7 Y/i 

//v - 7/ 

y2 - Q 5- 

NASA CASE NO. I AR 1.1890-1 

PRINT FIG. 3 

The invention disclosed in this document resulted from research in aeronautical 

and space activities performed under programs of the National Aeronautics and Space 

Administration. The invention is owned by NASA and is, therefore, available for 

licensing in accordance with the patent licensing regulations applicable to U.S. 

Government-owned inventions (37 CFR 404.1 et seq.). 

To encourage commercial utilization of NASA-owned inventions, it is NASA 

policy to grant licenses to commercial concerns. Although NASA encourages 

nonexclusive licensing to promote competition and achieve the widest possible 

utilization, NASA will provide the necessary incentive to the licensee to achieve early 

practical application of the invention. 

Address inquiries and all applications for license(s) for this invention to the 

Technology Applications Team, NASA Langley Research Center, Code 200, Hampton, 

Virginia 2368 1-0001. 

Serial No.: 08/31 7,491 

Filed: 09/27/94 LaRC 

(NASA-Case-LAR-13890-1) CAPACITIVE 

ACCUSTIC WAVE DETECTOR AND METHOD 

OF USING SAME Patent Ap~lication 

(NASA, Langley Research Center) 

18 P 
AWARDS ABSTRACT 

CAPACITIVE ACOUSTIC WAVE DETECTOR 

AND METHOD OF USING SAME 

According to the present invention, a capacitor having two substantially parallel 

conductive faces is acoustically coupled to a conductive sample end such that the 

sample face is one end of the capacitor. A non-contacting dielectric may serve as 

a spacer between the two conductive plates. The formed capacitor is connected 

to an LC oscillator circuit such as a Hartley oscillator circuit producing an output 

frequency which is a function of the capacitor spacing. This capacitance oscillates 

as the sample end coating is oscillated by an acoustic wave generated in the 

sample by a transmitting transducer. The electrical output can serve as an 

absolute indicator of acoustic wave displacement. This invention is an improvement 

to mechanical end joint systems, and more particularly, to a mechanical end joint 

system useful for the transverse connection of numerous strut elements to a common 

node to permit the rapid assembly and disassembly of diverse skeletal framework 

elements. 

Novel aspects of the present invention include performing absolute displacement 

measurements to detect acoustic waves while avoiding acoustic interaction with 

the waves. 

Inventor: William T. Yost 

797 Fawn Lane 

Newport News,VA 23602 

Employer: 

ss: 

NASA 

- 

LaRC 

Initial Evaluator: Joseph S. Heyman 

Serial No.: 08/31 7,491 

Filed: 9/27/94 

C:\WPWINGO\WPDOCS\13890-1 ABS 
LAR 13890-1 -1 - PATENT APPLICATION 

CAPACITIVE ACOUSTIC WAVE DETECTOR 

AND METHOD OF USING SAME 

The invention described herein was made by an employee of the 

United States Government and may be used by or for the Government for 

governmental purposes without the payment of any royalties thereon or 

therefor. 

10 

d of the hention 

The present invention relates generally to the detection of ultrasonic 

waves and more particularly to a capacitive detection apparatus and method 

for detecting acoustic waves in a sample. 

2 af the RWd Art 

The detection of ultrasonic waves is important in a wide variety of 

applications including materials characterization and medical analysis. 

Current transducers fall into two broad categories of piezoelectric elements 

or magnetic concepts. Well-damped piezoelectric transducer elements are 

normally designed to operate from 100-250 KHz up to 250 MHz. 

Specifically shaped piezoelectric elements designed for point contact use 

low frequency responses from 50 KHz to approximately 2MHz. Both types 

of transducers are not true specimen displacement sensors since the voltage 

generated is a result of compression of the transducer by the ultrasonic 
LAR 13890-1 -2- PATENT APPLICATION 

waves. In addition, these transducers acoustically interact with the 

ultrasonic wave, thereby altering the detected wave. 

The magnetic units employ eddy currents and magnetic fields to 

detect both bulk waves and surface waves. A strong, homogeneous 

5 magnetic field is required to measure absolute amplitudes. Both sufficient 

strength and homogeneity are difficult to maintain. 

