Ultrasonic leak detecting method and apparatus

Leaks or thin spots in containers, as well as mechanical faults, e.g. worn bearings, are detected by an ultrasonic energy indicator that includes a modulator that heterodynes ultrasonic signals received by a transducer and creates an audio signal related thereto. The transducer includes a series arrangement of piezoelectric crystals feeding a field-effect transistor amplifier and the modulator operates in an amplitude modulation, suppressed-carrier mode which can be stabilized by a feedback loop. For indicating the presence of the audio signal, either headphones or a meter or both are employed. When leaks in a container are being detected, an ultrasonic generator is used which varies its output frequency slowly over a band of frequencies. The leak detection technique can be improved by applying a thin layer of a liquid to the container surface so that pressure introduced into the container causes the formation and bursting of bubbles in the liquid that results in the production of ultrasonic energy.

BACKGROUND OF THE DISCLOSURE 
This invention relates to ultrasonic apparatus and more particularly, to 
methods and apparatus for detecting leaks and malfunctions of mechanical 
parts by ultrasonic means. 
It is well known that ultrasonic generators and detectors can be used to 
locate leaks, e.g. in pipes. Such a system is shown in U.S. Pat. No. 
3,978,915 to Harris. In that arrangement ultrasonic generators are 
positioned in a chamber through which the pipes pass. At the ends of these 
pipes, exterior to the chamber, ultrasonic detectors are located. At the 
point where a leak occurs in the pipe or the pipe wall is thin, the 
ultrasonic energy will enter the pipe and travel along the pipe to the end 
where the detector resides. Thus the detector will receive a signal 
indicating the existance of this leak or weak spot. Since ultrasonic 
energy used for such purposes is generally in the range of 40 KHz, it is 
too high in frequency to be heard by a human being. Thus, means are 
provided for heterodyning or frequency shifting the detected signal into 
the audio range and various schemes are available for doing this. 
By locating an ultrasonic generator in a closed chamber, a standing wave 
pattern with peaks and nodes, is established. If a node occurs at the 
position of a leak or weak spot, no ultrasonic energy will escape and the 
defect will not be detected. 
In certain instances, e.g. in detecting the malfunction of bearings, an 
ultrasonic detector is mechanically coupled to the casing of the bearings 
so that the vibrations caused by the malfunction can be mechanically 
transmitted to it. With such an arrangement the frequency is not set by an 
ultrasonic generator, but is created by the mechanical vibration itself. 
In such a case the ultrasonic detector circuit must be capable of sweeping 
over a band of frequencies to locate the one that is characteristic of the 
malfunction. This is usually accomplished by a heterodyning circuit which 
can be tuned to various frequencies, much in the manner of a radio 
receiver. 
Ultrasonic transducers generally produce a low voltage output in response 
to received ultrasonic energy. Means have been proposed for increasing 
this output. For example, in U.S. Pat. No. 3,374,663 to Morris it is 
suggested that an increase in the voltage output can be achieved by 
serially arranging two transducers. It has been found, however, that with 
such an arrangement a typical transistor preamplifier loads the 
transducers to such an extent that the gains achieved by stacking them 
serially are lost. The Morris patent proposes the use of a triple 
Darlington configuration in order to produce a sufficiently high input 
impedance to prevent this degradation in the signal produced by the stack 
of transducers. Unfortunately, the transducers in this arrangement are not 
placed so that they both readily receive ultrasonic energy. Thus the 
Morris arrangement is not entirely satisfactory. 
SUMMARY OF THE INVENTION 
The present invention is directed to providing improved methods and 
apparatus for detecting leaks and mechanical faults by ultrasonic means. 
This object is achieved by using an ultrasonic generator which has a 
variable frequency output and by using an ultrasonic detector which 
includes a series stack of transducers driving a field-effect transistor 
preamplifier. In addition, means are provided for frequency shifting the 
received signal into the audio band through the use of an amplitude 
modulated, suppressed carrier heterodyning device. 
In an illustrative embodiment of the invention the ultrasonic frequency 
generator typically includes a transducer crystal set for the frequency of 
interest. This crystal is in the feedback loop of an amplifier which 
drives it such that oscillations at the resonant frequency of the crystal 
are established. The frequency is varied about the selected frequency so 
as to avoid standing wave nodes at the location of a leak. This is 
accomplished by applying a ramp signal to the biasing network of the 
active element in the oscillator. This ramp signal is generated by a 
conventional RC circuit which has an LED flasher circuit connected across 
the capacitor element of that circuit. 
