Method and system for structural health monitoring with frequency synchronization

Abstract

Structural health monitoring (SHM) methods, apparatus and techniques involve building deformation fields maps (amplitude and phase related to excitation) on the surface of the structural component under monitoring based on a network of strain measurements by fiber Bragg grating sensors.

Claims

1. A method for monitoring the structural health (SHM) of a structural component of a type that has plural fiber Bragg grating (FBG) sensors distributed in a network of FBG sensors attached to a surface of the structural component, the method comprising: a) with at least one actuator, exciting the structural component using CW (continuous waves) across a predetermined frequency range to generate a dynamic deformation field thereby imposing sinusoidally defined variation at the same frequency of a potential difference on plural fiber Bragg grating (FBG) sensors distributed in the network of FBG sensors attached to the surface of the structural component; b) sensing a deformation field using the plural fiber Bragg grating (FBG) sensors distributed in the network of FBG sensors attached to the surface of the structural component, including synchronizing actuation of said at least one actuator with sensing using the plural fiber Bragg grating sensors to obtain in-phase and out-of-phase measurement resolution for strain measurements using the plural fiber Bragg gratings; c) filtering the sensed deformation field to select only that portion of the sensed deformation field associated with the sinusoidal actuation generated by the at least one actuator; d) in response at least in part to the filtered sensed deformation field, generating a two-dimensional deformation field map indicating the amplitudes and phases of surface strains, said deformation field map being based on the amplitude and phase sensed by the plural sensors; e) repeating steps (a)-(d) for multiple excitation frequencies of the at least one actuator to provide additional corresponding two-dimensional deformation field map(s); f) comparing the two-dimensional deformation field maps obtained by the different excitation frequencies to detect structural damage; and g) performing computational analysis of the two-dimensional field deformation field maps with the aid of pattern recognition to identify structural damage.

2. The method according to claim 1, wherein the frequency of actuation is different based on frequency ranges associated with primary and secondary loads as well as based on temperature variations in the said structural component.

3. The method according to claim 1, wherein said filtering excludes not only the amplitude but also phase, and selects only a specific frequency of a sinusoidal signal used in the feeding of the at least one actuator.

4. The method according to claim 1, wherein the method further includes obtaining a baseline two-dimensional deformation field map for comparison when the structural component under monitoring is free from defects or bears known defects.

5. The method according to claim 1, wherein the method is substantially unaffected by changes in structural component temperature.

6. A system for the structural health monitoring (SHM) of a structural component, comprising: a computer configured to (i) control inspections by executing software; and (ii) analyze deformation signals to obtain deformation field maps and compare the deformation field maps with a reference map, for detecting, locating and quantifying damage in the structural component or monitoring growth of some previously detected damage; a set of at least two Bragg gratings written along at least one optical fiber, the Bragg gratings forming sensors, the sensors being longitudinally positioned on the said structural component under monitoring, the Bragg gratings being attached to a surface of the structural component and configured to effect monitoring, and to provide strain measurements; a set of piezoelectric actuators comprising at least one actuator, attached to the surface of the structural component or embedded in the structure of the structural component and fed by a CW signal; a tunable laser used as a narrow band light source for interrogating the optical fiber sensors, the laser being configured to sweep a wide band of wavelengths to interrogate the Bragg grating sensors installed in the component under monitoring; an optical circulator providing at least first and second outputs, the first output sending a light signal emitted by the tunable laser towards the Bragg grating sensors, the second output sending the signal reflected by the sensors to a photo detector; a lock-in amplifier configured to perform a double function: (i) to use its own reference signal to provide a harmonic signal for excitations of the set of piezoelectric actuators; and (ii) to recover the amplitude and phase of the sinusoidal strain signals, at the same frequency component, produced by the harmonically excited piezoelectric actuators; a power amplifier configured to increase the excitation signal provided by the lock-in amplifier; a multiplexer configured to control the distribution of the excitation harmonic signal (phase and amplitude) by the piezoelectric actuators; the photo detector configured to detect the light signal reflected by the optical fiber sensors, turning the detected light signal into an electrical signal and conveying the signal to the lock-in amplifier; and an optical fiber multiplexer structured to access more than one optical fiber in the case the sensors are distributed on more than one optical fiber.

