Gershberg, David N. (Rockville, MD)
Lee, Alex Y. (Arlington, VA)

05/273596

04/02/1974

07/20/1972

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E-Systems Incorporated (Dallas, TX)

340/516

342/52, 367/94, 342/28, 340/554, 340/522

G01S13/56; G01S13/86; G08B13/16; G08B13/24; G01S13/00; G08B13/16; G08B29/00

340/258C,258D,258A,258R,409,411 343/7ED,7A,5PD,17.7

3603973

COMBINATION FIRE AND BURGLAR ALARM SYSTEM

September 1971

Hough

Caldwell, John W.

Curtis, Marshall M.

Richards, Harris & Medlock

What is claimed is

1. An ultrasonic-microwave intrusion system including an alarm for producing an indication of movement within a specified area, the combination comprising:

2. An ultrasonic-microwave intrusion system as set forth in claim 1 wherein said ultrasonic detector includes:

3. An ultrasonic-microwave intrusion system as set forth in claim 2 wherein said transmitting transducer includes:

4. An ultrasonic-microwave intrusion system as set forth in claim 2 wherein said frequency generator includes an R.C. driven operational amplifier, and a radiating element coupled to one terminal of the operational amplifier to lock the frequency output of said generator to the frequency-temperature characteristics of said radiating element, said radiating element is also employed as a transmitting element in the overall system.

5. An ultrasonic-microwave intrusion system as set forth in claim 4 wherein said frequency generator further includes a complimentary symmetry power output switch coupled to the output of said operational amplifier.

6. An ultrasonic-microwave intrusion system as set forth in claim 1 wherein said microwave detector includes:

7. An ultrasonic-microwave intrusion system as set forth in claim 6 wherein said balanced mixer includes:

8. An ultrasonic-microwave intrusion system as set forth in claim 7 including an RF choke coupled to said transmission line at the radiating element end thereof.

9. An ultrasonic-microwave intrusion system as set forth in claim 8 wherein said balanced mixer further includes a D.C. level sampling connection for generating a fault condition signal to said test circuit means.

10. An ultrasonic-microwave intrusion system including an alarm for producing an indication of movement within a specified area, the combination comprising:

11. An ultrasonic-microwave intrusion system as set forth in claim 10 wherein said transmitting transducers include:

12. An ultrasonic-microwave intrusion system as set forth in claim 11 wherein said fault responsive circuit includes:

13. An ultrasonic-microwave intrusion system as set forth in claim 11 wherein said frequency generator includes an R.C. driven operational amplifier, and a radiating element coupled to one terminal of the operational amplifier to lock the frequency output of said frequency generator to the frequency-temperature characteristics of said radiating element.

14. An ultrasonic-microwave intrusion system as set forth in claim 10 wherein said balanced mixer includes:

15. An ultrasonic-microwave intrusion system as set forth in claim 14 including an R.F. choke coupled to the radiating element end of said transmission line.

16. An ultrasonic-microwave intrusion system as set forth in claim 15 wherein said balanced mixer further includes a D.C. level sampling connection for generating a fault condition signal to said test circuit means.

17. In an intrusion alarm system wherein an ultrasonic detector and a microwave detector each generate a signal varying with movement within a specified area, said detector signals combined in circuit means for providing an alarm when the detector signals simultaneously exists at a level indicating movement within the specified area, wherein the microwave detector comprises in combination:

18. In an intrusion alarm system as set forth in claim 17 wherein said balanced mixer includes:

19. In an intrusion alarm system as set forth in claim 18 wherein said balanced mixer further includes an RF choke coupled to the radiating element end of said transmission line.

20. In an intrusion alarm system as set forth in claim 19 wherein said balanced mixer further includes a D.C. level sampling connection for generating a fault condition signal to a test circuit.

21. In an intrusion alarm system wherein an ultrasonic detector and a microwave detector each generate a signal varying with movement within a specified area, said detector signals combined in circuit means for providing an alarm when the detector signals simultaneously exist at a level indicating movement within the specified area, wherein said ultrasonic detector comprises in combination:

22. In an intrusion alarm system as set forth in claim 21 wherein said frequency generator includes an R.C. driven operational amplifier, and a radiating element coupled to one terminal of the operational amplifier to lock the frequency output of said generator to the frequency-temperature characteristics of said radiating element.

23. In an intrusion alarm system as set forth in claim 21 wherein said transmitting transducer elements include:

24. In an intrusion alarm system as set forth in claim 23 wherein said fault responsive circuit includes:

The present invention relates to motion detection systems operating on the doppler principle, and more particularly, to a combination of ultrasonic and microwave doppler motion detection systems for reliable movement evaluation.

