Acoustic properties and their changes in a sample of liquid, gas, plasma or solid are measured by applying a variable frequency source to the sample by means of a transducer to produce sound waves within the sample. The application of the variable frequency source to the sample is periodically interrupted for a short duration. Means are connected to the transducer for receiving the resulting acoustic signals during the interruptions for producing a control signal indicative of a difference in the frequency of the output of the variable frequency source and the frequency of a mechanical resonant peak in the sample. The control signal is applied to the variable frequency source to maintain its output frequency at the frequency of the mechanical resonant peak. The change in frequency of the variable frequency source indicates the shift in frequency of the mechanical resonant peak and the amplitude of the acoustic signals indicates the attenuation of the acoustic signals in the sample

Heyman, J. S.

English

NASA Technical Reports Server

[54] PSEUDO CONTINUOUS WAVE 
INSTR WEPJT 
[75] Inventor: 
[73] Assignee: 
[21] Appl. No.: 
[22] Filed: 
Joseph S. Heyman, Gloucester, Va. 
The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
858,763 
Dec. 8,1977 
[51] Int. Cl.2 ............................................. GOlN 29/00 
[52] U.S. Cl. ......................................... 73/579; 73/589 
[58] Field of Search ................. 73/579, 659, 580, 581, 
73/582, 583, 599, 88 F, 589 
[561 References Cited 
U.S. PATENT DOCUMENT§ 
3,345,862 10/1967 Rowe ..................................... 73/579 
4,014,208 3/1977 Moore et al. .................... 73/88 F X 
Primary Examiner-Jerry W. Myracle 
Attorney, Agent. or Firm-William H. King; Howard 9. 
Osborn; John R. Manning 
18 
\ 
ABSTRACT 
A device for measuring acoustic properties and their 
changes in a sample of liquid, gas, plasma or solid. A 
variable frequency source is applied to the sample by 
means of a transducer to produce sound waves within 
the sample. The application of the variable frequency 
source to the sample is periodically interrupted for a 
short duration. Means are connected to the transducer 
for receiving the resulting acoustic signals during the 
interruptions for producing a control signal indicative 
of a difference in the frequency of the output of the 
variable frequency source and the frequency of a me- 
chanical resonant peak in the sample. The control signal 
is applied to the variable frequency source to maintain 
its output frequency at the frequency of the mechanical 
resonant peak. The change in frequency of the variable 
frequency source is indicative of the shift in frequency 
of the mechanical resonant peak and the amplitude of 
the acoustic signals is indicative of the attenuation of the 
acoustic signals in the sample. 
P71 
10 Claims, 2 Drawing Figures 
.I2 /I3 , \ 
VOLTAGE 
SUMMING GATED 
--9 BUFFER 
AMPLIFIER 
-
- "o"s",:,","~~~'," -P AMPLIFIER 
t 1 1 1  t 
OSCILLATOR iy 
3P 35 
INTEGRATOR 
DETECTOR 
I -  uI 28 
BUFFER 
INVERTER 
-27 I 
VARIABLE 
PHASE 
32 
r- I TR:NSDUCER 
a MIXER 
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US. Patent OCL 3,1978 Sheet 2 of 2 4,4.67,73 1 
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4,117,731 
2 
combined to form a CW signal that has a DC compo- 
PSEUDO CONTINUOUS WAVE INSTRUMENT 
ORIGIN OF THE INVENTION 
nent and an FM componenthaving the same frequency 
as the FM signal. The detected CW signal is in phase 
with the FM signal if the outvut of the VCO is at a Y 
The invention described herein was made by an em- 5 frequency that is on the rising slope of the frequency 
response curve of a mechanical resonant peak of the 
samde: and the detected CW signal is out of phase with 
ployee of the United States Government and may be 
manufactured and used by or for the Government for 
governmental purposes without the payment of any 
royalties thereon or therefore. 
BACKGROUND OF THE INVENTION 
This invention relates generally to ultrasonics and 
more specifically concerns the measurement of acoustic 
properties and their changes in liquids, gases and solids. 
suring devices are pulse echo devices and continuous 
wave (CW) devices. The disadvantages of the pulse 
echo devices are the acoustic waves are nonmono- 
chromatic and noncoherent, the measurements suffer 
from broadband and duty cycle effects and the devices 20 
usually require very fast rise time electronics and high 
peak powers leading to instrumentation complexity and 
expense. 
