Abstract:
A metal detecting device for a crop conveyor has several closely spaced permanent magnets fixed within a hollow feed-roller in parallel with the shaft thereof. One pole of each magnet is directed radially outwardly such that its magnetic field penetrates through the hollow feed-roller and an adjoining crop-feed slot from inside to out, and wherein said magnetic field also passes axially through a respective sensor winding that is likewise arranged within the hollow feed-roller.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to agricultural machinery having crop conveyors, and more particularly to a metal detecting device for a crop conveyor. In a preferred embodiment, several closely spaced permanent magnets are fixed within a hollow feed-roller in parallel with the shaft thereof. 
     A metal detecting device of this type is known from DE 43 01 611 A1. A plurality of permanent magnets are arranged in a row, transversely of the crop feed path, within a hollow feed-roller. One magnetic pole of each magnet is directed toward the feed-roller and is located close to the inner surface thereof. The permanent magnets are surrounded by respective sensing coils and the other poles of the magnets are accommodated in a U-shaped yoke whose two ends terminate close to the inner face of the feed-roller. By this arrangement, the basic sensor field extends through the feed-roller into the adjacent feed-slot to a depth corresponding approximately to half the width of the yoke. The signals from the sensors are added to form the wanted signals and, to compensate for interference signals which arise, in particular, from the movement of the feed ribs of the feed-roller in each sensing field, they are subtracted from one another and then used as reference values. A disadvantage of this arrangement is the relatively small range covered by the sensing fields since there are limits to the amount by which the yoke can be enlarged. This is because the rotating members, which interfere heavily with the sensors, are located at the entry point of the feed-slot. Another disadvantage is that the detection process at the output of the feed-slot occurs too late to allow the conveyor to be stopped before a detected piece of metal reaches the subsequent chopper. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a metal detecting device for a crop conveyor which overcomes one or more of the deficiencies discussed above. 
     Another object is to improve the metal detecting device in such a way that the effective sensing field will extend deeper into the feed-slot and that the signal-to-noise ratio of the sensor signals will be increased. 
     In accordance with the invention there is provided a metal detecting device for use with a crop conveyor having a hollow feed roller mounted on a shaft, and an opposite roller spaced from the feed roller to provide a crop feed slot therebetween; the device including a plurality of magnets fixed within the hollow feed roller in parallel with the roller shaft, each magnet having a pole directed radially outwardly to an outer end within the roller, each magnet including a pole shoe arranged on the outer end and extending outwardly toward the roller such that a magnetic field of the magnet penetrates through the roller and the crop feed slot from inside to out, a sensor winding on the pole shoe and within the roller whereby said magnetic field also passes axially through the sensor winding. 
     A respective pole shoe is arranged on each of the radially outwardly directed poles of the permanent magnets so as to extend up close to the hollow feed-roller, and in that each said pole shoe penetrates the appertaining sensor winding and has a free pole face which is narrower than that of the corresponding permanent magnet so as to produce an enhanced field per unit area through the material of the pole shoe at the free pole face. 
     The fields of the permanent magnets are preferably oriented in parallel. The signal levels occurring under similar conditions when a standardized metallic body passes through the various regions of the slot covered by the individual sensors are higher at the center than in the lateral side regions so that it is advantageous to normalize the signal levels and to logically combine these normalized signals in order to compensate for interference. 
     These, and other objects and advantages of the present invention, will become apparent as the same becomes better understood from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is now made more particularly to the drawings which illustrate the best presently known mode of carrying out the invention and wherein similar reference characters indicate the same parts throughout the several views. 
     FIG. 1 is a longitudinal section through a feed roller and showing an installed sensor device in side view. 
     FIG. 2 is an end view of the feed roller and installed sensor device, and of an adjacent pressure roller. 
     FIG. 3 is a top view of the sensor device. 
     FIG. 4 is a schematic of a signal evaluating circuit. 
    
    
     DETAILED DESCRIPTION 
     As shown in FIG. 2, crop conveyor includes a hollow feed roller W and an adjacent pressure roller AW. The two rollers are spaced to provide a variable crop feed slot SP which has an average width WS. 
