Patent AU-2020102628-A4 (AU)
Title: A low frequency piezoelectric underwater transducer based on fold-back structure
Inventor: TENG DUO
IPC: H04R1/02
Filed: 2020-10-07
Granted: 2020-11-26
Abstract: A low frequency piezoelectric underwater transducer based on fold
back structure 
TECHNICAL FIELD 
[01] The invention relates to an underwater transducer, in particular to a low 
frequency piezoelectric underwater transducer based on fold-back stacks, which 
belongs to the field of electroacoustic sensor. It is one type of electroacoustic devices, 
which finishes the transformation between acoustic energy and electrical energy in 
water to be used as low frequency projector and hydrophone.  
BACKGROUND 
[02] So far, the sound waves are the most effective information carrier for 
underwater detection, and the underwater transducer is an indispensable key device.  
With the further development of modem sonar technology and the continuous 
improvement of application requirements, the underwater transducer with low 
frequency and small size is urgently needed. The relevant research has become the 
important developing trend. Currently, the most widely used low frequency transducers 
include: giant magnetostrictive underwater transducer, flextensional transducer and its 
improved versions, flexural transducer, free-flooded ring transducer and so on.  
[03] The literature (Steohen C. Butler, A 2.5 kHz magnetostrictive Tonpilz sonar 
transducer design, Smart Structures and Materials 2002: Active Materials: Behavior 
and Mechanics, Vol. 4669, 2002, P510-521.) disclosed a low frequency longitudinal 
underwater transducer based on the composite bar, whose active material is terbium
dysprosium-iron alloy rare earth giant magnetostrictive material. This kind of 
transducer takes full advantage of the low sound velocity characteristics of functional 
material terbium-dysprosium-iron alloy to significantly reduce its resonance frequency, 
but the transducer is 15 kg of the weight.  
[04] The literature (Kenneth D. Rolt, History of the flextensional electro
acoustic transducer, The Journal of the Acoustical Society of America, March 1990, 
Vol. 87, No. 3, P1340-1349.) and U.S. Patent US 4922470 both introduced the 
underwater transducer based on the certain theories. In order to obtain the low
frequency acoustic radiation, the reasonable mechanical structure was utilized to realize
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the transformation between the extensional vibration modes of active element and the 
other vibration modes, such as the flexural vibration of thin shell. This kind of 
transducer can easily obtain low frequency with resonance lower than 3 kHz, however, 
most of them have a heavy weight of more than 3 kg and a relatively large size.  
[05] The U.S. patent US 4709361 disclosed a flexural disk underwater 
transducer, which applied the flexural vibration mode to achieve low-frequency 
resonance, while the transducer is greatly affected by the boundary support conditions, 
so its application is greatly limited.  
[06] The literature (Teng Duo, Chen Hang, Zhu Ning, Finite Element Analysis 
of Free-flooded Segmented Ring Transducer, Torpedo Technology, December 2008, 
Volume 16, Issue 6, P44-47.) introduced the theory of the free-flooded ring by utilizing 
the Helmholtz resonance liquid cavity to achieve its low-frequency resonance. The free
flooded ring transducer has the low-frequency Helmholtz resonance of 2.3 kHz. On the 
other hand, its weight exceeds 4 kg.  
[07] In summary, the above-mentioned underwater transducers can easily 
achieve low-frequency electroacoustic transformation at the frequency of 1-3 kHz, 
but they have some common disadvantages in practical applications: (1). Heavy mass 
(generally several kilograms or even more than ten kilograms); (2). Large size; (3).  
Limited application (especially, they are unsuitable for array arrangement); (4). High 
cost. Therefore, strictly speaking, the above-mentioned transducers all have the 
characteristics of low frequency, while they do not have the absolute advantages in 
weight and size.  