The detector arrangement used with the present invention preferably has 
more than one transducer connected in series and spacially aligned in one 
plane such that each transducer receives the energy generated. To avoid 
degradation of the signal from this arrangement due to loading from the 
preamplifier, a field-effect transistor is used as the input to the 
preamplifier. When a particular frequency is expected from an ultrasonic 
generator, a tank circuit can be located at the input to the field-effect 
transistor to intensify that signal and to eliminate signals which differ 
therefrom. 
The shift of the ultrasonic frequency into the audible range, so that a 
user may hear it or a meter may display it, can be accomplished with an 
integrated circuit function generator arranged to provide AM suppressed 
carrier operation. When such an integrated circuit is biased to the middle 
of its range, the carrier signal it is intended to produce, i.e. a signal 
slightly different than the received ultrasonic signal, will be 
suppressed. By adding the ultrasonic signal to the biasing level for this 
circuit the output will be only the sum and difference signals between the 
ultrasonic input and the carrier generated by the circuit. The sum signal 
is filtered out and the difference signal is used to drive headphones with 
which one can listen for leaks, or a meter by which to indicate them. The 
bias for establishing the suppressed carrier operation is under the 
control of a feedback loop which senses when carrier signal is present in 
the output signal, integrates the sensed carrier signal over a period of 
time to arrive at an average value, and uses the average value to vary the 
bias such that the carrier signal is eliminated. 
In a preferred embodiment of the method according to the present invention, 
leaks are detected not only by the passage of frequency-shifting 
ultrasonic energy through a hole in a container, but also by the sound 
made by the bursting of bubbles created by a liquid which is applied over 
the surface of the container after it is pressurized. This liquid has a 
surface tension which causes the formation and the bursting of the bubbles 
when they are at a size which produces ultrasonic energy when they burst, 
which energy is readily detected by the circuitry.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT 
FIG. 1 represents a block diagram of an apparatus and method for 
ultrasonically detecting mechanical faults, for example bearing failures 
and leaks in containers. By way of example, a container 10 is illustrated. 
Located within the container is an ultrasonic generator 12 which produces 
ultrasonic sound waves in the vicinity of, e.g., 40 KHz. According to the 
present invention this generator produces an output which is frequency 
modulated about its 40 KHz center frequency at a much lower frequency, for 
example 5-20 Hz. By means of this variation in the output of the 
generator, the standing wave pattern created within the container varies 
such that the standing wave nodes do not remain constant at one position. 
At position 10' there is a small hole in the container 10 from which 
ultrasonic energy may escape and be detected by an ultrasonic transducer 
14. The output of the transducer 14 is applied to a field-effect 
transistor preamplifier 16 whose gain is under the control of sensitivity 
control 16', which varies the supply voltage to the preamplifier as shown 
in FIG. 3. 
From the preamplifier 16 the ultrasonic signal is delivered to a transistor 
driver amplifier 20 which amplifies it again and in turn capacitively 
couples it to the input (pin 1) of a function generator 22 via capacitors 
29, 29'. The generator 22 acts to frequency shift the amplified ultrasonic 
signal into the audible frequency range. This audio signal from the output 
(pin 2) of circuit 22 is applied to a headphone transistor amplifier 24 
which may include a Darlington transistor arrangement as indicated by the 
letter D on the drawing. From the amplifier 24 it passes via a filter and 
transformer 25 to a set of headphones 26. The filter acts to eliminate the 
carrier and any extraneous frequencies, e.g. the sum signal, from the 
signal applied to the headphones. Circuit 22 also has its output applied 
through a transistor driver amplifier 23 to a frequency-compensated meter 
amplifier circuit 35. Filter elements in circuit 35 also act to cancel the 
carrier signal and any extraneous signal, leaving only the audio signal. 
Following the amplifier 35 there is a temperature compensation circuit 36 
and a meter signal conditioning circuit 37 whose output is applied to a 
meter 38. Circuit 37 acts to put the signal in proper condition for 
display by the meter 38. 
Temperature compensation circuit 36 includes a thermistor 36' which is part 
of a voltage divider network leading to the meter signal condition circuit 
37. As a result, an increase in temperature will cause the proportion of 
voltage applied to the meter circuit to be changed in such a way as to 
compensate for the change in gains due to the temperature increase. 
