Abstract:
To provide a radar module used for a speed measuring device or the like, in which dispersion of intensity distribution of electromagnetic waves emitted from the radar module via a lens is small, the radar module using a substrate with a plane antenna formed on a surface of the substrate includes: a lens having one end face that is plane and another end face that is spherical. In the radar module, a plane side of the lens is disposed to contact the plane antenna, and a spherical side of the lens is disposed in a remote field of the plane antenna.

Description:
TECHNICAL FIELD 
     The present invention relates to a radar module used for a speed measuring device that calculates a speed by emitting an electromagnetic wave of, e.g., a millimeter wave band or a micro wave band toward a ground and measuring a frequency change quantity of a reflected wave for non-contact measurement of a groundspeed of a transport machine. 
     BACKGROUND ART 
     To detect a groundspeed of a transport machine such as an automobile or a railroad vehicle, a method for obtaining a speed by measurement of the number of rotations of wheels is generally known. Such a method is known to fail to measure a groundspeed at a time of wheel slip, and to cause a measurement error when a wheel diameter is changed due to deflation of a tire or wear of a wheel. 
     A speed measuring device is also known in which the device calculates a groundspeed by using a radar module of a millimeter wave band or a micro wave band, emitting a continuous electromagnetic wave from the radar module, receiving a reflected wave of the electromagnetic wave to measure a frequency change quantity of the reflected wave (see, e.g., Patent Literature 1). Since the device uses a non-contact type method, a groundspeed can be measured even at a time of slip and change of a wheel diameter causes no influence. 
     Such radar module using an electromagnetic wave generally has wide directivity of an antenna. Thus, the directivity has to be sharpened by a lens. For example, Patent Literature 2 proposes a radar module that seals an MMIC (monolithic microwave integrated circuit) chip that has an active circuit, e.g., an oscillator or a mixer and an antenna mounted on a same semiconductor substrate in a resin package, and mounts a lens by bonding at a position above the antenna on a surface of the resin package. Patent Literature 3 proposes a configuration mounting a lens in contact with an opening of a tapered slot antenna. Patent Literature 4 proposes a configuration in which a dielectric oscillator is disposed near an end of a plane dielectric line to propagate an electromagnetic wave, an electromagnetic wave is emitted to air by a resonance phenomenon and a lens is arranged with a gap above the oscillator. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent Publication (Kokai) No. 2006-184144A 
         Patent Literature 2: Japanese Patent Publication (Kokai) No. 2003-315438A 
         Patent Literature 3: Japanese Patent Publication (Kokai) No. 2000-31727A 
         Patent Literature 4: Japanese Patent Publication (Kokai) No. 10-341108A(1998) 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In Patent Literature 2, however, an inaccurately determined lead frame position in resin-sealing of the MMIC chip easily causes dispersion of spaces between an incident face of the lens and an antenna face among modules, and further, due to an antenna-side end face of the lens in a near field or an area of the near field, an electric field pattern greatly varies depending on a distance. Thus, Patent Literature 2 has a problem that such a minor deviation may cause dispersion of intensity distribution of electromagnetic waves emitted from the radar module via the lens among the modules. Also, Patent Literature 2 has a problem that since the bonded face of the lens is an impedance mismatching face, a transmittance is deteriorated, a problem that since the MMIC chip in the package is bonded, the position of the MMIC chip is unfixed, which may cause dispersion of the position of the antenna axis among the packages, and further, a problem that since an opaque package cannot adjust the positions of the lens axis and the antenna axis, an emission direction of a millimeter wave may deviate from the lens axis. If such radar module in which a lens axis and an antenna axis are deviated is used as a speed measuring device, measurement dispersion occurs among the devices. 
     Patent Literature 3 has a problem that a length in an emission axis direction of an electromagnetic wave is required, and a problem that determining or fixing a position of the lens is difficult because the lens is necessarily disposed at an almost linear part of an end face of the substrate. 
     The problems in Patent Literature 4 are as follows. To utilize a resonance phenomenon, the upper and lower faces of the oscillator is required to have a gap for ensuring occurrence of a resonance phenomenon, and thus, the lens is necessarily arranged with a gap. However, to obtain the effect of the lens, whole of the lens is required to be arranged in a remote field considerably apart from a near field area, and thus, a large gap is required. Furthermore, since an electromagnetic wave from the oscillator is emitted at a wide angle, the lens has to be large. 
     The object of the present invention is to provide a radar module used for, e.g., a speed measuring device, in which dispersion of intensity distribution of electromagnetic waves emitted from the radar modules via a lens is small among the modules. 
