阅读说明：本技术 用于检测飞机结构损坏的系统及相关方法 (System for detecting structural damage to an aircraft and related method ) 是由 劳伦斯·E·帕多 于 2019-07-05 设计创作，主要内容包括：一种用于检测飞机结构损坏的系统及相关方法,包括飞机结构和用于独立于温度监测飞机结构的健康状况的结构健康监测(SHM)系统。SHM系统包括致动器,该致动器结合到飞机结构并且被配置为生成具有参考振幅的参考振动信号,该参考振动信号在第一时间传播通过飞机结构,并且致动器生成具有比较振幅的比较振动信号,该比较振动信号在第一时间之后的第二时间传播通过该飞机结构。比较振幅表示由飞机结构在第一时间和第二时间之间引起的损坏。SHM系统包括结合到飞机结构的传感器,其接收参考振动信号和比较振动信号,并且还包括处理器,其被配置为计算作为参考振幅除以比较振幅的函数的增益损坏指数。(A system and associated method for detecting damage to an aircraft structure includes an aircraft structure and a Structural Health Monitoring (SHM) system for monitoring a health condition of the aircraft structure independent of temperature. The SHM system includes an actuator coupled to an aircraft structure and configured to generate a reference vibration signal having a reference amplitude that propagates through the aircraft structure at a first time, and the actuator generates a comparison vibration signal having a comparison amplitude that propagates through the aircraft structure at a second time after the first time. The comparison amplitude is indicative of damage caused by the aircraft structure between the first time and the second time. The SHM system includes a sensor coupled to the aircraft structure that receives a reference vibration signal and a comparison vibration signal, and further includes a processor configured to calculate a gain damage index as a function of the reference amplitude divided by the comparison amplitude.)

1. A system for detecting structural damage to an aircraft, the system comprising: an aircraft structure (102); and a structural health monitoring system (100) configured to monitor a structural health condition of the aircraft structure, the structural health monitoring system comprising:

an actuator (104a) coupled to the aircraft structure and configured to: generating a reference vibration signal having a reference amplitude, the reference vibration signal propagating through the aircraft structure at a first time, and generating a comparison vibration signal having a comparison amplitude, the comparison vibration signal propagating through the aircraft structure at a second time after the first time, the comparison amplitude being indicative of damage caused by the aircraft structure between the first time and the second time;

a sensor (104b, 104c) coupled to the aircraft structure and configured to receive the reference vibration signal and the comparison vibration signal; and

a processor (116) configured to calculate a gain damage index as a function of the reference amplitude divided by the comparison amplitude, the gain damage index enabling the structural health monitoring system to monitor a structural health condition of the aircraft structure independent of temperature.

2. The system of claim 1, wherein the gain damage index indicates the presence or absence of a crack in the aircraft structure (102).

3. The system as recited in claim 1, wherein the aircraft structure (102) includes a composite structure having a plurality of plies.

4. The system as recited in claim 1, wherein the actuator (104a) includes a lead zirconate titanate transducer.

5. The system of claim 1, wherein the processor (116) is further configured to remove a phase shift in the comparison vibration signal due to a temperature change between the first time and the second time.

6. The system of claim 1, wherein the processor (116) is further configured to calculate the gain impairment index as a function of the reference amplitude multiplied by the comparison amplitude divided by a square of the comparison amplitude.

7. A method (400) of detecting damage in a structure (102), the method comprising:

a step (410): retrieving a reference vibration signal of the structure from a memory, wherein the reference vibration signal was previously collected;

step (420): exciting a first transducer coupled to the structure to generate a comparative vibration signal that propagates through the structure;

step (430): receiving the comparative vibration signal at a second transducer coupled to the structure;

step (440): performing a gain damage index algorithm using the reference vibration signal and the comparison vibration signal to calculate a gain damage index as a function of the amplitude of the reference vibration signal multiplied by the amplitude of the comparison vibration signal divided by the square of the amplitude of the comparison vibration signal; and

a step (450): identifying damage in the structure when the gain damage index is positive.

8. The method (400) of claim 7, further comprising:

detecting a phase shift in the comparison vibration signal due to a change in temperature at the time the comparison vibration signal was received in step (430) relative to the temperature at the time the reference vibration signal was previously collected; and

removing the phase shift from the comparison vibration signal prior to performing the gain impairment exponent algorithm of step (440).

9. The method (400) of claim 7, further comprising:

exciting a third transducer to generate the reference vibration signal, the third transducer being bonded to the structure at the approximate location of the first transducer and prior to bonding the first transducer;

receiving the reference vibration signal at a fourth transducer, the fourth transducer being bonded to the structure at the approximate location of the second transducer and prior to bonding the second transducer; and

storing the reference vibration signal in the memory.

10. The method (400) of claim 7, wherein identifying the damage in the structure in step (450) comprises detecting a crack in the structure.

Technical Field

The field of the present disclosure relates generally to Structural Health Monitoring (SHM), and more particularly to systems and methods for temperature insensitive damage detection.