7. A system for monitoring the structural health (SHM) of a structural component of a type that has plural fiber Bragg grating (FBG) sensors distributed in a network of FBG sensors attached to a surface of the structural component, the system comprising: at least one actuator coupled to the structural component, the actuator exciting the structural component using CW (continuous waves) across a predetermined frequency range to generate a dynamic deformation field thereby imposing sinusoidally defined variation at the same frequency of a potential difference on the plural fiber Bragg grating (FBG) sensors distributed in the network of FBG sensors attached to the surface of the structural component; a detector that is coupled to the plural fiber Bragg grating sensors, the detector being configured to detect a deformation field using the plural fiber Bragg grating sensors in a manner that synchronizes detection by said plural fiber Bragg grating sensors with actuation of said at least one actuator to obtain in-phase and out-of-phase measurement resolution for strain measurements using the plural fiber Bragg gratings; a filter configured to filter the detected deformation field to select only that portion of the detected deformation field associated with the sinusoidal actuation generated by the at least one actuator; a map generator configured to generate, in response at least in part to the filtered detected deformation field, a two-dimensional deformation field map indicating the amplitudes and phases of surface strains, said two-dimensional deformation field map being based on the amplitude and phase sensed by the plural sensors; the at least one actuator, the detector, the filter and the map generator cooperating to use multiple frequencies to provide additional corresponding two-dimensional deformation field map(s); and at least one processor comparing the two-dimensional deformation field maps obtained by the different excitation frequencies to detect structural damage; the at least one processor operatively coupled to the comparator and the map generator, the at least one processor performing computational analysis of the two-dimensional field deformation maps with the aid of pattern recognition in order to identify structural damage.

8. The system according to claim 7, wherein the frequency of actuation is different based on frequency ranges associated with primary and secondary loads as well as based on temperature variations in the said structural component.

9. The system according to claim 7, wherein said filter is configured to exclude not only the amplitude but also phase, and to select only a specific frequency of a sinusoidal signal used in feeding of the at least one actuator.

10. The system according to claim 7, wherein the map generator is further configured to obtain a baseline two-dimensional deformation field map for comparison when the structural component under monitoring is free from defects or bears known defects.

11. The system according to claim 7, wherein the system is substantially unaffected by changes in structural component temperature.

12. A method for the structural health monitoring (SHM) of a structural component, comprising: using at least one computer: (i) controlling inspections by executing software; and (ii) analyzing deformation signals to obtain deformation field maps and compare the deformation field maps with a reference map, for detecting, locating and quantifying damage in the structural component or monitoring growth of some previously detected damage; effecting monitoring and providing strain measurements using a set of at least two Bragg gratings written along at least one optical fiber, the Bragg gratings forming sensors, the sensors being longitudinally positioned on the said structural component under monitoring, the Bragg gratings attached to a surface of the structural component; feeding a CW signal to a set of piezoelectric actuators comprising at least one actuator, attached to the surface of the structural component or embedded in the structure of the structural component; interrogating the optical fiber sensors using a tunable laser as a narrow band light source, including sweeping the laser across a wide band of wavelengths to interrogate all the at least two Bragg grating sensors installed in the component under monitoring; sending, from a first output of an optical circulator providing at least first and second outputs, a light signal emitted by the tunable laser towards the Bragg grating sensors, sending, from a second output of the optical circulator, a signal reflected by the sensors to a photo detector; using a lock-in amplifier to perform a double function of: (i) using its own reference signal to provide a harmonic signal for excitations of the piezoelectric actuators; and (ii) recovering the amplitude and phase of the sinusoidal strain signals, at the same frequency component, produced by the harmonically excited piezoelectric actuators; using a power amplifier to increase the excitation signal provided by the lock-in amplifier; controlling the distribution of the excitation harmonic signal (phase and amplitude) by the piezoelectric actuators with a multiplexer; using the photo detector to detect the light signal reflected by the optical fiber sensors, turn the detected light signal into an electrical signal, and convey the signal to the lock-in amplifier; and using an optical fiber multiplexer to access more than one optical fiber in the case the sensors are distributed on more than one optical fiber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The following detailed description of exemplary non-limiting illustrative embodiments is to be read in conjunction with the drawings of which:

