Abstract:
A method and apparatus for detecting for detecting intrusions, such as intrusions through a door or window of a room, in a manner which ignores movements in other adjacent regions, is provided. The method of detecting intrusions with respect to a monitored space includes exposing the monitored space to a passive infrared sensor having a first sensor element generating a positive polarity signal when its field of view senses an infrared-radiating moving object, and a second sensor element generating a negative polarity signal when its field of view senses an infrared-radiating moving object; generating a movement signal consisting of a positive polarity signal and a negative polarity signal when both have been generated within a first time interval such as to indicate the movement of an object within the monitored space; determining from the relative sequential order of the positive polarity signal and negative polarity signal in the movement signal the direction of movement of the detected object, and particularly whether the movement direction is a hostile direction or a friendly direction; and actuating an alarm when the direction of movement of the movement signal is determined to be in the hostile direction, but not when it is determined to be in the friendly direction.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to a method and apparatus for detecting objects by the use of passive infrared sensors. The invention is especially useful for detecting intrusions through a door or window, and is therefore described below with respect to such an application, but as will be pointed out below, the invention could be advantageously used in many other applications as well 
     Passive infrared (hereinafter “PIR”) sensors are widely used for detecting infrared (IR) radiating bodies, such as a person, particularly for detecting intrusions in monitored spaces. Where the ambient temperature is less than body temperature (e.g., above 36° C.), the radiation generated by the intruder can be sensed to trigger an alarm. 
     There are many applications, however, where it is desired to monitor a limited space, such as the region adjacent to a window or door of a room, in order to detect an intrusion through the window or door, but to ignore movements in other regions within the room. In such applications, it is necessary to install the PIR detector system very precisely in order to make it sensitive only to movements within the region to be protected and to make it insensitive to movements outside that region. In addition, it is also frequently necessary to use a PIR detector internally of the room and another one externally of the room in order to enable the system to detect movement in the “hostile” direction, (i.e., through the window or door into the room) and to make it insensitive to movements in the friendly direction, (i.e., within the room or towards window or door). 
     PIR detectors are also used in other applications, such as for controlling automatic doors. 
     OBJECT AND BRIEF SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a method, and also an apparatus, for detecting objects in a manner which enables the direction of movement of the object to be easily determined. Another object of the invention is to provide a method, and also an apparatus, for detecting intrusions, such as intrusions through a door or window of a room, in a manner which ignores movements in other adjacent regions. 
     According to one aspect of the present invention, there is provided a method of detecting intrusions with respect to a monitored space comprising: exposing the monitored space to a passive infrared sensor having a first sensor element generating a positive polarity signal when its field of view senses an infrared-radiating moving object, and a second sensor element generating a negative polarity signal when its field of view senses an infrared-radiating moving object; generating a movement signal consisting of a positive polarity signal and a negative polarity signal when both have been generated within a first time interval such as to indicate the movement of an object within the monitored space; determining from the relative sequential order of the positive polarity signal and negative polarity signal in the movement signal the direction of movement of the detected object, and particularly whether the movement direction is a hostile direction or a friendly direction; and actuating an alarm when the direction of movement of the movement signal is determined to be in the hostile direction, but not when it is determined to be in the friendly direction. 
     According to further features in the described preferred embodiment, the method further comprises disabling the actuation of the alarm during a second time interval after a generated movement signal has been determined to be in the friendly direction. More particularly, the disabling of the actuation of the alarm is effected by applying each movement signal, after a short time delay, to restart the second time interval during which the actuation of the alarm is disabled. 
     As will be described more particularly below, such a method actuates the alarm upon detecting a movement In the hostile direction (i.e., from a door or window into the interior of the room), but will be insensitive to movement in the friendly direction (i.e., from the interior of the room towards the window or door). Moreover, each time a movement has been detected, the time interval for disabling the alarm, if not already actuated by a hostile movement, is restarted so that the system will be insensitive to any movements during this time interval, whether in the friendly direction as well as in the hostile direction. 
