BACKGROUND OF THE INVENTION

This invention is concerned with electroacoustic transducers and, more specifically, with electroacoustic transducers for transmitting or receiving sound in a fluid medium.

The teachings of this invention can be used by one skilled in the art in a wide variety of transducer designs using many different methods of transduction for sound radiation or reception in fluid or gaseous mediums. Transducers using the teachings of this invention can be designed using a wide variety of transduction materials, such as magnetostrictive rods, piezoelectric crystals, and polarized ceramic elements. However, the teachings of this invention can be best used in conjunction with ultrasonic transducers designed with polarized ceramics for use in a gaseous medium. Such a transducer is described in U.S. Patent 3,928,777.

U.S. Patent 3,928,777 described an ultrasonic transducer employing a ceramic disc as the transduction material. For optimum performance, the transducer is operated in the vicinity of resonance. This disc could operate in the thickness mode of resonance, but in the preferred embodiment the ceramic disc operates in the radial mode of resonance, since a transducer of this design is smaller and less costly. The transducer further employs an acoustic transformer in the form of an acoustic impedance matching material inserted between the ceramic and the gaseous medium. This acoustic transformer material is characterized in that its acoustic impedance is less than the acoustic impedance of the ceramic, but greater than the acoustic impedance of the gaseous medium. The acoustic impedance of a material is its density, p , times the velocity of sound, c, in the material. In addition, the thickness of the acoustic transformer material is approximately one quarter of a wavelength. As shown in the referenced patent, a transducer utilizing the teachings thereof will be more sensitive over a broader frequency response, and the resultant acoustic radiation pattern will contain reduced secondary lobes. However, the beam angle from the transducer is fixed and controlled at the resonant frequency by the diameter of the ceramic disc.

In operation, the radiating face of the transducer described in US 3,928,777 must be exposed to the fluid medium in order to efficiently transmit and receive sound pulses with the desired acoustic radiation patterns. Having the face of the transducer exposed is usually not a problem in most echo- ranging applications. However, in certain cases, such as with sensors used in the automotive industry, having transducers visibly mounted onto the vehicle is not desirable because it interferes with the looks and style of the car.

Therefore, in automotive acoustic echo-ranging applications, such as park assist systems, it would be a great advantage to hide the transducers mounted onto the vehicle so that they are not visible. For example, in park assist systems, it would be desirable to hide the transducer behind the vehicle bumper.

BRIEF SUMMARY OF THE INVENTION

To overcome the limitation of the prior art that the beam angle from the transducer is fixed and controlled at the resonant frequency by the diameter of the ceramic disc, the present invention provides a novel design of an acoustic transformer with dimensions that are not the same as the dimensions of the ceramic disc. This acoustic transformer can therefore be designed to produce a wide variety of different acoustic radiation patterns. These radiation patterns can be conical in shape with different beam angles, or they can be fan shaped containing one beam angle in the horizontal plane and a different beam angle in the vertical plane. The transducer can also be designed to utilize a plastic housing that provides improved environmental protection. This invention also makes modifications to the forward acoustic transmission line between the vibrating surface of the transduction material and the medium into which the vibrations are to be transmitted. These modifications include fabricating the forward acoustic transmission line as a composite consisting partially of a portion of a continuous smooth sheet of material in a manner that results in the transducers being hidden and the acoustic beam being emitted from the continuous sheets of material.

Mounting means are provided to allow the transducer to be mechanically held in place behind the continuous sheet of material.

Generally speaking, the invention provides a method of mounting an electroacoustic transducer behind an exterior automotive skin such as a bumper, mirror housing or sheet metal. The method includes provisioning a quarter-wavelength or half-wavelength resonating structure having a frequency of resonance f _L , wherein the resonating structure includes a piezoelectric element coupled to one or more acoustic transmission lines; provisioning a portion of the skin that is sufficiently thin to not deleteriously interfere with the transfer of acoustic energy at frequencies in the vicinity of f _L ; attaching one of the one or more resonating structure acoustic transmission lines to the thin skin portion to thereby enable the transfer of acoustic energy from the resonating structure to the ambient air beyond the skin; and stiffening the skin that surrounds the thin skin portion and does not directly contact the acoustic transmission line in order to minimize radiation from the surrounding skin at frequencies in the vicinity of f _L .

The thickness of the thin skin portion is in the range of 0.005 to 0.25 inches, more preferably in the range of 0.01 to 0.1 inches, and most preferably in the range of 0.03 to 0.06 inches.

The stiffening may be provided by a thick portion of skin surrounding the thin skin portion. Alternatively, the stiffening may be provided by a housing element, insert or reinforcement disposed about the acoustic transmission line and mounted to the surrounding skin about the thin skin portion. A combination of the two techniques may also be utilized.

In preferred embodiments, the acoustic transmission line attached to the thin skin portion forms part of the housing element, insert or reinforcement and is connected thereto by a web having a width G, which is in the range of 20 thousands of an inch to 100 thousands of an inch.

The acoustic transmission line coupled to the thin skin portion may have a width dimension that is shorter than a corresponding width dimension of the piezoelectric element. This enables a fan shaped radiation pattern to be produced.

The resonating structure may be a quarter-wave structure having one acoustic transmission line. In this case, the length of the acoustic

transmission line in combination with the thickness of said thin skin portion being approximately one quarter of a wavelength at frequency _.

The resonating structure may alternatively and more preferably be a half-wave structure having a forward acoustic transmission line coupled between one side of the piezoelectric element and the thin skin portion and a rear acoustic transmission line coupled to an opposing side of the

piezoelectric element. In this case the length of the forward acoustic transmission line plus the thickness of the thin skin portion plus one half of the distance between the two opposing sides of the piezoelectric element are preferably approximately one quarter of a wavelength at frequency f _L .

