GB2313980A

GB2313980A - Anti-jamming radio-type burglar alarm

Info

Publication number: GB2313980A
Application number: GB9611699A
Authority: GB
Inventors: Walter John Aston; Brian M Stringer
Original assignee: Yale Security Products Ltd
Current assignee: Security Products UK Ltd
Priority date: 1996-06-05
Filing date: 1996-06-05
Publication date: 1997-12-10
Anticipated expiration: 2016-06-05
Also published as: GB2313980B; GB9611699D0

Abstract

A radio-type burglar alarm system includes at least one detector/transmitter 11,12 and a receiver 10. The receiver 10 comprises a jamming signal detector 15 and a CPU 14 which includes a counter for accumulating a count of the cumulative time duration during which jamming signals are detected, comparing means for comparing this count with a threshold level and resetting means for resetting the counter only after a predetermined period starting at the commencement of jamming detection and of a duration longer than the duration represented by the threshold level. When triggered the detector/transmitter transmits a series of randomly spaced signals spread over a period exceeding the above, predetermined threshold level. The receiver is arranged to ignore transmissions from the detector/transmitters for at least the predetermined threshold duration after the alarm has been set.

Description

RADIO-TYPE BURGLAR ALARM SYSTEMS This invention relates to radio-type burglar alarm systems, i.e. systems in which there are a plurality of detectors which are used to detect breaches of security and each detector includes a radio transmitter which transmits signals when a breach is detected, and a receiving and monitoring station which receives the radio signals.

British Standard Specification BS6799 which relates to such systems calls for an antijamming provision to be made in the receiver whereby a fault indication is produced by the receiver if a continuous blocking or interfering signal is received for 30 seconds. System manufacturers generally provide for such fault indication to trigger off the alarm, but this facility can be inhibited if the system is operating in an environment such that the prevailing interference level can cause frequent false alarms.

The simplest arrangement for meeting the BS6799 antijamming requirement is to provide a counter which is started when an interfering signal is detected and reset when such signal ceases. If the counter reaches a count representing 30 seconds the fault indication is triggered.

However, to conserve the batteries of the transmitters these are designed to transmit signals intermittently and jamming signals could theoretically be transmitted intermittently whilst still blocking the legitimate signals without ever causing the counter to reach its fault indication threshold.

It is one object of the present invention to provide a receiver for a radiotype burglar alarm system in which this disadvantage is overcome.

In accordance with this aspect of the invention there is provided a receiver for a radio-type burglar alarm system which includes jamming signal detecting means, counter means for accumulating a count of the cumulative time duration during which said jamming signal detecting means detects the presence of a jamming signal, comparing means for comparing the count with a threshold representing a predetermined threshold duration, and resetting means for resetting said counting means only after a predetermined period starting at the commencement of detection of jamming and of duration longer than said predetermined threshold duration.

Where, for example, the predetermined threshold duration is set to 30 seconds (the BS6799 limit), the predetermined period could be one minute, in which case the count threshold will be reached if jamming is detected for a total of 30 seconds in one minute, or 2 minutes, in which the count threshold will be reached if jamming is detected for a total of 30 seconds in two minutes. The BS6799 requirement is met and the risk of signals being jammed by intermittent jamming transmission is reduced significantly.

It is current practice for any transmitter which is triggered to send four groups of encoded signals with a randomly selected delay between 125mS and 2 seconds. Each encoded signal includes sixteen repeats of a 72 bit code. The maximum period over which signals are transmitted intermittently after triggering is therefore about six seconds, whereas 30 seconds of jamming is required to generate a fault condition at the receiver.

In accordance with another object of the invention there is provided a radio-type burglar alarm system including at least one detector/transmitter and a receiver, said receiver including a jamming detection means which signals a fault condition if a jamming signal is detected for a predetermined threshold duration, said detector/transmitter operating when triggered to transmit a series of randomly spaced encoded signals spread over a period exceeding said predetermined threshold duration, and said receiver including initial inhibiting means such that it does not act on transmissions received from the transmitter for at least said predetermined threshold during following setting of the alarm system.

In the accompanying drawings: Figure 1 is an overall diagram of a radio-type burglar alarm system; Figure 2 is a simplified block diagram of a receiver/alarm forming a part of the system; Figure 3 is a flow chart of a jamming detection routine employed in the receiver of one example of a system in accordance with the invention; Figure 4 is a flow chart of an alarm transmission routine used in the transmitter of the exemplary system, and Figure 5 is the flow chart of a start-up routine used in the receiver.

Referring firstly to Figure 1 the system shown includes a receiver/alarm module 10 and two remote transmitter units 11, 12 one of which incorporates an infra red movement detector of known form including a pyroelectric transducer behind a faceted lens, and the other includes a door switch. Any number of devices of either type may be used in the system. The transmitters are FM transmitters operating at a 418MHz centre frequency as is usual and each has its own code assigned to it which it transmits repeatedly, as explained hereinafter, when triggered.