(2) FIG. 1 is an example non-limiting system embodiment of the example non-limiting embodiment for the structural health monitoring (SHM) of an exemplary structural component.

(3) FIG. 2 is a flowchart illustrating an overall method of the example non-limiting embodiment for the structural health monitoring (SHM) of an exemplary structural component.

(4) FIG. 3 is an example plot which represents the reflection spectrum of five FBG sensors, when irradiated with a wide band optical source.

(5) FIG. 4 shows the shift of the reflection spectrum of a FBG sensor and the variation of the amplitude in the inferred signal, when a mechanical excitation on the structure of interest is caused by a PZT actuator.

(6) FIGS. 5a & 5b illustrate example three-dimensional and two-dimensional field deformation maps.

DETAILED DESCRIPTION OF EXAMPLE NON-LIMITING EMBODIMENTS

(7) According to the present example non-limiting embodiment, the expression structural damage means delamination, debonding, cracks, peeling, corrosion, wear, crushing, bearing, loss of mass and/or loss of rivet.

(8) The example non-limiting embodiment is directed to a method and a system for the structural health monitoring of composites as well as of metal substrates.

(9) As regards the vibrations designed to induce strains, the system can be operable by vibrations caused from piezoelectric actuators, shape memory alloy, etc., or external vibrations, magnetic fields, acoustic fields, etc.

(10) Example non-limiting embodiments will now be described by relation to the Figures.

(11) FIG. 1 depicts one non-limiting example non-limiting embodiment of a system (100) designed for the structural health monitoring (SHM) of an exemplary structure. According to FIG. 1, (1) is a tunable laser connected to an optical circulator (2) provided with two outputs: a first output is connected to optical fibers (8) on which are connected a number of fiber Bragg gratings or sensors (9a, 9b, 9c); and a second output is connected to a photo detector (3). The photo detector (3) is in turn linked by any known means to a lock-in amplifier (4). The tunable laser (1) and the lock-in amplifier (4) are connected to a computer or other control processor 50. The lock-in amplifier (4) is also linked to a power amplifier (5) where is connected a number of actuators (6a, 6b, 6c). There can be any number of actuators (6), but for the sake of simplicity only three actuators (6a, 6b, 6c) are represented in this figure.

(12) The actuators (6) are linearly positioned in the boundary of the extremity of structural component (7).

(13) In the system (100) depicted in FIG. 2, the FBG sensors (9) sense the amplitude and the phase of vibrational characteristics of structural component 7. These sensed amplitudes and phases encoded in laser light are selected by optical circulator 2, one light signal at a time, to photodetector 3. Photodetector 3 converts the received light signal to an analog electrical signal and provides the analog electrical signal to lock-In amplifier (4). Lock-in amplifier (4) comprises a conventional analog dual phase lock-in amplifier that measures the amplitude and phase of signals using a synchronous detection process to recover the signals. Lock-in amplifier (4) acts as a narrow-bandpass filter that removes unwanted noise while allowing through the signal that is to be measured. The frequency of the signal to be measured and thus the passband region of the filter is set by a reference signal, which is supplied by reference signal generator 52 to the lock-in amplifier along with the signal detected by the photodetector 3. The reference signal the reference signal generator 52 generates is at the same frequency as the modulation of the photodetector signal to be measured since the reference signal is also supplied to actuators 6. The lock-in amplifier (4) thus compares the frequency of the signal measured by photodetector (3) with the reference signals generated for the actuators (6). In this way the signals obtained by sensors (9) and the signals generated by the actuators (6) are synchronized. For each frequency of excitation, all the information generated by sensors (9) is stored in storage device 54 so as to be analyzed in order to evaluate the integrity of the structural component (7).

