Mobile object discriminating system

Abstract: An antenna system (1), comprising an antenna (10,11) for transmitting a radio wave of a frequency, and a radio wave absorber (2) intercepting and absorbing part of the radio wave to reduce the gain of the antenna. Thereby, the antenna system narrows down the communication area of the radio wave independently of the frequency. The antenna system is applicable to a movable object discriminator using a radio wave.

Description: The present invention relates to a movable object discriminating system having an interrogator transmitting and receiving radio wave to and from a responder according to the preamble of claim 1. Generally, when a person desires to communicate with a particular movable object in radio communication, he must employ an antenna of a directivity. Antennas of directivity comprise a Yagi-Uda antenna, an array antenna, a horn antenna, a parabolic antenna and the like. When the frequency of a radio wave belongs to a milliwave or EHF (Extremely High Frequency) band of 30 GHz or more (e.g. 30-300 GHz), these antennas can have a high directivity although they are small. However, when they are conventionally applied to a radio communication using a radio wave of a frequency of less than 30 GHz, e.g., microwave or UHF (i.e. 300 MHz to 3 GHz) band, they must be large. This causes the following problems on movable object discriminators transmitting an interrogatory radio wave of a frequency of 2.45 GHz and receiving a responding radio wave of the frequency of 2.45 GHz. Mobile object discriminating systems of that type are known in the art; these known systems, generally, are low power consumptive and have a very short communication distance (approximately 2 m at maximum). The representative system utilizing this technology is a wicket gate checker for checking tickets in a rail road gate, an amusement park gate, a sky lift gate or the like. Each person carrying a responder passes through the wicket gate, and an interrogator installed on the gate recognizes date returning from the responder. If necessary, data can be newly written into the responder. The most important things in this system are: i) to recognize only a responder of a person who just passes in front of the gate, without responding to other responders, and ii) to accurately capture the responder of the targeted person. More specifically, if the communication area of an antenna is too broad, the system will erroneously respond to other responders of non-targeted persons. On the contrary, if the communication area is too narrow, the system will force each passenger to stop at the gate to show his/her responder closely to the interrogator, resulting in deterioration of efficiency in the wicket gate check system. A typical example of the communication area for such a wicket gate system is as follows: Depth of communication area approximately 100 cm (gate width); width of communication area not less than 30 cm; and person-to-person interval not larger than 60 cm. the conventionally available systems cannot adjust or narrow their communication areas so as to fit to the above required size. In order to narrow down or squeeze the communication area, a directional antenna such as an array antenna is generally employed. However, the array antenna, for example, must comprise a great number of antenna elements arrayed in a predetermined pattern in order to sharply narrow down or squeeze the communication area toward a particular direction. This inappropriately increases the size of the array antenna so largely that the communication area cannot be realized in such a short distance (approximately 1 m). In the document "Institute of Electrical and Electronics Engineers AP-S Int. Symposium 1977, Stanford, California pages 324-327", there is disclosed a horn antenna using absorber tunnels. By this technology it becomes possible to reduce the level of sidelobe and, as a result, the width of mainlobe can be suppressed in both E- and H plane patterns at substantially the same level. However, using this technology for narrowing down the communication area to obtain the above required size will encounter with the problem of undesirable increase of the physical features of the antenna. Furthermore, the vicinity of the antenna is subjected to multiple reflections and the like which disturb the electromagnetic field. Thus, it is difficult to form a communication area which is smaller than the physical size of an antenna. Consequently, it is not preferable to use this technology for narrowing down the communication area for the short distance communication such as a wicket gate system. In FR-A-2,390,027, FR-A-2,512,280 and US-A-3,377,593, there are disclosed antenna systems having similar absorber means. In the unexamined Japanese patent 61-2432/1986 there is disclosed a technology of adjusting an antenna. There is provided a throttle between the primary radiator and the parabolic reflector. By this technology it is possible to reduce the transmission power. However, reflections are returned to the transmitter by the mismatching of antenna due to presence of the throttle. If this technology is used for a wicket gate system, a part of the transmission power is reflected by the throttle and returned to the