The primary disadvantage of CW devices is “cross- 
talk” (electrical leakage) which complicates measure- 25 
ment interpretation. This problem requires care to iso- 
late the receiving transducer from the transmitting 
transducer to minimize the cross-talk. A partial solution 
to this problem requires a complex transducer having 
high acoustic coupling yet high electrical isolation. 30 
Such a complex transducer is expensive and fragile. 
A sampled continuous wave (SCW) device disclosed 
in a paper entitled A “Sampled-Continuous Wave” 
Ultrasonic Technique and Spectrometer by J. G. Miller 
and D. I. Bolef published in The Review of Scientific 35 
Instruments, Vol. 40, No. 7, pages 915-920, July 1969, 
gates off the CW source and waits for the acoustic 
decay szmpling as desired. This comes close to a CW 
ultrasonic measuring device with no cross-talk; how- 
ever, it has the disadvantage that it is duty cycle bound. 40 
That is, the ratio of signal time to time for a complete 
cycle is low. 
It is, therefore, a primary object of this invention to 
provide an ultrasonic measuring instrument that has no 
cross-talk. 45 
Another object of this invention is to provide an 
ultrasonic measuring device that has nearly one-hun- 
dred percent duty cycle. 
A further object of this invention is to provide an 
ultrasonic measuring device that requires only a very 50 
simple transducer. 
Still another object of this invention is to provide a 
simple, inexpensive, portable, yet accurate means for 
measuring acoustic properties and their changes in liq- 
uid, gases, plasmas and solids. 
Other objects and advantages of this invention will 
become apparent hereinafter in the specification and 
drawings. 
SUMMARY OF THE INVENTION 
10 
The two general classes of prior art ultrasonic mea- 15 
55 
hn 
the +hi signal if the output oft& VCO is at a-frequency 
that is on the downward slope of the frequency re- 
sponse curve of the mechanical resonant peak. The 
phase of the CW signal is compared with the phase of 
the FM signal and if the two signals are in phase a posi- 
tive DC voltage is integrated by an integrator and then 
added to the FM signal before it is applied to the VCO. 
If the two signals are out of phase a negative DC volt- 
age is applied to the integrator and added to the FM 
signal to lower the DC level of the output of the inte- 
grator before it is applied to the VCO. Consequently, 
the frequency of the VCO is held approximately at the 
frequency of the mechanical resonant peak. The fre- 
quency change of the VCO is indicative of the shift of 
the frequency of the mechanical resonant peak and is 
measured by measuring the output frequency of the 
VCO or by measuring the DC level of the voltage at the 
output of the integrator. The attenuation of the sample 
is determined by measuring the DC level of the CW 
signal just after it is detected, sampled and held. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a block diagram of the embodiment of the 
invention selected for illustration in the drawings; and 
FIG. 2 shows a mechanical resonance response curve 
for the purpose of explaining how the invention oper- 
ates. 
DETAILED DESCRIPTION OF THE 
INVENTION 
Turning now to the embodiment of the invention 
selected for illustration in the drawings the number 11 
in FIG. 1 designates a varactor tuned voltage controlled 
oscillator (VCO) which provides an initial radio fre- 
quency of for example, 5 x lo6 Hz. The output of the 
VCO is highly amplified in a gated amplifier 12 which 
has an on to off gain ratio greater than 70 dB. The out- 
put of the gated amplifier 12 is buffered by a buffer 13 to 
drive a low impedance and then fed to a transducer 14 
which converts the electrical radio frequency signals to 
acoustic waves which are introduced into a bolt 15. 
Even though bolt 15 is shown in FIG. 1, it is to be 
understood that this invention is operable on any sample 
of liquids, gases, plasmas or solids. VCO 11 is frequency 
modulated by a 1 KHz oscillator 16 whose square wave 
output is converted to a sine wave by means of a band- 
pass amplifier 17 and then applied through a summing 
amplifier 18 to VCO 11. 
