About Ultrasonics
Ultrasonic sensors are used around the world, indoors and outdoors in the harshest
conditions, for a variety of applications. Our ultrasonic sensors, made with piezoelectric
crystals, use high frequency sound waves to resonate a desired frequency and convert
electric energy into acoustic energy, and vice versa. Sound waves are transmitted to and
reflected from the target back to the transducer. Targets can have any reflective form,
even round. Certain variables, such as target surface angle, changes in temperature and
humidity, and reflective surface roughness, can affect the operation of the sensors.
There are two types of ultrasonic sensors
Proximity Detection: An object passing within the preset range will be detected and
generate an output signal. The detect point is independent of target size, material or
reflectivity.
Ranging Measurement: Precise distance(s) of an object moving to and from the sensor
are measured via time intervals between transmitted and reflected bursts of ultrasonic
sound. Distance change is continuously calculated and outputted.
15 Applications Using Ultrasonic Sensors:
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Loop control
Roll diameter, tension control, winding and unwind
Liquid level control
Thru beam detection for high-speed counting
Full detection
Thread or wire break detection
Robotic sensing
Stacking height control
45° Deflection; inkwell level detection; hard to get at places
People detection for counting
Contouring or profiling using ultrasonic systems
Vehicle detection for car wash and automotive assembly
Irregular parts detection for hoppers and feeder bowls
Presence detection
Box sorting using multi-transducer ultrasonic monitoring system
Frequency is characterized as the number of signals or waves that may occur at a
fixed time. Hertz units for the frequency are (Hz). Based upon the frequency values,
these frequencies are broken into many ranges. There is Very Low Frequency
(VLF), Low Frequency (LF), Medium Frequency (MF), High Frequency (HF), Very
High Frequency (VHF), Ultra High Frequency (UHF), Super High Frequency (SHF),
and Highly High Frequency (SHF) (EHF). Depending on the type of frequency, the
frequency range can vary. The VLF frequency spectrum varies between 3 and 30
kHz. The LF frequency spectrum varies between 30 kHz and 300 kHz. The MF
frequency spectrum varies between 300 and 3000 kHz.
One type of sound-related transducer is the ultrasonic transducer. The electrical
signals are transmitted to the target by these transducers and after the signal
reaches the object, it returns to the transducer. This transducer tests the distance
of the object in this method, not the amplitude of the signal. For the calculation of a
few parameters, these transducers use ultrasonic waves. In different regions, it has
a wide variety of uses. The ultrasonic wave frequency spectrum is over 20 kHz.
These are primarily used in applications that measure distance. The ultrasonic
transducer is indicated in the following illustration.
The HF frequency spectrum varies between 3 MHz and 30 MHz. The UHF frequency
spectrum varies between 300 MHz and 3000 MHz. The SHF frequency spectrum
varies from 3 GHz up to 30 GHz. The EHF frequency spectrum varies between 30
GHz and 300 GHz. A description of the ultrasonic transducer and its function is
discussed in this article.
I. Ultrasonic Transducer Working Principle
This vibrates throughout the particular frequency spectrum when an electrical signal
is added to this transducer and produces a sound wave. These sound waves fly and
these sound waves will reflect the transducer's echo knowledge if some barrier
appears. And this echo transforms into an electric pulse at the end of the transducer.
The time interval between transmitting the sound wave to the receiving echo signal
is determined by the transducer here. At 40 kHz, the ultrasonic transducer gives an
ultrasonic pulse that passes through the air. Such transducers are safer than
infrared transducers because dust, black materials, etc. are not influenced by these
ultrasonic transducers/transducers. In suppressing noise distortion, ultrasonic
transducers exhibit excellence.
Ultrasonic transducers are primarily used to use ultrasonic waves to assess the size.
The following formula will calculate the distance:
D=½*T*C
Here, the distance is indicated by D
The time gap between transmitting and receiving ultrasonic waves is shown by T
C is a sonic velocity indication.
II. Ultrasonic Transducer Features
1. Performance
The core of the ultrasound probe is a piezoelectric chip in its plastic or metal
jacket. There are many kinds of materials that make up the wafer. The size of the
wafer, such as diameter and thickness, are also different, so the performance of
each probe is different, we must know its performance before use. The main
performance indicators of ultrasonic transducers include:
2. Working Frequency
The working frequency is the resonance frequency of the piezoelectric wafer.
When the frequency of the AC voltage applied to its two ends is equal to the
resonance frequency of the chip, the output energy is the highest and the
sensitivity is the highest.
3. Operating Temperature
Because the Curie point of piezoelectric materials is generally relatively high,
especially the ultrasonic probe used for diagnosis uses low power, the operating
temperature is relatively low, and it can work for a long time without failure. The
temperature of medical ultrasound probes is relatively high and requires separate
refrigeration equipment.
4. Sensitivity
Mainly depends on the manufacturing wafer itself. The electromechanical coupling
coefficient is large and the sensitivity is high; on the contrary, the sensitivity is low.
5. System Components
It is composed of sending transducer (or wave transmitter), receiving
transducer (or wave receiver), control part, and power supply part. The transmitter
transducer is composed of a transmitter and a ceramic vibrator transducer with a
diameter of about 15mm. The function of the transducer is to convert the electric
vibration energy of the ceramic vibrator into super energy and radiate into the air;
while the receiving transducer is transduced by the ceramic vibrator The
transducer is composed of an amplifier and an amplifier circuit. The transducer
receives the wave to produce mechanical vibration and converts it into electrical
energy, which is used as the output of the transducer receiver to detect the
transmitted super. In actual use, the ceramic vibrator of the transmitter is also
used. It can be used as the ceramic vibrator of the receiver transducer company.
The control part mainly controls the pulse chain frequency, duty cycle, sparse
modulation, and counting and detection distance sent by the transmitter. The
ultrasonic transducer power supply (or signal source) can be DC12V ± 10% or 24V
± 10%.
6. Operating Mode
Ultrasonic transducers use the acoustic medium to perform non-contact and wearfree detection of the detected object. Ultrasonic transducers can detect
transparent or colored objects, metal or non-metal objects, solid, liquid, and
powdery substances. Its detection performance is hardly affected by any
environmental conditions, including smoke and dust environments and rainy days.
7. Advantages & Disadvantages
There are benefits and a few pitfalls to every system. The perks of the ultrasonic
transducer will be discussed here.
 In any form of material, these ultrasonic transducers can be tested. All
sorts of textures they can detect.
 The temperature, water, dust, or any of the ultrasonic transducers are not
affected.
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Ultrasonic transducers can operate in a good way in every form of
environment.
 It may also measure elevated sensing distances.
The following are the drawbacks of these transducers:
 Ultrasonic transducers are susceptible to the change in temperature. The
ultrasonic reaction will alter this temperature variance.
