METAL DETECTION IN THE VICINITY OF FERROUS BOUNDARIES

Abstract: A forage harvester has a metal detector with permanent magnets (18-20, 21-23) located within a crop feed roll (10), additional permanent magnets (30, 32) being positioned beyond the ends of the feed roll and at a distance greater than the radius of the feed roll from the axis of rotation of the feed roll. The additional magnets (30, 32) cooperate with magnets (18, 23) within the feed roll (10) to provide a more uniform flux density across the width of the feed roll, even in regions near ferrous side sheets (26, 28) defining the lateral limits of the harvester feed path. Tramp metal is thus detected with equal facility regardless of its trajectory past the feed roll (10).

Description: This invention relates to an improvement in metal detectors used in forage harvesters, and more particularly to apparatus for producing a more uniform flux density across the feed path of the harvester, even in regions bounded by ferrous parts. U.S.-A-4433528 discloses a metal detector located in the lower front feed roll of a forage harvester. The metal detector generates a magnetic field between the lower front feed roll and an upper front feed roll so that a ferrous tramp metal object passing between the feed rolls induces a signal in the metal detector. The metal detector then produces an output signal to arrest the feed rolls thus preventing damage to the cutterhead in the harvester by the metal object. While the metal detector arrangement disclosed in U.S.-A-4433528 is admirably suited to its intended purpose, and is widely used, it suffers a disadvantage when used in a harvester having ferrous side sheets defining the lateral limits of the crop feed path. These ferrous parts distort the magnetic field generated by the metal detector so that there are regions of reduced magnetic intensity near the side sheets at each end of the feed rolls. If a ferrous metal object of a given size passes through one of these regions there is less likelihood that it will be detected than if its trajectory takes it between the feed rolls nearer the centre of the crop feed path. A primary object of the present invention is to provide a simple, economical solution to this problem, the solution requiring no physical change to the metal detector and only a minimum modification of the harvester side sheets. The present invention is defined in the appended claims and provides an improvement in metal detectors of the type located within a feed roll of a forage harvester and having permanent magnets for generating a magnetic field outside the feed roll for detecting ferrous tramp metal passing in the vicinity of the feed roll, the improvement comprising a plurality of permanent magnets located outside the feed roll and cooperating with magnets in the metal detector to provide a sensing field of generally uniform magnetic intensity across the feed path of the harvester, even in regions bounded by ferrous metal parts. In accordance with the principles of this invention, a forage harvester having a feed path limited on top and bottom by two feed rolls and bounded at the sides by ferrous side sheets, and a permanent magnet metal detector located inside one of the feed rolls, is provided with additional permanent magnets supported by the side sheets at the sides of the feed path. The additional magnets cooperate with permanent magnets inside the feed roll to provide a magnetic field intensity near the side sheets which increases the likelihood of detection to be substantially the same as near the middle of the feed path. With uniform detection likely across the entire width of the feed path, a ferrous object induces substantially the same magnitude signal in the metal detector regardless of whether it passes between the feed rolls near the centre of the feed path or at one side thereof. The present invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a top view of a forage harvester feed roll and shows the location of the permanent magnets of a metal detector inside the feed roll and additional magnets outside the feed roll; Figure 2 is a front view of a forage harvester feed path showing one way in which additional magnets in accordance with the invention may be supported by side sheets; Figures 3 and 4 are diagrammatic illustrations of magnetic fields produced by a metal detector without (Figure 3) and with (Figure 4) the addition of permanent magnets outside the feed roll; Figure 5 illustrates an alternative positioning of the additional permanent magnets; and Figure 6 is a view similar to Figure 1 showing an arrangement of four additional permanent magnets. Figure 1 is a top view of the lower front roll 10 of a forage harvester. The feed roll 10 has a metal detector mounted therein as fully described in U.S.