ITUB20152754A1 - Method and system for monitoring building construction - Google Patents

Description

DESCRIPTION

The present invention generally relates to the monitoring of building constructions, for example in the field of seismic or hydrogeological risk assessment, or in the field of the protection of fine arts. In particular, the present invention relates to a method and a system for monitoring deformations and / or displacements to which a building construction can be subjected.

In the following of the present description and of the claims, the expression "building construction" will indicate an artifact or structure having dimensions of the order of magnitude of 1 m or more (up to tens of meters), intended for any area of civil use ( construction, geotechnical, infrastructural, hydraulic, electrical, structural, urban planning), such as for example a civil or industrial building, a bridge or viaduct, a tunnel, a barrier, a dam, an embankment, an aqueduct, a sewer , a canal, a pylon, a wind turbine or an electric pylon.

The monitoring of deformations or displacements to which a building construction may be subjected is useful in various areas, such as the assessment of seismic or hydrogeological risk, civil protection or the protection of fine arts. Monitoring the movements (for example the oscillations, in terms of amplitude and frequency) of a building, for example, allows to evaluate the characteristics and any structural defects of the building, or to evaluate the response of the building to environmental factors (earthquakes , winds, rains and other natural phenomena) or anthropogenic factors (car or rail traffic, underground works, etc.).

Currently, the monitoring of the deformations or displacements to which a building construction is subject is typically carried out by means of suitable sensors (seismometers), which are placed in different points of the building and / or its surface.

Although seismometers by themselves provide accurate results, they have some drawbacks.

First of all, access to the building for the positioning of seismometers can be difficult, especially if the building is damaged or is subject to particular regulations that limit access (for example as a building of artistic or historical interest). In addition, each seismometer provides data relating to the movement (in particular, the acceleration along the three axes) of the single point of the building in which it is located. Therefore, if it is desired to monitor the construction as a whole, it is necessary to provide a plurality of seismometers positioned in appropriate points of the construction. Furthermore, current seismometers are able to detect periodic deformations or displacements (i.e. oscillations) with a frequency greater than a certain minimum frequency, and do not allow instead to detect other types of deformations or displacements to which the construction may be subject, for example displacements. non-periodic due to structural failures that occur on even relatively long time scales (hours or days).

The object of the present invention is therefore to provide a method and a system for monitoring a building construction which solve at least one of the aforementioned problems.

In particular, the object of the present invention is to provide a method and a system for monitoring a building construction that allow the construction to be monitored remotely (i.e. without requiring the installation or placement of any sensor inside the building or on its surface), and / or that allow you to monitor any point of the construction (or the construction as a whole) and / or that are able to detect deformations or periodic displacements (oscillations) of arbitrarily low frequency, and also deformations or displacements of the construction non-periodic, occurring over relatively long time scales (hours or days).

According to embodiments of the present invention, these and other objects are achieved by a method and a system which carry out the monitoring of the building construction by means of a digital holography technique. In particular, through digital holography a sequence of construction phase images is reconstructed in real time, which are subjected to an appropriate numerical processing to reconstruct the temporal trend of deformations or displacements to which the construction is subjected.

The accuracy of the reconstruction is equal to approximately one hundredth of the wavelength used. According to preferred embodiments of the invention, the wavelength used is in the mid-infrared spectrum, for example 10.6 µm. This wavelength spectrum is particularly advantageous compared to others (for example, the visible spectrum), as on the one hand it allows to obtain a field of view large enough to contain a significant portion of the construction (if not the entire construction) , and on the other hand to reduce the sensitivity to environmental vibrations to a level that makes measurement possible.

The method and the monitoring system according to the present invention therefore allow the building construction to be monitored, potentially in its entirety, remotely, without the need to place any sensor inside or on the external surface of the building. The times and costs of monitoring are therefore significantly reduced compared to known techniques that use seismometers.

Furthermore, the method and the monitoring system according to the present invention are able to detect not only periodic deformations and / or shifts (oscillations) of arbitrarily low frequency, but also non-periodic deformations and / or shifts of the construction, which occur on even relatively long time scales (hours or days).

According to a first aspect, a system is provided for monitoring a building construction, the system comprising:

- a laser source capable of emitting infrared radiation;

- an interface metric structure configured to divide the infrared radiation into an object beam configured to illuminate at least a portion of the building construction and to be diffused by the at least one building portion, and a reference beam configured to interfere with the object beam diffused from the at least one building portion so as to create at least one hologram of the at least one building portion;

- a sensor adapted to detect a temporal sequence of holograms of the at least one construction portion; And

- a processing unit configured to reconstruct a temporal course of a deformation and / or a displacement to which the at least one construction portion is subjected by numerically processing the sequence of holograms of the at least one construction portion.

Preferably, the infrared radiation has a wavelength between 3 μm and 30 μm.

Preferably, the interferometric structure comprises a first lens adapted to widen the object beam before it illuminates the at least one portion of the building construction and a second lens adapted to widen the reference beam before it interferes with the object beam diffused by the at least one portion. of building construction.