 During the reading of reflections from small objects, thin and soft objects,
it can face issues.
III. Ultrasonic Transducer Types
Based on factors like piezoelectric crystal arrangement, footprint, and frequency,
there are different types of ultrasonic transducers available. They are:
Linear ultrasonic transducers - The structure of piezoelectric crystals is linear in
this type of transducers.
Normal Ultrasonic - Transducers-Convex transducers are also known as this form.
The piezoelectric crystal of this type is in a curvy shape. These are superior to in depth tests.
Phased Array Ultrasonic Transducers - There is a limited footprint and low
frequency of phased array transducers. (2 MHz-7 MHz)
The ultrasonic transducers again have distinct forms for non-destructive studies.
Contact transducers, transducers of angle beams, transducers of delay lines,
transducers of immersion, and transducers of dual components.
IV. Ultrasonic Transducer Applications
The Ultrasonic Transducers implementations are
In diverse fields, such as automotive, medical, etc, these transducers have many
applications. Owing to ultrasonic waves, they have more uses. This helps to locate
the targets, to determine the distance of the objects to the target, to find the
object's location, to quantify the level, and to support the ultrasonic transducers.
In the medical area, the ultrasonic transducer is used for diagnostic tests, surgical
instruments for cancer care, internal organ testing, heart checkups, ultrasonic
transducers for eyes and uterus checkups.
Ultrasonic transducers have few major uses in the industrial sector. Via these
transducers, in manufacturing line management, liquid level monitoring, wire break
detection, people detection for counting, car detection, and many more, they can
determine the distance of such objects to prevent a collision.
Acoustic Waves[edit | edit source]
Acoustic waves are mechanical and longitudinal waves (same direction of vibration as
the direction of propagation) that result from an oscillation of pressure that travels
through a solid, liquid or gas in a wave pattern. These waves show numerous
characteristics including wavelength, frequency, period and amplitude. Acoustic
waves are perceived by the ear as sound.
Wavelength[edit | edit source]
Distance between two consecutive points with the same spacing from the
equilibrium positions and the same oscillatory movement. It depends on the media
in which the wave is propagating. The SI unit is meter (m).
Frequency[edit | edit source]
Frequency is the number of oscillations, or waves, per unit of time. Sound waves
with higher frequencies have higher pitches than those with lower frequencies. It
depends only on the frequency of oscillation of the emitting source. The SI unit is
hertz (Hz).
Period[edit | edit source]
The time interval between the emission of two pulses is given by the period,
which is reciprocal to the frequency. This characteristic only depends on the
period of oscillation of the emitting source. The SI unit is the second (s).
Amplitude[edit | edit source]
Amplitude is the maximum deviation in the oscillation relatively to the
equilibrium position. The greater the amplitude of an acoustic wave, the
louder the sound. It depends on the amplitude of the emitting source and on
the propagation medium. The SI unit is the meter (m).
Speed of Sound[edit | edit source]
Acoustic waves travel with the same speed as sound. Sound speed is the
distance travelled by the sound during a given time and is dependent on
temperature and pressure conditions. At Normal Temperature (15 °C) and
Pressure (NTP) conditions sound speed has a value of 340 ms-1.
Sound and motion
When we are moving, or a source producing a sound
is moving, we hear things differently. You may have noticed that a train whistle gets lower
as it passes you. The whistle is not changing pitch
, but you are hearing a change. This principle is known as the Doppler effect
. The Doppler effect
is named after the Austrian physicist, Christian Johann Doppler, who discovered it.
What did Christian Johann Doppler discover?
Doppler claimed that if a sound
is getting closer to you, either because its source is approaching you or because you are
going towards the source, the sound
will seem higher than it really is. If you are heading away from a source or it is going away
from you, he believed the sound
would seem lower than its actual pitch
. To test his theory, scientists hired 15 trumpeters to play on a moving train. As the train
passed by them, they heard a drop in pitch
, just like Doppler predicted.
The Doppler effect
happens because distance affects the amount of time it takes you to hear the sound
. Imagine you are playing in the park and your friend rolls a ball to you. The ball would reach
you sooner if you walked towards it and later if you moved away from it. The same is true
for sound
. Remember that frequency
is wavelengths per time. If you hear a frequency
in a shorter amount of time, it seems like you are hearing a higher frequency
. For example, say you heard a sound
that had 50 wavelengths by the time it reached you, it would have taken it 5 seconds to reach
you. The frequency
of that sound
is 50 divided by 5, or 10 Hertz
. Imagine you heard the same sound
, but this time you were moving towards its source and it only took 2 seconds for 50
wavelengths to reach you. Now the frequency
you hear is 50 divided by 2, or 25 Hertz
. The frequency
seemed higher because you were moving. If you were not moving, after 2 seconds, only 20
wavelengths would have reached you and the frequency
would still sound
like 10 Hertz
.
The opposite happens when the distance between you and a source of sound
widens. Now it takes longer for you to hear a certain amount of wavelengths. Therefore,
the frequency
seems lower. The Doppler effect
makes a pitch
appear to change when you, or the source, are in motion.
Review:
1. The doppler effect
is an effect in which when a sound
source is moving it appears as if the frequency
is higher when the source is moving towards you and as if the frequency
is lower when the source is moving away from you.
Acoustic Impedance (Z)- The resistance of a material to the passage of sound
waves. The value of this material property is the product of the material density and
sound velocity. The acoustic impedance of a material determines how much sound
will be transmitted and reflected when the wave encounters a boundary with another
maAcoustic Impedance (Z)- The resistance of a material to the passage of sound
waves. The value of this material property is the product of the material density and
sound velocity. The acoustic impedance of a material determines how much sound will
be transmitted and reflected when the wave encounters a boundary with another
material. The larger the difference in acoustic impedance between two materials, the
large the amount reflected will be.
Sound
travels through materials under the influence of sound
pressure. Because molecules or atoms of a solid are bound elastically to one another, the
excess pressure results in a wave propagating through the solid.
The acoustic impedance
(Z) of a material is defined as the product of its density
(ρ) and acoustic velocity
(V).
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Acoustic impedance
is important in
1. the determination of acoustic transmission and reflection at the boundary
of two materials having different acoustic impedances.
2. the design of ultrasonic
transducers.
3. assessing absorption
of sound
in a medium.
Wave Interference
When two or more sound
waves from different sources are present at the same time, they interact with each other to
produce a new wave. The new wave is the sum of all the different waves. Wave interaction is
called interference
. If the compressions and the rarefactions of the two waves line up, they strengthen each other
and create a wave with a higher intensity
. This type of interference
is known as constructive.