Background

At least some SHM systems detect damage in a structure, for example, by exciting a piezoelectric transducer or actuator coupled to the structure to generate a vibration signal or wave, and then receiving the vibration signal with another piezoelectric transducer coupled to the structure at a different location. The damage present in the structure in the path of the propagating wave affects the characteristics of the propagating wave, such as, for example, amplitude and phase. Thus, comparing the amplitude and phase of the received vibration signal (comparison signal) with the amplitude and phase of the previously received vibration signal (reference signal) enables detection of damage occurring in the duration between the reference signal and the comparison signal. Furthermore, this comparison enables detection of damage at a location in the structure along the path of the propagating wave.

The propagation of waves through the structure is also affected by environmental parameters, including, for example, the ambient temperature at which the SHM system operates on the structure. Generally, the amplitude of the vibration signal propagating through the structure increases at warmer temperatures and decreases at cooler temperatures. The ambient temperature may further change the phase of the propagating wave or shift it. Such environmental effects may mask damage, i.e. false negatives, in the structure, or at least interfere with its detection. In contrast, environmental influences can produce false positive damage detection.

At least some SHM systems utilize algorithms or other signal processing to quantify damage in a given structure to be tested and mitigate the effects of environmental disturbances, such as temperature disturbances. The quantification of damage is called the Damage Index (DI). Such quantification enables periodic monitoring of structural health in a given structure over time. One such algorithm for calculating DI is to calculate the Root Mean Square (RMS) of the "error" signal, or in other words, the RMS of the difference between the comparison signal and the reference signal. Various methods exist for mitigating environmental interference. For example, one approach is to collect reference signals at various temperatures and select the appropriate reference signal to compare with the comparison signal. However, this approach is time consuming and requires coolers, ovens, and other ambient temperature control devices to produce various temperatures over a range of temperatures, such as from 13 degrees fahrenheit to 120 degrees fahrenheit. Another example approach is to use a calculated correlation between the comparison signal and the reference signal that mitigates at least some amplitude interference, but may mask detection impairments. Yet another exemplary method is to measure and correct for phase shift caused by environmental interference, as can be found in U.S. patent No. 8,892,384. However, there is a need for a DI-generating SHM system that mitigates the effects of temperature disturbances while reducing false positives and false negatives.

This background section is intended to introduce the reader to various aspects of art that may be related to the present disclosure, which is described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

Disclosure of Invention

One aspect relates to a system for detecting damage in an aircraft structure. The system includes an aircraft structure and a Structure Health Monitoring (SHM) system having actuators, sensors, and a processor. The actuator is coupled to the aircraft structure and configured to generate a reference vibration signal having a reference amplitude, the reference vibration signal propagating through the aircraft structure at a first time. The actuator is further configured to generate a comparative vibration signal having a comparative amplitude, the comparative vibration signal propagating through the aircraft structure at a second time after the first time. The comparison amplitude is indicative of damage caused to the aircraft structure between the first time and the second time. The sensor is coupled to the aircraft structure and configured to receive a reference vibration signal and a comparison vibration signal. The processor is configured to calculate a gain impairment index as a function of the reference amplitude divided by the comparison amplitude. The gain damage index enables the SHM system to monitor the structural health of the aircraft structure independent of temperature.

Another aspect is a method of detecting damage in a structure. The method includes retrieving a reference vibration signal of the structure from a memory, wherein the reference vibration signal was previously collected. The method includes exciting a first transducer coupled to the structure to generate a comparative vibration signal that propagates through the structure. The method includes receiving a comparative vibration signal at a second transducer coupled to the structure. The method includes performing a gain damage index algorithm using the reference vibration signal and the comparison vibration signal to calculate a gain damage index as a function of the amplitude of the reference vibration signal multiplied by the amplitude of the comparison vibration signal divided by the square of the amplitude of the comparison vibration signal. The method includes identifying a damage in the structure when the gain damage index is positive.

Yet another aspect includes a Structural Health Monitoring (SHM) system having a plurality of transducers, Data Acquisition (DAQ) circuitry, and a processor. The plurality of transducers are configured to be distributed over a region of the structure and bonded to the structure. The plurality of transducers includes a first transmitting transducer and at least one sensor transducer. The DAQ circuit is coupled to the plurality of transducers and configured to actuate the first transmit transducer to generate a reference vibration signal that propagates through the structure at a first time. The DAQ circuit is configured to receive a reference vibration signal via the at least one sensor transducer. The DAQ circuit is configured to actuate the first transmitting transducer to generate a comparative vibration signal that propagates through the structure at a second time after the first time. The DAQ circuit is configured to receive a comparative vibration signal via the at least one sensor transducer. The processor is configured to calculate a respective gain impairment index for the at least one receiving transducer as a function of the amplitude of the reference vibration signal divided by the amplitude of the comparison vibration signal, the gain impairment index enabling the SHM system to monitor the structural health of the aircraft structure independent of temperature.