An ultrasonic sensor constructed in accordance with the foregoing is also described.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features which are characteristic of the invention are set forth with particularity in the appended claims. However, the invention itself, both as to its organization and method of operating, together with further objects and advantage thereof, will best be understood by reference to the description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic cross-sectional representation of a cylindrical transducer resonant element using a thin radial resonant ceramic disc for the transduction material with an acoustic transmission line that radiates sound into the medium that has a diameter that is equal to the diameter of the ceramic disc.

FIG. 2 is a schematic cross-sectional representation showing the radial resonant structure of FIG. 1, except the diameter of the acoustic transmission line that radiates sound into the medium is smaller than the diameter of the radial resonant disc, therefore producing a conical radiation pattern with a larger beam angle.

FIG. 3 is a top view of the structure shown in FIG. 2.

FIG. 4 is a plot showing how the beam angle of the acoustic radiation pattern changes as a function of the dimension of the diameter of the acoustic transmission line radiator divided by the wavelength of the sound in the medium.

FIG. 5 is a schematic cross sectional representation of a cylindrical transducer resonant element that is a half wavelength resonator, which uses a thin non-resonant ceramic disc as the transduction material with an acoustic transmission line that radiates sound into the medium that has a diameter that is equal to the diameter of the ceramic disc.

FIG. 6 is a schematic cross-sectional representation showing the half wavelength resonant structure of FIG 5, except the acoustic transmission line for radiating sound into the medium is rectangular in shape, which enables it to produce a fan shaped acoustic radiation pattern containing one beam angle in the horizontal plane and another angle in the vertical plane.

FIG. 7 is a top view of the structure shown in FIG. 6. FIG. 8 is a schematic cross-sectionai representation of a rectangular transducer resonant element that is a half wavelength resonator and produces a fan shaped acoustic radiation pattern.

FIG 9 is a top view of the structure shown in FIG. 8.

FIG. 10 is a schematic cross-sectional view of a transducer in a housing employing the teachings of this invention.

FIG. 11 is a sectional view taken along the line A-A of FIG. 10.

FIG. 12 is a plot showing the measured horizontal acoustic radiation pattern of a transducer similar to the structure shown in FIGS. 10 and 11.

FIG. 13 is a plot showing the measured vertical acoustic radiation pattern of a transducer similar to the structure shown in FIGS. 10 and 11.

FIG. 14 is a schematic cross-sectional representation, similar to FIG. 6, showing a half wavelength resonant element of a transducer in which the forward acoustic transmission line for radiating sound into the medium is rectangular in shape, which enables it to produce a fan shaped acoustic radiation pattern containing one beam angle in the horizontal plane and another in the vertical plane.

FIG. 15 is similar to FIG. 7 of the co-pending application and shows a top view of the structure shown in FIG. 14.

FIG. 16 is a schematic cross-sectional representation showing the half wavelength resonant structure of FIG. 14, except the forward acoustic transmission line for radiating sound into the medium has been modified to be a composite structure that is comprised of a portion of the rectangular forward transmission line of FIG. 14 and a thin portion of a sheet of material that has a much larger area than the area of the front surface of the rectangular forward transmission line.

FIG. 17 is a top view of the structure shown in FIG. 16.

FIG. 18 is a cross-sectional view of a transducer in a housing employing the teachings of this invention that includes only the rectangular portion of the forward acoustic transmission line. FIG. 19 is a top view of the structure shown in FIG. 18.

FIG. 20 is a cross-sectional view showing the transducer structure of FIG. 19 that has been mounted onto a thin portion of a sheet of material such as an automobile bumper.

FIG. 21 is a system block diagram of an ultrasonic object detection system.

DETAILED DESCRIPTION OF THE INVENTION

Referring more particularly to the figures, FIG. 1 shows a schematic cross-section of a representation of a cylindrical resonant element of a transducer that uses the teachings of U.S. Patent 3,928,777. The

transduction material essentially comprises a thin piezoelectric ceramic disc 1 , which may be any one of the well known polarized ceramic materials such as, for example, lead-zirconate-titanate or barium titanate. The flat surfaces of the ceramic disc 1 are coated with metallic electrodes 2 and 3. They could be silver, electroless nickel, or other material as well known in the art. The ceramic disc 1 will resonate in the radial mode, also called the planar mode, and the frequency of the radial resonance, f _R , will be inversely proportional to its diameter, D _c , as is also well known in the art. To keep the thickness resonant frequency of the ceramic from interfering with the transducer operating at f _R , the thickness of ceramic disc 1 must be kept small enough so that the frequency of the thickness resonance of the ceramic is much higher than the frequency of the radial resonance. Electrical lead 4 is attached to electrode surface 2. This connection could be made using solder, electrically conducting cement, or any other means well known in the art. Electrical lead 5 is similarly connected to electrode surface 3.

A cylindrical acoustic transmission line 6 is attached to the front of ceramic disc 1. This attachment can be by any method that produces a strong mechanical bond, such as vulcanizing or cement. In the resonant element embodiment of FIG. 1, this transmission line 6 can be made of a wide variety of materials, such as potting compounds, rubbers, or plastics, but it is preferred that its specific acoustic impedance is greater than the specific acoustic impedance of the fluid or gas transmission medium into which the sound is to radiate, and less than the specific acoustic impedance of the ceramic material. The specific acoustic impedance is defined as the product of its density times the velocity of sound in the material. As was described in U.S. Patent 3,928,777, the length of the transmission line 6 should be a quarter wavelength, λ/4 , at frequency f _R , where for this structure λ is the wavelength of sound in the transmission line material.

When an electrical voltage is applied across the electrical leads 4 and

5 at a frequency equal to the radial resonant frequency, f _R , of the ceramic disc 1 , electrode surfaces 2 and 3 will alternately move towards each other and away from each other with maximum amplitude at that same frequency. The quarter wavelength transmission line 6 will then amplify the magnitude of the vibration that occurs at its surface that is attached to the front of electrode 2 of ceramic disc 1 and produce a much larger magnitude of vibration at its opposite surface 7. This large vibration of surface 7 will then radiate the sound from the transducer into the transmission medium at a higher sound pressure than would be produced by the ceramic radiating into the

transmission medium by itself. Because the transducer is reciprocal, the same increase in sensitivity is obtained when the transducer is used as a receiver. The response magnitude and Q of the transducer can be changed by using materials with different properties for the transmission line. If the material has greater internal mechanical losses, it will produce a transducer with a lower Q and lower sensitivity.