10 It is accordingly an object of the present invention to detect ultrasonic 

waves in a sample. 

It is another object of the present invention to perform absolute 

displacement measurements to detect acoustic waves. 

It is a further object of the present invention to detect ultrasonic 

15 waves while avoiding acoustic interaction with the waves. 

It is another object of the present invention to obviate the need for 

strong homogenous magnetic fields in ultrasonic wave detection. 

Additional objects and advantages of the present invention are 

apparent from the drawings and specification which follow. 

20 

Summarv 

The foregoing and additional objects are obtained by a capacitive 

detector according to the present invention. A dielectric having two 

25 substantially parallel faces is placed between a coated sample end 

(conductor) and an electrode such that a capacitor is formed comprising the 

sample end coating, the dielectric and the electrode. The formed capacitor 

is connected to an LC oscillator circuit such as a Hartley oscillator circuit 

producing an output frequency which is a function of the changing 
LAR 13890-1 -3- PATENT APPLICATION 

capacitance of the capacitor. This capacitance oscillates as the sample end 

coating is oscillated by an acoustic wave generated in the sample by a 

transmitting transducer. 

5 Brief De-ofthe Drawings . . 

FIG. 1 is a schematic of the present invention coupled to a 

conventional Hartley oscillator circuit to detect and analyze transmitted 

waves; 

10 FIG. 2 is a side view of a sample holding apparatus used in 

conjunction with the present invention; 

FIG. 3 is a schematic of a standard LC circuit; and 

FIG. 4 is a log-log plot of sound wave amplitude versus drive voltage 

of the transmitting transducer. 

15 

Referring to FIG. 1, the present invention is schematically represented 

with reference to a sample S such as ULE 7971 glass commercially 

20 available from Corning Glass Works. The sample has a first end S1 and a 

coated 2nd end S2. The second end S2 may be coated with a metallic such 

as gold, copper, silver, or chrome. A signal generator 10 such as an 

HP 3325 frequency synthesizer commercially available from 

Hewlett-Packard generates a desired frequency signal which is amplified by 

25 a broadband amplifier (bandwidth of greater than 30 MHz) 12 such as 

commercially available from EN1 (EN1 A1 50). The amplified signal drives a 

transmitting transducer 14 such as a 5 MHz lithium niobate compressional 

transducer. Transducer 14 is acoustically coupled to a first end S1 of 

sample S via transmission gels or otherwise as known in the art. The 
LAR 13890-1 -4- PATENT APPLICATION 

transducer sets up an ultrasonic wave in the sample S which propagates 

to the opposite second end. 

A dielectric layer 16 is provided having first and second substantially 

parallel opposite faces. The dielectric may have any suitable composition 

5 such as mica or Teflon. The purpose of the dielectric is to enhance the 

capacitance of the parallel conductive plates and to serve as a spacer. 

Because there is no acoustic couplant between the dielectric and the coated 

sample end S2, the sample end S2 behaves as a "free" boundary whose 

characteristics are well known to acousticians. An electrode 18 is located 

10 at the second dielectric face. Electrode 18 may be any conducting material 

such as copper. As the coating of the sample second end S2 oscillates in 

response to the acoustic wave at the free boundary, the capacitance 

between this coating and the electrode 18 also oscillates as described in 

greater detail below. This oscillating capacitance is the resonating 

15 capacitance in a Hartley or other LC oscillator circuit 20 which is maintained 

in oscillation by an active current element 22. This schematic is by way of 

illustration and in no way limits the present invention, which can be used 

to frequency modulate any LC (inductance-capacitance) oscillator circuit. 

The depicted circuit 20 is the classic Hartley LC oscillator which is driven 

20 by active circuit element 22 in the form of a J-FET (junction field-effect 

transistor). The Hartley oscillator and J-FET are well known in the art and 

described in detail in various textbooks in electronics such as Horowitz & 

Hill "The Art of Electronics". The capacitor formed by the sample end 

coating, dielectric 1 6 and electrode 18 is referred to as FMESAT (Frequency 

25 Modulation Electrostatic Acoustic Transducer) and serves to frequency 

modulate the output of the LC circuit. 

