Patent Grant
7092539
1. Field of the Invention
The present invention relates generally to an acoustic combination responsive to an acoustic wave, and more particularly, to an acoustic combination that can be utilized as a dynamic pressure sensor and a microelectromechanical systems (MEMS)-based acoustic array that utilizes the acoustic combination.
2. Background
As aircraft noise regulations become more stringent, the need for modeling and measuring aircraft noise phenomena becomes more important. In order to intelligently design quieter aircraft, the physical mechanisms of noise generation should be understood and any theoretical or computational noise model should be experimentally validated. One validation method is the comparison of the theoretical and measured acoustic far-field pressures. However, single microphone measurements of aeroacoustic sources in wind tunnels are hampered by poor signal to noise ratios that arise from microphone wind self-noise, tunnel system drive noise, reverberation, and electromagnetic interference. In addition, a single microphone cannot distinguish pressure contributions from different source locations. The need for more precise noise source characterization and localization has driven the development of advanced sound field measurement techniques. In particular, the development and application of directional (phased) microphone arrays have been documented as a means to localize and characterize aeroacoustic sources in the presence of high background noise.
Although knowledge of the acoustic field does not uniquely define the source, localization of a source and analyses of the spatial and temporal characteristics of its far-field radiation can provide insight into noise generation mechanisms. Modern acoustic arrays used in wind tunnel studies of airframe noise are typically constructed of moderate numbers (≦100) of instrumentation grade condenser microphones, and range in aperture size from several inches to several feet. Data collection, followed by extensive post-processing has been used to implement various beamforming processes, including conventional beamforming, array shading, shear-layer corrections, adaptive methods, etc. The resulting data files can be over 500 GB in size and require up to an hour of post-processing per data set.
Greater numbers of microphones in an array can improve the ability to characterize a sound field. A greater number of microphones enhances the signal to noise ratio of an array, defined as the array gain, given (in dB) by 10*log(M), where M is the number of microphones. In addition, a large number of microphones may be used to extend the frequency range of an array. The spatial resolution of an array is related to the product kD, where k=ω/c is the acoustic wavenumber, ω is the radian frequency, c is the speed of sound, and D is the aperture size. Thus, a larger aperture is needed to improve the spatial resolution of an array, of most concern at low frequencies. In contrast, the intersensor spacing must be kept less than one-half of the smallest wavelength of interest (highest frequency) to avoid spatial aliasing. The feasibility of scaling the current technology to multiple arrays with large numbers (hundreds or thousands) of microphones is limited by the cost per channel (microphone, amplifier, data acquisition), data handling efficiency (acquisition capabilities, signal processing complexity, storage requirements), and array mobility (size, weight, cabling). In addition, experiments performed in large wind tunnels are costly and require extensive setup. Thus, an array system that provides near real-time output would be advantageous.
Thus, there is a need to develop a new acoustic array system that, among other applications, can be utilized for aeroacoustic measurement.
In accordance with the purposes of this invention, as embodied and broadly described herein, this invention, in one aspect, relates to a combination responsive to an acoustic wave that can be utilized as a dynamic pressure sensor. For example, the combination in a form of acoustic array can be utilized as a dynamic pressure sensor for aeroacoustic measurements for airframe noise mechanism identification and source localization. In one embodiment of the present invention, the combination has a substrate having a first surface and an opposite second surface, a microphone positioned on the first surface of the substrate and having an input and a first output and a second output, wherein the input receives a biased voltage, and the microphone generates an output signal responsive to the acoustic wave between the first output and the second output. The combination further has an amplifier positioned on the first surface of the substrate and having a first input and a second input and an output, wherein the first input of the amplifier is electrically coupled to the first output of the microphone and the second input of the amplifier is electrically coupled to the second output of the microphone for receiving the output signal from the microphone. The amplifier is spaced from the microphone with a separation smaller than 0.5 mm.
