U.S. patent number 3,641,549 [Application Number 05/049,794] was granted by the patent office on 1972-02-08 for electro-optical perimeter intrusion alarm.
This patent grant is currently assigned to Sanders Associates, Inc.. Invention is credited to Victor A. Misek, Robert Myers.
United States Patent |
3,641,549 |
Misek , et al. |
February 8, 1972 |
ELECTRO-OPTICAL PERIMETER INTRUSION ALARM
Abstract
An electro-optical perimeter intrusion alarm system wherein a
pulsed beam of collimated optical energy from a transceiver is
directed sequentially to a plurality of partially retroreflective
stations. Each station may have an additional reflector for
directing energy transmitted by that station in a desired direction
to enclose a perimeter with a beam or beams of energy. An intruder
interrupting the beam blocks the pulsed energy which would
otherwise be returned to the transceiver by any retroreflectors
further along the perimeter. The area between stations which is
crossed by an intruder is determinable from the characteristics of
the energy retroreflected by the plurality of partially
retroreflective stations.
Inventors: |
Misek; Victor A. (Hudson,
NH), Myers; Robert (Chelmsford, MA) |
Assignee: |
Sanders Associates, Inc.
(Nashua, NH)
|
Family
ID: |
21961778 |
Appl.
No.: |
05/049,794 |
Filed: |
June 25, 1970 |
Current U.S.
Class: |
340/526; 340/557;
356/5.08; 250/221 |
Current CPC
Class: |
G08B
13/184 (20130101) |
Current International
Class: |
G08B
13/184 (20060101); G08B 13/18 (20060101); G08b
013/00 () |
Field of
Search: |
;340/258R,258B,276
;356/4,5 ;250/221 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Caldwell; John W.
Assistant Examiner: Slobasky; Michael
Claims
Having described what is new and novel and desired to secure by
Letters Patent, what is claimed is:
1. An electro-optical intrusion detection system comprising
means for transmitting a pulsed beam of collimated optical energy
in a predetermined direction,
partially retroreflecting means remotely disposed with respect to
said transmitting means and intercepting said beam for
retroreflecting a portion of said transmitted optical energy and
transmitting the remainder thereof,
terminal retroreflecting means remotely disposed with respect to
said partially retroreflecting means for retroreflecting that
portion of said optical energy transmitted by said partially
retroreflecting means,
means colocated with said transmitting means for receiving optical
energy pulses retroreflected by said partially and terminal
retroreflecting means and for producing a pulsed electrical output
signal in response to said received optical energy pulses,
signal-processing means coupled to said receiving means and
producing an electrical output signal when less than a preselected
number of retroreflected pulses are received within a preselected
period of time from each transmitted pulse, thereby indicating the
presence of an intruder between said transmitting means and said
terminal retroreflecting means, and
an alarm coupled to said signal processing means and responsive to
the electrical output signal therefrom to indicate the presence of
an intruder.
2. Apparatus as recited in claim 1 wherein
said partially retroreflecting means comprises a focusing first
lens, a transparent partially reflective surface disposed
substantially at the focal plane of said focusing lens, and a
collimating second lens having the focal plane thereof
substantially coincident with said partially reflective
surface.
3. Apparatus as recited in claim 2 further including
a planar reflecting means disposed with respect to said partially
retroreflective means such as to direct said transmitted portion of
said optical energy in a desired direction.
4. Apparatus as recited in claim 2 further including
a third lens disposed adjacent said collimating lens for providing
parallelism between said retroreflected and transmitted optical
energy.
5. Apparatus as recited in claim 1 wherein
said terminal retroreflecting means comprises a focusing lens and a
substantially totally reflective surface disposed substantially at
the focal plane of said focusing lens.
6. Apparatus as recited in claim 1 further including
a PRF-clock coupled to said transmitting means to trigger the
transmission of said optical pulses at a preselected rate.
7. Apparatus as recited in claim 6 wherein said signal-processing
means includes
a pulse-counting means coupled to said receiving means and to said
PRF-clock whereby said counting means is cleared simultaneously
with the transmission of each said optical pulse, said counting
means operative to count said electrical output signal pulses
produced by said receiving means in response to said retroreflected
optical energy pulses.