Various refinements exist of the features noted in relation to the above-mentioned aspects. Other features may also be included in the above aspects. These refinements and additional features may exist individually or in any combination. For example, various features discussed below in relation to any of the illustrated embodiments may be incorporated into any of the above-described aspects, alone or in any combination.

Drawings

FIG. 1 is a schematic block diagram of an exemplary SHM system;

FIG. 2A is a graph of an example reference vibration signal from an SHM system;

FIG. 2B is a graph of an example comparative vibration signal from an SHM system;

FIG. 2C is a graph of example error signals from the reference vibration signal and the comparison vibration signal shown in FIGS. 2A and 2B, respectively;

FIGS. 3A and 3B are a series of graphs of an example reference vibration signal and an example comparative vibration signal collected at various temperatures; and

FIG. 4 is a flow diagram of an example method of detecting a damage in a structure.

Although specific features of various embodiments may be shown in some drawings and not in others, this is for convenience only. Any feature of any drawing may be referenced and/or claimed in combination with any feature of any other drawing.

Unless otherwise indicated, the drawings provided herein are intended to illustrate features of embodiments of the present disclosure. These features are believed to be applicable to a variety of systems that include one or more embodiments of the present disclosure. Accordingly, the drawings are not intended to include all of the conventional features known to those of ordinary skill in the art for practicing the embodiments disclosed herein.

Detailed Description

Embodiments of the described system include a Structural Health Monitoring (SHM) system that calculates a gain Damage Index (DI) that is capable of detecting damage in a structure, such as, for example, an aircraft structure, independent of temperature. The SHM system described herein includes a plurality of transducers (transducers) distributed over a region of a structure and bonded to a surface of the structure. Each transducer is configured to be excited to generate a vibration signal that propagates through the structure and is then received at each other transducer of the plurality of transducers. When excited, the transducer is referred to as an actuator or transmitter. Thus, the other transducers of the plurality of transducers are referred to as sensors or receivers. The actuator transducer first generates a reference vibration signal that propagates through the structure and is received by the at least one sensor transducer, collected by a Data Acquisition (DAQ) circuit, and stored in a memory. The actuator transducer then later generates a comparative vibration signal that propagates through the structure and is received by the sensor transducer and collected by the DAQ circuit. The processor then calculates a gain DI as a function of the amplitude of the reference vibration signal divided by the amplitude of the comparison vibration signal. More specifically, the processor calculates respective gains DI for the plurality of sensor transducers, each gain DI calculated as a function of the amplitude of the reference vibration signal multiplied by the amplitude of the comparison vibration signal divided by the square of the amplitude of the comparison vibration signal. When the gain DI is positive, damage occurring within a duration between a time of generating the reference vibration signal and a time of generating the comparison vibration signal is identified. Positive gain generally refers to a gain greater than 1. In some embodiments, the gain DI is normalized to zero by subtracting 1 from the gain value, such that "positive gain" instead refers to a gain value greater than zero. Each gain DI value represents a quantification of the state of health of a structure (e.g., a metal structure or a composite structure) along a path between an actuator transducer and a sensor transducer for that gain DI value. Thus, the quantification enables periodic monitoring of the structural health of the structure over time.

Fig. 1 is a schematic diagram of an exemplary SHM system 100 for detecting damage (such as cracks or delamination) in a structure 102. The SHM system 100 includes a plurality of transducers 104 bonded to a surface of the structure 102. The transducers 104 are distributed over an area of the structure 102 and are arranged, for example, in an array 106 or any other suitable arrangement for testing the structure 102. For example, for aircraft structures, the transducers 104 may be arranged such that they are concentrated on high stress areas of a given aircraft structure, which may be constructed of, for example, composite or metallic materials, or laminations, with multiple layers. The transducer 104 may comprise a ceramic piezoelectric transducer, such as, for example, a lead zirconate titanate (PZT) transducer. When a voltage is applied to one of the transducers 104 coupled to the structure 102, the transducer is actuated and generates a vibration or vibration signal that propagates through the structure 102. The resulting waveform exhibits some amplitude, phase and frequency. In the embodiments of the SHM system 100 described herein, the transducer 104 typically generates a vibration signal having a frequency in the range of about 200 kilohertz to about 450 kilohertz. However, in alternative embodiments, transducers 104 that generate vibration signals outside of this range may be utilized, and these transducers 104 are within the scope of the present disclosure.