SUMMARY 
[08] In order to solve the technical problem of the coexistence of low frequency 
as well as light weight and small size of the underwater transducer, the proposed 
invention provides a low frequency piezoelectric underwater transducer based on fold
back structure. Such a folding underwater transducer makes full use of the space of 
transducer and the compact structure design, and reasonably combines the longitudinal 
vibration mode and the flexural vibration mode of different components, while 
implementing the characteristics of low frequency as well as light weight and small
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size. It has the advantages of wide application, high electro-acoustic efficiency, low 
cost, convenient application as projector and hydrophone and reliable operation.  
[09] In order to solve the above technical problems, the technical scheme of the 
proposed invention provides a low-frequency underwater transducer including a 
radiating head, a tail mass, two bending beams, two front piezoelectric stacks, two rear 
piezoelectric stacks, a middle anti-phase piezoelectric stack and a housing. The whole 
structure is symmetrical.  
[010] The piezoelectric stack is formed by an even number of piezoelectric 
ceramic pieces in series. The polarization directions of two adjacent piezoelectric 
ceramic pieces are opposite. On the adhered surfaces of the piezoelectric ceramic piece, 
the electrodes are led out by the electrode plates. The same polarized electrode plates 
of the piezoelectric stacks are electrically connected in parallel, and two terminals are 
led according to the "positive" and "negative" polarization directions. There are 
insulating shims at both ends of the piezoelectric stacks, and the appropriate prestress 
is applied by the prestressed bolt. It should be noted that in order to always ensure that 
the front and rear piezoelectric stacks are in opposite phase with the middle anti-phase 
piezoelectric stack, it is necessary to connect the "polarization +" and "polarization -" 
terminals of the front and rear piezoelectric stacks with the "polarization -" and 
"polarization +" terminals of the middle anti-phase piezoelectric stack. At the same 
time, in order to ensure the feasibility of the structure, the number of piezoelectric 
ceramic pieces in the middle anti-phase piezoelectric stack should be less than that of 
the front and rear piezoelectric stacks; The bending beams together with the radiating 
head and the tail mass should be respectively glued to the two ends of the piezoelectric 
stacks, so that the piezoelectric stacks can be formed as a "Z" shaped fold-back structure 
like a folding ruler.  
[011] The front radiator with light metals such as duralumin or aluminum
magnesium alloy is made into a structure of truncated cone for increasing the radiation 
area; The tail mass is of a cylindrical structure, with heavy metals such as steel and 
brass; The purpose of this design is to obtain a larger vibration velocity ratio of the front 
and rear surfaces, so as to increase the acoustic radiation of the front radiation surface.
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[012] The proposed invention provides a low frequency piezoelectric underwater 
transducer based on fold-back structure, which solves the technical problem of the 
coexistence of low frequency as well as light weight and small size by reasonably 
utilizing the longitudinal vibration mode of the piezoelectric stacks and the flexural 
vibration mode of the bending beams. Such a folding underwater transducer has the 
following beneficial effects: the fold-back structure makes full use of the space of 
transducer, effectively increases the sound power capacity and improves the 
transmitting ability and receiving sensitivity of the underwater transducer; The low 
frequency piezoelectric underwater transducer based on fold-back structure also has the 
advantages of simple structure, convenient manufacture, low cost, convenient assembly 
and dis-assembly, and wide application. In particular, the way of transmitting the 
acoustic energy form the front radiating surface will facilitate the application of the 
transducer, making it more widely used and more suitable for array arrangement.  
BRIEF DESCRIPTION OF THE FIGURES 
[013] Hereinafter, a low frequency piezoelectric underwater transducer based on 
fold-back structure of the proposed invention will be described in further detail with 
reference to the attached drawings and examples.  
[014] Fig. 1 is a schematic diagram of the structure of the low frequency 
piezoelectric underwater transducer based on fold-back structure of the proposed 
invention.  
[015] Fig. 2 is a schematic diagram of the structure and the cascade relationship 
of the middle anti-phase piezoelectric stack of the folding transducer of the proposed 
invention.  
[016] Fig. 3 is a schematic diagram of the structure and the cascade relationship 
of the front piezoelectric stacks of the folding transducer of the proposed invention.  
