GB2246891A

GB2246891A - Alarm system

Info

Publication number: GB2246891A
Application number: GB9103079A
Authority: GB
Inventors: Richard Davies
Original assignee: ARROWAXE Ltd
Current assignee: ARROWAXE Ltd
Priority date: 1990-04-27
Filing date: 1991-02-13
Publication date: 1992-02-12
Also published as: GB2243739A; GB9103079D0; GB9009510D0

Description

ALARM SYSTEM This invention relates to an alarm system comprising at least

one target unit, and a monitor unit arranged to receive signals from the target unit and to generate an alarm in response to the signals.

In one aspect of the invention, the monitor unit derives from the signal a measure of the distance to the target unit and generates an alarm in response to a condition when the distance exceeds an acceptable range, or the distance is changing.

One example of an application for such a device is as a theft alarm. In a theft alarm system, the target unit (or each target unit) is positioned at a portable item such as an article of luggage, a strong box or a babies pram. The monitor unit is positioned at a predetermined distance (for example on the person of the owner of the valuables) and periodically or continuously measures the distance to the target unit. An alarm is generated when the target unit distance changes, or exceeds a predetermined threshold.

Three examples of such GB2112600A.

devices are shown in However, where multiple target devices are used the signals can mutually interfere and lead to false alarms or failure to recognise geniune alarms.

According to one aspect of the invention there is therefore provided an alarm system in which a first unit signals to a second unit, which signals in reply, including means for preventing interference between such signals.

In another aspect, the target unit includes or is connectable to a sensor (for example a theft sensor such as a Mercury switch) and the signal received by the monitor unit is responsive to an alarm condition sensed by the sensor so that the monitor unit is responsive to such an alarm condition.

In many applications, such as the protection of valuables, the size of the target unit is constrained. Since the target unit is to be portable, power for the signal to be generated is derived from a stored power 1 1 1 t i source, (eg a battery) and over time the power source will be become drained leading inevitably either to false alarm conditions or to an absence of protection depending upon how the monitor unit responds to an absence of signal from the target unit.

Accordingly, a third embodiment of the invention provides an alarm signalling unit, and a system including such a unit, including means for conserving battery power; preferably by ceasing transmission between sparse bursts. This also conserves radio channel usage.

This enables the target unit to remain inactive and thus consume no power for a considerable period of time. Additionally, it assists in reducing collision between signals to and from proximate target units.

According to one particular aspect of the invention, the monitor unit is normally not transmitting and transmits a burst of signal of duration sufficient for a distance measurement to be performed only at such times as it requires to know the distance of the target. The target unit only transmits when it is receiving a signal from the measurement unit allowing f - 4 both units to realise a consumption.

reduction in power According to a further aspect of the invention, the monitor unit transmits a burst of signal in accordance with a defined time schedule allowing the transmitter and receiver sections of one or both units to be switched off between bursts realising a reduction in power consumption.

i According to a further aspect of the invention, the modulation signal is itself modulated by to give a communication means from the monitor unit to the target unit. In a particular aspect of the invention, this communication is used to address specific targets allowing operation with a single measurement unit and multiple targets. In a further particular aspect, transmission in randomly spaced bursts and addressing using the communications link is used to prevent interference between co-located systems.

Other aspects of the invention, and preferred embodiments thereof, are as described and/or claimed hereafter.

i i 1 p 1 i i 1 1 i 1 i i i j i i f - The invention will now be illustrated, by way of example only, with reference to the accompanying drawings in which; Figure 1 shows schematically an alarm system; Figure 2 shows schematically a monitor unit according to one embodiment of the invention; Figure 3 shows schematically an associated target unit; Figure 4 shows in greater detail an element of Figure 2; Figure 5 shows in greater detail an element of Figure 3; Figure 6 shows in greater detail further elements of Figure 2; Figure 7 shows in greater detail further elements of Figure 3; Figure 8 shows schematically the structure of a message according to the invention; Figure 9 shows in greater detail a portion of the message of Figure 8; Figure 10 'shows schematically the structure of transmission within a system according to this embodiment of the invention; Figure 11 shows in greater detail a further element of t 1 - 6 i i j 1 i i 1 i i Figure 3; Figure 12 shows schematically a control unit of Figure 3 or Figure 2; Figure 13 shows schematically the flow of operation of the controlunit of Figure 2; Figure 14 shows schematically the flow of operation of the control unit of Figure 3; Figure 15 shows schematically the application of the invention to movable personal property; Figure 16 shows schematically the application of the invention to child minding; Figure 17 shows schematically the application of the invention to electronic detention; Figure 18 shows schematically the application of the invention to protection of unattended equipment.

