OPTICALLY BASED INTRUSION DETECTOR

Abstract: A beam of light (14) is caused to scan a room, creating light pulses as it strikes retroreflective elements (12) strategically placed in the area to be protected. This spatial pattern of pulses is detected and compared to a standard pattern. The absence of a pulse is used to indicate the presence of an intruder or a missing article. An unexpected pulse is used to indicate an open door, drawer, window, etc. The machine (10) may be easily taught a new standard pattern, allowing the same device to be used in a wide variety of applications.

Description: OPTICALLY BASED INTRUSION DETECTOR Technical Field This invention relates to techniques for initiating alarms based on the detection of a reflected scanned light beam when the path of the beam is altered, such as by intruders, movement of an item in the protected area, or if a window, door, drawer, etc. has been inadvertently left open, etc. Background Art It is a well known technique to utilize light beams to detect the presence of an intruder along an optical path, to activate automatic door openers, and the like. Similarly, it is known to scan light beams to enable detection of objects, intruders, etc. within a protected zone. For example, U.S. Patent 3,120,654 (Lee) depicts an entry detection system utilizing a narrow light beam which is scanned and reflected from multiple, strate¬ gically positioned, stationary reflectors to define the perimeter of a protected zone. The reflected light is then directed onto a single detector. Entry into the zone, and hence interruption of the beam, is detected by comparing individual pulses produced by the detector with individual reference pulses each of which corresponds to the position of one of the reflectors. Figure 1 thereof depicts a system wherein a conical zone is defined by a light source and adjacent detector forming the apex of the cone and by a rotating plate which allows a beam of light from the source to pass through an aperture thereon, the circular path of the aperture thus establishing a ray which describes a cone of light as the plate is rotated. As the scanned light beam thus impinges on a surface normal to the center position of the beam, it may be seen to describe a circular path, i.e. to scan about a single angular di en- sion. The multiple reflectors are positioned at various locations in the path of the rotating beam to reflect the light back through the aperture and onto the detector. A plurality of position markers are also located about the disc to provide pulses each of which instantaneously corresponds to a given one of the multiple reflectors and which when instantaneously compared to the light reflected from a given reflector, provides an alarm in the event of a mismatch, Such a system would necessarily require precise positioning and alignment of the mirrors, which would preclude its use in most environ¬ ments wherein movement of the reflectors would likely occur, and is incapable of determining where in the scanne zone a missing reflection resulting in a mismatch has occurred. Further, such a system only defines the perimeter of a protected zone, and does not provide detection throughout the protected zone. U.S. Patent 4,063,085 (Montanvert) also depicts a system for optically scanning a zone to detect intrusion therein. In that patent, an array of light sources are electronically commutated to, in essence, optically scan a limited protected zone, such as a vertical optical zone between a press and its operator. In one embodiment, one side of the zone is formed by a linear array of LED-photo- transistor modules, and the other by a catadioptric mirror, an alarm being initiated in the event a disagree¬ ment in a given sequence of signals from the array is detected. Due to the use of a single reflector, such a system is extremely limited in the size of the zone to be monitored. ' Summary of the Invention The present invention is directed to a system fo generating an optical pattern within a surveillance zone and for producing an alarm in response to changes in that pattern. Like that of Lee (U.S. Patent No. 3,120,654), th present system includes a source of a beam of light, and means for cyclically scanning the light beam through the zone. A plurality of retroreflecting elements are furthe provided, each of which is adapted to be positioned in th zone to retrore lect a pulse of light- oward the source as the light beam impinges thereon. The reflected pulses are detected by detector means and a corresponding electrical rignal is processed to activate an alarm in the event a change in !the electrical signal is detected. Unlike Lee, however, wherein an alarm is produced in the event any signal derived from the detector does not occur during the time a corresponding signal results from the reference elements, the system of the present invention provides an alarm in the event the optical pattern derived from the detector during a complete cyclical scan fails to correspond to the pattern derived during a different complete cyclical scan. The present system, therefore, further comprises means for detecting the optical pattern formed by the retroreflecting light pulses during each cyclical scan and for generating an electrical signal corresponding thereto, together with means for storing a representation of the electrical signal produced during one