[017] Fig. 4 is a schematic diagram of the structure and cascade relationship of 
the rear piezoelectric stacks of the folding transducer of the proposed invention.  
[018] Component in the figures:
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[019] 1. Radiating head; 2. Sealing ring; 3. Front bending beam; 4. Front 
piezoelectric stack; 5. Middle anti-phase piezoelectric stack; 6. Rear piezoelectric stack; 
7. Housing; 8. Rear bending beam; 9. Tail mass; 10. Positioning bolt; 11. Positioning 
hole of the housing; 12. Positioning screw hole; 13. Output cable; 14. Insulating shim; 
15. Electrode plate; 16. Electrode connecting wire; 17. Piezoelectric ceramic plate; 18.  
Insulating sleeve; 19. Spring shim; 20. Prestressed bolt.  
DESCRIPTION OF THE INVENTION 
[020] This example demonstrates a low frequency piezoelectric underwater 
transducer based on fold-back structure including a radiating head (1), a tail mass (9), 
a front bending beam (3), front piezoelectric stacks (4), a middle anti-phase 
piezoelectric stack (5), rear piezoelectric stacks (6), a rear bending beam(8), a housing 
(7) and an output cable (13).  
[021] As shown in Figure 1, the radiating head (1) of the proposed invention is of 
the truncated cone structure with a sealing ring (2); The tail mass (9) is cylindrical with 
a sealing ring (2); The housing (7) is cylindrical, which is combined with two sealing 
rings (2) to complete underwater sealing; The positioning hole (11) of the housing is 
provided on the housing (7), and the positioning screw hole (12) is provided on the tail 
mass (9); The positioning bolt (10) is for the positioning of the housing (7); The output 
cable (13) connects the electrodes of the piezoelectric stacks to the external driving 
source, passing through the tail mass (9); There are three types of piezoelectric stacks, 
namely front piezoelectric stacks (4), rear piezoelectric stacks (6) and middle anti-phase 
piezoelectric stack (5), which combine with the front bending beam (3) and the rear 
bending beam (8) to realize a "Z" shaped fold-back structure like a folding ruler. The 
total number of the front piezoelectric stacks (4) which are glued parallelly between the 
radiating head (1) and the rear bending beam (8) is two; The total number of the rear 
piezoelectric stacks (6) which are glued parallelly between the tail mass (9) and the 
front bending beam (3) is two; There is only one middle anti-phase piezoelectric stack 
(5) glued between the front bending beam (3) and the rear bending beam (8).  
Meanwhile, the front bending beam (3) and the rear bending beam (8) are perpendicular 
to each other is necessary.
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[022] The above-mentioned three types of piezoelectric stacks all need to be 
properly prestressed by the prestressed bolt (20). The prestressed bolt (20) is inserted 
in insulating sleeves (18) and combined with the spring shim (19). All piezoelectric 
stacks are formed by glueing an even number of piezoelectric ceramic pieces (17) in 
series. There is an insulating shim (14) at each end of the piezoelectric stacks. The 
polarization directions of the two adjacent piezoelectric ceramic pieces are opposite.  
Moreover, the electrodes are led out by electrode plates (15) on the adhered surface, 
and the identical electrode of each piezoelectric stack are electrically connected in 
parallel by electrode connecting wires (16).  
[023] There are two points need to be noted. One is that in order to always ensure 
that the front piezoelectric stacks (4) and rear piezoelectric stacks (6) are in opposite 
phase with the middle anti-phase piezoelectric stack (5), it is necessary to connect the 
"polarization +" and "polarization -" terminals of the front and rear piezoelectric stacks 
with the "polarization -" and "polarization +" terminals of the middle anti-phase 
piezoelectric stack (5), and they are connected to the same external driving source. The 
other is that the number of piezoelectric ceramic pieces of the middle anti-phase 
piezoelectric stack should be less than that of the front and rear piezoelectric stacks for 
the feasibility of the structure.  