Transducer 104 is coupled to DAQ circuit 108 by a wiring harness 110. The DAQ circuit 108 may include a plurality of analog input/output channels for applying or measuring potentials on the transducer 104. For example, the measured potential may be converted to a digital value by one or more analog-to-digital converters within the DAQ circuit 108. The DAQ circuit 108 controls the excitation of the transducer 104 to generate a vibration signal and receives the vibration signal through the sensor transducer 104. More specifically, for example, the DAQ circuit 108 selects the actuator transducer 104a to operate as a transmitter while the other transducers 104 operate as receivers, or as sensor transducers 104b and 104c, for example. The DAQ circuit 108 applies a voltage to the actuator transducer 104a that causes a reference vibration signal to be generated that propagates through the structure 102 along, for example, paths 112 and 114 leading to the sensor transducers 104b and 104c, respectively. The sensor transducers 104b and 104c receive the reference vibration signal, thereby generating an electrical potential or voltage across the terminals of the device. The voltage present over time on a given sensor transducer represents a reference vibration signal for that sensor transducer, and more specifically, a reference vibration signal representing a corresponding path between the actuator transducer and the sensor transducer, such as, for example, paths 112 and 114 between actuator transducer 104a and sensor transducers 104b and 104c, respectively.

SHM system 100 includes a processor 116 and a memory 118. The processor 116 executes a series of computer-executable instructions, program code, or software stored in a memory, such as the memory 118. By executing such program code, the processor 116 is configured to perform various steps of calculating the gain DI. For example, upon execution of program code stored in the memory 118, the processor 116 is configured to receive a reference vibration signal from the DAQ circuit 108. The processor 116 is also configured to write the reference vibration signal to the memory 118. In some embodiments, the processor 116 may process the reference vibration signal before writing to the memory 118. In an alternative embodiment, the reference vibration signal may be written directly from the DAQ circuit 108 to the memory 118.

The DAQ circuit 108 controls the transducer 104 such that a reference vibration signal is generated and collected at a first time, for example, when the structure is new or otherwise known to be undamaged. The DAQ circuit 108 controls the transducer 104 to generate a comparative vibration signal at a second time after the first time. The SHM system 100 may be periodically utilized to test the structural health of the structure 102. For example, the duration between collecting the reference vibration signal and comparing the vibration signal may be one or more years of operation. Alternatively, the duration may be much shorter, for example, on the order of days, weeks, or months. The SHM system 100 is able to detect damage independent of the duration between collections or "scans". This detection is limited to the duration between the collection of the reference vibration signal and the comparison vibration signal. The duration is typically selected based on expected wear and fatigue in the life cycle of the structure 102. In certain embodiments, the transducer 104 remains in place, bonded to the structure 102, for the duration between collections. In other embodiments, the transducer 104 is bonded to the structure 102 for collecting the reference vibration signal, and is thereafter removed for normal operation or use of the structure 102. The sensor 104 is then coupled to the structure 102 again at a later time to collect a comparative vibration signal. Typically, the transducers 104 should be placed in approximately the same location for collecting the reference vibration signal and comparing the vibration signals.

Fig. 2A is a graph of an example reference vibration signal 202 from a SHM system (such as SHM system 100 shown in fig. 1) and a given structure under test. The reference vibration signal 202 is plotted as an amplitude 204 over time 206. The amplitude 204 is expressed in volts and the reference vibration signal 202 ranges from about 1 volt to about-1 volt for a duration of about 25 microseconds. The reference vibration signal 202 has a frequency of about 200 kilohertz (or about 5 microsecond period).

Referring again to fig. 1, to collect the comparative vibration signals, the DAQ circuit 108 selects the actuator transducer 104a to operate as a transmitter while the other transducers 104 operate as receivers, or as sensor transducers 104b and 104c, for example. The DAQ circuit 108 applies a voltage to the actuator transducer 104a that causes a comparative vibration signal to be generated that propagates through the structure 102 to the sensor transducers 104b and 104c along, for example, paths 112 and 114, respectively. The sensor transducers 104b and 104c receive the comparative vibration signal, thereby generating an electrical potential or voltage across the terminals of the device. The voltage present over time on a given sensor transducer represents a comparative vibration signal of that sensor transducer, and more particularly, a comparative vibration signal representing a corresponding path between the actuator transducer and that sensor transducer, such as, for example, paths 112 and 114 between actuator transducer 104a and sensor transducers 104b and 104c, respectively.

Fig. 2B is a graph of an example comparative vibration signal 208 from a SHM system (such as SHM system 100 shown in fig. 1) and a given structure under test. The comparative vibration signal 208 is plotted as the amplitude 204 over time 206. The amplitude 204 is expressed in volts and the comparative vibration signal 208 ranges from about 1.1 volts to about-1.1 volts for a duration of about 25 microseconds. The comparative vibration signal 208 has a frequency of about 200 kilohertz (or about 5 microsecond period). Note that the amplitude and phase of the comparison vibration signal 208 are distorted relative to the reference vibration signal 202 shown in fig. 2A.

Referring again to FIG. 1, the damage that occurs within the structure 102 during the duration between collections is represented in the comparison vibration signal as a change in the amplitude and phase of the comparison vibration signal relative to the corresponding reference vibration signal. Furthermore, variations in amplitude and phase may also be caused by differences in ambient temperature when the reference vibration signal is collected and when the comparison vibration signal is collected.

