Abstract:
The present invention relates to a monitoring device ( 1 ) with a transmitter unit ( 2 ) and a receiver unit ( 3 ) for monitoring an area. Slotted cables serve as antennas. According to the invention, a first slotted cable is arranged as a transmitting an antenna ( 4 ) for transmitting a pulsed high-frequency signal and a second slotted cable is arranged as a receiving antenna ( 9 ) for receiving the reflected signal. The slotted cables are arranged in association with each other and essentially parallel to each other, and the transmitter unit ( 2 ) is connected to the first slotted cable at one end of the antenna arrangement and the receiver unit ( 3 ) is connected to the second slotted cable at the other end of the antenna arrangement.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a device for monitoring an area, which device comprises a transmitter unit for transmitting a signal and a receiver unit for receiving a reflected signal via an antenna.  
         [0002]     The installation of a monitoring device in the form of an active sensor for monitoring an area in front of a vehicle is already known. The sensor has a poor range and the radiation from the sensor in the area that is to be monitored can be regarded as originating from a point source. When the sensor is located in the centre of the front of the vehicle, acceptable values can be obtained for the relative speed between the vehicle and a reflecting object that is centrally in front of the vehicle in the direction of movement of the vehicle. However, if the reflecting object is displaced sideways relative to the direction of movement of the vehicle, an unacceptable error in the speed can easily arise. There is also the risk that the object will leave the sensor&#39;s field of vision when there is a short distance between the reflecting object and the vehicle. In order to increase the accuracy when determining speed, it would be possible to install several individual sensors along the front of the vehicle. The use of several sensors with associated arrangement for signal processing and installation means that the monitoring device runs the risk of being both complex and expensive.  
         [0003]     A monitoring device in the form of a security alarm is already known from U.S. Pat. No. 3,947,834. The alarm is provided with a combined transmitting and receiving antenna in the form of a cable with groups of slots. The antenna is supplied at one end with a radar frequency which, when it is reflected from moving objects, gives rise to a Doppler frequency signal which under certain conditions causes the alarm to be activated. The security alarm detects movement but carries out no direct determination of speed or distance. Due to the not insignificant suppression that is introduced along the length of the antenna, the antenna has different sensitivity along its length, with greater sensitivity close to the supply end. In addition, the length of the signal wave varies in the antenna. Reflections close to the supply end of the antenna have a shorter signal wave in the antenna than reflections close to the end of the antenna that is furthest away from the supply end. Among other things, the circumstances described above make it difficult, if not impossible, to determine the relative speed and distance of an object with any accuracy, if so required.  
         [0004]     A device according to the first section of the description is previously known from U.S. Pat. No. 4,612,536. In this case a security alarm is again in question. The intrusion detector of the security alarm here comprises two parallel antenna cables preferably located in the ground in connection to an object to be protected. The antenna cables are in opposite ends connected to a transmitter unit and a receiver unit, respectively and the asymmetry occurring in the transmitter antenna lobe due to attenuation along the cable will be balanced out of a corresponding reversed asymmetry in the antenna lobe of the receiver antenna. The intention is to detect objects if any passing above the antenna cables and if desired to determine the location of the passage. However, there is no determining of the speed of an object approaching the object to be protected.  
       SUMMARY OF THE INVENTION  
       [0005]     The object of the present invention is to achieve a monitoring device that does not have the abovementioned limitations, is simple in design and can be produced and installed at a low cost. The object of the invention is achieved by means of a monitoring device characterized in that the relative speed between the monitoring device and a reflecting object is determined by utilizing the Doppler principle, a mixer being arranged with one input connected to the transmitting antenna and the other input connected to the receiving antenna, which mixer generates at its output the difference frequency between the transmitted signal and received signal. The monitoring device according to the invention has, in addition, the advantage of being able to function independent of the cable length of the antennas.  