transmitter; this causes a significant amount of receiving noises. Moreover, the communication area itself is not narrowed down, but is rather widened. The US-A-3,501,766, finally, discloses a technology of adjusting the direction of an antenna. However, this technology is not effective to change the configuration or size of the communication area. It is therefore the object of the present invention to improve a mobile object discriminating system according to the preamble of claim 1 in such a way that the communication area can be easily narrowed down. This object, according to the present invention, is solved by the advantageous measures indicated in claim 1. Advantageous further developments of the invention are indicated in the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1A is a perspective view of an array antenna system according to a first embodiment of the present invention; FIG.1B is a plan view of the array antenna system of FIG.1A; FIG.2A is a perspective view of a prior-art two-element array antenna; FIG.2B is a perspective view of a first configuration of an array antenna system employing the array antenna of FIG.2A; FIG.2C is a perspective view of a second configuration of the array antenna system employing the array antenna of FIG.2A; FIG.3A is a diagrammatic plan view of the communication area of the array antenna of FIG.2A; FIG.3B is a diagrammatic plan view of the communication area of the array antenna system of FIG.2B; FIG.3C is a diagrammatic plan view of the communication area of the array antenna system of FIG.2C; FIG.4 is a plan view of an antenna system according to a second embodiment of the present invention; FIG.5 is a plan view of an antenna system according to a third embodiment of the present invention; FIG.6A is a perspective view of a dipole antenna system according to a fourth embodiment of the present invention; FIG.6B is a plan view of the dipole antenna system of FIG.6A; FIG.7 is a schematic diagram of an article delivery system employing the array antenna system of FIG.2C; FIG.8 is a diagrammatic plan view of the communication area of the dipole antenna system of FIG.6A; FIG.9A is a perspective view of an array antenna system according to a fifth embodiment of the present invention; FIG.9B is a plan view of the array antenna system of FIG.9A; FIG.10A is a plan view of an array antenna system according to a sixth embodiment of the present invention; FIG.10B is a perspective view of the array antenna system of FIG.10A; and FIG.11 is a plan view of an array antenna system according to a seventh embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described with reference to the drawings hereinafter. As shown in FIGS.1A and 1B, an array antenna 1 in the form of rectangular board of an array antenna system 10 comprises a plurality of antenna elements 11 in the form of square board arrayed vertically in line. Each antenna element 11 receives electric power from an electric power feeder 12 and transmits radio waves of UHF and SHF (Super High Frequency) bands (i.e. 3-30 GHz). Right-hand and left-hand side edges of the front surface of the array antenna 1 have radio wave absorbers 2 in the form of rectangular board connected to the array antenna 1 by means of hinges 3 and extending forward from the array antenna 1. As shown in FIG.1B, the hinges 3 enable each of the radio wave absorbers 2 to be pivoted toward right and left. The radio wave absorbers 2 and the hinges 3 constitute a means for changing the radio wave transmission area S of the array antenna system 10. Each of the radio wave absorbers 2 absorbs a radio wave transmitted from the array antenna 1 toward an undesired direction. When the array antenna system 10 is applied to a movable object discriminator having a frequency of 2.45 GHz, the radio wave absorbers 2 are made of a composite of a ferrite plus an epoxide, or a ferrite plus a rubber, or conductive fibers plus urethane foam. Operation of the array antenna system 10 will be described hereinafter. When all of the antenna elements 11 have received electric power from the electric power feeder 12, the array antenna 1 transmits a radio wave. If the array antenna 1 would have no radio wave absorber, the front surface of the array antenna 1 transmits a radio wave in directions in a radiation pattern as shown in FIG.3A. However, the radio wave absorbers 2 of the present embodiment intercept and absorb part of the radio wave transmitted from the array antenna 1 since the array antenna 1 actually has the radio wave absorbers 2. The arrangement of the array antenna 1 and the radio wave absorbers 2 causes the radio wave absorbers 2 to mainly intercept and absorb part of the transmitted radio wave propagating substantially transversely to the array antenna 1 and cut the transverse propagation of the radio wave, thus producing a communication area in the form of a lobe. As shown in FIG.1B, the degree of opening or setting angle α of each radio wave absorber 2 to the array antenna 1 is variable, the array antenna system 10 can optionally change which part of the radio wave transmitted by the array antenna 1 is absorbed by the radio wave absorbers 2, so that a desired radiation pattern for radio communication can be produced and the communication area can