One class of motion detector systems employs a sensitive receiver in conjunction with a transmitter to receive and measure an electric field. If an intruder or foreign object disturbs the electric field there results a variation in the field strength which is detected by the receiver and used to trigger an indicator or alarm system. Another class of motion detector systems in the space alarm system characterized by the transmitting of energy into a specified space to be protected, or the space surrounding an object to be protected, and subsequently receiving that portion of the transmitted energy that is reflected by the surroundings. An alarm is triggered upon detection of a disturbance, i.e., frequency change, in the reflected energy caused by an intruder within the area. Any frequency change of the reflected energy, as compared to the transmitted energy, will indicate an object is moving within the area being monitored. This is the principle of operation of the well known "doppler" effect. This type of system detects a doppler frequency shift in radiation reflected by moving objects within a specified area.

The present invention pertains to a space alarm system of the doppler type, and more particularly, to a doppler system in which the energy radiated is ultrasonic energy from one set of transducers and microwave electromagnetic radiation from a monostatic antenna.

The basic parameter in the optimization of any motion detection system is the attaining of the highest probability of detection of motion, with the lowest probability of false aarm. Although numerous systems have been devised to reliably extract a doppler signal from a received wave, false alarms continue to exist. Since an intruder moves a much slower rate than do the transmitted radiations, the doppler effect of the moving intruder causes a very small percentage variation in the frequency of the received radiation. For example, if energy having a frequency of 20 KHz is reflected from an intruder moving at one foot per second, the doppler effect of the motion of the intruder will cause the received frequency to differ from the transmitted by only 37 Hz. This is a very small shift to reliably detect. An optimized detector will, however, reliably detect the doppler effect while rejecting extraneous signals which can be confused with a motion generated doppler signal.

A motion detection system in accordance with the present invention has a lower false alarm rate than existing systems of comparable complexity by combining an alarm signal from a microwave subsystem with the alarm signal from an ultrasonic subsystem. Only if an output signal exists from both subsystems simultaneously will a system alarn be sounded indicating motion within a specified space.

In many applications, motion detection systems are called upon to protect an extended area. In protecting such areas, certain problems are encountered. For example, an antenna unit of an alarm system which is sensitive to intruders at great distances from the antenna usually has an excessively high probability of responding to extraneous disturbances which originate near the antenna. On the other hand, a unit designed to have a more uniform sensitivity both near and far from an antenna is more likely to respond to disturbances originating beyond the confines of the area being protected. Therefore, for the uniform coverage of an extended area, it is generally desirable to employ a plurality of units distributed about the area to be protected. A drawback of many microwave doppler types of intrusion detectors is the need for expensive and complex systems utilizing separate transmit and receive antennas for protection of extended areas or, if a single antenna is used, expensive duplexer networks are used to protect the receiver from damage by the high power signal being transmitted. An advantage of the system of the present invention is the utilization of multiple ultrasonic transducers for coverage of large specified volumes wherein the sensors are interconnected in parallel by inexpensive bandpass filtering networks. Further, a microwave subsystem, for improving system reliability, utilizes a single omnidirectional transmit/receive (monostatic) antenna in conjunction with miniaturized RF circuitry to provide balanced doppler mixing without complex radio frequency devices.

One area of intrusion alarm systems which has heretofore received little investigation is the problem of fault detection within the system. More presently available intruder systems may be simply disenabled by an intruder prior to his entry into a protected area. In addition, a failure of one or more of the transducer assemblies may render the complete system ineffective in a manner not detectable by an operator. With the present invention, both the ultrasonic and microwave portions of the system are provided with automatic self-test and tamper monitoring and external self-testing circuits. The ultrasonic automatic self-testing and tampering circuitry employs techniques wherein the monitoring signal is transmitted on the same two wire cable that carries the ultrasonic power to the ultrasonic transmitting elements. The microwave portion of the system provides an automatic self-testing capability by use of a D.C. level sampling connection in a balanced mixer circuit having an output that supplies the automatic self-testing and tamper capability.

Both the ultrasonic and microwave subsystems feed the automatic self-testing and tampering signals through an automatic self-testing and tampering logic network. These signals, as derived from the ultrasonic oscillator and the microwave transmission oscillator, will vanish if: (1) the microwave power fails, (2) the ultrasonic power fails, or (3) if the lines supplying ultrasonic power to the ultrasonic transmitting elements are cut or shorted. The automatic self-testing and tampering combining logic network alarms if any one of the tamper signals vanishes.