Figure 2 is a block diagram of the receiver. This includes an RF stage 1 3 with an appropriate tuning arrangement and a demodulation circuit enabling received data to be extracted and passed to a CPU 14. To detect attempts to jam the receiver, the RF stage 13 also provides output to an interference/jamming signal detection circuit 15. The output of this is sampled by a sampling circuit every 100mS to assess whether interference or jamming signals are present. The samples (which are binary signals) are passed to the CPU.

The routine used in the CPU to analyse the signals from the jamming detection circuit is shown in Figure 3. The output of the jamming detector is sampled every 100mS (20). If jamming is present (21), the state of a timer is examined (22). If the timer has not been started, it is started (23) and in either case a sample counter is incremented (24). If the sample count has reached a value N corresponding to the threshold duration (25) a fault is signalled (26) and the routine is finished. The number N is in the region of 300 so that if this number is reached in consecutive cycles, 30 seconds of continuous jamming will have been detected. The value of N may be set lower to give a shorter duration when the receiver is used in an environment where interference is uncommon. If the sample count has not reached N a 100mS delay (27) is introduced before the next sample (20).

If step 21 indicates that no jamming is present, the timer is checked (28) to see if its set period has expired. If the timer period has expired the timer is reset (29) and the sample counter is reset (30) before the next sample in 100mS. If the timer period has not expired the timer and the counter are not reset.

The routine therefore monitors the jamming detector output until the sample count reaches its threshold value within the timer period. The timer period is set to duration longer than the predetermined threshold duration (30 seconds) and could be one minute or more. Intermittent jamming over the timer period can therefore be detected.

Turning now to Figure 4, the routine shown therein is employed in each transmitters CPU to control the sending of coded signals when the associated detector is triggered. On triggering, a timer is started (40) and a packet count PCOUNT is set (41) to zero. The first of ten packets of digitally coded signal is then sent (42), the code being a 72-bit code (identifying the transmitter) sent 16 times in each packet. PCOUNT is incremented (43) and a delay count DCOUNT is set (44) to zero. A randomly selected delay value is set in step 45. This delay can be up to 16 counts for the first phase of operation during which four packets are to be sent within six seconds of triggering. For the second phase of operation, during which another four packets are to be sent during 24 seconds, the delay can be up to 48 counts. For the third and final phase, during which another two packets are to be sent within five seconds, the delay can be up to 12 counts.

If PCOUNT is less than 5 (46), DCOUNT is incremented (47) and DCOUNT is then compared with Q, the delay. If DCOUNT is equal to Q, the routine loops back to step 42 and another packet is transmitted.

If not the routine loops back to decision 46 after a 125mS delay 49.

When PCOUNT is 5 or more in step 46, decision 50 is implemented to test whether PCOUNT is less than 9. If it is then the timer is checked to see if the first phase has yet ended. If the first phase has not ended (i.e.

six seconds have not expired) the routine returns to decision 46 after a 125mS pause (49). If the first phase has ended, DCOUNT is incremented, DCOUNT is checked against Q and so on as in the first stage.

When PCOUNT is 9 or more, a check is made in decision 52 to establish whether PCOUNT has reached 10. If it has, all packets have been sent, and the routine ends. In the case of an infra red movement detector the transmitter is inhibited for three minutes or until no more movement is detected in the zone protected as in the conventional system. If PCOUNT has not reached 10 the timer is checked (53) to see if the final phase has started (i.e. 30 seconds have elapsed), the further packets being sent at the appropriate time.

Since the total period during which packets are transmitted exceeds 30 seconds, the likelihood of intermittent jamming of the receiver blocking all of the randomly spaced packets is very small.

To allow users to escape from a protected zone after setting the alarm system in its active state, the routine shown in Figure 5 is used in the receiver. This inhibits input for 30 seconds after setting and again for 30 seconds after opening the final entry door.

It will be understood that the packet structure, the number of packets to be transmitted in each phase and the number of phases can all be varied by simple changes to the receiver CPU software and the values given are by way of example only.

Claims

1. A receiver for a radio-type burglar alarm which includes jamming signal detecting means, counter means for accumulating a count of the cumulative time duration during which said jamming signal detecting means detects the presence of a jamming signal, comparing means for comparing the count with a threshold representing a predetermined threshold duration, and resetting means for resetting said counting means only after a predetermined period starting at the commencement of jamming detection and of duration longer than said predetermined threshold duration.

2. A radio-type burglar alarm system including at least one detector/transmitter and a receiver as claimed in claim 1, said detector/transmitter operating when triggered to transmit a series of randomly spaced signals spread over a period exceeding said predetermined threshold duration, and said receiver including initial inhibiting means such that it does not act on transmissions received from the transmitter for a least said predetermined threshold duration following setting of the alarm system.

3. A radio-type burglar alarm system including at least one detector/transmitter and a receiver, said receiver including a jamming detection means which signals a fault condition if a jamming signal is detected for a predetermined threshold duration, said detector/transmitter operating when triggered to transmit a series of randomly spaced signals spread over a period exceeding said predetermined threshold duration, and said receiver including initial inhibiting means such that it does not act on transmissions received from the transmitter for a least said predetermined threshold duration following setting of the alarm system.