US2554575A - Magnetic detector - Google Patents

Description

y 1951 J. KURTZ ETAL MAGNETIC DETECTOR 2 Sheets-Sheet 1 Filed March '7, 1944 5 m m m w.

Julius [0112 Francis J. Paelsclz y 29, 1951 J. KURTZ mm. 2,554,575

MAGNETIC DETECTOR Patented May 29, 1951 Julius Kurtz Toqele, Utah, and Francis J. Paetsch, Rio Tinto,-Nev., assignors to Anaconda Copper Mining'Co'mpany, New York; N. Y., a" corporationof Montana Application March -7,' 1944,.Serial "No. 525;462

(Cl. l9'2125) 6 Claims. 1

This: invention relates to magnetic detection andparticularly to the detection of metallic materialj to permit the separation of magnetic from nonemagnetic material and metallic from semimetallic ornon-metallic material.

The invention isespecially applicable to the detection of tramp metal on conveyors which feedymaterialzsuch as ore or coal to crushers or other machinermand to this end the invention includesameans for detecting the presence of metallic or. magnetic material, for producing resulting changesin an electric current, and by change: in current, for controlling the desired apparatusewhioh may include, for example, the motor driving the conveyor, a suitable signaling orlothendevice, or both.

Occasionally; in the material (hereinafter re ferred. tonas ore) which is conveyed to the crusher, pieces of metal, usually iron or steel, are'ioundi Such pieces of metal are likelyv to beipartsioi broken tools or machinery which have cordance'. with. the present invention, as thetrampg-metal. approaches the crusher, it is causedtocpassthrough a.- magnetic field set up by an energizing or rexciter coil. The presence of tramp metal in the magnetic field whether the tramp metal betof magnetic or non-magnetic material, w-ill distort the field. This distortion of the fieldwill inducaa-apotential or a change in potentialdn anpickup or detector coil suitably positioned-.imthe field. The pick-up or detector coil issconnected toxthe input of a suitable amp1ifier, the -output-of which is connected to control ..circuitswhichwill control the desired apparatus,--.such.- for-example as the motor which drives theconveyor belt or a signaling device, or 110511;.

A better understanding of the invention can behadirom consideration of the following specification. taken. together with the drawings,

wherein Fig. 1 is-a-perspective view showing the rela-- tion ofathe exciter and pickup coilstoa-conveyor belt. in accordance with the invention;

Fig...1a. isa.modification of Fig. 1 showing the. exciten.,coilpositioned with its diagonalaxisspere pen'dicular to-the longitudinal axis of the belt;

Fig.2isaneleizationalview of a conveyor belt. and. suitable supportin structures .for the coils;

If such pieces of 2 Fig. 3 is a sectional view in elevation taken along lines 33 ofFig. 4; v A

Fig. 4 is a sectional view taken along line 4-4 of Fig. 3; and H v Fig. 5 is a diagram of the detection and con trol circuits.

Fig. 1 shows the general arrangements fthei coils with respect to conveyor belt 21 which may be assumed to move from right to left, as seen in the drawing, and to be carrying ore to a crusher,

for example. This belt should be of non-mag netic and preferably of non-metallic material. A piece of tramp metal3'l is represented as ap proachin coils 25 and 25 as it travelsion belt 2'5; Coil 2:2 is the eXciter coil and is shown in the drawing as being rectangular and positioned with its longer axis at right angles to the longitudinal axis of the belt, althoughit may be square- However, in certain cases it may be [16" sirable that the exciter coil be positioned so thatits diagonal axis is perpendicular to the longitudinal axis of the belt as shown in Fig. la. In the embodiment herein described theexciter coil 24 may comprise approximately 481 turns of No. 14 insulated wire and will consume about 2.2 kv.-a., at very low power factor, when con-' nected through leads 26 to an alternatingcur rent power source of 480 volts and cycles."