It is often desirable to design transducers to produce different acoustic radiation patterns. A transducer could be designed so that the radiation pattern of the sound it produces is omni-directional, or it could be designed to produce higher levels of sound pressure in a particular direction, usually perpendicular to the vibrating surface. This lobe of high sound pressure output can be made very broad, very narrow, or anywhere in between. The beam angle of the acoustic radiation pattern produced by a transducer is defined as the angle subtended by the two points on the lobe where the sound pressure radiated is 3 dB lower than the maximum level of the lobe. The beam angle at any specific frequency is a function of the dimensions of the radiating surface divided by the wavelength of sound in the transmission medium, λ _Μ , at that frequency. A transducer with a circular radiating surface with a diameter D will produce a conical acoustic radiation pattern. The beam angle of the conical pattern is inversely proportional to the ratio D/A _M .

Therefore, for a given frequency the beam angle will decrease as the diameter increases.

The radiating surface of the resonant element shown in FIG. 1 is surface 7 of the transmission line 6. This radiating surface is circular and has a diameter equal to D _T , which is also equal to the diameter of the ceramic, D _c . Since the radial resonant frequency, fR, of the ceramic is inversely

proportional to the diameter of the ceramic, Dc, and since DT is equal to Dc, both diameters will increase as the frequency f _R decreases, but

λ _Μ will also increase by the same proportion as the frequency decreases.

Therefore, the ratio of D _T / _M will stay the same for an element that is designed to resonate at any frequency, provided it uses the same ceramic material and radiates into the same transmission medium. The radiation patterns for all resonant frequencies will be conical with the same beam angles. It was shown in U.S. Patent 3,928,777 that for the range of the most common types of ceramic materials, the beam angle of the transducer radiating into air will be between approximately 8° to 12° at the radial resonant frequency of the ceramic discs.

It is possible to change the radiation pattern at its resonant frequency of a transducer using a resonant element similar to the structure illustrated in FIG. 1. FIG. 2 shows a schematic cross-sectional representation of a modification of the transducer resonant element of FIG. 1 that would change the beam angle of the transducer. FIG. 3 is a top view of the structure shown in FIG. 2. This structure contains the same ceramic disc that was used in the resonant element of FIG. 1 ; therefore, the diameter of the ceramic disc 1 , Dc, is the same for both. The length of the transmission line 6a of FIGS. 2 and 3 is a quarter wavelength long at f _R and is also approximately the same as the length of transmission line 6 of FIG. 1. The diameter of the transmission line 6a, Da-r _, however, is less than the diameter of the transmission line 6, D _T , of FIG. 1.

Since the ceramic disc is the same for both structures, the radial resonant frequency f _R is the same and λ _Μ is the same. Because the diameter of the transmission line is decreased, the ratio D _T /A _M also decreases, and the beam angle of the conical acoustic radiation pattern of the transducer therefore increases. FIG. 4 shows a plot of the beam angle of the acoustic radiation pattern produced by a transducer as a function οίΌ _τ /λ _Μ .

By changing the diameter of the transmission line 6a of FIGS. 2 and 3, it is therefore possible for one skilled in the art to fabricate transducers that use the same ceramic disc 1 in their resonant elements, and operate at the same resonant frequency, f _R , but produce different conical beam angles. To obtain the desired beam angle, the diameter of the transmission line 6a is adjusted to produce the required value of D _T /A _M shown in the graph of FIG. 4.

For some applications it is often desirable to make the resonant element of the transducer a half wavelength resonator. This type of device produces a single clean resonance with one resonant frequency and one anti- resonant frequency. In the transducer resonant elements of FIGS. 1, 2, and 3, the radial resonance of the ceramic produce one resonant frequency and one anti-resonant frequency; however, the quarter wavelength resonance of the transmission line 6 is superimposed onto the resonant frequency of the ceramic. These two different resonant frequencies in the resonant element can change at different rates as the temperature of the transducer is changed. This is not the case with a half wavelength element. Half wavelength transducers are therefore more temperature stable and consistent, and their electroacoustic responses are more immune to degradation if dirt or other material attaches to the radiating surface.

FIG. 5 shows a schematic cross-section of a representation of one preferred embodiment of a cylindrical resonant element that uses a half wavelength resonator. The thin ceramic disc 1a has a radial resonant frequency and a thickness resonant frequency that are both higher than the half wavelength resonant frequency of the transducer element. Two cylindrical acoustic transmission lines are used in the structure, the forward transmission line 8 and the rear transmission line 9. When designing half wavelength resonating structures such as this, a wide variety of materials can be used for the two transmission lines. Different materials will produce transducers with different characteristics, such as different Qs, different sensitivities, different maximum stresses, different front to back vibration ratios, and different temperature responses. The resonant element is designed to operate at the frequency of half wavelength length resonance, f _L . To accomplish this, the length of the forward transmission line 8 plus half of the thickness of the ceramic disc 1a is designed to be a quarter of a wavelength long at ή_, and the length of the rear transmission line 9 plus half the thickness of the ceramic disc 1a is also made a quarter wavelength. This makes the structure one half wavelength long at frequency f|_.