Preferably, the interferometric structure also comprises a mirror adapted to deflect the object beam before it illuminates the at least one portion of the building construction, the mirror being movable to adjust a direction of incidence of the object beam on the construction portion.

Preferably, the processing unit is configured for:

- processing the sequence of holograms of the at least one construction portion so as to obtain a sequence of phase images of the at least one construction portion; And

- calculating a displacement in time of at least one point of the at least one construction portion represented by a certain pixel in the phase image sequence of the at least one construction portion, as a function of pixel phase differences between successive phase images of the phase image sequence of the at least one construction portion.

Preferably, the processing unit is configured to calculate the displacement in time of the at least one point of the at least one construction portion along a direction N, normal to the surface surrounding the at least one point of the at least one construction portion, keeping account of the inclination of the direction of incidence of the object beam on the at least one portion of construction with respect to the direction N.

Preferably, the processing unit is also configured to process the sequence of holograms of the at least one construction portion so as to obtain a sequence of images of width of the at least one construction portion.

According to a second aspect, a method for monitoring a building construction is provided, the method comprising:

- provide infrared radiation;

- dividing the infrared radiation into an object beam and a reference beam, illuminating at least a building construction portion with the object beam so that the object beam is diffused by the at least one building construction portion, and making the beam interferes with the object beam diffused by the at least one building portion so as to create at least one hologram of the at least one building portion;

- detecting a temporal sequence of holograms of the at least one construction portion; And

- reconstructing a temporal course of a deformation and / or a displacement to which the at least one construction portion is subjected by numerically processing the sequence of holograms of the at least one construction portion.

The present invention will be illustrated in greater detail thanks to the accompanying drawings, provided by way of non-limiting example, in which:

Figure 1 schematically shows a building construction monitoring system according to an embodiment of the present invention;

Figure 1a schematically shows the building construction illuminated by a beam coming from the system of Figure 1;

Figure 2 is a flow diagram of the operation of the system of Figure 1;

- Figures 3a and 3b schematically show a portion of the system of Figure 1; And

- Figure 4 shows in graphic form the results of a comparative test performed by the inventors.

Figures 1, 1a, 3a and 3b are schematic representations not to scale, which are not indicative of the preferred values of distances and angles shown therein. Quantitative evaluations on distances and angles of the system schematically shown in Figures 1, 1 a, 3a and 3b are reported in the following of the present description.

Figure 1 shows a system 1 for monitoring a building construction 9 (a building, by way of non-limiting example) according to an embodiment of the present invention. Figure 1a schematically shows an exemplary building of the building construction 9 illuminated by a beam coming from system 1.

The system 1 preferably comprises a laser source 2, a beam splitter 3, a first lens 4, a second lens 5, a variable attenuator 6 and an infrared sensor 7.

The laser source 2 is preferably adapted to emit a radiation in the infrared spectrum. In particular, the emission wavelength of the laser source 2 is preferably in the so-called "mid-infrared region" or "long-wavelength infrared region", which conventionally ranges from 3 pm to 30 pm. More preferably, the emission wavelength of the laser source 2 is comprised between 8 pm and 12 pm. The choice of the preferred range for the emission wavelength of the laser source 2 is preferably dictated by the following criteria: wavelengths of the typically available infrared sources, spectral response of the typically available infrared sensors, absorption spectrum atmospheric in the infrared region, sensitivity of system 1 to environmental vibrations and of the system itself (which decreases as the wavelength increases and assumes an adequate value for the application of system 1 for wavelengths in the medium infrared) and desired size of the field of view (which increases with increasing wavelength, as will be discussed in more detail below).

The laser source 2 is preferably a continuous source. Alternatively, a pulsed laser source can be used.

Preferably, the minimum emission power of the laser source 2 is 30 W, more preferably 40 W, even more preferably 60 W. The preferred range for the emission power of the laser source 2 depends mainly on the distance d between the building portion 9 that you want to monitor and the surface of the sensor 7, from the maximum size of the portion of the building 9 that you want to monitor and from the detection sensitivity of the sensor 7. Assuming you want to irradiate a portion of the building 9 (neglecting the absorption of 'air), the inventors have estimated that, if the distance d increases by a factor n, the emission power of the laser source must be increased by a factor n <2> in order to maintain the same signal level on the sensor. The inventors have verified that, to monitor a portion of a building of about 4m <2> placed at a distance d of about 18m from the surface of the sensor 7, an emission power of about 60W is sufficient. On the other hand, the emission power of the laser source 2 is preferably such that the energy density on the irradiated portion of the building surface 9 does not exceed a threshold beyond which it can damage the surface of the building 9.

Furthermore, the radiation emitted by the laser source 2 is preferably linearly polarized. The inventors performed some positive tests using an RF CO2 laser source from Universal Laser Systems (Scottsdale, Arizona, United States) capable of emitting a linearly polarized radiation with an emission wavelength of 10.6 μηη and a maximum emission power of 60 W.