The output from a 10 KHz oscillator 19 is applied 
through a buffer 20 to gated amplifier 12 to gate ampli- 
fier 12 off for a brief period, for example, 10 micro- 
seconds each cycle of oscillator 19. During the times 
-- that amplifier 12 is gated off some of the acoistic energy 
A voltage controlled oscillator (VCO) controlled by in sample 15 is converted back to electrical signals by 
an FM signal has its output applied through a trans- transducer 14 and passes through a mixer 21. The out- 
ducer to the sample whose acoustic properties are to be put from oscillator 19 is also applied through an inverter 
measured. Several times during each cycle of the FM 22 to mixer 21 only while gated amplifier 12 is gated off. 
signal the output from the VCO to the transducer is 65 The signals passed through mixer 21 are detected and 
interrupted momentarily. During these interruptions amplified by a detector amplifier 23 and then applied to 
the resulting CW acoustic waves set up in the sample a sample hold circuit 24. The output of oscillator 19 is 
are detected. The detected CW acoustic waves are also applied through a delay logic 25 to ensure that the 
4,117,73 1 
3 
signals are not sampIed during transients after gated 
amplifier 12 is gated off. Each signal sampled by the 
sample hold circuit 24 is held until the next signal is 
sampled. The output of sample hold circuit 24 is applied 
through a bandpass amplifier 26 resulting in a sine wave 
on a DC voltage. This signal is applied through a vari- 
able phase shifter 27 to a phase detector 28. 
The output of oscillator 16 is applied through a buffer 
29 to phase detector 38. If the phase of the signal at the 
output of variable phase shifter 27 is in phase with the 
signal at the output of buffer 29 then phase detector 28 
applies a positive DC voltage through a switch 35 to an 
integrator 20 which integrates the D C  voltage. If the 
two signals are out of phase, phase detector 28 applies a 
negative DC voltage to integrator 30. Consequently, 
integrator 30 produces a DC voltage level that is con- 
stantly varying. The DC voltage level is a pseudo con- 
tinuous wave (PCW). This DC voltage is added to the 
output from amplifier 17 by means of summing ampli- 
fier 18 and then applied to VCO 11. The output of 
integrator 30 is also passed through a low pass filter 31 
and then measured by a D C  voltmeter 32 to give an 
indication of the frequency shifts of a mechanical reso- 
nant peak of sample bolt 15. The output of sample hold 
circuit 24 is measured by a D C  voltmeter 33 to give an 
indication of the attenuation of bolt 15. The frequency 
shifts of the mechanical resonant peak can also be mea- 
sured by use of a frequency readout 34 connected to the 
output of VCO 11. 
In the operation of this invention, with the sample 
bolt 15 being untightened and no strain on it the system 
is put into operation and the variable phase shifter 27 is 
adjusted to give maximum reading on voltmeter 33. 
This ensures that the output frequency of VCO 11 is 
equal to the frequency of a mechanical resonant peak 
for the sample bolt 15. Switch 35 is then closed thus 
locking the PCW to a bolt resonance. Thereafter when 
the sample bolt 15 is tightened and a strain is put on it 
the frequency of the mechanical resonant peak will 
shift. If the shift is such that the sine wave at the output 
of bandpass amplifier 17 is on the rising slope of the 
frequency response curve as shown by curve “A” in 
FIG. 2, then the sine wave generated at the output of 
variable phase shifter 27 is shown by curve “E”. As can 
be noted, curve “B” is in phase with curve “A”. Hense 
phase detector 28 will apply a positive D C  voltage to 
integrator 30 which will increase the DC voltage at the 
output of integrator 30. This increase in the DC voltage 
applied to VCO 11 increases the frequency of the out- 
put of VCO 11 such that its frequency approaches the 
frequency of the peak of the frequency response curve. 
Conversely, when the sine wave at the output of band- 
pass amplifier 17 is operating on the down slope of the 
mechanical resonant peak of sample bolt 15 as shown by 
curve “C” in FIG. 2, the sine wave produced at the 
output of variable phase shifter 27 is shown by curve 
“D”. It will be noted that curve “D” is 180” out of phase 
with curve “C”. Hence, phase detector 17 will apply a 
negative D@ voltage to integrator 30 which will de- 
crease the DC voltage level at the output of integrator 
30. This decrease in DC voltage applied to VCO 11 
decreases its output frequency in the direction of the 
frequency of the peak of the frequency response curve. 
Hence, the output of YCO 11 will always be approxi- 
mately at a frequency corresponding to the frequency 
of the mechanical resonant peak of sample bolt 15. The 
shift in frequency as measured by voltmeter 32 or fre- 
quency readout 34 is indicative of the strain on bolt 15. 
4 
All of the electrical components disclosed in FIG. 1 
are old and wellknown and are commercially available. 
Hence, the details of these components are not disclosed 
in this application. 
The advantages of the present invention over prior 
ultrasonic measuring devices are numerous. It does not 
suffer from being a broadband measurement containing 
many frequencies and phases as do pulse echo devices. 