         [0006]     According to an advantageous embodiment of the monitoring device according to the invention, a delay is introduced between the transmitting antenna and the receiver unit. By this means, the area that is to be monitored can be moved out from the antennas and the monitoring area can be adapted to suit the application concerned. In a simple embodiment, the delay is carried out by a coaxial cable. A delaying device designed to be able to vary the delay can advantageously also be introduced. A monitoring device with the ability to vary the delay has the advantage that the monitoring area can be moved. It is thereby possible to monitor a larger area.  
         [0007]     Another embodiment of the invention is a device for monitoring an area, comprising: a combined transmitting and receiving antenna configured to transmit a transmitted signal and to receive a reflected received signal; a transmitter unit configured to generate the transmitted signal; a receiver unit configured to receive the reflected received signal; and wherein the transmitter unit is connected to one end of the combined transmitting and receiving antenna, the receiver unit is connected to an other end of the combined transmitting and receiving antenna, and a relative speed between the monitoring device and a reflecting object is determined by utilizing the Doppler principle.  
         [0008]     Yet another embodiment of the present invention is a device for monitoring an area comprising: a first slotted cable configured as a transmitting antenna for transmission of a high-frequency transmitted signal; a transmitter unit configured to transmit the transmitted signal via the first slotted cable and connected to a first end of the first slotted cable; a second slotted cable configured as a receiving antenna for receiving a reflected received signal; a receiver unit configured to receive the received signal via the second slotted antenna; and connected to a first end of the second slotted cable; and a mixer configured to generate a mixer output signal that is a difference frequency between the transmitted signal at first mixer input and the received signal at a second mixer input, wherein the first and second slotted cables are configured in association with each other and essentially parallel to each other, a first mixer input is connected to a second end of the first slotted cable, a second mixer input is connected to a second end of the second slotted cable furthest away from the first end of the first slotted cable, and a relative speed between the monitoring device and a reflecting object is determined by utilizing the Doppler principle.  
         [0009]     According to an advantageous further development of the monitoring device, a processor is arranged to identify the highest Doppler frequency and base the determining of the relative speed upon that. Suitably the processor is arranged to apply a Fourier transform to the generated difference signal and to identify the highest Doppler frequency from the generated Fourier-transformed signal.  
         [0010]     According to another advantageous further development of the monitoring device, the transmitter unit and the receiver unit each comprise an interacting switch, whereby reflections from objects outside the monitored area can be excluded by means of the interaction of the switches. Radiation reflected from an object immediately outside the monitoring area is stopped by the receiver unit&#39;s switch which assumes an open, non-transmitting position when the radiation reaches the receiving antenna. In this way, reflections from large reflecting objects outside the monitored area are stopped in a very efficient way.  
         [0011]     The slotted cable of the transmitting antenna and the receiving antenna consists advantageously of a slotted coaxial cable. A suitable number of slots can be four to six per meter of cable. For example, in a suitable embodiment for mounting on a vehicle, the first and the second slotted cables each comprise 3-20 slots, distributed along the length of the cable.  
         [0012]     In order to determine the distance between the monitoring device and a reflected target, a means is advantageously arranged for measuring the delay time interval of a reflected pulse. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0013]     In the following, the invention will be described in greater detail in exemplified form with reference to the attached figures, in which:  
         [0014]      FIG. 1  shows schematically an example of a monitoring device according to the invention.  
         [0015]      FIG. 2   a  shows schematically the antenna beam of a transmitting antenna incorporated in the monitoring device.  
         [0016]      FIG. 2   b  shows schematically the antenna beam of a receiving antenna incorporated in the monitoring device.  
         [0017]      FIG. 2   c  shows the resulting antenna beam comprising the antenna beam of the transmitting antenna according to  FIG. 2   a  and the antenna beam of the receiving antenna according to  FIG. 2   b.    
         [0018]      FIG. 3  illustrates a number of examples of reflections between the monitoring device and a reflecting object.  
         [0019]      FIG. 4  shows schematically an example of a slotted coaxial cable.  
         [0020]      FIG. 5  shows schematically the monitoring device according to the invention mounted on a vehicle.  
         [0021]      FIG. 6  shows schematically the monitoring device installed in association with a section of road.  