The beam splitter 3 is adapted to divide the light radiation emitted by the laser source 2 into a first light beam (hereinafter called "object beam O") and a second light beam (hereinafter called "reference beam R"). The beam splitter 3 is preferably configured so that the object beam O diffused by the building surface 9 and the reference beam R are received by the sensor 7 with comparable intensity (the variable attenuator 6 is also provided for this purpose , as will be discussed in more detail below). More specifically, the beam splitter 3 is configured so that the power of the object beam O is at least 80% of the total power incident on the beam splitter 3, for example 90% of the total power incident on the beam splitter 3 In the system 1 shown in Figure 1, the reflected portion of the radiation incident on the beam splitter 3 constitutes the object beam O, while the transmitted portion constitutes the reference beam R. This however is not limiting and, according to other embodiments, it is not shown in the drawings, the object beam O and the reference beam R can correspond respectively to the transmitted portion and to the reflected portion of the radiation incident on the beam splitter 3 (provided that the aforementioned proportions are maintained between the power of the object beam O and the power of the reference R).

The first lens 4 is preferably arranged on the optical path of the object beam O. The first lens 4 is preferably adapted to increase the dimension of the object beam O so as to irradiate a sufficiently large portion of the building 9. The first lens 4 can have different shapes (plano-convex, biconvex, meniscus, etc.). Preferably, the first lens 4 is a diverging lens. Alternatively, the first lens 4 can be a converging lens, which first focuses and then widens the object beam O. Alternatively, instead of using a single lens 4, to increase the size of the object beam O, a lens system can be used, comprising two or more lenses, so as to be able to vary at will the size of the object beam at a certain distance and effectively irradiate more or less large portions of the building 9. The inventors have carried out positive tests using a system with two converging biconvex lenses, which advantageously it allowed to adjust the size of the object beam O within a significantly wide range.

The second lens 5 is preferably arranged along the optical path of the reference beam R. The second lens 5 is preferably adapted to increase the size of the reference beam R. The second lens 5 can have different shapes (plano-convex, biconvex, a meniscus, etc.) and may include a converging lens or a diverging lens. Preferably, the second lens 5 is a diverging lens. Alternatively, the second lens 5 is a converging lens, which first focuses and then widens the reference beam R.

The variable attenuator 6 is preferably arranged on the optical path of the reference beam R, more preferably between the beam splitter 3 and the second lens 5. The variable attenuator 6 is adapted to adjust the power of the reference beam R so as to optimize the visibility of the interference pattern between the reference beam R and the object beam O, as will be described in greater detail below. According to alternative variants, the variable attenuator 6 is replaced by a polarizer, which makes it possible to obtain the same attenuation result as the reference beam R.

The sensor 7 is preferably arranged so as to detect the interference pattern between the object beam O and the reference beam R. The sensor 7 is preferably a thermal imaging camera comprising a two-dimensional matrix of NxM elements or pixels. The inventors performed positive tests using the Thermoteknix Miricle (UK) 307 K micro-bolometric camera (640x480 pixel matrix), with an acquisition frequency of 25 frames / sec, a pixel size of 25 μπη < χ> 25 μπη and a spectral response between 8 prn and 12 pm.

The system 1 can further comprise one or more mirrors able to define the optical path of the object beam O and / or of the reference beam R.

In the embodiment shown in Figure 1, by way of non-limiting example, the system 1 comprises a first mirror 81 disposed between the laser source 2 and the beam splitter 3, a second mirror 82 disposed between the beam splitter 3 and the first lens 4, and a third mirror 83 arranged between the first lens 4 and the building 9. The third mirror 83 can preferably be moved by means of actuators (not shown in the figure for simplicity), so as to be able to remotely modify the direction of the beam object O emitted by the system 1. In particular, the actuators of the movable mirror 83 can be used to direct the object beam O onto the portion of the building 9 to be investigated. The first lens 4, being positioned upstream of the movable mirror 83, advantageously does not hinder the adjustment of the direction of the object beam O.

Furthermore, in the embodiment shown in Figure 1, by way of non-limiting example the system 1 also comprises a fourth mirror 84 arranged between the beam splitter 3 and the variable attenuator 6, a second mirror 85 arranged between the variable attenuator 6 and the second lens 5, and a sixth mirror 86 arranged between the second lens 5 and the sensor 7, which deflect the reference beam R onto the sensor 7 so as to direct the reference beam R with the correct angle of incidence Θ.

The system 1 can comprise other optional elements arranged along the optical path of the object beam O and / or of the reference beam R.

For example, according to a variant not shown in the drawings, the system 1 also comprises one or more cylindrical lenses arranged along the optical path of the object beam O. The cylindrical lens gives the object beam O an elongated shape (i.e. an ellipsoidal shape), which it is particularly suitable if the building 9 also has an elongated shape (in height or in width).

