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Inventors
Kobayashi, Hiroshi
Obayashi, Hiroaki
Application #
737937
Filed
May-28-1985
Published
Apr-26-1988
Current US Class
181/123
181/139
181/144
181/148
181/151
181/152
181/160
181/177
367/140
367/99
International Classes
G01S 015/00; H05K 005/00; G10K 013/00; H04R 007/00; 169; 210; 151; 152; 160; 177
Field of Search
181/108 181/138 181/139 181/140 181/141 181/144 181/145 181/148 181/123 181/124 181/182 181/186 181/189 181/190 181/195 181/198 181/199 181/206 181/232 367/87 367/91 367/92 367/99 367/118 367/162 367/173 367/176 367/910
Assignee
Nissan Motor Co., Ltd. (Yokohama, JP)
Examiners
Kyle; Deborah L.
Attorney, Agent or Firm
Schwartz, Jeffery, Schwaab, Mack, Blumenthal & Evans
US Patent References
| 4463049 |
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Referenced by:
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Citation
Cite This Patent
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Abstract
An ultrasonic rangefinder capable of detecting a distance between a vehicle and an object with high accuracy is shown. The measurements for microphones for transmitting and receiving ultrasonic waves are chosen so that detour waves are restricted.
Claims
What is claimed is:
1. An ultrasonic rangefinder comprising:
ultrasonic wave transmitting means having a transmitting surface,
ultrasonic wave receiving means having a receiving surface,
at least one of the wave transmitting means and the wave receiving means further including:
(a) a horn; and
(b) a straight tubular section provided between at least one of the transmitting surface and receiving surface and said horn, said straight tubular section being shaped to satisfy the condition x=k(r.sup.2 /.lambda.)
wherein x is the length of the straight tubular section, r is the radius of the at least one of the transmitting surface and receiving surface, .lambda. is the wave length of the ultrasonic wave signal and k is a constant.
2. An ultrasonic rangefinder of claim 1 wherein the constant k is approximately 1.5.
3. An ultrasonic rangefinder of claim 1 wherein the diameter of the larger aperture of the horn is approximately a multiple of the wave length.
4. An ultrasonic rangefinder of claim 3 wherein the diameter of the larger aperture is approximately twice the wave length.
5. An ultrasonic rangefinder of claim 1 further comprising:
a wave transmitting housing means, for housing the wave transmitting means, having a signal transmitting port with an aperture surface for an ultrasonic wave signal produced by the wave transmitting means; and
a wave receiving housing means, for housing the wave receiving means, having a signal receiving port with an aperture surface for introducing a reflected wave to the ultrasonic receiving means, wherein the aperture surface of the signal transmitting port lies in an X, Y plane and the aperture surface of the reflected wave signal receiving port lies in an X, Y plane which is displaced in a Z direction relative to the aperture surface of the transmitting port.
6. An ultrasonic rangefinder of claim 5 further comprising a connecting section for connecting said wave transmitting housing means to said wave receiving housing means.
7. An ultrasonic rangefinder of claim 6 wherein the aperture surface of the signal transmitting port lies in an X, Y plane that is closer in the Z direction to the surface of an object whose range is to be detected than the aperture surface of the reflected wave signal receiving port.
8. An ultrasonic rangefinder of claim 7, further including an insulator surrounding the at least one of the transmitting or receiving means except for the at least one of the wave transmitting surface or wave receiving surface.
9. An ultrasonic rangefinder of claim 7, further including an insulator interposed between said transmitting and receiving means.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an ultrasonic rangefinder which is used to transmit and receive an ultrasonic wave signal for the detection of the distance from an object and, in particular, to improvements of an ultrasonic rangefinder for preventing a detour wave from reducing the accuracy of the detection.
There are already several ultrasonic rangefinders in existence, for example the one described in Utility Model Published Application No. Sho 57-68574. Such a device emits ultrasonic waves from an ultrasonic wave transmitter and receives pulses reflected from an object by an ultrasonic wave receiver. The distance is found from the phase difference between the emitted pulse and the received pulse.
As technically discussed in the art, received waves on a receiver include a detour wave which reduces the accuracy of detection. In a rangefinder, paticularly in a rangefinder which detects the vehicle height (the distance from the undersurface of the body of the vehicle to the ground), as disclosed in Japanese Patent Published Application No. Sho 57-182544, the reflected wave and the detour wave partially overlap, because the detected distance is short, (15 to 40 cm).
