US 3641549 A
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.
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[ 1 Feb. 8, 1972 ELECTED-OPTICAL PERIMETER INTRUSION ALARM Pn'mary Examiner-John W. Caldwell Assistant Examiner-Mich'ael Slobasky  Inventors: Victor A. Misek, Hudson, N.H.; Robert Myers, Chelmsford, Mass. Attorney Fouls Eflmger  Assignee: Sanders Associates, Inc., Nashua, NH.  v ABSTRACT  Filed: June 25, 1970 An electro-optical perimeter intrusion alarm system wherein a pulsed beam of collimated optical energy from atra'nsceiver is  Appl' 49794 directed sequentially to a plurality of partially retroreflective 1 stations. Each station may have an additional reflector for  US. Cl. ..340/258 B, 250/221, 356/4 dimgfing energy transmitted by that station in a desired 1 Cl- "608) direction to enclose a perimeter a beam o beams ofener.  Field of Search ..340/258 R, 258 B, 276; 356/4, intruder interrupting the beam blocks the pulsed em 356/5; 250/221 gy which would otherwise be returned to the transceiver by any retroreflectors further along the perimeter. The area  References cued between stations which is crossed by an intruder is determina- UNITED STATES PATENTS ble from the characteristics of the energy retroreflected by the plurality of partially retroreflective stations. 3,335,285 8/1967 Gally, Jr. et a1. ..340/258 B 3,365,572 1/1968 Strauss ..340/ 258 R 14 Claims, 2 Drawing Figures I l 12 I6 23 2s 24 M l H I8 20 22 2e 30 IO COUNT OPTICAL O 32 D'SPLAY MASTJAL TRANSCEIVER RESET 34 AND GATES 46 PR F CLOCK 42 DELAY COUNTER 36 CLEAR MATRIX ALARM 44 INHIBIT GATE PAIENTEBFEII 8 I972 SHEET 1 [IF 2 lo 48 L, I COUNT OPTICAL DISPLAY m TRANSCEIVER 1 T T RESET 46 AND GATES 1 t T ALARM *44 COUNTER 36 p CLEAR CLOCK 4o I I I I4 I INHIBIT DELAY MATRIX GATE 38 //VVENTOR$ VICTOR A. MISEK ROBERT MYERS EM AM AGE/VT PATENTEDFEB 8l872 3.641.549
SHEET 2 [1F 2 *3 LR INVENTORS I VICTOR AMISEK ROBERT MYERS AGENT 1 ELECTRO-OPTICAL PERIMETER INTRUSION ALARM BACKGROUND OF THE INVENTION 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 solidstate 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 itis 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.
BRIEFDESCRIPTION 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 dov not change direction due to any misalignment; i.e., the input and output beams aremaintained 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 difi'rcult to eliminate with plane reflectors as used in the prior art.
The receiver portion of the transceiver l0 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 enei'gy pulse received by the transceiver 10.
When the PRF-clock l4 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 l 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 elec' tro-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. I 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 comer 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 applicants have provided a new and improved electro-optical perimeter intrusion alarm system whereby the objectivesset 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.
I-Iaving 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 signalprocessing 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.
l 1. 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.