According to other variants not shown in the drawings, the system 1 can comprise other optical elements, such as polarizers, Brewster windows, etc. In any case, the beam splitter 3, the lenses 4 and 5 and the variable attenuator 6 form an "off-axis" and lensless internal metric structure, i.e. a system metric integer in which two interfering beams (ie the object beam O and the reference beam R) are reciprocally inclined by a non-zero angle when they affect the surface of the sensor 7 ("off-axis") and in which no system is present optical for image formation in front of sensor 7 ("lensless").

The system 1 further comprises a processing unit 8 cooperating with the sensor 7. The processing unit 8 is preferably configured to receive from the sensor 7 the detected interference patterns or holograms in discretized form, to store and process them, as will be discussed in detail below. The processing unit 8 is also preferably provided with a screen 8a for displaying the processing results.

The system 1 can be implemented as a portable apparatus, which can be transported and placed near the building to be monitored, as shown schematically in Figure 1a. In particular, the system 1 (with the exception of the processing unit 8) is preferably mounted on a platform which can be oriented in the horizontal direction and in the vertical direction. For example, the various components of the system 1 (with the exception of the processing unit 8) can be mounted on a plate of portable dimensions. The inventors carried out positive tests using a plate measuring 90 cm x 60 cm.

The operation of the system 1 will now be described in detail with reference to the flow chart of Figure 2.

During a first acquisition phase 30, the system 1 is brought near the building 9 to be monitored. The distance d between system 1 and the portion of building 9 that you want to monitor depends on the environmental conditions (accessibility of the land surrounding the building), on any safety measures (if building 9 is unsafe, it will not be allowed to approach beyond a certain limit) and the size of the portion of the building to be monitored (the linear field of view of system 1, as will be discussed in greater detail below, increases as the distance d increases). System 1 is then oriented, through the adjustable platform, so that the portion of the building 9 that you want to monitor falls within the field of view of system 1. Therefore, the direction of the object beam O emitted by the system 1 is preferably adjusted (through the actuators of the mirror 83) in order to select a specific portion of the building 9 within the visual field. The direction of the object beam O (hereinafter also called "direction of irradiation") therefore forms, with respect to the direction N normal to the surface of the building 9 around the measuring point, an angle ψ (see Figure 1 a).

Then, the laser source 2 is turned on and begins to emit infrared radiation. The infrared radiation is divided into the object beam O and the reference beam R by the beam splitter 3.

The object beam O is widened by the first lens 4 and irradiates the surface (or a portion of the surface) of the building 9. The extent of the irradiated surface depends on the distance d between the system 1 and the portion of the building 9 that is desired monitor and by the lens 4 or lens configuration used to expand the object beam O. The object beam O is scattered from the irradiated surface of the building 9, and thus reaches the sensor 7. On the other hand, the reference beam R - after possibly having been attenuated by the variable attenuator 6 - it is widened by the second lens 5 and directed on the sensor 7. The reference beam R reaches the sensor 7 with a low intensity and an approximately spherical wavefront whose radius of curvature depends on the focal length of the lens 5 and the distance of the lens 5 from the sensor. The focal length and the position of the second lens 5 are preferably selected so that the reference beam R illuminates, in a substantially uniform manner, the entire surface of the sensor 7.

Therefore, the object beam O diffused by the building 9 and the reference beam R interfere with each other on the surface of the sensor 7, thus creating an interference pattern or hologram, which is detected by the sensor 7.

Preferably, during step 30 the sensor 7 acquires a temporal sequence of holograms. The sequence of holograms is preferably acquired at the acquisition frequency of the sensor 7. If, for example, the acquisition frequency of the sensor 7 is 25 frames / sec, 25 holograms per second are acquired. The acquisition of a sequence of holograms allows to reconstruct a temporal trend of the deformations or displacements to which the illuminated portion of the building 9 is subject, as will be discussed in greater detail below.

Each acquired hologram has interference fringes having a certain fringe spacing. Each hologram can be described in terms of two-dimensional intensity distribution according to the following equation:

H (x, y) = | R | <2> + | 0 | <2> + R <*> - 0 + R-0 * [1]

where x and y are the two orthogonal coordinates of the sensor surface 7, while R * and O <*> are respectively the conjugate complexes of the reference beam R and the object beam O.

Preferably, during phase 30 the variable attenuator 6 is adjusted so that, on the surface of the sensor 7, the power of the reference beam R is substantially equal to the power of the object beam O diffused by the building 9. This allows to maximize the visibility of the interference fringes of the hologram.

Since, thanks to the use of infrared radiation, the system 1 has a reduced sensitivity to vibrations, during the course of step 30, advantageously, anti-vibration measures are not necessary. Furthermore, since the sensor 7 is sensitive only to infrared radiation, the visible components of artificial light and sunlight do not disturb the operation of the system 1 during the acquisition phase 30. The infrared components of artificial light and sunlight do not disturb either. the operation of the system 1, because they are inconsistent with respect to the object beam O and the reference beam R, therefore they represent only a background noise.

The sequence of holograms acquired by the sensor 7 during step 30 is then stored by the processing unit 8.