In accordance with the present invention, an ultrasonic-microwave intrusion system including an alarm for producing an indication of movement within a specified area includes an ultrasonic sensor responsive to movement within the specified area and providing an output signal varying with such movement. A microwave sensor also responds to movement within the specified area and provides an output signal varying therewith. Connected to the ultrasonic sensor and the microwave sensor is a combining circuit that generates a signal to the system alarm when the output of both sensors exists simultaneously at a level indicating movement within the area of interest. Test circuitry also connects to the ultrasonic sensor and microwave sensor and responds to fault conditions within the system to generate a fault indication signal to the system alarm.

A more complete understanding of the invention and its advantages will be apparent from the specification and claims and from the accompanying drawings illustrative of the invention.

Referring to the drawings:

FIG. 1 is a physical layout of an ultrasonic-microwave intruder alarm system using three ultrasonic assemblies and a single microwave antenna;

FIG. 2 is a block diagram of the intrusion alarm system of the present invention wherein signals from an ultrasonic sensor and a microwave sensor are combined to produce a system alarm signal;

FIG. 3 is a block diagram of the microwave sensor portion of the system utilizing a microstrip oscillator connected to a balanced mixer;

FIG. 4 is a schematic of a microstrip oscillator coupled to a H microstrip balanced mixer;

FIG. 5 is an equivalent circuit of the microwave portion of the system illustrating that a matched antenna presents a resistive load with no target present;

FIG. 6 is a block diagram of the ultrasonic sensor portion of the system of FIG. 2 employing a plurality of transmitting radiating elements and a plurality of receiving elements;

FIG. 7A and 7B are schematics showing circuitry for combining the receiving elements and the transmitting elements, respectively;

FIG. 8 is a schematic of an oscillator, distribution and tampering monitoring circuitry for the ultrasonic transmitting elements;

FIG. 9 is a plot of frequency in KHz as a function of temperature for a typical ultrasonic radiating element;

FIG. 10 is a schematic of a radiating element controlled oscillator and driver circuitry; and

FIG. 11 is a schematic of a test modulator and mixer circuitry for the ultrasonic sensor portion of the system of FIG. 2.

Referring to FIG. 1, there is shown an intrusion alarm system for protecting a specified area, for example, a room 100 feet by 50 feet, employing a central detector unit 10 and outboard transducer units 12 and 14 all mounted approximately 9 feet above floor level by means of pipe brackets 16 which also serve as cable conduits. The central detector unit 10 includes four ultrasonic receiving elements 18 (only two shown) and a microwave antenna 20 which functions in both the transmit and receive modes, as will be explained. In each of the outboard transducer units 12 and 14, there are ultrasonic transmitting elements 22 (only two shown). For some small areas to be protected, one or more ultrasonic transmitting elements 22 and the ultrasonic receiving elements 18 may all be incorporated into a central detector unit eliminating the need for the outboard transducer units. The present embodiment actually incorporates one ultrasonic transmitting element (not shown) in the central detector unit. This transmitting element controls an oscillator frequency, as to be described. Operationally, when using outboard transducer units or a composite central detector unit, the system functions in the manner to be described.

Signals to and from the central detector 10 and the outboard transducer units 12 and 14 are provided from a central controller 24 containing system electronics. A master control switch 26 controls operation of the system to provide intrusion protection when required and to disenable the system for alarm free movement within the specified area to be protected. Alarm signals from the central controller 24 are supplied to a monitoring console 28, which as shown includes four rows of alarm annunciators of any commercially available type. Since more than one specified area may be monitored from the same console 28, a plurality of central controllers 24 will connect to the console 28, one for each of the annunciator positions if full utilization of the console is anticipated.

Referring to FIG. 2, there is shown a block diagram of the doppler intrusion alarm system of FIG. 1 including the ultrasonic subsystem 30 that incorporates both the transmitting elements 22 and the receiving elements 18 along with the microwave subsystem 32 that comprises the microwave antenna 20. Each of the subsystems 30 and 32 are complete doppler intrusion alarm systems within themselves and will be subsequently described.

An output alarm signal from each of the subsystems 30 and 32 is applied to inputs of a detector combining logic 34 within the central controller 24 for supplying an alarm signal on a line 36 to the monitoring console 28. Typically, outputs of the subsystems 30 and 32 may be logic level signals varying between an upper and lower state and applied to a NAND gate within the combining logic 34 for producing a logic varying signal on line 36. In accordance with standard logic network operation, an alarm signal appears on a line 36 whenever both inputs to the detector combining logic 34 from the subsystems 30 and 32 are at the same logic level. Thus, the detector combining logic 34 triggers only when both subsystems 30 and 32 alarm simultaneously indicating motion within the specified area to be protected. Hence, redundancy is provided to reduce the false alarm rate as often inherent in previous intrusion alarm systems.