The output frequency signal of the oscillating circuit 20 passes 

through a buffer amplifier 24 such as LH-0002 IC (integrated circuit) 

commercially available from National Semiconductor is used to buffer the 
LAR 13890-1 -5- PATENT APPLICATION 

output of the LC oscillator circuit 20 and to a spectrum analyzer 26 such as 

model HP 3585 AC commercially available from Hewlett-Packard which 

determines the amplitude of the ultrasonic wave detected by the dielectricelectrode arrangement based on the frequency of the LC circuit. Ferrite 

5 beads 25A and 256 are provided on the input and output sides of buffer 

amplifier 24 to suppress spurious oscillations in the LH 0002. 

This dielectric-electrode configuration is referred to as an F.M. 

capacitive detector. It is an ultrasonic transducer which permits absolute 

displacement measurements as described below. The configuration of 

10 electrodes permit detection of bulk waves impinging on the surface since 

the capacitance variation causes the electrical modulation. This 

configuration is set up to detect bulk compression waves reflected at the 

unbound (free) end of the sample. The detection of bulk compressional 

waves is described in detail. The capacitive detector can be designed to 

15 operate over a wide range of frequencies from approximately DC to a 

fraction of the resonant frequency of approximately 10 MHz of the Hartley 

oscillator ("front end" network) comprising the capacitive detector and the 

inductor. 

FIG. 2 shows one arrangement of the present invention. A sample 

20 holding apparatus 28 is provided for sample S. An actuator 30 maintains 

a constant downward mechanical force on electrode 18 to hold it firmly 

against the dielectric 16 and coated sample end S2 to form the capacitor. 

The electrode 18 is held in proximity with the coated sample end S2 by the 

dielectric 16 serving as a spacer. As shown, connecting wire 19A connects 

25 the electrode 18 to the circuit and wire 196 connects the circuit to the 

coated sample second end S2. 

FIG. 4 shows a basic LC circuit wherein L refers to the inductance 

of the coils, C refers to the capacitance of the capacitor, and the natural 

angular frequency w, for a resistance-less LC circuit is given by 
LAR 13890-1 PATENT APPLICATION 

If the distance between the capacitor plates, i.e., between the sample end 

coating and electrode 18, changes then 

wherein C, is the quiescent capacitance. The inductance L of the circuit 

of course remains constant in the present embodiment. For a parallel plate 

10 capacitor as in the present invention, the following relationship exists 

wherein S is the distance between the plates at capacitance C, and 4, is 

15 the initial distance between the plates at capacitance C,. We have from Eq. 

(2) and (3) 
LAR 13890-1 PATENT APPLICATION 

wherein AP is the change in distance between the plates. Combining Eqs. 

(4) and (2) into (1) and expanding, 

wherein w is the angular frequency of the LC circuit. 

The distance between the plates varies because of an ultrasonic wave 

10 impinging on the end of the sample according to 

15 wherein A is the amplitude of the acoustic wave, w, is the angular 

frequency of the sound wave, and t is time. Combining Eqs. (5) and (6) 

yields 
LAR 13890-1 -8- PATENT APPLICATION 

A o,, = wo(l + - cos wf) Q 0 

The output v(t) of the LC circuit can be expressed as 

v(t) = Vo sin (wot + yr) 

wherein V, is the quiescent voltage amplitude of the oscillator, i.e., the 

oscillator output voltage amplitude when the sound field is turned off 

(At = O), and q~ is the phase angle. 

10 Given that 

d aJ 0(t) = -(mot + lp) = wo + - - 

Wk wo+- cos w,t, dt dt 4 0 

results in 

dly woA -=- cos 0,t. dt Qo 
LAR 13890-1 

Integrating Eq. (10) results in 

PATENT APPLICATION 

ooA ~(t) = - sin w,t + 8, wSQ0 

5 Setting 8, =0 and substituting Eq. (1 1) into Eq. (8), 

v(Q = V, sin (mot + 6 sin w,Q 

wherein b is the deviation ratio expressed as 

Expanding in terms of single frequency components, 

v(f) = V, C J,, (6) sin (w, + nw,)t 
LAR 13890-1 -1 0- PATENT APPLICATION 

This provides a set of frequencies and associated amplitudes which are 

proportional to the Bessel functions, i.e., 

represents this set of frequencies and associated amplitudes. Letting 

v, = V, J, (a), where v, is the voltage of the nth sideband, then 

(1 6) 