The processing unit 8 then preferably carries out a numerical processing step 31 on the acquired hologram sequence. The numerical processing step 31 can be carried out at the end of the step 30, ie at the end of the acquisition of the entire sequence of holograms. Alternatively, since each hologram of the sequence is individually subjected to numerical processing, step 31 can start immediately after the acquisition of the first hologram of the sequence, and then continue in parallel with the acquisition of the subsequent holograms.

The numerical processing that the processing unit 8 performs on each acquired hologram preferably comprises a first sub-step 310, during which the hologram is filtered so as to cancel the zero diffraction order, i.e. the term | R | <2> + | 0 | <2> of equation [1]. Since the system 1 has an off-axis configuration (ie the object beam O diffused by each point of the irradiated portion of the building 9 and the reference beam R affect the sensor 7 at different angles), the term | R | <2> + | 0 | <2> is advantageously non-spatially superimposed on the other terms R <*> - 0 + R-0 \ and consequently can be filtered in the spatial frequency domain.

Then, during a second sub-step 311, the hologram is preferably subjected to a "zero padding" operation. This operation foresees to extend the matrix of NxM pixels of the discretized and filtered hologram by introducing at its edges a number of additional fictitious pixels, whose intensity is set equal to zero. Preferably, this operation conforms to what is described in EP 1 654 596.

As is known from the digital holography theory, to reconstruct the amplitude or phase image of an object starting from its acquired hologram, a mathematical algorithm is performed that implements the well-known Rayleigh-Sommerfeld formula. This formula basically contains a double integration of the digitized hologram multiplied by a numerical copy of the reference beam R and other terms. The resulting integral can be converted into Fourier transforms with a consequent reduction of the computational effort. Since the hologram is in discretized form, the Fourier transforms are also discrete, and can therefore be calculated relatively easily by means of the known FFT (Fast Fourier Transform) algorithms. To perform the transformation, in particular, two methods are known: the convolution method and the Fresnel method. Using for example the Fresnel method, the spatial resolution of the reconstructed phase image is quantified by the so-called "reconstructed pixel", whose dimensions along the x and y axes are given by the following equations:

A d A d

Δξ = Δη - [2]

Ν Δχ M Ay

where N and M are the pixel numbers of the sensor 7 along the x and y axes (and, therefore, of the discretized hologram), A is the emission wavelength of the laser source 2, d is the reconstruction distance (i.e. the distance between the investigated portion of the building 9 and the sensor 7), and Δ <χ> and Ay are the dimensions of each pixel of the sensor 7 along the x and y axes. From equation [2] it is evident that Δξ and Δη are proportional to the wavelength A and the reconstruction distance d, while they decrease as the total number of pixels NxM increases and the physical dimensions of the pixel increase. The spatial resolution of the reconstructed image can therefore be worse than that obtained by working at the minimum distance allowed by the sampling theory.

The aforementioned "zero padding" operation advantageously allows to increase the spatial resolution of the reconstructed phase image. More specifically, by adding dummy pixels of zero intensity to the NxM matrix of the acquired hologram, Δξ and Δη are decreased, and the spatial resolution is thus increased. Preferably, the dummy pixels are added along the edges of the scanned hologram, so as not to mix with the actual pixels. This ensures that the reconstructed phase image does not have any spurious frequencies resulting from discontinuities introduced between the effective pixels. The number of dummy pixels added depends on the resolution you want to achieve for the reconstructed phase image. The maximum resolution obtainable in any case is limited by the diffraction limit.

Although the "zero padding" operation has been discussed above with reference only to the Fresnel method, it can be used in combination with other methods, such as the angular spectrum method or the convolution method.

Then, during a third sub-phase 312, the discretized hologram (filtered and possibly "enlarged" by fictitious pixels) is processed to reconstruct a phase image of the portion irradiated by the laser of the building 9.

For this purpose, the processing unit 8 preferably first performs a numerical focusing of the hologram, which provides for the application to the discretized hologram (filtered and possibly "enlarged" by fictitious pixels) of a mathematical algorithm which implements the aforementioned formula of Rayleigh-Sommerfeld, which basically simulates the diffractive effects of the propagation of a numerical copy of the reference beam R through the hologram. The application of this algorithm results in the reconstruction of the wave front of the beam object O, focused at a distance d. Preferably, the algorithm is based on the aforementioned Fresnel method, which is particularly simple and fast compared to other known methods. However, according to other variants, it is possible to use other focusing methods, for example the angular spectrum method or the convolution method.

The numerical focusing performed at substep 312 provides a reconstructed complex field, i.e. a matrix in which each element or pixel is a complex number. From this matrix, at the sub-phase 312 the processing unit 8 preferably directly derives the phase image of the portion irradiated by the laser of the building 9 as a phase of the reconstructed complex field. In particular, the processing unit 8 preferably calculates the phase of each complex number of the matrix, thus obtaining a corresponding matrix in which each element or pixel is a phase. The matrix thus obtained contains the phase image of the portion irradiated by the laser of the building 9 at a certain instant.

Sub-step 312 preferably also comprises filtering the conjugate complex of the reconstructed complex field.

