Inductive reactance proximity alarm system for bulky movable objects

Claim 2

An alarm system for protecting movable objects having at least a metallic portion comprising oscillator means in a protected location remote from the object to be protected; tuneable circuit means coupled to said oscillator means for energization thereby, said circuit means having movable inductive sensing coil means adapted to be mounted on the metallic portion of the movable object to be protected, so that the proximity of said coil means to the metallic portion of thE object determines the inductive reactance of the coil means, and tuning means in said protected location for tuning said circuit means to a predetermined resonant or non-resonant operating condition with said coil means mounted on the metallic portion of the object, so that removal of the coil means from the metallic portion of the object causes the inductive reactance of the coil means to change and said circuit means to be detuned; mounting means for removably mounting said sensing coil means on the metallic portion of the object to be protected; detector means in said protected location coupled to said tuneable circuit means for producing an output signal in response to a change in the inductive reactance of said coil means; and alarm signal means in said protected location coupled to said detector means for producing an alarm signal in response to said detector means output signal.

Claim 3

An alarm system as claimed in claim 1 wherein said tuneable circuit means is a series circuit tuned to a resonant operating condition, and said detector means is coupled across said sensing coil means and responsive to the voltage thereacross.

Claim 4

An alarm system as claimed in claim 2 wherein said detector means comprises a bridge rectifier circuit having the input thereof coupled across said sensing coil means, an electrical energy source, and relay means having the control winding thereof coupled to the output of said rectifier circuit for energization thereby and normally-closed relay contact means for coupling said alarm signal means to said electrical energy source whenever the output of the rectifier circuit falls below a predetermined level.

Claim 5

An alarm system as claimed in claim 1 wherein said mounting means comprises a permanent magnet, and means for securing said sensing coil means to said permanent magnet.

Claim 6

An alarm system as claimed in claim 4 wherein said permanent magnet is a C-shaped permanent magnet having a recessed portion between the pole pieces thereof and, said sensing coil means is disposed within the recessed portion of said C-shaped magnet.

Claim 7

An alarm system as claimed in claim 4 wherein said sensing coil means comprises an annular sensing coil, said permanent magnet comprises a ceramic permanent magnet disposed within said annular sensing coil, and said means for securing the sensing coil means to the permanent magnet comprises a potting compound encapsulating said sensing coil and said permanent magnet.

Claim 8

An alarm system as claimed in claim 5 wherein said mounting means further comprises a ring of magnetic material having a diameter substantially the same as the distance between the pole pieces of said C-shaped permanent magnet for magnetically engaging said C-shaped magnet when the sensing coil means is to be mounted on the metallic portion of the object to be protected, and means for securing said ring to the metallic portion of the object to be protected.

Claim 9

An alarm system as claimed in claim 6 wherein said mounting means further comprises a disc of magnetic material removably disposed within said annular sensing coil and adapted to magnetically engage said ceramic permanent magnet when the sensing coil is to be mounted on the object to be protected, and means for securing said disc to the metallic portion of the object to be protected.

Claim 10

An alarm system as claimed in claim 8 wherein said disc of magnetic material comprises a ceramic permanent magnet.

Claim 11

An alarm system as claimed in claim 1 wherein the object to be protected has at least one hinged door thereon having at least a metallic portion adjacent the metallic portion of the remainder of the protected object, and said mounting means is adapted to mount said sensing coil means in a position wherein the sensing coil means overlaps at least the metallic portion of the door and the metallic portion of the remainder of the protected object.

Claim 12

An alarm system as claimed in claim 10 wherein said sensing coil means comprises a sensing coil having a FIG. 8 winding configuration.

Claim 13

An alarm system as claimed in claim 10 wherein said sensing coil means comprises a pair of spaced-apart annular sensing coils having the windings thereof connected in series circuit.

Claim 14

An alarm system for protecting a plurality of movable objects each having at least a metallic portion comprising an oscillator circuit in a protected location remote from the objects to be protected; a plurality of tuneable circuit means coupled to said oscillator circuit for energization thereby, each of said tuneable circuit means being associated with a different one of the objects to be protected and having movable inductive sensing coil means adapted to be mounted on the metallic portion of the object to be protected associated therewith, so that the proximity of said coil means to the metallic portion of that object determines the inductive reactance of the coil means, and tuning means in said protected location for tuning said circuit means to a predetermined resonant or non-resonant operating condition with said coil means mounted on the metallic portion of the protected object associated therewith, so that removal of the coil means from the metallic portion of the object causes the inductive reactance of the coil means to change and said circuit means to be detuned; a plurality of mounting means for removably mounting said plurality of sensing coil means on the metallic portion of the bodies to be protected; a plurality of detector means in said protected location, each of said detector means being associated with a different one of said plurality of tuneable circuit means and being coupled to the tuneable circuit means associated therewith for producing an output signal in response to a change in the inductive reactance of the coil means associated therewith; and a plurality of alarm signal means in said protected location, each of said alarm signal means being coupled to a different one of said plurality of detector means and being operable to produce an alarm signal in response to the output signal of the detector means associated therewith.

Claim 15

An alarm system for protecting a plurality of movable objects each having at least a metallic portion comprising an oscillator circuit in a protected location remote from the objects to be protected; a plurality of tuneable circuit means coupled to said oscillator circuit for energization thereby, each of said tuneable circuit means being associated with a different one of the objects to be protected and having movable inductive sensing coil means adapted to be mounted on the metallic portion of the object to be protected associated therewith, so that the proximity of said coil means to the metallic portion of that object determines the inductive reactance of the coil means, and tuning means in said protected location for tuning said circuit means to a predetermined resonant or non-resonant operating condition with said coil means mounted on the metallic portion of the protected object associated therewith, so that removal of the coil means from the metallic portion of the object causes the inductive reactance of the coil means to change and said circuit means to be detuned; a plurality of mounting means for removably mounting said plurality of sensing coil means on the metallic portion of the bodies to be protected; a plurality of detector means in said protected location, each of said detector means being associated with a different one of said plurality of tuneable circuit means and being coupled to the tuneable circuit means associated therewith for producing an output signal in response to a change in the inductive reactance of the coil means associated therewith; and a plurality of alarm signal means in said protected location, each of said alarm signal means being coupled to a different one of said plurality of detector means and being operable to produce an alarm signal in response to the output signal of the detector means associated therewith.