When the compressions and rarefactions are out of phase, their interaction creates a
wave with a dampened or lower intensity
. This is destructive interference
. When waves are interfering with each other destructively, the sound
is louder in some places and softer in others. As a result, we hear pulses or beats in the sound
.
Dead spots
Waves can interfere so destructively with one another that they produce dead spots,
or places where no sound
at all can be heard. Dead spots occur when the compressions of one wave line up with the
rarefactions from another wave and cancel each other. Engineers who design theaters or
auditoriums must take into account sound wave interference
. The shape of the building or stage and the materials used to build it are chosen based
on interference
patterns. They want every member of the audience to hear loud, clear sounds.
Sound Traveling Between Materials
Remember that sound
travels faster in some materials than others. Sound
waves travel outward in straight lines from their source until something interferes with their
path. When sound
changes mediums, or enters a different material, it is bent from its original direction. This
change in angle of direction is called refraction
. Refraction
is caused by sound
entering the new medium at an angle. Because of the angle, part of the wave enters the new
medium first and changes speed. The difference in speeds causes the wave to
bend. Refraction
is also discussed more in the following page.
Critical Angle
The angle of refraction
depends on the angle that the waves has when it enters the new medium. As the angle from
the wave to the barrier between the two mediums gets smaller, the angle of refraction
also gets closer to the barrier. When the wave’s entering angle reaches a certain point, called
the critical angle
, the refraction
is parallel to the dividing line between the mediums. The critical angle
depends on the two mediums the sound
is coming from and going to. The speed of sound
is different in every medium. Because of this, even if the sound
hits at the same angle, the angle of refraction
will vary for different mediums. The greater the difference in speed between the two
mediums, the greater the critical angle
will be.
If sound
hits the new medium with any angle smaller than the critical angle
, it will not be able to enter. Instead it will bounce off, or be reflected, from the dividing line.
When a wave is reflected, it returns with an angle equal to the one with which it hit.
Whenever sound
hits a new medium, part of it is reflected back. The rest enters the new medium and is
refracted. Imagine sound
is traveling through the air and hits the wall of a brick building. Some of the wave is
reflected, but much of it enters the brick. The part of the wave going through the brick is now
going faster than the part in the air. This is because brick is a solid whose molecules are
closer together and can transmit sound
more quickly. This difference in speeds caused the wave to bend, or be refracted. Suppose
that the wave hits the building with an angle that is smaller than its critical angle
. This time, the wave cannot enter the brick and all of it is reflected. If the wave struck the
wall with an angle of 15 degrees, it would reflect back with the same angle from the other
side. Since there are 180 degrees total, the reflected angle would be 165 degrees, 15 degrees
measured from the other direction.
In the field of aerospace, real-time monitoring and accurate
measurement of liquid fuel consumption in fuel tanks are very
necessary [1,2]. Therefore, the research and development of a liquidlevel sensor are particularly important.
There are two types of liquid-level measurement technologies, which
are invasive and non-invasive [3]. The invasive types include
capacitive [4], resistive [5], float-type [6], magnetostriction type [7],
optical fiber liquid-level meter [8], and many more. As the fuel tank is
a closed container, its internal environment is high pressure, low
temperature, etc., and its internal liquid fuel is inflammable and
explosive. Therefore, it is not suitable to use a contact sensor
introduced into the container to measure the liquid level [9,10].
Ultrasonic non destructive testing (NDT) technology has gradually
become the mainstream for liquid-level detection [11,12].
There are some liquid-level measuring devices based on ultrasonic
propagation characteristics, which are mainly divided into three
categories: interface reflection method [13], penetrative method [14],
and attenuation method [15]. The detection accuracy of the interface
reflection method and the penetrative method is greatly affected by
the temperature of the internal medium. For large containers with
diameters over 1 m, the long transmission distance and bubbles or
impurities in the liquid will seriously affect the transmission of
ultrasonic waves. The penetration attenuation characteristics of liquid
medium will also seriously affect the reliability of measurement
[16,17]. Attenuation is a relatively new technique that requires only
an ultrasonic transducer to be installed on one side of the container
wall. When the internal medium at the measurement point is different,
the attenuation range of ultrasonic echo energy on the container wall
is different. According to the time from the reception of the echo to the
attenuation, it can distinguish whether the internal liquid level
reaches the detection point, so as to play the role of liquid-level
monitoring [18,19]. Therefore, the ultrasonic attenuation method has
relatively good measurement accuracy and reliability.
The ultrasonic transducer emits a beam of ultrasonic waves, but due
to the existence of the near field, the effective reflection echo cannot
be received, resulting in inaccuracy of the measurement. Therefore,
when using ultrasonic waves for measurement, it is necessary to
ensure that the measured surface is in the far-field area of the sound
pressure to obtain an effective signal [20]. Buffer blocks are widely
used in ultrasonic applications. At present, two kinds of rods with
cylindrical and cone structures are used by researchers. Zhang et al.
[21] studied the shape and boundary conditions of the buffer block
and proposed a high-performance rod with shape based on a cone
reference surface. Hoppe et al. [22] found an optimized geometry of a
buffer rod for an ultrasonic density sensor. They can measure the
amplitude with high accuracy and low noise. Fischer et al. [23] used a
conical buffer element with a combination of two materials to obtain a
reference for the pulse amplitude of the emitted signal. The buffer
material connected to the transducer is polymethyl methacrylate
(PMMA), and the material in contact with the measured liquid is highgrade steel. However, the acoustic impedance of the buffer block
material is not close to that of the measured liquid, so the sensitivity is
low. Liu et al. [24] made a detailed comparison description of the
buffer block materials and drew the curve of the sound velocity in
PMMA varying with frequency and temperature. Combined with other
physical properties of PMMA, it is finally proposed that PMMA is most
suitable for the measurement experiment of liquid acoustic
properties.
To sum up, most of the researchers studied the material, shape,
boundary conditions, and internal noise of the buffer block. For the
length of the near-field area of ultrasound, the researchers only say
that the acoustic beam range should be more than 3 times the length
of the near field when using the p-wave testing [22]. However, if the
length of the buffer block is too short, the near field region cannot be
avoided, and if it is too long, it may cause ultrasonic attenuation. At
present, no team has proposed an exact value of the optimal and the
minimum size of the buffer block required to avoid the near-field area.
In conclusion, based on the attenuation method, this paper builds a
fixed-point liquid-level monitoring system. This method is based on
the ultrasonic impedance method: the ultrasonic transducer emits a
group of continuous ultrasonic waves to monitor whether the height
of the liquid level is higher than the transducer by measuring the
energy values of the received echo of the container wall. In this paper,
a buffer block is added between the probe and the container wall. We
used different lengths of buffer blocks to conduct experiments and
studied the relationship between the length of the near field of the
ultrasonic wave and the amplitude of the received echo. Finally, the
experiment was conducted to find the minimum size of the ultrasonic
probe and buffer block that can get effective results when using this
method for liquid-level monitoring. The research in this paper
provides an effective solution to avoid the near-field area for
experiments such as liquid-level measurement based on ultrasound. It
also provides a powerful basis for the selection and design of
ultrasonic probes in other experiments.