It has been found that different plastic and rubber materials similar to those used in the transmission lines for the radial resonant elements are preferred for use in the forward transmission line 8 of FIG. 5. Metals such as aluminum or steel are best used for the rear transmission line 9 to ensure that there is a much larger vibration amplitude produced at surface 7b that radiates the sound into the transmission medium than occurs at the open surface of the rear transmission line 9. Since the resonant frequency of the element shown in FIG. 5 is controlled by the lengths and material choices of the forward transmission line 8, and the rear transmission line 9, and the thickness of the ceramic disc 1a, the diameters of the forward and rear transmission lines and the ceramic disc can be made any value desired, provided all other resonances in these structures are kept well outside the operating frequency region of ή_. Therefore, the transducer element can be designed to produce any reasonable conical beam angle for the acoustic radiation pattern by making the diameter of the forward transmission line 8 conform with the curve of FIG. 4.

Since the rear transmission material is usually metallic, it is an electrical conductor. Therefore, to make fabrication easier, the rear transmission line 9 can be attached to the bottom electrode of the ceramic disc 3a using conductive cement, and the electrical lead 5 can then be electrically attached to the bottom of the rear transmission line 9, as shown. The lead 5 could obviously also be attached to the ceramic directly. In a structure such as that shown in FIG. 5, the diameters of the forward transmission line 8, the rear transmission line 9, and the ceramic can all be different, but it is usually easier to assemble the transducer if all three diameters are approximately the same.

It is often desirable to have an ultrasonic transducer produce a fan shaped radiating beam. For example, in an obstacle detection system for a robot or a vehicle, a transducer with a broad horizontal radiation pattern and a narrow vertical radiation pattern is ideal because the narrow vertical angle will not detect back scatter from the road surface and the broad horizontal pattern will require fewer transducers to cover detection of objects over the desired angular azimuth. A transducer with a rectangular radiating surface will produce a fan shaped acoustic radiation pattern that is broad in the plane around the width of the radiating surface, which I will call the horizontal plane, and narrow in the plane around its length, which I will call the vertical plane. If the width and length dimensions of the radiating surface are W and L respectively, then the beam angle of the acoustic radiation pattern in the horizontal plane, which is perpendicular to the radiating surface, bisects the length and is parallel to the planes formed by the two shorter ends, will produce a beam angle that is inversely proportional to the ratio W/X _M . The beam angle in the vertical plane which is perpendicular to the radiating surface, bisects the width and is paralleled to the planes formed by the two longer ends, will produce a beam angle that is inversely proportional to the ratio L/X _M . The beam angle of the radiation pattern in the horizontal plane will therefore be broader than the beam angle in the vertical plane, thus producing the fan shaped beam. For a given frequency the beam angle in each plane will decrease as W and L increase, and vice versa. The relationship of beam angle to W/X _M and L/X _M is very similar to the relationship of beam angle to D _T /X _M shown by the curve in FIG. 4.

FIG. 6 shows a cross sectional view of a modification to the resonant element structure of FIG. 5 that will produce a fan shaped radiation beam. FIG. 7 shows a top view of the structure of FIG. 6. In this embodiment the forward transmission line 8a has been shaped into a rectangular structure that is L long and W wide. The surface 7c radiates sound into the transmission medium. Typically L would be the same dimension as the diameter of the ceramic disc 1 , but it can be smaller as shown. If L is equal to the ceramic diameter, the short ends of the forward transmission line 8a could form an arc that follows the circular curve of the ceramic. This would typically make the transducer easier to fabricate. Since W is smaller than L, the ratio L/X _M is less than the ratio WjX _M . This will therefore produce a large beam angle for the radiation pattern in the horizontal plane around the width of the

rectangular radiating surface, and a narrow beam angle in the vertical plane around its length. FIG. 8 and FIG. 9 show a schematic cross-sectional and planar view of another embodiment of the resonant element structure shown in FIGS. 6 and 7, in which the front transmission line 8b, the rear transmission line 9a, and the ceramic 1 b, are made rectangular. The surface 7d of the forward transmission line 8b radiates sound into the transmission medium. A structure of this nature would be a little more expensive to produce, but the transducer would also fit into a smaller size envelop.

The schematic resonator element structures shown in FIGS 1 , 2, 3, 5, 6, 7, 8 and 9 illustrate modifications to the basic resonator element design to produce transducers with different radiation patterns. However, these structures are not usable unless they can be incorporated into a housing that will allow the transducer to be protected and mounted without affecting its electroacoustic responses. FIG. 10 is a cross-section of one preferred embodiment of a complete cylindrical transducer employing the teachings of this invention, and FIG. 11 is a sectional view of the structure taken along line A-A of FIG. 0. This transducer employs a similar resonator assembly to the one shown in FIGS. 6 and 7, except the forward transmission line 8c is incorporated into the transducer housing 10. The housing 10 would typically be a molded plastic piece. The length of the forward transmission line 8c plus half of the thickness of the ceramic 1a is equal to a quarter wavelength at the transducer resonant frequency, _. Likewise, the length of the metallic rear transmission line 9 plus half of the ceramic thickness is equal to a quarter wavelength at fi.. The top electrode of the ceramic disc 2a can be attached to the forward transmission line 8c by using any of a wide variety of cements that are commercially available. The rear transmission line 9 is also attached to the bottom electrode of the ceramic disc 3a using a commercially available cement, but in this configuration, the cement should be electrically conducting, or conducting particles should be mixed into the cement. This will allow lead 5 to be electrically attached to the rear transmission line 9which will make it also electrically attached to electrode 3a of ceramic disc 1a. Lead 4a is electrically attached to electrode 2a of the ceramic disc 1a. It contains an insulating jacket to insure that an electrical connection cannot be inadvertently made to electrode 3a or rear transmission line 9. Leads 4a and 5 are electrically attached to each conductor of cable 12, as shown.

The structure of FIG 10 contains a separation disc 11 which fits over the back of the resonating element structure and mounts into the shoulder in housing 10. Separation disc 11 contains holes that will allow leads 4a and 5 to pass through. It can be made from a wide variety of materials, but, it is usually best for it to be made from a material with high acoustic losses, such as certain plastics, rubbers, or corprene. The purpose of separation disc 11 is to form a dam to allow potting compound 13 to be poured into the back of the transducer while keeping it from flowing into the interior space of the transducer. After the potting compound cures, it forms a seal over the back of the transducer and also provides a strain relief for the leads 4a and 5 and the cable 12. The walls of housing 10 are made relatively thin to reduce the amount of acoustic reverberation within the structure after the transducer transmits a sound pulse.