The substrate comprises a semiconductor material such as silicon. The microphone can be chosen from a variety of available devices. In one embodiment, the microphone comprises a piezoresistive microphone that has a semiconductor substrate having a cavity, a diaphragm covering the cavity, and at least one piezoresistor positioned on top of the diaphragm, wherein when the cavity receives the acoustic wave, the diaphragm moves in response to cause stress in the piezoresistor, and the piezoresistor generates an electric signal responsive to the acoustic wave. The piezoresistor is electrically coupled to the first and second outputs of the microphone to cause an electrical voltage as output signal from the electric signal responsive to the acoustic wave. The combination may further have at least one resistor electrically coupled to the piezoresistor. In one embodiment, the microphone comprises four piezoresistors forming a Wheatstone bridge. Each piezoresistor can be a single-crystal silicon piezoresistor, wherein the piezoresistor may be silicon-dioxide encapsulated. The diaphragm may be a silicon-nitride membrane.
The combination may further include a channel in fluid communication with the cavity and the ambient atmosphere, wherein the channel is positioned on the surface of the semiconductor substrate and terminated at one end with a vent hole in fluid communication with the ambient atmosphere.
In another aspect, the present invention relates to a method of assembling a combination responsive to an acoustic wave. In one embodiment, the method includes the steps of providing a substrate having a first surface and an opposite second surface, positioning a microphone on the first surface of the substrate, and positioning an amplifier on the first surface of the substrate spaced from the microphone with a separation smaller than 0.5 mm. The microphone has an input and a first output and a second output, and the amplifier has a first input electrically coupled to the first output of the microphone and a second input electrically coupled to the second output of the microphone and an output. The substrate comprises a silicon layer. The microphone can be a piezoresistive microphone.
In a further aspect, the present invention relates to a combination responsive to an acoustic wave. In one embodiment, the combination includes a microphone having an input and a first output and a second output, wherein the input receives a biased voltage, and the microphone generates an output signal responsive to the acoustic wave between the first output and the second output. The combination further includes an amplifier having a first input and a second input and an output, wherein the first input of the amplifier is electrically coupled to the first output of the microphone and the second input of the amplifier is electrically coupled to the second output of the microphone for receiving the output signal from the microphone. Additionally, the combination includes a first high pass filter electrically coupled between the first input of the amplifier and the first output of the microphone, and a second high pass filter electrically coupled between the second input of the amplifier and the second output of the microphone, wherein the output signal from the microphone has a DC component and an AC component, and the first and second high pass filters substantially block the DC component and allow the AC component of the output signal from the microphone to pass, thereby to allow the amplifier to generate a low impedance signal at the output.
In one embodiment, the microphone comprises four piezoresistors forming a Wheatstone bridge that has a first arm, an opposing second arm, a third arm, and an opposing fourth arm, the first arm being electrically coupled to the input of the microphone, the opposing second arm being electrically coupled to ground, the third arm being electrically coupled to the first input of the amplifier, and the opposing fourth arm being electrically coupled to the second input of the amplifier. The amplifier is a preamplifier. The first high pass filter comprises a resistor and a capacitor, the capacitor being electrically coupled in series between the third arm of the Wheatstone bridge and the first input of the amplifier, and the resistor being electrically coupled in parallel between the second arm of the Wheatstone bridge and the capacitor. Moreover, the second high pass filter comprises a resistor and a capacitor, the capacitor being electrically coupled in series between the fourth arm of the Wheatstone bridge and the second input of the amplifier, and the resistor being electrically coupled in parallel between the second arm of the Wheatstone bridge and the capacitor. The resistor of the first high pass filter and the resistor of the second high pass filter are jointly coupled to the second arm of the Wheatstone bridge. The resistor of the first high pass filter and the resistor of the second high pass filter are substantially identical to each other. For example, each of the resistor of the first high pass filter and the resistor of the second high pass filter can be a low profile surface mount resistor. The capacitor of the first high pass filter and the capacitor of the second high pass filter are substantially identical to each other. For example, each of the capacitor of the first high pass filter and the capacitor of the second high pass filter is a low profile surface mount capacitor.