In a case of the type, the overlapping portions mutually interfere, causing the signal level to vary. In addition, when using the rangefinder on snow-covered roads and sandy soil, giving full consideration to the fact that the reflected wave signal is diminished because the ultrasonic wave signal is absorbed, there is a great necessity to set a low threshold level, and for this reason it is especially easy to produce the abovementioned error.
It has been proposed, as shown in Japanese Patent Published Application No. Sho 53-21953, to provide horns on a wave transmitter and a wave receiver for improving the directionality of the wave transmitter and receiver.
However, even with the improvement in directionality provided by the horns in such an ultrasonic rangefinder, when an ultrasonic transmitter and an ultrasonic receiver are installed in close proximity, a detour wave of an appreciable level which is directly incident from the wave transmitter and not reflected from the target object, is received in addition to a reflected wave.
This detour wave, especially when an object at a short distance is being detected (such as in the case of gauging the height of an automobile, as previously mentioned), partially overlaps the reflected wave and creates an obstruction, and is apt to cause a detection error by mixing with the reflected wave.
Furthermore, in a low temperature ambience the type of insulators normally used for damping the detour wave which transmits machanically from the transmitter to the receiver inside the rangefinder suffer a decrease in elasticity, and because their noise prevention effectiveness deteriortes, the level of the detour wave becomes higher than when at normal temperatures.
SUMMARY OF THE INVENTION
An object of the present invention is to provide, with due consideration to the drawbacks of such conventional devices, an ultrasonic rangefinder in which the detour wave is effectively damped.
A further object of the present invention is to improve the detection performance of an ultrasonic rangefinder by effectively damping the detour wave through an improvement in the shape or positions of transmitting and/or receiving port of the rangefinder.
A still further object of the present invention is to provide an ultrasonic rangefinder which can prevent the effect of the detour wave even in a low temperature ambience.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a block diagram showing an example of the electrical configuration of an ultrasonic rangefinder.
FIG. 2A and FIG. 2B are waveform diagrams showing transmitted and received signals of a conventional rangefinder.
FIG. 3 is a sectional view showing the inside of a conventional ultrasonic rangefinder.
FIG. 4A and FIG. 4B are waveform diagrams showing a transmitting wave and the generated status of the detour wave.
FIGS. 5A to 5C and FIGS. 6A to 6C are waveform diagrams showing the waveforms of the detour wave and reflected waves when they overlap, and the time difference change.
FIG. 7 is a sectional view showing the configuration of a first embodiment of the present invention.
FIG. 8 is a graph showing the relationship between the horn aperture diameter and S/N of an ultrasonic rangefinder, obtained by actual test.
FIG. 9 is a graph showing the relationship between the length of the straight tubular section of the horn and S/N, obtained by actual test.
FIG. 10 is a graph showing the relationship between the radius of the wave transmitting/receiving surface and the optimum length of the straight tubular section, obtained by actual test.
FIG. 11 is a graph showing the results of actual tests to determine the relationship between the wave length of the ultrasonic wave signal and the optimum length of the straight tubular section.
FIG. 12 is a diagram of the directional characteristics of the wave transmitting means of the first embodiment of the present invention.
FIG. 13 is a diagram of the directional characteristics of the wave receiving means of the first embodiment of the present invention.
FIG. 14 is a sectional view showing the configuration of a second embodiment of the present invention.
FIG. 15 is a plan view showing the upper surface of the second embodiment of the present invention.
FIG. 16 is a sectional view showing the configuration of a third embodiment of the present invention.
FIG. 17 is a sectional view showing the configuration of a fourth embodiment of the present invention.
FIG. 18 is a characteristic diagram for the fourth embodiment of the present invention, showing the change in rigidity versus temperature for insulators housing transmitting and receiving means.
FIG. 19 is a characteristic diagram for the fourth embodiment of the present invention showing the effect of damping on the detour wave versus change in temperature.
FIG. 20 is a sectional view showing the configuration of a fifth embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
To facilitate the understanding of the present invention, the conventional ultrasonic rangefinder will be briefly discussed.
The configuration of the electrical components of such an ultrasonic rangefinder is shown in FIG. 1.