         [0022]      FIG. 7  shows schematically another example of a monitoring device according to the invention.  
         [0023]      FIG. 8  shows schematically yet another example of a monitoring device according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0024]     The monitoring device  1  shown schematically in  FIG. 1  comprises a transmitter unit  2  and a receiver unit  3 . The transmitter unit is connected to a mixer  5  in the receiver unit  3  via a transmitting antenna  4 . A delaying device  16  in the form of a coaxial cable is connected between the transmitting antenna  4  and the receiver unit  3 . The transmitter unit comprises an oscillator  6  and a switch  7 . A receiving antenna  9  is connected to the mixer  5 , the output of which is connected to a switch  8 . The switch is in turn connected to a threshold detector  26  and a processor  25 . The mixer  5 , the switch  8 , the processor  25  and the threshold detector  26  are regarded as being part of the receiver unit  3 . The transmitting antenna and the receiving antenna consist of slotted cables and preferably slotted coaxial cables.  
         [0025]     An example of a slotted coaxial cable is shown in  FIG. 4 . The cable  10  comprises a central conductor  11  surrounded by an insulating layer  12 . On the outside surface of the insulating layer  12  there is a conductive screen  13  covered by an outer insulating layer  14 . Slots  15  are arranged along the longitudinal direction of the coaxial cable, preferably at regular intervals. The slots are achieved by creating openings in the conductive screen by removing parts of the screen or by pushing the screening material aside. The shape of the openings can vary, depending among other things upon the frequency range concerned and the required beam shape.  
         [0026]     In the embodiment shown in  FIG. 1 , the transmitting antenna  4  and the receiving antenna  9  have each been provided with five slots,  4 . 1 ,  4 . 2 ,  4 . 3 ,  4 . 4 ,  4 . 5  and  9 . 1 ,  9 . 2 ,  9 . 3 ,  9 . 4 ,  9 . 5  respectively. A suitable number can, for example, be from three up to 20 slots and preferably four to six slots per meter, depending among other things upon the frequency concerned. The slots behave essentially as dipole antennas.  
         [0027]     During operation, the oscillator  6  emits a high-frequency signal that the switch  7  converts into a pulsed high-frequency signal, which is fed into the transmitting antenna  4 . In order to reduce the bandwidth, the pulses are given a relatively large pulse length.  FIG. 2   a  shows schematically a continuous area  17  for the transmitting antenna  4  where the signal strength exceeds a particular value. Due to suppression along the transmitting antenna, the area  17  will be asymmetrical, with a larger range at the input side of the transmitting antenna. The area constitutes the transmitting antenna&#39;s combined asymmetrical antenna beams. In a corresponding way, an asymmetrical area  18  is also obtained for the receiving antenna  9 , see  FIG. 2   b . In this asymmetry, the area has a larger range on the receiver side. In ideal conditions with identical slotted cables as antennas, the asymmetrical area  18  is the inverse of the asymmetrical area  17 .  FIG. 2   c  shows how the asymmetrical areas of the transmitting antenna  4  and the receiving antenna  9  interact and create an essentially symmetrical area  19 .  
         [0028]     In the mixer  5 , a version of the signal that is transmitted by the transmitting antenna  4  delayed by the delaying device  16  is combined with a reflected signal received by the receiving antenna. At the output of the mixer difference frequencies are obtained, caused by the Doppler effect that arises with reflections from an object when the object is moving in relation to the monitoring device. The further processing of the difference signal is described in greater detail below in connection with the description of  FIG. 3 . The switch  8  is controlled based on the condition of the switch  7 , so that the monitoring area is limited in distance. Strongly reflecting objects at a great distance can thus be excluded and the subsequent signal processing can be made easier.  
