Abstract:
An ultrasonic sensor is described having a housing which has a circumferential side wall and a base surface. A transducer element for generating ultrasonic vibrations is mounted on the base surface. The side wall includes a lower side wall section, in which the side wall has an essentially rotationally asymmetrical profile in a plane parallel to the base surface, and an upper side wall section in which the side wall changes to an essentially rotationally symmetrical profile toward an upper edge of the side wall. In other respects, the present system provides a parking assistance system for a vehicle, having a control unit and such an ultrasonic sensor, as well as a method for manufacturing an ultrasonic sensor.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an ultrasonic sensor, a parking assistance system for a vehicle having an ultrasonic sensor, and a corresponding method for manufacturing an ultrasonic sensor. 
     BACKGROUND INFORMATION 
     Ultrasonic sensors are used in motor vehicles for parking assistance systems, for example, which during parking measure the remaining distance from obstacles bordering a parking space. Such parking assistance systems typically include one or multiple ultrasonic sensors and a control unit, the ultrasonic sensors containing an ultrasonic transducer which is used both for transmitting and receiving ultrasonic signals. In particular, a key functional requirement for the ultrasonic sensors is a so-called proximity measuring capability at a distance range of less than 30 cm. 
     German patent document DE 3 441 684 A1 discusses an electroacoustic transducer having a cap- or pot-shaped housing whose base surface is designed as a diaphragm. A piezoelectric ceramic body as a transducer element rests on the diaphragm, and is covered by a damping foam layer on the side opposite from the diaphragm. In order to generate sound radiation having a particularly small aperture angle in a vertical plane, but sound radiation having a relatively large aperture angle in a horizontal plane, the contour of the diaphragm and of the side wall surrounding same has an approximately elliptical, i.e., rotationally asymmetrical, design in a plane perpendicular to the direction of sound radiation. In such designs it is disadvantageous that, in particular in pulsed operation, not only the diaphragm but also the side wall is excited to oscillations of secondary modes. 
     German patent document DE 197 27 877 A1 discusses an ultrasonic transducer having a pot-like housing which has an annular wall, and a base surface as a vibrating diaphragm. A stabilizing ring is provided on the exterior of the wall to impart greater rigidity to the housing and limit the ultrasonic vibration essentially on the pot base as a vibrating diaphragm. However, in particular for pulsed transmission excitation of the ultrasonic transducer, this design results in excitation of secondary modes, which typically correspond to tilting and crumpling motions of the diaphragm pot wall. These secondary modes impair the proximity measuring capability of the ultrasonic sensor, since they prolong the attenuation time after transmission excitation, and beat patterns overlap with small reflected ultrasonic signals from nearby objects. 
     It is therefore desirable to further reduce the excitation of secondary modes in the transmission mode of ultrasonic sensors, thus improving the proximity measuring capability. 
     SUMMARY OF THE INVENTION 
     Accordingly, an ultrasonic sensor is provided which has a housing having a circumferential side wall and a base surface, i.e., having a pot shape or cap shape. A transducer element for generating ultrasonic vibrations, a piezoelectric element, for example, is mounted on the base surface. The side wall includes a lower side wall section in which the side wall has an essentially rotationally asymmetrical profile in a plane parallel to the base surface. The side wall also includes an upper side wall section in which the side wall changes to an essentially rotationally symmetrical profile toward an upper edge of the side wall. 
     As a result of the design of the two side wall sections which are distinguished by the rotationally asymmetrical profile in the lower side wall section which directly surrounds the base surface, and by the transition to a rotationally symmetrical profile at the upper edge of the side wall, the undesired indirect excitation of, for example, secondary oscillation modes as the result of tilting and/or crumpling vibrations of the side walls upon excitation of a main or working oscillation mode of the base surface is significantly reduced by the transducer element in comparison to conventional ultrasonic sensors. 