A blocking oscillator 1, as shown in FIG. 1, generates a high frequency signal S.sub.1 for a specified time interval t.sub.1, every period t.sub.2, and supplies an ultrasonic wave transmitting means 2. As a result, an ultrasonic wave pulse signal from the ultrasonic wave transmitting means 2 is transmitted toward a target object D.
Then, the ultrasonic wave pulse signal relfected from the target object D enters an ultrasonic wave receiving means 3, upon which the received signal is amplified in an amplifier 4, then the amplified signal S.sub.2 is input into a time difference measuring curcuit 5.
This time difference measuring circuit 5 determines the time interval t.sub.3 (shown in FIG. 2B) from the starting point of the high frequency wave signal S.sub.1 output from the blocking oscillator 1, to the starting point of the previously mentioned received signal S.sub.2 after amplification, and treats this as data giving the distance to the target object.
The starting point of the received signal S.sub.2 is set at the point where the level of the received signal S.sub.2 exceeds a specified threshold level A.
The ultrasonic wave transmitting means 2 and the ultrasonic wave receiving means 3, as shown in FIG. 3, are housed in a common casing 8.
The casing 8 is formed from a plastic such as a synthetic resin. As well as housing the ultrasonic wave transmitting means 2 and the receiving means 3, this casing 8 is formed to provide a transmitting port K.sub.2 and a receiving port K.sub.3, both of which are horn-shaped, and which lead to a transmitting surface 2a and a receiving surface 3a, respectively.
The detour wave includes a wave which transmits sonically inside the rangefinder from the transmitting means 2 to the receiving means 3 as well as a wave which propagates and is detoured through the air from the transmitting port K.sub.2 to the receiving port K.sub.3.
A plurality of insulators 7A and 7B, made from rubber or some such elastic material, are interposed between the casing 8 and the transmitting and receiving means 2 and 3 for prevention transmitting signals from enterring the receiving means inside the casing 8.
As shown in FIG. 3, when the ultrasonic wave transmiting means 2 and the ultrasonic wave receiving means 3, are in close proximity and are housed in the casing 8, the vibrations from the wave transmitting means 2 are transmitted into the casing 8. As shown in FIG. 4B, the signal received in the receiving means 3 includes the reflected wave signal S.sub.2 from the target object D and the reception level of the detour wave S.sub.3. Thus, the detour wave S.sub.3 arriving at the wave receiving means 3 cannot be ignored.
In particular, the reflected wave S.sub.2 and the detour wave S.sub.3 partially overlap in the equipment detecting the vehicle height (the distance from the undersurface of the body of the vehicle to the ground), because the detected distance is short, (15 to 40 cm).
In a case of this type, the overlapping portions mutually interfere, causing the signal level received by a time measuring circuit to vary. This overlap, indicated by section B, is shown in more detail in FIGS. 5 and 6.
Furthermore, as can be seen in FIG. 4B, the pulse width of the detour wave S.sub.3 is distorted according to the route by which it reached the wave receiving means 3 from the transmitting means 2.
Specifically, in the case where the detour wave S.sub.3 and the reflected wave S.sub.2 are received in the same phase (shown in FIG. 5A), the levels of their component waves are added, as shown in FIG. 5B, and the starting point of the reflected wave S.sub.2 as received by a time measuring circuit (the point where the threshold level A is exceeded) advances (by .DELTA.t' as shown in FIG. 5C).
When the signals are received as opposite phases (shown in FIG. 6A), the levels of their component waves are subtracted, as shown in FIG. 6B), and the starting point of the reflected wave S.sub.2 is retarded (by .DELTA.t" as shown in FIG. 6C).
Accordingly, at this sort of a starting position for the reflected wave S.sub.2, there is an overlapping, and, as shown in FIG. 5C and FIG. 6F, an error range is produced for .DELTA.t' or .DELTA.t" in the time difference t.sub.3, and the accuracy of detection diminishes.
Hereinafter some embodiments of the present invention will be described with reference to the accompanying drawings in which the same references are given for the same parts as in FIGS. 1 and 3.
A first embodiment of the present invention is shown in FIG. 7. Referring to the figure, ultrasonic wave transmitting means 2 and an ultrasonic wave receiving means 3 are housed side by side in a housing chamber of a casing 10, which is formed of a synthetic resin such as a plastic.