Refering to Figure 1, an alarm system comprises a monitor unit 10 and at least one target unit 20A, 20B, 20C, 20D. The monitor is equipped with a signal transducer 12, as are the target units 20A, 20B, 20C, 20D (22A, 22B, 22C, 22D respectively). The monitor unit 10 receives signals transmitted by the or each target unit 20A, 20B, 20C, 20D in some cordless fashion; although infra-red or ultrasonic transmission could be used, it is preferred to employ 1 I i 1 1 I 1 - 7 radio communication since this gives greater immunity to intervening obstructions such as walls, doors and persons. Especially preferred are sub-microwave frequencies, at which the communications are substantially omnidirectional so the monitor unit does not need to be aligned towards the target unit.

Referring to Figure 2, in one embodiment of the invention the monitoring unit 10 comprises a radio frequency antenna 12 connected to receive a radio frequency transmission signal from a radio frequency transmitter unit 14. The antenna 12 is also connected to a radio frequency receiver unit 16 (a separate receive antenna is not required for reasons discussed below). From the received signal, a range assessing unit 18 generates a signal dependant upon the distance between the target unit 20 and the monitor 10. A signalling unit 23 is arranged to control the radio frequency transmission unit 14, to produce a modulated radio frequency signal carrying information to the taget unit 20. The signalling unit 23 is preferably also connected to derive from the r-adio frequency receiver unit 16 information transmitted by the target unit 20.

J A control unit 24 is connected to the range assessing unit 18 so as to receive the range dependent signal and, if necessary, generate an alarm condition. Preferably the alarm condition is signalled acoustically by an electro-acoustic transducer 26 (for example a piezo electric tweeter loudspeaker) driven from the control unit 24, and/or visually on a display screen 28 (for example, a liquid crystal display) driven from the control unit 24.

A user input 30, for example a keypad or switch is provided to allow a user to control the state of the control unit 24 (for example, to disable the alarm or to reset the alarm). A power supply unit 32 such as a battery provides electrical power to the monitoring unit 10.

Referring to Figure 3, a target unit to operate with the monitor unit of Figure 2 comprises a radio frequency antenna 22 connected to a transponder unit 34 comprising a radio receiver and a radio transmitter. The receiver and transmitter of the transponder 34 are connected to a signalling unit 36 arranged to derive from the received radio signal information transmitted by the monitor unit 10. Where i i 1 i i 1 i the monitor unit 10 is arranged to receive signals, the signalling unit 36 of the target unit 20 is arranged to transmit signals by modulating the transmitter within the transponder 34.

A control unit 38 is connected to the signalling unit 36 and is arranged in response to a detected received signal to take actions discussed in greater detail below. Where the signalling unit 36 is also capable of transmitting signals, the control unit is arranged to actuate the signal unit 36 so to do.

Connected to the control unit 38 is an alarm indicating device 40 preferably comprising an electro-acoustic transducer 40 (for example a piezo electric tweeter loudspeaker). In preferred embodiments, the target unit 20 includes a sensor 42 for example a vibration or motion sensitive element such as a mercury switch, to allow the control unit to generate an alarm or transmit an alarm signal in response to a sensor output. A further input socket or port 44 may be provided to allow the connection of external sensors for the same purpose.

f A power supply unit 46, for example a battery, provides electrical power to operate the target unit 20; preferably, the control unit 38 is connected to the power supply unit 46, or to the supply lines therefrom, so as to be able to interrupt or reduce the power supply to part or all of the target unit 20, so as to allow power saving whilst the unit is not in use. A manual enable/disable switch (not shown) may also be provided.

