Passive infrared intrusion detector employing correlation analysis

• US 4,746,910 A
• Filed: 10/03/1986
• Issued: 05/24/1988
• Est. Priority Date: 10/01/1982
• Status: Expired due to Fees

• Current Assignee: Cerberus ABP Investor LLC (Cerberus Capital Management LP)
• Original Assignee: Cerberus ABP Investor LLC (Cerberus Capital Management LP)
• Inventors: Wagli, Peter, Pfister, Gustav
• Primary Examiner(s): Swann, III, Glen R.
• Application Number:
  US06/915,057
• Time in Patent Office: 599 Days
• Field of Search: 340/567, 340/552, 340/522, 250/340, 250/395
• US Class Current: 340/567
• CPC Class Codes: G08B 13/19 using infrared-radiation detection systems G08B13/194 takes precedence
• CPC Class Codes: G08B 29/046 prevention of tampering with detection circuits
• CPC Class Codes: G08B 29/188 Data fusion; cooperative systems, e.g. voting among different detectors
• CPC Class Codes: Y10S 250/01 Passive intrusion detectors

Abstract

For reducing the susceptibility to false alarms and for increasing the detection probability of a passive infrared detector, the actual signals obtained from a first sensor element are continuously compared in a correlator with reference or set signals stored in a read-only memory and/or with the actual signals obtained from a second sensor element monitoring the near region. The correlator delivers an output signal which corresponds to the correlation of both signals which are compared with one another. An alarm signal is triggered when the correlation exceeds a predetermined value, for instance 0.7, and the amplitude has reached a predetermined threshold. The infrared detector affords high security against giving of false alarms and a high detection probability, even in the presence of signals possessing a great amount of noise, but also delivers an alarm signal in the event the detector is attempted to be sabotaged, for instance by covering the inlet optical system.

Claims

1. A passive infrared detector for determining the presence of an intruder possessing a temperature differing from the ambient temperature, comprising:

at least one sensor element generating an electrical signal as a function of infrared radiation impinging upon said at least one sensor element;

at least one optical system for focusing the infrared radiation upon said at least one sensor element;

said at least one optical system imaging infrared radiation upon said at least one sensor element which infrared radiation emanates from a number of predetermined separate fields of view;

an evaluation circuit operatively connected with said at least one sensor element and delivering an output signal dependent upon changes in the impinging radiation;

said evaluation circuit comprising;

a correlator; and

storage means for storing a predetermined number of reference signals which are representative of typical movement patterns of intruders;

said correlator correlating by means of a correlation method said electrical signals generated by said at least one sensor element with said reference signals stored in said storage means;

said correlator delivering an output signal corresponding to the correlation of the electrical signal generated by said at least one sensor element and the reference signals; and

said evaluation circuit further comprising an alarm stage arranged in circuit after the correlator for delivery an alarm signal when the correlation between the electrical signal generated by said at least one sensor element and at least one of said stored reference signals as well as the amplitude of said electrical signal simultaneously exceed a predetermined value.

2. The infrared detector as defined in claim 1, wherein:

said storage means comprises a read-only memory.

3. The infrared detctor as defined in claim 2, wherein:

5. The infrared detector as defined in claim 1, wherein:

said at least one sensor element comprises a first and second sensor element;

said at least one optical system comprises a first and second optical system, said second optical system focusing radiation upon said second sensor element;

both of said optical systems being structured such that monitoring regions thereof overlap only in the immediate vicinity of the detector;

said correlator being operatively connected to said storage means, to said first sensor element and to said second sensor element; and

said correlator being structured and operable such that it selectively compares the electrical signals obtained from the first sensor element with any one of either (i) said reference signals stored in the storage means or (ii) electrical signals generated by the second sensor element.

7. The infrared detector as defined in claim 5, wherein:

said preset first threshold value amounts to approximately 0.35.

9. The infrared detector as defined in claim 7, wherein:

said preset second thereshold value amounts to approximately 0.7.

12. The infrared detector as defined in claim 1, wherein:

said predetermined value for the correlation between the electrical signal generated by the at least one sensor element and the at least one stored reference signal amounts to approximately 0.7 based upon a variation in the occurrence probability of a correlation signal as a function of the magnitude of said correlation; and

said predetermined value for the amplitude of said electrical signal amounts to approximately twice the RMS-value of the noise.

13. The infrared detector as defined in claim 1, wherein:

said correlator comprises a predetermined number of shift registers;

each one of said shift registers containing a fixed location at which one of said reference signals is stored;

said at least one sensor element being operatively connected to at least one analog-to-digital converter which converts the electrical signal generated by the at least one sensor element into at least one digital actual monitoring signal;

said predetermined number of shift registers being connected parallel to said at least one sensor element and receiving said at least one digital actual monitoring signal at a predetermined temporal offset from the reference signals stored at said fixed locations in said shift registers; and

said correlator correlating said at least one digital actual monitoring signal and said reference signals stored at said fixed locations of said shift registers in accordance with the relationship ##EQU3## wherein;

S_i represents the digital actual monitoring signal,

REF_i the reference signal stored at the fixed location of one of the shift registers,

λ

the integration variable, namely time,

t the temporal offset between the digital actual monitoring signal and the reference signal stored at the fixed location of the one shift register, and

-T_o /2, +T_o /2 the integration limits.

4. said reference signals correspond to different speeds of movements of intruders.

6. The infrared detector as defined in claim 4, wherein:

said second optical system contains imaging means imaging a region near the infrared detector on said second sensor element; and

said alarm stage is structured such that it delivers a disturbance signal when the correlation between the electrical signals received from the first sensor element and the electrical signals received from the second sensor element exceeds a preset first threshold value based upon a variation in the occuurence probability of a correlation signal as a function of the magnitude of said correlation.

8. The infrared detector as defined in claim 4, wherein:

said second optical system contains imaging means imaging a region near the infrared detector on said second sensor element; and

said alarm stage is structured such that it delivers an alarm signal when the correlation between the electrical signals received from the first sensor element and the electrical signals received from the second sensor element exceeds a preset second threshold value based upon a variation in the occurrence probability of a correlation signal as a function of the magnitude of said correlation.

10. The infrared detector as defined in claim 4 wherein:

both of said first and second sensor elements are located upon a chip.

11. The infrared dector as defined in claim 4, further including:

a common housing means for both of said sensor elements.