MULTI FREQUENCY ACOUSTIC EMISSION MICROMACHINED TRANSDUCERS FOR NON-DESTRUCTIVE EVALUATION OF STRUCTURAL HEALTH

Abstract

A MEMS AE transducer system is provided that takes advantage of the low power consumption and lightweight characteristics of MEMS AE transducers, while also achieving higher sensing sensitivity. To address the problem of low sensitivity typically associated with MEMS AE transducers, electrical responses of multiple MEMS AE transducers operating at different frequency ranges are combined to increase the bandwidth and sensitivity of the MEMS AE transducer system. As the frequencies are constructive, the combined response on a single channel is the actual summation of two signals with an improved signal to noise ratio. Additionally, each frequency can be decomposed because they are well separated from each other due to the super narrowband response and high Quality factor of MEMS AE transducers.

Claims

1. A microelectromechanical systems (MEMS) acoustic emissions (AE) transducer system comprising: at least first and second MEMS AE transducers integrated together in a chip, the first and second MEMS AE transducers operating over at least first and second frequency ranges, respectively, that are separate from one another, each frequency range comprising one or more frequencies, the first and second MEMS AE transducers generating first and second electrical responses in response to receiving acoustic emissions in the first and second frequency ranges, respectively, the first and second electrical responses being output from the chip.

2. The MEMS AE transducer system of claim 1, wherein the first and second electrical responses are electrically combined into a combined electrical response on the chip and are output from the chip over a single electrical channel of the MEMS AE transducer system.

3. The MEMS AE transducer system of claim 2, wherein the first and second MEMS AE transducers are electrically coupled together in series such that the first and second electrical responses are combined into the combined electrical response prior to being output from the chip on the single electrical channel.

4. The MEMS AE transducer system of claim 2, further comprising: a data acquisition system in communication with the chip, the data acquisition system receiving the combined electrical response output on the single channel, the data acquisition system being configured to perform a signal processing algorithm that processes the combined electrical response to extract the first and second electrical responses from the combined electrical response.

5. The MEMS AE transducer system of claim 1, wherein the first and second MEMS AE transducers are tuned to the first and second frequency ranges, respectively, by fabricating the first and second MEMS AE transducers according to first and second sets of design parameters, respectively.

6. The MEMS AE transducer system of claim 5, wherein the first and second MEMS AE transducers each comprise: one or more electrically-conductive semiconductor layers comprising a first electrode; one or more metal layers comprising a second electrode; and one or more piezoelectric layers disposed in between and in contact with the first electrode and the second electrode.

7. The MEMS AE transducer system of claim 6, wherein said one or more electrically-conductive semiconductor layers comprise one or more layers of N-doped silicon and wherein said one or more piezoelectric layers comprise one or more layers of aluminum nitride.

8. The MEMS AE transducer system of claim 6, wherein at least one of the first and second MEMS AE transducers has a four-beam design, each beam being formed in said one or more electrically-conductive semiconductor layers, each beam having a first end that is coupled to a central disk-shaped portion of said one or more electrically-conductive semiconductor layers and a second end that extends away from the first end and is coupled to an outer portion of said one or more electrically-conductive semiconductor layers.

9. The MEMS AE transducer system of claim 6, wherein at least one of the first and second MEMS AE transducers has a diaphragm design, a diaphragm portion of said one or more electrically-conductive semiconductor layers comprising the first electrode of the MEMS AE transducer, said one or more layers of piezoelectric material being disposed on top of the diaphragm portion, said one or more layers of metal being disposed on top of said one or more layers of piezoelectric material to form the top electrode of the transducer on top of the diaphragm portion, said one or more layers of piezoelectric material being clamped about its circumference by the diaphragm portion of said one or more electrically-conductive semiconductor layers.

10. The MEMS AE transducer system of claim 6, wherein the first MEMS AE transducer has a four-beam design and the second MEMS AE transducer has the diaphragm design, each beam of the first MEMS AE transducer being formed in said one or more electrically-conductive semiconductor layers, each beam having a first end that is coupled to a central disk-shaped portion of said one or more electrically-conductive semiconductor layers of the first MEMS AE transducer and a second end that extends away from the first end and is coupled to an outer portion of said one or more electrically-conductive semiconductor layers of the first MEMS AE transducer, wherein a diaphragm portion of the second MEMS AE transducer is formed in said one or more electrically-conductive semiconductor layers and comprises the first electrode of the second MEMS AE transducer, said one or more layers of piezoelectric material of the second MEMS AE transducer being disposed on top of the diaphragm portion, said one or more layers of metal of the second MEMS AE transducer being disposed on top of said one or more layers of piezoelectric material of the second MEMS AE transducer to form the top electrode of the second MEMS AE transducer on top of the diaphragm portion, said one or more layers of piezoelectric material of the second MEMS AE transducer being clamped about a circumference of the said one or more layers of piezoelectric material by said one or more electrically-conductive semiconductor layers.

