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

Yost, William T.

English

NASA Technical Reports Server

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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 )  C A P A C I T I V E  
A C C U S T I C  W A V E  DETECTOR AND METHOD 
OF U S I N G  S A M E  P a t e n t  Ap~lication 
( N A S A ,  L a n g l e y  Research  C e n t e r )  
18  P 
https://ntrs.nasa.gov/search.jsp?R=19950016504 2020-06-17T23:51:24+00:00Z
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 
ss: -
Employer: 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. 
1 0  
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 R W d  Art  
The detection of ultrasonic waves is important in a wide variety of 
applications including materials characterization and medical analysis. 
Current transducers fall into t w o  broad categories of  piezoelectric elements 
or magnetic concepts. Well-damped piezoelectric transducer elements are 
normally designed t o  operate from 100-250 KHz up t o  250 MHz. 
Specifically shaped piezoelectric elements designed for point contact use 
low frequency responses from 50  KHz t o  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 t o  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 
wi th  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 1 4  such as a 5 MHz lithium niobate compressional 
transducer. Transducer 14  is acoustically coupled t o  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 dielectric- 
electrode 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 
1 0  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 w f )  
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 + - - W k  w o + -  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 
b y  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 t o  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 
1 0  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. 





Capacitive acoustic wave detector and method of using same

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