R.F. SYSTEM FOR DETECTING UNAUTHORIZED TRAVEL OF ARTICLES THROUGH A SELECTED ZONE

• US 3,818,472 A
• Filed: 05/26/1972
• Issued: 06/18/1974
• Est. Priority Date: 05/26/1972
• Status: Expired due to Term

• Current Assignee: Kenneth G. Mauk, Peter J O'Neal
• Original Assignee: Kenneth G. Mauk, Peter J O'Neal
• Inventors: Mauk, Kenneth G., O'Neal, Peter J.
• Primary Examiner(s): Caldwell, John W.
• Assistant Examiner(s): Swann, III, Glen R.
• Application Number:
  US05/257,131
• Time in Patent Office: 753 Days
• Field of Search: 340/280,258C,408,224,261 343/6.8R
• US Class Current: 340/572.4
• CPC Class Codes: G08B 13/2414 using inductive tags
• CPC Class Codes: G08B 13/2471 Antenna signal processing by receiver or emitter
• CPC Class Codes: G08B 13/2474 Antenna or antenna activator geometry, arrangement or layout loop antennae H01Q1/22

Abstract

A theft detection system and method for detecting the passage of articles through a monitored zone including affixing a relay transponder to the articles, transmitting a preselected frequency signal to the zone for pickup by the transponder and retransmission to the ambient atmosphere, receiving and detecting the retransmitted signal, and providing simultaneous digital control to both the transmitting and the receiving means, whereby the latter may be alternately energized during monitoring operation to provide maximum sensitivity and minimum average power dissipation within the system. The relay transponder may advantageously be an LC parallel connected resonant tank circuit tuned to the preselected frequency of the transmitter and providing by ringing action the retransmitted signal detected in the receiver.

Claims

1. A theft detection system for detecting the passage of protected articles through a selected area, said system comprising:

a. a transmitter for transmitting, when activated, radio frequency signal bursts at a predetermined frequency to said selected area;

b. a parallel connected inductance capacitance tank circuit tuned to said predetermined frequency and adapted for affixation to articles passing through the selected area;

c. a superheterodyne receiver for detecting, when activated, radio frequency signal bursts retransmitted thereto from said tank circuit at said predetermined frequency;

d. control means connected to both said transmitter and said receiver and operative to alternately activate same at a chosen control frequency;

e. alarm means comprising a main alarm signal channel including a main alarm and an error alarm signal channel including an error alarm, said alarm means including pulse shaping means connected to said control means for reception of control signals therefrom and connected also to both said alarm signal channels, said pulse shaping means incorporating timing means for sorting the radio frequency bursts detected by said receiver into said main alarm channel if such bursts are detected During a predetermined time interval and into said error alarm signal channel if such bursts are detected at times other than said predetermined interval, and counting means for counting a predetermined number of radio frequency bursts for energizing at least one of said alarms.

2. A theft detection system as set forth in claim 1 wherein said pulse counting means comprises:

a. first counting means in said main alarm signal channel for counting a predetermined number of radio frequency bursts for energizing said main alarm means to signal passage of a protected article through said selected area; and

b. second counting means in said error alarm signal channel for counting a predetermined number of radio frequency bursts for energizing said error alarm to signal detection of spurious radio frequency bursts.

3. A theft detection system as set forth in claim 2 wherein each of said alarm signal channels includes a respective AND gate, each of said AND gates having a first input for receiving detected radio frequency bursts and a second input interconnected with said pulse shaping means.

4. A theft detection system as set forth in claim 3 wherein said first and second counting means comprise respective shift registers.