Fig. 2C is a graph of an example error signal 210 from the reference vibration signal 202 and the comparison vibration signal 208 shown in fig. 2A and 2B, respectively. The error signal 210 is plotted as the amplitude 204 over time 206. The amplitude 204 is expressed in volts and the error signal 210 ranges from about 0.25 volts to about-0.25 volts for a duration of about 25 microseconds. Error signal 210 represents a coarse DI, where a larger amplitude generally represents greater damage. It is noted that the error signal 210 does not take into account, for example, temperature variations between the collected reference vibration signal 202 and the comparison vibration signal 208.

Referring again to fig. 1, for each path between transducers 104 (e.g., paths 112 and 114), processor 116 retrieves a reference vibration signal from memory 118 and receives a corresponding comparison vibration signal from DAQ circuit 108. The processor 116 is configured to calculate the gain DI as a function of the amplitude of the reference vibration signal (reference amplitude) divided by the amplitude of the comparison vibration signal (comparison amplitude). The gain DI represents the "gain" required to match the amplitude of the comparison vibration signal to the amplitude of the reference signal. Gains greater than 1 are referred to as "positive" gains in terms of the typical "gain" sign, since applying such gains increases the amplitude of the resulting signal. Also, gains less than 1 are referred to as "negative" gains, as applying such gains reduces the amplitude of the resulting signal. Under this notation, a "positive" gain DI indicates that the comparison amplitude is less than the reference amplitude, and also indicates that damage occurred in the structure 102 along the path between the transducers (e.g., path 112 or 114). In general, as a wave propagates through structure 102, the damage suppresses the amplitude of the wave. Also, the temperature of the collected comparison vibration signal affects the amplitude and phase of the comparison vibration signal as compared to the temperature of the collected reference vibration signal.

Fig. 3A and 3B are a series of graphs 302, 304, 306, 308, 310, and 312 of example reference signals and example comparison signals collected at various temperatures using a SHM system (such as SHM system 100 shown in fig. 1) and a given structure under test. It is noted that the reference signal and the comparison signal are collected almost simultaneously without any disturbing wear, fatigue or other damage to the structure to be measured. Thus, graphs 302, 304, 306, 308, 310, and 312 illustrate the effect of phase and amplitude under temperature changes between the collected reference vibration signal and the collected comparison signal, which is represented as voltage 314 over time 316. Graph 302 shows a reference vibration signal 318 collected at 64 degrees Fahrenheit (F) or about room temperature, and a comparative vibration signal 320 collected at-9 degrees Fahrenheit. The cooler temperature of the collected comparative vibration signal 320 reflects at least a reduction in the amplitude of the comparative vibration signal 320 relative to the amplitude of the reference vibration signal 318. The comparison vibration signal 320 also exhibits some negative phase shift when compared to the reference vibration signal 318.

Graph 304 shows a reference vibration signal 318 collected at 64 degrees fahrenheit or about room temperature, and a comparative vibration signal 322 collected at 26 degrees fahrenheit. The cooler temperature of the collected comparative vibration signal 322 reflects at least a reduction in the amplitude of the comparative vibration signal 322 relative to the amplitude of the reference vibration signal 318, albeit to a lesser degree than the comparative vibration signal 320 shown in the graph 302. The comparative vibration signal 322 also exhibits some negative phase shift when compared to the reference vibration signal 318, although again to a lesser degree than the comparative vibration signal 320 shown in the graph 302.

Graph 306 shows a reference vibration signal 318 collected at 64 degrees fahrenheit or about room temperature, and a comparative vibration signal 324 also collected at 64 degrees fahrenheit. The equal temperatures render the reference vibration signal 318 and the comparison vibration signal 324 indistinguishable in the graph 306.

Graph 308 shows a reference vibration signal 318 collected at 64 degrees fahrenheit or about room temperature, and a comparative vibration signal 326 collected at 80 degrees fahrenheit. The higher temperature of the collected comparative vibration signal 326 reflects at least an increase in the amplitude of the comparative vibration signal 326 relative to the amplitude of the reference vibration signal 318. The comparison vibration signal 326 also exhibits some small positive phase shift when compared to the reference vibration signal 318.

Graph 310 shows a reference vibration signal 318 collected at 64 degrees fahrenheit or about room temperature, and a comparative vibration signal 328 collected at 100 degrees fahrenheit. The higher temperature of the collected comparative vibration signal 328 reflects at least an increase in the amplitude of the comparative vibration signal 328 relative to the amplitude of the reference vibration signal 318, albeit to a greater degree than the comparative vibration signal 326 shown in the graph 308. The comparative vibration signal 328 also exhibits some positive phase shift when compared to the reference vibration signal 318, although again to a greater degree than the comparative vibration signal 326 shown in the graph 308.

Graph 312 shows a reference vibration signal 318 collected at 64 degrees fahrenheit or about room temperature, and a comparative vibration signal 330 collected at 120 degrees fahrenheit. The higher temperature at which the comparative vibration signal 330 is collected reflects at least an increase in the amplitude of the comparative vibration signal 330 relative to the amplitude of the reference vibration signal 318, albeit to a greater degree than the comparative vibration signals 326 and 328 shown in graphs 308 and 310, respectively. The comparative vibration signal 330 also exhibits a positive phase shift when compared to the reference vibration signal 318, although again to a greater extent than the comparative vibration signals 326 and 328 shown in graphs 308 and 310, respectively.