In yet another aspect, the present invention relates to a combination responsive to an acoustic wave. In one embodiment, the combination includes a semiconductor substrate having a first surface and a second surface, a microphone positioned on the first surface of the semiconductor substrate and having an input and a first output and a second output, wherein the input receives a biased voltage, and the microphone generates an output signal responsive to the acoustic wave between the first output and the second output, an amplifier positioned on the first surface of the semiconductor substrate and having a first input and a second input and an output, wherein the first input of the amplifier is electrically coupled to the first output of the microphone and the second input of the amplifier is electrically coupled to the second output of the microphone for receiving the output signal from the microphone, a first capacitor and a first resistor forming a first RC pair positioned on the first surface of the semiconductor substrate and being electrically coupled between the first input of the amplifier and the first output of the microphone, and a second capacitor and a second resistor forming a second RC pair positioned on the first surface of the semiconductor substrate and being electrically coupled between the second input of the amplifier and the second output of the microphone. The output signal from the microphone has a DC component and an AC component, and the first and second RC pairs substantially block the DC component and allow the AC component of the output signal from the microphone to pass, thereby to allow the amplifier to generate a low impedance signal at the output, and wherein the amplifier is spaced from the microphone with a separation smaller than 0.5 mm.
In one embodiment, the microphone comprises four piezoresistors forming a Wheatstone bridge that has a first arm, an opposing second arm, a third arm, and an opposing fourth arm, the first arm being electrically coupled to the input of the microphone, the opposing second arm being electrically coupled to ground, the third arm being electrically coupled to the first input of the amplifier, and the opposing fourth arm being electrically coupled to the second input of the amplifier. The first capacitor is electrically coupled in series between the third arm of the Wheatstone bridge and the first input of the amplifier, and the first resistor is electrically coupled in parallel between the second arm of the Wheatstone bridge and the capacitor. Additionally, the second capacitor is electrically coupled in series between the fourth arm of the Wheatstone bridge and the second input of the amplifier, and the second resistor is electrically coupled in parallel between the second arm of the Wheatstone bridge and the capacitor. The first resistor and the second resistor are jointly coupled to the second arm of the Wheatstone bridge. The semiconductor substrate comprises a silicon layer, which can be passivated with silicon dioxide. The combination may further comprise a plurality of metal bond pads for receiving components positioned on the silicon layer, and a plurality of conductive traces for interconnecting corresponding metal bond pads.
The combination may further comprise a package positioned underneath the semiconductor substrate. In one embodiment, the package has a body for supporting the semiconductor substrate, and a plurality of pins, each pin being conductive and penetrating through the body. The body is electrically coupled to ground for electromagnetic shielding. The combination can additionally have a plurality of metal wire bonds, wherein each metal wire bond may make an additional electrical coupling to the metal bond pads and the pins. Each metal wire bond may be a gold wire bond.
In another embodiment, the combination may further have a package positioned underneath the semiconductor substrate. The package has a body for supporting the semiconductor substrate, and a peripheral portion extending away from the package body. The body is electrically coupled to ground for electromagnetic shielding. The combination further has a lid positioned above the semiconductor substrate. The lid includes a body, and a peripheral portion extending away from the body. The lid body is complementarily sized such that when the lid is positioned over the semiconductor substrate, the peripheral portion of the lid matches to and supported by the peripheral portion of the package, and the body covers the semiconductor substrate and components positioned on the semiconductor substrate. The lid may be made from a material that is substantially transparent to the acoustic wave. Additionally, the lid is electrically coupled to ground for electromagnetic shielding.
In another aspect, the present invention relates to a printed circuit board array responsive to an acoustic wave. In one embodiment, the printed circuit board array includes a printed circuit board having a first surface and an opposing second surface, a plurality of sockets distributed over the first surface of the printed circuit board, a laser diode positioned on the first surface of the printed circuit board for aiming the printed circuit board array toward to the acoustic wave, and a plurality of microphone packages, wherein each microphone package is complimentarily sized and received in a corresponding socket and contains at least one microphone responsive to the acoustic wave.
The printed circuit board array may further include at least one SMB-type coaxial cable connector positioned on the second surface of the printed circuit board. An SMB-type coaxial cable connector is a type of connector usually used for coaxial cable connections for RF (radio frequency) applications, as known to people skilled in the art. Moreover, the printed circuit board array has at least one support layer positioned above the first surface of the printed circuit board, and at least one support layer positioned below the second surface of the printed circuit board. Each of the support layers may comprise a garolite stiffening layer, which may be bonded to the printed circuit board.
