WO 2023/079449 PCT/IB2022/060544

METHOD FOR MECHANICALLY CHARACTERIZING A STRUCTURE AND

CONTROL UNIT AND SYSTEM THEREOF

DESCRIPTION

The present invention relates to a method for mechanically characterizing a structure, in accordance with the preamble of claim 1. In particular, disclosed herein are a method, a control unit and a system for mechanically characterizing a structure, e.g. for the purpose of continuously or periodically monitoring its structural condition.

The present invention can be used for mechanically characterizing any flexible structure, whether artificial or natural, i.e. for determining a set of mechanical parameters (in particular, dynamic/modal ones) which can reveal the structure’s response to mechanical stresses. Within the frame of the present invention, a structure is any manufacture, whether a building, a church, a stadium, a shopping center, a monument or any infrastructure, e.g. a bridge, a dam, an industrial plant, or any natural structure showing some deformability. The invention may also be useful for characterizing mechanical and industrial components that, in addition to being perceptibly flexible, have such an extension as to justify an interaction between a data processing unit and a measurement system (e.g. big wind turbines, off-shore structures, flexible ducts and pipes, plants, power stations, aerospace structures, etc.).

Several systems are currently known which permit a mechanical characterization of a structure, such as, for example, the ARTeMIS system (https ://svsbs .co /). Such system makes it possible to acquire and process data coming from sensors positioned on a structure, and to display some results obtained by processing such data in order to make a structural identification and determine some modal parameters mechanically characterizing the structure.

The above-mentioned system suffers from a number of drawbacks, which will be illustrated below.

A first drawback lies in the fact that such a system does not permit an easy characterization of large structures, in that for large structures the system must necessarily use a large number of sensors, all of which have to be managed simultaneously.

A second drawback comes from the fact that the use of a large number of sensors, for large structures, inevitably implies some latency in the signals measured by sensors positioned very far from one another. In fact, latency times may introduce errors when processing the data, possibly affecting the reliability of the mechanical characterization results. A further drawback of the above-mentioned system lies in the fact that the use of a large number of sensors, for large structures, inevitably implies increased computational complexity of such a system. In fact, the system must necessarily introduce some corrective algorithms taking into account the position of each sensor in use, so as to reduce the adverse effects of latency times upon the mechanical characterization results.

Another drawback of the above-mentioned system comes from the fact that the use of a large number of sensors for large structures inevitably results in increased management and logistic complexity of such a system. In fact, the system must necessarily be able to manage a large number of sensors, with a consequent increase in the system’s management and logistic complexity, e.g. for positioning and/or wiring sensors on the structure.

Yet another drawback of the above-mentioned system lies in the fact that the use of a large number of sensors for large structures inevitably results in increased management and logistic costs. In fact, the system must necessarily be able to manage a large number of sensors, with a consequent increase in the costs incurred for purchasing and managing the sensors.

It is therefore one object of the present invention to solve these and other problems suffered by the prior art, and particularly to provide a method, a control unit and a system that facilitate the mechanical characterization of a structure by using a smaller number of sensors than prior-art characterization systems.

It is a further object of the present invention to provide a system, a control unit and a related method that avoid the introduction of latency times in the signals measured by sensors positioned at great mutual distances within the system.

It is another object of the present invention to provide a system, a control unit and a related method that permit reducing the computational complexity of the mechanical characterization system.

It is a further object of the present invention to provide a control unit, a system and a related method that permit reducing the logistic complexity concerning the placement and management of the sensors used by the mechanical characterization system.

It is yet another object of the present invention to provide a system, a control unit and a related method that permit reducing the costs incurred for managing the system.

In brief, the invention described herein consists of a system, a control unit and a related method that make it possible to obtain a mechanical characterization of a structure by aggregating data acquired by a limited number of sensors during data acquisition campaigns carried out at different times. The sensors are positioned on the structure in such a way that some sensor acquisition positions are shared by different acquisition campaigns.

Further advantageous features of the present invention are set out in the appended claims, which are an integral part of the present description.