The pickup or detector coil 25 may also-berectangular and is preferably positioned as shown in Fig. l with its longer axis perpendicular to the longitudinal axis of the belt and symmetrical with respect to thebelt. Inthe em-' bodiment referred to this pickup coil may comprise approximately l'l85 turns of No. 18-insulated wire and its inside dimensionsbeapproxi mately 34.- inches by 8 inches. This coil is suitable foruse with Gil-cycle exciting current and A different energizing frequency or belt width-would,- of

rectly.supportedwith relationto each other and to---the belt.- In the contemplated embodiment these coils may normally be spaced 'approximately 24 inches from each other at their nearset points. It is preferable that the support for the detector coil he adjustable to permit-the spacing between the belt and detector coil to be changed when required, so as to accommodate different depths of material on the belt In accordance with the invention, the supporting structure for the exciting coil includes a suitable casing to protect it from damage, and the supporting structure for the detector coil includes certain adjustable features which will be described in connection with Figs. 3 and 4. It will be noted in connection with Fig. 1 that the belt 21 passes between coils 24 and 25 and not through the center of either coil. This arrangement permits repair and changing of the belt without interfering with the coils or detection apparatus, and also permits the mentioned adjustment of the coil which is on the upper side of the belt. The positions of the exciter and detector coils may be reversed, although it is preferable that the detector coil be positioned on the upper side of the conveyor belt, viz., the side on which the ore is carried.

Fig. 2 illustrates the relation of the coil-supporting structures to the conveyor system. The figure shows a conventional conveyor system as used to feed an ore crusher, and in general comprises a conveyor belt 21 supported on suitable rollers 29. The belt is inclined in order to raise the ore so that it may drop by gravity into the crusher, and the rollers are arranged to dish the belt as shown in Fig. 4. The upper end of the belt passes around a traction wheel 30 which is driven by an electric motor it. At a suitable position along the belt before it reaches the traction wheel, the magnetic exciter and detector coils are supported.

The coil-supportin structures are more clearly shown in Figs. 3 and l. Exciter coil 24 is shown enclosed in a non-magnetic box 32, the sides and top of which may be of heavy plywood and the bottom of wooden planks, say 2 inches thick. If the top of the box is hinged the coil will be readily accessible but protected from injury and from dirt. The supporting platform 42 should be so located that the top of the box will be as close as possible to the bottom of belt 2'? without being hit by it. Thus exciter coil 24 will be positioned close enough to the belt to assure that the magnetic field produced by the coil will permeate the ore conveyed on the belt.

The supporting structure for pickup coil 25 may, as shown in Fig. 3, comprise two A-frames 33 made of lumber of suitable dimensions. The legs of each of these frames are fastened together at the bottom by tie pieces 43, and both frames are fastened together at the tops by stringers 35 which straddle the conveyor belt. At the center of each A-frame is secured a vertical rail 38 which extends above the frame a few inches. Stringer pieces 35 are bolted to the extensions of these rails as shown, so that the stringers are spaced from each other a distance corresponding to the thickness of the rails 36, as shown in Fig. 3. A coil frame 3 5, which may also be made of wood, supports coil 25. This coil frame includes two extensions 44 on each end which slide along both sides of rails 36, permitting the entire frame 34 with its detector coil 25 to be moved vertically with respect to belt 21. To adjustably support the coil frame, two U-shaped metal straps 31 are secured around the sides and top of the frame near the ends thereof. To the centers of these straps are secured threaded rods or bolts 38 which pass through the space between stringer pieces 35. Across the tops of the stringer pieces are metal bearing plates M3. Nuts '39 engaging the threaded rods 38 bear on these plates 40. Thus by turning the nuts 39 the frame 34 with its detector coil 25 may be adjusted vertically, viz., perpendicularly to the surface of the conveyor belt 21. Connecting leads 4| (Fig. l) connect coil 25 to the input terminals l' and 2 of the amplifier shown in Fig. 5.

An amplifier and control apparatus according to the invention are shown in the circuit dia gram of Fig. 5. Vacuum tube 3, i and 5 together with their associated circuit elements comprise a high gain amplifier which, in connection with coils 25 and 24, has been found to provide a sensitive, stable and readily adjustable magnetic detector system. The specific types of tubes which may be here employed and suitable constants for the associated circuit elements are given below.

