Dual channel infrared intrusion alarm system

Abstract

An infrared intrusion alarm system utilizes two sensing elements and two signal processing channels arranged such that an intruder produces signals of opposite polarities in the two channels. An alarm signal is delivered only in the event that the two signals of opposite polarities are present simultaneously. Disturbances, such as component noise, which affect only one channel cannot give rise to an alarm, nor can power supply disturbances which produce signals of the same polarity in both channels.

Description

BACKGROUND OF THE INVENTION

This invention pertains generally to intrusion alarm systems and more particularly to a system in which the presence of an intruder is detected by infrared heat energy emitted by his body.

Infrared intrusion alarm systems heretofore provided generally utilize a sensing element which produces an electric signal corresponding to the level of infrared energy impinging thereon from an area to be protected. The signal is processed by suitable circuitry, and an alarm is actuated in the event of an abrupt change in the signal, as occurs when a warm bodied intruder enters the protected area. This type of system is not affected by gradual temperature changes in the protected area or by disturbances such as air currents, mechanical shock and vibration, and noises which cause false alarms in other types of systems.

Infrared alarm systems of the prior art are, however, susceptible to false alarms arising from disturbances within the system itself, for example noise generated by the sensing element or components in the signal processing circuitry. Disturbances in the power supply, such as RF pickup and power line transients can also produce false alarms despite efforts to eliminate the disturbances by filtering.

SUMMARY AND OBJECTS OF THE INVENTION

The invention provides an infrared alarm system which is relatively immune to the disturbances which have caused false alarms in infrared systems in the past. This system utilizes two sensing elements and two signal processing channels arranged such that an intruder produces signals of opposite polarities in the two channels. An alarm signal is delivered only in the event that the two signals of opposite polarities are present simultaneously. Disturbances, such as component noise, which affect only one channel cannot give rise to an alarm, nor do power supply disturbances which produce signals of the same polarity in both channels.

It is in general an object of the invention to provide a new and improved infrared intrusion alarm system.

Another object of the invention is to provide an alarm system of the above character utilizing two channels and signals of opposite polarities to discriminate against extraneous disturbances which might otherwise give rise to false alarms.

Another object of the invention is to provide an alarm system of the above character utilizing an improved infrared detector having two side-by-side sensing elements.

Additional objects and features of the invention will be apparent from the following description in which the preferred embodiment is set forth in detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view, partly broken away, of one embodiment of an infrared intrusion alarm system according to the invention.

FIG. 2 is an enlarged fragmentary sectional view, partly broken away, illustrating the mounting of the detector assembly in the system of FIG. 1.

FIG. 3 is an enlarged plan view of the detector assembly in the system of FIG. 1.

FIG. 4 is a block diagram of a dual channel signal processing circuit utilized in the alarm system of FIG. 1.

FIG. 5 is a circuit diagram of the tuned amplifier for one channel of the processing circuit of FIG. 4.

FIG. 6 is a circuit diagram of the level detectors and output gate of the processing circuit of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As illustrated in FIG. 1, the alarm system includes a generally cylindrical housing 11 mounted on a base 12 by means of a swivel assembly 13 which allows the housing to be aligned as desired when the base is mounted on a suitable support such as the wall of a room to be protected. One end of the housing is open, and this end is provided with a window 14 which is fabricated of a material, such as polyethylene film, which is transparent to infrared radiation and serves to keep air currents and dust out of the housing.

A mirror assembly 16 and an infrared detector assembly 17 are mounted coaxially within housing 11. The mirror assembly comprises a plurality of spherical mirror segments which collect infrared energy from a plurality of spaced-apart fields of view and focus this energy onto the detector assembly. In the embodiment illustrated, the mirror assembly includes five vertically extending mirror segments 21-25 and two horizontally extending segments 26-27 disposed above the vertical segments. The mirror segments are of such area that they provide substantially equal sensitivity for each of the fields of view, and in the embodiment illustrated, the mirror segments are mounted on a base having an annular rim 31 and a generally semispherical axially facing wall 32. In the preferred embodiment, the mirror base is molded of a suitable material such as plastic, and the mirror segments are formed by coating portions of the generally semispherical wall with crome, aluminum or another material having a relatively high reflectivity in the infrared spectrum.

Detector assembly 17 is mounted at the center of focus of the mirror assembly by means of a mounting bracket 36. This bracket includes a hub 37 in which the detector assembly is disposed, a semicircular rim 38, and a plurality of spokes 39 extending between the hub and rim. The rim of the mounting bracket is affixed to the rim of the mirror assembly by suitable means, such as cementing, to form an integral structure.