desirably be narrowed down. The radiation pattern of the array antenna system 10 which is embodied into an array antenna system 10a of an interrogator of the movable object discriminator will be described hereinafter. The array antenna system 10a transmits a radio wave of the frequency of 2.45 GHz of UHF band allotted to the movable object discriminator. FIG.2A is a prior-art two-element array antenna 21 used for determination of communication area. The array antenna 21 in the form of rectangular board includes two antenna elements 11 arrayed vertically in line and has no radio wave absorber. FIGS.2B and 2C show first and second configurations of the array antenna system 10a with two-element array antenna 21. The first configuration of the array antenna system 10a of FIG.2B has a setting angle α 1 (=90°) of the radio wave absorbers 2 to the array antenna 21. Therefore, an opening defined by the front edges of the pair of radio wave absorbers 2 is equal to the front surface of the array antenna 21. The second configuration of the array antenna system 10a of the FIG.2C has a setting angle α 2 (i.e. an acute angle) of the radio wave absorbers 2 to the array antenna 21. Therefore, an opening defined by the front edges of the pair of radio wave absorbers 2 is narrower than the front surface of the array antenna 21. FIG.3A is a diagrammatic plan view of a communication area or radiation pattern of the array antenna system of FIG.2A. FIG.3B is a diagrammatic plan view of a communication area or radiation pattern of the first configuration of the array antenna system 10a of FIG.2B. FIG.3C is a diagrammatic plan view of a communication area or radiation pattern of the second configuration of the antenna system 10a of FIG.2C. The prior-art array antenna system, as shown in FIG.3A, has a hatched communication area 4a. The first configuration of the array antenna system 10a of FIG.2B, as shown in FIG.3B, has a hatched communication area 4b. The second configuration of the array antenna system 10a of FIG.2C, as shown in FIG.2C, has a hatched communication area 4C. Each of the radio wave absorbers 2 of FIGS.2B and 2C is made of a material absorbing 99 % and reflecting 1 % of a radio wave transmitted thereto. As shown in FIG.3A, the communication area 4a has a width of 180 cm at the distance of 1 m(i.e. substantially a half of the maximum distance of the communication area) forward from the front surface of the array antenna 21 of the prior-art antenna system. As shown in FIG.3B, the communication area 4b has a width of 90 cm at the distance of 1 m forward from the front surface of the array antenna 21 of the first configuration of the antenna system 10a. As shown in FIG.3C, the communication area 4c has a width of 60 cm at the distance of 1 m forward from the front surface of the array antenna 21 of the second configuration of antenna system 10a. Thus, the width of the communication area 4b of the first configuration of the array antenna system 10a is 1/2 of that of the prior-art array antenna system at the distance of 1 m forward from the array antenna 21. The width of the communication area 4c of the second configuration of the antenna system 10b is 1/3 of that of the prior-art array antenna system at the equal distance. It is important to the movable object discriminator to narrow down the communication area of the radio wave. A case where an array antenna system having a narrowed communication area is applied to a movable object discriminator of an article delivery system will be described with reference to FIG.7 hereinafter. In the article delivery system, all of articles 14 entrusted to be delivered have responders 15 attached thereto, articles 14 which have been collected are loaded on a plurality of belt conveyors 13, an interrogator 16 for each belt conveyor 13 has the second configuration of the array antenna system 10a of FIG.2C and reads delivery data from each responder 15, and a classifier (not shown) classifies the articles 14 by destinations. If the interrogators 16 have the prior-art array antenna systems of FIG.2A instead of the array antenna systems 10a, the interrogators 16 experience a radio interference with a plurality of responders 15 since many articles 14 are densely loaded on the belt conveyors 13. Thus, the interrogators 16 possibly establish a radio communication with a responder 15 not targeted (including a responder 15 attached to an article 14 loaded on an opposite belt conveyor 13), so that the article delivery system misfunctions. For example, the interrogators 16 misreads data from the responder 15 so that the classifier mistakes a destination of an article 14. The array antenna system 10a of the present embodiment can appropriately narrow down the communication area although it employs a radio wave of UHF band. Thus, each of the interrogators 16 communicate with the responders 15 one to one at a time, so that the article delivery system of FIG.7 can preclude the above-described misfunction. In addition, since the radio wave absorbers 2 of the second configuration of the antenna system 10a produce the narrowed communication area 4c, the array antenna 21 need not narrow down the communication area only by means of a configuration thereof including arraying conventionally a great