U.S. patent number 3,928,843 [Application Number 05/482,783] was granted by the patent office on 1975-12-23 for dual channel infrared intrusion alarm system.
This patent grant is currently assigned to Optical Coating Laboratory, Inc.. Invention is credited to Herbert L. Berman, James Cole Sprout.
United States Patent |
3,928,843 |
Sprout , et al. |
December 23, 1975 |
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.
Inventors: |
Sprout; James Cole (Los Altos,
CA), Berman; Herbert L. (Los Altos Hills, CA) |
Assignee: |
Optical Coating Laboratory,
Inc. (Santa Rosa, CA)
|
Family
ID: |
23917444 |
Appl.
No.: |
05/482,783 |
Filed: |
June 24, 1974 |
Current U.S.
Class: |
340/567; 250/342;
250/371; 250/DIG.1 |
Current CPC
Class: |
G01J
5/20 (20130101); G08B 13/19 (20130101); Y10S
250/01 (20130101) |
Current International
Class: |
G08B
13/19 (20060101); G08B 13/189 (20060101); G01J
5/20 (20060101); G08B 013/18 () |
Field of
Search: |
;340/258D,258B,228R,227R,276 ;250/342,371 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Swann, III; Glen R.
Attorney, Agent or Firm: Flehr, Hohbach, Test, Albritton
& Herbert
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.
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.
* * * * *