Referring again to fig. 1, and more specifically, the gain DI is calculated as a function of the reference amplitude multiplied by the comparison amplitude and divided by the square of the comparison amplitude (which yields the "instantaneous" gain DI). The reference and comparison vibration signals are typically collected as amplitude samples over a period of time called a time series. The gain DI of the reference time series and the comparison time series is represented by the following algorithm, each time series having N samples:

where Ref denotes a reference time series, Comp denotes a comparison time series, and n is a coefficient of the time series. At least some SHM systems use the gain calculation shown in equation 1 as a pre-processing step for calculating DI. For example, some SHM systems utilize Mean Square Error (MSE) DI, which utilizes gain calculations similar to those shown in equation 1 as an intermediate step, rather than DI itself. Referring again to the gain DI, in general, a "positive" gain DI indicates the presence of damage. The location of the damage in the structure follows a given path between the actuator transducer and the sensor transducer. A "negative" gain DI indicates no damage along a given path. The negative gain DI is typically associated with an increase in temperature when the comparison vibration signal is collected as compared to the temperature (e.g., room temperature) at which the reference vibration signal is collected, because damage to the structure 102 typically does not result in an increase in the amplitude of the comparison vibration signal. Conversely, positive gain DI may be associated with a decrease in temperature of the collected comparative vibration signal and damage occurring along the path between transducers 104. However, the colder temperature affects the amplitude of the comparative vibration signal to a lesser extent than the damage, so that effects can be distinguished. Thus, the gain DI is more capable of more positively detecting the presence of damage, such as cracks or delamination, in the structure under test. In contrast, DI, such as MSE and correlation coefficient, does not consider the direction or sign of the amplitude difference between the reference vibration signal and the comparison vibration signal.

To make the gain DI evaluation more intuitive, in some embodiments, the gain DI is normalized to zero:

gain DI ═ Gain' -1 equation 2

Thus, a "positive" gain DI corresponds to a gain value greater than zero, while a "negative" gain DI corresponds to a gain value less than zero.

In certain embodiments, the processor 116 is further configured to measure and remove a phase shift in the comparison vibration signal due to a temperature change between the time the reference vibration signal is collected and the time the comparison vibration signal is collected. This phase shift correction enables a timely and accurate comparison of the reference vibration signal and the comparison vibration signal. For example, when the comparison vibration signal is out of phase with the reference vibration signal, the amplitude of a given peak in the comparison vibration signal will deviate in time from the corresponding peak in the reference vibration signal. Generally, as described in U.S. patent No. 8,892,384, temperature produces a progressively larger phase shift over time, where the phase shift grows or "stretches" linearly over time, particularly in homogeneous structures such as metals. Heterogeneous structures, such as composite materials, tend to produce non-linear increases in phase shift over time. This stretching is sometimes referred to as phase shift as measured by time delay. Damage tends to produce a phase shift that grows non-linearly over time. Thus, the phase shift in the comparison vibration signal caused by temperature is approximated and corrected before the processor 116 calculates the gain DI according to equation 1 above.

More specifically, to correct for phase shifts in the comparison vibration signal due to temperature, the processor 116 divides or otherwise samples the comparison vibration signal and the reference vibration signal over multiple time windows. The processor 116 then calculates a cross-correlation between the comparison vibration signal and the reference vibration signal for each time window. The cross-correlation is determined by calculating the amount of time shift required to maximally correlate the comparison vibration signal with the reference vibration signal within a given time window. A weighted regression is then performed over time to estimate the time delay, wherein the weights are based on the relative amount of signal energy from the reference vibration signal within each time window. Weighted regression using time-windowed energy as a weighting function maximizes the effectiveness of phase shift compensation by focusing on the most important parts of the signal. Weighted regression yields the relationship between time and time shift as a quadratic or higher order equation. This relationship then enables the processor 116 to correct for the phase shift in the comparison vibration signal.

Fig. 4 is a flow diagram of an example method 400 for detecting a damage in a structure using a SHM system (such as the SHM system 100 shown in fig. 1). At a first time, the SHM system 100 excites, for example, the actuator transducer 104a to generate a reference vibration signal that propagates through the structure 102 and is received, for example, at the sensor transducers 104b and 104 c. The collected reference vibration signals are then stored in the memory 118 until a subsequent test with the SHM system 100 is required. The previously collected reference vibration signal is retrieved 410 from the memory. The actuator transducer 104a is again excited 420 to generate a comparative vibration signal that propagates through the structure 102. The comparative vibration signal is then received 430 at the sensor transducers 104b and 104 c. The processor 116 receives the reference and comparison vibration signals for each path 112 and 114 and performs 440 a gain DI algorithm (such as the algorithm shown in equation 1) using the corresponding pair of reference and comparison vibration signals to calculate the gain DI. When the corresponding gain DI for that path is positive, damage along, for example, paths 112 and 114 is identified 450.