A plurality of insulators 11 and 12, which are made from a material which effectively insulates sound, such as a flexible rubber, is interposed between the casing 10 and the transmitting and receiving means 2 and 3, with the exception of a transmitting surface 2a and a receiving surface 3a.
In the bottom surface of the casing 10 are provided a plurality of horns 13 and 14 which are formed as orifices communicating with the transmitting surface 2a and the receiving surface 3a respectively.
The horns 13 and 14 comprise straight tubular sections 13a and 14a, formed in a cylindrical shape pointing in a downward direction from the peripheries of the transmitting surface 2a and the receiving surface 3a respectively, and conical sections 13b and 14b, formed in a cone shape, opening in the downward direction from the bottom edges of the straight tubular sections 13a and 14a toward the bottom surface of the casing 10.
The length x of the straight tubular sections 13a and 14a is set to satisfy the relationship.
x=1.5 r.sup.2 /.lambda. (1)
where r is the radius of the transmitting and receiving surfaces 2a and 3a, and .lambda. is the wave length of the ultrasonic signal produced by the wave transmitting means 2.
Also, the aperture diameter y of the horns 13, 14 is set to that
y=2.lambda. (2)
By shaping the horns 13, 14 so that they satisfy the relationships shown in the above equations (1) and (2), the detour wave is considerably damped, and it is possible to avoid its effect.
The results of tests performed by the inventors of the present invention to derive the above equations (1) and (2) are given below.
The wave transmitting and receiving means 2, 3 used in these tests have a center frequency of 40 KHz (wave length .lambda.=8.5 mm), and the radius r of the transmitting and receiving surfaces 2a, 3a is 5.4 mm. (N.B. Because the normal oscillation mode of the transmitting and receiving means is a bending mode, here the effective radius of the vibrating surfaces is considered to be 5.0 mm).
The test results shown in FIG. 8 give the variation in S/N for variations in the aperture diameter y of the horns 13, 14 (where S/N is the comparative strength of the reflected wave and the detour wave with an aluminum plate placed 30 cm away from the aperture surface).
From the same drawing it can be determined that S/N is a maximum when the aperture diameter y is 18.5 mm (which is about double the wave length .lambda.).
The test results shown in FIG. 9 give the S/N ratio for different values of x, the length of the straight tubular section, with the horns 13, 14 having an aperture diameter of 18.5 mm, which is determined to be the optimum diameter from the test results of FIG. 8.
From the same drawing it can be determined that S/N is a maximum when the length of the straight tubular section x is approximately 4.5 mm.
Next, considering the radius r of the transmitting and receiving surfaces and the wave length .lambda. of the ultrasonic wave signal as parameters related to the optimum value of the length x of the straight tubular section, the following expression is obtained.
x=f(r, .lambda.) (3)
The test results shown in FIG. 10 give the length x of the straight tubular section with S/N at a maximum (which is the optimum straight tubular section length) when the radius r of the transmitting and receiving surfaces is varied with the wave length .lambda. at a fixed value (8.5 mm).
The test results shown in FIG. 11 give the length x of the straight tubular section when the wave length .lambda. is varied while holding the radius r at a fixed value (5 mm).
From these two drawings it can be determined that the optimum length x of the straight tubular section is proportional to the square of the radius and is inversely proportional to the wave length .lambda..
Accordingly, equation (3) may be expressed as
x=k(r.sup.2 /.lambda.) (4)
(where k is a constant).
Here, if the optimum length of the straight tubular section, determined from the test results shown in FIG. 9 as x=4.5 mm, and .lambda.=8.5 (mm), r=5 (mm) are substituted in equation (4), the value of the constant k is calculated as k=1.5 and the equation (1) is derived.
Similarly, the optimum value of the length x of the straight tubular sections 13a, 14a can be obtained from equation (1) for diverse values of .lambda. and r.
The results of the tests on the damping of the detour wave, provided by the shape of the horns, are shown in FIG. 12 and FIG. 13. FIG. 12 shows the directional characteristics of the wave transmitting means 2, while FIG. 13 shows the directional characteristics of the wave receiving means 3. In the diagram, P indicates the characteristics of the present invention, and Q indicates the characteristics of a means not using the horns.