cyclical scan, and means for comparing the stored representation with that produced during a subsequent cyclical scan. In a preferred embodiment, the scanning means includes means for cyclically scanning the light beam over two dimensions, such as by deflecting the beam along botn orthogonal coordinates, or along both angular coordinates so as to cause the beam to impinge substantially through¬ out an extended surveillance zone during each complete cyclical scan. Such an embodiment thus differs in large measure over that of Lee, in that any change throughout the zone which alters the reflected pattern will cause an alarm, rather than just such changes about the periphery of the zone. The system of the present invention can also be easily installed, as power need only be provided to one point in the protected area. The retroreflective _4_ elements, strategically placed in the protected area, act as transponders and eliminate the need to install wiring to each point of protection. Furthermore, since retroreflective elements are used, the detector being positioned proximate the light source, and since no reference ' signal elements as in Lee are necessary, there is no need to either precisely position and/or align the reflecting elements. The system may be used to detect intruders, to identify doors or windows that are, not in their correct position when the system is energized, or to recognize the absence or even the repositioning of an object in the area of coverage. The same scanner may be used in many applica- tions, eliminating the need to customize each installa¬ tion. With this technique, the customization is done by placement of the retroreflective elements. In a further embodiment, two scanners can be utilized to overlap in coverage, and even share retro- reflectors, without interfering with one another. This is of significant advantage in providing coverage over large or odd shaped surveillance zones. The ease of installation also makes practical temporary coverage. For instance, at construction sites o loading docks, a combined scanning light source and detector assembly may be mounted on a tripod and retro- reflective strips placed in the area of coverage. Once th optical pattern is stored, the system may be activated to provide an alarm if the pattern subsequently changes. Similarly, a meeting room could be inspected to be free of illicit listening devices, the scanning assembly installed on a temporary tripod, and retroreflective strips located about the room. Once the system stores the optical pattern, it may be set to scan until the room is to be used, at which time the system is removed, leaving no devices with wires attached to them in the room — only th retroreflective strips. Also, the system may be utilized to divide the protected zone into many small angles and to store informa¬ tion indicative of which angle segment reflections occur, and in which segments the reflections do not occur. While it is possible to depend on time from some starting mark on tl<e scanning assembly, it is preferable to detect angular position, i.e., to have a tachometer coupled to a scanner drive assembly strobe the reflection data into memory. This eliminates variations in the angular velocity of the drive assembly as a source of error. Brief Description of Drawings FIGURE 1 is a cut-away perspective view of one embodiment of the system of the. resent invention; FIGURE 2 is a diagrammatic view of a mechanically implemented scanner assembly used in the system of the present invention; FIGURE 3 is a cut-away view of the scanning pattern provided by the embodiment set forth in Figure 2; FIGURE 4 is a combined simplified block and schematic view of an electrically implemented scanning assembly for use in the present invention; FIGURE 5 is a cut-away view showing the scanning pattern obtained with the scanning assembly of Figure 4; FIGURE 6 is a simplified view of a further scanner assembly for use in the present invention; FIGURE 7 is a block diagram of a comparator circuit for use with one embodiment of the present invention; and FIGURE 8 shows representative signals obtained with the circuit shown in Figure 7. Detailed Description Retroreflective devices are commonly employed in a variety of applications, ranging from vehicle license plates and highway signs to reflectorized clothing. Electronic re roreflective detector assemblies are used in such applications as counting boxes moving along a conveyor belt, and in locating and reading universal bar codes. In typical detector assemblies, light, which may or may not be visible, emanates from a source and is reflected from a half-silvered mirror positioned 45 degrees to the axis of the light rays. The source may be of any form, including a light emitting diod (LED) , incandescent light, or a laser. After being thus reflected, the light rays emanate from the assembly toward a retroreflective surface, whence they are reflected back along the same axis to the mirror. A portion of the rays thus return to the source, while the remainder is reflecte onto a photosensor, such as a photo cell, phototransistor, or any other device that converts light energy into an electrical signal. The output of the photosensor will change accordingly if some non-retroreflective surface interrupts or alters the optical path, for instance a box moving on a conveyor between the detector