic splitter (3) comprised in the second device (1b).

7. The system (1 , T) according to any of the preceding claims, further comprising a cooling system (2a) configured to cool the laser source (2) during operation, said cooling system (2a) being housed in the first device (1a).

8. The system (1 , T) according to any of the preceding claims, wherein the data processing unit (8) is positioned in the first device (1a) or in the second device (1 b).

9. The system (1 , T) according to any of the preceding claims, further comprising a display (8a) suitable for displaying data indicative of said deformation or said displacement of said at least a portion of said building structure (9) as provided by the data processing unit (8), the display (8a) being positioned on the second device (1b) or in a third device other than the first device (1a) and second device (1 b).

10. The system (1 , T) according to any of the preceding claims, wherein the second device (1 b) is provided with a gimbal system (5).

11. The system (1 , T) according to any of the preceding claims, wherein the second device (1 b) further comprises at least one of:an optical system (11) suitable for maximizing the optical power of the object beam (0) scattered by the at least a portion of the building structure (9) and directed to the infrared detector (7); a beam splitter (12) located on the optical path of the reference beam (R) and configured to combine the reference beam (R) and the object beam (0) scattered by the at least a portion of the building structure (9); an adjustable collimator (13) located on the optical path of the object beam (0) and suitable for adjusting the size of the object beam (0) directed onto at least a portion of the object (9); a further adjustable collimator (14) located on the optical path of the reference beam (R) and suitable for adjusting the size of the reference beam (R); and a variable attenuator (15) located on the optical path of the reference beam (R) and suitable for adjusting the intensity of the reference beam (R) such that the reference beam (R) and the object beam (0) scattered by the at least a portion of the building structure (9) are received at the infrared detector (7) with comparable intensities.

12. The system (1 , T) according to any of the preceding claims, wherein said data processing unit (8) is configured to: provide a sequence of phase images of said at least a portion of the building structure (9) by processing said time sequence of holograms; and calculate a displacement over time of at least one point of said at least a portion of the building structure (9) represented by a certain pixel in said sequence of phase images of said at least a portion of the building structure (9), as a function of phase differences of said pixel betweenconsecutive phase images of said sequence of phase images of said at least a portion of the building structure (9).

13. The system (1 ) according to claim 12, wherein said data processing unit (8) is configured to calculate said displacement over time of said at least one point of said at least a portion of the building structure (9) along a direction (N) perpendicular to a surface of said at least a portion of the building structure (9) and to divide said displacement along said direction (N) by a factor (k) which takes into account the inclination of a direction of incidence of said object beam (0) on said at least a portion of the building structure (9) relative to said direction (N).

14. The system (1 ) according to any of the preceding claims, wherein said data processing unit (8) is further configured to process said time sequence of holograms of said at least a portion of the building structure (9) so as to provide a sequence of amplitude images of said at least a portion of the building structure (9).

15. A method for detecting a displacement or a deformation of at least a portion of a building structure (9), said method comprising:- by a laser source (2) comprised in a first device (1a) connected to a first end of the optical fiber having said first end and a second end, emitting an infrared radiation into the first end of the optical fiber (2out);- by a fiber optic splitter (3) comprised in a second device (1 b) connected to the second end of the optical fiber (2out), receiving the infrared radiation from the second end of the optical fiber (2out), dividing the infrared radiation into an object beam (0) and a reference beam (R), directing the object beam (0) onto at least a portion of the building structure (9) and directing the reference beam (R) onto an infrared detector (7)also comprised in the second device (1 b);- by the infrared detector (7), detecting a time sequence of holograms formed by the reference beam (R) interfering with the object beam (0) scattered by the at least a portion of the building structure (9); and- by a data processing unit (8), reconstructing an evolution in time of said deformation or said displacement of said at least a portion of said building structure (9) by numerically processing said time sequence of holograms.

Images