Abstract:
A perimeter monitoring system intended for the enhanced safety of children includes a receiver communicating with a plurality of transmitters worn by the monitored individuals within a predefined perimeter area surrounded by a perimeter loop antenna. A periodic transmission is sent by each transmitter to the receiver as confirmation that the child is presently within the desired area. The system will alarm the operator and provide an indication identifying any one of the monitored children that either leaves the predefined perimeter or enters a restricted area.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from Provisional Application No. 60/171,985, filed on Dec. 23, 1999. 
     
    
     
       BACKGROUND  
         [0002]    The present invention relates to security systems. More particularly, the invention is directed to a child safety system.  
           [0003]    A typical home or commercial security system generally consists of a plurality of different monitoring devices, depending upon the type and extent of protection desired. The monitoring devices include motion sensitive detectors, closed circuit video cameras, light curtains and audio detectors. Motion sensitive detectors and light curtains may be setup to cover a particular area. An alarm will be triggered if movement is detected within the monitoring area. Likewise, audio detectors will monitor for intruders by detecting all sounds within a defined area and activating an alarm if the sounds exceed a predetermined threshold.  
           [0004]    Video monitoring devices such as closed circuit cameras are typically installed in areas where direct visual monitoring is difficult or when it is desired to observe several areas from a single location. However, video monitoring devices require constant visual surveillance of the display to determine whether any changes have occurred.  
           [0005]    Electronic entry monitoring devices may be installed at all doors, windows or other access points within a home or commercial establishment. These devices utilize a closed current loop, whereby current is continuously circulated through the current loop as long as the door or window remains closed. Upon opening the monitored door or window, the current will be discontinued and the discontinuity triggers an alarm condition.  
           [0006]    Although these prior art devices are useful for many applications, they may not be suitable in certain circumstances. For example, to implement a security system for children, such as in a daycare center to monitor whether children leave a predefined area or enter a restricted area, if only electronic entry monitoring devices or motion sensitive devices are used, an alarm will be triggered even if an adult or teacher opens a monitored door or enters a monitored area. There is a need for a system to enhance the security and safety of a daycare center or a home environment to monitor the whereabouts of every child.  
           [0007]    U.S. Pat. No. 4,136,339 to Antenore discloses a perimeter alarm apparatus that includes a loop of wire to be placed around an area, and electrical circuitry which is connected to the loop to monitor a mobile signal sender within the loop. This system is designed to monitor one signal transmitter within the loop. Although the system may be modified to monitor more than one transmitter, it is necessary to duplicate the RF circuit tuned to the respective transmitter frequencies. This prior art design can only monitor a very limited number of transmitters because each transmitter, and thus each receiver, requires its own frequency range. It is costly and impractical to repeat circuitry for each additional transmitter. Moreover, the prior art does not disclose how to switch between the monitoring of different frequencies transmitted by different transmitters.  
         SUMMARY  
         [0008]    It is an objective of the present invention to enhance the safety of children in a predefined area, whereby each child can be individually monitored.  
           [0009]    This and other objectives are achieved by providing a system having receiver communicating with a plurality of transmitters attached to target objects within a predefined perimeter area surrounded by a perimeter loop antenna. The system includes a scheme for identifying individual transmitters and for processing the detection of multiple identification signals sent therefrom. The system will alarm the operator if one of the monitoring objects either leaves the predefined perimeter or enters a restricted area.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a perspective overview of a system made in accordance with the present invention having at least one transmitter and a receiver.  
         [0011]    [0011]FIG. 2 is a functional block diagram of the transmitter portion of the system of FIG. 1.  
         [0012]    [0012]FIG. 3 is a functional block diagram of the receiver portion of the system of FIG. 1.  
         [0013]    [0013]FIG. 4A is a flow diagram of the operation of the transmitter.  
         [0014]    [0014]FIG. 4B is a flow diagram of the operation of the receiver. FIGS. 5A and 5B are diagrammatic views of the transmitter flux lines relative to the perimeter and restricted loops. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    The preferred embodiment will be described with reference to the drawing figures wherein like numerals represent like elements throughout.  