8. Apparatus as recited in claim 7 further including
means coupled between said receiving means and said counting means
for digitizing the pulsed electrical output signal of said
receiving means.
9. Apparatus as recited in claim 7 further including
a delay means coupled to said PRF-clock for delaying the electrical
output signal therefrom by a time greater than the round trip time
of optical pulses from said transmitting means, to said terminal
retroreflecting means and back to said receiving means, and
an inhibit gating means having inputs coupled to said counting
means and to said delay means and an output coupled to said alarm
whereby said PRF-clock electrical output signal is applied to
actuate said alarm only if less than a preselected number of pulses
are counted within said delay time.
10. Apparatus as recited in claim 9 further including
a matrix having inputs coupled to said counting means, an output
coupled to said inhibit gating means, and having an electrical
output only when said counting means counts less than said
preselected number of pulses.
11. Apparatus as recited in claim 7 further including
display means coupled to said pulse counting mans for displaying
the number of said retroreflected pulses counted for each said
transmitted pulse.
12. Apparatus as recited in claim 9 further including
a display means, and a gating means having inputs coupled to said
counting means and to said inhibit gating means and an output
coupled to said display means whereby the number of said
retroreflected pulses counted for each said transmitted pulse is
displayed only when said number of pulses counted within said delay
time is less than said preselected number of pulses.
13. Apparatus as recited in claim 1 wherein
said transmitting means, partially and terminal retroreflecting
means and said receiving means are disposed with respect to one
another such that said optical energy beam defines a closed
perimeter.
14. Apparatus as recited in claim 1 further including
a plurality of said partially retroreflecting means interposed in
said optical energy beam between said transmitting means and said
terminal retroreflecting means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates most generally to the field of
electro-optics and more particularly to a new and novel
electro-optical perimeter intrusion alarm.
2. Description of the Prior Art
Prior to the present invention a wide variety of intrusion alarm
systems have been developed including radar networks, television
surveillance, passive acoustic systems and electromagnetic sensors.
Typical, however, of the prior art in electro-optical perimeter
defining intrusion alarm systems is a detection system described in
U.S. Pat. No. 3,370,285 which issued on Feb. 20, 1970 to P. M.
Cruse et al. This system involves the use of an infrared source and
a remotely disposed infrared receiver. A beam of infrared energy is
set up between the source and receiver which, when interrupted by
an intruder triggers an alarm. Several disadvantages attend this
approach to the intrusion detection problem. A very significant
problem arises in aligning apparatus such as that shown by Cruse et
al. Even if the source and receiver are used to define a single
line the very narrow beam widths typical of the solid-state lasers
require very precise alignment in order to cause the beam of
infrared energy to impinge upon the receiver. This alignment
problem is even further compounded when a series of reflectors are
used with a single source and receiver to define a closed
perimeter. The second drawback of the prior art approach,
particularly as applied to intrusion detection around a closed
perimeter, lies in the fact that although interruption of the beam
does trigger an alarm, no information is provided to indicate at
what point the intrusion was detected. Thus it is necessary to
investigate the entire perimeter on the detection of any intrusion.
Finally the apparatus of the prior art, once discovered by an
intruder, may be countered by inserting an appropriate energy
source in the beam during the penetration of the beam. In this
manner the receiver is still caused to receive appropriate energy
and the intrusion goes without detection.
An alternative prior art approach to perimeter intrusion detection
involves the use of a colocated transmitter and receiver and
remotely disposed plane mirror. It will be apparent that this
approach exhibits identical practical shortcomings as the remote
source-receiver systems set forth above.
OBJECTS AND SUMMARY OF THE INVENTION
From the foregoing it will be understood that among the objectives
of the present invention are:
To provide a new and novel electro-optical perimeter intrusion
alarm;
To provide apparatus of the above-described character employing a
plurality of partially retroreflective stations;
To provide apparatus of the above-described character which
provides an indication of the portion of the perimeter in which an
intrusion is detected;
To provide apparatus of the above-described character which is
substantially immune to countermeasures;
To provide apparatus of the above-described character which
operates with a low level of radiated power;
To provide apparatus of the above-described character having
improved ease of alignment; and
To provide apparatus of the above-described character wherein
remote stations require no source of power.
