Metal detecting apparatus for conveyor belt

Claim 2

Apparatus for detecting the fortuitous presence of a metallic body traveling at a known rate through a detection zone, comprising an excitation coil for setting up an alternating magnetic field in said zone, means for supplying an alternating excitation current to said coil, a pickup coil means in said zone, the entry of a metallic body into said field resulting in the setting up of eddy currents in such body and the consequent inducing of a signal voltage in said pickup coil means; means for establishing an alternating bucking voltage controllable in phase and amplitude and of the same frequency as said excitation current, means for algebraically adding said bucking voltage to the voltage induced in said pickup coil means to produce a resultant voltage, means responsive to said resultant voltage for automatically controlling the phase and amplitude of said bucking voltage to maintain said resultant voltage at a minimum, said last-mentioned means having a time constant in excess of the time required for a metallic body, moving at said rate, to travel through said zone; and an indicating circuit responsive to said resultant voltage.

Claim 3

Apparatus in accordance with claim 1, wherein said pickup coil means comprises a pair of coils connected in series-aiding relationship and positioned in overlapping relation with respect to said excitation coil but on respective opposite sides of the critical coupling position with respect thereto.

Claim 4

Apparatus in accordance with claim 1, wherein said alternating bucking voltage establishing means comprises means coupled to said alternating excitation current supplying means for deriving from such current four voltages in phase quadrature, and separate voltage-controllable means for determining the amplitude of each of said quadrature voltages fed to a summing means to provide said bucking voltage.

Claim 5

Apparatus as defined in claim 3, wherein said phase and amplitude controlling means comprises means responsive to said resultant voltage for producing a pair of control voltages whose polarities and magnitudes depend upon the relative phase and magnitude of said resultant voltage, and means for applying said control voltages to said voltage-controllable means.

Claim 6

Apparatus as defined in claim 3, wherein said phase and amplitude controlling means comprises a pair of synchronous rectifiers for rectifying said rEsultant voltage, means for driving said rectifiers respectively by two voltages of the same frequency as said excitation current and in phase quadrature with respect to each other, and means for applying the output voltages of said rectifiers as control voltages to said voltage-controllable means.

Claim 7

Apparatus according to claim 5, wherein said time constant is provided by series resistors and shunt capacitors connected in the circuits of said synchronous rectifiers.

Claim 10

Apparatus in accordance with claim 7, wherein said last-mentioned means operates to reduce the sensitivity of said indicating circuit to a degree commensurate with the amplitude of said signal voltage, thereby to prevent operation of said indicating means in response to said signal voltage.

Claim 11

Apparatus in accordance with claim 7, wherein said last-mentioned means has incorporated therein a time delay such as to maintain the sensitivity of said indicating circuit reduced for a time interval slightly in excess of the time required for the longest metallic nonuniformity to pass said pickup coil means.

Claim 12

Apparatus in accordance with claim 7, wherein said detecting means is positioned ahead of said pickup coil means, with reference to the direction of travel of said belt.

Claim 13

Apparatus for detecting the fortuitous presence of a metallic body moving at a known rate along a metallically-reinforced conveyor belt known-length portions of which are nonuniform as to the amount of metal contained therein, comprising an excitation coil for setting up an alternating magnetic field in a zone located along the length of said belt, means for supplying an alternating excitation current to said coil, a pickup coil means in said zone, the entry of any nonuniform metallic body into said field resulting in the inducing of a signal voltage in said pickup coil means; means for establishing an alternating bucking voltage controllable in phase and amplitude and of the same frequency as said excitation current, means for algebraically adding said bucking voltage to the voltage induced in said pickup coil means to produce a resultant voltage; means responsive to said resultant voltage for automatically controlling the phase and amplitude of said bucking voltage to maintain said resultant voltage at a minimum, said last-mentioned means having a time constant in excess of the time required for a nonuniform metallic body, moving at said rate, to travel through said zone; an indicating circuit of controllable sensitivity responsive to said resultant voltage, means positioned adjacent said zone for detecting the approach of one of said metallic nonuniformities to said zone and for producing a voltage in response thereto, and means for utilizing said last-mentioned voltage to reduce the sensitivity of said indicating circuit.

Claim 14

Apparatus for detecting the fortuitous presence of a metallic body moving at a known rate along a metallically-reinforced conveyor belt known-length portions of which are nonuniform as to the amount of metal contained therein, comprising an excitation coil for setting up an alternating magnetic field in a zone located along the length of said belt, means for supplying an alternating excitation current to said coil, a pickup coil means in said zone, the entry of any nonuniform metallic body into said field resulting in the inducing of a signal voltage in said pickup coil means; means for establishing an alternating bucking voltage controllable in phase and amplitude and of the same frequency as said excitation current, means for algebraically adding said bucking voltage to the voltage induced in said pickup coil means to produce a resultant voltage; means responsive to said resultant voltage for automatically controlling the phase and amplitude of said bucking voltage to maintain said resultant voltage at a minimum, said last-mentioned means having a time constant in excess of the time required for a nonuniform metallic body, moving at said rate, to travel through said zone; an indicating circuit of controllable sensitivity responsive to said resultant voltage, means positioned adjacent said zone for detecting the approach of one of said metallic nonuniformities to said zone and for producing a voltage in response thereto, and means for utilizing said last-mentioned voltage to reduce the sensitivity of said indicating circuit.

Claim 15

Apparatus in accordance with claim 11, wherein said pickup coil means comprises a pair of coils connected in series-aiding relationship and positioned in overlapping rElation with respect to said excitation coil but on respective opposite sides of the critical coupling position with respect thereto.

Claim 16

Apparatus in accordance with claim 11, wherein said alternating bucking voltage establishing means comprises means coupled to said alternating excitation current supplying means for deriving from such current four voltages in phase quadrature, and separate voltage-controllable means for determining the amplitude of each of said quadrature voltages fed to a summing means to provide said bucking voltage.

Claim 17

Apparatus in accordance with claim 11, wherein said last-mentioned means operates to reduce the sensitivity of said indicating circuit to a degree commensurate with the amplitude of a signal voltage induced in said pickup coil means due to the entry of one of said metallic nonuniformities into said field, thereby to prevent operation of said indicating means in response to said last-mentioned signal voltage.

Claim 18

Apparatus in accordance with claim 11, wherein said last-mentioned means has incorporated therein a time delay such as to maintain the sensitivity of said indicating circuit reduced for a time interval slightly in excess of the time required for the longest one of said metallic nonuniformities, moving at said rate, to pass said pickup coil means.