Example technical effects of the methods, systems, and apparatus described herein include at least one of: (a) mitigating the effects of temperature on the amplitude and phase of the comparative vibration signal relative to the reference vibration signal; (b) detecting damage in the structure to be tested independent of temperature variations between collecting the reference vibration signal and collecting the comparison vibration signal; (c) improving false positive and false negative rates of damage detection in structures such as metals and composites; and (d) reducing the time required to collect the reference vibration signal by eliminating the need to collect the reference vibration signal at more than one temperature.

Further, the present disclosure includes embodiments according to the following clauses:

clause 1: a system for detecting structural damage to an aircraft, the system comprising: an aircraft structure; and a Structural Health Monitoring (SHM) system configured to monitor structural health of an aircraft structure, the SHM system including: an actuator coupled to the aircraft structure and configured to: generating a reference vibration signal having a reference amplitude and propagating through the aircraft structure at a first time, and generating a comparative vibration signal having a comparative amplitude and propagating through the aircraft structure at a second time after the first time, the comparative amplitude being indicative of damage caused by the aircraft structure between the first time and the second time; a sensor coupled to the aircraft structure and configured to receive the reference vibration signal and the comparison vibration signal; and a processor configured to calculate a gain damage index as a function of a reference amplitude divided by a comparison amplitude, the gain damage index enabling the SHM system to monitor a structural health of the aircraft structure independent of temperature.

Clause 2: the system of clause 1, wherein the gain damage index indicates the presence or absence of a crack in the aircraft structure.

Clause 3: the system of clause 1, wherein the aircraft structure comprises a composite structure having a plurality of plies.

Clause 4: the system of clause 1, wherein the actuator comprises a lead zirconate titanate (PZT) transducer.

Clause 5: the system of clause 1, wherein the processor is further configured to: removing a phase shift in the comparison vibration signal due to a temperature change between the first time and the second time.

Clause 6: the system of clause 1, wherein the processor is further configured to calculate the gain damage index as a function of the reference amplitude multiplied by the comparison amplitude divided by the square of the comparison amplitude.

Clause 7: the system of clause 6, wherein the positive gain damage index indicates that damage is present.

Clause 8: a method of detecting damage in a structure, the method comprising: retrieving a reference vibration signal of the structure from a memory, wherein the reference vibration signal was previously collected; exciting a first transducer coupled to the structure to generate a comparative vibration signal that propagates through the structure; receiving the comparative vibration signal at a second transducer coupled to the structure; performing a gain damage index algorithm using the reference vibration signal and the comparison vibration signal to calculate a gain damage index as a function of the amplitude of the reference vibration signal multiplied by the amplitude of the comparison vibration signal divided by the square of the amplitude of the comparison vibration signal; identifying damage in the structure when the gain damage index is positive.

Clause 9: the method of clause 8, further comprising: detecting a phase shift in the comparison vibration signal due to a change in temperature at the time the comparison vibration signal is received relative to a temperature at the time the reference vibration signal was previously collected; and removing the phase shift from the comparison vibration signal prior to performing the gain damage index algorithm.

Clause 10: the method of clause 9, wherein detecting and removing the phase shift comprises: dividing the comparison vibration signal and the reference vibration signal over a plurality of time windows; calculating a cross-correlation between the comparison vibration signal and the reference vibration signal within each of a plurality of time windows by calculating an amount of time shift required to maximally correlate the comparison vibration signal and the reference vibration signal; performing a weighted regression to estimate the time shift as a function of time; and correcting a phase shift in the comparison vibration signal as a function of time.

Clause 11: the method of clause 8, further comprising: exciting a third transducer to generate the reference vibration signal, the third transducer being bonded to the structure at the approximate location of the first transducer and prior to bonding the first transducer; receiving the reference vibration signal at a fourth transducer coupled to the structure at an approximate location of the second transducer and prior to coupling the second transducer; and storing the reference vibration signal in the memory.

Clause 12: the method of clause 11, wherein a first temperature at which the fourth transducer receives the reference vibration signal differs from a second temperature at which the second transducer receives the comparison vibration signal by at least 20 degrees fahrenheit.

Clause 13: the method of clause 8, wherein identifying the damage in the structure comprises detecting a crack in the structure.

Clause 14: a Structural Health Monitoring (SHM) system, comprising: a plurality of transducers configured to be distributed over a region of a structure and to be bonded to the structure, the plurality of transducers including a first transmitting transducer and at least one sensor transducer; a Data Acquisition (DAQ) circuit coupled to the plurality of transducers and configured to: actuating the first transmitting transducer to generate a reference vibration signal that propagates through the structure at a first time; receiving the reference vibration signal via the at least one sensor transducer; actuating the first transmitting transducer to generate a comparative vibration signal that propagates through the structure at a second time after the first time; and receiving the comparative vibration signal via the at least one sensor transducer; and a processor configured to calculate a respective gain damage index for the at least one sensor transducer as a function of the amplitude of the reference vibration signal divided by the amplitude of the comparison vibration signal, the respective gain damage index enabling the SHM system to monitor a structural health of the structure independent of temperature.

