Ultrasonic transducer

An ultrasonic transducer comprises a diaphragm formed by a metal plate provided with a bulged portion in its center and a piezoelectric element bonded to the inner surface of the bulged portion thereby to make the diaphragm vibrate in a bending mode. A node line is present outside the bulged portion in vibration of the diaphragm, and the diaphragm is sandwiched by first and second elastic members along the node line. The first and second elastic members are held between a case and a base fixed thereto so that the diaphragm is elastically retained through the first and second elastic members.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an ultrasonic transducer, and more 
particularly, it relates to an improvement in the structure of an 
ultrasonic transducer employing a piezoelectric element, such that the 
transducer can be reduced in thickness. 
2. Description of the Prior Art 
A drop-proof ultrasonic transducer may be employed in a sensor in an 
automobile for detecting an obstruction and informing the driver of the 
same, in case of backing the automobile, for example. Such a transducer 
performs the functions of transmitting and receiving ultrasonic waves such 
that, when some obstruction is present in a direction to which the 
ultrasonic wave is transmitted, the ultrasonic wave transmitted from the 
transducer is reflected by the obstruction and is received by the same 
transducer, which thereby detects the presence of the obstruction. 
Further, the distance between the obstruction and the transducer can be 
determined by measuring the time from transmission of the ultrasonic wave 
to receiving of the same by way of the reflection. 
A piezoelectric ultrasonic transducer of background interest to the present 
invention is disclosed in U.S. Pat. No. 4,556,814 issued on Dec. 3, 1985 
and granted to Ito et al. In said U.S. Patent, a piezoelectric element is 
bonded to the inner surface of a top wall of a housing member which has an 
inverted U-shaped cross-section, while the bottom surface of the housing 
member is closed by a cover plate. Lead wires are respectively connected 
to two electrodes of the piezoelectric element for inputting and 
outputting signals, which lead wires are respectively connected to a pair 
of terminal pins passing through the cover plate. 
The aforementioned transducer operates through bending vibration of the top 
wall of the housing member. Therefore, the vertical size of the side wall 
of the housing member must be as large as possible in order to reduce 
leakage of the bending vibration to the minimum. This prior art patent 
also discloses an embodiment in which a tubular member having relatively 
high acoustic impedance is connected to the side wall in order to reduce 
the decay time caused by transmission of vibration generated in the top 
wall to the side wall and the cover plate mechanically fixed to the same, 
thereby to improve a vibration damping effect in the side wall of the 
housing member. 
In the prior art as disclosed in the aforementioned U.S. Patent, the size 
of transducer is increased since the side wall of the housing member has a 
large height. Further, assembly of the transducer is complicated in the 
structure provided with the tubular member. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide an ultrasonic 
transducer which can be reduced in size, particularly in thickness, and 
which can be readily assembled. 
According to the present invention, employed is a diaphragm of metal, which 
is characterized in its configuration and support mode. The diaphragm has 
oppositely directed first and second major surfaces and a bulged portion 
provided in a substantially central portion to outwardly direct the first 
major surface. The bulged portion is formed by a first annularly extending 
bent line for outwardly projecting the first major surface and a second 
annularly extending bent line positioned to encircle the outer side of the 
first bent line to outwardly project the second major surface. A 
piezoelectric element is bonded to a region on the second major surface of 
the diaphragm encircled by the first bent line. Thus, the diaphragm is 
made to vibrate in a bending mode. In such vibration of the diaphragm, a 
first node line is present inside the first bent line and a second node 
line is present outside the second bent line. First and second elastic 
members are prepared to support the diaphragm. The first and second 
elastic members are arranged to be respectively in contact with the first 
and second major surfaces along the second node line of the diaphragm. A 
base and a case are prepared to hold the diaphragm through the first and 
second elastic members. The base is adapted to support the diaphragm 
through the second elastic member. The case has a peripheral wall portion 
encircling the peripheral edge portion of the base and being substantially 
fixed to the base and a top wall portion having an opening for receiving 
the bulged portion of the diaphragm and being in contact with the first 
elastic member. Thus, the diaphragm is held between the base and the top 
wall portion through the first and second elastic members, to be 
elastically retained. 