assembly and the reflector. Electronic detector assemblies such as described above may also be combined with a scanner device to scan the light beam through a given zone and thus intercept retroreflective elements positioned in the zone. In such an adaptation, the emanating light beam may be directed toward a conventional mirror coupled to a motor shaft for rotation such that as the mirror rotates, the light beam i swept through a sector. The specific electronic retroreflective detector assemblies used in the system of the present invention and the exact nature of the light source and detector therein are not of critical importance. The description is provided simply to aid in understanding the present system A simplified embodiment of the present invention is shown in Figure 1, wherein a combined detector scanner assembly 10 is depicted installed in a room together with several retroreflective elements 12. The beam 14 creates spot 16 on the wall, shown "frozen" in time. The dashed line on the walls shows the track of the spot as it moves around the room. Each time the spot 16 hits a retroreflec¬ tive element 12, light is retroreflected to the detector assembly 10, causing an output from the photosensor therein. As set forth in detail hereafter, the present invention also includes means for storing the output from the photosensor during each complete scan and for comparing the stored output with that produced during subsequent scans. Accordingly, when the spatial pattern of retro- reflected pulses is changed, such as by an intruder, an alarm is created. Assume, for example, that the scanning mirror is rotated at 1800 rpm, thus causing each retro- reflector to be polled 30 times a second. This is so quick with respect to human motion that the effect is virtually the same as having a permanent beam to each retroreflector. If the room in Figure 1 were viewed from the top, down through the ceiling, the beam would fan out from the scanning assembly 10 to each of the retroreflectors 12. If the track of the beam were visible, it would scribe a cone with its apex at the scanner. In a preferred embodiment, the scanner assembly of the present invention is adapted to cover or protect the entire volume of a room by vertically modulating the beam as well as scanning it horizontally. Figure 2 sets forth a diagrammatic view of one such scanning assembly. As shown therein, a motor 20 is positioned to rotate a mirror 22 and also to drive a gear 24. A tachometer 25 is positioned to sense the angular position or the rotational velocity of the mirror. The gear 24 is coupled through a linkage member 26 to a cam 28, which bears on the detector assembly 30 against a spring 32, causing the assembly 30 to be periodically tilted about a pivot 34 as the cam 28 rotates. As is also there shown, the detector assembly 30 may desirably include a light emitting diode, light source 36, a phototransistor 38, and a half-silvered mirror 40 appropriately mounted within the housing. -Z_- Figure 3 sets forth a portion of typical scanning pattern thus provided. As there shown, the first time the beam scans the room it follows track 42. The ratio of the various gears, etc. within the linkage member 26 may be so arranged that the succeeding tracks(44, 46, etc.) precess the first track 40. Eventually, after a number of revolu¬ tions set by the gear ratio, a complete cyclical scan will have resulted such that the beam will retrace track 42. The retroreflective elements may thus be placed at random 'throughout the room to intersect these paths such as at location 48 to enable detection of the opening of a door, or at location 50 to detect the movement of an art object. A complete cycle will begin again when the light spot starts retracing the track 42. The spatial pattern repre- sented by the pulse train formed as a result of the reflections from the retroreflective elements repeats each time the spot begins to retrace track 42. Figure 4 shows an electronic technique for achieving a similar vertical modulation. In this embodi- ment, a motor 52 rotates a mirror 54, the rotation of which is sensed by a tachometer 56. By use of the tachometer 56 and associated drive circuit 58, the vertical modulation is caused to be a function of the angular position of the motor (and therefore the beam). The circuit 58 drives a voice coil 60, such that a change in the current there¬ through causes a mirror 62 to pivot about * axis 64, thus vertically deflecting the beams from and toward the detector assembly 66. Figure 5 shows the track of the beam thus produced by incrementally stepping the vertical modulation once per revolution of the horizontal scanning mirror 54. The first revolution of the mirror 54 causes track 68. When the current through voice coil 60 is increased by a step function, the beam courses the track 70. After completing the second revolution, the current through the voice coil 60 is further increased in a step function. causing the beam to course the track 72 on the third revolution, and so on. Figure 6 sets forth an additional embodiment for effecting a multidimensional scan, As in the previous embodiment, a motor and tachometer are employed, but on the motor shaft 74, the previously employed mirror is replaced with a polyhedron 76. Each face 78 of the polyhedron 76! is successively more sloped. These sloped faces 78 are mirrored surfaces. These