Clause 15: the SHM system of clause 14, wherein the processor is further configured to: performing a gain damage index algorithm using the reference vibration signal and the comparison vibration signal to calculate a respective gain damage index for the at least one sensor transducer as a function of the amplitude of the reference vibration signal multiplied by the amplitude of the comparison vibration signal divided by the square of the amplitude of the comparison vibration signal; and identifying damage in the structure when at least one corresponding gain damage index is positive.

Clause 16: the SHM system of clause 14, wherein the DAQ circuit includes a plurality of analog input/output channels corresponding to the plurality of transducers and includes an analog-to-digital converter.

Clause 17: the SHM system of clause 14, wherein the processor is further configured to: detecting a phase shift in the comparison vibration signal due to a change in temperature at the time the comparison vibration signal is received relative to a temperature at the time the reference vibration signal was previously collected; and removing the phase shift from the comparison vibration signal prior to calculating the corresponding gain impairment index.

Clause 18: the SHM system of clause 17, wherein the processor is further configured to detect a phase shift in linear progression between the reference vibration signal and the comparison vibration signal.

Clause 19: the SHM system of clause 14, wherein the processor is further configured to: identifying an amplitude difference of the comparison vibration signal relative to the reference vibration signal due to a change in temperature at the time the comparison vibration signal was received relative to the temperature at the time the reference vibration signal was previously collected when at least one of the respective gain impairment indices is negative.

Clause 20: the SHM system of clause 14, wherein the processor is further configured to identify a location of a damage in the structure based on the corresponding gain damage index being positive.

Some embodiments involve the use of one or more electronic processing or computing devices. As used herein, the terms "processor" and "computer" and related terms (e.g., "processing device," "computing device," and "controller") are not limited to just those integrated circuits referred to in the art as a computer, but broadly refer to a processor, a processing device, a controller, a general purpose Central Processing Unit (CPU), a Graphics Processing Unit (GPU), a microcontroller, a microcomputer, a Programmable Logic Controller (PLC), a Reduced Instruction Set Computer (RISC) processor, a Field Programmable Gate Array (FPGA), a Digital Signal Processing (DSP) device, an Application Specific Integrated Circuit (ASIC), and other programmable circuits or processing devices capable of performing the functions described herein, and these terms are used interchangeably herein. The above are examples only and are thus not intended to limit in any way the definition or meaning of the terms processor, processing device and related terms.

In the embodiments described herein, memory may include, but is not limited to, non-transitory computer-readable media such as flash memory, Random Access Memory (RAM), Read Only Memory (ROM), Erasable Programmable Read Only Memory (EPROM), Electrically Erasable Programmable Read Only Memory (EEPROM), and non-volatile RAM (nvram). As used herein, the term "non-transitory computer readable medium" is intended to mean any tangible computer readable medium, including, but not limited to, non-transitory computer storage devices, including, but not limited to, volatile and non-volatile media, and removable and non-removable media, such as firmware, physical and virtual storage, CD-ROMs, DVDs, and any other digital source, such as a network or the internet, and digital means not yet developed, with the sole exception being a transitory, propagating signal. Alternatively, a floppy disk, a compact disk-read only memory (CD-ROM), a magneto-optical disk (MOD), a Digital Versatile Disk (DVD), or any other computer-based device, such as computer-readable instructions, data structures, program modules and sub-modules, or other data, implemented in any method or technology for short-and long-term storage of information may also be used. Thus, the methods described herein may be encoded as executable instructions, e.g., "software" and "firmware," embodied in a non-transitory computer-readable medium. Further, as used herein, the terms "software" and "firmware" are interchangeable, and include any computer program stored in memory for execution by a personal computer, workstation, client and server. When executed by a processor, the instructions cause the processor to perform at least a portion of the methods described herein.

Also, in the embodiments described herein, additional input channels may be, but are not limited to, computer peripherals associated with operator interfaces such as a mouse and a keyboard. Alternatively, other computer peripherals may also be used, which may include, for example, but are not limited to, a scanner. Further, in the embodiments described herein, additional output channels may include, but are not limited to, an operator interface monitor.

The systems and methods described herein are not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein.

Although specific features of various embodiments of the disclosure may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the present disclosure, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing.

As used herein, an element or step recited in the singular and proceeded with the word "a" or "an" should be understood as not excluding plural elements or steps, unless such exclusion is explicitly recited. Furthermore, references to "one embodiment" or "an example embodiment" of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.

This written description uses examples to disclose various embodiments, including the best mode, to enable any person skilled in the art to practice those embodiments, including making and using any devices or systems and performing any incorporated methods. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.