         [0016]    An overview of a monitoring system  1  embodying the present invention is shown in FIG. 1. The monitoring system  1  generally comprises one or more receivers  2 , which are in communication with a plurality of transmitters  12 ,  18 ,  20  and  22  via a perimeter antenna loop  8 . The perimeter loop  8  defines an interior area  4  and an exterior area  6 , and separates the interior area  4  from the exterior area  6 . The perimeter loop  8  is an RF receiving antenna, which receives all RF signals transmitted from the transmitters  12 ,  18 ,  20 ,  22 . As should be recognized by those of skill in the art, the length of a receiving antenna must be equal to, or longer than, the wavelength of the RF frequency to receive the RF signals.  
         [0017]    Preferably the RF frequency band used in the present invention is appropriately 100 Khz. However, this is a design choice which may be changed to suit the particular application. The transmitter antenna is a ferrite core antenna; the receiver antenna comprises one or more loops around the designated perimeter.  
         [0018]    A plurality of smaller restricted areas  14 ,  15  can also be setup within the interior area  4  by surrounding each restricted area  14 ,  15  with its own loop of wire. Each restricted area loop antenna also functions as a loop antenna  16 ,  17 , hereinafter called a restricted loop antenna. As will be explained in further detail hereinafter, the restricted loop antennas  16 ,  17  are also connected to the perimeter loop  8 .  
         [0019]    The perimeter loop  8  receives a periodically-transmitted individually-identifiable low frequency RF signal from each of the transmitters  12 ,  18 ,  20 ,  22  and forwards these signals to the receiver  2 . The receiver  2  will receive no signals (or weaker signals) transmitted by a transmitter from the exterior area  6  because the magnetic field of the transmitters within the perimeter loop  8  will induce voltage in the perimeter loop  8  that will cause the current to flow in the loop in a direction tending to set up an opposing magnetic field. The induced voltage in the perimeter loop  8  is reduced if the transmitter is outside the perimeter loop  8 , such as transmitter  22 .  
         [0020]    For example, as shown in FIG. 5A, a transmitter  12  in the center of both perimeter loop  8  and one of the restricted loops  17  is transmits a signal which induces current in the restricted loop  17 , as well as the perimeter loop  8 . The induced current will be perpendicular to the field, (dashed lines). Due to the location of the transmitter  12 , the currents induced in the loops  8 ,  17  are clockwise since the fields are oriented in the same direction. If point A is connected to point B, the currents between two loops  8 ,  17  cancel each other. Therefore, the receiver  2  will receive no signals transmitted by a transmitter  12  that has entered a restricted area  14 ,  15 . On the other hand, if the transmitter  18  is located outside one of the restricted loops  16 ,  17  but within the perimeter loop  8 , as shown in FIG. 5B, the restricted loops  16 ,  17  will detect a signal having a very small magnitude because the current within the restricted loop  17  will self-cancel. In essence, one half of the restricted loop  17  will have current induced in one direction while the other half of the restricted loop  17  will have current induced in the opposite direction. Therefore, the signal will come from the perimeter loop  8 .  
         [0021]    In operation, the transmitters  12 ,  18 ,  20 ,  22  periodically transmit RF signals, each including a unique identification number (UID) to that transmitter  12 ,  18 ,  20 ,  22 . Once a transmitter  12  moves from the interior area  4  into a restricted area  12 , the receiver  2  receives no signal, (or an extremely weak signal). Concurrently, the receiver  2  continuously receives signals from transmitters  18  and  20  which stay within the perimeter loop area  4 . If a transmitter leaves the perimeter area  4  and enters the exterior area  6 , such as transmitter  22 , the receiver  2  will receive no signal, (or an extremely weak signal), from that transmitter  22 . Based upon the presence or absence of a signal from each transmitter  12 ,  18 , 20 ,  22 , the receiver  2  can immediately identify whether any transmitters have left the interior area  4  or entered a restricted area  14 ,  15 , and can also identify which transmitter  12 ,  18 ,  20 ,  22  has done so.  