     Solution to Problem 
     To achieve the aforementioned object, in the present invention, a radar module using a substrate with a plane antenna formed on a surface of the substrate includes: a lens having one end face that is plane and another end face that is spherical, wherein a plane side of the lens is disposed to contact the plane antenna, and a spherical side of the lens is disposed in a remote field of the plane antenna. 
     Advantageous Effects of Invention 
     The present invention allows, in a radar module used for, e.g., a speed measuring device, dispersion of intensity distribution of electromagnetic waves emitted via a lens to be small among the modules. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an example of a radar module of the present invention. 
         FIG. 2  illustrates an example of a lens of the present invention. 
         FIG. 3  illustrates an example of a section of the radar module of the present invention. 
         FIG. 4  illustrates an example of a speed measuring device of the present invention. 
         FIG. 5  illustrates an example of a section of the speed measuring device of the present invention. 
         FIG. 6  illustrates a schematic example of a circuit of the speed measuring device. 
         FIG. 7  illustrates an example of the radar module of the present invention. 
         FIG. 8  illustrates an example of the radar module of the present invention. 
         FIG. 9  illustrates an example of the radar module of the present invention. 
         FIG. 10  illustrates an example of a section of the speed measuring device of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     (Embodiment 1) 
     Hereinafter, a first embodiment of the present invention will be described with reference to  FIG. 1  to  FIG. 6 . A speed measuring device as an example of the present embodiment uses a radar module utilizing an electromagnetic wave (millimeter wave) of a 77 GHz band. 
       FIG. 1  is a diagram illustrating an example of a configuration of a millimeter wave radar module  1  as a main part of the speed measuring device. The millimeter wave radar module  1  is mainly formed of a substrate  20 , an MMIC chip  15 , and a lens  10 . In  FIG. 1 , the lens  10  before fixed on the substrate  20  is illustrated. 
     On the substrate  20 , a plane antenna  22 , a feeder  21  and a series of wirings  23  that is used for connection with an outside circuit are formed. Further, on the substrate  20 , an IC mount cavity  14  that mounts the MMIC chip  15  by bonding and lens mount cavities  25 A to  25 D that determine a mount position of the lens  10  are formed. A ceramic multilayer substrate or a printed substrate is used as an example of the substrate  20 , a patch antenna as an example of the plane antenna  22 , and a microstrip line as an example of the feeder  21 . A GND plane parallel to the plane antenna  22  spreads over an inner layer of the substrate  20 . 
     To supply power to the MMIC chip  15  from outside and to input/output a signal between the MMIC chip  15  and the outside, a wire bonding  19  connects the MMIC chip  15  to the series of wirings  23  so that the MMIC chip  15  connects to an outside circuit through a pad portion  23 A. A wire bonding  18 A connects a millimeter wave band signal terminal of the MMIC chip  15  to the feeder  21  extending from the plane antenna  22 . A wire bonding  18 B and a wire bonding  18 C connect the GND plane on the inner layer of the substrate  20  to a GND terminal of the MMIC chip  15 . The connections allow a millimeter wave band transmission signal generated by the MMIC chip  15  to be emitted as an electromagnetic wave into air and allow an electromagnetic wave from air to be received, through the plane antenna  22 . (An antenna or a lens can be configured to be same both for transmission and reception, due to its reversibility. In the following descriptions, the antenna is an antenna for transmission, unless noted otherwise.) 
     Since a directional angle of an electromagnetic wave emitted from the plane antenna  22  is wide, the lens  10  is used to sharpen the directivity by a focusing effect. 
       FIG. 2  is a diagram illustrating an example of a structure viewed from the bonded face of the lens  10 . Examples of a material of the lens  10  include a resin with a dielectric constant of about  2  to  10 . The lens  10  is formed of a curved part  10 A and a plane part  10 B. The plane part  10 B contacts directly with the plane antenna  22 . 
     The lens  10  further includes a flange  11 A and a flange  11 B that have a structure made of the same material as the lens  10  and elongated integrally. The flange  11 A and the flange  11 B have positioning bosses  12 A to  12 D. The positioning bosses  12 A to  12 D are disposed at the same positions as the aforementioned lens mount cavities  25 A to  25 D of the substrate  20  in  FIG. 1 , respectively. 
     With reference to  FIG. 1  again, the lens  10  is fixed to the substrate  20  by bonding between the positioning bosses  12 A to  12 D and the lens mount cavities  25 A to  25 D, respectively. When the lens  10  is fixed to the substrate  20 , the flanges  11 A and  11 B do not overlap with the feeder  21 . 