-A-4433528, the disclosure of which is incorporated herein by reference. As disclosed in that patent, the feed roll 10 is supported for rotation on a drive shaft 12 which is affixed to the feed roll, and a fixed hollow shaft 14. The components of the metal detector, including a pair of coils 16 (Figure 2), a plurality of magnets 18-23, and a ferrous carrier 15 joining the bottoms of the magnets to complete the magnetic circuit, are mounted in a fixed position on the fixed hollow shaft 14. Leads connected to the metal detector are fed through the hollow shaft. The lower front feed roll 10 and an upper front feed roll 24 (Figure 2) compress crop material and feed the compressed material in the direction of arrow 25 to a cutterhead which chops it into smaller pieces. Side sheets 26 and 28 define the lateral limits of the crop feed path. The feed roll 10 is made of a non-magnetic material but the side sheets are ferrous metal. The metal detector generates a magnetic field in the feed path between the feed rolls 10 and 24 to detect the presence of tramp metal in the crop material. As shown in Figure 1, the metal detector is provided with two rows of permanent magnets, the rows extending transverse to the direction of crop feed and parallel to the axis of rotation of feed roll 10. The first row includes magnets 18-20 and these magnets have their south poles uppermost as the magnets are viewed in Figure 2. The second row includes magnets 21-23 and these magnets have their north poles uppermost. The magnets 18-23 produce a magnetic field which covers most of the region between the feed rolls 10 and 24. However, there is a region at each end of the feed roll 10 through which little magnetic flux extends, hence there is a reduced likelihood that a piece of tramp metal passing through either of these regions will induce a signal of sufficient magnitude in coils 16 reliably to produce a detection signal. Figure 3 illustrates the flux field associated with magnets 18 and 21. The principle flux, depicted by solid lines, extends between the south pole of magnet 18 and the north pole of magnet 21 and provides the sensing field for metal objects passing between the feed rolls over the region where these magnets are located. In addition, a minor flux extends between the south and north poles of magnet 18 and its mounting carrier. This minor flux is influenced by the ferrous side sheet as shown in dashed lines. The density of this minor flux does not extend upwards sufficiently to permit reliable detection of a piece of metal passing through the region A near side sheet 26. A similar region exists at the opposite end of the feed roll where the flux of magnet 23, as influenced by the ferrous sheet 28, is similarly insufficient for reliable detection. In accordance with the principles of the present invention, a preferred embodiment of the metal detector is provided with at least two additional permanent magnets 30 and 32 located above the feed roll 10 and outwardly from the respective ends of the feed roll. Magnet 30 is aligned with the first row of magnets 18-20 and is mounted in an opening 29 provided in the side sheet 28. Magnet 32 is aligned with the second row of magnets 21-23 and is mounted in an opening 31 provided in the side sheet 26. Magnet 30 is positioned such that its south pole faces the first row of magnets while its north pole is at the end farthermost from the row. Magnet 32 is positioned such that its north pole faces the second row of magnets while its south pole is at the end farthermost from the row. In Figure 1, ferrous metal brackets 33 are provided to support the magnets 30 and 32 and complete the magnetic circuit. However, the magnets 30 and 32 may be supported by any suitable manner. As illustrated in Figure 2, the side sheets may be provided with holes 29 and 31 and the magnets 30 and 32 embedded within an epoxy material 34 within ferrous housing 36 and 38 which are in turn attached to the side sheets 26 and 28, respectively, to complete the magnetic circuits. This arrangement permits use of the invention without having to relocate the side sheets 26 and 28. However, it will be understood that the magnets 30 and 32 may be mounted inboard of the side sheets if the side sheets are moved outwardly from the ends of the feed roll. This arrangement, for one end only, is illustrated in Figure 5 where 26′ indicates a former location of the side sheet and 26 indicates its new position. It will be noted that the relative distances between the magnets 32 and 18 is the same in Figures 4 and 5. Figure 4 illustrates the flux pattern between the magnet 18 and the additional magnet 32. The flux density in the region A is increased