number of antenna elements. Thus, the present embodiment of the invention can reduce the size of the antenna system including the array antenna 21 and narrow down the width of the communication area and the maximum range or distance of the communication area independently of frequency of radio wave. As shown in FIGS.3B and 3C, a simple change in the setting angle of the radio wave absorbers 2 to the two-element array antenna 21 changes a radio wave transmitting area S to easily change the width and the maximum distance of the communication area. Thus, the antenna system of the present embodiment can change the communication area by uses and by environments of use and allows a fine adjustment in a scene of use of the antenna system. This overcomes the problem in the conventional array antenna system that the number of arrayed antenna elements determines a communication area so that the conventional antenna system must be changed by uses and by environments of use. In addition, narrowing down the width of the communication area reduces the communication distance forward from the front surface of the two-element array antenna 21 of the array antenna system 10a. This indicates that narrowing down the width of the communication area reduces the gain of the antenna system 10a. Therefore, when the gain G of a configuration of the array antenna system 10a producing the largest communication area is selected to be no more than the legal largest gain (e.g. 20 dB for a movable object discriminator), a gain of the antenna system 10a when the radio wave absorbers 2 extremely narrows down the communication area is simply increased to the gain G. Thus, even if the directivity of the antenna system 10a is high, the antenna system 10a may legally be used. FIG.4 shows an antenna system according to a second embodiment of the present invention. This antenna system 10b has no hinge connecting an array antenna 1 in the form of rectangular board to a pair of radio wave absorbers 2 in the form of rectangular board. A pair of radio wave absorbers 2b is fixedly placed to the side edge surfaces of the array antenna 1 so that the front surfaces of the array antenna 1 and the radio wave absorbers 2b are in the same plane, and the radio wave absorbers 2a are disposed in front of an assembly of the array antenna 1 and the radio wave absorbers 2b and movable transversely to the array antenna 1. Moving the radio wave absorbers 2a toward right and left, changes the radio wave transmitting area of the antenna system 10b to change the communication area of the antenna system 10b. As shown in FIG.4, the cross section of each of the radio wave absorbers 2a has a form in which the thickness of the radio wave absorber 2 decreases from its rear edge to its front edge so that the radio wave absorbers 2a effectively absorb astray radio waves substantially transversely transmitted from the array antenna 1 together with the fixed radio wave absorbers 2b. Thus, the antenna system 10b transmits a radio wave of a high directivity to produce the communication area sharply narrowed down forward from the array antenna 1. FIG.5 shows an array antenna system 10c according to a third embodiment of the present invention. This array antenna system 10c has no hinge connecting a pair of radio wave absorbers 2c to an array antenna 1 in the form of rectangular board. Rear edges of the radio wave absorbers 2c made of a plastic material of a ferrite plus a plastic rubber are joined to the right-hand and left-hand edges of the array antenna 1 by means of fasteners 3a (e.g. rivets) so that the rear edges of the radio wave absorbers 2c are attached to the rear surface of the array antenna 1 and rear parts of the radio wave absorbers 2c are bent around the right-hand and left-hand edges of the array antenna 1. Transverse positions of free front edges of the radio wave absorbers 2c extending forward from the array antenna 1 are changed and fixed there by the plasticity of the radio wave absorbers 2c to change the radio wave transmitting area and the communication area of the array antenna system 10c. If the radio wave absorbers 2c are made of a material of a ferrite plus a rubber lacking plasticity, a suitable fastener means (not shown) is used to releasably fix a radio wave transmitting area defined by the free front edges of the radio wave absorbers 2c. FIGS.6A and 6B show a dipole antenna system 10d according to a fourth embodiment of the present invention. The dipole antenna system 10d employs a vertical dipole antenna 51 and a pair of radio wave absorbers 2 in the form of rectangular board. The radio wave absorbers 2 vertically extend and are arranged symmetrically with respect to the dipole antenna 51. As shown in FIG.6B, the radio wave absorbers 2 are horizontally movable. As shown in FIG.8, the dipole antenna system 10d produces a pair of symmetrical communication areas 4 in the form of a lobe in front and rear of the dipole 51. The radio wave absorbers 2 cut part of a communication area of the dipole 51 to transversely extend so that the dipole antenna system 10d produces the pair of communication areas 4 narrowed down in the form of the lobe. The communication areas 4 depend on horizontal positions of the radio wave absorbers 2. FIGS.9A and 9B show