Patent Publication Number: US-9417111-B2

Title: Parabolic antenna with an integrated sub reflector

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from, and is a divisional application of U.S. Ser. No. 13/761,416 filed Feb. 7. 2013 which in turn relates to European Ser. No. 13 150 192.6 filed Jan. 3, 2013, the entire contents of which are incorporated herein fully by reference. 
    
    
     FIGURE FOR PUBLICATION 
     
       FIG. 1 
     
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sensor for measuring the fill level in a container. More particularly, the present invention provides a parabolic antenna with a sub reflector integrated into the radome. 
     2. Description of the Related Art 
     Sensors designed for measuring the fill level in a container that is filled with a liquid or with bulk material by determining the delay time of high-frequency signals are known in the art, wherein the sensor is embodied as a parabolic antenna which comprises a main reflector, a sub reflector, and a hollow waveguide, which is connected to the main reflector and which is oriented with its front end toward the sub reflector for coupling electromagnetic waves in and/or out. 
     Parabolic antennas embodied as sensors can therefore be used for measuring the fill level of liquids or for measuring the fill level of bulk material or for determining the level of liquids or bulk material. Sensors of this type are frequently required to withstand high pressures or high temperatures, and frequently are used in environments that contain aggressive media. To protect their components against corrosion and destruction, most such sensors are therefore equipped with a protective shell or a radome. 
     In the measurement of fill levels by determining delay times of high-frequency signals, high-frequency pulses are transmitted via an antenna and are reflected on the surface of a medium. The reflected high-frequency pulses are then received by the sensor. From the time difference between transmission of the high-frequency pulse and reception of the reflected high-frequency pulse, the distance of the sensor from the surface of the respective medium can be determined. Fill level measuring devices embodied as parabolic antennas for monitoring and determining fill levels in a container are known in the prior art. 
     DE 10 2009 000 733, for example, the entire contents of which are incorporated herein by reference, discloses a fill level measuring device, which has a special temperature and a hermetic seal that is stable under pressure. In this case, a process separation unit is proposed, which is equipped at least with a radiation structure that is transparent to high-frequency signals, a retention clip, and a coupling sleeve, wherein the radiation structure encompasses the hollow waveguide flush in a predefined region on the exterior side of the hollow waveguide, and wherein a radial enclosure of the region of the radiation structure that encompasses the hollow waveguide is embodied as a mounted coupling sleeve. The radiation structure is thereby pressed in a positive connection against the exterior of the hollow waveguide, forming a tight seal, and the metallic retention clip is preferably attached to the metallic sub reflector and to the mounted metallic coupling sleeve. However, part of the electromagnetic wave exiting the waveguide is reflected and deflected by this retention clip, so that the emitted electromagnetic radiation field is disrupted. However, during measurement, such undesirable reflections lead to an objectionable reduction in the signal-to-noise ratio, which can in some cases be substantial, and which can cause the echo to disappear into noise, making measurement of the fill level impossible. 
     DE 10 2005 049 242, for example, the entire contents of which are also incorporated herein by reference, attempts to solve this problem by employing a diffusion disk for removing the interfering portion of the electromagnetic waves laterally, past the exciter. In this process, the electromagnetic waves transmitted by the exciter are concentrated only partly by the parabolic mirror and radiated to the bulk material. The other part of the electromagnetic waves transmitted by the exciter is not concentrated, and is instead removed laterally by the diffusion disk. In this manner, the signal-to-noise ratio can be improved particularly in the short range, and the quality and sensitivity of measurement can be increased. The diffusion disk is arranged at the center of the parabolic mirror. In the embodiment examples of DE 10 2005 049 242, the exciter is coupled directly to the hollow waveguide and securely connected thereto, and is optionally protected by a radome. However, this device has the disadvantage that, on one hand, only part of the electromagnetic radiation is available for measuring the fill level, and on the other hand, by mounting the exciter with the hollow waveguide, additional interference with the electromagnetic field can occur. 
     The present invention therefore addresses the problem of providing a sensor for a fill level measuring device, which has no elements that interfere with the antenna array of the sensor, but which at the same time is easy to handle. 
     Accordingly, there is a need for an improved sensor, method of providing a sensor and a desire to overcome at least one of the detriments noted above. 
     ASPECTS AND SUMMARY OF THE INVENTION 
     In response, it is now recognized that the proposed invention addresses one of the desires noted herein. 