Referring to Figures 4 and 5, the operation of the range assessing unit 18 of the monitor unit 10, together with the radio transmitter and receivers 14, 16 and the transponder 34 of the target unit 20, will now be discussed in greater detail. The range assessing unit preferably use s a continuous wave frequency modulation technique. In this technique a carrier is frequency modulated with a time varying modulation signal and transmitted towards a target. The signal returned from the target is received and mixed with the transmitted signal to give an output which is dependent upon the round trip delay, and hence the distance between the monitor unit 10 and the target. However, a problem which occurs with this technique is that even if separate transmit and 1 1 i i 1 receive antennas are employed, the signal from the transmitter can leak to the receiver when sub-microwave frequencies are employed (which is desirable to reduce the directivity of the monitor unit 10). Instead of relying upon reflection or retransmission of the same signal frequency from the target unit 20, therefore, in this embodiment of the invention the transponder 34 of the target unit 20 is arranged to receive the transmitted signal and transmit back a frequency shifted signal. The frequency shift enables the received signal to be separated electrically from the transmitted signal, and thus allows the use of a common antenna 12, 20 for both transmission and reception.

Accordingly, the radio transmitter 14 comprises a radio frequency amplifier operative at frequencies around a few hundred megahertz (sufficiently low that the signal is transmitted substantially nondirectionally by the antenna 12) and the receiver 16 is correspondingly a radio frequency amplifier in the same range. In the UK, the 173 MHz and 450 MHz bands are available for such use. The carrier frequency supplied to the transmitter amplifier 14 is generated by a radio frequency oscillator 48. The i - 12 frequency at which the oscillator 48 operates is determined by a control voltage applied thereto from the output of a modulation oscillator 50 operating at a relatively low modulation frequency (around 1 kilohertz). The output of the modulation oscillator 50 is sinusoidal. The peak magnitude of the modulation oscillator 50 output determines the frequency excursion /\ f of the carrier frequency generated by the carrier frequency oscillator 48.

The signal transmitted by the monitor unit 10 via the antenna 12 therefore comprises a signal of frequency F; F f + /\ f sin[2-xf tl tx TX o.. mod This signal is received by the antenna 22 of the target unit 20, amplified by a radio frequency receiver amplifier 52 and fed to a single sideband mixer 54 where it is mixed where an offset frequency generated by an offset frequency oscillator 56. The offset frequency is large enough to prevent instability due to either the receiver 16 of the monitor unit 10 or that of the target unit 20 picking up its own transmitted signal, but otherwise its i 1 1 i 1 i 1 i i i 1 i 1 i i 1 1 i 1 1 E i i is - 13 signal is not of great importance; for example, for a transmission frequency Ftx of a few hundred megahertz, the offset frequency might typically be 100 kilohertz. The output of the mixer 54 therefore comprises a signal at a frequency of; f f +f Cf (t-2r/c-t RX o offset+ A.fsin[2T mod d This signal is received, amplified by a radio frequency transmitter amplifier 58 and transmitted via the target unit antenna 22.

Upon re-reception of the signal from the transponder 34 by the antenna 12 of the monitor unit 10, the received signal F rx is amplified by radio frequency amplifier 16 and supplied to a single sideband mixer 68 which is connected also to the output of the carrier frequency oscillator 48, and produces a corresponding difference frequency output of; f X =f TX- f RX 1 - 14 =f offset+2 /\ fsin[2'J-( f mod (r/c+t d /2)]cos[2 f mod( t-r/c-t d /2)] using standard trigonometric identities. This corresponds to a carrier signal of frequency foffset which is frequency modulated by the sinusoidal modulation f mod produced by the modulation oscillator 50. The modulation is subject to a (negligibly small) phase shift (r/c-t d /2) with respect to the output of the modulation oscillator 50, and has a peak deviation (FM modulation amplitude) of; \ f out" 2 /\ fsin[2-Wf mod (r/c+t d /2)l Since the bracketed term is much less than unity, the value of the sign tends to the value of the bracketed term and consequently; L\ fout=47r/\ ff mod (r/c+t d /2) 1 i i Thus, the degree of frequency of modulation of the signal generated by the mixer 68 is directly proportional to the range to the target unit. This is therefore the basis on which the range assessing unit 18 detects the target range.