From these two diagrams, in the case of this embodiment of the present invention, the amount of damping from the means with the horns is large on the side of the horn as shown at the angle of 90.degree. in FIGS. 12 and 13. Specifically it is determined that adequate damping of the detour wave is carried out.
Further, in this embodiment, the provision of the insulators 11, 12 results in the noise eliminating effect. Also, by housing the wave transmitting and receiving means 2, 3, the positioning error as a result of the elasticity of the insulators 11, 12 is controlled by the casing.
Further, in the present invention, as shown in FIG. 7, the wave transmitting and receiving means 2, 3 can obviously be applied for a rangefinder having separate casings side by side.
As has been explained in detail above, in the present invention, the detour wave can be extensively damped by improving the shape of the horn, thereby improving the detection of the reflected wave.
In addition, the high efficiency of the horn in damping the detour wave makes it possible to install the wave transmitting and receiving means in close proximity, so that the ultrasonic rangefinder itself can be reduced in size.
FIG. 14 and FIG. 15 show a second embodiment of the present invention. A casing 20, formed from synthetic resin such as a plastic, comprises a wave transmitting means housing section 20A which houses a wave transmission means 2, a wave receiving means housing section 20B which houses a wave receiving means 3, a connecting section 20C which connects the housing sections 20A and 20B, and a circuit housing section 20D.
A signal transmitting port K.sub.2, which transmits an ultrasonic signal produced by the wave transmitting means 2, is formed in the wave transmitting means housing section 20A, and a signal receiving port K.sub.3, which introduces the ultrasonic signal of the reflected wave, is formed in the wave receiving means housing section 20B. The aperture surfaces of the signal transmitting port K.sub.2 and the signal receiving port K.sub.3 are positioned at the same height.
A signal-processing circuit 21 is housed in the connecting section 20C. In addition, a concave section 20E is provided which is formed so that its surface is more concave than the aperture surfaces of either the signal transmitting port K.sub.2 or the signal receiving port K.sub.3. The bottom surface and wall surfaces of the concave section 20E are formed roughly at right angles.
A signal-processing circuit 23 is housed in the signal housing section 20D. In addition, the entire upper surface of the casing 20 is covered with a lid 24, and around its circumference this lid 24 is sealed to the circumferential edge of the casing 20.
A rubber or synthetic plastic insulator 22 is interposed between the side circumference and top surfaces of both the wave transmitting means 2 and the receiving means 3.
FIG. 15 is a plan view showing the second embodiment of the present invention.
As a result of this type of configuration, in this embodiment of the present invention, through the separation of the signal transmitting port K.sub.2 and the signal receiving port K.sub.3 by the connecting section 20C, together with the utilization of the concave section 20E, damping of the detour wave is achieved and it is possible to prevent the detour wave from entering the receiving means 3.
The reason that the concave section 20E damps the detour wave is because damping occurs in a plurality of angular sections 20F and 20G, which are formed at the bottom edge of the wall surface of the concave section 20E, as a result of the dispersion of the detour wave. The damping which results from this dispersion is large, and first occurs at the angular section 20F of the signal transmitting port K.sub.2, then at the angular section 20G of the signal receiving port K.sub.3.
A damping effect is also produced by the interference which occurs when the detour wave enters the concave section 20E.
Next, a third embodiment of the present invention is shown in FIG. 16. In the same way as in the second embodiment, a casing 30 is provided with a connecting section 30C, a wave receiving means housing section 30B, and a circuit housing section 30D, in addition to a wave transmitting means housing section 30A, formed as the signal transmitting port K.sub.2, which has a aperture surface at a different height than the signal receiving port K.sub.3.
A concave section 30E is formed at the bottom surface of the connecting section 30C in the same way as in the second embodiment.
As a result of this type of configuration, in this embodiment of the present invention, the same effect is obtained as for the second embodiment. In addition, as a result of the height of the signal transmitting port K.sub.2 being different from the signal receiving port K.sub.3 being different, it is possible to prevent the detour wave from entering the receiving means 3 through diffraction to obtain an even greater damping of the detour wave.