Both the ultrasonic and microwave subsystems are continuously monitored for proper operation and in addition to tampering by someone attempting to bypass the system, self-test signals are applied to the subsystems 30 and 32 over a line 38 from the central controller 24. Both the ultrasonic and microwave subsystems provide self-testing and tampering signals to an automatic self-testing and tamper testing combining logic 40. These signals are derived from the ultrasonic and microwave transmission oscillator outputs and will vanish if the microwave power fails, the ultrasonic power fails or if the line supplying power to the ultrasonic transmitting transducers is cut or shorted. The automatic self-test and tamper testing combining logic 40 provides an alarm signal through the monitoring console 28 on a line 42 if any one of the tamper signals or self-testing signals vanishes. The automatic self-testing and tamper testing combining logic 40 may comprise an arrangement of NAND gates or OR gates as required to produce an alarm signal on line 42 whenever any of the self-test and tamper signals vanishes.

Referring to FIG. 3, there is shown a block diagram of the microwave subsystem 32 including the transmit/receive antenna 20 coupled to a transmit/receive balanced mixer 44, the latter also coupled to a microwave frequency oscillator 46. A doppler frequency signal from the balanced mixer 44 is applied to a doppler filter-preamplifier 48 that builds up the signal strength from the mixer and defines the doppler frequency pass-band. The doppler frequency passband rolls off at about 12 db per octave on the lower end and 36 db per octave on the upper end. This rapid upper end roll off improves rejection at 120 Hz, which is the frequency associated with false alarms due to the plasma generated by florescent lamps.

The doppler filter preamplifier 48 is provided with a resistive attenuator 50 for injecting an external test signal in accordance with a command on the line 38 from the central controller 24. This signal simulates the doppler frequency effect from the balanced mixer 44 as produced by a moving intruder and causes a normally functioning system to alarm while the test signal on line 38 is being applied to the attenuator 50.

Signals from the filter preamplifier 48 are applied to a filter-postamplifier 52 for further amplification of the doppler frequency signals. An output from the postamplifier 52 is applied to a rectifier/integrator 54 that renders the doppler signals unidirectional by means of a standard full wave rectifier and then couples this unidirectional signal to an integrator circuit. The unidirectional signal is integrated with respect to time and when the doppler signal has existed for a sufficient length of time for the integrator to have integrated to an established level, a signal applied to an alarm level detector 56 triggers the detector to produce an alarm signal on a line 58 to the detector combining logic 34.

Returning to the microwave subsystem "front end" including the balanced mixer 44 and the oscillator 46, there is shown in FIG. 4 these components fabricated using microstrip techniques. The balanced mixer 44 employs a symbolic H microstrip structure to miniaturize the assembly, reduce false alarms, improve reliability and provide greater economy. A mixer as illustrated provides balanced doppler mixing with a single transmit/receive "monostatic" antenna 20 without utilizing conventional circuitry involving circulators, hybrids, power amplifiers and additional circuit complicating components.

Considering first the microstrip oscillator 46, a transistor 60 has a grounded collector electrode, an emitter electrode coupled to an inductance coil 62 and a base electrode tied to a microstrip line 64. Also coupled to the microstrip line 64 is a resistor 66 and adjustable capacitors 68 and 70. The adjustable capacitor 68 provides a means for tuning the frequency of oscillation of the oscillator 46 and the capacitor 70 provides maximum power coupling between the oscillator 46 and the mixer 44. Also included as part of the oscillator 46 is a resistor 72 conected to the inductance coil 62 and a line capacitance 74 at a terminal 76 for roviding a D.C. voltage to the oscillator.

Microwave transmission frequencies from the oscillator 46 are applied to the H microstrip line 78 through the capacitor 70. Tied to the output end of the microstrip line 78 is the transmit/receive antenna 20 and an inductance coil 80. Coupled to the junction of line stubs in the main transmission line of the H microstrip line 78 are peak detector diodes 82 and 84. Peak detector diode 82 mixes to pass doppler frequency signals to a resistor capacitor network including a resistor 86 and a capacitor 88. This is the so-called doppler frequency produced by movement within the specified protected area. The peak detector diode 84 mixes to pass a phase-inverted doppler frequency signal to a resistance capacitance network including a resistor 90 and a capacitor 92. Resistors 86 and 90 are interconnected to an output terminal 94 that provides an interconnection to the filter-preamplifier 48. A line capacitor 96 is associated with the line interconnecting the resistors 86 and 90.