Go to:
2. Theory and Methods
2.1. Principle of Ultrasonic Impedance Method
This paper builds an experimental system for liquid-level monitoring
based on the ultrasonic impedance method. Ultrasonic waves can
propagate in any medium in the form of a wave. It propagates along a
straight line [25]. In the process of transmission, diffraction,
refraction, reflection, attenuation, and other phenomena will occur
when encountering obstacles in the path [26]. When the ultrasonic
transducer emits a beam of ultrasound and reaches the interface
between the inner wall of the container and the internal medium,
transmission and reflection will occur. The sound intensity
reflectance, R, and sound intensity transmittance, T, can be calculated
by Equations (1) and (2) [27]:
R=IaI=(Z2−Zi)2(Z2+Zi)2
(1)
T=ItI=1−R=4Z2Zi(Z2+Zi)2
(2)
where Ia is the reflected sound intensity, W/m2, It is the transmitted
sound intensity, W/m2, I is the incident sound intensity, W/m2, Z2 is the
acoustic impedance of the tested container, Mrayl, and Zi is the
acoustic impedance of the internal medium, Mrayl. According to
Equation (2), transmittance and reflectance have an inverse
relationship, the more ultrasonic waves transmitted into the
container, the less echo energy reflected, and vice versa.
Ultrasonic waves can propagate in solids as longitudinal waves and
transverse waves. Acoustoelastic effect means that in an isotropic
solid medium, due to the effect of stress, the material has the
characteristic of acoustoelasticity. That is, the ultrasonic wave velocity
changes with the change of the stress state. But, ultrasonic waves can
only propagate in the form of longitudinal wave in the liquid and gas
medium, so the acoustoelastic effect is not considered.
2.2. Ultrasonic Near-Field and Far-Field Areas
A beam of ultrasound emitted by an ultrasonic transducer includes
both near-field and far-field areas [28]. The sound pressure near the
wave source fluctuates sharply due to the interference of the wave
and a series of sound pressure maximum and minimum appears,
which is cylindrical in shape. At this time, the sound pressure is
irregular, and the ultrasonic propagation is unstable [29]. The
distance between the last sound pressure maximum value and the
sound source is called the near field length, which is expressed by N,
and the area within the N is called the near-field area. The region
where the distance from the axis of the wave source to the wave
source is greater than the length of the near-field region is divergent
and is called the far-field region [30]. Its sound field diagram is shown
in Figure 1. The ultrasonic near-field area can be calculated by
Equation (3) [31]:
N≈D2/4λ=Aπλ
(3)
where D is the ultrasonic sensor diameter, m, A is the sensor area, m2,
and λ is the wavelength of ultrasonic wave propagation in the
medium, which can be calculated using Equation (4):
λ=cf
(4)
where c is the wave velocity of ultrasonic wave propagation in the
medium, m/s, and f is the ultrasonic frequency, Hz. Therefore, the
near-field length of a beam of ultrasound is related to the diameter
(area) of the piezoelectric plate and the speed and frequency of the
ultrasound propagation in the medium. At a certain frequency and
speed, the larger the diameter, the longer the near-field length.
Figure 1
Ultrasonic sound field.
The choice of buffer block material needs to consider several factors,
of which the robustness, durability, and sensitivity are particularly
important [21]. Puttmer et al. found that a low impedance material is
more sensitive when its acoustic impedance is the same order of
magnitude as the measured liquid [32]. The comparison of acoustic
impedance of common buffer materials and water is shown in Table 1.
Table 1
Comparison of acoustic impedance of common buffer materials and water. PMMA:
polymethyl methacrylate.
Materials
Material Types Acoustic Impedance (Mrayl) Reflectance (R)
Water
Liquid
1.48
100%
PMMA [34]
Polymer
3.26
37%
Quartz glass [35]
Glass
13.1
79.50%
Glass ceramics [22]
Glass
16.5
83.30%
Aluminum [36]
Metal
17.3
84%
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Polymers have a lower speed of sound than glass, ceramics, or metals,
and even the thickness of the buffer block is small, and the delay effect
is also good. Moreover, the polymer’s characteristic acoustic
impedance is close to water, making it more sensitive [33].
Considering that the buffer block should have lower acoustic
impedance and more regular acoustic characteristics, therefore,
polymethyl methacrylate (PMMA) is selected as the buffer block in
this paper. The characteristic acoustic impedance of PMMA is only
3.26 Mrayl, which is particularly suitable for measuring liquid acoustic
characteristics using reflection technology.
Angled Beam Transducers:
Miniature Angle Beam Transducers and Wedges are used primarily for testing of weld
integrity. Their design allows them to be easily scanned back and forth and provides a short
approach distance. Angle beam transducers are single element transducers used with a
wedge to introduce a refracted shear wave or longitudinal wave into a test piece.
Advantages:

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Three-material design of our Accupath wedges improves signal-to-noise characteristics while providing excell
High temperature wedges available for in-service inspection of hot materials
Wedges can be customized to create nonstandard refracted angles
Available in interchangeable or integral designs
Contouring available
Wedges and integral designs are available with standard refracted angles in aluminum
Applications:


Flaw detection and sizing
For time-of-flight diffraction transducers see page 25

Inspection of pipes, tubes, forgings, castings, as well as machined and structural components for weld defects
Focused Transducers:
A focused transducer can improve the sensitivity and axial resolution by
concentrating the sound energy to a smaller area. Immersion transducers are
typically used inside a water tank or as part of a squirter or bubbler system in
scanning applications.
Ultrasonic
Ultrasonic
transducers are manufactured for a variety of applications and can be custom fabricated
when necessary. Careful attention must be paid to selecting the proper transducer
for the application. A previous section on Acoustic Wavelength and Defect Detection gave a
brief overview of factors that affect defect
detectability. From this material, we know that it is important to choose transducers that have
the desired frequency
, bandwidth, and focusing
to optimize inspection capability. Most often the transducer
is chosen either to enhance the sensitivity
or resolution
of the system.
Transducers are classified into groups according to the application.

Contact transducers
are used for direct contact inspections, and are generally hand manipulated.
They have elements
protected in a rugged casing to withstand sliding contact with a variety of
materials. These transducers have an ergonomic design so that they are easy
to grip and move along a surface. They often have replaceable wear
plates to lengthen their useful life. Coupling
materials of water, grease, oils, or commercial materials are used to remove
the air gap
between the transducer
and the component being inspected.