     According to one aspect of the present invention, a sensor for a fill level measuring device for determining the fill level in a container by determining the delay time of high-frequency signals comprises a main reflector having an edge region and an interior region, a sub reflector, which is connected to the edge region of the main reflector via a radome, an inner space, which is encompassed by the radome and the main reflector, and a hollow waveguide which is connected to the main reflector and has a front end, which is oriented toward the sub reflector for coupling electromagnetic waves in and/or out. The radome can be connected to the sub reflector via adhesive bonding or screws. Preferably, however, the radome is provided and embodied as integral with, or unitary, or monolithic with the sub reflector. Consequently, the previously customary attachment of the sub reflector can be dispensed with. And as a result, the active antenna surface is enlarged, and the efficiency of the radiated power is increased, since the antenna array of the detector can no longer be interfered with by projecting connectors. This advantage has an impact particularly at higher frequencies, for example, in the range between 77 and 81 GHz, that is, in a range in which the retaining connectors or other attachments become quite large in proportion to the wavelength. 
     The sub reflector and the radome are preferably made of a first dielectric material, wherein the part of the dielectric material that forms the sub reflector has a reflective coating, preferably a metal coating, on its side that faces the inner space. This allows the radome and the sub reflector to be produced together in a simple process, and allows a modular structure for the sensor. The space inside the sensor is preferably sealed, pressure-tight and/or vacuum-tight, in relation to an outer space that surrounds the sensor, so that corrosive vapors from liquids, the fill level of which is to be determined, cannot damage the sensor, and bulk material cannot soil the sensor. 
     The edge region of the main reflector can be embodied as a collar which encompasses the inner region, and which is preferably embodied as a tubular collar that encompasses the inner region and projects in the direction of radiation. The beam direction of the sensor can thereby be improved. The radome can also have a collar that is embodied such that the radome can be screwed onto, screwed into, or operatively fit onto the collar of the main reflector in a positive and operative connection. For this purpose, the collar of the main reflector can have a corresponding threading, onto which the radome can be screwed, or into which the radome can be screwed. A secure mounting of the radome onto the main reflector can thereby be guaranteed. 
     To accomplish sealing of the inner space in relation to the outer space, the edge region of the main reflector can have a sealing surface for providing a sealed connection between the radome and the main reflector. For this purpose, optionally as a sealing means but not limited thereto, at least one O-ring can be provided between the edge region of the main reflector and the radome for the purpose of sealing the inner space in relation to the outer space. 
     The main reflector of the sensor can be embodied as parabolic or paraboloid, and it can either be made of a metal or can have a metal coating on the inner region of its side that faces the inner space. High efficacy of the radiated power can thereby be achieved. The sub reflector can be elliptical or hyperbolic in shape. It conducts the power radiated from the hollow waveguide to the main reflector and thereby ensures the desired radiation. Advantageously, the sensor is embodied as a Cassegrain antenna or as a Gregory antenna, and it is understood that those of skill in the particular art will recognize these phrases as being fully enabling based upon the skill in the art. The sub reflector can be embodied as concave according to the Gregory principle, or it can be embodied as convex according to the Cassegrain principle. 
     To increase mechanical stability, the radome is advantageously embodied as convex and/or conical, but is not limited to these sole geometric shapes. The radome can be made of a plastic, for example, but may be pursued of any other operatively advantageous material. Advantageously, it is made of polypropylene (PP) or polytetrafluoroethylene (PTFE). These materials are characterized by a dielectric constant that is suitable for the W-band range, and absorb the transmitted radiation only weakly. On the other hand, however, the materials are not entirely optimal with respect to temperature resistance and expansion, and therefore can be used only conditionally for determining fill level, for example, in pressure less or low-pressure applications and/or at temperatures that are not excessively high. In environments in which the sensor is exposed to high pressures, or in environments in which it must have a particularly high stability, the radome is advantageously made of a glass fiber reinforced plastic or a high-performance plastic, Materials that are particularly suitable as plastic materials in these cases include polyetherether ketone (PEEK) and polyether imides (PEI). 
     In one further adaptive and advantageous embodiment, the thickness of the radome is such that it corresponds to a whole-number multiple of half the dielectric wavelength of a coupled-in electromagnetic wave, wherein the wavelength preferably corresponds to a wavelength of the W-band, particularly preferably to a wavelength of between 77 GHz and 81 GHz. Interfering reflections back to the surfaces of the radome are thereby avoided. 