The input of this amplifier, as has already been mentioned, is connected to terminal l and 2, and the output thereof is coupled across resistors R18, R11 and R22 to a gas-filled grid-controlled trigger tube 6. This tube has the characteristic that with a positive potential on the anode 41 and a suitable negative control potential on the grid 48, the gas in the tube is not ionized and no anode current flows. However, when the negative control potential on the grid is de creased beyond a certain critical point the gas in the tube ionizes and anode current flows in the anode circuit wherein a suitable control device is connected. As soon as the gas is ionized the grid loses its control so that in order to open the anode circuit, viz., to stop anode current from flowing, the anode potential must momentarily be reduced substantially to zero.

The control device connected in the anode circuit of tube 8 comprises a relay L1 having a winding which when energized by current in the anode circuit opens contacts II. The control system also includes a second relay L2 having a singlecontact armature l2 and a double-contact armature l3. This relay is so constructed that when current passes through its coil the contact I2 opens and aramture l3 breaks its connection with contact l5 and closes its connection with contact M.

A reset-device 1 together with its associated contact elements 8, 9 and it, is provided and suitably connected to the relays so that operation of the reset device restores the entire system to its original condition after the system has been actuated as a result of the presence of tramp metal on the conveyor belt. Motor [6 of Fig. 5 is the same motor represented in Fig. 2, and, if desired, may be automatically disconnected to stop the conveyor belt when a piece of tramp metal is detected on the belt. Additional motors may be similarly controlled. The horn, or howler [9 comprises an audible signal or indicating device which may be actuated when tramp metal is detected and which continues to sound until the reset device is operated. Any other desired type of indicating or control apparatus may be added to or substituted for this audible indicator 19.

The full-wave rectifier tube 2| is connected to suitable windings on a power transformer 22 and to a filter comprising choke L4 and condensers C8 to provide a suitable source of direct-current potential to the other tubes in the system. Hesistors R12, R6 and R121 are connected as a voltage divider to provide the required potentials to the tubes. The entire system is energized from terminals 20 which may be connected to a suitable alternating current power source such as volts, 60 cycles. The power for motor 16 is.

auger-s It is to be understood that the particular circuit arrangement and circuit elements and constants herein set forth, are given only by way of example in connection with the present description of a preferred embodiment of the invention, and that the scope of the invention is not to be limited thereby.

Operation The operation of the magnetic detector system abovedescribed is as follows: A direct-current potential of about 200 volts, derived at the tap. between resistors R12; and R6, is; applied to the anode. 41 of tube 6, and: a negative bias such as to block the tube is derived from potentiometer 3 R11 and is impressed on the grid 48 of tube 6. Thus, in the absence of a suiiicien-t alternatingcurrent potential super-imposed on the grid of tube 6, the control system does not operate. How: ever, when a potential or change of potential of correct sign and" magnitude, due to a disturbance in the magnetic field impressed on detector coil 25, is impressed across input terminals 1-, 2 of the amplifier, the negative bias on the grid 48 of tube 6 will beinstantaneously decreased below its critical value, permitting the gas in the tube to ionize; thus establishing anode current in the anode circuit thereof: and actuating relay L1 connected in the anode circuit. When relay L1 is actuated its armature is locked in actuated position by means of latch 46, holding contacts H closed. Contacts II are. connected; in series with the actuating coil. of relay L2. across the power line 20. Hence, v closure of contacts H operates relay L2, opening. contacts I2 and l3- -i5,

and closing the connection between armature l3 and contact M. The opening of contacts 12 opens the anode circuit of gas tube 6, thus deionizing that tube and permitting grid 48 to resume control upon subsequent closing of contacts l2. Assuming switch IT to be closed and switch 18 open, such operation of relay Laalso opens the circuit to traction motor l6 and closes the circuit to howler l9, thus immediately stopping the conveyor belt 21 and simultaneously producing an audible signal calling the attention of an attendant to the fact that tramp metal has been detected on the conveyor-belt. These con.- ditions will persist until the reset device I is operated. The operation just described is pracg tically instantaneous, so that the attendant will find the detected tramp metal under or just beyond the detector coil at, its position over'the conveyor belt.

When the system operates with switches l1 but howler 19-- will give an audible signal as. the tramp metal passes with-in the hold of the detec tor-coil 25'and will continue to sound until the:

reset device I is operated; 'In' this: manner the: attendant can remove the piece 'of tramp metal without interrupting the operation of the conveyor. Should no' audible signal be: desired; switches l 'l and I8 may be kept-open, in which: event the conveyor: tractionmotor- 16 will. auto-.

matically be stopped when tramp meta-l is detected, and will remain so untilthe: reset device Tisoperat'ed; although the howler will not sound.