In one embodiment, the printed circuit board comprises a double-sided copper clad. The printed circuit board has a center, and the plurality of microphone packages are distributed over the first surface of the printed circuit board along a plurality of concentric rings, each ring having a radius measured from the center of the printed circuit board different from the radius of any other ring. An equal or unequal number of the microphone packages are distributed over each ring, wherein each microphone package has at least one microphone responsive to the acoustic wave and at least one amplifier coupled to and spaced from the microphone with a separation smaller than 0.5 mm.
These and other aspects will become apparent from the following description of various embodiments taken in conjunction with the following drawings, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and the corresponding experimental results and together with the description, serve to explain the principals of the invention.
The present invention is more particularly described in the following examples that are intended to be illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. As used in the specification and in the claims, the singular form “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise.
Additionally, as used in the specification and in the claims, an acoustic array is a collection of spatially distributed microphones used to measure an acoustic field. The time signals from each microphone are selectively weighted and delayed through a signal processing technique known as beamforming. Beamforming provides the array with a directional response, which is electronically “steered” in space. As illustrated in
Referring generally to
Referring first to
The combination 100 further has an amplifier 30 positioned on the first surface 12 of the substrate 10. In the embodiment as shown in
The substrate 10 comprises a semiconductor material such as silicon. The microphone 20 can be chosen from a variety of available microphones. All microphones are based on the electronic measurement of a pressure-induced structural deflection. The microphone is an electro-mechanical-acoustic transducer that transforms acoustical energy into electrical energy. Most commercial (non-MEMS) microphones employ capacitive sensing schemes. The implementation of a capacitive scheme in solid-state sensors requires on-chip electronics to minimize the effects of parasitic capacitance. The piezoresistive transduction scheme, which includes measuring the strain on the top surface of a deflected diaphragm is less expensive to develop, simpler to fabricate, and potentially more robust than a capacitive device. Furthermore, theoretical studies indicate that piezoresistive sensing schemes can transduce a lower minimum detectable pressure than capacitive schemes for diaphragm edge-lengths of less than 0.5 mm. These factors, along with concerns about environmental stability (e.g., condensation) and ease of fabrication, leads the inventors to select a piezoresistive sensing scheme in one embodiment of the present invention.
In one embodiment, the microphone can be a piezoresistive microphone 320 as shown in
The piezoresistive microphone 320 may further include a channel 330 in fluid communication with the cavity 322 and the ambient atmosphere. The channel 330 is positioned on the surface 312 of the semiconductor substrate 310 and terminated at one end with a vent hole 332 in fluid communication with the ambient atmosphere. The channel 330 is terminated at another end 324 in fluid communication with the cavity 322.
As an example and as shown in
Referring now to
Still referring to
The semiconductor substrate 10 comprises a silicon layer, which can be passivated with silicon dioxide. The combination 100 may further comprise a plurality of conductive leads 16 for interconnecting corresponding components positioned on the layer 10.
Referring now to
The combination 500 further has a lid 570 positioned above the semiconductor substrate 510. The lid 570 includes a body 572, and a peripheral portion 574 extending away from the body 572. The lid body 572 is complementarily sized such that when the lid 570 is positioned over the semiconductor substrate 510, the peripheral portion 574 of the lid 570 matches to and supported by the peripheral portion 554 of the package 550, and the body 572 covers the semiconductor substrate 510 and components positioned on the semiconductor substrate 510. The lid 570 may be made from a material that is substantially transparent to the acoustic wave. Additionally, the lid 570 is electrically coupled to ground for electromagnetic shielding.
In another aspect, the present invention relates to a printed circuit board array responsive to an acoustic wave. Referring now to
Referring now to
The printed circuit board 710 may comprise a double-sided copper clad. In the embodiment as shown in
The printed circuit board array 680 or 780 can be utilized for aeroacoustic measurements for airframe noise mechanism identification and source localization. As shown in
The invention will be better understood by reference to the following illustrative samples and corresponding testing results, which are constructed and performed according to the present invention.
Hybrid Microphone-Amplifier Packages
A hybrid microphone-amplifier package scheme combines a micromachined piezoresistive silicon microphone and an Analog Devices AD624 low-noise differential amplifier into a 16-pin, 0.6″ TO-8 semiconductor package. The differential outputs of the microphone Wheatstone bridge are AC-coupled to the inputs of the amplifier via two resistor-capacitor (RC) pairs as shown in
The construction of the hybrid package including four layers as shown in
Printed Circuit Board Array
An illustrative array of the invention has 16 microphones arranged as partially shown in
Signal Processing
The signal processing involves continuously sampling data from the array, computing the Fast Fourier Transform (FFT) on blocks of data, and using conventional frequency-domain beamforming methods to obtain the array pressure response over a scanned region in space.