     The front end of the hollow waveguide can be embodied as horn-shaped, and it can preferably be filled with a second solid dielectric material. In this manner, the wave impedance of the hollow waveguide can be adapted to the wave impedance of the open space. 
     The inner space can be filled with a third, preferably solid dielectric material. The sensor is thereby made stable to pressure, and the diameter of the hollow waveguide can be chosen as smaller. The dielectric constants of the first, second and third materials may be, but are not required to be, equal. 
     Of course, the front end of the hollow waveguide can also be embodied as a hollow waveguide that projects into the inner space, and is open in the direction of the sub reflector if, for example, the entire radiated energy is to be effectively deflected to the main reflector via the sub reflector. The sensor can be used in a fill level measuring device for detecting the fill level in a container by determining the delay time of high-frequency signals for radiating and receiving electromagnetic radiation. 
     The above and other aspects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustrative side sectional view of a sensor in a first optional embodiment, with a flat radome and a hollow waveguide end piece which is expanded in the shape of a horn. 
         FIG. 2  is an illustrative alternative embodiment of the sensor with a conical radome. 
         FIG. 3  is a further illustrative third embodiment of the sensor with a collar piece drawn toward the front side thereof. 
         FIG. 4  is a further illustrative embodiment of the sensor with a hollow waveguide that projects into the inner space. 
         FIG. 5  is a further illustrative view from the side of a vacuum-tight sensor with an O-ring seal arranged radially between radome and edge region of the main reflector, and 
         FIG. 6  is a pressure-tight embodiment of the sensor having one O-ring seal arranged axially and one O-ring seal arranged radially between radome and the edge region of the main reflector. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to embodiments of the invention. Wherever possible, same or similar reference numerals are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form and are not to precise scale. The word ‘couple’ and similar terms do not necessarily denote direct and immediate connections, but also include connections through intermediate elements or devices. For purposes of convenience and clarity only, directional (up/down, etc.) or motional (forward/back, etc.) terms may be used with respect to the drawings. These and similar directional terms should not be construed to limit the scope in any manner. It will also be understood that other embodiments may be utilized without departing from the scope of the present invention, and that the detailed description is not to be taken in a limiting sense, and that elements may be differently positioned, or otherwise noted as in the appended claims without requirements of the written description being required thereto. 
     Various elements, steps, features, or operations may be described as multiple discrete details in turn, and in a manner that may be helpful in understanding embodiments of the present invention; however, the order of description will not be construed to imply that these elements, steps, features or operations are order dependent. 
       FIG. 1  shows a schematic sectional illustration of one optional sensor  1  with a hollow waveguide  3 , the front end  4  of which is extended in the shape of a horn, but is not limited thereto, and is filled with a dielectric material. The front end  4  of the hollow waveguide  3  is connected to a main reflector  7 . An inner region  10  that forms the front surface of the main reflector  7  is bordered by an edge region  12  of the main reflector. The inner region  10  has a surface that is reflective to electromagnetic radiation, and is preferably metallic. A sub reflector  17 A located opposite the front end  4  of the hollow waveguide is connected to the edge region  12  of the main reflector  7  via a radome  20 A made of dielectric material. On its side that faces the hollow waveguide end piece  4 , the sub reflector  17  at least has a metallic coating or is made entirely of metal. The radome  20 A is connected to the sub reflector  17 A, supporting the reflector, either via a screw connection or an adhesive connection, or is advantageously embodied as integral with said reflector. The main reflector  7 , with the radome  20 A and the sub reflector  17 A, encompasses an inner space  30 , which is hermetically sealed in relation to the exterior space  40  that surrounds the sensor  1 , for example by means of O-rings  43 ,  46  arranged between edge region  12  and radome  20 F as discussed in relation to  FIG. 6 . It will be understood that the sealing means is operatively effective to encompass inner space  30  as will be discussed further below. 
     As is illustrated by way of example in  FIG. 5 , such a seal can be implemented by an O-ring  43  arranged radially between edge region  12  and radome  20 . In  FIG. 5 , the sub reflector  17 E is embodied as integral with the radome  20 E and is equipped with a metal coating  48 . Here, radome  20 E in  FIG. 5  is made of a first dielectric material. The hollow waveguide end piece  4  extended in the shape of a horn is filled with a second dielectric material  50 , as shown in  FIG. 1 . 