         [0029]      FIG. 3  illustrates a number of paths of propagation between the transmitting antenna  4  and the receiving antenna  5  via a reflecting object  20 . The transmitting antenna and the receiving antenna have been shown with five slots each,  4 . 1 - 4 . 5  and  9 . 1 - 9 . 5 , respectively. An example of the longest occurring path of propagation is indicated by the path of propagation along lines  21  and  22 . The slot  4 . 5  of the transmitting antenna  4  transmits a signal that is reflected by the reflector  20  before it reaches the slot  9 . 1  of the receiving antenna. The lines  23 ,  24  indicate the shortest path of propagation. In this case the signal is transmitted from the slot  4 . 3  and is received in the slot  9 . 3  after being reflected by the reflector  20 . This latter path of propagation indicated by the lines  23  and  24  corresponds in principle to twice the actual distance from the monitoring device  1  to the reflecting object  20 . By studying the geometry in  FIG. 3 , a number of additional paths of propagation can be identified. These additional paths of propagation have a delay time interval between the two paths of propagation discussed above.  
         [0030]     In order to identify the actual distance d, a Fourier transform can suitably be applied to the difference signal emitted by the mixer  5  according to  FIG. 1 , for example an FFT transform. The transform can be applied by the processor  25 , which can also be used to identify the highest frequency from the Fourier-transformed signal, which highest frequency corresponds to the path of propagation according to the lines  23  and  24 . The threshold detector  26  is used in order to prevent minor interference. In order to determine the distance to the object, there is a means for measuring the delay time interval of a reflected pulse. The processor  25  can be used for this measurement.  
         [0031]      FIG. 5  shows the front of a vehicle which has been provided with a monitoring device according to the invention. The slotted coaxial cables of the transmitting antenna  4  and the receiving antenna  9  have been mounted in or on the vehicle&#39;s bumper  28  in the longitudinal direction of the bumper. The internal space of the vehicle, for example the engine compartment or the passenger compartment, can be used to house other parts comprised in the monitoring system.  
         [0032]      FIG. 6  shows another application. The transmitting antenna  4  and receiving antenna  9  of the monitoring device have been installed along a road. By road is not only meant here a vehicular road, but also for example a railroad. According to the embodiment shown in  FIG. 6 , a transmitting antenna  4  and a receiving antenna  9  are installed along a section of road  29  close to the edge of the road. In the embodiment shown, the transmitting antenna and the receiving antenna are shaped to follow the shape of the road, which is here somewhat curved. In principle, the antennas  4 ,  9  can be laid directly on the ground. It is, however, also possible to fix the antennas in some other way, for example, to some form of road barrier.  
         [0033]     Another embodiment of the monitoring device  1 ′ is shown schematically in  FIG. 7  which comprises a transmitter unit  2  and a receiver unit  3 . The transmitter unit  2  is connected to a mixer  5  in the receiver unit  3  via a combined transmitting and receiving antenna  31 . The transmitter unit comprises a controllable oscillator  6 ′ and a modulator  30 . The mixer  5  has a first mixer input signal  32  and a second input mixer signal  34 . The output of the mixer  5  is connected to a threshold detector  26  and the output of the threshold detector  26  is connected to a processor  25 . The mixer  5 , the threshold detector  26  and the processor  25  are regarded as being part of the receiver unit  3 .  
         [0034]     Yet another embodiment of the monitoring device  1 ″ is shown schematically in  FIG. 8  which comprises a transmitter unit  2  and a receiver unit  3 . The transmitter unit  2  is connected to a mixer  5  in the receiver unit  3  via a combined transmitting and receiving antenna  31 . The transmitter unit comprises a controllable oscillator  6 ′ and a modulator  30 . The mixer  5  has a first mixer input signal  32  and a second input mixer signal  34 . The output of the mixer  5  is connected to a threshold detector  26  and the output of the threshold detector  26  is connected to a processor  25 . The mixer  5 , the threshold detector  26  and the processor  25  are regarded as being part of the receiver unit  3 .  