Immersion

transducers
do not contact the component. These transducers are designed to operate in
a liquid environment and all connections are watertight. Immersion
transducers
usually have an impedance matching
layer that helps to get more sound
energy into the water and, in turn, into the component being
inspected. Immersion transducers
can be purchased with a planer, cylindrically focused or spherically focused
lens. A focused transducer
can improve the sensitivity
and axial
resolution
by concentrating the sound
energy to a smaller area. Immersion transducers
are typically used inside a water tank or as part of a squirter
or bubbler
system in scanning
applications.
More on Contact Transducers
Contact transducers
are available in a variety of configurations to improve their usefulness for a variety of
applications. The flat contact transducer
shown above is used in normal beam inspections of relatively flat surfaces, and where near
surface resolution
is not critical. If the surface is curved, a shoe
that matches the curvature of the part may need to be added to the face of the transducer
. If near surface resolution
is important or if an angle beam inspection is needed, one of the special contact transducers
described below might be used.
Dual element
transducers
contain two independently operated elements
in a single housing. One of the elements
transmits and the other receives the ultrasonic
signal. Active elements
can be chosen for their sending and receiving capabilities to provide a transducer
with a cleaner signal, and transducers for special applications, such as the inspection of
course grained material. Dual element transducers
are especially well suited for making measurements in applications where reflectors are very
near the transducer
since this design eliminates the ring down effect that single-element transducers experience
(when single-element transducers are operating in pulse
echo
mode
, the element cannot start receiving reflected signals until the element has stopped ringing
from its transmit function). Dual element transducers
are very useful when making thickness measurements of thin materials and when inspecting
for near surface defects. The two elements
are angled towards each other to create a crossed-beam sound
path in the test material.
Delay line
transducers provide versatility with a variety of replaceable options. Removable delay line
, surface conforming membrane, and protective wear
cap options can make a single transducer
effective for a wide range
of applications. As the name implies, the primary function of a delay line
transducer
is to introduce a time delay
between the generation of the sound
wave and the arrival of any reflected waves. This allows the transducer
to complete its "sending" function before it starts its "listening" function so that near
surface resolution
is improved. They are designed for use in applications such as high precision thickness
gauging of thin materials and delamination
checks in composite
materials. They are also useful in high-temperature measurement applications since the delay
line
provides some insulation to the piezoelectric element
from the heat.
Angle beam
transducers
and wedges are typically used to introduce a refracted shear
wave into the test material. Transducers can be purchased in a variety of fixed angles or in
adjustable versions where the user determines the angles of incidence and refraction
. In the fixed angle versions, the angle of refraction
that is marked on the transducer
is only accurate for a particular material, which is usually steel. The angled sound
path allows the sound
beam to be reflected from the backwall to improve detectability of flaws in and around
welded areas. They are also used to generate surface waves
for use in detecting defects on the surface of a component.
Normal incidence
shear
wave transducers are unique because they allow the introduction of shear waves
directly into a test piece without the use of an angle beam wedge
. Careful design has enabled manufacturing of transducers with minimal longitudinal
wave contamination. The ratio of the longitudinal
to shear
wave components is generally below -30dB.
Paint brush transducers are used to scan wide areas. These long and narrow
transducers are made up of an array of small crystals that are carefully matched to
minimize variations in performance and maintain uniform sensitivity
over the entire area of the transducer
. Paint brush transducers make it possible to scan a larger area more rapidly for
discontinuities. Smaller and more sensitive transducers are often then required to further
define the details of a discontinuity
.
As a beam of ultrasound travels outwards from the surface of the
transducer, the distribution in space of the ultrasonic energy
undergoes change. Axially, the intensity of the beam diminishes
gradually with distance along the central axis of the beam, while
laterally, at any plane perpendicular to the beam direction, the
intensity decreases rapidly with distance from the central axis.
Generally, the ultrasound beam spreads out, or
undergoes divergence, as it moves away from the transducer. The
term "ultrasound beam shape" is commonly used to describe the
manner in which the spatial distribution of the beam changes with
distance from the source. The beam shape has very significant
effects on the quality of the ultrasonic image, and on the tissue
depths that can be usefully interrogated using a particular beam.
This section examines the factors which influence ultrasound beam
shape, and the associated implications for ultrasonic imaging.
GENERAL SHAPE OF THE ULTRASOUND BEAM
It is helpful to consider first the general shape of the ultrasound
beam, and to introduce some terminologies used in describing the
beam, before examining the various factors which modify this
general shape.
The typical manner in which the ultrasound beam spreads out with
increasing distance from the transducer, T, is shown in Fig 1
Initially, between T and the plane P along the beam path, the beam
is narrow, with a small beam width, d, equal to about the diameter
of the piezoelectric crystal. This part of the beam is referred to as
the near field, or the Fresnel zone. Beyond P, the beam spreads
out (diverges) over a larger and larger area, with increasing beam
widths which result in a rapid deterioration of spatial resolution of
the image. This part of the beam is known as the far field, or
the Fraunhofer zone. The distance from the transducer to the plane
P is sometimes called the transition distance (in reference to the
change from Fresnel zone to Fraunhofer zone).
The length, D, of the Fresnel zone, and the beam width, d, at a given
plane across the beam, are important parameters which influence,
respectively, the practical tissue depth that can be interrogated with
the beam, and the spatial resolution in the ultrasonic image. The
narrow beam associated with the near field is desirable for good
spatial resolution. The length of this part of the beam therefore
determines the approximate tissue depth which, in practice, can be
investigated using the beam.
FACTORS INFLUENCING BEAM SHAPE
The shape of the ultrasound beam is affected by:

the size and shape of the ultrasound source

the beam frequency,

beam focusing.
1. Effect Of Source Size
The size of the ultrasound source affects the beam width, the length
of the Fresnel zone, and the angle of divergence beyond the near
field. For a transducer in which no focusing is applied, the length, D,
of the Fresnel zone is determined by the diameter of the transducer
and the wavelength of the ultrasound beam according to the
relation:
where r = radius of the transducer, 'A = wavelength of the
ultrasound beam and d = 2r is the diameter of the transducer.
Within the near field, the beam width is approximately equal to the
transducer diameter. We infer from the above equation that for an
unfocused transducer, the length of the Fresnel zone increases
rapidly as the beam width (or transducer diameter) is increased.
Conversely, the length of the Fresnel zone diminishes rapidly as the
transducer diameter is reduced. In addition, a small transducer
diameter results in a large angle of divergence beyond the near
field (see Fig 2 (a) and (b)), thereby diminishing the lateral
resolution rapidly.
An important practical implication of these observations is that,
although a narrow beam gives us good image resolution, narrow
beams should not be obtained only by making the transducer
smaller, as this would also reduce the depth of tissue interrogation.