The invention will now be described in detail through some non-limiting exemplary embodiments thereof, with particular reference to the annexed drawings, wherein:

- Figure la schematically shows a system for mechanically characterizing a structure, comprising a control unit and a plurality of sensors, according to one embodiment of the present invention;

- Figures lb, 1c and Id schematically show an illustrative arrangement of the sensors on the structure of Figure la, according to one embodiment of the present invention;

- Figure 2 schematically shows a block diagram of the control unit of Figure la;

- Figure 3 shows an illustrative flow chart of a method for mechanically characterizing the structure of Figure la, in accordance with the present embodiment of the invention;

- Figure 4 shows an example of three modal shapes corresponding to the configurations of Figures lb, 1c and Id, respectively;

- Figure 5 shows an example of aggregation of the three modal shapes shown in Figure 4, corresponding to the configurations of Figures lb, 1c and Id, respectively.

Figure la schematically shows a system 100 for mechanically characterizing a structure 101, wherein the system 100 comprises a control unit 200 and a plurality of sensors adapted to communicate with the control unit 200.

For example, the structure 101 may be a building, e.g. a skyscraper, lying on a supporting surface 102, e.g. a ground surface, etc. The structure 101 can be divided into portions 110 mechanically connected to one other, e.g. each portion 110 may be a floor of the building. For example, the structure 101 may comprise twelve portions 110, i.e. twelve floors mechanically connected to one another.

In accordance with Figure la, at least two sensors 120 of the plurality of sensors are operationally connected to the structure 101, e.g. by fastening means like brackets, screws, bolts, etc., so as to be integral with such structure 101. One sensor 120 of the plurality of sensors can be operationally connected to a portion 110 of the structure 101 at an acquisition point, e.g. it may be connected to a slab, a column, a wall, a beam, etc. of a floor of the building. The sensor 120 may comprise at least one accelerometer adapted to measure an acceleration value in an acquisition direction, e.g. a direction parallel to the supporting surface 102, at the acquisition point where it is operationally connected to the structure 101. Together, the acquisition point and the acquisition direction define an acquisition position of the sensor 120. The sensor 120 may comprise communication means adapted to communicate with the control unit 200, wherein such communication means may comprise at least one of the following interfaces: a WiFi interface, a Bluetooth interface, a GSM interface, an LTE interface, a 5G interface, a CANBUS interface, and an Ethernet interface. For example, each sensor 120 can communicate with the control unit 200 using loT technology, e.g. Sigfox, LoRa or the like. The acceleration value measured by the sensor 120 can be sent to the control unit 200 by means of an analog or digital signal representative of such acceleration value.

The control unit 200 is adapted to communicate with the plurality of sensors, in particular with at least two sensors 120 of the plurality of sensors operationally connected to the structure 101. Consequently, the control unit 200 can receive at least one signal representative of an acceleration of a portion 110 of the structure 101, such signal being transmitted by at least one sensor 120 operationally connected to the structure 101. The control unit 200, which will be further described below with reference to Figure 2, makes it possible to mechanically characterize the structure 101 by analyzing the data obtained from the sensors 120 in accordance with the method of the present invention, which will be described below with reference to Figure 3. In addition, the control unit 200 allows a user to configure and/or manage the system 100 for characterizing the structure 101, e.g. by means of a specific application wherein one or more parameters can be set which pertain to the plurality of sensors and/or the structure 101. In addition, the control unit 200 can communicate with a server (not shown in Figure la) adapted to record data relating to the system 100, such as, for example, the signals transmitted by the sensors 120 and/or data resulting from the mechanical characterization of the structure 101.

In accordance with the present embodiment of the invention, Figures lb, 1c and Id show an illustrative arrangement of the sensors 120 on the structure 101, respectively in a first, a second and a third data acquisition campaigns conducted in order to acquire data about the structure 101.

The first, second and third data acquisition campaigns are conducted at different times, particularly at three different times.

Figure lb shows a first configuration of at least two sensors 120a, 120b operationally connected to the structure 101, e.g. installed by the user, based on a first positioning scheme, which may comprise at least two acquisition positions in which the two or more sensors 120a, 120b of the plurality of sensors are positioned. Such at least two acquisition positions may be referred to a system of coordinates of the structure 101. For example, the system of coordinates may be a three-axis Cartesian coordinate system (not shown in Figure lb) with two axes coplanar to the supporting surface 102 and the third axis perpendicular to the supporting surface 102, while the origin of the Cartesian coordinate system may coincide with an edge of the structure 101, lying on the supporting surface 102.