As illustrated in FIGS. 2 and 3, detector assembly 17 comprises a cup-shaped case 41 similar to a TO-5 case commonly used in the manufacture of semiconductors. This case has a base wall 42 in which openings 43 are provided. Leads 44, spaced in quadrature, extend axially of the case and pass through openings 43. The case is filled with an electrically insulative material 46, such as epoxy, which serves to hold leads 44 in place. The detector assembly also includes a pair of infrared sensing elements 47, 48 which are mounted externally of case 41 adjacent to base wall 42. In the preferred embodiment, the sensing elements are flakes of a thermistor material having an electrical resistance which varies inversely with temperatures. These flakes are on the order of 40 mils long, 40 mils wide and 1-2 mils thick. The sensing elements have leads 47a, 48a which are soldered to leads 44 to provide electrical connections and support for the sensing elements. The sensing elements are disposed side-by-side and spaced away from base wall 42. They are disposed vertically of each other and close to the axis of the housing 11 so that infrared radiation from an intruder reflectd by each of the vertically extending mirror segments 21-25 will strike both sensing elements simultaneously. In this regard, it will be noted that leads 47a, 48a of the respective sensing elements are positioned inwardly of leads 44.

Detector assembly 17 is mounted in a recess 51 in the hub of mounting bracket 36, with sensing elements 47, 48 facing away from the open end of housing 11 and leads 44 passing through an opening 52 in the mounting bracket hub. Electrical connections to these leads are made by a suitable cable, not shown, which passes through an opening 53 in the semispherical wall 32 of the mirror base. A cup-shaped cover 56 is disposed coaxially of case 41 in recess 51 and provided with an opening 57 through which sensing elements 47, 48 are exposed. A filter 59 is mounted in front of opening 57 so that all energy reaching the sensing elements must pass through the filter. The filter passes intermediate and long wave length infrared radiation and blocks other forms of visible and near infrared energy from sunlight, incandescent lamps and flourescent lights.

As illustrated in FIG. 4, sensing elements 47, 48 are connected electrically in series with load resistors 61, 62 and a suitable source of voltage, such as 18 volts DC, at the inputs of a two-channel signal processing circuit. As discussed more fully hereinafter, the sensing element and load resistor in each channel function as a voltage divider, producing an input signal corresponding to the level of infrared radiation impinging on the sensing element. In the first channel resistor 61 is connected between sensing element 47 and the positive terminal of the voltage source, and in the second channel resistor 62 is connected between sensing element 48 and ground. The input signals are taken across sensing element 47 and load resistor 62 and are of opposite polarities.

In the first amplifier, the input signal is applied to the input of a tuned amplifier 63, and the output of this amplifier is connected to a level detector 64 which is adapted to fire when the signal from the amplifier reaches a first predetermined level. In the second channel, the input signal is applied to the input of a tuned amplifier 66, and the output of this amplifier is connected to the input of a level detector 67 which is adapted to fire when the signal from amplifier 66 reaches a second predetermined level. The outputs of level detectors 64, 67 are connected to the inputs of an output gate 68 which delivers an alarm signal only upon conjoint receipt of signals of opposite polarities from the two level detectors. In the preferred embodiment, the signal processing circuit is constructed in the form of a module 68 which is mounted behind mirror assembly 16 in housing 11.

Tuned amplfiers 63, 64 are identical, and as illustrated in FIG. 5, each comprises a source follower input stage 71 and a pair of tuned amplifier stages 72, 73. The source follower provides an impedance match between the sensing element and the first amplifier stage, and it comprises a field effect transistor 76 and a source resistor 77. The input signal is applied to the gate of the FET, and the drain of the FET is connected to a positive voltage source, e.g. +18 volts.

Each amplifier stage comprises an operational amplifier 81, with resistors 82, 83 connected in series between the input to the stage and the non-inverting input of the amplifier. A capacitor 84 is connected in parallel with resistors 82, 83, and a capacitor 86 is connected between the junction of the resistors and ground. A capacitor 87 and a resistor 88 are connected in series between the input of the stage and the inverting input of the amplifier, and a resistor 91 and a capacitor 92 are connected in parallel between the output of the amplifier and the inverting input. Each stage functions as an active band pass filter, with the low frequency cut-off point determined by capacitors 86, 87 and the high frequency cut-off point determined by capacitors 84, 92. In the preferred embodiment, the capacitors are chosen to provide a pass band on the order of 0.2 to 2 Hz, with a peak frequency on the order of 0.5 Hz. This frequency response corresponds to the rate at which a person walks, and it has been found to be particularly suitable for discriminating between changes in the level of infrared radiation produced by an intruder and gradual changes such as room or ambient temperature changes.

