US3378834A - Perimeter intrusion detection system - Google Patents

Description

April 16, 1968 P. I. CORBELL PERIMETER INTRUSION DETECTION SYSTEM Filed May 10, 1966 3 Sheets-Sheet 1 4 I I DETECTOR 7 I II|IIIIIIIIIIIIIIIIIIIIIIIIIIII.

LOW PASS FILTER ALARM TEMPERATURE OOMPENSATOR I l I I l l I I I l I l l I I I l I l J- DEMODULATOR HIGH PASS FILTER SUPERVISOR Z R-F 6 SOURCE SUPPLY CONVERTER 0.0 VOLTAGE INVENTOR. PAUL 1 (meta Jna s Sim-1V2 Jf/a/I'e 5 April 16, 1968 P. CORBELL PERIMETER INTRUSION DETECTION SYSTEM 3 Sheets-Sheet 2 Filed May 10, 1966 INVENTOR. 840; I (aRBe-LL Affa/we s April 16, 1968 P. I. CORBELL 3,378,834

.PERIMETER INTRUSION DETECTION SYSTEM Filed May 10, 1966 5 Sheets-Sheet 25 DC TO DC CONVERTER C 4 A DETECTOR 9/ D7LAY AND A; w

7; 77 5/ AGC CIRCUIT Q SQUARE wAvE AMPLITUDE SIGNAL 0 DETECTOR SUPERK'SORY ALARM CIRCUIT ALARM x I NVENTOR v PAUL CoRBHL BY dyndrus I Jimflffagyys United States Patent 3,378,834 PERIMETER INTRUSION DETECTION SYSTEM Paul I. Corbell, Milwaukee, Wis, assignor to .lohnson Service Company, Milwaukee, Wis, a corporation of Wisconsin Filed May 10, 1966, Ser. No. 548,983

- 17 Claims. (Cl. 343-) ABSTRACT OF THE DISCLOSURE A perimeter type control controls entrance to an area and includes a transmitter at one end and a receiver at the opposite end. A beam of high or radiofrequency energy of the order of 200 megacycles or higher is established. The receiver, such as a radiofrequency detector, is fed into a high gain amplifier the output of which is fed through a temperature stabilizing circuit. An alarm circuit and a supervisory circuit are parallel connected to the amplifier. The alarm circuit is selected such that it will not detect the change in the Doppler frequency signal established by slow movement within the beam. The supervisory circuit however detects the continued proper operation of the circuit and the state of the carrier signal. Slow moving targets substantially decrease the carrier signal which in turn will trigger the alarm and indicate the malfunctioning of the circuit and/or the presence of a slow moving target. Further the amplifier section is provided with a feedback circuit including an amplitude detector and a delayed automatic gain control circuit. The output of the amplifying section can be fed into a timing capacitor to introduce a preselected time delay in the transfer of a proportional signal which will be fed into a control transistor having a variable impedance to vary the gain of the control transistor and thereby the amplifying section.

This invention relates to a perimeter intrusion detection system and particularly to such a system for producing an output signal by detecting the presence of the intruder.

Detection of intrusion into restricted areas has suggested the use of various forms of radiated energy into the area with the intruding person or object causing a change in the energy distribution. This change in energy distribution is sensed and caused to actuate a suitable alarm or the like. A highly sensitive and reliable intrusion detection system is disclosed in applicants copending application entitled, Intrusion Detection System, which was filed on Apr. 20, 1962, with Ser. No, 189,170, now US. Patent 3,242,- 486, and assigned to a common assignee wherein a radio frequency energy field is established and the creation of a Doppler frequency as a result of movement within the field provides a means for detecting movement of foreign bodies. The system disclosed therein particularly suggests the use of the reflected energy from the body to a receiver where it is mixed with a portion of transmitted energy which is also fed to the receiver. That system in contrast to the prior art integrating system employs an amplitude discriminating means such that it will respond to a single cycle of a Doppler signal of a selected magnitude. Although such system has been found to produce unusually satisfactory results for indoor detection or for limited outdoor areas, the sensitivity is such that when employed to cover large outdoor areas, the generation of spurious alarm signals presents a severe problem. Thus, as the area to be protected is extended, the target distance from the receiver increases. In order to make the system sensitive to movement at the outer extremes, the transmitted energy level must the substantially increased or the receiver must operate at a fairly high level of sensitivity. As a result, the energy immediately adjacent the transmitter and the receiver increases to such level that precipitation, blowing "ice paper, leaves, small animals and the like have a tendency to cause a triggering of the alarm. If the sensitivity is reduced such that such spurious alarm signals do not occur, the motion of an object in the outer periphery area of the field will not be detected.