For example, in accordance with the present embodiment of the invention, the first positioning scheme makes use of all the sensors 120 of the plurality of sensors, for example comprising a total number of seven sensors 120. Subsequently, each sensor 120a, 120b, operationally connected to the structure 101, can then transmit to the control unit 200 at least one first signal representative of an acceleration of a portion 110 to which such sensor 120a, 120b is operationally connected.

Similarly, Figure 1c shows a further configuration of at least two sensors 120a, 120c operationally connected to the structure 101, for example installed by the user based on a further positioning scheme, which may comprise at least two further acquisition positions in which the two or more sensors 120a, 120c of the plurality of sensors are positioned. Such at least two further acquisition positions may be referred to the system of coordinates of the structure 101, previously described herein with reference to Figure lb.

Similarly, Figure Id shows a further configuration of at least two sensors 120a, 120d operationally connected to the structure 101, for example installed by the user based on a further positioning scheme, which may comprise at least two further acquisition positions in which the two or more sensors 120a, 120d of the plurality of sensors are positioned. Such at least two further acquisition positions may be referred to the system of coordinates of the structure 101, previously described herein with reference to Figure lb.

In accordance with the present embodiment of the invention, the further positioning scheme of Figure 1c makes use of just a part of the sensors 120a, 120c of the plurality of sensors, i.e. it only uses six sensors 120a, 120c of the seven sensors 120 of the plurality of sensors. Likewise, the further positioning scheme of Figure Id makes use of just five sensors 120a, 120d of the seven sensors 120 of the plurality of sensors.

Subsequently, with reference to Figure 1c, each sensor 120a, 120c operationally connected to the structure 101 can transmit to the control unit 200 at least one further signal representative of an acceleration of a further portion 110 to which it is operationally connected.

Subsequently, with reference to Figure Id, each sensor 120a, 120d operationally connected to the structure 101 can transmit to the control unit 200 at least one further signal representative of an acceleration of a further portion 110 to which it is operationally connected.

In accordance with the present invention, the predefined first positioning scheme (shown in Figure lb) and the further predefined positioning scheme (shown in Figure 1c) comprise at least one common acquisition position of the sensors 120a, preferably at least two common acquisition positions of the sensors 120a. For example, with reference to Figures lb, 1c and Id, three sensors 120a (indicated in black) have the same acquisition positions in all three configurations.

In another embodiment of the invention, the configurations of Figures lb and 1c may have at least one common acquisition position of the sensors 120a, preferably at least two common acquisition positions of the sensors 120a, whereas the configuration shown in Figure Id may have at least one acquisition position of the sensors 120a, preferably at least two acquisition positions of the sensors 120a, in common with the configuration lb or 1c.

In another embodiment of the invention, one may advantageously consider as few as just two configurations, or any number of configurations depending on the dimensions of the structure 101 and/or the total number of sensors 120 of the plurality of sensors.

It will be apparent to those skilled in the art that the present invention advantageously permits using, at different times, a smaller number of sensors 120 than prior-art techniques. In fact, prior-art techniques would require, in order to mechanically characterize the structure 101, twelve sensors 120, each one located in an acquisition position, acquiring data simultaneously. The acquisition positions of the sensors 120 can be determined, for example, during an interaction phase and an optimization phase that will be described hereinafter.

With the present invention, on the other hand, it is possible to mechanically characterize the structure 101 by using a smaller number of sensors, i.e. no more than seven sensors 120 acquiring data simultaneously. Advantageously, the sensors 120 can be so positioned as to measure at least two distinct parts of the structure 101 at at least two different times. In the example shown in Figures lb, 1c and Id, the data coming from the sensors 120 are acquired at three different times.

Figure 2 shows an illustrative block diagram of the control unit 200 for mechanically characterizing a structure 101. The control unit 200 may comprise communication means 230, interface means 220, memory means 240, and processing means 250. Such means may be interconnected via a communication bus 201.