Reset device I, electrically is in thenature of a push button, but in. an installation ofthetype. herein considered it: may preferably take. the form of a large and-rugged switching. means orv device preferably secured'above the. head of the, attendant and arranged to be: operated manu-i ally by pulling on a; cord suspended from the 1 device' Operation of reset device 1 first. closes contacts 8 and 9, and as the cord is pulled further down also closes contacts 9 and Ill. Suite able springs restore. the. contacts to open-circuit position upon release of the cord. Closure of'contacts 8 and 9 actuates relay Lzwhen con-. tacts II of relay L1 are open. Thus, as longv as. contacts. 8 and 9' are. held closed the. motor Ht will. remain stopped; and howler [9 will sound providing switch I! is closed. This; is. a con-- venient manner in which to test the control; and signaling apparatus aswell as, to permit the ate tendant to: stop, the conveyor belt in an emergency. As reset device I is further actuated. to close contacts 9 and ll], release coil L3. is. energized, releasing latch 46 and. allowing contacts H to be opened bya spring; Thus, immediately upon therelease of reset device 1', opening con.- tacts 8, 9: and I0, relay is no longer. energized, whereby. contacts I2 will closethe anode circuit of tube 5 and restore-the control system to its normal condition. Also, armature [3' will make connection with contact [5 thereby connecting driving motor I6 to the line 45 and. restoring the conveyor belt to operation;

The adjustable resistor R11, which is preferably-of thepotentiom eter type, should be set at a point which will permit stable operation when the reset device is operated, i. e;, when the reset device 1 is released and contacts l2 close, the anode voltage is suddenly applied to" tube 6 and may have a tendency to trigger this tube unless sufiicient margin of grid bias is impressed on grid 48. Ifhe value of this grid bias is adjusted by resistor Rn and with the proper setting thereqf the, reset device 1 maybe operated and released with no danger of triggering the tube 6,

The system may be initially adjusted as follows: Adjustable resistor R7, which may also be of the potentiometer type, may first be set about midway of its adjustment range; then resistor R11 is adjusted to increase the bias on grid 48 until repeated operation of the reset device does not trigger tube 6. Resistor R7 is then varied to adjust the, gain of the amplifier to a point where the entire detector system will operate specifically to, detect metal of the type and minimum proportions which it is desired to detect. It, with resistor R7 at such adjustment, it be found that with no metal near coil 25 the residual balance voltage in coil 25 tends to trigger tube 6, such tendency can be overcome by increasing the bias on grid 48 by further adjustment of resistor Rn, or by adjusting the spatial and hence the balance relation between coils 25, 24; After these adjustments have been effected, the system accord- 7i ing to the present invention will detect with complete reliability all pieces of metal down to the minimum proportions of that for which the system was initially adjusted.

The present invention has been described in connection with a system for detecting tramp metal on a conveyor belt, but it is not limited thereto, as those skilled in the art will at once appreciate, for it has many difierent and additional applications.

., What is claimed is:

1. In a system for detecting tramp metal among other material on a conveyor, an alternating-current exciter coil located on one side of said conveyor so that said material passes through the field of said coil, a detector coil located on the other side of said conveyor in the field of said exciter coil, an alternating current vacuum tube amplifier having an input and an output, said detector coil being coupled to said input so as to impress a voltage thereon, a control system, said output being coupled to said control system, a motor for driving said conveyor, control means connected in said control system operable in response to change of current in said output, switching means actuated by said control means, connections between said switch ing means and said motor, said switching means being operable to open the connections to said motor upon operation of said control means, locking means retaining said connections open, and manually operable means for releasing said locking means.

2. In a system for detecting tramp metal among other material on a conveyor, an alternating-current exciter coil located on one side of said conveyor so that said material passes through the field of said coil, a detector coil located on the other side of said conveyor in the field of said exciter coil, an alternating current vacuum tube amplifier having an input and an output, said detector coil being coupled to said input so as to impress a voltage thereon, a control system, said output being coupled to said control system, a driving motor for said conveyor, a detection-indicating device, switching means in said control system connected to said motor and to said device and operable in response to change of current in said output, connections between said switching means, said motor and said device whereby operation of said switching means may automatically control said motor and said device, and second switching means for selectively connecting said motor or saiddevice,

or both, to said control system for automatic control by said first mentioned switching means.