It is necessary that the rectangular forward transmission line 8c is acoustically isolated from the rest of housing 10. This is accomplished by designing the peripheral portions of the front of housing 10 to have a large thickness to ensure that it is very stiff at the resonant frequency f _L . The rectangular forward transmission line 8c is disconnected from the stiff front of the housing by undercut 13. This undercut has a width w, and the thickness of the plastic in front of the undercut is t, as shown in FIG. 10. It is important that t and w be designed so that the resonance between the stiff portion of housing 10 and the resonant element is well below f|_, thus causing a complete mechanical decoupling of the forward transmission line 8c from the rest of housing 10 when the transducer is driven at frequency _.

If undercut 13 is properly designed, only the front portion of the housing directly in front of rectangular forward transmission line 8c will vibrate, and the rest of the front surface of the housing will be relatively stationary. The acoustic radiation pattern from the transducer will therefore be the same as that produced by the resonant element shown in FIGS. 6 and 7. If the undercut is not properly designed, there will also be motion in the rest of the front surface of the housing beyond the surface that is in the front of the rectangular forward transmission line 8c, and the radiation pattern will therefore become distorted.

An exemplary transducer utilizing the structure shown in FIG. 10 was fabricated and tested. In this unit that resonated at 62 kHz, the width and length of rectangular forward transmission line 8c were 0.2 inches wide by 0.54 inches long. The width w of undercut 13 was .062 inches and the thickness t was .012 inches. The thickness of the front of the housing beyond the area in front of transmission line 8c was 0.18 inches. The measured broad horizontal radiation pattern in the plane around the width of the forward transmission line 8c is shown in FIG. 12, and the measured narrow vertical radiation pattern in the plane around its length is shown in FIG. 13.

It is possible for anyone skilled in the art to employ the technique of this invention to design an ultrasonic transducer to operate at any desired frequency and to produce any reasonably desired acoustic radiation pattern. The transducer housing forms a continuous surface in its front portion, thus providing optimal environmental protection.

This design also will be relatively unaffected by dirt attaching to the radiating surface, particularly if the resonant element is a half wavelength resonator. It will also be appreciated that any of the resonant elements shown in FIGS. 1 , 2, 3, 5, 6, 7, 8 and 9 can be substitutes for the resonator in the embodiment shown in FIGS. 10 and 11.

FIG. 14 shows a schematic cross-sectional of a half wavelength resonant element of a transducer that will produce a fan shaped radiation beam. This is similar to the half wavelength resonant element shown in FIG. 6. FIG. 15 shows a top view of the structure of FIG. 14. The transduction material consists of a thin piezoelectric ceramic disc 101 , which may be any one of the well known polarized ceramic materials such as, for example, lead- zirconate-titanate or barium titanate. The flat surfaces of the ceramic disc 101 are coated with metallic electrodes 102 and 103. They could be silver, electroless nickel or some other material as is well known in the art. The thin ceramic disc 101 has a radial resonant frequency and a thickness resonant frequency that are both higher than the half wavelength resonant frequency, f _L , of the transducer element. Two acoustic transmission lines are used in the structure, the forward transmission line 108 and the rear transmission line 109. In the embodiment, the rear transmission line 109 is cylindrical, but the forward transmission line 108 has been shaped into a rectangular structure that is W wide and L long.

The cross-section of the forward transmission line, the rear

transmission line, and the ceramic in the transducer could ber circular, rectangular or any other shape. The surface 107 radiates sound into the transmission medium. The wavelength of sound in the transmission medium at the resonant frequency _ is X _L and is equal to the speed of sound divided by t. As was described above, typically L would be the same dimension as the diameter of the ceramic disc ,101 , but it can be smaller as shown. If L is equal to the ceramic diameter, the short ends of the forward transmission line 108 could form an arc that follows the circular curve of the ceramic. This would typically make the transducer easier to fabricate. Since W is smaller than L, the ratio L/ λι, is less than the ratio W7 X _L . This will therefore produce a large beam angle for the radiation pattern in the horizontal plane around the width of the rectangular radiating surface, and a narrower beam angle in the vertical plane around its length.

When designing half wavelength resonating structures such as this, a wide variety of materials can be used for the two transmission lines. Different materials will produce transducers with different characteristics, such as different Qs, different sensitivities, different maximum stresses, different front to back vibration ratios, and different temperature responses. The resonant element is designed to operate at the frequency of half wavelength length resonance, ft- To accomplish this, the length of the forward transmission line

108 plus half of the thickness of the ceramic disc 101 is designed to be a quarter of a wavelength long at f _L , and the length of the rear transmission line

109 plus half the thickness of the ceramic disc 101 is also made a quarter wavelength. This makes the structure one half wavelength long at frequency f _L .

It has been found that different plastic and rubber materials are preferred for use in the forward transmission line 108 of FIG 14, as is discussed above. Metals such as aluminum, brass, or steel are best used for the rear transmission line 109 to ensure that there is a much larger vibration amplitude produced at surface 107 that radiates the sound into the transmission medium than occurs at the open surface of the rear transmission line 109. Since the resonant frequency of the element shown in FIG. 14 is controlled by the lengths and material choices of the forward transmission line 108, and the rear transmission line 109, and the thickness of the ceramic disc 101, the diameter of the rear transmission line and the length and width of the forward transmission line and the ceramic disc can be made any value desired, provided all other resonances in these structures are kept well outside the operating frequency region of ft.