Although capacitance, sonic and ultrasonic field systems have also been suggested for outdoor protection, such systems are not sufficiently tolerant of wind, environmental noise and the like and thus suffer from similar disadvantages as those described above. Beamed energy between receivers and transmitters using relatively low frequencies, generally below 2000 megacycles have also been suggested. Generally, the frequency is held low because higher frequencies generally detect small objects as well as rain and other forms of precipitation and consequently have been considered undesirable. Such systems should be provided with high sensitivity in order to provide effective motion detection.

The present invention is particularly directed to a perimeter type detection unit wherein a transmitter and receiver are positioned at opposite ends of a perimeter line to provide a beam of high or radio frequency energy which is selected to be what would normally be considered of an undesirably high frequency; that is, in the order of 2000 megacycles or higher. This system employs highly directional and relatively narrow beams with a crystal receiver or the like which has a relatively low sensitivity, The output circuit is designed to respond to the Doppler effect and the high transmitted frequency provides a sufficient diiference in the motion detection signal for convenient signal manipulation. The high frequency signal is modulated either by modulating the transmitted signal or the received signals. The output of the receiver or R-F detector is followed by a very high gain amplification at the 1 modulating frequency. This is then demodulated and divided into a Doppler difierence signal which is extracted from the motion detected signal and a fail-safe signal.

The system may be adjusted such that very slow motions will not (be detected; that is, maximum motionsensing capability is not necessarily employed to detect a moving target. Rather, the present invention relies on detection of the carrier signal as a part of a fail-safe circuit to trigger an alarm for exceedingly slow targets. The failsafe signal level may be amplitude detected to either side of a normal level. Generally, however, it has been found desirable to detect a decrease in signal level of 20% or more. If an intruder were to move very slowly through the energy beam, the motion detecting means may not receive a sufiicient signal to trigger the alarm in the normal motion detecting channel. However, there would be a substantial loss in the carrier signal and consequently the failsafe circuit would be triggered. Thus, in the present invention, the fail-safe circuitry performs the dual functions of fail-safe protection and motion intrusion detection, especially extremely slow motion. This has the distinct advantage of eliminating the many problems inherent in previously suggested solutions such as increasing of power levels, circuit gains and sensitivity and the like.

A further problem encountered in motion detection, particularly where extremely slow motion can be detected, is false triggering as a result of slowly changing environmental conditions such as accumulations of snow, debris and the like. In accordance with another aspect of the present invention, the receiver includes means to modify the detected sisignal to eliminate the efiects of extremely slowly changing conditions without adversely reducing the sensitivity to human movement within the field. Thus, the receiver normally includes an amplifying section which is provided with a feedback circuit including an amplitude detector and delayed automatic gain control ircuit. The control circuit may include a control transistor as a variable impedance element connected in the bias circuit 3 of an input stage of the amplifying section. The control transistor is in turn biased by the output of the amplifying section with a timing capacitor introducing a preselected delay in change of the gain of the control transistor and therefore the amplifying section.

The present invention provides a highly improved perimcter type protection system which can be employed in outdoor environments without the usual problems encountered by spurious generating signal elements.

The drawings furnished herewith illustrate one mode of carrying out the present invention clearly disclosing the above advantages and features as well as others which will be clear from the following description.

In the drawings:

FIG. 1 is a block diagram illustrating the perimeter control intrusion detection system constructed in accordance with the present invention;

FIG. 2 is a schematic circuit diagram of a suitable transmitter unit;

FIG. 3 is a schematic circuit diagram of a suitable receiver unit.

FIG. 4 is a block diagram of a modified system; and

FIG. 5 is a schematic circuit diagram of FIG. 4.

Referring to the drawings and particularly to FIG. 1, the radio frequency source or transmitter 1 includes an antenna 2 which is adapted to establish and transmit a high or radio frequency energy beam preferably having a frequency of the order of 10,525 megacycles. In accordance with the present invention, the radio frequency or the transmitter 1 creates a pulsed radio frequency energy beam shown diagrammatically in FIG. 1 by the pulse lines 3. The energy field is a highly directional and relatively narrow beam and is directed to an aligned receiving antenna 4 of a radio frequency detector 5 such as the known crystal detector device to transmit the received energy and proper amplification and demodulation to a motion detection channel 6 and a fail-safe channel 7, although for crystal detectors and the like, this limits the level of detection and it has generally been found that a length of 100 feet can be effectively detected with the receiver located as high as five or six feet above the ground level.