11. The MEMS AE transducer system of claim 2, wherein a highest frequency of the first frequency range is lower than a lowest frequency of the second frequency range, and wherein a center frequency of the second frequency range is not a multiple of a center frequency of the first frequency range to ensure that the first and second electrical responses combine constructively when combined into the combined electrical response.

12. The MEMS AE transducer system of claim 11, wherein a largest dimension of the chip is smaller than a smallest wavelength corresponding to the first and second frequency ranges.

13. A microelectromechanical systems (MEMS) acoustic emissions (AE) transducer system comprising: an array of N MEMS AE transducers integrated together in a chip, where N is a positive integer that is greater than two, at least first and second MEMS AE transducers of the array operating over at least first and second frequency ranges, respectively, that are separate from one another, each frequency range comprising one or more frequencies, the first and second MEMS AE transducers generating first and second electrical responses in response to receiving acoustic emissions in the first and second frequency ranges, respectively, the first and second electrical responses being combined into a combined electrical response and output from the chip on a single channel of the chip.

14. The MEMS AE transducer system of claim 13, wherein at least the first and second MEMS AE transducers are electrically coupled together in series such that the first and second electrical responses are combined into the combined electrical response prior to being output from the chip on the single electrical channel.

15. The MEMS AE transducer system of claim 14, wherein each MEMS AE transducer comprises: one or more electrically-conductive semiconductor layers comprising a first electrode; one or more metal layers comprising a second electrode; and one or more piezoelectric layers disposed in between and in contact with the first electrode and the second electrode.

16. The MEMS AE transducer system of claim 15, wherein said one or more electrically-conductive semiconductor layers comprise one or more layers of N-doped silicon and wherein said one or more piezoelectric layers comprise one or more layers of aluminum nitride.

17. The MEMS AE transducer system of claim 15, wherein at least one of the first and second MEMS AE transducers has a four-beam design, each beam being formed in said one or more electrically-conductive semiconductor layers, each beam having a first end that is coupled to a central disk-shaped portion of said one or more electrically-conductive semiconductor layers and a second end that extends away from the first end and is coupled to an outer portion of said one or more electrically-conductive semiconductor layers.

18. The MEMS AE transducer system of claim 17, wherein at least one of the first and second MEMS AE transducers has a diaphragm design, a diaphragm portion of said one or more electrically-conductive semiconductor layers comprising the first electrode of the MEMS AE transducer, said one or more layers of piezoelectric material being disposed on top of the diaphragm portion, said one or more layers of metal being disposed on top of said one or more layers of piezoelectric material to form the top electrode of the transducer on top of the diaphragm portion, said one or more layers of piezoelectric material being clamped about its circumference by the diaphragm portion of said one or more electrically-conductive semiconductor layers.

19. The MEMS AE transducer system of claim 15, wherein the first MEMS AE transducer has a four-beam design and the second MEMS AE transducer has the diaphragm design, each beam of the first MEMS AE transducer being formed in said one or more electrically-conductive semiconductor layers, each beam having a first end that is coupled to a central disk-shaped portion of said one or more electrically-conductive semiconductor layers of the first MEMS AE transducer and a second end that extends away from the first end and is coupled to an outer portion of said one or more electrically-conductive semiconductor layers of the first MEMS AE transducer, wherein a diaphragm portion of the second MEMS AE transducer is formed in said one or more electrically-conductive semiconductor layers and comprises the first electrode of the second MEMS AE transducer, said one or more layers of piezoelectric material of the second MEMS AE transducer being disposed on top of the diaphragm portion, said one or more layers of metal of the second MEMS AE transducer being disposed on top of said one or more layers of piezoelectric material of the second MEMS AE transducer to form the top electrode of the second MEMS AE transducer on top of the diaphragm portion, said one or more layers of piezoelectric material of the second MEMS AE transducer being clamped about a circumference of the said one or more layers of piezoelectric material by said one or more electrically-conductive semiconductor layers.