It is for this reason that, in multicrystal transducers where many
small crystal elements are used, the crystals are not pulsed
individually, but in small groups of neighbouring crystals which then
provide an instantaneous beam wide enough to give a sufficiently
long length of the Fresnel zone.
In summary, the effects of source size on beam shape are:
(i) a small source provides a narrow beam initially, is associated
with a short Fresnel zone, and the beam diverges rapidly
beyond the near field.
(ii) a large source provides a broader beam initially, gives a
longer Fresnel zone, and the beam diverges more gradually,
thus providing better resolution of deeper structures.
2. EFFECT OF BEAM FREQUENCY
The above equationcan be modified by substituting the wavelength
of the ultrasound beam by
where v = velocity of ultrasound in the transmitting medium, and f
= beam frequency.
From this expression, we conclude that the length of the Fresnel
zone increases as the beam frequency is increased. Also, the
angle of divergence beyond the near field diminishes with
increasing frequency. The effect of higher frequencies is therefore
not only improved image resolution but also an increase in the
length of the useful near field. In practice, however, some of this
advantage is taken away by increased beam attenuation at higher
frequencies.
3. FOCUSING OF THE ULTRASOUND BEAM
The shape of the ultrasound beam can be influenced to varying
extents by applying different focusing methods.
(i) Shape of the crystal Element : The crystal element can be
suitably shaped by concave curvature to focus the ultrasound beam
(Fig 3 (a)). This is an internal focusing method, because it is
effected in the crystal itself. The degree of focusing will depend on
the extent of curvature (radius of curvature) of the crystal.
(ii) Acoustic Lenses : A concave mirror can be used to focus
ultrasound by reflection (Fig 3 (c)). Again, the degree of focusing
will depend on the radius of curvature. Acoustic lenses and mirrors
provide external focusing.
Fig 3: Mechanical methods of focusing a beam of ultrasound
(iii) Electronic focusing : Electronic focusing is employed in
multicrystal transducers. In such transducers with many crystal
elements, movement of the ultrasound beam across the plane of
interest in the subject is effected electronically by pulsing small
groups of crystal elements at a time. By applying a pulsing
programme with carefully controlled time delays between different
crystal elements, ultrasound waves from all the crystals in the array
can be made to arrive in phase at one particular point (the focus),
where they reinforce to produce a high intensity zone. The time
delay programme can also be applied during reception of echoes.
Electronic focusing offers the advantage of providing variable
focus, or dynamic focus, as opposed to the other methods which
provide fixed focus. Variable focusing is achieved by altering the
time delay programme.
(iv) Focus of a transducer, focal zone : The focus, F, of a
transducer is that point along the central axis of the beam which is
equidistant in time from all points on the surface of the transducer.
The times of flight of the ultrasound waves are equal for all linear
paths between the surface of the transducer and F. The waves
therefore arrive at F in phase and reinforce each other by
constructive interference. Attractive beam properties are associated
with the point F: the beam has its narrowest width, greatest
intensity, and best spatial resolution. The focus of a transducer is
not sharply defined. Areas within the beam close to F will have
properties which will closely match those at F itself. The region
around F over which these conditions prevail is called the focal
zone of the transducer (see Fig 4).
Classification of focusing:
The degree of focusing may be classified into three categories as
follows:

strong focusing (or short focusing)

medium focusing

weak focusing (or long focusing)
In all cases, fixed focusing gives a focal point which is nearer to the
transducer than the transition distance (length of the Fresnel zone).
Strong focusing brings the focal point very close to the transducer,
typically 2 - 4 cm. It achieves a high degree of beam narrowing, but
the beam diverges rapidly beyond the focal distance. It can only be
applied to transducers for high resolution examinations of small
parts. Weak focusing gives a focal point further away from the
transducer - typically more than 8 cm - and a gentle divergence of
the beam beyond the focus. It is preferred in diagnostic applications
because it provides an extended useful, narrow beam.
OPTIMIZATION OF SPATIAL RESOLUTION WITH TISSUE DEPTH
The shape of the ultrasound beam is of great significance in
ultrasonic imaging. Deliberate efforts are therefore required during
transducer design to control the beam shape to suit the desired
applications. Generally, a narrow beam would be desirable to
maximize spatial resolution of the image, as would be an extended
length of the near field in order to facilitate imaging to adequate
tissue depths. To achieve these goals requires that the size and
shape of the ultrasound source, the beam frequency, and focusing
of the transducer, be suitably chosen. In ultrasonic imaging, efforts
to enhance one desirable feature quite often works in opgosition to
another desirable feature. Thus, we have seen that increasing the
beam frequency improves image resolution, but also reduces beam
penetration due to increased attenuation. A large source of
ultrasound at the transducer may extend the useful range of the
beam, but it will diminish resolution in the near field. This means
that compromises must be made when conflicting interests come
into play. The process of balancing opposing interests is referred to
as optimization. Making the most appropriate choices concerning
beam shape characteristics involves optimizing spatial resolution
with beam penetration. Special purpose transducers can be
designed to suit specific applications. For example, in
ultrasonography of small parts, high frequencies can be employed
to enhance resolution, because the tissue depths of interest are
small, but in examinations of large body sections, lower frequency
transducers will be necessary to achieve adequate beam
penetration. In the latter case, the demand for high resolution must
be compromised to some extent. The optimum choice of frequency
would be the highest frequency compatible with the tissue depth
requirements. An interesting development in this connection has
been the introduction of broad band transducers which offer mixed
frequency beams to exploit a bit of the advantages of both low and
high frequencies.
Beam steering refers to altering the angle of the ultrasound beam with
respect to the transducer without moving the probe. Beam steering
allows a point on an image to be insonated from multiple angles from a
single probe and a single position of the probe. Beam steering is
accomplished by adding delays to the transmit and receive timing of the
ultrasound beam. In linear arrays beam steering can be used
in compound imaging to reduce speckle and improve image quality.
Medical Imaging Transducers :Introduction
Medical ultrasound imaging for diagnosis has advantages, such as
reasonable cost, real-time imaging, portability, and its harmless effect,
over computerized tomography (CT) and magnetic resonance imaging
(MRI) [1]. However, the resolution of the ultrasound imaging system
is usually lower than that of CT and MRI systems. The ultrasonic
imaging system consists of ultrasonic transducers and an imaging
system. The imaging system controls the ultrasonic transducer in
order to transmit and receive the ultrasound, and creates an
ultrasound image with a set of data from the transducer. Depending
on the type of the transducer and the imaging system, the images may
be either two-dimensional (2D) or three-dimensional (3D).
Ultrasound imaging technology has benefited from increasingly
sophisticated computer technology, and system integration has
ensured better image quality, data acquisition, analysis, and display.