an array antenna system 10e according to a fifth embodiment of the present invention. The front surface of the array antenna 1 in the form of rectangular board has a light transmitter (e.g. an electric lamp or LED) 17 fixed to the centerline thereof near the antenna elements 11. The light transmitter 17 is lit if necessary. Thus, a beam of light 5 from the light transmitter 17 passing through the radio wave transmitting area defined by the front edges of the radio wave absorbers 2 in the form of rectangular board, as shown in FIG.9B, roughly indicates the communication area of the antenna system 10e so as to facilitate an adjustment of the communication area (i.e. the width of the communication area and thus the maximum distance of the communication area) of the array antenna system 10e. FIGS.10A and 10B show an array antenna system 10f according to a sixth embodiment of the present invention. As shown in FIG.10A, two array antennas 1 and 1' in the form of rectangular board are mounted on the front and rear surfaces of a conductive support board 18 opposite each other. Right-hand and left-hand edges of the front and rear surfaces of the support board 18 have two pairs of radio wave absorbers 2 in the form of rectangular board pivotally mounted thereto. A setting angle of the radio wave absorbers 2 of each pair to the front or rear surface of the support board 18 is acute, so that the front radio wave absorbers 2 are arranged tapering forward from the support board 18 and the rear radio wave absorbers 2 are arranged tapering rearward from the support board 18. A housing 22 contains all of the array antennas 1 and 1', the support board 18, and the radio wave absorbers 2. The central portions of the outer surfaces of the opposite sidewalls of the housing 22 define vertical grooves 23. Radio interference guards 19 in the form of rectangular board are fitted in the grooves 23 by suitable fixing means 20. The radio interference guards 19 are made of a conductive solid board or a conductive network and guard radio waves transmitted by the front and rear array antennas 1 and 1' from a radio interference. Thus, the front half of the array antenna system 10f including the front array antenna 1 and the rear half of the array antenna system 10f including the rear array antenna 1' operate independently of each other without radio interference. The array antenna system 10f can determine whether there is a responder 15 on a front or rear side of the support board 18. FIG.11 shows an array antenna system 10g according to a seventh embodiment of the present invention. The array antenna system 10g comprises only the same front half of the array antenna system 10f of the sixth embodiment including the radio interference guards 19. The radio interference guards 19 guard a radio wave transmitted by the array antenna 1 from interfering with a radio wave transmitted by an antenna system near the array antenna system 10f. Therefore, if there is no antenna system near the array antenna system 10f, the radio interference guards 19 may be eliminated. If the radio interference guards 19 are in electrical contact with the support board 18 on large contact surfaces between each of the radio interference guards and the support board 18, the operation of the radio interference guards 19 is enhanced. The above embodiments employ the array antenna 1 and the dipole antenna 51. The present invention may alternatively employ a horn antenna, a parabolic antenna, a Yagi-Uda antenna and an antenna including a single antenna element in the form of board. The radio wave absorbers 2 may alternatively be mounted to the top and bottom ends of the array antenna 1 and the dipole antenna 51 instead of the right-hand and left-hand sides of the array antenna 1 and the dipole antenna 51. The radio wave absorbers 2 may alternatively be mounted to all of the top and bottom ends and the right-hand and left-hand sides of the array antenna 1 and the dipole antenna 51. The embodiment in which the present invention is applied to the movable object discriminator of the article delivery system has been described above. The present invention is also applicable to a parking-place control system opening and closing a gate or door in response to a radio wave from a responder, to a room entrance and exit control system and to a ticket examination system. The present invention is also applicable to an automotive radar for sensing a vehicle-to-vehicle distance or the position of an obstacle. The automotive radar transmits a radio wave to a preceding automotive vehicle, receives a reflected radio wave from the preceding automotive vehicle and then determines a vehicle-to-vehicle distance between the preceding automotive vehicle and an automotive vehicle having this automotive radar. Thus, it . is often desirable that the communication area of the radio wave has a relatively short range rather than a long range. In this case, an automotive radar antenna system of the present invention serves to detect only vehicles in a short range without increasing noises of detecting vehicles in a long range and without increasing the size of the radio transmitting antenna system since it narrows down the radio communication area with decreasing the gain of the antenna system.