The communication means 230 are adapted to establish a communication channel with at least two sensors 120 of said plurality of sensors. In addition, the communication means 230 can establish a further communication channel with the server. The communication means 230 may comprise, for example, a USB, CANBUS, ETHERNET, WiFi, Bluetooth, GSM, etc. interface.

The interface means 220 are adapted to receive and transmit input/output information from the control unit 200 to the user, for example in order to allow the user to manage the system 100. The interface means 220 may comprise, for example, a display, a keyboard, a touchscreen, etc.

The memory means 240 are adapted to store the information and instructions of the control unit 200 for mechanically characterizing a structure 101 according to the present embodiment of the invention, and may comprise, for example, a Flash-type solid-state memory. Such information may comprise data and parameters relating to the structure 101 and/or the plurality of sensors, and may comprise the signals received from the sensors 120. The instructions stored in the memory means 240 will be further described below with reference to the flow chart of Figure 3.

The processing means 250 are adapted to process the information and the instructions stored in the memory means 240, concerning the communication means 230 and the interface means 220, and may comprise, for example, a multicore ARM processor, and Arduino microcontroller, etc. The processing means 250 can establish a communication between the control unit 200 and the server using the communication means 230.

The communication bus 201 is adapted to interconnect said communication means 230, interface means 220 and memory means 240 with the processing means 250.

The control unit 200 may be implemented, for example, as a computer program product comprising portions of software code, which can be loaded into a memory of a terminal, e.g. a smartphone, a tablet, a laptop or a computer equipped with interface means such as, for example, a USB, ETHERNET, WiFi, Bluetooth, GSM, etc. interface.

As a whole, the control unit 200 is adapted to receive said at least one first signal representative of an acceleration of a portion 110 of the structure 101 , or is adapted to receive said at least one further signal representative of a further acceleration of a further portion 110 of the structure 101. Moreover, the control unit 200 is adapted to determine a first set of modal parameters based on at least the first signal, and to determine a further set of parameters based on at least one further signal. In addition, the control unit 200 is adapted to aggregate the first set of modal parameters and at least one further set of modal parameters in order to obtain a single set of modal parameters useful for mechanically characterizing the entire structure 101.

It will be apparent to those skilled in the art that the present invention advantageously permits obtaining a single set of modal parameters for mechanically characterizing the structure 101, starting from at least two partial data acquisition campaigns conducted at different times by the plurality of sensors, which have been positioned on the structure 101 in accordance with at least two distinct positioning schemes having at least one common acquisition position of said sensors 120.

With reference to Figure 3, the following will describe a method for mechanically characterizing the structure 101, in accordance with the present embodiment of the invention.

At step 300, a phase of initializing the control unit 200 is carried out in order to put the latter in operation. For example, during this step the processing means 250 verify the operational state of the communication means 230, interface means 220 and memory means 240.

At step 310, a first phase of positioning at least two sensors 210a, 210b of the plurality of sensors on the structure 101 is carried out by the user. During this phase, the control unit 200 can indicate to the user, e.g. via the interface means 220, at least two acquisition positions in which each one of the at least two sensors 120a, 120b of the plurality of sensors should be positioned according to a first positioning scheme. In this way, the at least two sensors 120a, 120b of said plurality of sensors can be operationally connected to the structure 101 based on the first positioning scheme. For example, each sensor 120 of the plurality of sensors may comprise a mini-drone equipped with anchoring means and at least one accelerometer comprising a wireless interface, e.g. an loT interface. Upon a user’s command, the control unit 200 can indicate to each sensor 120a, 120b where it should position and anchor itself on the structure 101 according to the first positioning scheme.

At step 320, a first acquisition phase is carried out by said at least two sensors 120a, 120b. During this phase, each sensor 120a, 120b operationally connected to the structure 101 transmits to the control unit 200 at least one first signal representative of an acceleration of a portion 110 of the structure 101. During this step, the control unit 200 receives said at least one first signal, e.g. via the communication means 230. At step 330, a further phase of positioning at least two sensors 120a, 120b of the plurality of sensors on the structure 101 is carried out by the user. During this phase, the control unit 200 can indicate to the user, e.g. via the interface means 220, at least two further acquisition positions in which each one of the at least two sensors 120a, 120b of the plurality of sensors should be positioned according to a further positioning scheme. In this way, the at least two sensors 120a, 120b of said plurality of sensors can be operationally connected to the structure 101 based on the further positioning scheme, e.g. as described at step 310.