Although any suitable components can be employed, the transmitter or source 1 includes a D.C. (direct current) supply 8 which is connected to a D.C. to D.C. converter to provide D.C. voltages suitable for operation of transmitter 1 which is modulated to provide the desired modulating or pulse frequency. The receiving system includes a pro-amplifier 10 connected to the output of the detector 5 and selected to provide very high gain at the modulating frequency. In the illustrated embodiment of the invention, a temperature compensator circuit 11 is connected to modify the amplified signal and compensate for the wide variation in temperatures which will be encountered in many outdoor environmental installations. In certain climates for example in the northern part of the United States, temperatures may vary from about 30 to about 120. The response of the various components of the system particularly employing solid state units which are required for iugged and reliable operation cause the system to be temperature sensitive.

The compensated signal is impressed upon the channels 6 and 7 in parallel. Channel s includes a demodulator 12 to remove the modulating frequency and a low pass filter 13 which passes only the Doppler components. The filtered Doppler components are again amplified by an amplifier 14 to a suitable operating level for triggering a motion alarm circuit 15 or other suitable output.

The fail-safe channel 7 similarly includes a demodulator 16 and a high pass filter 17 to remove the low frequency Doppler signals to the transmitter by a relatively steady D.C. signal component which is amplified by an amplifier 18 and fed to a supervisory and alarm circuit 19.

More particularly, the component shown in block diagram may take the form of the circuitry shown in FIGS. 2 and 3 which respectively show the transmitters and in schematic form. particularly to FIG. 2, the illustrated transincl' cs a D.C. to D.C. converter circuit .r of switchin transistors 20 connected to the D.C. supply 8 and coupled by a transformer 21 to a low voltage D.C. filament power circuit 22 for a klystron tube 23 having a filament 24 connected to circuit 22. Tube 23 is a well znown device for generating R-F energy and includes a reflector 25, beam grids 26 and a cathode 27. A. rectifier is separately connected to the output of the D.C. to D.C. converter transformer 21 in a three wire output having a common center tap or ground line 29, a 250- volt positive line and a 50- to 200-volt negative line A filter circuit 32 is connected to the output side of the D.C. supply between lines 38 and 31 to provide a relatively constant D.C. voltage which is applied as the beam voItage between grids 26 and cathode 27 of the klystron tube 23. The reflector voltage of the klystron tube 23 is taken between lines 29 and 30 which includes a specially selected poor filter 33 which maintains a D.C. output having a selected ripple component superimposed thereon. In the illustrated embodiment, filter 33 includes a capacitor 34 connected between the D.C. lines 29 and 31 with a resistor 35 connected in series in line 29. A pair of voltage dividing resistors 3-6 and 37 is connected in parallel with capacitor 34 and resistor 35 between lines 29 and 31 to complete the filter circuit. A potentiometer 38 is connected across the resistor 37 and includes a potentiometer tap 39 connected to reflector 25. Potentiometer 38 may be adapted to vary the reflector voltage between 50 to 200 volts. The filter circuit 33 may be selected to maintain a substantial A.C. type modulation generally at two times the switching rate of the converter circuit 20-. Thus, the circuit may operate at a switching rate of Z lcilocycles (kc.) and produce a ripple frequency of 4- kc. The reflector voltage is adjusted to cause the klystron tube 23 to switch in and out of oscillation and thereby provide modulation of the transmitted signal. This provides a very convenient and inexpensive manner of providing the pulsed radio frequency signal and field between the transmitting unit 1 and the receiving unit 5.

Other reams of modulation and other means of generating a radio frequency may of course be employed and, as more fully described hereinafter with respect to FIG. 4, it is sometimes highly desirable to positively modulate the reflector volta e by a superimposed square wave modulating signal or the like. Thus, the output signal might be modulated employing a mechanical flapper or an electronic switch operating at the microwave level.

At the receiving end, the antenna 4 impresses the transmitted energy upon a crystal detector 5 or the like, the output of which is connected to the high gain pro-amplifier 10. K

The pre-amplifier 19 is preferably a multiple stage unit. The illustrated circuit is a well known unit employing a first stage transistor 40 connected in a common emitter configuration and with a boot-strap type feedback connection. A capacitor and resistor .1 connect the emitter-base circuit to provide the desired feedback for a boot-strap circuit. The multiple stage pro-amplifier 10 with the boot-strap construction is highly desirable in that such circuits have a high input impedance and permits ready cascading thereof without loading of succeeding stages. This provides a very simple means of providing very high gain for example in the order of 200,000.