In operation of the oscillator 46 and the balanced mixer 44, relatively low operating frequency (e.g., 915 MHz) is established by the oscillator 46. By employing a relatively low operating frequency the effective frequency reflective cross section of a target within the specified area is small when the target dimensions are small with respect to the operating wavelength of the oscillator 46. Hence, for short wavelengths at X-band frequencies false alarm could occur from small targets such as cats and mice; whereas, at the longer wavelengths corresponding to 900 MHz false alarms from small targets of this type are lowered. A second advantage of operating the oscillator 46 at a relatively low frequency is economical in that it may be generated by a simple transistorized oscillator fabricated in microstrip. To obtain equivalent power at X-band, it is usual to employ a cavity-diode oscillator. Also, diodes and other circuit components of the mixer 44 at the relatively low operating frequencies are more economical than in equivalent X-band mixers. A third advantage of a relatively low operating frequency from the oscillator 46 is that indoor microwave systems respond to the 120 Hz plasma produced by florescent lighting tubes. Consequently, it is advantageous to provide a microwave frequency which places 120 Hz outside the doppler frequency passband for target speeds of interest.

The transmission frequency from the oscillator 46 is applied to the input terminal of the balanced mixer 44 which may best be understood in operation by treating a moving target within a specified area in terms of a time-varying impedance reflected thereby back into the antenna 20. Such an approach is considered valid so long as the target speed is negligible with respect to the velocity of wave propagation. For intrusion alarm systems, this is hardly a problem. Physically, a target creates a weak spatial VSWR pattern which is dragged along as it moves. This pattern couples into the microwave antenna 20 as a time-varying impedance.

With reference to the equivalent circuit of FIG. 5, the microwave antenna 20 presents a resistive load 20a (Z _o ) with no target present. If the antenna terminal voltage of the transmitted wave is represented by E and the antenna terminal voltage of a target return wave given by V, the microwave range equation is as follows:

V = EGλ√σ/(4π) ³ /2 x ² e ^{- j4} π x/ λ (1)

where x = distance between the antenna 20 and a target 98,

σ = microwave cross section of the target 98,

λ = operating wavelength of the system, and

G = the gain of the antenna 20.

The time-varying terminal impedance Z(t) of the antenna 20 may be derived from the equation:

Z(t) = Z _o (1 + Γ)/(1 - Γ) (2)

where

Γ= V/E = Gλ√σ/(4π) ³ /2 x ² e ^{- j4} π

If the target 98 moves at a speed S then:

x = St (4)

and

e ^{- j4} π x/ λ = e ^{- j2} π (2S/ λ )t (5)

where the term 2S/λ is the expression for doppler frequency.

Returning to the general case of time-varying impedances, the net radio frequency voltage B across the terminal impedance Z(t), see FIG. 5, is given by the equation:

B = 2EZ(t)/[Z _o +

Solving this equation for the present impedance Z(t) results in:

B = E [1 + Gλ√π/(4π) ³ /2 S ² t ² e ^{- j} ωt ] (7)

connected providing

where ω = 2π times the doppler frequency.

From the equation (7), a doppler output frequency is obtained from the peak detector diode 82 which produces a voltage proportional to the absolute magnitude of the radio frequency voltage B. At the diode 82 an output is produced as given by the expression:

│B│ α E [1 + Gλ√σ/(4π) ³ /2 S ² t ² cos ωt] (8)

This expression contains a D.C. level signal with a weak amplitude modulation at the doppler frequency imposed thereon. The modulation intensity is inversely proportional to S ² t ² , which is the normal range attenuation for microwave signals.

In most intrusion alarm systems of the doppler frequency type, the doppler signal must be amplified on the order of 90 db to bring it to usuable level. Since the D.C. level of the peak detector 82 can be several volts, capacitive coupling is employed between the detector 82 and the doppler amplifier chain including the preamplifier 48. This prevents D.C. saturation of the amplifier chain, however, it does not isolate amplifiers 48 and 52 from a time variation of the D.C. voltage level. Such a variation arises from amplitude modulation and noise on the oscillator 46 and can produce false alarms from the radar subsystem 32.

To minimize the effects of D.C. level variation, the peak detector diode 84 is connected to the microstrip network 78 and interconnected with the peak detector diode 82. This cancels out the D.C. voltage at the terminal 94 to provide balanced mixing. It follows, that the doppler frequency signals will also cancel out unless the doppler output of the diode detector 84 is phase inverted with respect to the output of the diode detector 82. In the balanced mixer of FIG. 4, this is accomplished by connecting the detector diode 84 across the transformed radio frequency impedance Z'(t) in accordance with the equation:

Z'(t) = Z _o ² /Z(t) (9)

This transformation is performed by a four terminal radio frequency network known as an impedance inverter.