     The ratios of the characteristics of the working mode to the interfering secondary modes are ascertainable, among other ways, by electrical measurements at sensor housings which are not damped by a foam filling or similar material. The individual modes are each characterized by an equivalent electrical circuit diagram. The characteristics of the secondary modes may be assessed on the basis of the ratios of the damping of the individual secondary modes to the main mode. Comparative measurements using conventional ultrasonic sensors having a pot-like housing with a constant side wall profile along the height of the side wall, and optionally a stabilizing ring on the exterior of the side wall, show an increase in the damping of the secondary modes by a factor of 5 to 10. 
     For ultrasonic sensors which are damped by a foam filling or the like and, depending on the application, optionally incorporated into additional casings which enclose the housing during operation, the characteristic shapes of the individual modes and the increased damping of the secondary modes compared to the working mode may be detected with the aid of laser interferometry. The increased damping of the secondary modes also results in a directly detectable improved reduction in interfering beats in the electrically amplified reception signal of the ultrasonic sensor which compete with the small reflecting ultrasonic signals of nearby objects, thus allowing improved proximity measuring capability of the ultrasonic sensor according to the present invention. 
     According to one refinement of the ultrasonic sensor, the side wall in the upper side wall section is at least predominantly tapered with respect to the lower side wall section; i.e., an average wall thickness of the side wall in the upper side wall section is less than in the lower side wall section. According to another refinement, the upper side wall section extends over more than 50% of an overall height of the housing. The upper side wall section may extend over less than 80% of the overall height of the housing. In these refinements the damping of the interfering secondary modes is particularly greatly increased. 
     According to one refinement, in the lower and upper side wall sections the housing has an essentially cylindrical outer contour. Such an ultrasonic sensor is particularly easy to install, since the lower and upper side wall sections are formed by the shape of the inner contour of the housing, while the outer contour may be provided, for example, in a shape which is identical to a conventional type of ultrasonic sensor. Thus, it is not necessary to modify the shape of surrounding parts of a vehicle, for example. 
     According to one refinement, the side wall has a circumferential outer thickening in the upper side wall section. This thickening acts as a stabilizing ring which results in greater rigidity of the housing, which reduces the occurrence of secondary modes in which the side walls are deflected. In addition, the mechanical robustness of the housing is improved. The outer thickening may extend along the upper edge of the side wall. 
     According to one refinement, the upper side wall section includes an inclined shape of an inner contour of the housing in a plane perpendicular to the base surface. For a cylindrical or generally rotationally symmetrical outer contour of the housing, for example, the plane perpendicular to the base surface may be a plane which passes through the axis of symmetry of the outer contour, a segment of the intersection line of the inner contour with the plane perpendicular to the base surface forming a straight line which is inclined with respect to the axis of symmetry of the outer contour. The segment of the intersection line may be inclined by approximately 45° relative to the base surface, so that, for example, the inner contour in places extends along the lateral surface of a cone having an aperture angle of 90°. 
     According to another refinement, the upper side wall section includes a curved shape of an inner contour of the housing in a plane perpendicular to the base surface. For a cylindrical or generally rotationally symmetrical outer contour of the housing, for example, the plane perpendicular to the base surface may be a plane which passes through the axis of symmetry of the outer contour, a segment of the intersection line of the inner contour with the plane perpendicular to the base surface forming a curved line. The inner contour may include an essentially quarter-circle-shaped curvature; i.e., a segment of the intersection line of the inner contour with the perpendicular plane extends in the shape of a quarter circle. Particularly advantageous vibration behavior of the housing with particularly high damping of secondary modes may be achieved in these two refinements. 
     According to one refinement, the upper side wall section includes a stepped shape of an inner contour of the housing in a plane perpendicular to the base surface. For a cylindrical or generally rotationally symmetrical outer contour of the housing, for example, the plane perpendicular to the base surface may be a plane which passes through the axis of symmetry of the outer contour, a segment of the intersection line of the inner contour with the plane perpendicular to the base surface forming a horizontal line, i.e., a line which is parallel to the base surface. Such a shape may be provided in a particularly simple manner. 
     The exemplary embodiments and/or exemplary methods of the present invention are explained below with reference to specific embodiments and the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a top view of an ultrasonic sensor according to one specific embodiment of the present invention. 
         FIG. 1B  shows a sectional view of the ultrasonic sensor from  FIG. 1A . 