Since the rear transmission material is usually metallic, it is an electrical conductor. Therefore, to make fabrication easier, the rear transmission line 109 can be attached to the bottom electrode of the ceramic disc 103 using a conductive cement, and the electrical lead 105 can then be electrically attached to the bottom of the rear transmission line 109 as shown using many different types of attachment means, such as solder or using electrically capsulating cement. The lead 105 could obviously also be attached to the ceramic directly. Electrical lead 104 is electrically attached to the top electrode of the ceramic disc 102. Ultrasonic transducers are often used in echo-ranging distance measuring systems to detect the presence of objects and to determine the distance of the objects from the transducers. In these systems, the electronic circuitry causes a transducer to transmit a short pulse of ultrasonic sound into the air. The sound reflects from any objects in its path and returns to the transmitting transducer or to another transducer in the system. The receiving transducer converts the echo to an electronic signal that is detected by the electronic system. By measuring the time between transmission of the acoustic signal and the detection of the echo, the distance to the target can be calculated. In conventional ultrasonic transducers, the radiating surface of the transducer is directly in contact with the transmission medium, such that the transducers are either mounted on the exterior surface of the structure holding the system, or mounted into holes in the external surface of the structure holding the system. In short, the radiating surface of the transducer is visible.

In many applications, such as automobile park assist systems, it is desirable for the ultrasonic transducers to be hidden so that they do not interfere with the lines of the smooth surfaces of the car bumpers or doors. FIG. 16 shows a schematic cross-section of a representation of a modification of the transducer shown in FIG. 14. FIG. 17 shows a top view of the structure of FIG. 16. This modification allows the transducer to be attached to the inside surfaces of the plastic bumper or the metal surface of the door or the side of the car's body. A transducer using this modification is capable of efficiently transmitting and receiving acoustic pulses with controlled radiation patterns through the continuous surface of thin a sheet of material. The forward quarter wavelength long transmission line of this transducer is a composite comprised of half of the thickness of the ceramic disc 101, the length of the rectangular structure 108a and the thickness of the large thin sheet of material 115. The dimensions of these are chosen so that the entire length of the composite structure is a quarter wavelength at the resonant frequency f _L . The rectangular structure 108a is the same as the rectangular structure 108 of FIG. 14, except it is slightly shorter. The sheet of material 115 is typically part of the smooth extension surface of the device or vehicle onto which the transducer is mounted, such as the plastic bumper of the metal surface of the door. The rectangular structure 108a is mechanically attached to the thin sheet of material 115 by bonding and coupling means 116. This bonding means must acoustically couple the two structures and provide the necessary mechanical strength to hold the transducer structure together. It could be any of the many different epoxies and cements that are well known in the art. If external clamping means are provided to mechanically hold the transducer structure together, then coupling means 116 could be a viscous fluid that won't evaporate, such as grease or oil. In most cases the relative thickness of coupling means 116 is small compared to the length of the rectangular structure 108a and the thickness of the thin sheet material 115, so it will have minimal influence of the resonant frequency of the forward quarter wavelength long transmission line.

In the structure shown in FIGS. 16 and 17, the motion of the molecules in the forward transmission line are primarily controlled by the rectangular structure 108a, because it makes up the major portion of the length of the composite structure. The thickness of the thin sheet of material 115 is therefore a relatively small portion of the quarter wavelength long composite structure. Because of this, when the transducer resonates the major motion of the molecules in the thin sheet of material 115 occurs only in the

rectangular portion of the material that is connected to the rectangular structure 108a, which is W wide and L long. The large front surface of the thin sheet of material 115 is continuous and smooth, and the transducer structure behind it is not visible. However, in operation only the molecules of this thin sheet of material that are mechanically connected to the rectangular structure 108a will vibrate when the transducer is operated in the vicinity of the resonant frequency f _L . This will therefore produce a fan shaped acoustic beam from the large surface of the thin sheet of material 115 that has its horizontal and vertical beam angles controlled by the width W and length L of the rectangular structure 108a that is attached to the inside surface of the thin sheet of material.

It has been found in operation that unless the sheet of material 115 is extremely thin, the beam pattern can become distorted. The molecular motion can spread beyond the width W and the length L, and in addition, a larger portion of the sheet of material 115 can buckle and move causing further distortion to the acoustic radiation pattern. These unwanted motions can be reduced and eliminated by attaching a clamping means to the thin sheet of material 115 around the portion of the sheet that is W wide and L long and attached to the rectangular structure 108a.

In the schematic resonator element structures shown herein, the basic resonator element design can be modified by one skilled in the art to produce hidden transducers with different radiation patterns. However, these structures are not usable unless they can be incorporated into a housing that will allow the transducer to be protected and mounted without affecting the electroacoustic responses. The housing structure should also provide the required clamping means to control the motion of the thin sheet of material 115.

FIG. 18 shows a schematic cross-sectional view of a transducer that is a preferred embodiment of this invention, and FIG. 19 is a top view of the structure shown in FIG. 8. This transducer structure only contains the rear transmission line 109, the ceramic 101 and the rectangular member 108a of the half wavelength transducer. The entire quarter wavelength long forward transmission line is not completed until this transducer is mounted onto the thin sheet of material 115, as shown in FIG. 20. The transducer shown in FIG. 18 contains a housing 110, which would typically be a molded plastic piece. The rectangular structure 108a that forms the major portion of the forward transmission line is part of the housing, but it is acoustically decoupled from the rest of the housing by the thin web of plastic material 117. The web is designed so that its stiffness is low enough to ensure that the resonant frequency caused by the mass of the outer housing coupled by the web to the mass of the length resonator inside the housing will be much lower than the resonant frequency of the transducer f _L .