     FIG. 1 shows the hollow feed roller W in longitudinal section and a side view along the longitudinal edge of a sensor device  10 . The sensor device  10  includes three permanent magnets M 1 -M 3  arranged side by side with their fields extending in parallel directions. The North and South poles of the magnets are indicated by “N” and “S” in FIG. 1. A respective pole shoe P 1 -P 3  surrounded by a sensor winding S 1 -S 3  is disposed on the north pole surfaces “N”. A magnet (e.g. M 1 ), a pole shoe (e.g. P 1 ) and a sensor winding (e.g. S 1 ) are advantageously coupled together form an individual replaceable module. These modules are mounted in a closely packed series on a plate-like support T of magnetically soft material which is fixed to a stationary shaft A of the hollow feed-roller W. The connections for the windings S 1 -S 3  are fed out from one end  12  of the shaft A. It is very desirable for the pole shoes P 1 -P 3  and the support T to have a construction to reduce eddy current losses in the wanted signal. For this purpose the pole shoes P 1 -P 3  and the support T consist of a stack of mutually insulated transformer laminations which are either packed so as to extend in the radial direction as indicated by a short section  14  of the pole shoe P 2 , or, are layered in the axial direction as illustrated by a short section  16  of the pole shoe P 3 . Preferably the individual laminations are aligned radially or such that they are approximately radial. 
     FIG. 2 shows an end view of the sensor device  10 . The flat support T which supports the respective modules (comprising, e.g., the magnet M 1 , the pole shoe P 1  and the sensor winding S 1 ) is mounted on the stationary shaft A. The modules are conveniently surrounded by a housing G 1 . The free face F 1  of the pole shoe extends closely adjacent the hollow roller W. The width B of the free face F 1  of the pole shoe is approximately ⅓ smaller than the width BM of the magnet thereby concentrating the field accordingly. Because the free face F 1  of the pole shoe is narrower than the pole face of the magnet M 1  and as a result of the elongation of the magnetic arrangement along the line of the field that is produced by this pole shoe, the field will penetrate deeper into the feed-slot SP. Moreover, the sector of the roller W located with the magnetic field at any one time is smaller than it would be without the narrow free face F 1  of the pole shoe P 1  so that the interference signals produced by the rotating metal ribs of the roller as they pass through the field are smaller relative to the wanted signal and, in addition, these signals are more alike in the various sensor devices so that they are easier to compensate. The duration of the interference signals produced by the ribs WR of the roller when passing through the field is thereby reduced. 
     The total length of the magnet and pole shoe arrangement preferably corresponds to the average width of the feed slot SP so that, when a normalized metallic foreign body is passing there through, it will produce an easily detectable signal even when the crop throughput is at its greatest and the width WS of the slot is at its largest and even if the foreign body is in its most extreme position. As shown in FIG. 2, a length L of the pole shoes P 1 -P 3  corresponds approximately to the width of the magnets M 1 -M 3 . The total length comprising the length L of the pole shoe and a length LM of the magnet corresponds approximately to the average width WS of the variable feed slot SP formed between the feed-roller W and the co-operating spring-loaded pressure roller AW. As a result of this arrangement, a foreign metallic body of a defined minimum size will be penetrated by a sufficiently large component of the magnetic field even when it is passing close to the pressure roller, and consequently, the foreign metallic body will be detected. 
     FIG. 3 is a top view of a section of the arrangement including some of the three adjacently mounted modules, here, the free pole faces F 2 , F 3  and the sensor windings S 2 -S 3 . The housings G 2 -G 3  enclose the modules only along their longitudinal edges. The magnets and sensor windings S 2 -S 3  are located very close together. 
     FIG. 4 is a schematic of a circuit for evaluating output signals A 1 -A 3  taken from the sensor windings S 1 -S 3 . These are normalized to a common output level in respective pre-amplifiers V 1 -V 3  using appropriately adjusted feedback networks R 1 -R 3 . One of the level-normalized signals N 1 -N 3  and the complementary signals K 1 -K 3  of the other level-normalized signals are added in an adding network  1 R,  2 R,  2 R′ in such a way that the components of the interference signal are extensively compensated. The resulting sum signal SS is compared with a predefined threshold value SW in a threshold value comparator SV and a discriminator signal D 1  is passed on for further processing whenever the threshold is exceeded. If, as shown, there are three sensors, the resistors  2 R,  2 R′ of the adding network to which the complementary signals K 1 -K 3  are applied have a resistance value which is twice that of the resistor  1 R carrying the wanted signal N 1 . The same arrangement applies in respect of the other adders and comparators which are not illustrated in detail. 
     While a preferred embodiment of the invention has herein been illustrated and described, this has been done by way of illustration and not limitation, and the invention should not be limited except as required by the scope of the appended claims.