       FIG. 6  shows a further possibility for sealing the inner space  30  from the outer space  40 . In this example, the edge region  12  of the main reflector is embodied as forming an L-shape around its inner region. The part of the edge region  12  that is directly adjacent to the inner region provides an additional sealing surface for sealing the inner space  30  in relation to the outer space  40 . In  FIG. 6 , sealing is accomplished with an O-ring  43  that surrounds the radome  20 F radially and sub reflector  17 F, as was already illustrated in reference to  FIG. 5 , and with an O-ring  47  arranged axially between radome  20 F and edge region  12 , and surrounding the inner region  10  of the main reflector  7 . Of course, sealing arrangements are also conceivable in which, for example, sealing is accomplished only by means of one axially arranged O-ring  47 . 
     To attach the radome  20  (namely any radome  20 A- 20 F embodiment discussed herein or otherwise) to the main reflector  7 , and to simultaneously seal the inner space  30  in relation to the outer space  40 , the radome can be operatively screwed into a main reflector  7  embodied according to  FIG. 6 , for example, by means of screw threading (not shown), if the edge region  12  embodied as an L-shaped collar has a continuous indentation at its front end, for example, or a corresponding mating thread. In this operative case, for example, sealing can be accomplished solely via the O-ring  47  that surrounds the inner region  10  of the main reflector  7  axially. Here, it is necessary for the edge of the radome  20 F only to be shaped such that the radome rests flat against the O-ring  47  when it is screwed into the main reflector. Of course, other operative sealing means may be used without departing from the scope and spirit of the present invention. 
     As will be understood by those of skill in the art, in each of the  FIGS. 1, 2, 5 and 6 , the front end  4  of the hollow waveguide  3  is embodied in the shape of a horn, and is optionally and operatively filled with a solid dielectric material, with which the inner space  30  can also be sealed in relation to the interior of the waveguide  3 . 
       FIG. 3 , in contrast, shows a sensor  1  with a hollow waveguide end piece  4  embodied in the shape of a horn, which is not filled with a solid dielectric material. In this example, the inner space  30  also extends into the interior of the hollow waveguide  3 . In these cases, the inner space  30  can optionally be sealed in relation to the outer space  40 , for example, on the connection piece of the hollow waveguide  3  on the signal emitter side. 
     Whereas, in  FIG. 1 , the radome  20 A is embodied as planar or flat,  FIGS. 2 to 6  show alternatively shaped (here convex) radomes  20 B,  20 C,  20 D,  20 E, and  20 F (all as shown), which can be conical in shape, for example. Operatively, those of skill in the art will recognize that a radome  20  embodied as such has greater mechanical stability than the flat variant. The front end  4  of the hollow waveguide  3  in  FIGS. 2 and 3  is expanded in the shape of a funnel or horn such that the wave impedance of the hollow waveguide is adapted to the inner space  30 . The inner space  30  can be filled with a solid, liquid or gaseous dielectric material, or can be empty, all without limitation to the present invention. The wall  60  of the hollow waveguide  3 ,  4  can be embodied as a support, made of metal, for example, and can mechanically support the reflector  7  with the sub reflector  17  and the radome  20 . To the extent that the inner wall of the hollow waveguide  3  is optionally metalized, the wall  60  can also be made of a non-metallic material. This discussion is provided to illustrate that those of skill in the art will recognize that variations may be provided without departing from the scope and spirit of the present invention. 
       FIG. 3  and  FIGS. 4-6  show further exemplary embodiments of a sensor for a fill level measuring device for determining the fill level in a container. In this example, the edge region  12  of the main reflector is arranged in a generally tubular shape around the inner region  10  (the images are cross-sectionally shown). Such an arrangement can be provided, for example, as added mechanical protection for the radome and can stabilize the sensor. 
     The edge region  12  of the main reflector  7  is preferably made of metal, or preferably has a metallic coating, so that the radiation reflected by the surface of the liquid or by the surface of the bulk material, which would strike the sensor at an angle, cannot return to the hollow waveguide. In this manner, interferences in determining the delay time of the signal can be avoided. 