The amplified output is fed to temperature compensating circuit 11 which in the illustrated embodiment of the invention includes a voltage dividing branch including an input resistor 42 and a temperature compensating thermistor 43. The thermistor 4-3 has a substantially greater resistance when cold than when hot. The output of the simple divider network supplies an output voltage which is maximum at cold temperatures and minimum at hot temperatures. This will provide some compensation for the temperature characteristics of the usual solid state amplifier 10 and the crystal detector 5 forming the immediately preceding stages of the receiving unit. The illustrated compensating circuit 11 includes amplification means including an isolating emitter-follower transistor 44, an unbypassed emitter-resistor amplifier transistor 45 and an output emitter follower transistor 46. Transistors 44 and 46 isolate the signal from the adjacent sections and transistor 45 is unbypassed to aid in its temperature stabilization. The output of the emitter-follower 46 is capacitive coupled to channels 6 and 7.

Referring particularly to channel 6, the illustrated demodulator includes a diode 47 connected to the emitterfollower 46 by a variable potentiometer 48 having an adjustable output tap 49. The setting of the potentiometer tap 49 controls the operative incoming signal level for amplitude discrimination as hereinafter described. The output of the demodulator 12 is filtered by the low pass filter 13 including a capacitor 50 and a resistor 51 connected in parallel with the signal transmitting circuit. The demodulator in conjunction with the associated filter circuit removes the four kilocycle carrier and passes only the low frequency Doppler component which is generated by movement within the beamed field. This Doppler signal is fed to amplifier 14 which includes a pair of transistors 52 and 53. Transistor 52 is shown as a bootstrap unit having high input impedance. Another advantage of this circuit is that the high input impedance permits the use of moderate sizes of electrolytic capacitors in the amplifying circuit. A capacitor 54 is connected between the collector of transistor 52 and ground to remove any high frequency noise which might interfere with the desired operation of the circuit. Transistor 53 is connected as an emitter follower to avoid loading of the motion detector or alarm circuit 15.

Circuit 15 may be of any desirable form but is preferably similar to that disclosed in applicants previously referred to patent and includes a Schmitt trigger circuit 55 coupled to energize an alarm relay 56 or other suitable detection means. As more fully described in the above patent, the illustrated Schmitt trigger circuit includes a pair of transistors 57 and 58 having an initial stable state wherein transistor 57 is conducting and transistor 58 is quiescent or cut off. When the incoming input signal applied to the transistor 57 rises above the threshold level, it will cause a reversal in the state of the transistors 57 and 58. When such signal drops below the threshold level, the circuit will reverse to its standby or initial starting position. Relay 56 has a capacitor 59 paralleled with its winding 60 and connected in parallel with the output of the transistor 58 and consequently is energized when the transistor 58 is cut E and is deenergized when the transistor :58 is turned on. The relay 56 is spring loaded or otherwise biased to an alarm position and it is held in an off or non-alarm position by energization. This is desirable to provide the normal fail-safe operation such that in the event of failure of the motion detection channel the relay will be released and move to the alarm position to indicate the fact that the intrusion system is not operative.

Addiionally, in the il lustratedcircuit, a diode 61 is paralleled with a resistor 62 between relay 56 and the collector of transistor '58. This decreases the reset time of the relay circuit.

The operation of the marized as follows.