In meter signal condition circuit 37 there are two diodes, one in the form 
of a germanium transistor 39 which is wired as a diode. The effect of this 
diode is to rectify the AC signal received so as to generate a DC signal 
for application to the meter. This DC signal is ordinarily applied through 
resistor 37' to the meter. When the meter is to be operated in a 
logarithmic mode, a switch 37" is closed, placing a diode 39' and resistor 
37"' across resistor 37'. Resistor 37"' is significantly smaller than 
resistor 37' and thus changes the scale for the meter. In addition the 
diode 39' in series with resistor 37"' acts to crete a voltage threshold 
which eliminates noise in the circuit when it is in the more sensitive 
logarithmic scale. 
As a result of the arrangement of FIG. 1, ultrasonic signals leaking from 
container 10 are picked up by transducer 14, amplified and frequency 
shifted so that a user will have an indication of the existence of a leak 
through the sound heard in headphones 26 and level displayed on meter 38. 
The actual frequency shift of the ultrasonic signal is accomplished in 
function generator 22. This generator may be a commercially-available 
integrated circuit, such as the EXAR 2206, which has been wired to produce 
sine wave outputs at a frequency determined by tuning resistor 30 
connected to pin 7 of the circuit as well as capacitor 22' connected 
between pins 5 and 6. One characteristic of this circuit is that a 
particular bias applied to its input (pin 1) will cause it to produce an 
amplitude-modulated (AM), suppressed-carrier output. The bias to obtain 
this suppressed-carrier modulation is derived from variable resistor 28. 
If capacitor 22' and resistor 30 are selected to produce a carrier signal 
that differs from the ultrasonic signal by a frequency in the audio band, 
the output of circuit 22 will be an audio signal related to the input 
ultrasonic signal and a much higher signal. In particular the output 
signal will be equivalent to the sum and difference frequencies of the 
ultrasonic signal and the carrier signal generated by circuit 22, but the 
carrier signal itself will not be present in the output. If, for example, 
resistance 30 is set so that circuit 22 generates a 42 KHz signal and the 
ultrasonic signal applied through capacitors 29 and 29' to circuit 22 is 
at 40 KHz, the output will be at 2 KHz and at 82 KHz. Since only the audio 
band signal is desired, filter circuits 25 and 35 are designed to 
eliminate the 82 KHz sum signal. 
Although a proper bias on the input to circuit 22 will eliminate or 
suppress the carrier generated by that circuit, it has been found that 
this adjustment is critical and some carrier may leak through due to 
temperature and voltage variations. Also as the carrier frequency is 
changed due to changes in the setting of resistor 30 there are changes in 
the circuit operation that may cause the carrier to appear in the output 
unless there is an adjustment of the bias. In order to provide this 
adjustment a servo or feedback network is provided. In particular the 
output of circuit 22 (pin 2) is applied through driver circuit 23 to a 
highpass filter 26 which passes only frequencies above 20 KHz. The output 
of filter 26 is applied to a Darlington transistor arrangement that forms 
amplifier 27. Amplifier 27 acts to amplify and integrate or average, via 
capacitor 27', the carrier signal received. This signal is also inverted 
in the amplifier so as to vary the bias from resistor 28 in such a way as 
to correct for the presence of the carrier in the output signal. Although 
some carrier may always exist, sufficient to create an error signal, this 
level will be low enough to avoid erroneous indications in the earphones 
26 and the meter 38. 
In one embodiment of the present invention the transducer is used to detect 
vibrations, for example from bearings, through a direct mechanical 
connection between the noise source, e.g., the bearing housings, and the 
transducer crystals. In such a case the bandwidth of signals detected may 
range from 20 KHz to 100 KHz, and particular bands of this frequency range 
may be selected by changing the position of variable resistor 30 so as to 
create a carrier frequency in circuit 22 which moves over this range. 
In another embodiment the transducer is located in space at some distance 
from a hole in a container that has 40 KHz generator 12 within it. In such 
a case the preferred 40 KHz mode of operation of the detector is used. 
When operating in this acoustic pick-up mode where the transducer is held 
in space, the main tuning resistance 30 is rotated to a detent at one end. 