A further advantage of the H structured mixer 44 of FIG. 4 is that it may be considered a pseudo-lumped LC impedance inverter network. The detector diodes 82 and 84 at the output and input of the network, respectively, operate as high impedance peak detectors and draw very little radio frequency current. Thus, their effect on the radio frequency impedance may be neglected. Since the antenna terminal impedance Z(t) and the transformed radio frequency impedance Z'(t) reduces to the open circuit impedance Z _o with no target 98 present, the radio frequency voltage magnitude is the same for both diode detectors 82 and 84, resulting in equal D.C. voltage level out of both detectors. Considering the diode polarity and the interconnections thereof with the load resistors 86 and 90 a zero D.C. output voltage at the terminal 94 appears with no target 98 present.

Two additional important features of the mixer 44 are the RF inductance coil 80 at the output terminal to ground and a D.C. voltage level sampling connection at the anode of the diode detector 82. This sampling connection includes a resistor 99 and a line capacitor 100. This circuit supplies an automatic self-testing and tamper testing output from the "front end" of the microwave subsystem 32. Thus, the automatic self-testing and tamper testing output provides a continuous check on the microwave "front end," while the externl test signal provided to the attenuator 50 provides periodic checks of the remaining system components. The RF inductance coil 80 acts as a D.C. return for the diode detectors 82 and 84 and as a ground shunt for low frequency (60 Hz) disturbances that might couple to the stub 78a from the system environment.

Considering now the utrasonic subsystem 30, FIG. 7 is a block diagram of a complete ultrasonic doppler intrusion alarm system generating an alarm signal on line 102 to the detector combining logic 34. The four ultrasonic receiving transducers 18 are coupled to a transducer combining circuit 104 having an output applied to a filter-preamplifier 106. The ultrasonic transmitting transducers 22 of the outboard detector 12 are coupled to a transducer combining circuit 108 receiving transmission energy and sending test signals over a line 110 connected to a monitor circuit 112. Similarly, the ultrasonic transmitting transducers 22 of the outboard detector 14 are coupled to a transducer combining circuit 114 receiving transmission energy and sending test signals over a line 116 to the monitor circuit 112. An important feature of the ultrasonic subsystem 30 is the circuitry for sending ultrasonic power and automatic self-testing and tamper testing signals over the same two wires cables 110 and 116 to the combining circuits 108 and 114, respectively.

An amplified signal from the filter-preamplifier 106 is mixed with a modulation frequency generated by an oscillator 118 in a mixer circuit 120. Modulated signals from the mixer 120 are amplified in a filter-amplifier 122 where they are raised to a suitable level. Signals out of the filter-amplifer 122 are applied to a rectifier-integrator 124 wherein a full wave rectifier converts the doppler signals into unidirectional signals and applies them to a conventional integrator circuit. The integrator integrates the unidirectional signal with respect to time, and at a predetermined level triggers an alarm detector 126 for generating an alarm signal on the line 102 to the combining logic 34.

External self-testing and tamper testing of the ultrasonic subsystem 30 is provided by a test modulator 128 coupled to the oscillator 118. A self-test control signal on a line 130 triggers the test modulator 128 to provide a test signal on line 132 to the filter-preamplifier 106. The external self-test control signal on line 130 cannot be coupled directly to the ultrasonic doppler filter-amplifier 122 to provide a reliable test of the ultrasonic subsystem 30. Such a test would bypass the preamplifier 106 and balanced mixer 120 completely, giving a "good" system indication even if one or both of these components were in a fault condition. Consequently, the ultrasonic signal modulated at a frequency within the doppler passband is coupled into the ultrasonic preamplifier 106 in order to test the complete system chain.

The test signal on line 130 is a square wave from the central controller 24 and the test modulator 128 comprises a simple switch producing bursts of ultrasonic signals at a test signal rate. These bursts, as generated on line 132, are suitably attenuated for a proper test of the ultrasonic preamplifier and then coupled to the amplifier 106 input. The amplified test bursts from the preamplifier 106 are applied to the balanced mixer 120, which periodically becomes unbalanced to produce an output signal to the amplifier 122. An important consideration is that the ultrasonic test bursts on line 132 and the ultrasonic mixer drive signal from the oscillator 118 not be in phase quadrature otherwise very little mixer unbalance and output signal will be produced.

Considering now the transducer combining circuits 104, 108 and 114, the combining circuit 104 is shown in FIG. 7A and the combining circuits 108 and 114 are shown in FIG. 7B. The simplest approach for combining multiple transducers would be a parallel connection; unfortunately, parallel-connected transducers seldom give balanced performance because their resonant frequencies and impedances are seldom matched in a practical situation. For example, if the parallel transducers are driven as a transmitter, the transducer or transducers having series resonant frequencies the closest to the drive frequency will consume a large portion of the power because the remaining transducers exhibit higher impedances. Thus, some type of isolation/broad banding circuit is required for successful multiple-transducer large volume intrusion alarm operation.