If a moving body is present in the high energy beamed field between antennas 2 and 4, the alternating current signal detected by the detector includes a Doppler frequency. The Doppler modulated signal amplified by amplifier and compensated by circuit 11 reduces the effects of temperature variation. The amplified signal is then applied to channels 6 and 7. The potentiometer 48 of circuit 6 transmits a selected proportion of the sigdetection system is briefly sumnal to filter 13 which removes high frequencies components and transmits the Doppler frequency component to amplifier 14. The amplified motion-related or Doppler signal is applied to the Schmitt trigger circuit and if of suificient amplitude, causes the circuit to reverse its state wherein transistor 58 is turned on. As a result, relay 56 is de-energized. The relay 56 may control an alarm system having. means, not shown, to maintain the alarm indication after the trigger signal is removed at the end of the cycle and the relay 56 reverts to the normal energized position. The amplitude of the Doppler signal for any given object will be essentially independent of the position of the object between the transmitter 1 and the detector 5. For example, if the object is immediately adjacent the transmitter 1, the energy intercepted will be at or near a maximum. However, the Doppler-shifted signal will have to move through essentially the complete length of the path being protected. Consequently, the losses reduce the signal to a selected level. On the other hand, if the intrusion is made adjacent the detector 5, the intercepted energy is of course relatively low as a result of the losses in energy transmission. However, the Doppler-shifted signal travels a very short distance to the detector 5 and is received thereat at substantially the same level as in the initial case wherein the moving object was adjacent the transmitter. In summary, the level of the intercepted signal varies inversely with the distance from the transmitter 1. The speed or rate of movement of the that the Doppler signal travels a distance which similarly varies inversely with the distance between the body and the transmitter 1. The speed of rate of movement of the target however directly effects the frequency and the determination of the Doppler signal and the duration that the Doppler signal is present. In accordance with the present invention, the motion detection channel 6 may not detect exceedingly slow motion of even large bodies such as a human being. To design the system with sufiicient power, gain and sensitivity creates corresponding sensitivity to unwanted spurious signals from precipitation, small objects and the like.

The present invention employs the dual channels 6 and 7 with the supervisory channel 7 also serving to detect the exceedingly slow motion of a large body.

The amplified and temperature compensated signal from the compensator 11 is coupled to channel 7 by a small capacitor 63 in series with a potentiometer 64 having a tap 65. A diode 66 connects tap 65 to the amplifier 18 with the high pass filter 17 connected therein. Capacitor 63 is a small unit which essentially blocks the low frequency Doppler modulation such that only the subcarrier frequency signal created by the 4 kc. pulsing of the energy field appears in channel 7. The filter 17 is a capacitor-resistor unit which further insures removal of any extraneous signals and provides a relatively steady D.C. component corresponding to the level of the incoming carrier signal. The rectified carrier signal is amplified by amplifier 18 which is shown as a single stage transistor 67. The amplified signal is fed to a low level Schmitt trigger circuit 68 and forming a part of supervisory circuit 19. Thus, circuit 68 includes a pair of transistors 69 and 70. Transistor 69 is normally biased on or conducting by the incoming signal from transistor 67 and the transistor 70 is correspondingly biased off. A supervisory relay 71 is connected in shunt with the output circuit of the transistor 70 between the collector of the latter and the positive side of the power source, as shown. Consequently, relay 71 is normally energized and holds an alarm in the off position. If the modulating signal is lost for any reason, the transistors 67 and 69 will be cut off. When transistor 69 stops conduction, the other transistor 70 of the Schmitt circuit conducts. Transistor 70 then provides a very low resistance and an effective short circuit in parallel with relay 71 which is then deenergized and causes an alarm signal.

Although not absolutely necessary, a resistor 72 is shown in the connection of the emitters of the transistors 69 and 75B of the low level circuit 68. This increases the capability of discriminating input levels at very exceedingly low voltages.

The supervisory channel 7 is responsive to the presence of the modulating frequency signal and i s absence of a decrease below a selected level triggers the circuit 19. As previously described, the present invention employs the channel '7 to supervise the transmission of the radiofrcqucncy energy and further to detect ver slow moving targets. Thus, a very slow moving target of a size of a human or the like passing through the radio-frequency field results in a substantial reduction in the carrier signal transmitted to the receiving unit and therefore to channel 7 which will cause de-energization of relay 71. This is of substantial significance in eliminating the problems inherent in trying to adjust a system to otherwise detect all motion in a field and yet not detecting spurious signals generated by the surrounding environment and small objects moving through the field.

In the illustrated embodiment of the invention, the crystal detector employed may be energized with a slight direct current bias. This increases its sensitivity somewhat but more importantly tends to provide increased temperature stabilization.

Although the systems shown above provide highly satisfactory operation, it had been found that the slow alarm system may be affected and actuated by slowly changing normal conditions; for example, atmospheric changes, accumulations of debris, snow and the like. Further, when a plurality of the free running transmitting devices are employed in stacked relation, in order to increase the protective area, cross-talk effects may be introduced and cause malfunctioning.

Another embodiment of the present invention is shown in FIGS. 4 and wherein the above disadvantages are essentially eliminated.

