GB2340647A - Duel sensing intruder alarm system suitable for a vehicle - Google Patents

Abstract

A security system is disclosed which is suitable for a vehicle 10. The security system comprises a microwave transducer 16 which is arranged to provide an intrusion signal which is indicative of the presence of an intruder in the protected area 10. The intrusion signal is arranged to cause the activation of an active infra-red transducer 18, 20 which is used to check the protected area and to confirm whether the intrusion signal of the microwave transducer 16 is a false alarm. A security controller 12 is arranged to only activate an alarm 14 if the active infra-red transducer 18, 20 confirms the validity of the intrusion signal of the microwave transducer 16.

Description

2340647 - 1 A Security System This invention relates to security systems

and in particular to a security system which is suitable for use in a vehicle.

It is known to provide a security system which includes a detection means arranged in use to provide an intrusion signal which is indicative of an intruder in an area protected by the security system. Some such systems are used to protect vehicles and it is known to use a microwave transducer or an ulrasonic transducer as the detection means.

It is a problem with some such systems that they are sometimes known to trigger an alarm falsely. For example, microwaves generated by a vehicle volumetric microwave alarm are, in general, not restricted to the interior of the vehicle. Significant amounts of the signal can be passed through glass panels, such as windows or a sunroof, and this can result in false triggering, especially from the movement of large metallic surfaces such as lorries or garage doors near the vehicle.

Simply reducing the sensitivity of the vehicle transducer to eliminate false alarms tends to unacceptably reduce sensitivity to the movement of relatively less reflective human bodies, which may in turn result in an intruder not being detected.

In order to overcome the above problem, some manufacturers use active infra-red transducers instead of microwave sensors. These transducers emit an infra-red signal and look for changes in amplitude of the reflected signal. These infra-red transducers have their own problems, such as saturation by 5 bright sunlight and false alarms caused by insects landing on sensors and -effects caused by other sources of infra-red such as other infra-red remote controls like television controllers. If an area or object is left under the protection of an active infra-red system, quiescent current drain might cause pnoblems and so many systems which use infra-red transducers as primary transducers use a passive infra-red arrangement instead of an active one.

In an attempt to over-come the false alarming problems described above, it is known to use a second transducer to check the output of the primary transducer.

One system which uses a checking technique employing a second transducer is disclosed in US 4,882,567. The primary transducer is a passive infra-red transducer and, if it appears that there is an alarm situation, a microwave transducer is activated as a checking means. Because a microwave transducer needs to settle before its output can be trusted, such an arrangement suffers from a settling time delay between the initial (infra-red) alarm and any (microwave) confirmation signal.

A further example of transducer checking techniques employing infrared and microwave transducers is disclosed in WO 88100747. Intruders are again detected using passive infra-red techniques and the checking is performed using further infra-red or microwave transducers. In this arrangement, however, the checking transducers appear to be permanently powered-up so settling time is not such an issue. Such an arrangement is not suitable for use in a vehicle because it might consume too much power if the vehicle were to be left unattended for a long period.

1 It is an object of this invention to provide an improved security system which is suitable for use in a vehicle.

According to the invention there is provided a security system suitable for a vehicle, the system comprising a detection means arranged in use to provide an intrusion signal which is indicative of the likely presence of an intruder in a protected area, the system further comprising a checking means which is arranged in use to be activated substantially on or by the provision of a said intrusion signal and to provide a confirmation signal which is indicative of detection by the checking means of an intruder in the protected area, wherein the checking means comprises an active infrared transducer.

A light emitting stage of the active infra-red transducer may be driven with a high current and the current may be in the order of 300 mA.

The checking means may be activated for a time slot of a predetermined duration and the time slot may be in the order of 10 seconds. In the alternative, the checking means may be activated for a time slot of a duration which is variable between activation periods of the checking means.

In any case, the time slot may be of a minimum duration which is sufficient for the checking means to settle into proper operation and to then perform at least one detection scan of the protected area. The checking means may be activated for an initial time slot so that it can capture an initial level of a confirmation signal. The initial time slot may be in the order of 50 gs.

The checking means may be re-activated during a predetermined later re-check period after the initial time slot and the confirmation signal detected in that later re-check period is compared with the confirmation signal captured in the initial time slot, and wherein the security system is arranged to determine that an intruder is present in the protected area only if a difference threshold is exceeded between the initial confirmation signal and the confirmation signal captured in the later re-check period.

The detection means may, for example, comprise an ultrasonic transducer or a microwave transducer.

A vehicle is also provided which includes a security system according to the invention.

The invention will now be described by way of example with reference to the accompanying drawings, in which:

Figilre 1 is a schematic diagram of a vehicle including an alarm system according to the invention; Figure 2 is a simplified circuit diagram of part of the security system of Figure 1; Figure 3 is a flow chart showing the operation of the security system of Figure 1; and Figure 4 is a state transition chart which equates to the flow diagram of Figure 3.

Referring to the figures, a vehicle 10 includes a security system which comprises a security controller 12 which is arranged to selectively operate an alarm siren 14 if it 12 determines that an intruder is present in the vehicle 10.

The security system further comprises a detection means in the form of a microwave movement detection transducer 16 which is arranged to detect the presence of an intruder in the protected area of the vehicle 10 and to provide an intrusion signal to the security controller 12 which is indicative of the detection by it 16 of an intruder.

I The security system further comprises a checking means which is in the form of an active infra-red transmitter 18 and an associated infra-red receiver 20 which are under the control of the controller 12.