20. A method for performing non-destructive evaluation (NDE) of structural health of a structure: coupling an NDE chip package to the structure, the chip package comprising a chip having a microelectromechanical systems (MEMS) acoustic emissions (AE) transducer system comprising at least first and second MEMS AE transducers integrated together in the chip, the first and second MEMS AE transducers operating over at least first and second frequency ranges, respectively, that are separate from one another, each frequency range comprising one or more frequencies, the first and second MEMS AE transducers generating first and second electrical responses in response to receiving acoustic emissions in the first and second frequency ranges, respectively, the first and second electrical responses being combined into a combined electrical response and output from the chip package on a single channel of the chip package; with a data acquisition system in communication with the chip package, receiving the combined electrical response output on the single channel and performing a signal processing algorithm that processes the combined electrical response to extract the first and second electrical responses from the combined electrical response; and evaluating the first and second electrical responses to determine the structural health of the structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIGS. 1 and 2 are top perspective views of examples of different structures of the MEMS AE transducer.

[0038] FIG. 3 shows an example of the MEMS AE transducer system having a small footprint and comprising four transducers formed on a wafer.

[0039] FIG. 4 is a photograph of the system shown in FIG. 3 installed on a flat ceramic package and having several probes connected to the package leads.

[0040] FIGS. 5A and 5B are time domain plots of the amplitudes of the signals output from first and second MEMS AE transducers, respectively, operating at first and second frequencies or frequency ranges, respectively.

[0041] FIG. 5C is a time domain plot of the amplitude of the combined signals output from first and second transducers shown in FIGS. 5A and 5B.

[0042] FIG. 6 shows an example of a 10 mm×10 mm MEMS AE transducer chip manufactured by PiezoMUMPs that has been subdiced into four 5 mm×5 mm chips; the chips have different configurations of MEMS AE transducers on them that operate at different frequencies to allow testing different design configurations and reducing the sensor size.

[0043] FIG. 7 is a plot of frequency as a function of diameter of the diaphragm design shown in FIG. 2 that demonstrates that increasing the diameter tunes the transducer to a lower frequency, and vice versa, in a nonlinear manner.

[0044] FIG. 8 is a plot of targeted frequency as a function of diameter for the four-beam design shown in FIG. 1; each of the curves corresponds to a different width-to-length ratio, w/L for the beams.

[0045] FIG. 9 is a plot of the electrical charge produced by the transducer having the diaphragm design shown in FIG. 2 as a function of the radius, a, of the diaphragm.

[0046] FIGS. 10A-10C show first and second modes of operations for numerical models of the four-beam suspended mass design shown in FIG. 1 for different tuning frequencies.

[0047] FIGS. 11A-11C show first and second modes of operations for numerical models of the diaphragm design shown in FIG. 2 for different tuning frequencies.

[0048] FIGS. 12A and 12B show top plan and top perspective views respectively, of a MEMS AE transducer having the four-beam design shown in FIG. 1 in accordance with a representative embodiment.

[0049] FIGS. 13A and 13B show top perspective views of the MEMS AE transducer having the diaphragm design shown in FIG. 2 in accordance with a representative embodiment.

DETAILED DESCRIPTION

[0050] The present disclosure discloses a MEMS AE transducer system that takes advantage of the low power consumption and lightweight characteristics of MEMS AE transducers, while also achieving higher sensing sensitivity. To address the problem of low sensitivity typically associated with MEMS AE transducers, electrical responses of multiple MEMS AE transducers operating a different frequency ranges are combined to increase the bandwidth and sensitivity of the MEMS AE transducer system.

[0051] In the following detailed description, for purposes of explanation and not limitation, exemplary, or representative, embodiments disclosing specific details are set forth in order to provide a thorough understanding of an embodiment according to the present teachings. However, it will be apparent to one having ordinary skill in the art having the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatuses and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and apparatuses are clearly within the scope of the present teachings.

[0052] The terminology used herein is for purposes of describing particular embodiments only and is not intended to be limiting. The defined terms are in addition to the technical and scientific meanings of the defined terms as commonly understood and accepted in the technical field of the present teachings.

[0053] As used in the specification and appended claims, the terms “a,” “an,” and “the” include both singular and plural referents, unless the context clearly dictates otherwise. Thus, for example, “a device” includes one device and plural devices.