to an extent such that any tramp metal passing through this region is easily detected. The flux density in the region A depends on the magnets 18 and 32 and their relative positions and hence will vary depending upon which metal detector is used and the height of the magnet 32 above the feed roll 10. In a forage harvester such as the New Holland Inc. Model 790 equipped with an electronic metal detector, the additional magnets 30 and 32 may be positioned at a height h of about ½ to 1-½ inches (1.27 to 3.8 cms) above the periphery of feed roll 10 or h+r above the axis of rotation of the feed roll where its radius is r . The invention is not limited to the addition of two permanent magnets as shown in Figures 1 and 2. Figure 6 shows an arrangement where two additional permanent magnets 30, 30′ and 32, 32′ are provided at each end of the feed roll 10, the magnets being poled as shown in that figure and positioned in a plane which include the magnets in one row of magnets within the metal detector. From the foregoing description it is seen that the present invention provides an improvement in the detection capabilities of metal detectors used to detect tramp metal in forage harvesters. The improvement can be added to present equipment without physical alteration of the metal detector. The only alteration required in the forage harvester is the provision of holes 29 and 31 in the side sheets if the method of mounting shown in Figure 2 is employed.

Claims: A forage harvester having first and second feed rolls (10, 24) rotatable about first and second axes for feeding crop material through a crop feed path and a metal detector positioned within the first feed roll (10) for detecting the presence of tramp metal passing between the feed rolls, the metal detector including first and second rows of permanent magnets (18-20; 21-23) for producing a magnetic field between the feed rolls to detect tramp metal, characterised in that the metal detector further comprises: first and second additional permanent magnet means (30, 32), and support means (26, 28) for supporting the additional permanent magnets laterally of the crop feed path at a distance from the axis of the first feed roll (10) which is greater than the radius of the first feed roll. A forage harvester as claimed in claim 1, characterised in that the support means comprises first and second side sheets (26, 28) defining the lateral limits of the crop feed path. A forage harvester as claimed in claim 2, characterised in that the first and second side sheets are of ferrous material. A forage harvester as claimed in claim 3, characterised in that a hole is provided in each of side sheet (26, 28) and a ferrous cover (36) is attached to each side sheet on the side thereof opposite the crop feed path, each of the covers having potting material (34) therein for supporting one of the additional permanent magnets (30, 32). A forage harvester as claimed in any of the preceding claims, characterised in that the magnets of the first row (18-20) have south poles at their ends nearest the second feed roll (24) and the magnets of the second row (21-23) have north poles at their ends nearest the second feed roll, the first additional permanent magnet means (30) having a south pole at its end nearest said first row and the second additional permanent magnet means (32) having a north pole at its end nearest said second row. A forage harvester as claimed in claim 5, characterised in that the first additional permanent magnet means (30) coacts with one magnet (18) in the second row (18-20) to produce a magnetic field in a region near one lateral extent of the crop feed path, and the second additional permanent magnet means (32) coacts with one magnet (23) in the second row, to produce a magnetic field in a region near the other lateral extent of the crop feed path. A forage harvester as claimed in any of the preceding claims, characterised in that it further comprises third and fourth additional permanent magnet means (30′, 32′) the support means (26, 28) supporting the third and fourth permanent magnet means laterally of the crop feed path at a distance from the axis of the first feed roll (10) which is greater than the radius of the first feed roll. A forage harvester as claimed in claim 7 when appended to claim 5, characterised in that the third additional permanent magnet means (30′) has a south pole at the end nearest the first row of magnets (18-20) and the fourth additional permanent magnet means (32′) has a north pole at the end nearest the second row (21-23).

Inventors: BOHMAN, CARL E., CHOW, MARK K., STROSSER, RICHARD P.
Applicants: FORD NEW HOLLAND, INC. (A DELAWARE CORP.)
Application year: 1988
Main classification: A01D75
IPC codes: A01D75