Near this detent the resistance of main turning resistor 30 drops to near 
zero. Upon entering the detent, a switch 34 changes position, thereby 
connecting preset resistor 32 into the frequency control line for circuit 
22 and enabling a 40 KHz resonant filter 21. The resonant filter 21 acts 
to boost frequencies in the 40 KHz range and to eliminate those 
substantially outside this band of frequencies. Resistor 32 acts to set 
the frequency of circuit 22 at some value in the vicinity of 40 KHz, e.g. 
42 KHz. 
Power for the circuit of FIG. 1 is created by a battery 40 which is applied 
through power switch 50 to a DC-to-DC converter 42 that generates a 15 
volt output applied to the various parts of the circuit. Positioned across 
the battery input to the DC-to-DC converter is a recharging indicator 
circuit which includes elements 44-47. When the power switch 50 is closed, 
the voltage from battery 40 applies bias to transistors 45 and 47, which 
are Darlington transistor arrangements. Variable resistor 44 is set such 
that transistor 47 is turned on, thereby turning transistor 45 off. When 
the battery voltage drops, however, the bias on the base of transistor 47 
becomes insufficient to cause it to remain in the conduction state. As it 
turns off, it causes transistor 45 to turn on, which causes current to be 
drawn through light emitting diode 46. This diode indicates when the 
battery must be recharged. It has been found that through the use of 
cascaded arrangements of Darlington transistors, such as transistors 45 
and 47, the circuit has a sharp response so that a particular level of 
degradation in the amplitude of the battery voltage can be set by variable 
resistance 47 and it will be accurately indicated. 
Now that the details of the receiver and frequency conversion circuits have 
been described, reference will be made to the ultrasonic generator 12, the 
details of which are shown in FIG. 2. This generator is typically set to 
produce an output ultrasonic signal at approximately 40 KHz. To accomplish 
this a 40 KHz crystal transducer 114, such as that made by Panasonic 
Corp., is employed in the feedback circuit of an oscillator which includes 
transistor 110. When the circuit is turned on, current initially flows 
from the power supply through an amplitude control resistance 118 and the 
primary of a step-up transformer 112 to transistor 110. This creates a 
voltage level across the crystal transducer 114 which will begin to 
resonate at its fundamental frequency of 40 KHz. The resonant frequency 
from transducer 114 is passed to the base of transistor 110 through a 
filter network comprising capacitor 115, a parallel combination 116 of a 
capacitor and resistor, and capacitor 117. This filter network passes 
signals in the 40 KHz range and eliminates harmonics. Inversion of the 
feedback signal in order to have the positive feedback needed for 
oscillation is by means of the wiring of the secondary of transformer 112 
which is connected across transducer 114. With this arrangement the 
crystal will be driven at its resonant frequency of 40 KHz and will 
produce an ultrasonic wave at that frequency. 
If the generator is located within a container, such as container 10 shown 
in FIG. 1, the ultrasonic energy produced by the crystal will fill the 
container and will leak out of the container at openings, such as hole 
10'. When in a container, however, a constant output from transducer 114 
will result in the establishment of standing waves in the container. Thus 
it is possible that a null of the standing wave will occur at the position 
of the leak 10', which would result in failure to detect the leak. To 
compensate for this, generator 12 causes the standing wave pattern to vary 
slowly by varying its output frequency. This is accomplished by means of 
resistor 120, capacitor 122 and flasher light-emitting diode 24, e.g., a 
Litronix Flasher Mod. FRL-4403. With this arrangement current flows 
through resistor 120 and charges up capacitor 122. As it charges it 
changes the bias on transistor 110, which results in a frequency shift in 
the oscillator of perhaps 2-5 KHz. When the breakdown voltage of the 
flasher 124 is reached it will discharge the voltage across capacitor 122 
and the process will repeat. Typically the resistance 120 and capacitor 
122 are selected so that the variation in frequency occurs at a sub-audio 
rate, e.g. 5-20 Hz. However, any convenient frequency variation can be 
selected. 
The circuit of FIG. 2 can be altered by replacing the flasher LED with some 
other type of voltage breakdown device. Also, the secondary of transformer 
112 can be wired as shown in FIG. 2 so there is no voltage step-up. This 
arrangement lowers the impedance across the crystal, thereby lowering its 
Q and increasing the frequency variation achieved. 
By varying the frequency output at a slow rate, not only is the problem of 
node points avoided, it is psychologically easier for a listener to detect 
leaks. In addition the receiver circuit could be modified so as to respond 
only to this varying frequency, thus making it more sensitive and 
improving its signal-to-noise ratio. 