[0054] Relative terms may be used to describe the various elements' relationships to one another, as illustrated in the accompanying drawings. These relative terms are intended to encompass different orientations of the device and/or elements in addition to the orientation depicted in the drawings.

[0055] It will be understood that when an element is referred to as being “connected to” or “coupled to” or “electrically coupled to” another element, it can be directly connected or coupled, or intervening elements may be present.

[0056] Exemplary, or representative, embodiments will now be described with reference to the figures, in which like reference numerals represent like components, elements or features. It should be noted that features, elements or components in the figures are not intended to be drawn to scale, emphasis being placed instead on demonstrating inventive principles and concepts.

[0057] In the present disclosure, the electromechanical characterization of the MEMS AE transducers is presented. The independent performance of each MEMS AE transducer is compared with the case in which they are connected in series to detect simulated AE events. Taking into account size, power consumption and weight, the MEMS AE transducer system of the present disclosure has significant advantages as compared to conventional bulky AE transducer systems.

[0058] In accordance with an embodiment, the MEMS AE transducers are connected to transmit over a single channel. As will be described below, advantages of this single-channel approach as compared to a multi-channel approach include: (a) reducing costs as the more channels that are used increases the cost and complexity of the monitoring system; (b) increasing signal amplitude as the signal amplitude of an electrical response produced by a single MEMS AE transducer is typically not sufficient to bring the signal level above electronic noise; and (c) simultaneously collecting multiple electrical responses of the respective MEMS AE transducers associated with respective frequencies simplifies source characterization.

[0059] The advantages of the AE system and method disclosed herein include detecting the initiation of damage, pinpointing its location, qualitatively assessing the severity of damage, and classifying the damage mode using pattern recognition tools. Pinpointing the source location requires determining signal arrivals and wave velocity. In most structures, velocity depends on frequency. Such structures are commonly referred to as dispersive medium. A slight change in frequency can impact wave velocity and the localization result. Conventional AE systems and methods require wave velocity as input. In accordance with embodiments disclosed herein, the frequency bandwidth associated with each MEMS AE transducer is narrowed down, which improves the accuracy of selecting the correct wave velocity for each frequency.

[0060] The MEMS AE transducers disclosed herein do not require a bias voltage, in contrast to capacitive MEMS sensors, which makes the MEMS AE transducer system more attractive for field implementation. In addition, because the MEMS AE transducer system output can be over a single channel, the complexity and cost of data acquisition equipment can be reduced. Another advantage of the system is that multiple MEMS AE transducers can be coupled together to increase the bandwidth and the sensitivity of the system.

[0061] In accordance with inventive principles and concepts disclosed herein, an array of the MEMS AE transducers can be connected together and tuned to different frequencies or frequency ranges to generate data outputs that are combined into a single channel. An example of such an array is disclosed herein along with design variables that are used to tune the MEMS AE transducers to the desired frequencies or frequency ranges. In addition, an experimental implementation of the system is disclosed herein that incorporates 40 kHz and 200 kHz MEMS AE transducers.

[0062] Each MEMS AE transducer can be modeled as a mass-spring-damper system with the under-damped state. Considering a linear elastic model with the lumped mass assumption for individual mechanical resonator, the response is calculated as:

[00001]$\frac{D_{\dot{t}}\left(s\right)}{F\left(s\right)}=\frac{{\omega }_{\mathrm{ni}}^2}{k_{n\dot{t}}\left(s^2+\frac{{\omega }_{n\dot{t}}}{Q_{\dot{t}}}s+{\omega }_{\mathrm{ni}}^2\right)}$

where D is the displacement, F is the applied force, ω.sub.n is the natural frequency, k.sub.n is the elastic constant, Q is the quality factor, s is the Laplace variable and i is the number of resonators. Assuming the input signal is broadband, the total displacement output of m resonators is simply the linear summation of the individual displacement responses:

[00002]$D_{\mathrm{total}}\left(s\right)=F\left(s\right)\underset{\dot{t}=1}{\overset{m}{.Math.}}\frac{{\omega }_{n\dot{t}}^2}{k_{n\dot{t}}\left(s^2+\frac{{\omega }_{n\dot{t}}}{Q_{\dot{t}}}s+{\omega }_{n\dot{t}}^2\right)}$

The sensing mechanism is based on generating electrical charge associated with the structural deformation induced in the piezoelectric layer by vibrations in the adjacent semiconductor material layer.