(14) The example non-limiting embodiment of system 100 comprises a set of at least two Bragg gratings 9 written along at least one optical fiber 10. The Bragg gratings 9 are attached (mechanically coupled) to the surface of the structural component 7 or embedded within the structural component, in the region where are defined to effect the monitoring, and provide strain measurements.

(15) A set of piezoelectric actuators 6 made up of at least one actuator are attached (mechanically coupled) to the surface of the structural component or embedded in its structure.

(16) A tunable laser 1 is used as a narrow band light source for the interrogation of the optical fiber sensors 9. The laser light source 1 sweeps a wide band of wave lengths so as to interrogate all the Bragg grating sensors 9 installed in the structural component 7 under monitoring. Different FBGs 9 can be written to reflect different light frequencies such that sweeping the laser light source across a band of light frequencies allows the system 100 to acquire strain characteristics sensed by disparately-located FBGs 9. The capability of an FBG 9 to measure strain is well known in the art.

(17) Optical circulator 2 is provided with two outputs: a first output sends the light signal emitted by the tunable laser 1 towards the Bragg grating sensors 9, and another output sends the signal reflected by the sensors back along the same optical fibers 10 to a photo detector 3.

(18) Lock-in amplifier 4 has a double function: (i) to use its own reference signal (from reference signal generator 52) to provide a harmonic signal for excitations of the piezoelectric actuators 6; and (ii) to recover the amplitude and phase of the senoidal (sinusoidal) strain signals, at this same frequency component, produced exclusively by the piezoelectric actuators 6 harmonically excited.

(19) Power amplifier 5 is designed to increase the excitation signal provided by the Lock-in amplifier reference signal generator 52. In the proposed non-limiting system, the piezoelectric actuators are fed by a continuous wave (CW) signal.

(20) A multiplexer 58 is used to control the distribution of the excitation harmonic signal (phase and amplitude) by the several piezoelectric actuators 6.

(21) Photo detector 3 is used to find the light signal reflected by the optical fiber sensors 9, turning it into an electrical signal conveyed to the lock-in amplifier 4.

(22) An optical fiber multiplexer 2 can be used in order to access more than one optical fiber in the case the sensors are distributed on more than one optical fiber 10.

(23) According with the example non-limiting embodiment the system for the structural health monitoring (SHM) of a structural component, a computer or other processing element 50 designed to (i) control the inspections by means of software, hardware, firmware or a combination thereof; and (ii) analyze the deformation signals so as to obtain the deformation field maps and compare them with the reference map, with detecting, locating and quantification of the damages in the structural component or the monitoring of the growth of some previously detected damage. That is, artificial intelligence-based software for pattern recognition performs automatically the comparison between deformation field's maps.

(24) FIG. 2 is a simplified flow-chart of a method according to the example non-limiting embodiment, which may be executed by processor 50 under control of software code stored in storage 54. According to this flowchart, (101) means the structural component is under monitoring. Then at (102) signal analysis is performed by means of frequency of excitation as described above. A deformation field map is generated at (103). The comparison between deformation field map generated with a deformation field map of reference or baseline is performed at (104). The damage detection (difference between the two maps) is evaluated by (105). In the absence of damage, the method returns to (101) and repeats steps (101) to (104). If any damage is detected, the method performs operations leading to the assessment of the Location of Damage at (106), followed by the assessment of the Severity of Damage at (107) and response with location and severity of damage at (108).