If the microwave transducer 16 detects something which appears to be an intruder, it provides the intrusion signal to the controller 12. On receipt of the intrusion signal, a microprocessor 22 in the security controller 12 switches on the active infra-red transmitter 18. The security controller 12 uses the active infra-red transmitter 18 and its associated receiver 20 to check the validity of the intrusion signal initially detected by the microwave transducer 16.

The infra-red transmitter 18 comprises one or more infra-red light emitting diodes LEDs 24 which, because the checking means is only used for checking and is not therefore switched on for long enough to cause a serious risk of excessive power drain, are driven with a high current for example in the order of 300 mA. This results in very high illumination of the interior of the vehicle 10 which allows the infra-red receiver 20 to comprise a de5 sensitised infra-red photo diode 26 whose input passes through an amplifier 28 and into an analogue-to-digital converter 30 of the microprocessor 22.

When the transmitter 18 is first powered up, an initial level of reflected signal is captured. At intervals after that (typically in the order of 100 ms) the transmitter 18 is re-activated during a re-cheeking period and any reflected signal captured during the re-cheeking period is compared with the initial captured value. If a difference threshold which is preset in a memory of the microprocessor 22 is exceeded between the initial level of reflected signal which is captured and the level of reflected signal which is captured in the later re-check periods, the controller 12 is arranged to trigger the alarm sounder 14.

The period for which the infra-red transducer 18, 20 is enabled after receipt of an intrusion signal from the microwave transducer 16 is typically in the order of 10 seconds. After the 10 second period, the infra-red transducer 18, 20 is de-activated until receipt by the controller 12 of the next intrusion signal from the microwave transducer 16.

The reason for turning off the infra-red transducer 18, 20 in between intrusion signals from the microwave transducer 16 is so that the LED 24 can be driven at a higher current than is usual without it overheating. Furthermore, if the microwave transducer 16 and the infra-red device 18, 20 were both to be permanently activated, the quiescent current drain on the vehicle 10 could prove to be unacceptable.

It would be possible to use many known detection means such as a microwave transducer, an ultrasonic transducer, or a light operated device as the initial detection means 16. The use of an active infra-red transducer as the checking means overcomes the settling time problems which might arise if, for example, microwave or ultrasonic transducers were to be employed. The extra power used means that active infra-red sensing provides a more reliable detection source than its passive counterpart, but the extra power consumption does mean that this advantage is best employed as a checking means.where it can be safely assumed that it will only rarely need to be switched on.

It is preferable to use a detection means and a checking means which employ different technologies so as to reduce the likelihood that they will both be falsely triggered or miss a genuine detection for the same reason. If for example, a microwave alarm would trigger falsely once per week and an infra-red sensor once every three hours of accumulated operating time, then a false alarm would only be expected once in about every twenty years.

In a modification to the invention, it would also be possible to use the detection means 16 to trigger the checking means 18, 20 directly and to trigger the alarm 14 directly from the checking means 18, 20 and thereby to produce a system in accordance with the invention in a simple but still effective form.

In a further modification, the period for which the checking means is switched on is varied between its activation periods, and this makes it more difficult for a thief to attack the system. For example, a thief might try to record an infra-red signal which has resulted in the security controller determining that the protected area is safe. He or she might then try to reduce the effectiveness of the security system by covering over the protected area's infra-red transmitter and then, after triggering the microwave transponder, try to replay that captured signal into the protected area's infra-red receiver to try and fool the system into believing that the infra-red alarm has declared the area safe and thereby to hope to prevent the alarm from sounding. If the period between infrared activation periods is variable, a potential thief will have difficulty in predicting the length of the next activation period and consequently a recorded signal is of reduced value to his or her criminal intentions.

Claims (14)

-10Claims 1. A security system suitable for a vehicle, t he system comprising a detection means arranged in use to provide an intrusion signal which is indicative of the likely presence of an intruder in a protected area, the system further comprising a checking means which is arranged in use to be activated substantially on or by the provision of a said intrusion signal and to provide a confirmation signal which is indicative of detection by the checking means of an intruder in the protected area, wherein the checking means comprises an active infra-red transducer.

1

2. A security system according to Claim 1, wherein a light emitting stage of the active infra-red transducer is driven with a high current.

3. A security system according to Claim 2, wherein the current is in the order of 300 mA

4. A security system according to any preceding claim, wherein the checking means is activated for a time slot of a predetermined duration.

5. A security system according to Claim 4, wherein the time slot is in the order of 10 seconds.

6. A security system according to any one of Claims 1 to 3, wherein the checking means is activated for a time slot of a duration which is variable between activation periods of the checking means.

7. A security system according to any one of Claims 4 to 6, wherein the time slot is of a minimum duration which is sufficient for the checking means to settle into proper operation and to then perform at least one detection scan of the protected area.

8. A security system according to any preceding claim, wherein the checking means is activated for an initial time slot so that it can capture an initial level of a confirmation signal.

9. A security system according to Claim 8, wherein the initial time slot is in the order of 50 gs.

10. A security system acc ording to Claim 8 or Claim 9, wherein the checking means is re-activated during a predetermined later re-check period after the initial time slot and the confirmation signal detected in that later re-check period is compared with the confirmation signal captured in the initial time slot, and wherein the security system is arranged to determine that an intruder is present in the protected area only if a difference threshold is exceeded between the initial confirmation signal and the confirmation signal captured in the later recheck period.

11. A security system according to any preceding claim, the detection means comprising an ultrasonic transducer.

12. A security system according to any one of Claims 1 to 10, the detection means comprising a microwave transducer.

13. A security system substantially as described herein with reference to the accompanying drawings.

14. A vehicle including a security system according to any one of Claims 1 to 13.