It does not have cross-talk as do most CW devices. It is 
lo not duty cycle bound as is the previously mentioned 
SCW and it has a higher signal-to-noise ratio than the 
SCW since it samples the signal more frequently. 
The advantages of the present invention over the 
SCW are due primarily to the fact that in the SCW the 
l5 acoustic response in the sample has to build up from 
zero each time the transmitter is turned on. Whereas, in 
the present invention the acoustic response falls only 
slightly below CW equilibrium each time the transmit- 
What is claimed as new and desired to be secured by 
1. A device for measuring the acoustic properties and 
5 
2o ter is turned off. 
Letters Patent of the United States is: 
their changes in a sample comprising: 
25 a variable frequency source; 
transducer means for applying the output of said 
variable frequency source to said sample; 
means for periodically interrupting the application of 
said variable frequency source to said sample for a 
means connected to said transducer means for receiv- 
ing the resulting acoustic signals during said inter- 
ruptions for producing a control signal indicative 
of a difference in the frequency of the output of 
said variable frequency source and the frequency 
of a mechanical resonant peak of said sample; and 
means for applying said control signal to said variable 
frequency source to maintain the frequency of the 
output of said variable frequency source at the 
frequency of said mechanical resonant peak. 
2. A device for measuring the acoustic properties and 
their changes according to claim 1 wherein said vari- 
able frequency source is a voltage controlled oscillator. 
3. A device for measuring the acoustic properties and 
45 their changes according to claim 1 including means for 
measuring the change in frequency of said variable 
frequency source thereby giving an indication of the 
shift in frequency of said mechanical resonant peak. 
4. A device for measuring the acoustic properties and 
50 their changes according to claim 1 including means for 
measuring the amplitude of said control signal thereby 
giving an indication of the shift in frequency of said 
mechanical resonant peak. 
5. A device for measuring the acoustic properties and 
their changes according to claim 1 including means for 
measuring the amplitude of said acoustic signals thereby 
giving an indication of the attenuation of the acoustic 
signals in said sample. 
6. A device for measuring the acoustic properties and 
their changes according to claim 1 wherein said means 
for producing a control signal comprises: 
an FM signal, having a frequency several times less 
than the frequency of said interruptions, applied to 
the input of said variable frequency source to con- 
trol its output frequency; 
means for detecting said acoustic signals during each 
of said interruptions; 
30 short duration; 
35 
40 
55  
60 
65 
4.117.73 1 , ,  
5 
means for sampling and holding each of said detected 
signals to form a CW signal having a frequency 
equal to the frequency of said FM signal; 
means for comparing the phase of said CW signal 
with said FM signal and producing a positive DC 
voltage when the two signals are in phase and for 
producing a negative DC voltage when the two 
signals are out of phase; and 
means for adding the produced DC voltage to said 10 
FM signal to form said control signal. 
7. A device for measuring the acoustic properties and 
their changes according to claim 6 wherein said pro- 
duced DC voltage is integrated before it is added to said 15 
FM signal. 
8. A device for measuring the acoustic properties and 
their changes in a sample comprising: 
a voltage controlled oscillator; 
an FM signal for controlling said voltage controlled 2o 
oscillator; 
transducer means for applying the output of said 
voltage controlled oscillator to said sample; 
means for interrupting the application of the output 25 
of the said voltage controlled oscillator to said 
6 
sample several times during each cycle of said FM 
signal; 
means for detecting the amplitude of said acoustic 
signals in said sample during each of said interrup- 
tions and for forming a CW signal therefrom; 
means for comparing the phase of said CW signal 
with the phase of said FM signal and for producing 
a negative DC voltage when there is a difference in 
phase and for producing a positive DC voltage 
when the DC signal is in phase with the FM signal; 
and 
means for adding said DC signal to said FM signal 
before it is applied to said voltage controlled oscil- 
lator whereby the output frequency of said voltage 
controlled oscillator is maintained at the frequency 
of a mechanical resonant peak of said sample. 
9. A device for measuring the acoustic properties and 
their changes according to claim 8 wherein said pro- 
duced DC voltage is integrated before it is added to said 
FM signal. 
10. A device for measuring the acoustic properties 
and their changes according to claim 9 including means 
for measuring the amplitude of said integrated DC volt- 
age to give an indication of the shift in the frequency of 
the mechanical resonant peak in said sample. * + * * +  
30 
35 
45 
50 
55 
60 
65 


Pseudo continuous wave instrument

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