10 

Eq. (1 6) is solved over a specific range. Substituting Eq. (1 6) into Eq. (1 3) 

yields 

If w, w, and using the first sideband, 
LAR 13890-1 PATENT APPLICATION 

If w, < < w, the system becomes quite sensitive. Using any one or all of 

the sidebands, 6 is determined experimentally. Then 

5 

As compared with the static bias voltage case where 

FIG. 4 plots the calculated amplitude at low drive frequencies. Given 

the described instrumentation, the present invention can measure 

amplitudes of 9 x Angstroms at approximately 109 KHz. Specifically, 

a log-log plot of amplitude (meters) of the sound waves versus drive voltage 

15 Vpp (volts) of a 5 MHz lithium niobate transducer at a drive frequency of 

109.560 KHz is depicted for the cylindrical glass sample Corning ULE 7971 
LAR 13890-1 -1 2- PATENT APPLICATION 

described above having a diameter of 1.5" and a length of 2". A slope of 

1.05 was derived, showing experimentally that by using this technique, the 

displacement amplitude is linearly dependent on drive voltage, as predicted 

by theory. 

Table 1 shows the calculated amplitude for a 5 MHz lithium niobate 

transducer at a low drive frequency of 109.56 KHz and a drive voltage of 

55.5 VPP. 

Table 1 

10 

Amplitude average - 465 * 9.8 8\ 

The plot (FIG. 4) indicates a linear relationship between the drive 

voltage to transducer and the sideband voltage. There is a close agreement 

of absolute amplitudes calculated from various sidebands, with a standard 

deviation within 2.5%. 

The use of a dielectric enhances the sensitivity of a nondestructive 

evaluation technique. Surface preparation, e.g., smoothing to reduce 

irregularities which interfere with wave parameters under consideration, is 

less critical. Since the measurements are taken on an acoustically vibrating 

"free surface", where the surface displacement amplitude (that which is 

- 

Wave Amplitude 

(Angstrom) 

474 

Bessel Function 

J, 

442 

469 

467 

467 

Deviation Ratio 

1.635 

J1 

J2 

J3 

J4 

460 

469 

470 

474 

1.524 

1.61 7 

1.61 2 

1.61 1 

J-, 

J-2 

J., 

J4 

1.587 

1.61 7 

1.620 

1.636 
LAR 13890-1 -1 3- PATENT APPLICATION 

measured) is precisely equal to twice the displacement amplitude of the 

acoustic wave within the solid, such a surface can be precisely modeled. 

The present invention also allows absolute amplitude calibration of the 

capacitive detector since all of quantities on the right side of equation (1 90 

5 are known or can be measured. In addition, the invention results in 

enhanced sensitivity at low frequency ultrasonic waves (e.g., frequencies 

much smaller than the quiescent oscillator frequencies). 

Using coil probes, the capacitance can be held fixed and the induction 

fluctuation in the coil measured as a surface acoustic wave passes through 

10 the material close to the location of the probe. 

Many improvements, modifications, and additions will be apparent to 

the skilled artisan without departing from the spirit and scope of the present 

invention as described herein and defined in the following claims. 
LAR 13890-1 -pe PATENT APPLICATION 

CAPACITIVE ACOUSTIC WAVE DETECTOR 

AND METHOD OF USING SAME 

Abstract 

A capacitor having two substantially parallel conductive faces is 

acoustically coupled to a conductive sample end such that the sample face 

is one end of the capacitor. A non-contacting dielectric may serve as a 

spacer between the two conductive plates. The formed capacitor is 

10 connected to an LC oscillator circuit such as a Hartley oscillator circuit 

producing an output frequency which is a function of the capacitor spacing. 

This capacitance oscillates as the sample end coating is oscillated by an 

acoustic wave generated in the sample by a transmitting transducer. The 

electrical output can serve as an absolute indicator of acoustic wave 

15 displacement.