         [0022]    A block diagram of a transmitter  30  made in accordance with the teachings of the present invention is shown in FIG. 2. Preferably, the transmitter  30  is portable, such that it may be incorporated as part of an anklet or otherwise attached to the person to be monitored. The transmitter  30  includes a microcontroller  29 , a battery  31 , a random interval generator  34 , a baseband identification stream generator  36 , a modulator  38 , a filter and amplifier  40 , an RF upconverter antenna system  42 , an RF control circuit  44  and a self-diagnostic module  39 . The microcontroller  29  also includes a means for setting identification numbers  32 . Although this is shown in FIG. 2 as identification setting switches (such as DIP switch), this may also comprise a memory (not shown) which may be selectively programed with a keypad (not shown) to input a specific code desired by the user.  
         [0023]    The baseband identification stream generator  36  generates an identification stream, comprising a unique identification number (UID) for forwarding to the modulator  38 . The identification stream generator  36  reads the switch settings  32 , or receives the identification stored in memory which identifies the particular transmitter  30 . The modulator  38  receives the bit stream from the identification stream generator  36  and modulates the bit stream with the desired modulation scheme. As those skilled in the art would appreciate, the modulation scheme may be frequency shift keying (FSK) whereby the transmitter transmits one of two frequencies close together, one of which indicates a 0 and the other a 1. The modulation may also be any other type of known modulation scheme such as on-off keying (OOK), whereby the transmitter transmits a series of on off sequences which indicate a 1 or a 0, or amplitude shift keying (ASK), whereby the transmitter transmits one of two levels of signals indicating a 1 or a 0.  
         [0024]    The modulated bit stream is forwarded to the filter and amplifier  40  for filtering and amplifying the bit stream. The RF upconverter  42  upconverts the bit stream to RF for transmission. The antenna controller  44  controls both the power and the frequency at which the antenna  42  transmits.  
         [0025]    The random interval generator  34  generates a pulse at a random interval to the baseband identification stream generator  36  to minimize collision between transmissions from multiple transmitters occurring at the same time. Although collisions may occur, the random interval generator  34  ensures that if a collision does occur, the next transmission from each of the transmitters that were involved in the collision should occur at a different time. The pulse output from the random interval generator  34  activates the baseband identification stream generator  36 . Each time a pulse is sent from the random interval generator  34  to the baseband identification stream generator  36 , the baseband identification stream generator  36  generates a burst identification stream for transmission. Accordingly, the transmitter  30  will transmit periodic bursts, each burst containing only the UID of the particular transmitter. The RF upconverter  42  powers up only when the baseband identification stream generator  36  sends the UID, that is, at random time intervals controlled by the random interval generator  34 .  
         [0026]    The RF upconverter  42  may comprise a plurality of antennas which would be controlled by the RF control  44 . Multiple antennas may be necessary because of the low frequencies that are used. These low frequency signals are highly directional. By using multiple antennas, the transmitter  30  could transmit a sequence of identical signals using successive antennas, thus assuring at least one of the antennas is properly directed.  
         [0027]    The transmitter  30  has self-test mode executed by the self-diagnostic module  39 , which will sound an alarm if the battery  31  is low or any of the components within the transmitter  30  have malfunctioned. The self-diagnostic module  39  includes an energy storage unit (not shown) such as a back-up battery to ensure that in the event that the transmitter battery  31  is dead or malfunctions, the self-diagnostic module  39  will still be able to generate an alarm signal. Thus, failures, or potential anomalies, at the transmitter  30  will be known by the user of the system.  
         [0028]    A receiver  40  made in accordance with the present invention is shown in FIG. 3. The receiver  40  includes an RF downconverter  45 , a demodulator  46 , an identification decoder  48 , a plurality of timers  50   a - 50   d , a timeout detector  52 , and an alarm  54 . The receiver  40  receives incoming RF signals from the plurality of transmitters  12 ,  18 ,  20 ,  22  through the perimeter loop antenna  8 . The signals are downconverted by the RF downconverter  45  and forwarded to the demodulator  46 . The demodulator  46  demodulates the signal and forwards a baseband signal to the ID decoder  48 , which reads the UIDs from received RF signals. Collisions are not detected, but are significantly reduced since each transmitter transmits at a random time interval. In the event that a collision occurs between the signals sent from two transmitters, neither signal will be received. However, the likelihood of successive transmissions subsequently colliding again is reduced since the random interval generator  34  within each transmitter will pick a different (i.e., random) time at which to transmit its next signal.  