The foregoing as well as other objectives of the present invention
are achieved by providing a pulsed laser rangefinder transceiver
and a plurality of partially retroreflecting guard stations. Each
guard station comprises a first focusing lens, a transparent,
partially reflective surface at the focal plane of the lens and a
second collimating lens. A terminal guard station located for
example at the transceiver site may comprise a focusing lens with a
mirror disposed in the focal plane thereof. Each transmitted pulse
from the rangefinder is partially retroreflected by each guard
station and the energy received at the transceiver is thus a series
of pulses separated in time by an amount corresponding to the
distance between guard stations. An intruder penetrating the
perimeter interrupts the beam resulting in a reduction in the
number of retroreflected pulses which are received. Thus the fact
of an intrusion is detected as well as located to within a given
guard station pair.
These and other objects, features and advantages of the present
invention will become more apparent from the following detailed
description taken in conjunction with the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an intrusion alarm system in
accordance with the present invention.
FIG. 2 is an illustration of the manner in which the transceiver
and a plurality of guard stations may be deployed in operation to
define a closed perimeter.
DESCRIPTION OF PREFERRED EMBODIMENT
Turning now to FIG. 1 there is schematically illustrated on
electro-optical perimeter intrusion alarm in accordance with the
present invention. An optical transceiver 10 in the nature of a
laser rangefinder which is known in the art and preferably
operating in the invisible portion of the infrared spectral range,
emits collimated pulses of optical energy shown as rays 12 at a
rate controlled by a PRF-clock 14. The pulses of energy are
projected to partially retroreflective guard stations, two of
which, 16 and 16', are shown. Each guard station comprises a
focusing lens 18, a partially reflective surface 20 disposed at or
near the focal plane of lens 18 and a collimating lens 22. The
collimated energy 12 transmitted by guard stations 16 may be
redirected by a suitable plane mirror 24 such that the pulsed beam
may define a perimeter of any desired shape. Although the guard
station 16 is fully operative as shown, a guard station as
illustrated at 16' is preferred in the practice of the present
invention. The addition of a third lens 23 (illustrated in phantom)
operated at unity magnification may be used to ensure that the beam
which is retroreflected and that which is transmitted do not change
direction due to any misalignment; i.e. the input and output beams
are maintained parallel. A terminal retroreflecting guard station
16 comprising a focusing lens 28 and a reflecting surface 30 in the
focal plane thereof, defines the end of the perimeter and in
applications where it is desired to define a close area the
terminal retroreflector 26 may be colocated with the transceiver
10. Each transmitted pulse is thus partially retroreflected back in
the direction from which it came by each guard station 16. Each
guard station 16 thus contributes a retroreflected pulse of optical
energy and each pulse arrives back at the transceiver sequentially
displaced in time by an amount corresponding to the separation
between stations.
It will be appreciated that through the use of partially
retroreflective guard stations 16 the problems of alignment which
are so pronounced in the prior art are substantially reduced. It is
a property of a retroreflector to reflect energy back in the
direction from which it came regardless of the direction of
impingement relative to the optical axis of the retroreflector.
Thus, the retroreflected energy pulses are returned to the
transceiver regardless of angular alignment errors which are so
difficult to eliminate with plane reflectors as used in the prior
art.
The receiver portion of the transceiver 10 converts the
retroreflected pulses into video signals which are coupled through
a video amplifier 32 to a digitizer 34 which produces an electrical
output pulse for each optical energy pulse received by the
transceiver 10.
When the PRF-clock 14 triggers the transmission of a pulse by the
transceiver 10 it also operates to clear a counter 36. Thus by the
time the retroreflected optical energy pulses are received,
converted to electrical signals, amplified, digitized and coupled
to the counter 36, it is set to count the number of return pulses.