The web 117 must also be thin enough to ensure that any acoustic vibration in the frequency region of f _L is greatly attenuated as it tries to pass between the rectangular structure 108a and the rest of the housing 110. The web must be thick enough, however, to ensure that the structure of the transducer assembly is mechanically strong. This web also provides an environmental seal to the internal portion of the transducer assembly. The web can be located anywhere along the length of the rectangular structure 108a, but it is preferred to place the web closer to the ceramic disc 101. This is because in operation, the transducer creates at node with no motion at the center of the ceramic, and the displacement of the molecules along the rectangular structure 108a increase as the distance from the ceramic increases. Therefore, placing the web closer to the ceramic will be placing it where there is less motion within the rectangular structure 108a when the transducer is operating at f _L , which in turn will reduce the acoustic coupling between the rectangular structure 108a and the rest of the housing 110. Because of this construction, only the front surface of the rectangular structure 108a vibrates when the transducer is operated in the vicinity f _L . The front surface of the outside portion of housing 110 will remain stationary and result in little or no vibrating during the operation of the transducer because there is little or no acoustic coupling through web 117. The front surface of the outside portion of the housing 110 is separated from the rectangular structure 108a by a narrow gap with a width G, which is also the width of the web 117. The front surfaces of the rectangular structure 108a and the rest of the housing 110 are mechanically located in the same plane.

The transducer can be designed to operate at any one of a wide range of resonant frequencies, but for echo-ranging systems such as automobile park assist systems, it has been found that operation is best when the frequency f _L is within the band of approximately 40kHz to 60kHz. If f _L is much lower than approximately 40 kHz the transducer structure starts to become excessively large, and if f|_ is much higher than approximately 60 kHz the attenuation of sound in the transmission medium starts increasing to a level that the range of detection of targets is too short. For transducers operating in this frequency region, it has been found that webs with a thickness of approximately 15 thousands of an inch and a gap G of approximately 10 to 00 thousands of an inch, with 60 thousands being preferred.

As was discussed above, the top electrode 102 of the ceramic disc 101 can be attached to the rectangular structure 108a and the bottom electrode 103 can be attached to the rear transmission line 109 by using any of a wide variety of cements. Leads 104a and 105a are electrically attached to the ceramic electrodes and connected to electrical cable 112. Lead 104a

contains an insulation coating to ensure that it does not make electrical contact with the rear transmission line 109. An isolation cap 111 mechanically holds the rear transmission line inside the housing. This isolation cap would typically be made of an acoustically lossy material such as Butyl rubber, a lossy plastic, or corprene. The surface area where the isolation cap touches the housing should also be kept to a minimum. The cavity in the housing 110 behind the isolation cap 111 can then be encapsulated with a material such as cement or polyurethane that will environmentally seal the back of the transducer assembly and also provide a strain relief for the cable 112. This is obviously only one of many ways of mechanically holding the resonator and environmentally sealing the transducer that will readily come to the mind of one skilled in the art.

FIG. 20 shows a cross-sectional view of the transducer of FIG. 19 mounted onto the rear portion of a thin sheet of material 115a, that could be the plastic of the bumper, or the metal of the side of an automobile. The plastic material of an automobile bumper is typically acoustically lossy and is also relatively thick. Therefore, it has been found that the thickness of the plastic where the transducer is to be mounted will often have to be made thinner than the thickness of the rest of the bumper. This can be

accomplished by use of a secondary machining operation, or by molding that portion of the bumper to a smaller thickness. The cavity in the thin sheet of material 115a is made the proper diameter to allow the front of the housing 110 of the transducer shown in FIG. 18 to fit into it. The front surfaces of the transducer are attached to the inner surface of the cavity in the thin sheet of material 115a by using the bonding and coupling means 116a, which connects the front surface of the outer portion of housing 110 to the thin sheet of material 115a, and 116, which connects the front surface of the rectangular structure 108a to 115a. Bonding and coupling means 116 and 116a can be comprised of any number of materials, as discussed in connection with FIG. 16. The forward quarter wavelength transmission line of the assembly consist of half of the thickness of the ceramic disc 101, the rectangular structure

108a, and the thickness of the portion of the thin sheet of material 115a attached to the rectangular structure 108a, as shown in FIG. 20. This construction therefore causes the molecules of the portion of the thin sheet of material 115a attached to the rectangular structure 108a to vibrate, thus producing efficient transmission and reception of sound with a fan shaped radiation pattern with beam angles controlled by the width W and the length L of rectangular structure 108a. Because the external portion of housing 110 is acoustically disconnected from the vibrating portion of the transducer by the web 117, the front surface of the external portion of housing 110 does not vibrate, as was previously discussed. Because of this, attachment means 116a causes the outer portion of the housing 110 to clamp the portion of the thin sheet of material 115a that is in front of it. This clamping stabilizes the thin sheet of material 115a and reduces or eliminates the distortion of the acoustic radiation pattern that could occur without clamping as was previously discussed. It has been found that for the typical plastics used in automobile bumpers, the inventive transducer works best when the thickness of the material 115a in front of the surfaces of the transducers is between approximately .03 inches and .06 inches. If the thickness becomes less than approximately .03 inches, the structure becomes mechanically weaker, and as the thickness becomes greater than approximately .06 inches the acoustic performance of the transducer deteriorates. More generally however, depending on the type of material 1 5a, the thickness of the material 115a in front of the surfaces of the transducers may be in the range of 0.005 to 0.25 inches, more preferably in the range of 0.01 to 0.1 inches, and as discussed above, most preferably in the range of 0.03 to 0.06 inches