The degree of frequency modulation is extracted by passing the output of the mixer 60 to a limiter/discriminator circuit 70 having a gain K d V/hertz, which accordingly generates an output sinusoidal signal at the modulation frequency having an amplitude proportional to the range R. This signal is fed to a synchronous demodulator 64 (ie a mixer) in which it is mixed with the output of the modulation oscillator 50, and passed through a low pass filter 66 to remove the double frequency component, to give a DC output signal V 0 which is proportional to the range between the monitor unit 10 and the target unit 20; V =K 4-K /\ ff (r/c+t /2) o 'Y D mod d The offset error term due to t d is compensated by calibration or substraction of corresponding voltage, and the range voltage signal generated is supplied to the control unit 24.

j 14, The operation of the signalling units 23, 36 of the monitor unit 10 and target unit 20 will now be explained in greater detail. Although it would be possible to signal during the range assessing operation, this complicates the demodulation required and it is consequently preferred that the results of range assessment whilst signalling is occuring are not acted upon by the control unit 24, or that range assessment is not performed during signalling.

1 It would be possible to employ a separate signalling carrier frequency oscillator, or to couple a separate signalling modulator to the carrier frequency oscillator 48, but in practice it is found convenient to employ, as the modulation oscillator 50, a controllable oscillator and to generate a signal by frequency shift keying the modulation oscillator 50 (by applying a two level signal to its control input).

Referring to Figure 6, the signalling device 23 therefore comprises a device for generating a pulse train; typically a train of binary bit values. The signalling unit 23 may therefore comprise a signal transmission unit 23A including a message read only 1 1 i 1 i i i i C i 1 memory (ROM) containing a plurality of sequentially addressed bits of a message to be signalled, arranged in (typically) 8 bit words; an address circuit 60 coupled to the address bus of the ROM 68 and typically comprising a counter arranged to generate sequential addresses within the ROM; a parallel-in-serial-out device or shift register 72, coupled to the data bus of the ROM 68 so as to receive the word addressed by the address circuit 70 and product a corresponding bit stream output which is supplied to the control input of the modulation oscillator 50 at a suitable clock rate of, typically, a few hundred bits per second (for example 400 bits per second).

Referring to Figure 7, in the target unit, the transponder 34 comprises a message receiver comprising a limiter/discriminator 74 equivalent to that 62 in the monitor unit, connected to receive the amplified received signal from the radio amplifier 52. The output of the limiter/discriminator 74 therefore comprises a signal corresponding to the output of the modulation oscillator 50 of the monitor unit 10, and is correspondingly frequency shift keyed. The modulating digital signal is therefore recovered by passing this signal to a phase locked loop 76 the - 18 oscillator of which operates within the same range as the modulation oscillator 50 of the monitor unit 10, so that the output of the phase locked loop 76 comprises the binary bit stream generated at the monitor unit 10. This bit stream may be decoded to provide a parallel word output as discussed below in greater detail.

Referring to Figure 8, although synchronous communications could be used, it is preferred to employ asynchronous communication for the signalling, for simplicity. Accordingly, as shown in Figure 8A, a message transmitted by the monitor unit 10 comprises a synchronization portion 100 to allow the signalling unit 36 of the target unit 20 to synchronize its data recovery timing, and thereafter the data portion 102 making up the message.

ReferrIng to Figure 8B, the synchronization portion 100 comprises firstly a train 100A alternating mark and space bit levels, provided to allow the target unit 20 to derive sampling times. For example eight or so bit reversals may be provided. Subsequently, a start word 100B is transmitted to alert the target unit 20 to the initiation of the message; one i i i i i i 1 1 1 f i k, 19 suitable type of start word is the type having good autocorrelation properties (which, in this context means that the word is dis-similar - ie has a high signal distance - from time shifted versions of itself and consequently has low autocorrelation). A start word of length 16 bits is adequate.

is Referring to Figure 9, the data 102 comprising a message preferably contains one or more of the following fields;

An address field 102A, comprising an address uniquely identifying the target unit. An address unique within the target units of a single monitor unit is advantageous to allow one monitor unit to control several target units without the ranging or other signals from the target units colliding or interfering. The address word enables a target unit not corresponding to that address to take no further action in response to the received signal. An address word which is unique not only amongst the target units of a single monitor unit but amongst all other target units likely to be within the same area, is even more advantageous, since it enables the use of one or more monitor units within the same physical space without mutual intereference. This is especially useful in environments such as airport lounges where several passengers may wish to protect items of luggage. A thirty two bit address, for example, is sufficient to uniquely identify 2 32 = 65,536 target units. It may be advantageous to provide that the- address code comprises a prefix identifying the monitor unit 10 to which the target unit is assigned.