The number of counts should equal the number of guard stations 16
plus the terminal guard station 26. A matrix 38 is coupled to the
outputs of the counter 36 and is set to have a digital output "one"
when the count is equal to the number of guard stations and
operates to inhibit a gate 40. The output of the PRF-clock 14, in
addition to being applied to the transceiver 10 and counter 36, is
coupled to a delay means 42 which delays the clock output signal
for a time greater than the round trip time for pulses to be
retroreflected from the terminal guard station 26. When the clock
14 output pulse emerges from the delay means 42, two conditions may
exist; the matrix 38 may have a "one" output, thus inhibiting the
gate 40 and preventing the triggering of an alarm 44 or secondly
the matrix 38 has a "zero" output, allowing the delayed clock pulse
to pass through gate 40 thereby triggering the alarm 44. Under the
latter condition the delayed pulse is also coupled through gate 40
to AND-gates 46 to which the outputs of counter 36 are also
applied. In this manner the counter reading is gated into a count
display 48. The displayed counter reading gives the number of the
guard station immediately preceding the point at which an intruder
has interrupted the beam. A manual reset means 50 shown only
schematically is provided to reset the alarm and count display.
FIG. 2 illustrates a typical deployment of a three-beam
electro-optical perimeter intrusion alarm system. In a multiple
beam embodiment of the present invention an enlarged effective
cross section is provided with a resulting reduction in the chance
of an intruder defeating the system. A master station 52 includes a
mounting tower 53, first, second and third optical transceivers 54,
56 and 58 and signal processing electronics 60. Also to completely
close the perimeter about a position 61 to be guarded, the terminal
retroreflector units 62, 64 and 66, each identical to the more
detailed illustration shown at 26 of FIG. 1, may be mounted on the
master station tower 53. Guard stations 68, 70 and 72 may be
disposed such as to enclose the desired perimeter. Each guard
station is provided with three partially retroreflective assemblies
74, 76 and 78 of the type shown at 16 of FIG. 1 and a plane mirror
as shown in FIG. 1 at 24 for directing the beam about the
perimeter.
It will be apparent that the alarm 44 and count display 48 of FIG.
1 may readily be disposed at a location remote from the master
section 52 such as the guarded position 61 or any other desired
central control and monitoring location. The signal processing
electronics 60 in such instances may be coupled to the remote
station by either an RF or wire data link.
The transmitted optical energy pulse of a given amplitude and
duration is transmitted from each transceiver at the master station
52. This pulse is partially retroreflected at guard stations 68, 70
and 72 and the remaining energy is retroreflected by the terminal
retroreflectors mounted on the master station 52. Thus four energy
pulses of the same duration but lower amplitude than the
transmitted pulse are retroreflected to the transceiver. These
return pulses are spaced in time by an amount corresponding to the
distance between guard stations. Now assuming that an intruder 80
interrupts the beam at the point illustrated between guard stations
70 and 72; the transmitted pulse will be partially retroreflected
at guard stations 68 and 70 and the first two return pulses will be
received in the normal fashion. The transmitted pulse, however,
never reaches guard station 72 or the terminal retroreflector on
the master station 52. Thus in the time allotted for receipt of the
desired number of return pulses; i.e. the normal round trip transit
time from the transceiver to the terminal retroreflector, only two
of the normal four pulses are received and the alarm is activated
as discussed with reference to FIG. 1.
If the apparatus of the present invention were deployed to protect
an extensive perimeter, intermediate guard stations having
partially retroreflective assemblies but without a plane mirror may
be interposed between corner guard stations. In this manner the
point of beam interruption may be established with greater
precision.
It will be appreciated from the foregoing discussion that the
applicant's have provided a new and improved electro-optical
perimeter intrusion alarm system whereby the objectives set forth
hereinabove are efficiently achieved. Since certain changes in the
above construction may occur to those skilled in the art without
departure from the scope of the invention, it is intended that all
matter contained in the foregoing description or shown in the
appended drawings shall be interpreted as illustrative and not in a
limiting sense.
* * * * *