In this case, it is desirable to position the aperture surface of the signal receiving port K.sub.3 behind the aperture surface of the signal transmitting port K.sub.2 (above, in the drawing). It is also acceptable to eliminate the concave section 30E shown in FIG. 16, and position the bottom surface of the connecting section 30C at the same height as the aperture surface of the signal receiving port K.sub.3. Alternately, the bottom surface of the connecting section 30C may be positioned at a height midway between the aperture surface of the signal transmitting port K.sub.2 and the aperture surface of the signal receiving port K.sub.3.
As outlined in the foregoing detailed explanation, in the first, second and third embodiments of the present invention it is possible to damp the detour signal which is produced by the ultrasonic wave signal generated from the wave transmitting means, passing through space to enter the receiving means, and to prevent the detour signal from being received in the receiving means, so that an accurate detection action may be carried out.
Referring now to FIG. 17 a fourth embodiment of the present invention is shown.
This embodiment has been conceived for damping vibrations which are transmitted sonically from a transmitting means to a receiving means through an inside of the device.
In the ultrasonic rangefinder of this embodiment of the present invention, a plurality of insulators 37A and 37B cover the vibrating surfaces of an ultrasonic wave transmitting means 2 and an ultrasonic wave receiving means 3 other than a wave transmitting surface 2a and a wave receiving surface 2b. These insulators 37A and 37B are interposed between a casing 8 and the ultrasonic wave transmitting and receiving means 2 and 3.
The material of which the insulators 37A and 37B are constructed is a foamed elastic body of independent air bubbles on a substrate of an ethylene-propylene rubber such as ethylene-propylene terpolymer (EPDM) or thylene-propylene copolymer (EPM).
The insulators 37A and 37B made of this kind of material, as shown in FIG. 17, exhibit a smaller change in hardness with change in temperature than insulators formed of an unfoamed ethylene-propylene rubber. Even at low temperatures they do not lose their noise prevention effectiveness. In FIG. 18, the characteristics of the insulators 37A and 37B of the present invention are shown as I.sub.1, while the characteristics of insulators made of unfoamed ethylene-propylene rubber are shown as I.sub.2. The comparative modulus temperature TN is a temperature at which the rigidity becomes the N multiple.
FIG. 19 is a graph showing the relationship between the temperature and S/N, the ratio of the peak level of the reflected wave S.sub.2 to the peak level of the detour wave S.sub.3. The characteristics of this embodiment of the present invention are designated M.sub.1 and those of the conventional means shown in FIG. 3, M.sub.2.
From this graph it can be concluded that the damping effect of detour wave in this embodiment is large, and at low temperatures, as well, this effect is not diminished.
Accordingly, in this embodiment of the present invention a good damping effect on the detour wave S.sub.3 can be obtained, and the reflected wave S.sub.2 is always detected with good accuracy over a wide temperature range (-30 to +80.degree. C.).
In addition, the abovementioned ethylene-propylene rubber shows good resistance to weather and to chemicals, and is suitable for automobile parts.
The foamed elastic body used for the insulators 37A and 37B, because of its independent air bubble structure, does not become soaked with water by permeation when used in an environment where it is exposed to rainwater, unlike a continuously foamed material, and thus low performance because of freezing is prevented.
The foaming ratio of the insulators 37A and 37B is chosen at a rather high value, about 20%, compared to foaming materials which are broadly used. In addition, by use of an agent having small cold hardening it is possible to reduce the hardening of the insulators at low temperatures even more.
A fifth embodiment of the present invention, shown in FIG. 20, will now be explained.
In this embodiment, an insulator 37, which covers the ultrasonic wave transmitting and receiving means 3 and 2 are formed as one integral unit.
The material of construction of this insulator 37 is identical to that used in the fourth embodiment of the present invention shown in FIG. 17.
By this type of construction, the same effect is obtained as with the fourth embodiment, and, in addition, it is possible to reduce the assembly process and eliminate the positioning variance of the transmitting means 2 and receiving means 3.
In both fourth and fifth embodiments of the present invention ethylene-propylene rubber is used as the substrate for the insulators, but other materials having a noise deadening effect, such as urethane rubber, are also suitable for this application.
Herein before, several embodiments are described which show several structural features. Any structural feature of one embodiment can be applied for another embodiment so that the combination take a more effective configuration against detour waves.
It should be understood, of course, that the foregoing relates only to preferred embodiments of the present invention and that numerous modifications or alterations may be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.
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