According to the present invention, the diaphragm is elastically held and 
retained between the top wall portion of the case and the base through the 
first and second elastic members being in contact with the diaphragm along 
the second node line, whereby leakage of vibration of the diaphragm is 
reduced. Thus, there is no need to increase the height of a side wall 
portion of the case or provide a member for attenuating reverberation as 
soon as possible as in the conventional transducer. Consequently, the 
assembling structure is simplified and a thin transducer can be obtained. 
Further, the case is effectively sealed by the first and second elastic 
members, thereby to obtain structure suitable for a drop-proof transducer. 
In addition, the bulged portion of the diaphragm bonded with the 
piezoelectric element can be exposed from the opening of the top wall 
portion of the case, whereby a metal material forming the bulged portion 
can be adjusted in thickness after assembling. Thus, the frequency can be 
readily adjusted after assembling. 
These and other objects, features, aspects and advantages of the present 
invention will become more apparent from the following detailed 
description of embodiments of the present invention when taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, an ultrasonic transducer according to an embodiment of 
the present invention comprises a diaphragm 1 and a piezoelectric element 
2 bonded to its lower surface. Upper and lower surfaces of the diaphragm 1 
are sandwiched by first and second elastic members 3 and 4. The lower 
surface of the second elastic member 4 is covered by a base plate 5. The 
diaphragm 1, the first and second elastic members 3 and 4 and the base 
plate 5 are contained in a case 6. 
The diaphragm 1 is obtained by discoidally punching a plate of metal such 
as stainless steel to form a bulged portion 7 in its central portion 
through embossing. The bulged portion 7 is formed by a first annularly 
extending bend line 8 for providing an outwardly projecting portion of the 
upper surface of the diaphragm 1 and a second annularly extending bend 
line 9 outside the first bend line 8 for providing an outwardly projecting 
portion of the lower surface of the diaphragm 1. 
As shown in FIGS. 1 and 2, the piezoelectric element 2 is entirely on the 
form of a disc, and is provided in its upper and lower surfaces with 
electrodes 10 and 11 respectively. The piezoelectric element 2 is bonded 
to a region on the lower surface of the diaphragm 1 encircled by the first 
bend line 8 by a bonding agent (not shown) by the electrode 10. Thus, the 
piezoelectric element 2 is received in a space defined by the bulged 
portion 7 of the diaphragm 1. The bonding agent for bonding the electrode 
10 to the diaphragm 1 may be either a conductive type or an insulating 
type. Particularly when an insulating type bonding agent is employed, the 
electrode 10 is connected to the diaphragm 1 through capacitive coupling. 
The piezoelectric element 2 is adapted to make the diaphragm 1 vibrate in 
a bending mode. When the diaphragm 1, having a shape as shown, vibrates in 
a bending mode, a first node line 12 is positioned inside the first bend 
line 8 and a second node line 13 is positioned outside the second bend 
line 9 in the diaphragm 1. The diaphragm 1 is sandwiched by the first and 
second elastic members 3 and 4 in a support portion 14 of the diaphragm 1, 
which includes the second node line 13. 
The first elastic member 3 is formed of an elastic sheet material such as 
rubber, and is in the form of a ring for receiving the bulged portion 7 of 
the diaphragm 1. The second elastic member 4 is also formed by an elastic 
material such as rubber, and is in the form of a disc having a diameter 
substantially identical to the outer diameter of the support portion 14 of 
the diaphragm 1. A pair of holes 15 and 16 are formed in the peripheral 
edge portion of the second elastic member 4. The holes 15 and 16 may be 
replaced by notches. 
The base plate 5 is formed of an insulating material such as glass epoxy, 
and is substantially identical in diameter to the diaphragm 1. The base 
plate 5 is provided with holes 17 and 18 respectively communicating with 
the aforementioned holes 15 and 16. Around the peripheries of the holes 17 
and 18, conductive patterns 19 and 20 are formed on the lower surface of 
the base plate 5 to serve as terminals for connection with a circuit (not 
shown). 