     In the described preferred embodiment, the first time interval during which a positive polarity signal and a negative polarity signal must be received before a movement signal is generated, is measured in seconds, e.g., preferably about 4 seconds, which is a reasonable time to assume that both those signals were generated by the same moving object. The second time interval, during which the alarm is disabled after the generation of a movement signal (assuming the alarm has not been actuated by a hostile direction movement) is measured in tens of seconds or minutes, e.g., 40-60 seconds, which is a reasonable time to assume that no intrusion will be attempted through a door/window after a friendly body has been moving in the room. The short time delay imposed on each movement signal for restarting the disabling interval (the second time interval) is a small fraction of a second, e.g., 30 msec., merely to provide enough time for a detected movement in the hostile direction to actuate the alarm before the alarm can be disabled. Once the alarm is actuated, it continues to operate according to the alarm circuit, e.g., intermittently or for a predetermined time interval. 
     As briefly described above the invention is also applicable in other applications wherein it is necessary or desirable to detect not only the movement of an object within a monitored space, but also the direction of movement of the object. Such applications could include the controlling of automatic doors so as to automatically open the door when a person approaches the door, to automatically close the door when the person is moving away from the door, and to make the door control insensitive to movements parallel to the door. Another possible application of the invention would be to count traffic moving in each direction. 
     According to another aspect of the present invention, therefore, there is provided a method of detecting movement of an infrared-radiating object in a predetermined direction within a monitored space, comprising: exposing the monitored space to a passive infrared sensor having a first sensor element generating a positive polarity signal when its field of view senses an infrared-radiating moving object, and a second sensor element generating a negative polarity signal when its field of view senses an infrared-radiating moving object; generating a movement signal constituted of a positive polarity signal and a negative polarity signal when both have been generated within a first time interval such as to indicate the movement of an object within the monitored space; and determining from the relative sequential order of the positive polarity signal and negative polarity signal in the movement signal the direction of movement of the detected object. 
     According to a still further aspect of the present invention, there is provided apparatus for detecting moving infrared-radiating objects in a monitored space, comprising: a passive infrared sensor to be mounted to view the monitored space, the passive infrared sensor having a first sensor element generating a positive polarity signal when its field of view senses an infrared-radiating moving object, and a second sensor element generating a negative polarity signal when its field of view senses an infrared-radiating moving object; a time window circuit for receiving the generated positive polarity signals and negative polarity signals and for generating a movement signal when a positive polarity signal and negative polarity signal have been generated within a first time interval, such as to indicate the movement of an object within the monitored space; and a direction-determining circuit for determining, from the relative sequential order of the positive and negative polarity signals in the movement signal, the direction of movement of the detected object. 
     According to further features in the preferred embodiment of the invention described below, the apparatus is particularly useful for detecting intrusions and further comprises an alarm; and an alarm actuating circuit which actuates the alarm when the direction of movement of the movement signal is determined to be in a hostile direction, but not when it is determined to be in a friendly direction. 
     Further features and advantages of the invention will be apparent from the description below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
     FIG. 1 diagrammatically illustrates one application of the invention for providing protection against intrusion through a window or door of a room, without interfering with other movements within the room; 
     FIG. 2 schematically illustrates one form of passive infrared (PIR) detector used in the apparatus of FIG. 1; 
     FIG. 3 illustrates the positive and negative polarity signals generated by the PIR detector of FIG. 1 when detecting the movement of an IR-radiating body in one direction; 
     FIG. 4 a  schematically illustrates the main components of a typical PIR detector as commonly supplied for intrusion detection applications; 
     FIG. 4 b  illustrates the PIR detector of FIG. 3 but modified, by shifting its IR sensor elements laterally with respect to its lens, to better adapt it for use in the application of FIG. 1 in accordance with another feature of the invention; 
     FIG. 5 is a block diagram illustrating the functional blocks in one form of apparatus constructed in accordance with the present invention; 
     FIG. 6 is a circuit diagram illustrating an analog-circuit implementation of the invention; and 
     FIG. 7 is a flow chart illustrating a digital-circuit implementation of the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference first to FIG. 1, there is illustrated a room  2  or other enclosures having a door  3  and a window  4  to be protected against intrusion. For this purpose, a PIR detector, generally designated  10 , is mounted on a wall, or other suitable location, to view the space  5  immediately adjacent the door  3  and window  4  so as to detect objects moving within this space, but to be insensitive to objects moving within the remaining interior region  6  of the room  2 . 