(25) An example non-limiting method for the structural health monitoring (SHM) of a structural component having at least an actuator and at least a sensor or a group of actuator and group of sensors may thus comprise:

(26) a) Excitation in-phase or out-of-phase with a pre-determined frequency range to excite the said structural component 7, so as to generate a dynamic deformation field, with senoidal (sinusoidal) timed variation at the same frequency of the potential difference imposed on the piezoelectric elements 6;

(27) b) filtering the measurement of the deformation field to select only the range of the deformation field associated with the senoidal (sinusoidal) actuation generated by the actuators 6;

(28) c) obtaining a two-dimensional map with the amplitudes and phases of the surface strains, where these maps are based in the amplitude and in the phase of the several sensors 9 distributed in a network of sensors 9 attached to the surface of the structural component under monitoring;

(29) d) repeating the procedure for multiple frequencies range;

(30) e) comparing the two-dimensional maps obtained by the different frequencies in the analysis for detection of structural damage; and

(31) f) computational analysis of the so-obtained two-dimensional field deformation maps with the aid of pattern recognition methods in order to identify structural damage.

(32) FIG. 3 is a plot which represents the reflection spectrum of five FBG sensors, when irradiated with a wide band optical source. In the example embodiment, the photodetector 3 converts the reflection spectrum to an electrical signal that can be analyzed in the frequency domain. While it would be possible for controller 50 to provide a digital signal processor using FFT technology, the preferred non-limiting embodiment uses a simpler technique of a synchronous lock-in amplifier to synchronously detect the frequency and phase of the detected electrical signal.

(33) FIG. 4 shows the example shift of the reflection spectrum of an example FBG sensor and variation of the amplitude in the inferred signal, when a mechanical excitation on the structure of interest is caused by a PZT actuator 6a. If the PZT actuator excitation is sinusoidal, the resulting FBG-detected signal will show a phase shift that changes sinusoidally. This phase shift can be synchronously detected by lock-in amplifier (4). These signals can be stored in storage 54 along with other signals stored for other frequencies and other sensors 6. The processor 50 executes a stored program to analyze the stored signals and generate a graphical deformation map for display on graphical display 56.

(34) FIG. 5a illustrates an example 3D deformation map. The 3D map indicates a square delamination is at the center of the plate. As can be seen, the deformation map plots deformation (U.A.) against position (x, y). In this case, the deformation is represented by strain which is defined as the amount of deformation per unit length of an object when a load is applied. Strain is calculated by dividing the total deformation of the original length by the original length (L): =L/L. Typical values for strain are less than 0.005 inch/inch and are often expressed in micro-strain units (i.e., 10.sup.6). Note the scale on the right-hand side of FIG. 5a showing Strain in units of 0-1010.sup.6. In the example embodiment, the amount of Strain can be encoded in a visible color spectrum with for example higher strains of =1010.sup.5 showing red and lower strains of =0 showing violet, and strains in between distributed along the ROYGBIV colors of the rainbow. The FIG. 5a deformation map further graphically shows the topography of strain, with lower strains having lower elevation and higher strains having higher elevations or peaks. Through such visualization, it is possible to see which parts of the structural element are under how much strain. The FIG. 5b deformation map visualization is 2D and uses color encoding as described above to allow visualization of the amount of strain. Other representations are possible.

(35) The example non-limiting embodiment will now be illustrated by the Example shown the follows.

Example

(36) By means of numerical simulations were obtained the deformation maps related to the behavior of a composite plate. The composite plate has 16 superimposed layers, which were submitted to mechanical vibration caused by PZT actuators 6.

(37) Several delamination models were tested by varying dimensions (length and width). For each tested model, 40 excitation frequencies varying from 11.1 kHz to 15.0 kHz with a 0.1 kHz step were simulated. In this way, for each delamination model, 40 deformation maps were obtained.

(38) For this test, the frequency of excitation was =13.7 kHz. The deformation map 5a shows a square delamination is at the center of the plate (coordinate x=0.250 m and coordinate y=0.125 m), between layers 4 and 5.

(39) From FIG. 5a, 5b it is clear that a deformation pattern of the order of 10 microstrain is present on the delamination region.

(40) While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.