A mounting tube 119 is connected to the thin sheet of material 115a. This mounting tube can be fastened rigidly to the inner portion of the thin sheet of material 115a, as shown, by any number of means that are well known in the art. For example, it could be cemented in place, welded in place, or molded as part of the fabrication of the thin sheet of material 115a if it is a plastic bumper. The ID of the mounting tube 119 is designed to allow the transducer housing 110 to fit inside with a spring 118 placed between the housing and the ID of the mounting tube. The spring 118 rests against the lip in the front of the housing as shown. The spring is then compressed and the locking cap 120 is inserted around the housing 110 and over the edge of the mounting tube 119. The spring 118 is compressed by the locking cap 120, which therefore provides a constant force to hold the face of the outer portion of the housing 110 and the outer surface of the rectangular structure 108a rigidly against the inner surface of the thin sheet of material 115a. An environmental seal can be provided to protect the bonding and coupling means 116 and 116a. This can be provided in many different ways. For example, a gasket could be used or the O-ring 121 as shown in FIG. 20. This is only one of the many ways of mechanically mounting the transducer onto the thin sheet of material that will readily come to the mind of one skilled in the art. The inventive transducer used in combination with any of the many electronic echo-ranging systems that are well known in the art will produce a unique hidden transducer system that is highly desirable for automotive applications such as park assist systems. In such a system, hidden transducers such as those shown in FIG. 18 would be mounted into the front and rear bumpers of an automobile in the manner shown in FIG. 20. The length L and width W of the forward rectangular structures 108a are selected so that the horizontal beam angle of the radiation pattern of the transducers would be relatively broad and the vertical beam angle would be relatively narrow. This broad horizontal beam would allow the transducers to detect targets located in positions in a relatively large area over the horizontal plane behind or in front of the vehicle. The narrower vertical angle, however, while allowing for detection of actual targets that are located behind or in front of the vehicle, would greatly reduce or eliminate false targets that could be produced by backscatter echo reflections from the irregularities in the road surface if the horizontal beam was large enough to insonify the road. It has been found that horizontal beam angles of approximately 80-100 degrees and vertical beam angles of approximately 30-40 degrees work well for most applications.

Typically two to four transducers would be mounted on each bumper with the horizontal radiation patterns of adjacent transducers overlapping. This allows a target object to be within the detection patterns of multiple transducers. In operation, a single transducer can transmit a sound pulse and detect the return echo from a target, but an echo can also be detected by one or more adjacent transducers. In this manner, the redundancy of echoes from a single target received by multiple transducers will improve the probability of detection of targets that produce low amplitude echoes. In addition, because of the multiple transducers with multiple beams pointed in different directions, the system can determine the approximate side to side location of a target and report if it is directly in front of the vehicle, or if it is located to the left or right side. Additional transducers can be placed on the sides of the vehicle to determine the distance of targets such as walls or curbs to the side during parking.

FIG. 21 shows a schematic block diagram of one illustrative example of an automobile park assist system using the hidden transducers. In operation the control logic 135 produces an electrical tone burst signal in the vicinity of the resonant frequency, _, of the hidden transducer 130. This tone burst passes through the transmit drive circuit 132 to increase its voltage, which could include circuitry such as a power amplifier and a transformer. The high voltage electrical drive signal then passes through the Transmit/Receive (T/R) circuit 131 and is applied to the hidden transducer 130. This causes the transducer to produce an acoustic transmit pulse 137, which travels through the air and reflects off of target 136. The acoustic echo pulse 138 is reflected from the target and travels back to the hidden transducer 130, where it is converted into an electrical signal. This received electrical echo signal then passes through the T/R circuit 131 to the input of the amplifier 133. The T/R circuit 31 protects the amplifier 133 from the high voltages of the transmit pulse by using circuit components such as back to back diodes, as is well known in the art. The output of the amplifier is fed into the detection circuit 134. This circuit will detect the presence of an echo signal by using any one of a number of well known detection methods. For example, it could contain a simple peak detector and threshold detector, or it could use autocorrelation or crosscorrelation technology. When an echo pulse arrives, the detection circuit 134 sends a signal, such as a logic level pulse, to the control logic 135. The control logic then measures the time from when electrical transmit pulse was generated to when the acoustic echo was received, and since the speed of sound in the air is known, the control logic calculates the distance target 136 is from the hidden transducer 130.

As was discussed, the echo-ranging system shown in FIG. 21 can contain a number of additional transducers. There can be several

transducers located on the back bumper, several on the front bumper, and several on the sides of the vehicle. Each of these additional hidden transducers will be connected to the control logic 135 using similar circuit blocks as those just discussed. One additional transducer channel is shown in FIG. 21, as illustrated by hidden transducer 130a, T/R circuit 131a, transmit drive 132a, amplifier 133a, and detection circuit 134a. Even though only one additional transducer channel is shown, it is understood that it would be straightforward to add any number of transducer channels to the system. The control circuit 135 can operate each of these additional hidden transducer channels as stand alone echo-rangers. However, if any two hidden transducers have overlapping horizontal beams, than one transducer can transmit an acoustic pulse, but an echo can be reflected to and received by two or more transducers, as shown by acoustic echo pulses 138 and 138a in FIG. 21. Both of these echoes can then be detected and processed by control logic 135 to better determine the location of the target 136. All of this signal processing is well know in the art and is not unique in the inventive system. What is unique is the use of invisible transducers in combination with the electronics in the system, which solves a major problem that exists with all prior art systems where styling is important, such as in automobile park assist systems.

In a system such as that illustrated in FIG. 21 , the control logic 135 can use the detected echo signals from hidden transducers located on the rear bumper, the front bumper, or the sides of the vehicle to detect the presence of target objects and their distance and position relative to the vehicle. The control logic 135 will then produce output signals 140 that are used to display the location of these target objects to the driver. This display could be presented in many different forms, such as an audio output stating the relative positions of target objects, or a visual display in which the targets are shown pictorially relative to the vehicle. Electronic echo-ranging systems such as the one shown in FIG. 21 are well known in the art and are commonly used in automobile park assist systems. However, all current state of the art systems require the ultrasonic transducers to be exposed. This causes the face of the transducer to "break up" the smooth lines that were designed into the surface of the vehicle, which is a major disadvantage to these existing systems. The system shown in FIG. 21 is believed to be a significant improvement over existing state of the art systems because it produces an automobile park assist system that does not interfere with the smooth lines designed into the vehicle since it utilizes hidden transducers.

While a few specific embodiments of the present invention have been shown and described, it should be understood that various additional modifications and alternative constructions may be made without departing from the true spirit and scope of the invention. Therefore, the appended claims are intended to cover all such equivalent alternative construction that fall within their true spirit and scope.