Referring particularly to FIG. 4, the system is shown generally in block diagram with elements corresponding to those shown in FIG. 1 similarly numbered for simplicity and clarity of explanation. In FIG. 4, a modified transmitting system is shown particularly adapted for multiple stacked unit detection systems. Thus, in FIG. 4, the power supply for the klystron transmitter 2 is generally similar to that previously described with respect to FIGS. 1 and 2. In the embodiment of FIG. 4, however, the reflector supply is provided with a filtering stage to establish an essentially ripple free DC. signal at the tap 39 and thus without the ripple component employed in the embodiment of FIG. 2. Superimposed on this DC. signal is a square wave signal to provide the desired pulsing operation of the transmitter 2 by control of the reflector voltage. In the illustrated embodiment of the invention, a square wave signal source 75 is shown in block diagram and coupled into the reflector voltage lead by a suitable transformer 76. A second transformer '77 has its primary connected in parallel with the primary of the transformer 76 and thus provides a source to a second klystron transmitter 78. The power supply and receiver for transmitter 78 is not shown. 1

The transmitter of FIG. 4 operates in the same general manner as that previously described except that a synchronized pulsing of the energy is provided and maintained without adverse interaction between the two systems.

The receiving circuitry is also generally similar to that previously described and includes a suitable detector 5 having an amplifier for amplifying the detected signal and applying the signal simultaneously to the supervisory and alarm circuit 7 and to the alarm circuit 6. In addition, in the embodiment of FIG. 4, the amplifier It) is provided with a feedback system 79 to modify the output signal in a manner compensating for exceedingly slow amplitude changes in the received signal such that the circuit '7 is not triggered by very slow changing environmental conditions such as caused by changes in the weather, accumulating debris and the like. Generally, the feedback system '79 includes an amplitude detector 8:) and a time delay and automatic gain control unit 81 interconnecting the output of the amplifier 10 to an input stage, as more fully shown in FIG. 5.

In operation, the system responds to both slow and rapid movement within the field to trigger the supervisory and alarm circuit 7 or alarm circuit 6. However, the feedback systcm 79 established by the amplitude detector 80 and the delayed automatic ain control unit 81 provides automatic compensation for exceedingly slow amplitude variations in the received signal such that they cannot trigger the supervisory and alarm circuit 7.

More particularly, a preferred construction of the feedback system 79 and its connection is shown in detail in FIG. 5 along with modification of the supervisory and alarm circuit.

In FIG. 5, the amplifier Itl is shown having a modified input stage including a transistor 82 of an NPN variety connected in a common emitter configuration. The base of the transistor 82 is connected by a suitable coupling network to the output of the detector. The collectoremitter circuit is connected to suitable positive and negative power supply lines with the collector connected in the conventional manner. The emitter 83 of transistor 82 is connected to the negative line 84 through a paralleled resistor 85 and a capacitor 86 and a portion of the delay and automatic gain control circuit 81. The feedback system 79 modifies the emitter bias on the transistor 82 to compensate for slow variations in the changes in the output signal such as would be caused by changing weather conditions, accumulating debris and the like. The amplitude detector 80 of system '79 is shown including a large coupling capacitor 87 coupled to a diode detector circuit including a diode 88 in series with a Zener diode 89. A resistor-capacitor network 90 is connected to the output side of the Zener diode 89 to provide the desired high degree of rectification efiiciency and provide a direct current signal which is amplified by an amplifying transistor 91. The amplified signal is connected through a coupling circuit 92 to the time delay and automatic gain control circuit 81.

In the illustrated embodiment of the invention, the circuit 81 includes an emitter-follower transistor 93 as the input stage. A transistor 94 is connected to transistor 93 to amplify the follower signal and to bias the emitterfollower output transistor 95. The transistors 93, 94 and 95 are shown as NPN type transistors and the emitter 96 of the follower transistor 95 is connected by a large coupling capacitor 97 to the base 98 of the first stage transistor 93. The follower is also connected by a load and coupling network 99 to a control transistor 100.

The control transistor 100 has its emitter to collector circuit connected in series between the paralleled resistor 85 and capacitor 36 of the amplifier 10 and the negative signal power supply line 84. The transistor 100 constitutes a variable impedance element connected in the circuit of the emitter 83 of the transistor 82. This in turn controls the conductivity of the transistor 82 to compensate for slow moving signals as follows. The capacitor 97 provides a feedback path from the transistor 95 to the input of transistor 93. The capacitor 97 is relatively large and consequently prevents rapid changes in the input from affecting the output signal. However, over long periods of time, a charge on the capacitor 97 is established which effects the amplifying output of the circuit and thereby varies the input bias on the variable impedance transistor 100. The time constant is selected to prevent triggering of circuit 7 under changes occurring over relative long periods while maintaining the system sensitivity to unauthorized personnel movement which even though extremely slow will not approach that of the com- 9 pensated changes. As a practical matter, it has been found that a time constant of approximately five minutes or so provides a highly satisfactory control circuitry.