         FIG. 1C  shows another sectional view of the ultrasonic sensor from  FIG. 1A . 
         FIG. 2  shows a sectional view of an ultrasonic sensor according to another specific embodiment, having a quarter-circle-shaped inner contour. 
         FIG. 3  shows a sectional view of an ultrasonic sensor according to another specific embodiment, having an inner contour which is linear in places. 
         FIG. 4  shows a diagram of a signal curve received by an ultrasonic sensor according to one specific embodiment of the present invention, together with a comparative curve. 
         FIG. 5A  shows a frequency spectrum of an ultrasonic signal emitted by a conventional ultrasonic sensor. 
         FIG. 5B  shows a frequency spectrum of an ultrasonic signal emitted by an ultrasonic sensor according to one specific embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Unless explicitly stated otherwise, identical or functionally corresponding elements are denoted by the same reference numerals in the figures. 
       FIG. 1A  shows a top view of an ultrasonic sensor  100  which is suitable for use in a parking assistance system for a motor vehicle. The ultrasonic sensor includes a pot-like housing  101  having a base surface  104  and a side wall  102  which annularly encloses the base surface. Housing  101  is molded or milled from a metallic material such as aluminum, for example, and coated with a primer for purposes of corrosion protection and painting. 
     As viewed by the observer, the illustration shows a top view into the interior of housing  101 , with the inner side of base surface  104  facing the observer. An electromechanical transducer element  106 , in the present case a cylindrical disk-shaped piezoelectric element, for example, is affixed, for example glued, to the inner side of base surface  104  and contacted. The illustration of the contacting has been omitted in the drawing for the sake of clarity. The remaining space inside housing  101  is filled with a damping material, likewise not illustrated. Shown housing  101  may also be enclosed by further casings made of soft elastomers, for example. 
     Base surface  104  has the approximate shape of a rectangle having short, rounded sides. In the middle, the rectangle is widened by circular segment-shaped protrusions  132  which surround the mounting site of transducer element  106 . At its upper edge, in the observer&#39;s line of sight in  FIG. 1A , side wall  102  has an edge face which is parallel to the plane of the drawing and to base surface  104 , and which is delimited by two concentric circular lines whose midpoint lies on an axis of symmetry  134 , relative to which an outer contour  120  of housing  101  has a rotationally symmetrical design. 
       FIG. 1B  shows a sectional view of ultrasonic sensor  100  from  FIG. 1A  along a section plane, denoted by B-B in  FIG. 1A , which passes through axis of symmetry  134  of outer contour  120 . Outer contour  120  is formed over the predominant portion of an overall height  118  of housing  101  in the shape of a cylinder having an outer diameter  136 , the cylindrical axis lying on axis of symmetry  134 . Outer contour  120  deviates from a cylindrical shape, in that a thickening  122  which acts as a stabilizing ring is provided at upper edge  116  of housing  101 , and in addition, lower edge  138  of side wall  102  is rounded at the location where side wall  102  meets the outer side of base surface  104 . 
     While outer contour  120  is rotationally symmetrical with respect to axis of symmetry  134  in the specific embodiment shown, inner contour  124  deviates significantly from a rotationally symmetrical shape. In a lower side wall section  108  which directly adjoins base surface  104 , inner contour  124  is perpendicular  144 , so that in lower side wall section  108  side wall  102  has the shape of an extruded body, i.e., having a constant cross-sectional shape in a plane which is arbitrarily selectable within lower side wall section  108 , parallel to base surface  104  and extending above same. Since inner contour  124  follows the rotationally asymmetrical contour of base surface  104  illustrated in  FIG. 1A , the cross-sectional shape of side wall  102  in lower side wall section  108  is rotationally asymmetrical. In the present exemplary embodiment, in addition the thickness of side wall  102  is not constant in lower side wall section  108 . 