In accordance with the present invention, the first positioning scheme and the further positioning scheme comprise at least one common acquisition position of said sensors 120a, e.g. as described with reference to Figures lb, 1c and Id.

At step 340, a further acquisition phase is carried out by said at least two sensors 120a, 120b. During this phase, each sensor 120a, 120b operationally connected to the structure 101 transmits to the control unit 200 at least one further signal representative of a further acceleration of a further portion 110 of the structure 101. During this step, the control unit 200 receives said at least one further signal, e.g. via the communication means 230.

At step 350, the control unit 200 verifies if the first acquisition phase and said at least one further acquisition phase have been conducted in such a way as to comprise the whole structure 101. If not, the control unit will execute step 330, otherwise it will execute step 360.

At step 360, the control unit 200 determines a first set of modal parameters based on at least one first signal, and determines at least one further set of modal parameters based on said at least one further signal. The control unit 200 can determine the first set of modal parameters based on at least one first signal in accordance with a modal analysis of the structure 101. Likewise, the control unit 200 can determine the at least one further set of modal parameters based on the at least one further signal in accordance with the same modal analysis of the structure 101.

The modal analysis of the structure 101, whether constrained or unconstrained, permits determining a response of the structure 101 to externally applied mechanical stresses. Such stresses may be either natural, e.g. caused by telluric movements, or artificial, e.g. caused by heavy means of transport in motion, such as trains, tram cars and trailer trucks. The present invention may utilize any known modal analysis algorithm to obtain the first set of modal parameters and said at least one further set of modal parameters, which may comprise at least one of the following modal parameters relating to the structure 101 : a natural frequency, a modal damping, a modal shape.

The modal analysis provides natural frequency, modal damping and modal shape for each portion 110 of the structure 101 as a function of one or more elements of a mass matrix M, a damping matrix C and an elastic stiffness matrix K relating to the structure 101. The mass matrix M, the damping matrix C and the elastic stiffness matrix K can be determined on the basis of a mass, a damping constant and an elastic constant, respectively, relating to each portion 110 of the structure 101. In particular, the modal shape corresponds to an eigenvector having a number of components D matching the number of sensors 120 that transmit to the control unit 200 said at least one first signal or said at least one further signal, i.e. equal to the number of sensors 120 used in said first acquisition phase or in said at least one further acquisition phase. For example, such eigenvector can be determined by solving a matricial equation comprising the mass matrix M, the damping matrix C and the elastic stiffness matrix K.

For example, in accordance with the present embodiment of the invention, Figure 4 shows a first modal shape 410, a second modal shape 420 and a third modal shape 430 corresponding to, respectively, the configurations of Figures lb, 1c and Id. In particular, the abscissas indicate modal ratio values normalized to the mass matrix M, while the ordinates indicate the portions 110 of the structure 101, i.e. the floors of the skyscraper, to which the normalized modal ratio values correspond. In Figure 4, the black dots of such eigenvectors indicate the corresponding sensors 120a having common acquisition positions, i.e. being in the same acquisition positions, in the three configurations shown in Figures lb, 1c and Id.

At step 370, an aggregation phase is carried out, wherein the control unit 200 aggregates the first set of modal parameters and said at least one further set of modal parameters, determined as described in step 360, in order to obtain a single set of modal parameters mechanically characterizing the structure 101 in its entirety. For example, the single set of modal parameters resulting from the aggregation phase may comprise a single modal shape corresponding to a single eigenvector, such as, for example, the single eigenvector 510 shown in Figure 5, resulting from the aggregation of the first modal shape 410, the second modal shape 420 and the third modal shape 430, shown in Figure 4, respectively corresponding to the configurations shown in Figures lb, 1c and Id. In particular, Figure 5 shows the single modal shape (single eigenvector) 510 compared with a modal shape 520 obtained using twelve sensors 120 simultaneously.