The case 6 is entirely in the form of a cylinder as shown in FIGS. 2 and 3, 
and has a peripheral wall portion 21 and a top wall portion 22 having an 
opening in its center. Before the case 6 is thus assembled, the peripheral 
wall portion 21 extends straightly in the vertical direction as shown in 
FIG. 2. However, the top wall portion 22 is preferably formed to be at an 
angle slightly smaller than 90.degree. with respect to the peripheral wall 
portion 21, as shown in FIG. 1. The case 6 may be formed of metal or 
resin, the case 6 being preferably obtained of sheet metal or a metal 
plate. 
The electrodes 10 and 11 of the piezoelectric element 2 are electrically 
connected with the conductive patterns 19 and 20 in the following manner: 
First, the electrode 10 is electrically connected with the diaphragm 1 
directly or by a capacitance. An end of a lead wire 23 is soldered to a 
position close to the second bend line 9 of the diaphragm 1. The lead wire 
23 is soldered to such a position for the following reason: The second 
bend line 9 is outwardly projected from other portions to facilitate 
soldering, and is also located in the vicinity of the second node line 13, 
whereby such connection of the lead wire 23 exerts substantially no 
influence on vibration of the diaphragm 1. The lead wire 23 passes through 
the holes 15 and 17, and is soldered to the conductive pattern 19 on the 
lower surface of the base plate 5. On the other hand, an end of another 
lead wire 24 is soldered to the electrode 11. The lead wire 24 is 
preferably soldered in a position as close as possible to the first node 
line 12, so that vibration of the diaphragm 1 is not influenced by such 
connection of the lead wire 24. The lead wire 24 passes through the holes 
16 and 18, and is soldered to the conductive pattern 20 on the lower 
surface of the base plate 5. The solder for connecting the lead wires 23 
and 24 to the conductive patterns 19 and 20 is preferably applied to close 
the holes 17 and 18. 
The aforementioned first elastic member 3, the diaphragm 1 bonded to the 
piezoelectric element 2, the second elastic member 4 and the base plate 5 
are stacked in this order on the lower surface of the top wall portion 22 
of the case 6. In this state, the bulged portion 7 of the diaphragm 1 
projects through the opening of the top wall portion 22, thereby to 
facilitate the reducing in the thickness of the bulged portion 7 for 
adjusting the frequency after assembling. When it is not necessary to 
adjust the frequency after assembling, the upper surface of the bulged 
portion 7 may be flush with or lowered with respect to the upper surface 
of the top wall portion 22. Thereafter, a lower end portion 25 of the 
peripheral wall portion 21 of the case 6 is bent and thereby placed 
against the lower surface of the base plate 5 as shown in FIG. 1. Thus, 
the base plate 5 exerts force against the second elastic member 4, so that 
the diaphragm 1 is elastically held, by its support portion 14 by the 
pressure exerted by the base plate 5 and the top wall portion 22, on the 
first and second elastic members 3 and 4. The space defined in the bulged 
portion 7 of the diaphragm 1 is effectively sealed by the first and second 
elastic members 3 and 4 respectively, which are in pressure contact with 
the support portion 14 of the diaphragm 1. Such sealing can be further 
secured as shown in FIG. 1, by forming the top wall portion 22 to be 
connected with the peripheral wall portion 21 at an angle slightly smaller 
than 90.degree.. 
As shown in FIG. 1, a sealant 26 of, e.g., silicone rubber may be applied 
to seal the clearance between the diaphragm 1 and the peripheral edge 
portion defining the opening formed in the top wall portion 22, thereby to 
further improve the sealing performance. 
FIG. 4 shows a modification of the embodiment as shown in FIG. 1. In this 
embodiment, a first elastic member 3a is made to have a smaller outer 
diameter than the elastic member 3 as shown in FIG. 1, while a top wall 
portion 22a of a case 6a has a bent configuration. An annular and radially 
inward portion of the top wall portion 22a presses against the first 
elastic member 3a and is as parallel as possible thereto in the radial 
direction. A radially outward portion of the top wall portion 22a may be 
bent to form an angle slightly smaller than 90.degree. with the peripheral 
wall portion 21. Other structure of this embodiment is similar to that 
shown in FIG. 1, and hence detailed description thereof is omitted with 
similar reference numerals being assigned to corresponding elements. 