The message store 68 of the monitor 10 must, in this embodiment, contain the address words of all the target units which it is desired to monitor. Monitor and target: units may be sold together, or means may be provided for writing or registering the aCdress of a target unit into the monitor unit 10.

An "off time" code 102B indicates a period of time following the message in which the target unit 10 need expect no further message. The target unit 10 may consequently switch off all reception and transmission and other activities until the sleep time has elapsed. The "off time" word may comprise, for example, an eight bit number denoting a number of 10 millisecond 1 1 1 f 1 i 1 1 1 1 1 1 i 1 1 1 i 1 1 1 f 1 7.

- 21 time slots so as to specify a period between zero and 2.5 seconds before the next message.

A cyclical-redundancy check field 102C comprises a check sum (for example a 16 bit check sum) which enables the target unit 10 to check whether errors have ocurred in the preceding message transmission. The field 102C may employ an error correcting code. If an (uncorrected) error is detected, by the target unit 20, the preceding message is ignored.

The format for example, a one bit of a message may include further fields; an alarm indication field (which could be flag position) may be provided which, if set, causes the generation of an alarm at the target unit. Other commands, for example, instructing the target unit to switch off its power supply where required, may similarly be denoted by one bit flag fields in the message format. The position of the various fields is a matter of convenience, except that the error check field 102C should be the last, and instead of employing fixed format messages, each field may be prefixed by a field identifier code.

- 22 It is possible to provide a single word combining the start word and address words.

Referring to Figure 10, after the message has been transmitted it is convenient to transmit the modulated carrier generated by the range assessing unit 18 of the monitor unit 10. If the message field includes an address field 102A, the target unit 10 will be arranged only to transpond the ranging signal if the address transmitted is correct for that target unit (and, if the error check field 102C is present, if no errors are indicated).

Referring to Figure 11, in the target unit 10, the signal produced at the output of phase locked loop 76 is fed to.a timing circuit 110 and a sampling circuit 112 arranged on receiving the bit reversals signal 100A to derive the sampling intervals, and to derive points within the intervals at which to sample the signal (timing between adjacent rising and falling edges). Successive signal samples from the sampler 112 are fed to the input of a shift register 114 of length at least that of the start word, to the taps of which a logic comparison circuit 116 is connected so as to compare the received signal with the start word and i 1 i 1 i 1 i i 1 i 1 1 i 1 23 - hence detect the occurance of a start word. When the start word is detected, the comparison circuit 116 enables the connection between the output of the shift register 114 and a serial in parallel out register 118, the parallel output of which is fed to the control unit 38 of the target unit 20.

The signalling unit 36 in the target unit 20 may also be arranged to transmit a message back to the host unit 10. Where this is desired, the signalling unit 36 will,. include means for modulating the re-transmitted signal; typically, the signalling unit 36 includes a signal generating unit functionally equivalent to that 23A of the monitor unit 20, coupled to modulate the offset frequency modulator 56 (as shown in Figure 5) which in this embodiment comprises a controllable oscillator.

To decode messages from the target unit 10, which will typically be of the form shown in Figure 8, the monitor unit signalling unit 23 includes a signal receiving circuit 23B comprising a phase locked loop 124 receiving the output of the limiter/discriminator circuit 62 and generating a response a binary output in the same manner as the circuit 34 of Figure 7. The - 24 binary output is decoded by a circuit equivalent to that of Figure 11.

Alternatively, rather than providing a message transmission circuit at the target unit 10 anda message reception circuit 23B at the monitor unit 20, an alarm condition at the target unit 10 may be' signalled by simply providing that the target unit does not transpond the ranging signal transmitted from the monitor unit 10. This method is fail safe in that the monitor unit 10 will respond not only to a thus indicated alarm condition, but also to the circumstance where the target unit is inactive (due perhaps to power supply failure) and consequently unable to transpond. Likewise, the target unit 20 control unit 38 may be arranged to generate an alarm via the loudspeaker 40 if no transmissions received froni the monitor unit 10, a malfunction in the monitor unit 10.

are so as to indicate Referring to Figure 12, the control unit 30A (and the control unit 24) comprises a central processing unit (CPU) 120 and a memory device 122 including stored program sequences for operating the central processing unit 120, and associated data.