As illustrated in FIG. 6, level detector 64 comprises an operational amplifier 94. The output of tuned amplifier 63 is connected to the inverting input amplifier 94 by a capacitor 96, and a resistor 97 is connected between the inverting input and ground. A reference voltage is applied to the non-inverting input of the amplifier by a voltage divider comprising resistors 98 and 99.

Level detector 67 comprises an operational amplifier 101, and the output of amplifier 66 is connected to the inverting input of amplifier 101 by a capacitor 102. A biasing voltage is applied to the inverting input of amplifier 101 by resistors 103, 104, and a resistor 106 is connected between the non-inverting input and ground.

Output gate 68 comprises an operational amplifier 108, and the outputs of level detectors 64, 67 are applied to the inverting and non-inverting inputs of this amplifier by resistors 111 and 112, respectively. A biasing voltage is applied to the non-inverting input by resistors 113, 114, and a resistor 116 is connected between the inverting input and ground. The output of amplifier 108 is connected to an input terminal 117.

Operation and use of the alarm system can now be described. It is assumed that base 12 is mounted on the wall of a room to be protected and that housing 11 is oriented so that mirror segments 21-25 cover a plurality of horizontally spaced-apart fields of view. Mirror segments 26, 27 cover the region directly under the sensing unit and prevent an intruder from avoiding detection by attempting to cover or disable the unit.

The resistance of sensing elements 47, 48 varies inversely with the amount of infrared energy impinging thereon, and in the absence of an intruder, no signals are generated within the bandwidth of amplifiers 63, 66. In this situation, the output of level detector 64 is high, the output of level detector 67 is low, and the output of gate 68 is high, indicating the absence of an alarm.

When an intruder enters the room, the resistances of the sensing elements drop, producing a negative-going signal at the input of amplifier 63 and a positive-going signal at the input of amplifier 66. When the signal at the output of amplifier 63 decreases to the level determined by resistors 98, 99, level detector 64 fires and its output becomes low. Similarly, when the signal at the output of amplifier 66 rises to a sufficient level, level detector 67 fires, and its output becomes high. With the outputs of level detectors 64, 67 low and high, respectively, the output of gate 68 becomes low, indicating an alarm condition.

The system is immune to disturbances which affect only one channel, such as noise in one of the sensing elements, amplifiers or level detectors. Even though the disturbance is of sufficient magnitude to fire the level detector of the channel involved, output gate 68 will not deliver an alarm signal unless the level detector in the other channel has also fired.

The system also discriminates against disturbances in the power supply, such as RF pick up and power line transients, which produce spurious signals in both channels. Since these signals originate from the same source, they are of the same polarity, and only one of the level detectors will fire at a given time. Unless both level detectors fire at the same time, the state of the output gate will not change and no alarm signal will be given.

The invention has a number of important features and advantages. It is a passive infrared system which is not affected by gradual temperature changes in the protected area or by disturbances such as air currents, mechanical shock and vibration, and noises which cause false alarms in other types of systems. In addition, it discriminates against disturbances arising in the power supply as well as disturbances which affect only one channel.

It is apparent from the foregoing that a new and improved infrared intrusion alarm system has been provided. While only the preferred embodiment has been described, as will be apparent to those familiar with the art, certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims.

Claims

We claim:

1. In an intrusion alarm system: first and second sensing elements having electrical resistances dependent upon the level of infrared energy impinging thereon, a resistive element connected in series with each of the sensing elements and a voltage source, first amplifier means for providing an output signal corresponding to the signal developed across the first sensing element, second amplifier means for providing an output signal corresponding to the signal developed across the resistive element in series with the second sensing element, first and second level detector means for producing signals when the output signals from the amplifier means reach predetermined levels, and gate means for providing an alarm signal upon conjoint receipt of the signals from the level detector means.

2. The alarm system of claim 1 wherein the amplifier means comprise amplifiers having a frequency response in the range of 0.2 to 2 Hz.

3. The alarm system of claim 1 wherein the sensing elements comprise flakes of thermistor material disposed side-by-side on a support comprising a cup-shaped case, a plurality of axially extending leads spaced in quadrature and passing through openings in the base of the case, and insulative material in the case holding the leads in predetermined positions, the flakes of thermistor material being mounted on adjacent ones of the leads outside the case adjacent to the base and spaced therefrom.