     In  FIG. 4 , the end piece  4  of the hollow waveguide  3  is inserted into the inner space  30  in the direction of the sub reflector  17  (as shown). The end piece can be open or can be filled with a dielectric material (optionally). Operatively, a diffusion disk  65  ensures that the interfering portions of the electromagnetic waves reflected by the sub reflector to the main reflector can no longer be deflected back to the sub reflector, and therefore cannot be coupled back into the hollow waveguide. The thickness of the radome  20 D in  FIG. 6  is such that it corresponds to a whole number multiple of one-half its dielectric wavelength, for example, 2 to 10 times one-half its dielectric wavelength, or equal to one-half its dielectric wavelength. In this manner, the part of the reflection from the surface of the radome that faces the inner space  30  back to the hollow waveguide is minimized. The use of a thicker radome, as is indicated schematically in  FIGS. 4 to 6 , also increases the overall stability of the sensor to pressure, so that it can also be used in environments in which high mechanical stress on the sensor can occur. 
     The measurement method of determining a fill level using a sensor of the type described for determining the fill level in a container by determining the delay time of high-frequency signals is carried out as follows: 
     A signal source (not shown but understood by those of skill in the art) couples an electromagnetic wave into the hollow waveguide  3 . The electromagnetic signal is conducted through the hollow waveguide  3  toward the front end  4 , and in the inner space  30  is radiated toward the sub reflector  17  (e.g., sub reflectors  17 A- 17 F for corresponding radomes  20 A- 20 F). The sub reflectors  17  reflects the signal to the inner region of the main reflector  7 , such that said reflector sends the signal as an ideally parallel high-frequency burst, perpendicular to the surface of the liquid, or to the surface of the bulk material. The radome  20  is made of a dielectric material, and is operatively structured and arranged such that it interferes as little as possible with the electromagnetic signals radiated from the main reflector. The part of the electromagnetic signal which is reflected by the surface of the liquid or by the surface of the bulk material toward the sensor  1 , and which strikes the inner region  10  of the main reflector  7 , is reflected by the main reflector  7  to the sub reflector  17 , and is coupled by this into the front end  4  of the hollow waveguide  3 . In operation, the reflected signal is detected by a receiving unit, not specified in greater detail, and from the delay time of the signal, the distance between the sensor and the bulk material surface is determined. Customary methods are available for determining the delay time include the pulse delay method (pulse radar) or the FMCW radar method (frequency modulated continuous wave radar) within the skill in the art and such understanding is incorporated herein by reference. 
     The invention has been specified in greater detail in reference to preferred embodiment examples, without being restricted to these embodiment examples. The features of individual embodiment examples can be freely combined or exchanged with features of other embodiment examples, assuming they are compatible. 
     LIST OF REFERENCE SIGNS 
     
         
           1  sensor 
           3  hollow waveguide 
           4  front end of hollow waveguide 
           7  main reflector 
           10  inner region 
           12  edge region 
           17  sub reflector ( 17 A to  17 F) 
           20  radome ( 20 A to  20 F) 
           30  inner space 
           40  outer space 
           43  O-ring 
           47  O-ring 
           48  metal coating 
           50  second dielectric material 
           60  hollow waveguide wall 
           65  diffusion disk 
       
    
     It will be understood by those of skill in the art that the illustrative designations used herein, such as top, bottom, left and right, etc., are in no way intended as restrictive, even where they refer to preferred embodiments. For example, the term “leak-proof” is illustrative but means that in the case of a pressure difference of at least 100 mbar, no leakage will occur. The term “vacuum-tight” means that, at a pressure difference of up to one bar, no leakage will occur. The term “pressure-tight” is understood to mean that the sensor can be used in environments having an external pressure of at least 3 bar, and that the flexure of the radome at room temperature is less than 0.3 mm. As a result, these phrases are recognized and will be understood by those of suitable skill in the operative arts. It will also be understood, that as discussed here in where radome and sub reflector are noted as integral, this shall be understood to be operatively integrated into a unitary, monolithic, or operatively contiguous and continuous member effective to operate as discussed herein. 
     As another adaptive embodiment, and optionally without restriction thereto, it will be understood that a pressure-tight sensor as discussed herein can be used at an external pressure of at least 10 bar. 
     Having described at least one of the preferred embodiments of the present invention with reference to the accompanying drawings, it will be apparent to those skills that the invention is not limited to those precise embodiments, and that various modifications and variations can be made in the presently disclosed system without departing from the scope or spirit of the invention. Thus, it is intended that the present disclosure cover modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents. For a non-limiting example, while the description describes the system or apparatus details herein, the entire disclosure shall also be recognized as fully supporting a method of assembling or using the invention, as well as a method of operatively using the proposed invention without departing from the scope and spirit of the present invention.