       FIG. 3  is a diagram of the millimeter wave radar module  1  viewed from the side face. The plane part  10 B of the lens  10  contacts directly with the plane antenna  22 . That is, the plane part  10 B is disposed in a near field of the plane antenna  22 . In contrast, the curved part  10 A is disposed at a position from a center of the plane antenna  22  by about a wavelength of λ or more, i.e., in a remote field of the plane antenna  22 . Since the curved part  10 A is disposed in the remote field, a space impedance can be considered to be uniform. Thus, even if a distance between the curved part  10 A and the plane antenna  22  varies somewhat, dispersion in focusing effect of electromagnetic waves is too small to influence operation of the radar module  1 . Furthermore, since the flange  11 A and the flange  11 B are disposed at respective positions apart from a center axis of the lens  10  by θ or more, an effect that an effect of the lens  10  is not impaired by the flanges  11 A and  11 B can be provided. Since an emission angle of the plane antenna is about ±45°, θ is preferably 60° or more. 
       FIG. 4  and  FIG. 5  are a diagram illustrating an example of the configuration of the speed measuring device  2  and a section thereof, respectively. The speed measuring device  2  mainly includes the millimeter wave radar module  1 , a peripheral circuit  31 , an aluminum base  32 , a housing  33  and a cover  37 .  FIG. 4  illustrates the cover  37  before fixed to the housing  33 . 
     The peripheral circuit  31  mainly has a function to convert a supply voltage from the outside of the speed measuring device  2  to a desired voltage to supply power to the inside of the peripheral circuit  31  and the millimeter wave radar module  1 , a function to control the millimeter wave radar module  1  to convert a signal output from the millimeter wave radar module  1  to measured speed information, and a function to output the measured speed information to the outside of the speed measuring device  2 . 
     The aluminum base  32  has a fixing hole to fix the speed measuring device  2  to a transport machine and a function to radiate heat of the speed measuring device  2 . The millimeter wave radar module  1 , the peripheral circuit  31  and the housing  33  are fixed to the aluminum base  32 . The housing  33  includes a connector part  33 A used for connection with the outside. The housing  33  is bonded and fixed by fitting in a groove  32 B of the aluminum base  32 . The wire bonding  34  is used for electrical connection between the peripheral circuit  31  and the connector part  33 A. 
     The cover  37 , which includes lens structures  37 A and  37 B, can sharpen further the directivity of an electromagnetic wave emitted from the lens  10  by a focusing effect. The cover  37  is bonded and fixed by fitting in a groove  33 B of the housing  33 . Thus, since the millimeter wave radar module  1  and the peripheral circuit  31  are bonded to each of the aluminum base  32 , the housing  33  and the cover  37 , the millimeter wave radar module  1  and the peripheral circuit  31  can be protected from rainwater or dust. 
       FIG. 6  is a diagram illustrating an example of a schematic configuration of the circuit of the speed measuring device. The MMIC chip in the millimeter wave radar module  1  is mainly formed of a oscillator  134 , a transmission amplifier  110 , an isolator  119 , a reception amplifier  113  and a mixer  112 . A millimeter wave band signal is transmitted and received at a port  120  connected with the isolator  119 . The feeder  21  connects the port  120  to the plane antenna  22 , and thus, signal transmission is performed. 
     Descriptions of operation of the millimeter wave radar module  1  are as follows. A high-frequency signal of 77 GHz band generated by the oscillator  134  is amplified by the transmission amplifier  110 , and subsequently, is propagated to the plane antenna  22  through the isolator  119 , and emitted to air as an electromagnetic wave by the plane antenna  22 . The emitted electromagnetic wave is focused by the lens  10  and the cover  37  having the lens function to be incident on the ground. The millimeter wave is reflected by the ground. The frequency of the reflected wave changes by the Doppler effect in proportion to a groundspeed. The electromagnetic wave reflected by the ground is incident on the plane antenna  22  via the cover  37  and the lens  10 . A signal received by the plane antenna  22  is propagated to the reception amplifier  113  by the isolator  119 . The signal is amplified by the reception amplifier  113 , and mixed with a high-frequency signal output from the oscillator  134  at the mixer  112  to generate an IF (intermediate frequency) signal. The IF signal is input into an operation circuit  201 . The frequency of the IF signal is an absolute value of the frequency change by the Doppler effect. Main operations of the operation circuit  201  are to convert the IF signal into a digital signal with an AD converter, to obtain the frequency of the IF signal by FFT (fast Fourier transform) processing of the digital signal, and to convert the frequency to a speed v. If an angle between an incident direction of the millimeter wave to the ground and a direction opposite to a speed vector is θ, the speed v is expressed by the following equation:
 
 v= ( c/ 2 f   0 |cos θ|)× f   d ,  (Equation 1)
 
wherein c is a light velocity, f 0  is a frequency of a signal output by the oscillator, and f d  is a frequency change quantity by the Doppler effect.
 
     The aforementioned configuration can provide the following effects. 