In summary, the output of the amplifier 1G is continuously detected to energize the delay and automatic gain control circuit 81 which inserts a selected time delay into the response modification such that it will respond to only changes occurring over a very long period of time. In this manner, the response modification compensates for and effectively eliminates signals only for changes in the environment of the beam over periods substantially greater than the selected minimum motion of a target within the beam which shall actuate circuit 7.

The circuits 6 and 7 can be constructed in the same manner as that shown in the previous embodiment. In FIG. 5, an alternative embodiment of the supervisory and alarm circuit 7 is shown. In FIG. 5, the output signal of the amplifier 10- is connected through a suitable low pass filter circuit 1-01 to a transistor amplifier including an input bias transistor 102 and an output stage transistor 103. The relay 71 is connected in the collector circuit of the transistor 103. This circuit employs a bias emitter stage rather than a regenerative Schmitt trigger circuit such as shown in FIG. 3. The circuit of FIG. has been operated satisfactorily.

The transmitted signal may also be modulated either at the transmitter or the receiver employing a mechanical flapper or with an electronic switch.

Further, in the illustrated embodiment of the invention, the transmitted signal is shown as the modulated signal. Where extreme security is required, the pulse rate signal can be transmitted separately to the receiving station for synchronous demodulation. This will also improve the signal to noise characteristics. However, the added complexity and initial cost of the receiving system generally limits such applications having high security requirements such that cost considerations are generally secondary.

The present invention thus provides a perimeter type alarm system which can be employed in the outdoor environment without generating spurious signals as a result of small targets such as paper, leaves, small animals, precipitation and the like.

Various modes of carrying out the invention are con templated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

I claim:

1. In a perimeter type intrusion detection system for detecting the movement of a traget of a preselected minimum size,

means for radiating a directional beam of energy having a selected frequency characteristic, detection means mounted in spaced aligned relation to intercept said beam of energy and including response means to detect changes in the frequency characteristic in response to movement of a target and being constructed and arranged to require a predetermined minimum speed, such target movement within the beam changing the frequency characteristic and producing a Doppler frequency signal,

an output circuit means connected to the detection system and being responsive to the Doppler frequency signal and actuated by essentially a single cycle of said Doppler frequency signal of a selected minimum frequency and amplitude, said output circuit means including a signal amplifying section connected to the detection means, and a feedback circuit in the section and including a signal delay means, said feedback circuit modifying the amplifying section to compensate for changes in the environment of the beam over periods substantially greater than the selected minimum motion of a target within the beam.

2. The intrusion detection system of claim 1 wherein said amplifying sections include an input transistor connected in a common emitter configuration, a control transistor, said input transistor having an emitter connected to a supply means in series with said control transistor, and said feedback circuit being connected to bias the control transistor and including a time delay means.

3. The intrusion detection system of claim 1 wherein said feedback circuit includes an amplitude detector connected to the output of the amplifying section and a time delay automatic gain circuit connecting the amplitude detector to the input of the amplifying section.

4. In an intrusion detection system for detecting the movement of a target of a preselected minimum size,

source means for radiating a beam of energy having a selected frequency characteristic,

detection means mounted in spaced aligned relation to intercept said beam energy and including means to detect changes in the frequency characteristic movement of a target within the beam being effective to change the frequency characteristic to produce a Doppler frequency signal,

means to modulate the energy to provide a selected modulation characteristic in the detection means, and

a pair of parallel output circuits connected to the detection system, one of the output circuits being responsive to the Doppler frequency signal and the second output circuit being responsive to a selected change in the signal from the detection means.

5. The detection system of claim 4 wherein the second output circuit is responsive to loss of a signal of the modulation characteristic.

6. The detection system of claim 4 wherein means to modulate the energy forms a part of the means for radiat ing a beam of energy.