surfaces are sequentially pre- sented to the beam from the detector assembly 80 providing vertical modulation in planes 82, 84, 86, etc. of the same arc. In this embodiment, the coverage pattern is less than 360 degrees, and the exact angle of coverage is a function of the number of sides of the polyhedron 76. Further, one revolution of the motor in this embodiment is equal to one complete cycle since the resulting pattern will repeat once each revolution of the motor. The signal processing electronics utilized with this embodiment must have provision for removing ambiguities that may occur when the junctions of the adjacent sloped faces 78 are presented to the beam. This is done by counting, and then ignoring certain tach pulses. This can also be done with special marks on the tachometer. It would be appropriate to mount this version on the wall near the ceiling or in the corner of a room near the ceiling. Regardless of the particular detector-scanning assembly utilized, the signal processing circuit of the present invention is preferably arranged as shown in Figure 7. As there set forth, signals from a tachometer 88, which provides a large number of pulses (e.g. 1000) for each com¬ plete scan together with sync pulses from the tach sync mark 90, and signals from the detector assembly 92 are coupled to an AND gate 94. When signals from both are coincident, combined pulses are coupled through a "read-write" switch 96 and thence to one of two shift registers, register A or register B, 98 or 100, respectively. The contents of the two registers are compared in parallel via a comparator 102, and the output therefrom strobed out via an AND gate 104 under control of the tach sync pulses. Initially, the switch 96 is placed in the "write" position, indicated by the dotted line. Beginning with the tach sync pulse from the tach sync mark 90, the tachometer pulses begin clocking data into register A 98 via gate 94. If the tachometer pulse is coincident with a retroreflec- tive pulse, a one is outputted from the gate 94 and is entered into the shift register 98. When the tachometer pulse occurs with no coincident optical pulse, a zero is entered into register A 98. Once a complete cycle has occurred which may be multiple motor revolutions, the switch 96 is automatically changed to the "READ" position and the spatial pattern represented by a subsequent complete scan is stored in Register B 100, in the same manner as the data of the earlier scan was stored in register A 98. That is to say, if the tach pulse is coincident with a retroreflective pulse, a one is shifted into register 100, and if there is no coincidence, a zero is entered. The content of register B simultaneously is compared in parallel with the content of register A on each tach clock via comparator 102, and provides a digital one only when the two registers contain the same data. The output of the comparator 102 is sampled on each succeeding tach sync mark via AND gate 104. At this time, the output 106 will be a one provided the contents of register A and B are the same, while zero would indicate an interruption of the pattern, i.e., an alarm condition. The entries of "x" in register B 100 represent an indeterminate condition since the data is shown as having advanced only three positions since the last tach sync mark. Figure 8 shows typical patterns that would appear on the output of AND gate 94 which require coincidence between the tachometer pulses and pulses representing retro- reflective elements before the data would be entered into the appropriate register 98 or 100. To someone skilled in the art, it will be apparent that the registers 98 and 100, the comparator 104, and the AND gates 94 and 106 can be physically replaced with a microcomputer. Such a device can process the same pattern shown in Figure 8 and store the appropriate data at locations in memory where it expects optical pulses, these locations being assigned during the "WRITE" cycle. While not shown in Figure 7, it will be readily apparent that additional timing circuits may be provided to control the storage and comparison of signals representing the spatial patterns of various complete scanning patterns. Thus, for example, signals representing immediately sequen- tial scans may be compared to indicate a change such as might be caused by the movement or presence of an intruder in the protected zone. Alternatively, or in addition thereto, signals from one scan may be stored a relatively long time and then compared with signals representing the spatial pattern of a scan occurring minutes, hours, or even days later, thus providing an indication that objects to which a retroreflective marker is affixed have been moved within the zone or removed therefrom altogether. Similarly, the output from the gate 106 may be coupled to a variety of alarm monitors, flashing lights, cameras, etc., as is well known to those skilled in the art. The present invention is thus not restricted to only those particular embodiments set forth herein, which have been described only by way of example and without li ita- tion. On the contrary, all alternative forms of construc¬ tion in which different combinations of detector assem¬ blies, scanners, and signal processing circuits are employed to carry out the same functions in order to achieve the same results are likewise included in the present invention.