The receiving transducer elements 18 are combined by using the circuit of FIG. 7A, which is derived from a basic filter circuit, and wherein each of the transducers 18 is coupled into a resonant circuit with a resonant inductor 134. Thus, the transducers 18 are incorporated in bandpass filters and these filters are isolated from one another by resistive elements 136. Impedance ratios are chosen such that the resistive elements 136 have only slight effect on system efficiency.

The transmitting transducer elements 22 are combined in the system of FIG. 7B wherein each of the transducers 22 is in series with a resonant inductor 138 to form a basic filter circuit which again incorporates the transducers into a bandpass filter with the filters isolated from one another by a resistive element 140. The ultrasonic power source shown in FIG. 7B comprises the oscillator 118 of FIG. 6.

Referring to FIG. 8, each of the outboard transducer units 12 and 14 are coupled to the monitor circuit 112 which in turn receives a frequency signal on a line 142 from the oscillator 118. The monitor circuit 112 distributes ultrasonic power to the transmitting outboard units 12 and 14 and receives automatic self-testing and tamper testing information from the outboard units by means of the two wire cables 114 and 146. The inductors 138 and the resistive elements 140 in series with each of the transducers 22 constitute the isolation/broad banding network discussed previously.

The ultrasonic generator 118 is coupled to the cable 114 through a capacitor 148. Similarly, the oscillator 118 couples to the cable 146 through a capacitor 150. Coupled to the transducer side of the capacitor 148 is a fault indicating circuit including resistors 152 and 154, the latter in parallel with a capacitor 156. A fault indication signal on line 158 is coupled to the combining logic 40 of FIG. 2. Tied to the transducer end of the capacitor 150 is a fault circuit including resistors 160 and 162, the latter in parallel with a capacitor 164. A fault indication signal appears on the line 166 and is also applied to the combining logic 40 of FIG. 2.

Tied to the interconnection of the resistive elements 140 in each of the transducer units 12 and 14 is a diode 168. Since there is no D.C. path to ground from the transducer side of the capacitors 148 and 150 except through the fault circuit resistors 152, 154 or 160, 162, the diodes 168 in each outboard unit clamps its respective cable to a D.C. level. This level approaches the peak swing of the ultrasonic oscillator 118 resulting in the condition that the diodes 168 have little effect on operation of the transmitting transducers 22.

Each of the fault indicating circuits may be considered as a peak detector with a remotely located diode 168. Disconnecting of the diode 168 from either of the fault circuits, such as by a break in the cables 144, 146 opening the housing tamper switches 502 or shorting of the cables causes the D.C. detector output voltage to vanish. It should also be noted, that a failure of the ultrasonic oscillator 118 will produce the same result.

Considering the outboard transducer unit 12, the capacitor 148 constitutes the output capacitor of the peak detector for that portion of the system with the series resistance of the resistors 152 and 154 constituting the detector load resistor. The interconnection of the capacitor 156 across the resistor 154 filters out the ultrasonic frequency components. When the D.C. level at the junction of the resistors 152 and 154 vanishes, this constitutes a fault condition signal on the line 158 to the combining logic 40. Similarly, for the outboard transducer unit 14, the capacitor 150 constitutes the output capacitor of the peak detector circuit with the resistors 160 and 162 in series making up the detector load resistor. The capacitor 164 also functions to filter out the ultrasonic frequency component. When a D.C. signal at the junction of the resistors 160 and 162 vanishes, this constitutes a fault indication signal on line 166 as connected to the combining logic 40.

Ultrasonic elements for both the receiving transducers 18 and the transmitting transducers 22, have a frequency temperature characteristic shown by the curve 168 of FIG. 9. Since this curve represents the ideal transmitting frequency for the ultrasonic subsystem 30, maximum system sensitivity over a wide temperature range requires that the frequency signal from the oscillator 118 track this same curve, that is, for an increase in temperature the output frequency of the oscillator 118 should decrease along the curve 168.

Referring to FIG. 10, there is shown a schematic of the oscillator 118 wherein an additional ultrasonic transducer element 170, mounted in the central detector unit 10, connects to one input of an amplifier 172 through a resistor 174 to control the frequency output of the oscillator 118 as appearing at the terminal 176. In addition to its frequency control function, transducer 170 radiates sufficient ultrasonic power from detector unit 10 to render outboard units 12 and 14 unnecessary when relatively small areas are to be protected.

