INTRUSION DETECTION DEVICE UTILIZING LOW FREQUENCY SOUND WAVES AND PHASE DETECTION TECHNIQUES

• US 3,754,222 A
• Filed: 12/13/1971
• Issued: 08/21/1973
• Est. Priority Date: 12/13/1971
• Status: Expired due to Term

• Current Assignee: Harold Eisenberg
• Original Assignee: Harold Eisenberg
• Inventors: Eisenberg, Harold
• Primary Examiner(s): Caldwell, John W.
• Assistant Examiner(s): Curtis, Marshall M.
• Application Number:
  US05/207,466
• Time in Patent Office: 617 Days
• Field of Search: 340/1R,3D,3E,258A,258R
• US Class Current: 367/94
• CPC Class Codes: G08B 13/1609 using active vibration detection systems

Abstract

A signal in the low audio frequency range and preferably of about 400 to 600 hertz is radiated into one or more protected areas by one or more speakers, and each speaker also acts as a transducer of reflected audio energy. A substantially amplitude independent phase sensitive detector designed to operate at the audio frequency is coupled to the source of audio signals and to each speaker to produce an output signal when a reflected signal exhibits phase shift due to even relatively slight movements of an intruder in any direction in the protected area. A low band pass circuit couples a narrow low frequency band of signals from the detector to gating and integrating circuits to provide an alarm indication in response to phase changes characteristic of the intruder movement to be detected.

Claims

1. An intrusion alarm for detecting the movements of an intruder in a protectd area comprising:

signal generating means for generating an electrical signal having a frequency falling within the low audio frequency range;

transducer means for converting electrical energy into sound energy waves in the air and for simultaneously converting airborne sound energy waves into electrical energy;

phase detection means for developing an output signal whose magnitude fluctuates in response to changes in the phase relationship of two incoming signals;

first circuit means for conveying the signal generated by said signal generating means to said transducer means;

second circuit means for conveying a signal from said transducer means to said phase detection means;

third circuit means for conveying a signal generated by said signal generating means directly to said phase detection means;

amplification and filtering means connected to said phase detection means for amplifying only those sub-audio frequency output signals supplied by said phase detection means having frequencies falling within a frequency band of less than four octaves the center frequency of which band is adjusted to encompass the primary output signal frequencies generated by said phase detection means when a human moves adjacent said transducer means;

rectification and integration means connected to the output of said amplification and filtering means for integrating the strength and duration of signal perturbations which appear at the output of said amplification and filtering means; and

means coupled to said rectification and integration means for producing an alarm indication in response to the achievement of a predetermined integral value by said integration means.

2. An intrusion alarm in accordance with claim 1 wherein said transducer means comprises at least one loudspeaker having a single pair of electrical terminals.

3. An intrusion alarm in accordance with claim 1 wherein the electrical signal generated by the generating means has a frequency between 100 and 1,000 Hertz.

4. An intrusion alarm in accordance with claim 1 wherein the electrical signal generated by the generating means has a frequency between 400 and 600 Hertz.

5. An intrusion alarm in accordance with claim 1 wherein said first and second circuit means include limiter means for limiting the amplitude fluctuations in signals conveyed between said signal generating means and said phase detection means over said first and second circuit means, whereby said phase detector means is rendered relatively insensitive to signal amplitude fluctuations.

6. An intrusion alarm in accordance with claim 1 wherein the first and second circuit means, the transducer means, and the third circuit meAns have phase shift characteristics chosen to cause signals flowing from said signal generating means over said first and second circuit means to said phase detection means to be approximately in quadrature with signals generated by said signal generating means and flowing over said third circuit means to said phase detection means.

7. An intrusion alarm in accordance with claim 1 wherein the first circuit means includes saturated signal amplification means which limit the amplitude of electrical signals supplied to said transducer means.

8. An intrusion alarm in accordance with claim 1 wherein said rectification and integration means includes at its input full wave rectification means for supplying signal perturbations of either polarity at the output of said amplification and filtering means to said integration means as perturbations of the same polarity so that either positive or negative perturbations may be summed by said integration means.

9. An intrusion alarm in accordance with claim 1 wherein said amplification and filtering means comprises an active filter having a frequency bandwidth of approximately 3 octaves.

10. An intrusion alarm comprising in combination:

an oscillator operating at a frequency between about 400 and about 600 Hertz;

a speaker;

a first amplifier coupling said oscillator to said speaker;

a phase sensitive detector including a transformer having a primary winding and a tapped secondary winding;

a second amplifier coupling said oscillator to said primary winding;

means including said first amplifier coupling both said speaker and said oscillator to the secondary winding tap;

a sub-audio frequency band pass filter having a bandwidth in the neighborhood of three octaves and a center frequency of between 1 and 5 Hertz coupled to the output of said phase sensitive detector; and

alarm means coupled to the output of said band pass filter.

11. The alarm of claim 10 further comprising full-wave rectification and integrating means coupled between said band pass filter and said alarm means.

12. An intrusion alarm for detecting the movements of an intruder in a protected area comprising:

means including a tone generator and a transducer for generating a low audio frequency standing wave in the protected area;

means for generating an electrical signal whose amplitude changes represent phase changes in said standing wave;

means for filtering said electrical signal to eliminate frequency components lying outside of a predetermined sub-audio pass band;

electrical signal integration means for integrating incoming electrical signals;

a first circuit path including a unidirectional conductive device oriented in a first direction over which the filtered electrical signal is conveyed to said integration means;

a second circuit path including a unidirectional conductive device oriented in a second direction and also including inversion means for inverting the polarity of signals flowing over said second path over which the filtered electrical signal is conveyed to said integration means; and

alarm means coupled to said integration means for sounding an alarm when said integration means reaches a predetermined integral state in response to signals flowing over said circuit paths.

13. An intrusion alarm in accordance with claim 12 wherein each of said first and second circuit paths includes voltage doubling rectification means, of which the unidirectional conductive device forms a part, for rectifying signals so as to produce increased rectified signal output from each path.