An Agilent E1432A 16-channel, 16-bit, VXI-based digitizer is used to acquire the signals from the array. The digitizer is interfaced to a host computer (866 MHz Pentium III, 256 MB RAM) via a National Instruments MXI-2 interface bus. The host controls the operation of the digitizer, runs the beamforming algorithms, and displays and saves the results using MATLAB v.6.0. In operation, the digitizer samples all 16 channels with 16-bit resolution at a sampling rate of 25.6 kHz. The Agilent E1432A provides the capability to perform real-time Fast Fourier Transforms (FFT's) on the incoming data, significantly reducing the computational load on the host computer. A Hanning window is used in computing 1024-point FFT's, yielding a frequency resolution of 25 Hz. The digitizer internally compensates for the power lost in the windowing operation by scaling the output by the Hanning window weighting factor of √{square root over (8/3)}. Typically 400 non-overlapping blocks are used, corresponding to 16 sec of time data.
The cross-spectral matrices for all 512 bin frequencies are computed in real time for each successive block of FFT data that is transferred to the PC host. The time-averaged cross-spectral matrices are obtained by,
where vector Yk is a column vector containing the kth FFT coefficients for all M channels,
The data is converted to units of pressure squared (Pa2) by dividing by the square of the microphone sensitivity. The array power response is obtained using
{circumflex over (P)}k=e′k{circumflex over (R)}kek
where the quantity ek, known as the steering vector, contains the weights and phase shifts to be applied to the system and is given by
The weights and phase shifts are given by
where r and rm are the radial distances from the focus location to the array center and the mth microphone, respectively, and c is the speed of sound. The array power response is divided by the number of microphones squared (M2) to normalize the array output to that of a single microphone. The pressure response is given by the square root of this result. For most of the measurements made in calibrating the array, a 48 in.×48 in. grid of regularly spaced (1 in. increments) focal locations are used in a scan plane parallel to the array face at a distance of 36 in. The pressure response computation time for a single frequency bin is under 4 sec. The time-averaged cross-spectral matrices for all 512 frequency bins are stored to disk, resulting in file size of 2 MB.
Experimental Results
The experimental methodologies and results for calibrations of the hybrid package and array system according to one embodiment of the present invention are presented. The frequency responses of the hybrid microphone packages are obtained using a plane wave tube acoustic calibrator. Characterization of the array system is performed using an acoustic point source in an anechoic chamber.
Hybrid Package Calibration
Each hybrid microphone package is operated individually for amplitude and phase using a 1″×1″ square cross-section, normal incidence plane wave tube, designed to support plane waves up to 6.7 kHz. The microphone package and a ⅛-inch Bruel and Kjaer (B&K) 4138 reference microphone are flush mounted at the terminating end of the tube and subjected to normally incident plane waves. The frequency response of the hybrid package is determined with respect to the B&K microphone. The frequency response calibrations for the sixteen hybrid packages are shown in
For the purposes of array signal processing, the relative magnitudes or phases between sensor packages are important. The frequency response data is reformatted and shown in
Array Calibration
The array response to an acoustic point source was measured in an anechoic chamber, having a 100 Hz cutoff frequency, as a function of source frequency, source amplitude, and source location. Each measured response was compared to the theoretical response (array pattern) given by,
where the “primed” terms represent the radial distances from the actual source (located at {overscore (x)}′) and the “unprimed” terms represent the radial distances from the focus location (located at {overscore (x)}). The acoustic point source, provided by NASA Langley, consists of a JBL 2426J-compression driver mated to a 21 in. long, 0.75 in. inner diameter metal pipe. The pressure field generated by the device is modeled as a piston at the end of a pipe and performs suitably as a point monopole for frequencies below 11.5 kHz.
