Detector System

Abstract

An intruder detector system that utilizes both a Doppler effect detector system and an optical detector system monitoring the same space. An alarm output signal produced by either detector under alarm conditions is prevented from activating an alarm unless the other detector also is producing an alarm signal.

Description

BACKGROUND OF THE INVENTION

a. Field of the invention

This invention relates to electronic intruder detection devices.

B. Description of the prior art

One form of widely used device for intruder detection utilizes an accoustic generator that creates a pattern of standing waves in a space to be protected. Reflected waves are received by the detector, and if the reflected waves maintain a predetermined frequency, no alarm signal is created. However the motion of a person in the room or air currents due to heat from a fire will cause a change in the frequency received, and this change is utilized to actuate an alarm.

Another device of this type utilizes microwaves in the electro-magnetic spectrum, with the method of operation being substantially as described with respect to the acoustic system.

Although such devices have achieved considerable commercial success, they have the disadvantage of being extremely subject to false alarms from extraneous sounds from both inside and outside the protected space. For example, outside sounds such as automobile engine noises, sirens, thunder, airplanes, or construction equipment can contain sufficient high frequency components to cause an alarm response of the acoustic type of detector. Internally, sounds of thermostats or other devices with contacts, electric motors, telephones, doorbells, or the motion of animals such as cats, rodents, or guard dogs can cause an alarm response, as can air turbulence due to hot air heating systems.

In the case of the microwave type of detector, a false alarm may be caused by spurious R.F. signals or motion of a person or article in an area beyond the confined space, since the transmission passes through any wall not made of metallic material.

Although some of these causes of false alarms can often be prevented by the use of delay features in the alarm portion of the circuit, or by controlling the sensitivity of the device, such features reduce the effectiveness of the device in detecting intruders.

Another type of volumetric space surveillance system (as opposed to a line of sight type of detector) is an optical detector which utilizes the rate of change of light intensity to actuate an alarm. This type of device is also prone to false alarms, due to unpredictable sudden changes in ambient illumination such as head lights from passing cars, lightning, lightbulb failure or momentary power failure.

SUMMARY OF THE INVENTION

In accordance with the present invention a detector system comprises a Doppler effect intruder detector and a volumetric optical detection system combined to monitor the same space. The outputs of the detectors are connected to an AND gate, with the output of the AND gate being connected to alarm actuating means. Phenomena that cause false alarms in Doppler effect detectors and those that cause false alarms in optical detection are so different that the possibility is very slight that phenomena that affect each will occur simultaneously.

DESCRIPTION OF THE DRAWING

The single FIGURE of the drawing is a block diagram illustrating a detector system embodying the features of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

As illustrated in the drawing a detector system embodying the features of the invention comprises a Doppler effect detector 10 and a photo-electric detector 12 positioned for surveillance of the same space to be protected from intruders. The detectors 10 and 12 cooperate to energize an alarm on detecting an intruder or other condition in a manner to appear herein after.

Both Doppler effect detectors and optical detectors are known in the art and the specific types of detectors to be described are by way of example only, since other types may be used without departing from the scope of the invention.

The detector 10 comprises a tuned circuit resonating at, for example, 40 kHz which drives a piezo-electric tranducer transmitter, which radiates at 40 kHz into the protected space. When the detector is in use, a standing wave pattern of 40 kHz is established in the space, a portion of which is reflected back to the transducer, which also has a resonating frequency of 40 kHz. The output of the receiver transducer is fed into an amplifier and mixer which also receives a 40 kHz signal from the tuned circuit driver. If there is no disturbance in the ultra-sonic waves, the frequency received at the receiver transducer and therefore at the mixer is the same as the 40 kHz signal from the tuned circuit driver. However if there is any movement of a body in the protected space a different frequency is received at the receiver transducer, which when amplified and compared with the driver frequency in the mixer, produces a beat frequency. The detector allows this beat frequency to filter through to the delay integrator, which insures that the beat frequency is present for a predetermined time before transmitting a signal to the AND gate.

In the illustrated embodiment the optical detector system 12 comprises a photo-electric detector which is positioned to receive ambient light from the same space into which the Doppler effect system 10 is transmitting, an amplifier and discriminator which receives the signal from the photo-electric detector, and an output pulse generator which feeds an alarm pulse to the AND gate.

The photo-electric devices may be photo-resistive or photo-generative and may have various optical filters to make the device insensitive to light of certain band widths.

The discriminator responds to a predetermined change in illumination reaching the photo-electric detector to provide an output pulse to the AND gate.

As illustrated schematically, if the rate of change of the light received by the photo-electric detector is below a certain value, no signal is transmitted to the pulse output generator whereas if the rate of change is above said minimum value a signal is transmitted to the pulse output generator to produce an alarm pulse to the AND gate.

If both the Doppler effect detector and the optical detector provide an output signal at the same time, an output alarm signal will be transmitted to the alarm activating device, However the AND gate will not allow the alarm to be actuated if only one detector provides an output signal.

Although each detector in the above described system is subject to the false alarm conditions previously described, no single false alarm cause will cause a false alarm in both detectors simultaneously. The probability that a false alarm condition that affects the Doppler type detector will occur simultaneously with a false alarm condition that affects the optical detector, if not absolutely non-existent is very slight. The inhibiting of each detector by the other does not, however reduce the effectiveness of the detector in providing an alarm in response to an intruder.

In the illustrated embodiment of the invention, an entry and exit delay feature may be provided between the AND gate and the alarm actuating device to allow personnel time to leave the protected space after energizing the detector system, without actuating the alarm, and when re-entering, to allow them time to de-energize the system before the alarm is actuated by their presence.

Since certain obvious changes may be made in the embodiment described herein, it is intended that all matter contained herein be interpreted in an illustrative and not a limiting sense.

Claims

I claim:

1. An intruder detector system, comprising first and second detector devices arranged to monitor substantially the same confined space, each of said detector devices being responsive to the entry of an intruder to provide an output signal, one of said detectors being responsive to a Doppler effect to produce its output signal, the other being responsive to a change in light intensity to produce its output signal, means receiving the output of each of said detectors, said means producing an alarm output signal only when said detectors produce an output signal simultaneously.

2. A detector system comprising a detection device of the type utilizing a change in frequency to cause an alarm output signal and a detector device utilizing a rate of change in light intensity to cause an alarm output signal, said detector devices being arranged to monitor substantially the same confined space, an AND gate, the output of said detectors being connected to said AND gate, the output of said AND gate being connected to an alarm actuating device, whereby the AND gate transmit a signal to the alarm actuating device only if both alarm devices produce an alarm output signal simultaneously.

3. A detector system, comprising a first detector producing an alarm signal output in response to a change in frequency between a transmitted signal and a received signal, and a second detector arranged to monitor at least a portion of a confined space being monitored by the first detector, said second detector being responsive to a predetermined change of light intensity to cause an alarm signal output, and means preventing said alarm signal output from either detector from actuating an alarm unless the other detector also simultaneously generates an alarm signal output.