3'. In a system according to claim 1, adjustable supporting means for supporting, said detector coil in adjustable spatial relation to said exciter coil so as effectively to adjust the voltage from said detector coil which is impressed on the input of said amplifier.

4. In a system according to claim 2, support means for supporting said detector coil in fixed relation to said exciter coil, means for moving said detector coil on said support means to adjust the spacing between said detector coil and said eXciter coil and adjustable resistor means connected in the input of said amplifier so as effec tively to adjust the voltage from said detector coil which is impressed on the input of said amplifier.

5. In an inspecting system for detecting magnetic materials, said system being of the type in- .cluding an electric motor for moving material to plifier having an input and an output, means for coupling said input to a detector coil, a gas-filled grid-controlled tube having a grid circuit, an anode and an anode circuit, said grid circuit being coupled to said amplifier output, a source of abode potential connectible to said anode, a first relay having an actuating coil connected to be energized by current in said anode circuit and having contacts which are closed by actuation of said relay, a second relay having an actuating coil connected in series with said power source and the contacts of said first relay such that actuation of said first relay actuates said second relay, a first set of contacts on said second relay arranged to open on actuation and to close on de-actuation of said second relay, said first set of contacts being connected in series in said anode circuit between said source of anode potential and said anode, a second set of contacts on said second relay, a motor, an electric power source for said motor, and circuit connections between said motor power source, said motor and the second set of contacts on said second relay whereby closure of said second set of contacts connects said motor to said motor power source.

6. In a magnetic detector system of the type including an alternating current exciter coil and a detector coil positioned in the magnetic field of said exciter coil, the combination which comprises, a power source, a vacuum tube amplifier having an input and an output, means for coupling said input to said detector coil, a gas-filled grid-controlled tube having a grid circuit, an anode and an anode circuit, said grid circuit being coupled to said amplifier output, a source of anode potential connectible to said anode, a first relay having an actuating coil connected to be energized by current in said anode circuit and having contacts which are closed by actuation of said relay, a second relay having an actuating coil connected in series with said power source and the contacts of said first relay such that actuation of said first relay actuates said second relay, a first set of contacts on said second relay arranged to open on actuation and to close on deactuation of said second relay, said first set of contacts being connected in series in said anode circuit between said source of anode potential and said anode, a device to be controlled, 2. second set of contacts on said second relay connected to control the operation of said device, a latch associated with said first relay and arranged upon actuation of said first relay automatically to maintain said first relay in actuated position after the actuating coil thereof is deenergized, switching means having first and second switch contacts arranged so that upon actuation thereof the first switch contacts close before closure of the second switch contacts, a latch coil associated with said first relay and operative when energized to release said latch, circuit connections between said first switch contacts, said power source and the actuating coil of said second relay such that closure of said first switch contacts actuates said second relay, and circuit connections between said power source, said second switch contacts and said latch coil, such that closure of said second switch contacts energizes said latch coil and releases said latch 9 after said second relay is actuated, thus to restore said first relay to deactuating condition.

JULIUS KURTZ. FRANCIS J. PAETSCH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,564,815 Ballance Dec. 8, 1925 1,640,524 Augustine Aug. 30, 1927 1,724,924 Graham -1 Aug. 20, 1929 1,742,235 Cooley Jan. 7, 1930 1,783,234 Firestone Dec. 2, 1930 a 10 Number Name Date 2,032,514 Swart Mar. 3, 1936 2,124,410 Cockerell July 19, 1938 2,155,239 Hofimann Apr. 15, 1939 2,188,159 Rockwood Jan. 23, 1940 2,228,293 Wurzbach Ja l. 14, 1941 2,228,294 Wurzbach 14, 1941 2,290,930 Wurzbach July 28, 1942 2,339,750 Bartholy Jan, 25, 1944 10 2,400,069 Bloser May 14, 1946 OTHER REFERENCES 15 pages 12 and 13 and Figures 14, 15, arid 16.

G. Review, July 1929, pages 397, 398.

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