     (1) Since the position relationship of the plane antenna  22  and the lens  10  is accurately adjusted, dispersion of the emission direction of an electromagnetic wave can be made small among the modules. (2) Since the gap between the plane antenna  22  and the lens  10  is made zero, dispersion of the intensity of an electromagnetic wave emitted via the lens can be made small among the modules, and further, an electromagnetic wave emitted from the plane antenna can be incident on the lens  10  effectively. (3) Since the distance between the plane antenna  22  and the lens  10  is a minimum, the size of the lens  10  can be made small. Thus, the length of the radar module  1  in an emission axis direction of the electromagnetic wave can be made small, and further, even if an expensive material having an excellent property is used, the impact on the cost is small. 
     (4) The two separate flanges do not overlap directly with the feeder, which causes no influence on power supply to the plane antenna  22 . (5) A resin molding technique enables simultaneous forming of the lens  10 , the flanges  11 A and  11 B, the positioning bosses  12 A to  12 D. This takes a low cost for the processing. (6) If the resin is molded by injection molding, a gate can be placed at the flange  11 A or the flange  11 B, and distortion during the molding, which may influence on the focusing property of the lens  10 , can be suppressed. 
     (7) Since the curved part  10 A of the lens  10  is disposed in the remote field of the plane antenna  22 , the focusing effect of the curved part  10 A can be easily obtained. (8) The flanges  11 A and  11 B do not prevent an effect of the lens. 
     (Embodiment 2) 
     Next, a second embodiment of the present invention will be described with reference with  FIG. 7 . 
     In the configuration of a millimeter wave radar module  5  in  FIG. 7 , feeders to a plane antenna  52  in a substrate  50  are feeders  51 A and  51 B having different widths. A connection interface between the feeder  51 A and the feeder  51 B is disposed at a same position as the border of the lens  10 . 
     Although change of an impedance of the feeders is caused by change of a dielectric constant on upper faces of the feeders due to addition of the lens  10 , the present configuration allows such change of the impedance to be small. 
     (Embodiment 3) 
     Next, a third embodiment of the present invention will be described with reference to  FIG. 8 . 
     In the configuration of a millimeter wave radar module  6  in  FIG. 8 , a stepped structure  65  is provided in a substrate  60 . In the stepped structure  65 , a cavity structure  65 A surrounds a plane antenna  62 . The size of the cavity structure  65 A is same as the periphery of a lens  67 . A groove  65 B is further provided to prevent the stepped structure  65  from overlapping with a feeder  61 . The lens  67  is fitted in the cavity structure  65 A and bonded. The bonded part preferably corresponds to the periphery of the lens  67 . 
     The present configuration makes positioning of the lens  67  easy. 
     (Embodiment 4) 
     Next, a fourth embodiment of the present invention will be described with reference to  FIG. 9  and  FIG. 10 . 
     In the configuration of a millimeter wave radar module  7  in  FIG. 9 , on a substrate  70 , two plane antennas  73 A and  73 B and feeders  74 A and  74 B connected to the plane antennas  73 A and  73 B, respectively, are formed. The feeders  74 A and  74 B are connected to a millimeter wave band signal terminal of an MMIC chip  16  bonded to the substrate  70 . A lens array  71  having two lens structures  71 A and  71 B is bonded to the substrate  70 . 
       FIG. 10  is a section of a speed measuring device  80 . Electromagnetic waves emitted from the plane antennas  73 A and  73 B of the millimeter wave radar module  7  are incident on the lens structures  71 A and  71 B, respectively, and focused. The electromagnetic waves emitted from the lens structures  71 A and  71 B are emitted toward emission directions A and B, by the lens structure parts  37 A and  37 B of the cover  37 , respectively. Although two sets of an antenna and a lens are described herein, the number of the sets may be three or more. 
     The aforementioned configuration allows a single substrate to emit millimeter waves in multiple directions accurately. 
     In the above descriptions, a speed measuring device is an example. The present invention can be applied also to a general radar that modulates an electromagnetic wave to measure a distance from an object and a relative speed simultaneously. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  5 ,  6  Millimeter wave radar module 
           2 ,  80  Speed measuring device 
           10 ,  67  Lens 
           11 A,  11 B 
           12 A,  12 B,  12 C,  12 D 
           15 ,  16  MMIC chip 
           20 ,  50 ,  60 ,  70  Substrate 
           21 ,  51 A,  51 B,  74 A,  74 B Feeder 
           22 ,  52 ,  62  Plane antenna 
           25 A,  25 B,  25 C,  25 D Lens mount cavity 
           31  Peripheral circuit 
           32  Aluminum base 
           33  Housing 
           37  Cover 
           65  Stepped structure 
           71  Lens array