| 摘要 |
The device is primarily an anti-theft device or alarm for use in a vehicle and which can operate in a plurality of modes. The first mode may involve transmission of signals in the form of ultrasonic pulses from a transmitter (15) which are reflected from surfaces in the driver/passenger space (11) of the vehicle. The pulses are picked up by a receiver (16) and a pattern of reflections (34) is formed for comparison with a reference (33) based on the current level of background noise and environmental conditions in the space (11) and those associated with types of disturbance not intended to trigger an alarm. If an intrusion or disturbance occurs in the space which does not provide certainty that an alarm should be given, the device switches to a second mode where the transmitter (15) can transmit a continuous ultrasonic signal or a burst of ultrasound for a relatively long period. The reflected signal is picked up by the receiver (16) and compared to a reference level based on the current level of background noise and environmental conditions in the space (11) and those associated with types of disturbance not intended to trigger an alarm. If the reference level is exceeded an alarm is triggered. A third mode can be provided where there is no continuous or burst transmission from the transmitter and the system becomes passive so that the receiver (16) can pick up noise of breaking glass (Fig. 5). |