4. In an intrusion alarm system first and second sensing elements responsive to infrared energy from the body of an intruder, means for directing energy from a field of view simultaneously to both of the sensing elements, first and second amplifier means for providing output signals in response to infrared energy impinging upon the sensing elements, and means for providing an alarm signal upon conjoint receipt of the output signals from both of the amplifier means.

5. The alarm system of claim 4 wherein output signals of opposite polarities are provided in response to the infrared energy impinging simultaneously upon the sensing elements.

6. The alarm system of claim 4 wherein the amplifier means each include a tuned amplifier having a frequency response in the range of 0.2 to 2 Hz.

7. The alarm system of claim 4 wherein the sensing elements have resistances which vary in accordance with the level of infrared energy impinging thereon, each of the sensing elements being connected electrically in series with a resistive element and a source of voltage, the first amplifier means being responsive to signals developed across the first sensing element and the second amplifier means being responsive to signals developed across the resistive element in series with the second sensing element.

8. The alarm system of claim 4 wherein the means for providing an alarm signal comprises first and second level detectors for providing signals when the output signals from the amplifier means reach predetermined levels and gate means for providing an alarm signal upon conjoint receipt of the signals from the level detectors.

9. The alarm system of claim 4 wherein the sensing elements comprise flakes of thermistor material disposed side-by-side on a support.

10. The alarm system of claim 9 wherein the support comprises a cup-shaped case, a plurality of leads extending axially of the case, said leads being spaced in quadrature and passing through openings in the base of the case, and insulative material inside the case holding the leads in predetermined positions, the flakes of thermistor material being mounted on the leads outside the case adjacent to the base and spaced therefrom.

11. In an intrusion alarm system: a detector assembly comprising a cup-shaped case, axially extending leads spaced in quadrature and passing through openings in the base of the case, insulative material in the case holding the leads in predetermined positions, and a pair of infrared sensing elements mounted side-by-side on adjacent ones of the leads outside the case adjacent to the base and spaced therefrom; first and second amplifier means for producing output signals in response to infrared energy impinging upon the sensing elements; and means for providing an alarm signal upon conjoint receipt of the output signals from the amplifiers.

12. The alarm system of claim 11 wherein output signals of opposite polarities are provided in response to the infrared energy impinging on the sensing elements.

13. In an intrusion alarm system: an axially extending housing, first and second infrared sensing elements mounted side-by-side toward one end of the housing and facing inwardly of the housing, means within the housing for directing infrared energy from a plurality of spaced-apart fields of view outside the housing to the sensing elements, the sensing elements being positioned side-by-side in a direction normal to the axis of the housing and normal to the direction in which the fields of view are spaced, first and second amplifier means within the housing for providing output signals in response to infrared energy impinging on the sensing elements, and means within the housing for providing an alarm signal upon conjoint receipt of the output signals from the amplifier means.

14. The alarm system of claim 13 wherein the amplifier means are adapted to produce output signals of opposite polarities in response to the infrared energy impinging on the sensing elements.

15. A detector for a dual channel infrared intrusion alarm system comprising: a cup-shaped case, axially extending leads spaced in quadrature and passing through openings in the base of the case, a body of electrically insulative material in the case holding the leads in predetermined positions, and a pair of infrared sensing elements mounted side-by-side on adjacent ones of the leads outside the case proximate to the base and spaced therefrom.

16. The detector of claim 15 wherein the sensing elements comprise flakes of a thermistor material.

17. In an intrusion alarm system: an axially extending housing, first and second infrared sensing elements mounted side-by-side toward one end of the housing and facing inwardly of the housing, a plurality of reflective surfaces disposed horizontally of each other for directing infrared energy from horizontally spaced apart fields of view outside the housing to the sensing elements, the sensing elements being positioned side-by-side and vertically of each other, first and second amplifier means within the housing for providing output signals in response to infrared energy impinging on the sensing elements, and means within the housing for providing an alarm signal upon conjoint receipt of the output signals from the amplifier means.

18. In an intrusion alarm system: first and second sensing elements arrangend to provide signals of opposite polarities in response to infrared energy from the body of an intruder, means for directing energy from a field of view simultaneously to both of the sensing elements, first and second amplifier means for providing output signals in response to the signals of opposite polarities, and means for providing an alarm signal upon conjoint receipt of the output signals from both of the amplifier means.