As can be seen in Figure 4, since the first acquisition phase and the at least one further acquisition phase, relating to the configurations shown in Figures lb, 1c and Id, are conducted at different times, the normalized modal ratio values shown along the abscissas, corresponding to the sensors 120 having common acquisition positions (indicated in black in Figure 4), do not coincide. This is mainly due to the fact that the normalization of the values indicated on the abscissas (normalized modal ratios) is different, since only the masses of the portions 110 of the structure 101 have been considered during the first acquisition phase and during the at least one further acquisition phase. Moreover, systematic and/or environmental errors should also be taken into account, which might be introduced during the first acquisition phase and during the at least one further acquisition phase.

In order to tackle such problems, the aggregation procedure may comprise a first normalization phase, a second normalization phase, an averaging phase, and a concatenation phase. For example, let us indicate with Vl_i, V2J, V3_k the components of the first modal shape 410 (VI), second modal shape 420 (V2) and third modal shape 430 (V3), respectively, having a number Q of common acquisition positions, indicated in black in Figure 4. According to the example of Figure 4, Q is three (Q=3), while the corresponding number of components I, J and K for the first modal shape 410 (VI), second modal shape 420 (V2) and third modal shape 430 (V3) is, respectively, seven (1=7), six (J=6) and five (K=5).

During the first normalization phase, the components Vl_i, V2J, V3_k are normalized to the maximum component of each modal shape VI, V2 and V3, thereby obtaining first normalized vectors VI ’, V2’ and V3’ corresponding to Vl’=Vl/max{Vl_i}, V2’=V2/max{V2j} and V3’=V3/max{V3j}.

During the second normalization phase, each first normalized vector VI’, V2’ and V3’ is renormalized with reference to each component corresponding to the common acquisition positions, giving a first set of re-normalized vectors, i.e. (with reference to Figure 4):

W15=V17V1_5, W16=V1’/V1_6, W17=V1’/V1_7, W25=V2’/V2_5, W26=V2’/V2_6, W27=V2’/V2_7, W35=V3’/V3_5, W36=V3’/V3_6, W37=V3’/V3_7.

During the averaging phase, averages of the re-normalizations are determined, thereby obtaining vectors Wl*, W2* and W3* re- normalized to the number Q of common acquisition positions, i.e.: W1*=(W15+W16+W17)/Q, W2*=(W25+W26+W27)/Q and W3*=(W35+W36+W37)/Q.

During the concatenation phase, the single eigenvector 510 (shown in Figure 5) is determined (designated as Y in the following formula) by concatenating the components of the re-normalized vectors Wl*, W2* and W3*, wherein the components relating to the common acquisition positions (5, 6 and 7) have been further averaged, i.e.:

Y = {Wl*_l, Wl*_2, Wl*_3, Wl*_4, (Wl*_5+W2*_5+W3*_5)/Q, (Wl*_6+ W2*_6+W3*_6)/Q, (Wl*_7+W2*_7+W3*_7)/Q, W2*_8, W2*_9, W2*_10, W3*_l l, W3*_12}.

It is clear that this results in the single eigenvector 510 comprising twelve components, corresponding to all the floors of the building, i.e. the whole structure 101.

At step 380, the control unit 200 executes an output phase, wherein the single set of modal parameters, obtained at step 380, is made available to the user via the interface means 220. For example, the single set of modal parameters may comprise the single eigenvector 510, which can be represented on the display of the control unit 200.

At step 390, the control unit 200 executes a termination phase, wherein all the operations necessary for terminating the method for mechanically characterizing the structure 101 according to the present invention are carried out.

In addition, according to the present invention, said at least two acquisition positions and said at least two further acquisition positions can be determined in accordance with the interaction phase and the optimization phase.

In particular, during the interaction phase, at least one sensor 120 acquires a plurality of accelerations in accordance with a predefined measurement scheme, each acceleration of said plurality of accelerations concerning an acquisition position of the sensor 120 operationally connected to the structure 101, with reference to the system of coordinates.

For example, the predefined measurement scheme may comprise a plurality of acquisition positions of the sensor 120; such acquisition positions may, for example, be organized as a three-dimensional grid overlapping the structure 101, wherein at each node of the grid there is at least one acquisition position of the sensor 120, i.e. at one acquisition point the sensor 120 can be oriented in at least one acquisition direction that can be defined by a predefined angular value referring to the system of coordinates. Each acceleration acquired by the sensor 120 is rotated, by means of a rotation transform, so as to obtain three acceleration components relating to each axis of the system of coordinates, i.e. one rotated acceleration vector for each acceleration acquired by the sensor 120.