However, much of this progress has been derived from the
development of transducers that are in direct contact with patients,
which has expanded the possibilities for maximizing patient
diagnostic information. This paper reviews the structure, type, and
role of the transducers in realizing high-quality ultrasonic images.
There are different types of transducers used in various fields such as
cardiology, obstetrics, gynecology, urology, orthopedics, and
ophthalmology, as illustrated in Fig. 1. The position, size, and
properties of objects being observed determine the shape, size, type,
and frequency of the transducer required to achieve the field of view
appropriate for a specific application [2–6]. The transducers are
broadly classified into a one-dimensional (1D) array transducer,
mechanical wobbling transducer, and 2D array transducer. The 1D
array transducer, comprising several tens or hundreds of active
elements in a linear mode, generates a 2D planar image when all the
elements are operated simultaneously or in sequence. The mechanical
wobbling transducer is composed of a 1D array and a mechanism that
can control the precise position of the 1D array to form a 3D image by
combining several 2D images created with the 1D array. The 2D array
transducer produces a pyramidal beam pattern to acquire a
volumetric image instantly. This paper reviews detailed operation
principle, structure, and application of these transducers.
Fig. 1
Photograph of ultrasonic transducers [5]
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Structure of the transducer
The parameters of the transducer performance, which influence the
quality of ultrasound images, are the axial and lateral resolution and
sensitivity [7]. The axial resolution is determined mostly by the
frequency of the ultrasound wave. As the frequency increases, the
wavelength decreases, which is advantageous because it provides a
better distinction between a target and other objects. The lateral
resolution along the direction orthogonal to the axial direction is
determined by the beam profile of the transducer. A narrower beam
leads to better resolution along the lateral direction. The sensitivity of
the transducer determines the contrast ratio of the ultrasonic images.
A transducer with higher sensitivity can generate a brighter image of
the target. The transducer is designed to acquire high-quality images
by enhancing these performance parameters.
A typical 1D array transducer is composed of an active layer, acoustic
matching layers, a backing block, an acoustic lens, kerfs, a ground
sheet (GRS), and a signal flexible printed circuit board (FPCB), as
illustrated in Fig. 2. The active layer is usually made of a piezoelectric
material—mostly piezoceramic. The active layer generates an
ultrasound wave in response to an electric driving signal, receives the
wave reflected at the boundary of an organ, and converts the received
ultrasound wave to an electric signal by means of the piezoelectric
effect. However, the big difference in the acoustic impedance between
piezoceramic elements and a human body prevents the efficient
transfer of ultrasonic energy between the two media. The acoustic
matching layers are used to facilitate the transfer of ultrasound energy
[8]. Each matching layer has a thickness of one-quarter wavelength at
the center frequency of the transducer. The backing block is used to
absorb the ultrasound wave propagating backward from the
piezoelectric element. If the backward wave is reflected at the bottom
of the backing block and returned to the piezoelectric element, it can
cause noise in the ultrasound image. Thus, the backing block should
have a high attenuation. In addition to this material damping, several
structural variations have been implemented to increase the
scattering effects inside the backing block, e.g., inserting grooves or
rods in the block [9–11]. The backing block commonly has an acoustic
impedance between 3 and 5 Mrayl [12]. If the backing block has an
acoustic impedance that is too high, the acoustic energy generated by
the piezoelectric element will be wasted by the backing block and few
ultrasound waves will be transmitted to the human body. The acoustic
lens protects the ultrasonic transducer from exterior damage, and
focuses the ultrasound beam onto a specified point based on Snell’s
law [13]. Materials with low attenuation constants are preferred to
reduce the loss of ultrasound energy inside the lens [14, 15]. Typical
acoustic lenses are made of rubber materials for comfortable contact
between the transducer and patients. The kerf is a gap between
arrayed piezoelectric elements that isolates each element from its
neighboring elements to reduce the crosstalk between them. The
crosstalk seriously degrades the transducer performance. Therefore,
various shapes and materials of the kerf have been developed to
decrease the crosstalk [16, 17].
Fig. 2
Schematic structure of a 1D array transducer
To develop high-performance ultrasonic transducers, many
researches have been carried out to improve their structure and
components. The most significant effort is the use of a good active
layer. The most common piezoelectric materials used in commercial
transducers are piezoceramic materials that are cheap, easily
available, and well-characterized. However, since the efficiency of
piezoceramics for transmitting ultrasound waves to a human body is
low due to their high impedance, piezoelectric composite materials
have been developed to decrease the impedance [18, 19]. The
piezoelectric composite material consists of a piezoceramic arrayed in
a certain fashion and a low impedance polymeric material filled in
between the arrayed piezoceramic. This method also increases the
electromechanical coupling coefficient, which is a measure of the
conversion efficiency between acoustic and electrical energies [20].
Piezoelectric single crystals are another alternative for the active
layer, which has a superior electromechanical coupling coefficient but
a limited usable temperature range [21]. Additionally, a multi-layered
piezoelectric structure has been developed for a better electrical
impedance match with an imaging system [22]. It is fabricated by
laminating piezoelectric sheets along their thickness. Another
structure is related to a quarter wavelength resonant mode of a
piezoelectric layer [23, 24]. The use of a rigid material for the backing
block results in a node of deformation at the boundary between the
piezoelectric layer and the backing block. Therefore, the piezoelectric
layer is deformed toward the acoustic matching layer, and more
acoustic energy can be efficiently transmitted to the body.
For the active layers, apart from the piezoelectric materials, a
capacitive micromachined ultrasound transducer (CMUT) and a
piezoelectric micromachined ultrasound transducer (PMUT) have
been developed [25, 26]. A CMUT has a thin metalized membrane that
is suspended by insulating posts over a conductive silicon substrate.
When an alternating voltage is applied between the membrane and
substrate, the membrane is moved by Coulomb forces against the
surface tension of the membrane, which generates ultrasound waves.
Conversely, detection currents are generated by the change in the
capacitance when the biased membrane is moved by the reflected
waves. The CMUT has a higher electromechanical coupling coefficient
than the piezoelectric material, and it can be fabricated to a small size
by using photolithography processes. The PMUT has a structure
similar to that of the CMUT except that the PMUT has a piezoelectric
layer deposited on top of the silicon membrane.
To acquire a bright ultrasound image, the acoustic energy propagating
in the transducer and human body has to be increased by operating
the ultrasonic transducers with a high voltage. However, the increased
acoustic energy is converted to thermal energy due to various
attenuation mechanisms, which induces a temperature rise in the
transducer. The high temperature of the transducer may cause
patient’s skin to burn and degrade the transducer performance.
Therefore, thermally dispersive structures have been developed to
mitigate the temperature rise [27, 28].