As mentioned above, the numerical processing 31 is applied to all the holograms of the sequence acquired during step 30. The execution of the numerical processing described above on each hologram of the sequence then produces a sequence of phase images of the portion irradiated by the laser building 9.

Then, during a subsequent phase 32, the processing unit 8 preferably uses the sequence of phase images of the portion irradiated by the laser of the building 9 to reconstruct the time course of deformations or displacements to which the portion irradiated by the laser is subject. of the building 9. For this purpose, during the phase 32 the processing unit 8 preferably uses all the phase images of the sequence.

In particular, during the phase 32, the displacement along the normal N to the building 9 to which each point of the building 9 is subjected, irradiated by the laser and represented by a certain pixel of the phase image, 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 previous phase image in the sequence. By making phase differences of a certain pixel between successive phase images, it is possible to reconstruct the displacement, along the normal N, of the building point represented by that pixel, as time varies, according to the equation:

Δ3Ν - (1 / k) (λ · Δφ) / (4πη) - Sm / (2k), [3]

where Δ3Ν is the displacement of the point corresponding to the pixel, along the normal N to the surface surrounding the point itself, between two successive phase images, Sm is the variation of the building-sensor source optical path, Δφ is the phase difference of the pixel between two successive phase images, λ is the wavelength used, n is the refractive index of the air, k is a projection factor that depends on the inclination of the irradiation direction (i.e. the direction of the beam object O incident on the point in question) with respect to the normal N and is equal to cozy, where ψ is the angle by which the direction of irradiation must be rotated to superimpose it on the normal N. For displacements of 10 pm and for angles ψ less than 8 °, the correction according to the k factor it is lower than the sensitivity of the interfero-metric technique (estimated equal to 0.1 pm). Equation [3] is valid only in the approximation in which the positions of mirror 83 and sensor 7 are considered coincident and, therefore, the direction of irradiation substantially coincides with the direction of the sensor 7 joining the pixel in question.

By repeating the operation on a set of contiguous pixels (or on all the pixels of the phase image) - which represent at least a portion of the surface of the building 9 - it is therefore possible to reconstruct the displacement along the normal N of each point of the portion of building 9 represented by the pixels considered, i.e. the deformation or displacement to which this portion of building 9 is subject as a whole. If the signal to noise ratio of the single pixel is not sufficiently significant, the variation is optionally averaged phase over a larger number of adjacent pixels.

Optionally, for at least one of the holograms of the acquired sequence, the processing unit 8 also calculates the amplitude of the reconstructed complex field of the object beam O, by separately calculating the module of the complex value of each pixel of the matrix representing the field. This allows to reconstruct also an amplitude image (that is, a real image) of the building 9. This amplitude image can be advantageously used in the course of phase 30 to select the portion of the building 9 whose deformations are to be evaluated. or displacements. Once the pixels representing the portion of interest on the amplitude image have been selected, the processing unit 8 preferably selects the corresponding pixels in the phase images, and merely reconstructs the displacement along the normal N of these pixels based on the their values in the various phase images of the sequence.

Optionally, phase 32 can also include a Fourier analysis of the temporal trend of the optical path variation Sm relating to one or more points of the building 9 represented by the corresponding pixels of the phase image. Optionally, step 32 can also include the execution of a frequency filtering (band pass filter) to exclude any frequency components attributable to vibrations of the measurement system itself. This allows, in any case, to directly calculate the frequencies of the oscillations to which building 9 is subjected. Optionally, step 32 can also include performing a frequency filtering (band pass filter) to select a single frequency of interest and calculate the magnitude of the displacement relative to the selected frequency. The results obtained in step 32 are then stored and can be displayed on the screen 8a of the processing unit 8, for example in graphic form.

System 1 then uses digital holography to monitor deformations and displacements to which building 9 or its portions is subject. The use of digital holography has several advantages.

First, digital holography is an optical approach that allows for remote monitoring of the building. The system 1 is in fact positioned at a certain distance from the building, which as mentioned above can be adapted according to the accessibility and conditions of the area surrounding the building. The use of sensors or devices to be placed inside the building is not required. The system 1 therefore allows to easily and safely monitor building constructions whose access is limited for safety reasons or as buildings of artistic or historical interest. Moreover, thanks to the use of digital holography, the system 1 can monitor the building in a substantially continuous way, both from the spatial and temporal point of view. From a spatial point of view, digital holography makes it possible to detect the movements of any point in the building, provided it is illuminated by the object beam. The spatial resolution of the monitoring (i.e. the number and density of the building points that can be monitored separately) is substantially given by the resolution of the phase images reconstructed by system 1 (i.e. by the size of the reconstructed pixel, as discussed above). From the temporal point of view, system 1 allows to evaluate the displacements of a pixel-point of the building as time varies in a substantially continuous way. The temporal resolution of the monitoring (i.e. the time interval between two consecutive observations of the phase of the same pixel) is substantially given by the acquisition frequency of the sensor 7.