i 1 1 1 1 i i i 1 i 1 i 1 i k Referring to Figure 13, the operation of the control unit 24 in a preferred embodiment of the invention incorporated the above features will now be described. In this embodiment, the control unit 24 comprises, as shown in Figure 12, a central processing unit 120 and a memory 122. The memory 122 includes the message memory 68 of the signalling circuit 23A shown in Figure 6 and the central processing unit 120 functions as the address circuit 70. The control unit comprises also an analogue to digital converter 124 (for converting the output of the range assessing circuit 18 to a digital signal. The control unit 24 may comprise a commercially available 8 bit microprocessor or microcontroller device, preferably a CMOS device to reduce powdr consumption.

On commencement of operation, the monitor unit 10 selects a first target unit 20A and enables the power supply to the radio system 14, 16 and range assessing unit 18. The control unit 24 then accesses the memory 68 and determines the unique address of that target unit 20A. The control unit 24 then determines an off time for that target unit 20A, which sets the time for which that target unit will be inactive until the next transmission. This time is preferably not constant 1 1 for all target units and preferably varies over time for each target unit; a pseudo random number generator may be employed to derive variable off time lengths and to further ensure variation between times for different target units 22A 22D. The number may be generated in dependence upon the (unique) address of the target unit. This ensures that different monitor units in the same area do not produce the same random number sequence and hence cause repeated collisions. The use of essentially random spaced bursts, preferably derived in dependence upon a unique identifier for each target unit, thus greatly limits the possibility of collision between signals from ajacent monitor units within the same area.

The off time derived is added to an indication of the present time derived from a clock within the control unit 24, and the result (indicating the time at which that target unit 20A will next be addressed) is stored by the control unit 24.

The bit reversal synchronization signal and the start word (102A, 102B) are then read from the memory 68 and supplied for transmission, bit by bit, to the signalling unit 23. The address 102A and offtime 102B i 1 i i i 1 1 1 p 1 f f for the target unit 20A are next supplied to the signalling unit 23 for transmission. Finally a cyclical redundancy checkword 102C is calculated corresponding to the message transmitted, and is supplied to the signalling unit 23 for transmission.

Next, the ranging unit 18 is enabled so as to cause the transmission of a modulated ranging signal as discussed above. if the target unit 20A is functioning, a transponded signal should correspondingly be received by the radio receiver 16; if no such signal appears at the output of the mixer 60, the control unit 24 generates an alarm condition and actuates the loudspeaker 26 and display 28, preferably also indicating the identity of the target unit 20A which has given rise to the alarm condition.

The alarm is also signalled to the target unit 20A; this may be achieved simply by terminating the ranging signal prematurely.

if' on the other hand, a transponded signal is received from the target unit 20A, the range signal generated by the range assessing unit 18 is read via - 28 the analogue to digital convertor 124. If the range exceeds a predetermined threshold, or alternatively if the range has changed significantly from that previously assessed (and consequently stored) for that target unit 22A, the control unit 24 initiates an alarm condition which is indicated and signalled as before.

If on the other hand the range assessment does not indicate an alarm condition, the control unit terminates the power supply to the radio system 14, 16 and range assessing unit 18 (and any other power consuming components other than the control unit 24 itself) and selects the next target unit 20B. The control unit then reads the current time from the system clock, accesses the stored time for that target unit 20B, and compares the two. When they become equal, in other words, when the target unit 22B is expecting ' communication, the control unit 24 re-commences the process of Figure 13.

Referring to Figure 14, the operation of the control unit 38 of a target unit 20 will now be explained in greater detail.

1 1 1 i i i 1 i i 1 1 - 1 1 - 29 The radio receiver 52 of the target unit 20A is initially switched on and when a transmitted signal is detected (at the point at which the start word lOOB is detected), the control unit 38 reads the following received message bits as the target unit address 102A and compares them with its stored target unit address. If the two do not correspond, the control unit 38 ceases to read further bits from the signalling unit 36 and awaits the next transmission from the monitor unit 10.