     In an upper wall section  114  of side wall  102  which adjoins lower wall section  108 , inner contour  124  successively changes from the rotationally asymmetrical cross-sectional shape, which it has in lower side wall section  108 , to a rotationally symmetrical shape at upper edge  116 . In the present specific embodiment, upper side wall section  114  includes a transition section  110  and an edge section  112 , in which inner contour  124  is oriented perpendicularly and rotationally symmetrically with respect to axis of symmetry  134  of outer contour  120 , i.e., follows a cylindrical surface  142  whose cylindrical axis coincides with axis of symmetry  134 . The inner diameter of cylindrical surface  142  is selected in such a way that the cylindrical surface completely encloses the rotationally asymmetrical contour of base surface  104  in the projection along axis of symmetry  134 . It is apparent from  FIG. 1A  that short sides  130  of the approximately rectangular contour of base surface  104  are rounded specifically in such a way that in the projection along axis of symmetry  134  they coincide with cylindrical inner contour  124  in edge section  112 . 
     In contrast, in transition section  110  inner contour  124  extends in sections which are situated in the projection along axis of symmetry  134  outside the contour of base surface  104 , along a lateral surface  140  of a downwardly tapering cone whose vertex lies on axis of symmetry  134 . In the perpendicular section plane extending through axis of symmetry  134  of  FIG. 1E , this corresponds to a straight sectional contour which extends at an angle with respect to axis of symmetry  134  or base surface  104 . Aperture angle  146  of the cone may be 90°, for example, so that the lateral surface is inclined by an angle of 45° with respect to axis of symmetry  134 . On the other hand, in sections which lie in the projection along axis of symmetry  134  within the contour of base surface  104 , inner contour  124  extends perpendicularly in a continuation of its perpendicular shape in lower side wall section  108 . 
     Upper side wall section  114  extends over more than 50% of overall height  118  of side wall  102 , i.e., of housing  101 . Since the shape of inner contour  124  successively widens in upper side wall section  114  with respect to upper edge  116 , while outer contour  120 , with the exception of thickening  122  formed at upper edge  116 , extends essentially cylindrically, i.e., having a constant outer diameter  136 , the overall result is a successive tapering of outer edge  102  in upper side wall section  114 . 
       FIG. 1C  shows another sectional view of ultrasonic sensor  100  from  FIG. 1A  along a section plane denoted by C-C in  FIG. 1A , which likewise passes through axis of symmetry  134  of outer contour  120 . In alternative specific embodiments an edge section  112 , for example, in which inner contour  124  of side wall  102  extends cylindrically, may be dispensed with, so that upper side wall section  114  includes only a transition section  110 , and side wall  102  does not assume a rotationally symmetrical shape until it reaches upper edge  116 . 
       FIGS. 2 and 3  each show sectional views of further specific embodiments of ultrasonic sensors  100 , in which the shape of inner contour  124  in transition section  110  in the present described specific embodiment is different from conical surface  140  illustrated in  FIG. 1B . The section planes of the sectional views of  FIGS. 2 and 3  each extend through axis of symmetry  134  of outer contour  120  of housing  101 . Base surface  104  of housing  101 , the same as for the specific embodiment from  FIGS. 1A-C , has an approximately rectangular shape, resulting in essentially the same top view as in  FIG. 1A ; a separate illustration corresponding to top views of the specific embodiments of  FIGS. 2 and 3  has been omitted. The section planes of  FIGS. 2 and 3  extend along the longitudinal axis of the approximately rectangular base surface  104 , corresponding to the illustration in  FIG. 1B . 
     In the specific embodiment of  FIG. 2 , inner contour  124  in transition section  110  is designed in such a way that, in the sectional illustration shown, instead of the linear, angled sectional contour  140  in  FIG. 1B  it has a curved sectional contour  600  which smoothly adjoins cylindrical shape  142  of inner contour  124  in edge section  112 , and continues downward with a continuously decreasing inclination with respect to base surface  104 . Curved sectional contour  600  has a radius of curvature  602  which is constant over the curvature and which may be selected in such a way, for example, that at its lower end, curved sectional contour  600  is inclined parallel to base surface  104 , i.e., has an overall quarter circle shape. 