Also coupled to the noninverting input of the amplifier 172 is a resistor network including resistors 178, 180 and 182. A positive feedback path across the amplifier 172 includes a resistor 184 connected from the amplifier output to the non-inverting input thereof. For the inverting input of the amplifier 172, the drive circuit includes a resistor 186 in series with a capacitor 188 with a feedback path including a resistor 190 and a capacitor 192.

The output of the amplifier 172 drives a complimentary symmetry power output switch made up of transistors 194 and 196 through a coupling capacitor 198. Transistors 194 and 196 have a common collector connection to the output terminal 176 through a capacitor 200. An emitter electrode of the transistor 194 connects to ground and the emitter electrode of the transistor 196 connects to a D.C. supply source at a terminal 202. Base current for the transistor 194 is provided by the resistors 204 and 206 and a base current for the transistor 196 is established by resistors 208 and 210.

The oscillator 118 is an R.C. relaxation type, except that the transducer 170 shunts the noninverting amplifier input to ground through the resistor 174. The oscillator thus locks to the parallel resonant mode of the ultrasonic transducer 170, since the shunting effect on the positive feedback is least at this frequency. A feature of the oscillator 118 of FIG. 10 is that the ultrasonic power signal is a twelve volt peak-to-peak square wave, rather than a sine wave. Measurements indicate that radiated ultrasonic power from the transducers 22 is somewhat greater than when the twelve volt peak-to-peak sine wave signal drives the system.

Referring to FIG. 11, there is shown the test modulator 128 coupled to the preamplifier 106 having an output to the balance mixer 120. A frequency signal from the source 118, as connected to the terminal 212, is applied to both the mixer 120 and the test modulator 128. For the latter, the oscillator frequency signal is connected to a resistor 214 in series with a capacitor 216. Capacitor 216 connects to a junction 218 which also has interconnected thereto resistors 220 and 222 and the anode of a diode 224. Resistor 220 ties to a D.C. supply source set at a preestablished value. Resistor 222 connects to a test signal input terminal 226 and receives the self-test control signal over the line 130. Also connected to the resistor 222 is a resistor 228 in parallel with a capacitor 230.

When a test signal is applied to the terminal 226, the diode 224 conducts applying an input to the preamplifier 106 by means of a resistor 232 in series with a capacitor 234. A resistor 236 completes a divider network with the resistor 232. Also coupled to the input of the preamplifier 106 is a sensitivity control circuit including a variable resistor 238 in series with a resistor 240 to ground. The sensitivity control circuit receives signals from the combining circuit 104 on a terminal 242. The resistor 240 is connected in series with the sensitivity control variable resistor 238 so that the test circuit will still function with the control set for minimum sensitivity.

An output from the amplifier 106 is applied to the center tap terminal of a split secondary winding 244 of a mixer transformer 246 including a primary winding 248 connected to the terminal 212 through a resistor 250. Connected to the end terminals of the secondary winding 244 are diodes 252 and 254. These diodes are interconnected by resistors 256 and 258 with a path to ground through capacitors 260 and 262. The output of the amplifier 106 is also connected through a resistor 264 to ground. An output signal from the mixer 120 appears at a terminal 266 and is applied to the filter amplifier 122.

Operation of the test modulator is as follows: resistors 220, 222 and 228 form a D.C. voltage divider which back biases the diode 224 so long as no test signal is applied to the terminal 226. Resistor 214 and capacitor 216 and the resistor 222 constitute an ultrasonic frequency voltage divider/phase shifter which applies an ultrasonic voltage to the anode of the diode 224. However, the ultrasonic voltage magnitude is insufficient to overcome the D.C. back bias, and no current flows through the diode 224 to produce an output across load resistor 236. Application of a test signal across the resistor 228 periodically brings the diode 224 into conduction and applies bursts of ultrasonic signals across the resistor 236. These bursts are coupled to the input of the preamplifier 106 through the resistor 232 and capacitor 234. Resistor 232 is chosen to provide suitable attenuation in conjunction with the other circuit impedances.

Another important feature of the present invention, the ultrasonic transmitting and receiving transducers are selected to have a high directivity. This makes it possible to favor certain critical portions of the selected area to be protected. For example, a transmit/receive transducer pair is aimed directly at a cash register, window or other specific item, to give extra sensitivity to these regions. Conversely, it is possible to aim the transducers away from trouble spots such as space heaters, air conditioning outlets and other such sources of false signals.

While only one embodiment of the invention, together with modifications thereof, has been described in detail herein and shown in the accompanying drawings, it will be evident that various further modifications are possible without departing from the scope of the invention.