7. The detection system of claim 6 wherein said means to modulate is a pulsing means to create a pulsed modulated beam.

8. The intrusion detection system of claim 4 wherein said pair of parallel output circuits is respectively responsive to changes in the frequency characteristic and to signal changes of the modulation characteristic, said first output circuit being selected to respond to frequency characteristics generated by targets of a preselected size and rate of movement and said second output circuit being selected to respond to a selected loss of the signal of the modulation characteristic.

9. An intrusion detection system for detecting the movement of a body of a preselected minimum size, comprising source means to generate a radio frequency and adapted to be mounted adjacent one end of an area and to direct the radio frequency signal as a beam across an entrance to said area,

a detector adapted to be mounted in spaced alignment with the source means to intercept the beam at the opposite end of the entrance to said area,

the energy in said beam being essentially inversely proportional to distance from the source whereby the detector is energized at a similar level for any given target within the entrance,

a first demodulating channel connected to the detector to produce a signal corresponding essentially only to the Doppler components of the intercepted signal, and

a second demodulating channel connected to the detector to produce a signal corresponding essentially to only the carrier component of the intercepted signal.

10. The detection system of claim 9 having temperature compensating means connected to the output of the detector to modify the signal and compensate for environmental temperature variations.

11. The intrusion detection system of claim 9 wherein the first channel includes a diode detector connected to an adjustable potentiometer for presetting the minimum amplitude of the operative Doppler component.

l l 12. The intrusion detection system of claim 9 wherein the second channel includes a trigger circuit having a normally nonconducting trigger element and responsive to the presence of a signal from the detector to turn the trigger element on.

13. The intrusion detection system of claim 12 wherein the trigger circuit is a Schmitt trigger circuit.

14. The intrusion detection system of claim 13 having a transistor means connected as a variable impedance in the amplifying section to control the gain of the amplifying section and wherein said gain control circuit is connected to the transistor means and includes an amplifying means including a timing capacitor.

15. The intrusion detection system of claim 9 wherein an amplifying section connects the detector to said channels, said amplifying section including a feedback circuit including an amplitude detector and an automatic gain control circuit, said control circuit including a signal delay means whereby only changes in the environment of the beam over periods substantially greater than selected minimum motion of a target Within the beam.

16. An intrusion detection system for detecting the movement of a body of a preselected minimum size, comprising a klystron tube having a D.C. to D.C. converter connected to create a beam voltage and a reflector voltage, said converter selected to have a substantial ripple component in the reflector voltage to provide a pulsed radio frequency signal,

said tube being adapted to be mounted adjacent one end of an area and to direct the pulsed radio frequency signal as a beam across the area, motion of a target within the area generating a Doppler modulation of the signal,

a crystal detector,

said crystal detector being adapted to be disposed in spaced alignment with the klystron tube to intercept the beam at the opposite end of the area, the energy in said beam being essei ially inversely proportional to distance from the oscillator whereby the detector is energized at a similar level for any given object Within the area,

amplifier means conectcd to the detector to amplify the signal,

temperature compensating means connected to the output of the amplifier means to compensate for environmental temperature variations,

a first demodulating channel connected to the compensating means and including a demodulator and a filter to produce a signal corresponding essentially only to the Doppler components of the intercepted signal,

a second demodulating channel including a demodulator and a filter to produce a signal corresponding essentially to only the carrier component of the intercepted signal, and

load means connected to the output of the channels to detect the presence and absence of the corresponding signals in the respective channels.

17. The detection system of claim 16 wherein the converter includes a direct current filter in the ripple voltage supply and causing said klystron to be periodically switched in and out of oscillation to generate a pulsed radio frequency signal.

References Cited UNITED STATES PATENTS 2,649,538 8/1953 Marlowe et al. 340-258 2,903,683 9/1959 Bagno 340-258 3,111,657 11/1963 Bagno 340-258 3,149,318 9/1964 Bagno et al. 340258 3,270,339 8/1966 McEuen et al. 3435 3,292,096 12/1966 Deneen 33029 3,331,065 7/1967 McDonald 340258 RODNEY D. BENNETT, Primary Examiner.

C. L. WHITHAM, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,378,834 April 16, 1968 Paul I. Corbell It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 65, "sisignal" should read signal Column 6 line 28 "The speed or rate of movement of the" should read but this is compensated for by the fact line 31, "of" should read or Column 9, line 49, "traget" should read target Column 10, line 48, after "frequency" insert signal Column 11, line 8, "14." should read lS. same line 8, "13" should read l4 line l4,

"l5 should read 14 (SEAL) Signed and sealed this 4th day of November 196?.

Attest:

Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents

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