Furthermore, system 1 is able to monitor the building in real time. The numerical processing performed by the processing unit 8 is in fact rather simple, and can be performed for each hologram of the acquired sequence substantially in the time that elapses between the acquisition of two successive holograms. The displacement to which each point is subject can therefore be reconstructed substantially in real time, ie while the deformation or displacement to be detected is taking place.

The use of digital holography, and in particular of digital holography in the mid-infrared spectrum, also has other advantages.

First, the use of relatively long emission wavelengths makes it possible to reconstruct phase images of large objects such as buildings or other building constructions, as will be discussed in more detail below. Furthermore, C02 mid-infrared laser sources generally exhibit good spatial and temporal coherence properties. This makes it possible to significantly expand the object beam derived from them, so as to be able to illuminate significant portions of a building and at the same time obtain clearly visible interference fringes, even in the case of significant imbalance between the optical path length of the object beam and that of the beam. of reference.

As regards the maximum size of the buildings that can be monitored with system 1, an object (for example building 9) illuminated by the object beam O diffuses the light in different directions, and the contributions diffused by the various points of its surface and incident on the sensor surface 7 form different angles a with the direction of incidence on the sensor of the reference beam R. Each angle a is equal to θ + β, where Θ is the angle formed by the direction of the reference beam R and the direction T perpendicular to the surface of the sensor 7, and β is the angle formed by the direction of the contribution of the object beam O (see Figure 3a) with the normal to the sensor T. The larger the size of the irradiated building portion, the wider it is the range of angles a that the object beam O forms with the reference beam R. AN as the angle a increases, however, the spacing P of the interference fringes in the hologram decreases according to the following eq action:

<P _> 2sÌn (q / 2) '^

where λ is the emission wavelength of the laser source 2. Equation [4] applies in the hypothesis that Θ is approximately equal to β. In order for the Whittaker-Shannon sampling theorem to be satisfied, P> 2dp, where dp is the size of a single pixel of sensor 7. Therefore, the maximum value of a that allows to detect the interference fringes of the hologram is:

TO

^ max = 2sin <1> [5]

1 <4CI> PJ 2dP

The last equality is valid under conditions of small angles.

On the other hand, the maximum angle Pmax formed by the object beam O and the normal T at the surface of the sensor 7 depends both on the distance d between the portion of the building 9 to be monitored and the surface of the sensor 7, and on the lateral extension of the portion of the building irradiated D according to the following equation:

Pmax —artan (D / 2d) = D / 2d. [6]

The last equality is valid under conditions of small angles. Equation [6] is valid when the dimension D is much larger than the sensor surface 7.

Equations [5] and [6] therefore allow to calculate the maximum angle Θ between the reference beam R and the normal T at the sensor surface 7:

Smax - Climax "Pmax - A / (2dp) - D / 2d [7]

Given a certain distance d, it is however possible to decrease the angle Θ to increase the dimension D of the investigated portion, as seen in Figure 3b (since amax is fixed by the sampling theorem, decreasing Θ allows to increase Pmax, that is D). To keep the diffraction orders in the hologram separate, however, Θ must remain greater than D / 2d. By inserting this value in equation [7], the maximum dimension Dmax of the framed portion of the building 9 is obtained, i.e. the maximum visual field:

Dmax-Ad / (2dp). [8]

From equation [8] it is evident that Dmax is proportional to λ and to the distance d, while it is inversely proportional to the pixel size dp.

Whereas the wavelengths in the mid infrared are approximately 20 times greater than the wavelengths in the visible spectrum (10 μιτι instead of 0.5 pm) that the typical pixel size in an infrared sensor is typically 5 times greater than that of a sensor in the visible (25 pm instead of 5 pm), given a certain distance the use of infrared radiation allows to monitor rare objects of dimensions four times larger than those that can be monitored with visible radiation.

For example, digital holography with an exemplary visible wavelength of 532 nm at a distance d = 30 m using a pixel size dp = 5 pm would allow to monitor objects with a maximum size of approximately 1.6 m. The use of a wavelength in the medium infrared (ie about 10 microns) at the same monitoring distance and with a pixel size of dp = 25 pm would allow the maximum size of the object to be increased to about 6 meters.

As regards the amplitude of deformations and minimum detectable displacements, the inventors have estimated that, by optimizing the visibility of the fringes and the signal-to-noise ratio, the minimum displacement of each point-pixel detectable by the system 1 is substantially equal to one hundredth of the length wave. Using wavelengths in the mid-infrared (for example 10.6 pm), the minimum detectable displacement is therefore of the order of a tenth of a pm. On the other hand, as regards the maximum frequency of detectable periodic deformations or displacements, it is mainly limited by the sampling theorem, and essentially depends on the acquisition frequency of the sensor 7. For example, if the sensor 7 has an acquisition frequency of 25 frames / sec, the maximum frequency of detectable periodic deformations or oscillations is about 10 Hz. On the other hand, there is no lower limit to the frequency, so that system 1 is able to detect deformations or periodic displacements of arbitrarily low frequency, and also deformations and non-periodic shifts.