         [0029]    The receiver  40  has a plurality of timers  50   a - 50   d  and assigns an independent timer  50   a - 50   d  to each transmitter. All timers  50   a - 50   d  reset their count to zero when the receiver  40  is initially energized. The count of each timer  50   a - 50   d  continuously increments until the receiver  40  receives a valid UID for the transmitter  12 ,  18 ,  20 ,  22  corresponding to the particular timer  50   a - 50   d . When the UID is received and confirmed, the count of the timer  50   a - 50   d  will be reset to zero. The timeout detector  52  monitors all of the timers  50   a - 50   d . If a timer  50   a - 50   d  is not reset and its count exceeds a predetermined threshold, the timeout detector  52  detects the condition of the timer  50   a - 50   d  and notifies the alarm module  54 , which outputs an alarm. Although the operation of the timers  50   a - 50   d  has been explained with reference to counters, the timers  50   a - 50   d  may actually measure the amount of time that has elapsed and the timeout detector  52  will detect when a predetermined time limit has been exceeded. The alarm  54  will then be invoked if this predetermined time period has been exceeded.  
         [0030]    The UID is first checked for consistency by the ID decoder  48 . The UID includes a cyclical redundancy check (CRC) or at least one parody bit in the transmitted data to ensure the UID is received error-free. If the UID passes the consistency check, then the appropriate timer  50   a - 50   d  based on the received UID is reset to zero.  
         [0031]    Referring to FIGS. 4A and 4B, the operation of the system can be explained with reference to at least two concurrent-running modes: 1) the operation of the transmitter  30  as shown in FIG. 4A; and  2 ) the operation of the receiver  40  as shown in FIG. 4B. Referring to FIG. 4A, the operation of the transmitter begins at step  62  by assigning a UID to each of a plurality of transmitters operating with the same perimeter loop  8 . This may be either a manual or automatic task that is typically performed only upon initial energization of the system  1  or when a new transmitter  30  is added. The next two steps  64  and  66  are self diagnosis steps for the self-diagnostic module  39  within the transmitter  30 . Step  64  determines if the power of the battery  31  is low. If so, the self-diagnostic module  39  invokes an alarm  68  to report the defective condition. In step  66 , the self-diagnostic module  39  monitors all components within the transmitter  30  to determine whether a malfunction occurred, and activates alarm  68  to report any defective condition. At step  70 , the random interval generator  34  generates a timing pulse which prompts the baseband identification stream generator  36  to read the switches  32  or memory and generates the UID (step  72 ). The UID may include a CRC. The transmitter  30  then transmits an RF signal containing the UID (step  76 ) and transmitter  30  operation cycles back to step  64 .  
         [0032]    Referring to FIG. 4B, the operation of the receiver  40  will now be explained in detail. The operation of the receiver  40  assumes that the perimeter loop antenna  8  has been deployed along with one or more restricted loop antennas  16 ,  17 , which are optional. Each transmitter is assigned to a corresponding internal timer  50   a - 50   d  of the receiver  40 . At step  82 , the receiver  40  resets all its internal timers  50   a - 50   d  so that each timer count is equal to zero. The receiver  40  receives RF signals from the plurality of transmitters  30  through the perimeter loop antenna  8  (step  84 ). The received RF signals will be downcoverted and the UID&#39;s will be extracted (step  86 ). Once a received UID is verified (step  88 ), the internal timer  50   a - 50   d  corresponding to the verified UID will be reset to zero (step  90 ). If a timer  50   a - 50   d  does not get reset for a predetermined time period, or the count of the timer exceeds a predetermined value (step  92 ) then the alarm module  54  will be invoked at (step  94 ). Once an alarm is triggered, the operator can be notified that the particular transmitter left the predefined area or entered a restricted area. Finally, the receiver operation cycles back to step  84 .  
         [0033]    It should be understood that in order to improve the performance of the system, the perimeter loop antenna  8  and the restricted area antennas  16 ,  17  may comprise two or more loops superimposed upon each other. This will significantly improve the detection of transmitted signals, thereby permitting the system to be installed in larger areas and/or allowing weaker transmitter power. If weaker transmitter power is allowed, battery life of the transmitter will be greatly extended.  
         [0034]    While the present invention has been described in terms of the preferred embodiments, other variations which are within the scope of the invention as outlined in the claims below will be apparent to those skilled in the art.