Go to:
Types of transducers
Transducers for cross-sectional 2D images
As described in the Introduction, the 1D array transducer is used to
obtain cross-sectional 2D images, as illustrated in Fig. 3. The 1D array
is composed of piezoelectric elements arrayed one-dimensionally
along the azimuthal direction. The 1D array transducer is classified
into a linear array, a convex array, and a phased array in accordance
with the image shapes [7]. Basically, the linear array drives a few of
the piezoelectric elements to generate an ultrasound beam to scan a
line as illustrated in Fig. 4a. The beam profile along the azimuthal
direction can be changed by controlling the number of operated
elements. Thus, the ultrasound image of the linear array has a
rectangular shape. Since the linear array is normally used for precise
imaging, its operating frequency is high. In contrast, the convex array
is used to acquire a wide and deep ultrasound image at the cost of the
resolution. For this reason, the piezoelectric elements of the convex
array are arranged in a curved fashion along the azimuthal direction
as illustrated in Fig. 4b. The method of acquiring an image using a
convex array is the same as that when using a linear array but the
ultrasound image of the convex array has a fan shape. However, in the
case of a target object behind obstacles, such as a heart behind ribs, it
is difficult to obtain an ultrasound image using the linear array or
convex array. For this case, a phased array can be used for imaging by
steering the ultrasound beam, as illustrated in Fig. 4c. When all of the
piezoelectric elements are controlled to operate sequentially, the
phased array can steer the ultrasound beam. The ultrasound image of
the phased array has a circular cone shape.
Fig. 3
2D ultrasound image of a uterus using the 1D array transducer [5]
Fig. 4
Schematic of the 1D array transducer: a linear array, b convex array, and c phased array
However, none of the 1D array transducers mentioned above can be
used to control an elevation beam profile because the length of the
piezoelectric elements is fixed. The image is likely to be blurry in the
area other than the focal zone of the transducer because the
ultrasound beam is scattered outside the focal zone. For this reason,
1.25D, 1.5D, and 1.75D array transducers have been developed to
modify the ultrasound beam on the elevation and depth plane [29, 30].
These array transducers have a structure in which the piezoelectric
elements are arrayed along the elevation direction in addition to the
azimuthal direction to drive the piezoelectric elements in the
elevation direction as well. Although these transducers have better
ultrasound beam controllability along the elevation of the transducer,
they are just upgraded versions of the 1D array. Therefore, they retain
the limitations of 1D array transducers.
Transducers for 3D images
In order to acquire the volumetric image shown in Fig. 5, two different
types of 3D ultrasound imaging transducers are used: (1) a
mechanical wobbling transducer that generates a 3D image by
combining multiple 2D images from the 1D array, and (2) a 2D array
transducer that comprises several thousand piezoelectric elements
arrayed on a plane to transmit an ultrasound beam in a pyramid shape
[31].
Fig. 5
3D ultrasound image of a fetus using the 3D imaging transducer [5]
First, the mechanical wobbling transducer acquires the 3D image data
using a mechanical sequential scanning method, which means that the
3D information of the measured object is presented in multiple 2D
images. Thus, the 1D array and the mechanism to control the
movement of the 1D array compose the mechanical wobbling
transducer as shown in Fig. 6. The mechanism controls the rotational
behavior of the 1D array in a prescribed position by applying a
dynamic force generated from a servo or stepping motor to the 1D
array. A conical volume date set can be acquired as the 1D array
rotates in a semicircle around the central axis while a pyramidal data
set can be obtained as the 1D array moves in a fan-like arc according
to a prescribed angle [32, 33]. The technical issue is the optimization
of the scanning rate, scanning angle, long-term reliability, and
compliance for better performance of the transducer.
Fig. 6
Schematic structure of a mechanical transducer
The 2D array transducer can generate real-time 3D ultrasound images
through volumetric steering of the ultrasound beam. Since the 2D
array transducer consists of thousands of piezoelectric elements
arrayed along both the azimuthal and elevation directions as
illustrated in Fig. 7, the volumetric data set can be acquired instantly
through electronic control of the piezoelectric elements both
horizontally and vertically. The embodiment of the 2D array
transducer is a technically challenging issue that is related to electrical
wiring of all the piezoelectric elements and reducing the crosstalk
among the dense elements. All the piezoelectric elements fabricated in
the small footprint area are connected to a controlling electronic
circuit with a multi-layered FPCB or conductive backing block [34, 35].
Since it is not easy to handle thousands of cable bundles to connect the
2D array with an imaging system, it is necessary to implement specific
integrated circuit (ASIC) chips inside the transducer for preprocessing the image data [36, 37].
Fig. 7
Schematic structure of a 2D array transducer
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Application
Ultrasonic transducers of these basic structures can be modified in
terms of their shape, size, type, and operating frequency for various
imaging applications. In the field of cardiology, for instance, an
ultrasound image of the heart behind ribs can be obtained with a
transesophageal echocardiogram (TEE) transducer that includes a
small 1D phased array or 2D array transducer [38]. The TEE
transducer should be small so that it can be inserted in the patient’s
mouth and esophagus. The convex array and mechanical wobbling
transducers are used to obtain wide and deep images of a fetus,
uterus, and ovary through the abdomen in the field of obstetrics and
gynecology [39]. Breast is usually imaged with 1D linear array
transducers from skin surfaces. In the field of urology and endocrine
system, the linear array transducer is used to obtain an ultrasound
image of a prostate, bladder, testis, and thyroid from the skin surface.
Additionally, an endo-vaginal or endo-anal transducer having a thin
rod shape is also used to obtain the images of uterus and prostate
through the vagina or anus [40]. In the vascular system, an artery
image can be acquired with an intravascular ultrasound (IVUS)
transducer using a miniaturized ultrasonic transducer built-in
catheter [41]. Additionally, a 1D linear array transducer operating at
high frequencies is used to acquire high-resolution images of a tendon,
muscle, ligament, cornea, and eyeball in the field of orthopedics or
ophthalmology [42]. Continuing development of transducer
technology is playing a key role in enhancing the 3-D imaging
performance to replace current 2-D sonography by providing realtime capability and interactivity.
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Conclusions
In this paper, medical ultrasonic imaging transducers were reviewed
and their structure, type, and application fields were described. The
active and passive components of the transducer were described in
detail. The technical issues related to the development of each
component were also presented. Continuous development in signal
processing and precision machining technology offers new
opportunities for enhancing the ultrasound transducer’s performance.
In the future, more compact and integrated ultrasonic transducers will
be studied for generating high-resolution real-time images. It is
expected that 3-D ultrasound imaging will be a routine part of patient
diagnosis and management in the future. New applications of the
transducers are also expected through fusion with other imaging
modalities.