     It is particularly desirable to make the system illustrated in FIG. 1 sensitive only to movements in the direction from the window or door towards the center region  6  of the room (herein called the “hostile” direction) and to make the system insensitive to movements in the direction from the interior region of the room towards the door or window (herein called the “friendly” direction). For this reason, the system, as will be described more particularly below, not only detects movement within the protected space  5 , but also determines the direction of such movement so as to enable it to respond only to movements in the hostile direction, but to be insensitive to movements in the friendly direction. 
     FIG. 4 a  illustrates the construction of a typical PIR detector as commercially available for intrusion detection purposes. Such a detector  10  includes a housing  11  carrying a printed circuit board (PCB)  12  on which two (or four in a quad-type) IR sensor elements, shown as  13  and  14  in FIG. 2, are mounted for detecting IR-radiating moving objects. Housing  11  includes a window closed by a lens  15  which defines a field of view (FOV) for each of the sensor elements  13 ,  14 . Sensor element  13  generates a positive polarity signal when its field of view senses an IR-radiating moving object; and sensor element  14  generates a negative polarity signal when its field of view senses an IR-radiating moving object. 
     In the typical PIR detector used for intrusion monitoring, the outputs of the two sensor elements  13 ,  14  are connected together so that if the object is not-moving they cancel, thereby producing a zero output from the IR detector. Accordingly, the detector ignores non-moving IR-radiating objects, such as heat radiators within the detected space, sunlight entering the protected space, etc., and detects only moving IR-radiating objects, such as persons, by outputting a signal corresponding to the velocity of movement of the objects. 
     The embodiments of the present invention described below utilize such PIR detectors but include them in a system which not only detects moving objects, but also detects the direction of movement of such objects. 
     Also, in a typical PIR detector as presently available, the two sensor elements  13 ,  14 , are generally mounted symmetrically with respect to the lens  15 ; that is, as shown in FIG. 4 a,  the mid-line  16  between the two sensor elements passes through the optical center of the lens  15  and is perpendicular to the transverse axis  17  of the lens. 
     In accordance with another feature of the present invention, the PIR detector  10  illustrated in FIG. 1 is preferably slightly modified as shown in FIG. 4 b,  by shifting the printed circuit board  12 , laterally with respect to the fixed lens  15  so that the mid-line  16 ′ between the two sensor elements  13 ,  14  passing through the center of the lens  15 , is no longer perpendicular to the transverse axis  17  of the lens, but rather is slightly oblique with respect to that axis. As will be described more particularly below, such a modification enables a single detector  10  to be located within the room  6  and to efficiently monitor an attempted intrusion through the window or door, and eliminates the need for providing a separate detector externally of the room for this purpose. In a typical example, the distance between the two sensor elements  13 ,  14 , and the lens may be 10-30 mm, in which case the lateral shift would preferably be about 0.5-3 mm. In the example described below, this distance is 25 mm, and the lateral shift is 1-1.5 mm. 
     In FIG. 4, the area  13 F indicates the field of view of sensor element  13  of the PIR detector  10 , and the area  14 F indicates the field of view of the sensor element  14 . Sensor element  13  is connected to produce a positive polarity signal when detecting an IR-radiating moving body, and sensor element  14  is connected to produce a negative polarity signal when detecting an IR-radiating moving body. Accordingly, when a body moves In the hostile direction (indicated by arrow HD in FIGS. 1 and 4 b ) i.e., from the window  4  towards the center of the room, a positive polarity signal (A+) will first be generated in the field of view  13 F, and then a negative polarity signal (B−) will be generated in the field of view  14 F, as shown in FIG.  3 . On the other hand, if the body is moving in the friendly direction (indicated by arrow FD in FIGS. 1 and 4 b ) i.e., from the center of the room towards the window, such body will first generate a negative polarity signal (B−) in field of view  14 F, and then a positive polarity signal (A+) in field of view  13 F. 
     Thus, when both a positive polarity signal and a negative polarity signal are detected within a predetermined time interval indicating that both signals are generated by the same body, this determines that a body is moving within the field of view of the detector; and by examining the sequence of the two signals, this enables a determination to be made of the direction of movement of that body. 