[024] The specific assembly processes of the low frequency piezoelectric 
underwater transducer based on fold-back structure are as follows: 
[025] 1. Firstly, the piezoelectric ceramic plate (17) and the electrode plate (15) 
are glued together in cross series with epoxy resin, and then the insulating shim (14) is 
glued at both ends to form the middle anti-phase piezoelectric stack (5). The 
polarization direction of the piezoelectric ceramic plate is shown in Fig. 2; 
[026] 2. The middle anti-phase piezoelectric stack (5) is glued with the front 
bending beam (3) and the rear bending beam (8), which appropriate prestress is applied 
to by the prestressed bolt (20). The prestressed bolt (20) covered with insulating sleeves 
(18) needs to match with the spring shim (19). During the glueing process, it is 
necessary to ensure that the front bending beam (3) and the rear bending beam (8) are 
perpendicular to each other, which is as shown in Fig. 2;
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[027] 3. The piezoelectric ceramic plate (17) and the electrode plate (15) are glued 
together in cross series with epoxy resin, and the insulating shim (14) is glued at both 
ends to form the front piezoelectric stacks (4). The polarization direction of 
piezoelectric ceramic plate is shown in Fig. 3; 
[028] 4. Two groups of the front piezoelectric stacks (4) are glued parallelly 
between the radiating head (1) and the rear bending beam (8), which appropriate 
prestress is applied to by the prestressed bolt (20). The stress rod (20) covered with 
insulating sleeves (18) needs to match with the spring shim (19). The above is as shown 
in Fig. 3; 
[029] 5. The piezoelectric ceramic plate (17) and the electrode plate (15) are glued 
together in cross series with epoxy resin, and the insulating shim (14) is glued at both 
ends to form the rear piezoelectric stacks (6) for two groups. The polarization direction 
of piezoelectric ceramic plate is shown in Fig. 4; 
[030] 6. Two groups of the rear piezoelectric stacks (6) are glued parallelly 
between the front bending beam (3) and the tail mass (9), which appropriate prestress 
is applied by the prestressed bolt (20). The prestressed bolt (20) inserted in insulating 
sleeves (18) needs to match with the spring shim (19). The above is as shown in Fig. 4; 
[031] 7. The electrodes (15) of the above-mentioned piezoelectric stacks are 
connected in parallel, and the next step is to connect the "polarization +" and 
"polarization -" terminals of the front and rear piezoelectric stacks with the "polarization 
-" and "polarization +" terminals of the middle anti-phase piezoelectric stacks (5), 
which is as shown in Fig. 2, Fig. 3 and Fig. 4; 
[032] 8. The output cable (13) connects the electrodes of the above piezoelectric 
stacks, passing through the tail mass (9); 
[033] 9. The sealing rings (2) are evenly coated with lubricant and placed in the 
corresponding sealing grooves of the radiating head (1) and the tail mass (9);
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[034] 10. The housing (7) is sleeved from the back end to complete the underwater 
sealing of the transducer, which is positioned by one positioning bolt (10).  
[035] The main functions realized by the low frequency piezoelectric underwater 
transducer based on fold-back structure of the proposed invention are as follows: 
[036] The transducer has a better performance in both transmitting and receiving 
of underwater acoustic waves; the fold-back structure of transducer will significantly 
increase the acoustic power capacity, make full use of the space of transducer and 
reasonably combine the longitudinal vibration mode and the flexural vibration mode of 
different components, while implementing the characteristics of low frequency as well 
as light weight and small size; The way of transmitting acoustic energy by utilizing the 
piston transmitting head makes the application of the transducer more extensive 
applied, especially suitable for array arrangement; The waterproof design of the housing 
combined with the sealing rings makes the transducer more convenient in daily 
maintenance and repair.  
[037] Although the invention has been described with reference to specific 
examples, it will be appreciated by those skilled in the art that the invention may be 
embodied in many other forms, in keeping with the broad principles and the spirit of 
the invention described herein.  
[038] The present invention and the described embodiments specifically include 
the best method known to the applicant of performing the invention. The present 
invention and the described preferred embodiments specifically include at least one 
feature that is industrially applicable