Static magnetic field metal detector

Claim 2

In a forage crop harvester having rotatable machine elements including crop processing elements susceptible to damage by a tramp metal part the combination comprising: means for generating a substantially uniform unidirectional magnetic field on said harvester, multiple conductor magnetic field pick up coil means disposed in said unidirectional magnetic field having output conductor means, said coil means having first and second sections symmetrically located about a mid-plane of said coil means, cross over conductor means for connecting opposite conductors of said first and second sections at said mid-plane for providing a detection signal on said output conductor means in the presence of said tramp metal parts adjacent said first or second sections, threshold detector means coupled to said output conductor means, and utilization means responsive to said threshold means whereby said tramp metal part is detected substantially without interference due to spurious varying magnetic fields generated by said rotatable machine elements.

Claim 3

Apparatus as described in claim 1 wherein: said harvester has a substantial degree of symmetry about a vertical plane, and said mid-plane is disposed sufficiently close to said vertical plane in relation to said rotatable machine elements to provide substantial cancellation of spurious varying magnetic fields generated by said rotatable machine elements.

Claim 4

Apparatus as described in claim 1 additionally including band pass circuit means disposed in series circuit relation between said coil and said threshold detector means.

Claim 5

Apparatus as described in claim 3 including in series circuit relation between said coil means and said threshold detector means: band pass circuit means coupled to said output conductor means, and attenuator means responsive to said band pass circuit means.

Claim 6

Apparatus as described in claim 3 including in series circuit relation between said coil means and said threshold detector means: band pass circuit means coupled to said output conductor means, and full wave rectifier means responsive to said band pass circuit means.

Claim 7

Apparatus as described in claim 5 additionally comprising: low pass amplifier means responsive to said rectifier means for generating a control voltage, and means for supplying said control voltage to said threshold means for adjustably determining the threshold level of said threshold means.

Claim 8

In a forage crop harvester of the kind having rotatable machine elements including crop processing elements susceptible to damage by a tramp metal part, the combination comprising: means for generating a substantially uniform magnetic field in a space within said harvester through which said crop passes into said crop processing elements, first and second multiple conductor magnetic field pick up coil means disposed in stationary relation in said unidirectional magnetic field and having respective first and second output conductor means, each of said coil means having first and second sections symmetrically located with respect to first and second mid-planes of said respective coil means, first cross over conductor means for connecting opposite conductors of said first and second sections of said first coil means at said first mid-plane for providing a first detection signal on said first output conductor means in the presence of said tramp metal part adjacent said first or second section of said first coil means, and second cross over conductor means for connecting opposite conductors of said first and second sections of said second coil means at said second mid-plane for providing a second detection signal on said Second output conductor means in the presence of said tramp metal part adjacent said first or second section of said second coil means, said first and second coil means being off set longitudinally one relative to the other so that said first cross over means lies proximate said first section of said second coil means and said second cross over means lies proximate said second section of said first coil means for the purpose of providing output signals at said first or second output conductor means in the presence of said tramp metal part while affording substantial cancellation of spurious magnetic fields induced by said rotatable machine elements.

Claim 9

In a forage crop harvester of the kind having rotatable machine elements including crop processing elements susceptible to damage by a tramp metal part, the combination comprising: means for generating a substantially uniform magnetic field in a space within said harvester through which said crop passes into said crop processing elements, first and second multiple conductor magnetic field pick up coil means disposed in stationary relation in said unidirectional magnetic field and having respective first and second output conductor means, each of said coil means having first and second sections symmetrically located with respect to first and second mid-planes of said respective coil means, first cross over conductor means for connecting opposite conductors of said first and second sections of said first coil means at said first mid-plane for providing a first detection signal on said first output conductor means in the presence of said tramp metal part adjacent said first or second section of said first coil means, and second cross over conductor means for connecting opposite conductors of said first and second sections of said second coil means at said second mid-plane for providing a second detection signal on said Second output conductor means in the presence of said tramp metal part adjacent said first or second section of said second coil means, said first and second coil means being off set longitudinally one relative to the other so that said first cross over means lies proximate said first section of said second coil means and said second cross over means lies proximate said second section of said first coil means for the purpose of providing output signals at said first or second output conductor means in the presence of said tramp metal part while affording substantial cancellation of spurious magnetic fields induced by said rotatable machine elements.

Claim 10

Apparatus as described in claim 7 additionally comprising: first and second signal processing circuit means respectively responsive to signals at said first and second output conductor means, each said signal processing circuit means comprising in series circuit relation: band pass circuit means, and threshold detector means, OR circuit means responsive to said first and second signal processing means, and utilization means responsive to said OR circuit means.

Claim 11

Apparatus as described in claim 8 further comprising adjustable attenuator means interposed in series circuit relation between said band pass circuit means and said threshold detector means.

Claim 12

Apparatus as described in claim 9 further comprising full wave rectifier means interposed in series circuit relation between said band pass circuit means and said threshold detector means.

Claim 13

Apparatus as described in claim 8 wherein said utilization means comprises alarm means.

Claim 14

Apparatus as described in claim 8 wherein said utilization means comprises control means for stopping said rotatable machine elements subject to damage by said tramp metal part.