The transducer and preamplifier which detect the signal produced by the 
generator of FIG. 2 are shown in the schematic of FIG. 3. In FIG. 3 a 
group of series-connected ultrasonic transducers are shown for receiving 
ultrasonic energy and converting it into electrical energy. Each 
transducer may be a 40 KHz piezoelectric crystal, such as those 
manufactured by Panasonic Corporation. While three such crystals are shown 
in FIG. 3, the number of such crystals is not critical and two, or more 
than three, may be employed. By arranging these transducers adjacent to 
each other in one plane, each receives acoustic energy and the electrical 
signals created thereby are added because of the series connection, 
thereby giving a larger output than when a single transducer is used. The 
effectiveness of such an arrangement of transducers could be reduced if 
the impedance of the preamplifier is low. To avoid this a field-effect 
transistor preamplifier is used with an input stage field-effect 
transistor 220. The output of this transistor is applied through capacitor 
222 to field-effect transistor 230. The drain of transistor 230 is the 
output of the preamplifier that is connected to driver circuit 20 shown in 
FIG. 1. As explained previously, the receiver can be used in a mode which 
allows it to scan ultrasonic frequencies from 20 KHz to 100 KHz or it can 
be used in a preferred 40 KHz band. When used in the preferred 40 KHz 
band, capacitors 212 and 214 as well as inductance 216, are used to create 
a resonant circuit with a frequency response centered about 40 KHz. This 
will increase the size of the voltage at 40 KHz up to 40 times and will 
greatly attenuate frequencies removed from this center band. When this 
resonant circuit is used it is important that transistor 220 be a MOSFET. 
The gain of the preamplifier is controlled by variable resistance 16' shown 
in FIG. 1. The voltage derived from the variable resistance is applied to 
the drains of transistors 220, 230 through respective resistances. Because 
the output level is achieved in this manner, i.e. with the amplitude 
control being outside the signal path, any scratchiness in resistor 16' 
will not be heard in the microphones 38. In addition, any overload in the 
preamplifier is easily compensated by reducing the voltage level to the 
circuit. 
The transducers 210, regardless of how many are used, are serially-wired 
together and are spaced close together, preferably in a flat plane. Having 
these transducers arranged next to each other on a flat plane effectively 
focuses the overall transducer at infinity. If desired, the transducers 
could be located on a curved or parabolic surface such that they would be 
focused to receive energy originating from a source at any distance closer 
than infinity. However, in most cases this is not necessary. 
Referring once more to FIG. 1 the detection of a leak 10' can also be 
achieved without an ultrasonic source if the container is pressurized and 
its interior surface is covered with a liquid having a particular range of 
surface tension. In such a case the liquid is caused to form small bubbles 
by the passage of the pressurized gas through the leak, which bubbles 
break and reform when they have obtained a diameter of from 0.005 to 0.02 
inches. Breaking of bubbles of this size produces ultrasonic energy that 
can be detected by the transducer of circuit 14. If the liquid has too low 
a surface tension, e.g. water, then the bubbles will not form and no 
improvement in results will be achieved. On the other hand, if the surface 
tension is too great, such as that for a viscous soap, the bubbles grow 
too large for ultrasonic energy generation when they burst and the rate of 
generation is too slow. Therefore, a balanced surface tension between 
these extremes is necessary. However, suitable liquids can easily be found 
with this characteristic by experimentation. One liquid which has been 
found to be particularly useful consists of a 4% aqueous solution of 
sodium (2) ethyl hexyle sulfate, i.e., C.sub.4 H.sub.9 CH(C.sub.2 
H.sub.5)CH.sub.2 SO.sub.4 Na. With this liquid the popping noise created 
by the forming and bursting of the bubbles in an excellent indicator of 
the existance of a leak. This liquid has a surface tension at 25.degree. 
C. of 63 dynes per centimeter. Water has a surface tension of about 23 
dynes/cm. Thus liquids with surface tension in the range of 50-70 dynes/cm 
are preferable. 
With this liquid small leaks which cannot be detected will produce bubbles. 
When these bubbles break, however, they do produce ultrasonic energy which 
can be detected. 
While the present invention has been particularly shown and described with 
reference to a preferred embodiment thereof, it will be understood by 
those skilled in the art that various changes in form and details may be 
made therein without departing from the spirit and scope of the invention.