     FIG. 5 is a block diagram illustrating apparatus for using the foregoing mechanisms for detecting the presence of an object moving within the monitored space defined by the fields of view of detector  10  in the hostile direction HD (from the window towards the interior of the room) and to ignore movements of objects in the friendly direction FD (from the interior of the room towards the window or door) so as not to interfere with normal activities within the room. 
     Thus, as shown in FIG. 5, detector  10  feeds its output signals to an amplifier  20 , and then to a polarity detector  21  which determines the polarity of each received signal passing a predetermined threshold. Each positive pulse signal is outputted via port  21   a  to a time window circuit  22 , and each negative polarity signal is outputted via port  21   b  to time window circuit  22 . 
     Time window circuit  22  is open for a predetermined time interval, e.g., 4 seconds. If a positive polarity signal and a negative polarity signal have both been received within this time interval, the time window circuit generates a movement signal at its output port  22   a,  constituted of a positive polarity signal and a negative polarity signal as illustrated in FIG.  3 . The 4 second time interval during which this window is open shows that both pulses received are from the same moving body. 
     The sequential order of the positive and negative portions of the movement signal indicates the direction in which the body was moving. Thus, as described above, if a detected body was moving in the hostile direction HD (FIG.  4 ), the movement signal will be A+, B−; and if in the friendly direction FD, the movement signal will be of the opposite sequence, B−,A+. 
     The movement signal is outputted from time window circuit  22  into a direction identffication circuit  23 , which determines the direction of movement of the detected object, as described above. However, the above description for determining direction assured that the ambient temperature was below body temperature. Thus, when the ambient temperature is below body temperature (36° C.), the signal sequence is as described above, i.e., A+, B− for movement in the hostile direction HD, and B−,A+ for movement in the friendly direction FD. However, if the ambient temperature is above the body temperature (36° C.), the output of the sensor elements will be of the opposite polarity, so that the above sequence will be reversed for the two directions. 
     Accordingly, FIG. 5 illustrates a temperature measuring circuit  24  which measures the ambient temperature and controls the direction identification circuit  23  in accordance with the measured temperature. The absolute temperate need not be measured, as it is only necessary to determine whether the ambient temperature is above or below the body temperature (36° C.), and to control the direction identification circuit  23  accordingly. 
     The direction identification circuit  23 , therefore, receives an input from the time window circuit  22 . However, it also, as inputs, the positive polarity signal  21   a  and the negative polarity signal  21   b  from the polarity detector circuit  21 , and the temperature measurement signal from the temperature measuring circuit  24 . From these inputs, circuit  23  determines the direction of movement of the detected object. 
     If the direction is in the hostile direction HD as shown in FIGS. 2; and  4   b  (from the window or door towards the interior of the room), circuit  23  outputs an alarm signal to an alarm actuator circuit  25  which immediately actuates the alarm  26 . If, however, circuit  23  determines that the detected object was moving in the friendly direction FD (from the room interior towards the window or door), no alarm signal is outputted to the alarm circuit  25 , and therefore no alarm is actuated. 
     The system illustrated in FIG. 5 further includes an alarm disable circuit  27  for disabling the alarm circuit  25  in order to permit normal movements within the room for a predetermined time interval after a generated movement signal has been determined to be in the friendly direction FD. This disabling of the alarm circuit  25  is effected by each movement signal from the time window  22 , which is applied, after a short delay by delay circuit  28 , to the alarm disable circuit  27 . Each time such a movement signal is received from circuit  22  via delay circuit  28 , the time interval during which the alarm circuit  25  is disabled is restarted. 
     As typical examples, delay circuit  28  may impose a delay of about 30 msec in the time each movement signal from the time window  22  is applied to the alarm disable circuit  27 , and the disable circuit  27  may disable the alarm circuit  25  for a time interval of about 40-60 seconds. 
     The system illustrated In FIG. 5 thus operates as follows: 
     The space adjacent the window and door in FIGS. 1 and 4 b  is continuously monitored by detector  10 . Should an object move in the hostile direction HD, namely from the window or door towards the center of the room, a positive polarity signal (A+) will be generated by sensor element  13  in field of view A (FIG. 4 b ), and a negative polarity signal (B−) will then be generated by sensor  14  in field of view B. If the movement is in the friendly direction FD, the negative polarity signal B− will first be generated, and then the positive polarity signal A+. 