Several metrics were used to quantify the differences between the measured and theoretical responses. For this analysis, the measured array response is normalized by its peak value for direct comparison to the normalized array pattern. The first metric is comparison of the beamwidths of the mainlobe at 0.5 dB, 1 dB, 3 dB, 6 dB, and 9 dB down from the peak values. Because the mainlobe may not have cylindrical symmetry, an equivalent beamwidth is used. It is obtained by computing the diameter of a circle having the same area as enclosed by the respective contour curve. The second metric is to compute a weighted root mean squared (RMS) error for the measured response. It provides an estimate of the total relative error over K scan locations and is expressed,
where Tk and Mk represent the normalized theoretical and measured responses at the kth scan location, respectively. The error is weighted by the measured response Mk to account for the relative effect the errors would have in the measured response. A third metric is to compare the location of the measured peak response to the actual source location.
Array Response vs. Frequency
The array response was first examined as a function of frequency for a point source positioned at a distance of 36 in. on the z-axis of the array. Discrete tones were used to achieve an average sound pressure level of approximately 100 dB SPL at the array microphones. The measured array pressure responses are shown at 2 kHz, 6 kHz, and 10 kHz in
The equivalent 3 dB mainlobe beamwidth is shown in
Array Response vs. Sound Pressure
One benefit of an acoustic array is its improvement in the signal to noise ratio of the measured signal, referred to as the array gain. The use of multiple microphones enables the measurement of source signals that are below the noise floor of any one particular microphone. Thus, an important characteristic of an array is its performance as a function of incident sound pressure, or equivalently the signal to noise ratio of the microphones. For this experiment, the point source is fixed on the boresight of the array at a distance of 36 in. and the array response to a 6 kHz tone is measured at various sound pressure levels. As a reference, the array was removed and a single B&K 4138 microphone was used to measure the free-field sound pressure level at the location of the array origin for several sinusoidal voltage amplitudes supplied to the speaker. The array was then reinstalled and the responses were measured using the calibrated speaker voltage inputs.
Array Response vs. Location
Perhaps one of the most useful aspects of a directional array is its capability for source localization. As a one-dimensional verification, a 6 kHz source at a distance of 36 in. is translated in the x-direction in 3 in. increments to a distance of 24 in., and the performance of the array was examined. Ideally, the array response should be calibrated over a broad range of locations in space.
Of greater importance is the ability of the array to accurately locate a source in space.
The results from calibrations of the hybrid package and array confirm and verify the functionality of the system. From plane wave tube calibrations, the hybrid microphone packages show an average sensitivity of 831 μV/Pa with matched magnitude (±0.6 dB) and phase (±1 degree) responses. From tests conducted in an anechoic chamber, the array shows accurate source localization capabilities of ±0.3 in. It has a minimum detectable signal of 47.8 dB SPL for a 1 Hz bin at 6 kHz and a maximum input of 160 dB SPL. The usable frequency range is limited to 3 kHz to 8 kHz. A larger number of sensors can broaden the frequency range by increasing the overall array size while maintaining small intersensor spacings. The array noise floor was experimentally verified to be 11.6 dB below the noise floor of the individual microphones, as predicted by the theoretical array gain.
For this array system, an extensive calibration is needed to confirm determine the response over a larger parameter space. The calibration should include in situ calibrations of the microphones and a complete analysis of the directivity and accuracy of the array over a broad frequency range. An array, similar in construction, including more sensors could be constructed to examine the response over a larger frequency range. Additional efforts could be aimed at reducing the size and increasing the physical robustness of the hybrid packages and improving the construction techniques used for the array.
The invention has been described herein in considerable detail, in order to comply with the Patent Statutes and to provide those skilled in the art with information needed to apply the novel principles, and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modification, both as to equipment details and operating procedures can be effected without departing from the scope of the invention itself. Further, it should be understood that, although the present invention has been described with reference to specific details of certain embodiments thereof, it is not intended that such details should be regarded as limitations upon the scope of the invention except as and to the extent that they are included in the accompanying claims.
This application is a continuation-in-part and claims the benefit, pursuant to 35 U.S.C. § 119(e), of provisional U.S. patent application Ser. No. 60/253,525, filed Nov. 28, 2000, entitled “MEMS BASED ACOUSTIC ARRAY,” which is incorporated herein in its entirety by reference.
This invention was made partially with Government support under Grant No. NAG-1-2133 awarded by NASA Langley Research Center and the Government has certain rights in this invention.
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| 60253525 | Nov 2000 | US |