[0063] FIGS. 1 and 2 are top perspective views of examples of different structures of the MEMS AE transducers 10 and 20, respectively. Piezoelectric MEMS AE transducers can be made in a variety of ways, and therefore the inventive principles and concepts are not limited to the structures or shapes shown in FIGS. 1 and 2. In FIG. 1, the transducer 10 is configured to attach to a substrate (not shown) via four cantilever arms 2-5. In FIG. 2, the transducer 20 is configured as a thin circular diaphragm clamped around its circumference. In piezoelectric Multi-user MEMS Processes (piezoMUMPs), layer thicknesses are restricted by the manufacturing process. The dimensions shown in FIGS. 1 and 2 are variable within limits of specific ranges of minimum and maximum values provided by the manufacturer. In FIGS. 1 and 2, the major parameters that can be selected to tune the transducers 10 and 20 to the targeted frequencies or frequency ranges are shown. The resonant frequency of the four-beam design of the transducer 10 is correlated to the stiffness of the clamped beams 2-5 and to the mass of the moving membrane. For the four-beam design shown in FIG. 1, the stiffness of the device is assumed to be equal to the summation of the stiffnesses of the four cantilever beams. 2-5. Each transducer 10 can be tuned to a particular frequency by varying one or more of the diameter (D), width (W) and length (L) dimensions.

[0064] As an example, MEMS AE transducers manufactured using a Piezoelectric Multi-User MEMS Process (PiezoMUMPs) provided by MEMSCAP foundry are made of silicon mass with an Aluminum Nitride (AlN) layer underneath it that is attached to the four cantilever elements 2-5. This configuration reduces the total size of the transducer 10 for operating at a lower frequency as compared to the fully clamped diaphragm design of the transducer 20 shown in FIG. 2. The transducer 20 shown in FIG. 2 is tuned to the targeted frequency or frequency range by selecting the diameter D. It should be noted, however, that the inventive principles and concepts are not limited with respect to the process that is used to manufacture the MEMS AE transducer system of with respect to the manufacturer that performs the process.

[0065] FIG. 3 shows an example of the MEMS AE transducer system 30 having a small footprint and comprising four transducers 31-34 formed on a wafer 35 (e.g., a 5 mm×5 mm silicon wafer). FIG. 4 is a photograph of the system 30 shown in FIG. 3 installed on a flat ceramic package and having several probes connected to the package leads. It should be noted that the inventive principles and concepts are not limited with respect to the number of transducers that the system comprises or with respect to the size of the system or the size of the package in which it is installed. The small footprint that the MEMS AE transducer system can have is, however, one of the advantages of the MEMS AE transducer system over other known transducer systems that are used for structural health monitoring.

[0066] As indicated above, another advantage of the MEMS AE transducer system is that the responses of multiple MEMS AE transducers operating at multiple respective frequencies can be output on a single channel. This reduces system complexity and costs and produces a signal amplitude that is above the noise floor. FIGS. 5A and 5B are time domain plots of the amplitudes of the signals output from first and second transducers operating at first and second frequencies or frequency ranges, respectively. In this example, the first and second frequencies are 200 kHz and 300 kHz, respectively. The first and second transducers can have the configurations of the transducers 10 or 20 shown in FIG. 1 or 2, respectively. FIG. 5C is a time domain plot of the amplitude of the combined signals output from the first and second transducers onto a single channel of the system. As the frequencies are constructive, the combined response on a single channel as shown in FIG. 5C is the actual summation of two signals with an improved signal to noise ratio. Additionally, each frequency can be decomposed as they are well separated from each other due to the super narrowband response and high Quality (Q) factor of the MEMS AE transducer.

[0067] PiezoMUMPs allow subdicing a 10 mm×10 mm chip into four 5 mm×5 mm chips. FIG. 6 shows an example of a 10 mm×10 mm MEMS AE transducer chip 40 manufactured by PiezoMUMPs that has been subdiced into four 5 mm×5 mm chips 41-44. The chips 41-44 have different configurations of MEMS AE transducers on them that operate at different frequencies. This allows testing different design configuration and reducing the sensor size. The layout of chip 41 is a 2-D array of ultrasonic (UT) MEMS AE transducers that are electrically interconnected operating at 1 MHz. The layout of chip 42 is four different MEMS AE transducers tuned to 40 kHz, 55 kHz, 200 kHz and 300 kHz by changing the mass diameter and the size of the transducers. The layout of chip 43 is two different MEMS AE transducers tuned to 40 kHz and 200 kHz, a 2×5 array of UT MEMS AE transducers operating at 1 MHz and a strain gauge. The layout of chip 44 is multiple different MEMS AE transducers tuned to 1 MHz and 2 MHz and two strain gauges. Thus, it can be seen that the MEMS AE transducer system can have a variety of configurations while still achieving a very small footprint.