Referring to FIG. 5, a description is now made of still another embodiment 
of the present invention. In FIG. 5, elements corresponding to those of 
the embodiment as shown in FIG. 1 are indicated by similar reference 
numerals, to omit redundant description. As compared with the embodiment 
of FIG. 1, the embodiment as shown in FIG. 5 has the following 
characteristics: 
First and second elastic members 3b and 4b are made of an elastic material 
such as silicone rubber, and each have the form of a ring with a 
substantially circular section. 
A radially inward peripheral edge portion 27 for defining an opening in a 
top wall portion 22b of a case 6b is downwardly bent so as to locate the 
first elastic member 3b. On the other hand, a base plate 5b is provided 
with an upwardly projecting portion 28 for locating the second elastic 
member 4. 
Further, an upwardly projecting spacer 29 is integrally formed in the 
peripheral edge portion of the base plate 5b. The spacer 29 may be 
prepared independently of the base plate 5b, and thereafter placed on the 
base plate 5b. Such a spacer 29 facilitates maintaining a constant 
distance between the top wall portion 22b and the lower portion 25 of the 
case 6b, thereby to prevent irregular application of compressive force to 
the first and second elastic members 3b and 4b depending on the degree of 
bending of the lower portion 25 as shown in FIG. 5. 
In the embodiment of FIG. 5, further provided are a pair of terminal pins 
30 and 31 to serve as external terminals. The terminal pins 30 and 31 are 
respectively inserted in through holes formed in the base plate 5b, and 
fixed to the base plate 5b. Lead wires 23b and 24b are guided through the 
through holes receiving the terminal pins 30 and 31, wound around the 
terminal pins 30 and 31, and soldered thereto at the lower surface of the 
base plate 5b. The portions of the base plate 5b receiving the terminal 
pins 30 and 31 and the lead wires 23b and 24b are preferably sealed by 
appropriate means. The lead wires 23b and 24b may also be passed to the 
lower surface of the base plate 5b through other through holes (not shown) 
separated from those receiving the terminal pins 30 and 31. 
FIG. 6 shows a further embodiment of the present invention, which is 
similar to the embodiment as shown in FIG. 5. Thus, corresponding portions 
are indicated by similar reference numerals, thereby to omit redundant 
description. 
The embodiment as shown in FIG. 6 is characterized in that an elastic resin 
member 33 of silicone resin or the like is provided to fill in the spaces 
around a peripheral edge portion 32 of the diaphragm 1. The elastic resin 
member 33 is formed by introducing unhardened resin between a spacer 29 
and the first elastic member 3b, with the first and second elastic members 
3b and 4b and the diaphragm 1 having been placed on the base plate 5b 
before the base plate 5b is assembled in a case 6, for example. Such an 
elastic resin member 33 fills in a space defined by the first and second 
elastic members 3b and 4b, the base plate 5b and the case 6c, thereby to 
damp vibration of the diaphragm 1. Such a damping effect promptly causes 
attenuation of the vibration in both transmission and receiving of 
ultrasonic waves and reduces the decay time. 
In the embodiment as shown in FIG. 6c, further, a sound absorbing member 34 
is provided between the piezoelectric element 2 and the base plate 5b. The 
sound absorbing member 34 is formed of, e.g., a porous silicone rubber 
sheet, and is retained in a compressed state between the piezoelectric 
element 2 and the base plate 5b. The sound absorbing member 34 is in 
direct contact with the piezoelectric element 2, thereby to effectively 
absorb undesired spurious vibration generated by the piezoelectric element 
2. The sound absorbing member 34 is provided with a notch 35, through 
which a lead wire 24b is guided. 
In addition, a lower portion 25b of the peripheral wall portion 21 of the 
case 6c is further bent inwardly toward the peripheral edge portion of the 
base plate 5b. Thus, a container type space is defined by the lower end 
portion 25b and the base plate 5b. Resin such as epoxy resin or silicone 
resin is poured into said space, thereby to form a sealant 36. The sealant 
36 reliably prevents any clearance from being created by a difference in 
thermal expansion coefficient between the base plate 5b and the case 6c or 
the terminal pins 30 and 31. 
The terms "upward", "downward", and the like are used herein only for 
illustrative purposes with reference to the drawings, no limitation of the 
structure of the invention being intended. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.