Subsequently, for each acceleration of said plurality of accelerations, i.e. for each rotated acceleration vector, a spectral entropy value is determined as described in the paper entitled “Time-frequency signal analysis and processing: a comprehensive reference”, 2015, by Boashash B., Academic Press. For each acquisition point, one thus obtains an averaged spectral entropy value in a vertical direction, perpendicular to the plane 102, and an averaged spectral entropy value in a horizontal direction, coplanar to the plane 102, for each acquisition direction. An optimal local direction, having a greater information content, is determined by considering a minimum horizontal average spectral entropy. A minimum spectral entropy vector 5 is thus obtained, by concatenating a vector of vertical average spectral entropies and a vector of horizontal minimum averaged spectral entropies.

Lastly, a stochastic oscillator value so is determined on the basis of the minimum spectral entropy value s, e.g. according to the following formula: thereby obtaining a plurality of stochastic oscillator values so, said plurality of stochastic oscillator values being sorted in decreasing order, and wherein a maximum number of acquisition positions is selected in accordance with the order of said stochastic oscillator values.

The values thus sorted, in decreasing order, define a ranking of acquisition positions with high information content, i.e. with high stochastic oscillator values so.

Next, during the optimization phase, an algorithm based on the Modal Assurance Criterion (MAC), e.g. the algorithm shown in the article entitled “An optimal sensor placement strategy for reliable expansion of mode shapes under measurement noise and modelling error”, by Jaya, M. M. et al., 2020, published in Journal of Sound and Vibration, Elsevier, 487, p.115511, or the algorithm shown in the article entitled “Performance of sensor placement strategies used in system identification based on modal expansion”, 2018, published in 9th European Workshop on Structural Health Monitoring, EWSHM, is used in order to obtain said at least two acquisition positions and said at least two further acquisition positions from said maximum number of acquisition positions.

In particular, the MAC algorithm searches, starting from said maximum number of acquisition positions, for a solution characterized by a reduced number of acquisition positions, smaller than the maximum number of acquisition positions, which will minimize the off-diagonal terms of a matrix of the MAC algorithm. In accordance with the present embodiment of the invention, the reduced number of acquisition positions is twelve, i.e. the acquisition positions described with reference to Figures la, lb, 1c, Id, 4 and 5, from which at least two acquisition positions and at least two further acquisition positions can be selected in accordance with, respectively, the first positioning scheme and at least one further positioning scheme.

Advantageously, the interaction phase permits reducing the computational complexity of the optimization phase, which, instead of analyzing a number of combinations that can be calculated with the binomial coefficient based on a number of acquisition positions of the entire set of the measurement scheme, will analyze a smaller number of combinations that can be calculated with the binomial coefficient based on the maximum number of acquisition positions, which is smaller than the number of acquisition positions of the entire set of the measurement scheme.

The advantages of the present invention are apparent from the above description.

The present invention advantageously provides a system, a method and a control unit that facilitate the mechanical characterization of a structure by using a smaller number of sensors than prior-art characterization systems.

A further advantage of the present invention lies in the fact that it provides a system, a control unit and a related method that avoid the introduction of latency times in the signals measured by sensors positioned at great mutual distances within the system, by advantageously conducting two or more measurement campaigns at different times.

Another advantage of the present invention lies in the fact that it provides a control unit, a system and a related method that permit reducing the logistic complexity concerning the placement and management of the sensors used by the mechanical characterization system, by advantageously using a smaller number of sensors and advantageously aggregating the data acquired by them.

A further advantage of the present invention lies in the fact that it provides a system, a control unit and a related method that permit reducing the costs incurred for managing the system, by using a smaller number of sensors than prior-art mechanical characterization systems.

Of course, without prejudice to the principle of the present invention, the forms of embodiment and the implementation details may be extensively varied from those described and illustrated herein merely by way of non-limiting example, without however departing from the protection scope of the present invention as set out in the appended claims.