     In the specific embodiment of  FIG. 3 , in the sectional illustration, inner contour  124  in transition section  110  has a sectional contour which in places is composed of straight contour sections  140 ,  700 ,  140 ′. A first contour section  140 , which adjoins cylindrical shape  142  of inner contour  124  in edge section  112 , extends at an angle of 45° relative to base surface  104 . A first contour section  140  is adjoined by a second contour section  700  which extends parallel to base surface  104 , and a third contour section  140 ′ which once again extends at an angle of 45° relative to base surface  104 . 
     Alternative specific embodiments may also provide a strictly stepwise transition between edge section  112  and lower side wall section  108 , so that edge section  112 , without a transition section  110  of finite height, directly adjoins lower side wall section  108 . The shape of inner contour  124 , unlike that in  FIG. 3 , may be composed of sections which in places have a stepped shape, i.e., in a perpendicular sectional view as in  FIG. 1B , have a horizontal sectional contour, a conical shape, i.e., in a likewise sectional view, having a straight angled sectional contour, and/or a sectional contour having a circular or other curved shape. For example, a shape having a quarter-circle-shaped sectional contour may also be approximated by a shape of inner contour  124  which in places has conical and/or stepped sections. 
     During operation of ultrasonic sensor  100 , piezoelectric transducer element  106  is acted on by a control unit in an alternating, pulsed manner via an electrical excitation signal which generates a corresponding electrical field perpendicular to base surface  104 . When the polarization is suitably oriented, the electrical field generates, for example, a contraction of transducer element  106  transverse to the applied electrical field. This contraction of transducer element  106  tangential to base surface  104  causes base surface  104  to bend according to the so-called flexed arch principle. 
     For achieving the greatest possible deflections, it is advantageous to apply an actuating signal having a frequency which corresponds to a mechanically possible, for example approximately rotationally symmetrical, base mode, working mode, or upper mode of the base surface. Using suitable electrical wiring of transducer element  106  in the control unit, the mechanical bandwidth of the working mode is enlarged to the extent that short ultrasonic pulses requiring a large bandwidth may be transmitted. For pulsed transmission excitation of transducer element  106 , further secondary modes are excited which typically correspond to tilting and crumpling motions of side wall  102 . These modes are electrically uncompensated, and therefore have low bandwidth, i.e., large time constants. 
       FIG. 5A  shows a frequency spectrum of an ultrasonic signal emitted by a conventional ultrasonic sensor in the frequency range between 20 kHz and 80 kHz, recorded by laser interferometry. The frequency, in units of kHz, is plotted linearly along a horizontal axis  312 , while the logarithmic spectral intensity, in dB, is plotted along a vertical axis  310  in relation to a comparative output of 1 mW. In the illustrated spectrum, multiple individual curves illustrated using different line thicknesses are superimposed, the curves being obtained by measurements at various points of the vibrating base surface of the ultrasonic sensor. The frequency spectrum has a clear maximum  300  in the range of a working mode at 48 kHz, and further secondary maxima  302 ,  304 ,  306 , which correspond to interfering secondary modes 33 kHz, 67 kHz, and 75 kHz, respectively, which interfere with operation of the ultrasonic sensor. 
       FIG. 5B  shows a similarly obtained frequency spectrum of an ultrasonic signal emitted by an ultrasonic sensor according to one specific embodiment of the present invention. Compared to  FIG. 3A , maximum  300  in the range of the working mode at 48 kHz is present with unchanged intensity, including directly adjacent side bands  320 ,  322 . In contrast, the intensities of undesired secondary modes  302 ,  304 ,  306  are significantly reduced by factors of from 5 to 10. 
       FIG. 4  shows a diagram having two signal curves  200 ,  202  of two reception signals which are represented on a shared time axis  204  in different line thicknesses, a first reception signal  202  having been received by an ultrasonic sensor of conventional design, and a second reception signal  200  having been received by an ultrasonic sensor according to one specific embodiment of the present invention. Vertical axis  206  shows the particular signal level of the electrical voltage signal obtained. Clearly apparent in first reception signal  202  are beats  208  in reception signal  202  caused by secondary modes which impair the proximity measuring capability of the conventional ultrasonic sensor. Such beats are effectively suppressed in second reception signal  200 .