Claims: Mobile object discriminating system, comprising an interrogator (16) generating an interrogation signal, and a responder (15) attached on a mobile object (14) and returning a response signal to said interrogator (16) in response to said interrogation signal, said interrogator (16) comprising an antenna system for transmitting and receiving radio waves of said interrogation and response signals, characterized in that said antenna system comprises an array antenna (1) whose front surface transmits and receives radio waves; and a pair of movable radio wave absorbers (2) disposed at both sides of the front surface of said array antenna (1) for absorbing a part of the incoming and outgoing radio waves, each radio wave absorber (2) being boardlike and hingedly or slidably supported on said array antenna (1) in such a way that an opening area of the front surface of said array antenna (1) is substantially regulated by said movable radio wave absorbers (2), thereby changing a transmission area (S) of said antenna system in accordance with the opening area defined by distal edges of said movable radio wave absorbers (2). System according to claim 1, characterized by hinge means (3) for connecting base edges of said radio wave absorbers (2) to said array antenna (1), said hinge means (3) allowing to vary the transmission area (S) of said antenna system. System according to claim 2, characterized in that said hinge means (3) is supported about a fixed pivot so that each radio wave absorber (2) is swingable about said pivot, thereby arbitrarily varying the transmission area (S) by changing an intersecting angle (α) between said array antenna (1) and each radio wave absorber (2) at any place said antenna system is installed. System according to claim 1, characterized in that said pair of radio wave absorbers (2) are movable transversely with respect to the front surface of said array antenna (1) in such a way that the transmission area can be reduced when said absorbers (2) come close to each other and, on the contrary, increased when said absorbers (2) depart from each other. System according to claim 4, characterized in that said radio wave absorber (2) have a base edge having a width wider than that of the distal edge, so that a cross section of said radio wave absorbers (2) is configured into a trapezoid. System according to one of claims 1 through 5, characterized in that each of said radio wave absorbers (2) is made of a composite of ferrite and epoxide. System according to one of claims 1 through 5, characterized in that each of said radio wave absorbers (2) is made of a composite of ferrite and rubber. System according to one of claims 1 through 5, characterized in that each of said radio wave absorbers (2) is made of a composite of conductive fibers and urethane foam. System according to one of claims 1 through 5, characterized in that each of said radio wave absorbers (2) is made of a plastic material of ferrite and plastic rubber, and base edges of said radio wave absorbers (2) are fastened to said array antenna (1). System according to one of claims 1 through 9, characterized by two stationary radio wave absorbers (2b) fixedly disposed at both sides of said array antenna (1) transversely along the front surface thereof. System according to one of claims 1 through 10, characterized by a flat conductive support board (18), on the front surface of which said array antenna (1) is installed. System according to claim 11, characterized by a radio interference guard (19) made of conductive material and disposed at both sides of said conductive support board (18) transversely along the front surface thereof. System according to claim 12, characterized by a housing (22) accommodating said array antenna (2), said conductive support board (18)-and said radio wave absorbers (2), and by grooves (23) provided at both sides of said housing (22), each of said grooves (23) holding said radio interference guard (19). System according to claim 12, characterized in that said conductive support board (18) is electrically connected to said radio interference guard (19). System according to claim 12 or 14, characterized by: a second antenna having a front surface transmitting and receiving radio waves and installed on the rear surface of said conductive support board (18); secondary movable radio wave absorbers (2), wherein said second antenna and said secondary movable radio wave absorbers (2) are disposed in opposed relation to said array antenna (1') and said radio wave absorbers (2) across said radio interference guard (19).

Inventors: HIRATA TATSUYA, TOKITSU NAOKI, WATANABE ATSUSHI, HIRATA, TATSUYA, TOKITSU, NAOKI, WATANABE, ATSUSHI
Applicants: NIPPON DENSO CO, NIPPONDENSO CO., LTD.
Application year: 1992
Main classification: H01Q17
IPC codes: H01Q3, H01Q21, H04B7, H01Q17