[0068] Some of the benefits of the layouts shown in FIG. 6 include providing an incremental increase of amplitude for the transducers with the same frequency, providing a broadband response for the transducers with different frequencies, and providing single-channel acquisition.

[0069] As indicated above, the transducers can be designed as, for example, a diaphragm anchored from the circumference or as four-beam connected to the mass. The diaphragm design can be designed for a targeted frequency in accordance with the following equation:

[00003]$f=0.47\frac{t}{a^2}\sqrt{\frac{E}{\rho }}$

where f is the targeted frequency, a is the radius of the diaphragm D/2, t is the thickness of the diaphragm, E is Young's Modulus and p is density. Young's Modulus and density are controlled by the vibrating layer, which is typically silicon. FIG. 7 is a plot of frequency as a function of diameter of the diaphragm design shown in FIG. 2. As seen in FIG. 7, increasing the diameter tunes the transducer to a lower frequency, and vice versa, in a nonlinear manner.

[0070] As indicated above, for the four-beam design, the main design variables are beam length (L), the beam width (w) and the mass diameter (D). Beam length and width control stiffness (k). Mass diameter controls total mass (m). FIG. 8 is a plot of targeted frequency as a function of diameter for the four-beam design shown in FIG. 1 Each of the curves 81-85 corresponds to a different width-to-length ratio, w/L for the beams. Curve 81 corresponds to a 3/5 w/L ratio. Curve 85 corresponds to a 1/4 w/L ratio. Curve 83 corresponds to a 1/2 w/L ratio. Curve 84 corresponds to a 1/3 w/L ratio. Curve 82 corresponds to a 2/3 w/L ratio. These curves 81-85 demonstrate the manner in which the four-beam design can be tuned to different frequencies. The targeted frequency can also be expressed as:

[00004]$f=2\pi \sqrt{\frac{k}{m}}$

where k is the stiffness and m is the total mass. Thus, for the four-beam design, the beam length and width control the stiffness k and the mass diameter D controls total mass m).

[0071] The deformed shape of diaphragm can be represented by transverse displacement due to uniform loading. The charge produced by the transducer 20 shown in FIG. 2 can be expressed as:

q(r)=d.sub.13T.sub.piezo(r)

where d.sub.31 is the polarization coefficient and T.sub.piezo(r) is the force applied to the piezoelectric layer. Once the diaphragm vibrates due to external stimulus, it applies an axial force (T) to the piezoelectric layer that is converted into an electrical signal by piezoelectric polarization coefficient, d.sub.31. For the case of diaphragm that is fully anchored around its circumference of support, the axial force is compressed (negative) near the support, and tension (positive) near the middle. The transition point is called inflection point, which can be determined by an elastic displacement curve of the diaphragm. To prevent the cancellation of electrical current due to negative and positive axial forces, the piezoelectric film should be deposited between inflection points around the circumference.

[0072] FIG. 9 is a plot of the electrical charge produced by the transducer 20 as a function of the radius a of the diaphragm. There is an optimum radius, r.sub.opt, where only positive charge is produced. If the piezoelectric layer is deposited up to r.sub.opt, the influence of negative charge to the cumulative charge produced will be eliminated, as shown in FIG. 9.

[0073] This is not an issue for four-beam design shown in FIG. 1 as the entire mass is under tensile force; therefore, it is a preferred design for piezo MEMS AE transducers. FIGS. 10A-10C show the first and second modes of operations for numerical models of the four-beam suspended mass design shown in FIG. 1 for different tuning frequencies. The operational frequency of each transducer is computed using COMSOL Multiphysics software and eigenvalue analysis. The first mode generates the highest and the most uniform axial force on the mass where the piezoelectric film is deposited. Therefore, this is the mode of interest. By changing the total size of transducer from 860 um (FIG. 10A) to 300 um (FIG. 10C), the design frequency is changed from 130 kHz (FIG. 10A) to 600 kHz (FIG. 10C). In these examples, the “total size” is calculated as the distance from the far end of one of the cantilever beams to the far end of the opposite cantilever beam. It can be observed that there is a difference in the design frequency and the actual transducer frequency after manufacturing due to the influence of manufacturing variables, such as materials properties. The ratio between the actual frequency and the design frequency is identified as about 0.85. FIGS. 10A-10C demonstrate the manner in which the four-beam design can be tuned to different frequencies of operations by adjusting the aforementioned parameters, D, w and L.