         [0035]     In the embodiments shown in  FIG. 7  and  FIG. 8 , the combined transmitting and receiving antenna  31  consist of a slotted cable and preferably a slotted coaxial cable. An example of a slotted coaxial cable was shown in  FIG. 4 . The cable  10  comprises a central conductor  11  surrounded by an insulating layer  12 . On the outside surface of the insulating layer  12  there is a conductive screen  13  covered by an outer insulating layer  14 . Slots  15  are arranged along the longitudinal direction of the coaxial cable, preferably at regular intervals. The slots are achieved by creating openings in the conductive screen by removing parts of the screen or by pushing the screening material aside. The shape of the openings can vary, depending among other things upon the frequency range concerned and the desired beam shape. Each of the slots distributed along the cable  10  are each configured to look in the same direction.  
         [0036]     In the embodiments shown in  FIG. 7  and  FIG. 8 , the combined transmitting and receiving antenna  31  has been provided with five slots:  31 . 1 ,  31 . 2 ,  31 . 3 ,  31 . 4 , and  31 . 5 . A suitable number of slots can include, but are not limited to, from three up to 20 slots and preferably four to six slots per meter, depending among other things upon the frequency concerned. The slots behave essentially as dipole antennas.  
         [0037]     During operation of the devices  1 ′,  1 ″ of  FIG. 7  and  FIG. 8 , respectively, the controlled oscillator  6 ′ emits a high-frequency signal controlled by the modulator  7 ′. The signals output from the modulator  7  include, but are not limited to, waveforms for control signal inputs of the controlled oscillator  6 ′ that produce pulse modulated, frequency modulated and phase modulated signals at the output of the controlled oscillator  6 ′. The voltage range of the signals output from the modulator  7  will be of a sufficient magnitude to drive the inputs of a typical controlled oscillator  6 ′ device, as is well known from the background art. The output of the controlled oscillator  6 ′ is fed into the combined transmitting and receiving antenna  31 .  
         [0038]     As discussed above,  FIG. 2   a  shows a schematic of a continuous area  17  of coverage for a transmitting antenna, where the signal strength exceeds a particular value. This continuous area  17  corresponds to that provided by the transmitting antenna portion of the combined transmitting and receiving antenna  31 . Due to suppression along the transmitting antenna, the continuous area  17  will be asymmetrical, with a larger range at the input side of the combined transmitting and receiving antenna  31 . The area constitutes the transmitting antenna&#39;s combined asymmetrical antenna beams.  
         [0039]     In a corresponding way,  FIG. 2   b  shows a continuous asymmetrical area  18  of coverage is also obtained for a receiving antenna. This continuous asymmetrical area  18  corresponds to that of the receiving antenna portion of the combined transmitting and receiving antenna  31 . In this asymmetry, the area has a larger range on the receiver side. In ideal conditions with slotted cables as antennas, the asymmetrical area  18  is the inverse of the asymmetrical area  17 , as shown in  FIG. 2   a . Further, as discussed above,  FIG. 2   c  shows how the asymmetrical areas  17 ,  18  of the combined transmitting and receiving antenna  31  interact and create an essentially symmetrical area  19 .  
         [0040]     As shown in  FIG. 7  and  FIG. 8 , a version of the transmitted signal that is transmitted by the transmitting portion of the combined transmitting and receiving antenna  31  is combined with a reflected received signal that is received by the receiving portion off the combined transmitting and receiving antenna  31  in the mixer  5 . The difference in the embodiments of  FIG. 7  and  FIG. 8  is in the source that provides the second mixer input signal  34  to the mixer  5 . In particular,  FIG. 7  shows the second mixer input signal  34  is obtained from a separate line from the transmitter  2 . Alternatively,  FIG. 8  shows the second mixer input signal  34  is obtained from the first mixer signal input  32 .  
         [0041]     At the output of the mixer  36  difference frequencies, caused by the Doppler effect that arises with reflections from an object when the object is moving in relation to the monitoring device, are obtained. The further processing of the difference signal that is at the output from the mixer  36  was described in great detail in connection with the description of  FIG. 3 , as discussed above.  
         [0042]     The invention is not restricted to the embodiments described above, but can be modified within the scope of the following patent claims and invention concept. For example, there is a plurality of possible applications in addition to the ones described above.