Intrusion detection systems with turbulence discrimination

• US 4,003,045 A
• Filed: 10/01/1975
• Issued: 01/11/1977
• Est. Priority Date: 10/01/1975
• Status: Expired due to Term

• Current Assignee: Napco Security Systems Incorporated
• Original Assignee: Napco Security Systems Incorporated
• Inventors: Stockdale, Roy
• Primary Examiner(s): Swann, III, Glen R.
• Application Number:
  US05/618,692
• Time in Patent Office: 468 Days
• Field of Search: 340/258 A, 343/7.7, 343/5 PD
• US Class Current: 367/94
• CPC Class Codes: G08B 13/1627 using Doppler shift detection circuits

Abstract

An intrusion alarm system employs ultrasonic frequencies and operates in the presence of turbulence. The system includes a differentiator and an integrator, each coupled to a sample and hold circuit; the differentiated pulses determining the level from the integrator, which level is stored in the sample and hold circuit during a predetermined time interval. The level, as stored, will be relatively constant for the presence of an intruder and will exhibit excursions during turbulence. A threshold circuit is responsive to the steady level during a predetermined period to sound an alarm determinative of an intrusion. During turbulence, the signal stored fluctuates and is discriminated against by the threshold circuit to assure that the alarm will not occur for this condition.

Claims

1. An intrusion alarm system for detecting the presence of a moving target in the presence of interfering signals, comprising:

a. means for transmitting signals into a zone under surveillance,

b. means for receiving signals returned from said zone,

c. processing means operative in response to said received signals to provide an output signal, indicative of said moving target and said interfering signal,

d. first means for differentiating said output signal,

e. second means for integrating said output signal,

f. detector means coupled to said first and second means and responsive to said differentiated signal to store a magnitude proportional to said integrated signal during a predetermined period

g. threshold means coupled to said detector means and operative to provide an alarm signal when said stored magnitude is relatively constant during said predetermined period.

2. The intrusion alarm system according to claim 1 wherein said detector means comprises a sample and hold circuit having a storage input coupled to said second means for storing therein a level proportional to the magnitude of said integrated output signal and a control input coupled to said first means for determining the time said level is to be stored.

3. The intrusion alarm system according to claim 2 wherein said threshold means include:

a. an AC amplifier having an input coupled to the output of said sample and hold circuit and operative to amplify said stored level at an output terminal, and

b. a peak detector coupled to said output terminal of said AC amplifier and operative to provide a signal indicative of the peak value of said level as stored.

4. The intrusion alarm system according to claim 1 further comprising:

a. means coupled between said processing means and said threshold means for inhibiting said threshold means for output signal levels below a predetermined value.

5. The intrusion alarm system according to claim 1 wherein said means for transmitting signals into said zone include an ultrasonic oscillator circuit.

6. The intrusion alarm system according to claim 1 further including an alarm circuit coupled to said threshold means and responsive to said alarm signal for providing an alarm indication determinative of the presence of an intruder

7. The intrusion alarm system according to claim 3 wherein said threshold means further includes a Schmitt trigger.

8. The intrusion alarm system according to claim 1 wherein said output signal has a frequency range within 15 to 1000 Hz as determined by the rate of motion of said moving target.