If the received address matches the target unit address, the following bits are read as the time for which the target unit is to remain off subsequently. The check sum 102C transmitted from the monitor unit 10 is then compared with the address and offtime data received. If an error (or, in the case of an error correction code, an uncorrectable error) is indicated then the control unit 38 takes no further action and waits for the next transmission from the monitor unit 10. The control unit 38 next reads the states of the sensors 42,- 44 to determine whether an alarm has occured; preferably the control unit 38 receives the sensor output lines at latched inputs so that transitory activation of the sensor 42 is latched. If the sensors do not indicate an alarm condition, the control unit 38 enables the operation of the transponder'unit 34 to re-transmit the received signal with a frequency offset; otherwise, if an alarm condition oes exist, the control unit 38 inhibits this and indicates an alarm by activating the loudspeaker 40.

The control unit 38 monitors the length of the transmission from the monitor unit 10 and in the event that the transmission is prematurely truncated, indicating an alarm condition sensed by the monitor unit 10, the control unit 10 likewise indicates an alarm condition by activating the loudspeaker 40. Otherwise,' the control unit 38 switches off the transponder 34, together with any other power consuming components other than the control unit itself, and waits for the offtime period received before switching on its radio receiver once more to recommence the processor Figure 14.

As indicated above, the target unit 20 could be arranged to transmit signals and the monitor unit 10 to receive' signals rather than relying upon the absence of the ranging signal as an alarm indication.

i 1 1 1 i I i 1 1 i 1 1 1 - 31 The modifications to the processes of Figures 13 and 14 will be apparent to the skilled man.

is Various other modifications to the above sequence will be apparent to the skilled man. In preferred embodiments, the monitor unit 10 is arranged prior to generating-a transmission, to check whether a signal is being received. If a signal is detected, indicating for example that another monitor or target unit is present in the area, the control unit 24 does not initiate a transmission to a target unit but instead waits for the next transmission. This prevents confusing or garbled measurements due to reception of simultaneous transponded signals from two different target units allocated to different monitor units.

To prevent the monitor unit 10 from generating an alarm condition where a target unit 20 has failed to transpond because it detected an error in the received data, the monitor unit 10 may be arranged to initiate an alarm only after the target unit has failed to transpond on two more separate occasions.

c Accordingly, In another modification, the monitor unit is arranged not to respond to increases of distance which are due to movement of the monitor unit itself. For example, if the monitor unit is carried on the person, the carrier may wish to make a short excursion and later return to the vaccinitye of the target units 20.

in one embodiment the monitor units include means for disabling the alarm under corresponding conditions. The means may comprise a manually operated control, to which the control unit 24 is responsive, to set a period of time during which the monitor unit 10 will continue to interrogate target units 20 but will not generate an alarm merely because of an increase in distance to the target units 20, since this Increase in distance could be due to movement of the monitor unit 10. In another embodiment, where there are multiple target units 20, the monitor unit 10 may respond only to an increase in distance of a single target unit or a subset of the total number of target units, on the basis that if the distance to all target units is increasing this is probably due to movement of the monitor unit 10 rather than of the target units 20.

1 1 1 i 1 i 1 i Q' In other embodiments, the monitor unit control unit 24 is arranged to memorize a previous target unit range signal and to generate from the previous target unit range signal and the present target unit range signal a different signal representating a change in range to the target unit, and to be responsive to changes therein.

Referring to Figure 15, one application of the invention is in the protection of portable possessions such as briefcases, luggage, handbags, personal organisers, cash bags or other containers carried with the person. The person carries a monitor unit 10 which measures the distance to the target units 20A, 20B each fitted within or attached to each item to be protected 200A, 200B, and generates an alarm if the distance increases beyond a threshold of, for example, a few meters.

Referring to Figure 16, a further application of the invention is in child minding where the minder carries a monitor unit 10 and the children to be minded carry target units 20A, 20B. An alarm is sounded at the monitor unit 10 if one of children strays so the distance R exceeds a set value.

A further application of the invention is in electronic detention systems where a prisoner or detainee is fitted with a target unit 20 as shown in Figure 17. The monitor unit 10 is positioned in the area in which the detainee is to be confined and in the event that the detainee range R exceeds a predetermined value the monitor unit 10 indicates an alarm condition by signalling via a telecommunications chanel 202. In this case, the target unit 20 is adapted to signal any attempt to remove it or tamper with it to the monitor unit 10, for example by ceasing transponding. The monitor unit 10 responds to this condition by generating an alarm. Additionally, the monitor unit 10 may be arranged to signal an alarm condition via the telecommunications chanel 202 in the event of an attempt to tamper with it; for example, the monitor unit 10 may make periodic transmissions via the telecommunications chanel 202 if no alarm condition exists, and indicates an alarm by ceasing to make such periodic transmissions.