     The signals generated by detector  10  are amplified in amplifier  20 , and applied to polarity detector circuit  21 . Circuit  21  determines the polarity of each received signal passing a predetermined threshold, and outputs each positive signal via output  21   a,  and each negative signal via output  21   b.  These signals are received by the time window circuit  22 , and when both a positive signal and a negative signal have been received within a predetermined time interval (e.g., 4 seconds) circuit  22  determines that a movement has occurred in the detected object and outputs a movement signal to circuit  23 . The latter circuit receives, in addition to the movement signal from time window circuit  22 , also the positive and negative polarity signals from polarity detector  21 , and also the temperature signal from the temperature measuring circuit  24 , and from this confirmation determines whether the detected object was moving in the hostile direction HD, or in the friendly direction FD. If it was moving in the hostile direction HD, it immediately actuates the alarm circuit  25 , which actuates the alarm  26 . If, however, the detected direction of movement was in the friendly direction FD, no alarm signal is outputted to alarm circuit  25 , and therefore the alarm is not actuated. 
     The alarm disable circuit  27  prevents the alarm circuit from being actuated for a predetermined interval, e.g., 40-60 seconds, after a detected object has been found to be moving in the friendly direction. Thus, if an object after having moved in the friendly direction moves in the hostile direction within this time interval, the alarm is disabled from being actuated. For example, a friendly object may move in a friendly direction and thereafter, during this predetermined time interval, may move in many other directions within the monitored space, in which case it is not desired to have the alarm be actuated should the object move in the hostile direction during the time interval. 
     Thus, each movement signal produced by the time window circuit  22  will, after experiencing a short delay (e.g. 30 msec), be applied to the alarm disable circuit  27  to restart the disable period (40-60 sec). This disable will not be effective to prevent the actuation of the alarm  26  by the alarm circuit  25  when an object is detected outside of the disable period, since the actuation of the alarm is immediate when the movement signal is determined by circuit  23  to be in the hostile direction. However, where the alarm disable circuit  27  is not actuated, each movement signal thereafter from the time window circuit  22  will be subjected to a short delay imposed by the delay circuit  28  before it is applied to the alarm-disable circuit  27 . 
     Thus, once it has been determined that the detected object is moving in the friendly direction, alarm disable circuit  27  is effective to disable the alarm circuit  25  for a predetermined interval (e.g., 40-60 sec), and thereafter to restart that predetermined interval with each subsequent motion signal from circuit  22  so as not to interfere with normal activity during this disable period. 
     FIG. 6 is a circuit diagram illustrating an analog-circuit implementation of the system of FIG.  5 . As shown in FIG. 6, the elements within box  121  generally perform the function of the polarity detect circuit  21  in FIG. 5; the elements within box  122  generally perform the function of the time window circuit  22  in FIG. 5; the elements within box  123  generally perform the function of the direction identification circuit  23 ; the elements within box  124  generally perform the function of the temperature measuring circuit  24 ; the elements within box  125  generally perform the function of the alarm circuit  25 ; and the elements within box  127  generally perform the functions of the alarm disable circuit  27  and the delay circuit  28 . Transistor M 2  within box  127  functions to disable the actuation of the alarm circuit  25  for 40-60 seconds whenever two pulses of opposite polarity are outputted by the time window circuit  122  within the time interval (4 seconds) of the window. The two pulse of opposite polarity constitute a motion signal which, when occurring, produces a “1” in the inlet to circuit element UTD, which disables the alarm from actuation during the 40-60 second interval. 
     FIG. 7 is a flow chart illustrating a digital-circuit implementation of the system of FIG.  5 . Thus blocks  200 - 204  generally perform the functions of the polarity detector  21  which detects a positive polarity signal and a negative polarity signal passing a predetermined threshold; blocks  205 - 207  generally perform the function of the time window circuit  22 ; blocks  208 - 211  generally perform the function of the direction identification circuit  23 ; blocks  212  and  213  generally perform the function of the temperature measuring circuit  24 ; block  214  generally performs the function of the alarm detector  25 ; and block  215  generally performs the function of the alarm disable circuit  27 , 
     While the invention has been described with respect to one preferred embodiment, it will be appreciated that this is set forth merely for purposes of example, and that many other variations, modifications and applications of the invention may be made.