The inventors carried out a comparative test of system 1 under real conditions. In particular, the inventors used a monitoring system similar to system 1 to monitor the oscillations (periodic displacements) of a building about 20 m high with an approximately flat and vertical surface. System 1 was placed at a distance d of approximately 18 meters from the building, and was adjusted so that the direction of the beam object O was approximately 38 ° degrees with respect to the ground and contained in the plane normal to the surface of the building . The beam object O thus illuminated a portion of the building of about 4 m <2>, in correspondence with which - for comparison purposes - a seismometer was also placed. A sequence of phase images of 10 pixels x 10 pixels of an area corresponding to about 100 cm <2> of the 4 m <2> illuminated was then reconstructed.

The displacement of the building point in correspondence with which the seismometer was present as the time varied was therefore calculated both by system 1 and by the seismometer. The results are shown in Figure 4. Figure 4 in particular shows the data obtained from system 1 with digital holography (graph (a)) and the data obtained from the seismometer (graph (b)). As regards graph (a), the displacement with time has been calculated taking into account the factor k of inclination of the beam object O (division by cos (38 °)) according to equation [3]. On the other hand, as regards graph (b), among the data provided by the seismometer, only those relating to the displacement detectable by system 1, i.e. the displacement along the normal N to the surface of the building surrounding the measurement point, were considered. . From the comparison between the two graphs, it is evident that the results provided by system 1 are in complete agreement with those of the seismometer. System 1 is therefore able to provide results whose accuracy is comparable to that of traditional earthquake meters.

The method and the monitoring system according to the present invention therefore allow to monitor building constructions, possibly, in their entirety remotely and in real time, without the need to place any sensor inside or on the external surface of the buildings. The times and costs of monitoring are therefore significantly reduced compared to known techniques that use seismometers.

Claims (8)

CLAIMS 1. System (1) for monitoring a building construction (9), called system (1) comprising: a laser source (2) adapted to emit infrared radiation; an interferometric structure configured to divide said infrared radiation into an object beam (O) configured to illuminate at least a portion of said building construction (9) and to be diffused by said at least a portion of said construction (9), and a reference beam (R) configured to interfere with said beam object (O) diffused by said at least a portion of said construction (9) so as to create at least one hologram of said at least a portion of said construction (9); a sensor (7) adapted to detect a temporal sequence of holograms of said at least a portion of said construction (9); and a processing unit (8) configured to reconstruct a temporal course of a deformation and / or a displacement to which said at least a portion of said construction (9) is subjected by numerically processing said sequence of holograms of said at least a portion of said construction (9). System (1) according to claim 1, wherein said infrared radiation has a wavelength comprised between 3 pm and 30 pm. System (1) according to claim 1 or 2, wherein said interferometric structure comprises a first lens (4) adapted to widen said object beam (O) before it illuminates said at least a portion of said building construction (9) and a second lens (5) adapted to widen said reference beam (R) before it interferes with said object beam (O) diffused by said at least a portion of said building construction (9). System (1) according to any one of the preceding claims, wherein said interferometric structure also comprises a mirror (83) able to deflect said beam object (O) before it illuminates said at least a portion of said building construction (9), said mirror (83) being movable to adjust a direction of incidence of said object beam (O) on said portion of said construction (9). System (1) according to any one of the preceding claims, wherein said processing unit (8) is configured for: processing said sequence of holograms of said at least a portion of said construction (9) so as to obtain a sequence of phase images of said at least a portion of said construction (9); And calculate a displacement in time of at least one point of said at least a portion of said construction (9) represented by a certain pixel in said sequence of phase images of said at least a portion of said construction (9), as a function of phase differences of said pixel between successive phase images of said sequence of phase images of said at least a portion of said construction (9). System (1) according to claim 5, wherein said processing unit (8) is configured to calculate said displacement in time of said at least one point of said at least a portion of said construction (9) along a direction (N) normal to a surface surrounding said at least one point of said at least a portion of said construction (9) and to divide said displacement along said direction (N) by a factor (k) which takes into account the inclination of the direction of incidence of said beam object (O) on said at least a portion of said construction (9) with respect to said direction (N). System (1) according to any one of the preceding claims, wherein said processing unit (8) is also configured to process said sequence of holograms of said at least a portion of said construction (9) so as to obtain a sequence of images of width of said at least a portion of said construction (9). 8. Method for monitoring a building construction (9), said method comprising: provide infrared radiation; dividing said infrared radiation into an object beam (O) and a reference beam (R), illuminating at least a portion of said building construction (9) with said object beam (O) so that said object beam (O) is diffused by said at least a portion of said building construction (9), and causing said reference beam (R) to interfere with said object beam (O) diffused by said at least a portion of said building construction (9) so as to create at least a hologram of said at least a portion of said construction (9); detecting a temporal sequence of holograms of said at least a portion of said construction (9); And reconstruct a temporal course of a deformation and / or a displacement to which said at least a portion of said construction (9) is subjected by numerically processing said sequence of holograms of said at least a portion of said construction (9).