Referring 'to Figure 18, a similar arrangement may be employed to protect unattended equipment against theft vandalism or other tampering; in this case, the equipment 204 is fitted with a target unit 20 and, as 1 1 1 1 i f before, either excessive motionor any attempt to tamper with the target unit and preferably the monitor unit is signalled via the telecommunications channel 202.

It will be appreciated that the invention could also be used in local area positioning and tracking systems by measuring the range to fixed predetermined target units.

I.

- 36

Claims

1. A monitor unit for an alarm system comprising means for generating an alarm condition, means for transmitting a signal to at least one target unit and means for receiving an answering signal therefrom, in which the signal transmitted includes an identifier portion and the monitor unit is arranged to be able to transmit a plurality of different identifier portions corresponding uniquely to different target units.

2. An alarm system comprising a monitor unit according to claim 1 and a plurality of target units each target unit being arranged to respond only to a single predetermined identifier portion.

3. A target unit for an alarm system which comprises 'means for receiving a signal, means for comparing a portion of the signal with a stored identifier, and control means for responding to the signal in a first manner if the portion and the identifier co-incide and in a second manner if they do not.

1 i i i i 1 1

4. An alarm system comprising a first unit arranged to transmit a signal to a second unit spaced therefrom, in which the signal includes a portion indicating a time period and the second unit is arranged to be inactive after receipt thereof for a corresponding period.

5. A first unit for a system according to claim 4, the unit being arranged to generate successive signals representing differing time periods.

6. A unit according to claim 5 arranged to generate said time periods on a substantially pseudo-random basis.

7. A unit according to claim 5 or claim 6 arranged to generate time period for a plurality of signals to different second units, the length of each time period being determined in accordance with the identity of the corresponding second unit.

8. A second unit for a system according to claim 4 comprising means for receiving a signal, means for determiningtherefrom a time period and means for reducing the power consumption of the device within that period.

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9. A second unit for a system according to claim 4 comprising means for receiving a signal, means for deriving therefrom a time period, means for transmitting a signal, and means for inhibiting the transmission of such signals during the time period.

10. A distance measurement method from a monitor unit to a target unit using continuous wave frequency modulation, in which an active target unit is used which receives a signal from the monitor unit, shifts it in frequ: ency and re-transmits it, and in which the modulation signal is used to provide a signalling channel between the monitor unit and the target unit.

11. A distance measurement method as claimed in claim wherein the signalling channel means is used to allow multiple targets to be addressed by a single monitor unit, or to provide for co-located operation of multiple monitor units on the same frequency.

12. A method according to claim 10 or claim 11 further comprising the step of generating a distance related alarm from said measurement.

13. Application of the invention as claimed in any i i 1 i 1 1 i i i 1 i 1 i j 1 1 1 t one of claims 10, 11 and 12 to the protection of personal portable possessions, for example including briefcases, luggage, handbags, personal organisers, cash-bags and other containers.

14. Application of the invention as claimed in any one of claims 10 to 12 to child straying alarm.

15. Application of the invention as claimed in any one of claims 10 to 12 to an electronic detention system.

16. Application of the invention as claimed in any one of claims 10 to 12 for the protection of unattended goods, pieces of equipment or belongings.

17. A method of operating an alarm system which comprises spaced monitor and target units in two way communication, comprising providing periodic communication bursts at irregular intervals.

18. A method of operating an alarm system comprising spaced monitor and target units in communication, comprising providing communication in bursts and reducing power consumed by said units between bursts.

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19. A unit, system or method according to any preceding claim in which communication is at radio frequencies.

20. A unit, system or method according to claim 19 in which said frequencies are sufficiently low that the response of a given unit is substantially omnidirectional.

Published 1992 at The Patent Office. Concept House. Cardiff Road, Newport. Gwent NP9 IRH. Further copies mav be obtained from Cwrnfelinfach. Cross Keys. Newport, NP I 7RZ. Printed by Multiplex techniques ltd. St Marv Cray. Kent.

Sales Branch. Unit 6. Nine Mile Point.

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