Differential, multiple cell reflex cable intrusion detection system and method

• US 5,446,446 A
• Filed: 08/26/1994
• Issued: 08/29/1995
• Est. Priority Date: 12/09/1993
• Status: Expired due to Term

• Current Assignee: Southwest Microwave, Inc.
• Original Assignee: Southwest Microwave, Inc.
• Inventors: Harman, R. Keith
• Primary Examiner(s): Peng, John K.
• Assistant Examiner(s): Lee, Benjamin C.
• Application Number:
  US08/296,666
• Time in Patent Office: 368 Days
• Field of Search: 340/561, 340/566, 340/552, 340/550, 340/564, 340/554, 340/541
• US Class Current: 340/566
• CPC Class Codes: F02B 2075/025 two
• CPC Class Codes: G08B 13/169 using cable transducer means
• CPC Class Codes: G08B 13/2497 using transmission lines, e.g. cable

Abstract

A transducer cable includes an inner conductor, an outer conductor, and dielectric between the inner conductor and the outer conductor. A longitudinal passage and a sense wire loosely disposed therein extend through the dielectric. Movement, (e.g., intruder-caused vibration) of the transducer cable results in movement of the sense wire relative to the outer conductor, causing corresponding changes in characteristic impedance of the sense wire. A carrier signal transmitted down the inner conductor produces an electromagnetic field that couples energy to the sense wire. The change in characteristic impedance causes reflection of some of the coupled energy, which produces a corresponding signal that is sensed by a receiver circuitry. The receiver circuitry produces a signal indicating the occurrence of the intruder caused vibration. The receiver circuitry includes a system that digitizes the received signals at successive times that correspond to successive sections of the cable, performs digital filtering to eliminate a cable profile or clutter response, and produces target response values for sections of the cable that undergo changes in characteristic impedance. Target responses in several adjoining sections are interpolated to precisely locate the intrusion activity causing the vibrations.

Claims

1. A transducer cable comprising in combination:

(a) a first conductor;

(b) a second conductor generally parallel to the first conductor;

(c) solid non-magnetic dielectric material on opposed sides of the first conductor and between the first conductor and the second conductor,

(d) first and second longitudinal passages extending through the dielectric material, corresponding portions of the first and second longitudinal passages being located on opposed sides of the first conductor, respectively; and

(e) first and second sense wires extending through loosely fitting in the first and second passages, respectively, cross sections of the first and second sense wires being substantially smaller than that of their respective passages so that physical movement of the transducer cable results in free movement in generally opposed directions of the first and second sense wires relative to the the first conductor, resulting in corresponding changes in the characteristic impedance of a first transmission line formed by the first sense wire and the the first conductor and corresponding changes in the characteristic impedance of a second transmission line formed by the second sense wire and the the first conductor.

2. The transducer cable of claim 1 wherein the first and second conductors and the first and second sense wires are parallel.

3. The transducer cable of claim 2 wherein the first conductor is an inner conductor, the second conductor is an outer conductor, the outer conductor being generally tubular, the dielectric material being inside the outer conductor.

4. The transducer cable of claim 3 wherein the dielectric material is generally cylindrical, and the first and second passages are slots in an outer portion of the dielectric material.

5. The transducer cable of claim 4 further including a layer of dielectric material covering the first and second slots.

6. The transducer cable of claim 5 including a first termination matched to a characteristic impedance of the first transmission line and connected between the first sense wire and the outer conductor, and a second termination matched to the characteristic impedance of the second transmission line and connected between the second sense wire and the outer conductor, and a third termination matched to a characteristic impedance of a third transmission line formed by the inner conductor and the outer conductor and connected between the inner conductor and the outer conductor.

7. An intrusion detection system, comprising in combination:

(a) a transducer cable including

i. a first conductor,

ii. a second conductor,

iii. dielectric material on opposed sides of the second conductor and between the first conductor and the second conductor,

iv. first and second longitudinal passages extending through the dielectric material, corresponding portions of the first and second longitudinal passages being located on opposed sides of the second conductor, and

v. first and second sense wires extending through and loosely fitting in the first and second passages, respectively, so that physical movement of the transducer cable results in free movement of the first and second sense wires in generally opposite directions relative to the second conductor, causing corresponding changes in the characteristic impedance of a first transmission line formed by the second conductor and the first sense wire and in the characteristic impedance of a second transmission line formed by the second conductor and the second sense wire;

(b) a transmitting circuit adapted to transmit an RF signal down a third transmission line formed by the first conductor and the second conductor, an electromagnetic field being produced along the first conductor by the RF signal and coupling energy to the first and second transmission lines, an intrusion activity causing movement of the transducer cable resulting in movement of the first and second sense wires in the first and second passages, respectively, in generally opposite directions relative to the second conductor and changing characteristic impedances of the first and second transmission lines at portion thereof including the relative movement, the characteristic impedance changes causing reflection of a portion of the coupled energy back along the first and second transmission lines toward the transmitting circuit, the first and second sense wires carrying a differential first signal representative of the reflected energy; and

(c) a receiver circuit connected to receive the differential first signal and operative to produce a second signal representative of the occurrence of the activity.