Claims: Cla ims : 1. A system for generating an optical pattern within a surveillance zone and for producing an alarm in response to changes in said pattern, including a source of a beam of light, apparatus for cyclically scanning said beam of light through said zone, a plurality of retroreflecting elements, each of which is adapted to be positioned in said zone to retroreflect a pulse of light toward said source as said beam of light impinges thereon, a detector for generating an electrical signal corresponding to retroreflected light pulses, and a circuit for providing an alarm signal in the event of a change in said electrical signal, characterized by said detector further comprising a circuit for detecting the optical pattern formed by the retroreflecting light pulses during each complete cyclical scan and for generating an electrical pulse train corresponding to said pattern, and by said system further comprising a memory circuit for storing a representation of said electrical pulse train produced during a complete cyclical scan, a comparator for comparing a said stored representation with that produced during a subsequent cyclical scan for activating a said alarm signal providing circuit in the absence of correlation between the compared representations. 2. A system according to claim 1 further characterized by a circuit for providing a reference pulse corresponding to a given spatial location in each cyclical scan, and by said memory circuit comprising a subcircuit for storing representations of the relative spatial posi¬ tions of said retroreflected light pulses with respect to said reference pulse during each cyclical scan and by said comparator comprising a subcircuit for comparing the representations of said spatial positions produced during different cyclical scans. 3. A system according to claim 1, further characterized by a timer for activating said comparator after a predetermined number of scans have occurred. 4. A system according to claim 3, characterized by said timer including a circuit for activating said comparator to compare said representations of scans occurring with a relatively short time interval therebetween to indicate object and personnel movement within said zone. 5. A system according to claim 3, characterized by said timer including a circuit for activating said comparator to compare said representations of scans occurring with a relatively long time interval therebetween to indicate long term changes in the environment in said zone, such as removal and repositioning of objects therein. 6. A system according to claim 1, characterized by said scanning apparatus including a mechanism for cyclically scanning said beam of light in two dimensions such that during one complete cyclical scan said light beam impinges substantially throughout an extended surveillance zone. 7. A system according to claim 6 characterized by said scanning apparatus comprising a reflecting device positioned in the path of said light beam, apparatus for periodically varying the angle of said reflecting device with respect to said beam to cause said beam to be periodically scanned over one dimension, a second reflecting device positioned in the path of said one dimensionally scanned beam, and a mechanism for periodically varying the angle of said second scanning means with respect to said one dimensionally scanned beam to cause said beam to be periodically scanned over a second dimensional substantially orthogonal to said first dimension. 8. A system according to claim 7, characterized by said mechanism for periodically varying the angle of said second reflecting means comprising a drive device for incrementally varying said angle over a predetermined number of steps following each scan in said one dimension to provide a plurality of sweeps in both first and second directions during each cyclical scan. 9. A system according to claim 6, characterized by said scanning apparatus comprising a multifaceted motor driven mirror, onto the faces of which said light beam is directed, at least some of which faces are at different angles with respect to said light beam, such that as said mirror rotates, said faces successively deflect said light beam along one dimension and the differently angled faces result in said beam being scanned along parallel paths across a second dimension.

Inventors: NOTHAFT EUGENE M, NOTHAFT, EUGENE, M.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1982
Main classification: G08B13
IPC codes: G01V8, G08B13