[0074] FIGS. 11A-11C show the first and second modes of operations for numerical models of the diaphragm design shown in FIG. 2 for different tuning frequencies. These figures show three diaphragm designs tuned to different frequencies by changing the diaphragm diameter D. As compared to the four-beam design, the size of diaphragm should be significantly larger to tune the transducer to a lower frequency. For instance, the transducer size is 1100 um for tuning to 130 kHz using the diaphragm design shown in FIG. 11A while the size of the transducer is 860 um for the same frequency using the four-beam design shown in FIG. 10A. This means that more transducers can be connected in the same area using the four-beam design as compared to the diaphragm design.

[0075] FIGS. 12A and 12B show top plan and top perspective views, respectively, of a MEMS AE transducer having the four-beam design discussed above in accordance with a representative embodiment. The base of the MEMS AE transducer comprises one or more layer of an electrically-conductive semiconductor material 121 (e.g., N-doped silicon), the central disk-shaped portion 120 of which forms the bottom electrode of the transducer. One or more layers of piezoelectric material 122 are disposed on top of the central disk-shaped portion 120. One or more layers of metal 123 are disposed on top of the piezoelectric material 122 to form the top electrode of the transducer. Each cantilever beam 124-127 has a first, or proximal, end that is coupled to the central disk-shaped portion 120 and a second, or distal, end that extends away from the first end and is coupled to an outer portion of the semiconductor material 121. The transducer is tuned to a target frequency in the manner described above by selecting the appropriate design parameters.

[0076] FIGS. 13A and 13B show top perspective views of the MEMS AE transducer having the diaphragm design in accordance with a representative embodiment. The base of the MEMS AE transducer comprises one or more layer of an electrically-conductive semiconductor material 131 (e.g., N-doped silicon). A diaphragm portion 130 of the semiconductor material 131 forms the bottom electrode of the transducer. One or more layers of piezoelectric material 132 are disposed on top of the diaphragm portion 130. One or more layers of metal 133 are disposed on top of the piezoelectric material 132 to form the top electrode of the transducer. In accordance with this embodiment, the piezoelectric material 132 is anchored or clamped about its circumference by the semiconductor material 131 that forms the diaphragm portion 130. FIG. 13B shows the transducer after one or more layers of the semiconductor material 131 have been stripped away to define the outer diameter of the diaphragm.

[0077] It can be seen from the above discussion that the MEMS AE transducer system has several advantages over other types of sensors used in structural health monitoring. The MEMS AE transducers disclosed herein have highly narrowband responses, which have the advantages of accurate velocity selection in source localization and the ability to combine the responses into a single channel, which provides other advantages such as, for example, increased signal-to-noise ratio, reduced complexity and cost of signal acquisition circuitry, and increased bandwidth. However, combining multiple responses of multiple MEMS AE transducers on a single channel indicates that the selection of frequencies is important to preventing signal cancelling in adding their transient outputs. The selected frequencies should not be integers to prevent the signal cancellation. The foregoing discussion shows that individual frequencies can be successfully separated. The total area of the entire device should be less than the minimum wavelength to eliminate the aperture effect, which is related to the wavelength of the incoming wave causing the vibration of sensors. If total device size is larger than the wavelength of incoming wave, each transducer may respond separately, which may result in signal cancellation. The largest dimension of the MEMS AE transducer system (i.e., the largest dimension of the chip comprising the multiple transducers) should be smaller than the smallest wavelength of the incoming waves that the transducer system is tuned to sense.

[0078] It should be noted that the inventive principles and concepts have been described with reference to representative embodiments, but that the inventive principles and concepts are not limited to the representative embodiments described herein. Although the inventive principles and concepts have been illustrated and described in detail in the drawings and in the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art, from a study of the drawings, the disclosure, and the appended claims.