Apparatus and method for stabilising material transported on conveyor belts
BACKGROUND TO THE INVENTION 
     This invention relates to the stabilisation of material on moving conveyor surfaces, and particularly to a stabiliser unit for this purpose. It has especial application to sorting apparatus in which bulk material of relatively light specific weight is carried at speed along a conveyor to a sorting station where defective or rejected elements thereof are removed while the material is in free flight. 
     Where bulk material is to be transported on a conveyor belt at speed, upon delivery of the material to the belt it is subject to significant acceleration. This results in movement of the material on the conveyor and significant instability. While chutes or intermediate conveyor belts can be used to accelerate the material as it is delivered to the conveyor belt, it is not always possible to include a chute, and when the material is frozen or moist, chutes can be of limited use. While reference is made herein to conveyor belts, similar problems arise in the use of other conveyor surfaces, such as the peripheral surface of a rotating drum. 
     In the recognition that unstable material on a moving conveyor can create problems, various attempts have been made to provide an effective means by which such material can be stabilised. Soft rollers have been used, with the surface provided by foam or an airbag, but these have lead to some difficulties because the rollers were driven at least partly by contact with the material itself on the conveyor surface. In another proposal, a flexible open mesh is provided on a cylindrical surface mounted by a resilient core to a rotatable shaft. The shaft is rotated by a drive wheel on the shaft which engages the conveyer surface. Such a stabiliser is disclosed in U.S. Pat. No 4,830,180. While the use of a drive wheel to rebate the shaft reduces the demands on the surface contacting material on the conveyer, the downward force applied by the mesh to articles on the conveyer is essentially determined by the construction of the stabiliser. Thus, the stabilising material cannot be moved away from the conveyer, other than by contact with articles on the conveyer, without simultaneous removal of the drive wheel. 
     SUMMARY OF THE INVENTION 
     The present invention is directed at a stabiliser unit in which a stabiliser surface carried on a drum is driven at a speed substantially equal to the advance of a conveyer surface where it overlays the conveyer surface, to stabilise material thereon. The drum is rotatably mounted in a framework, and driven by a drive pulley adapted to engage the moving conveyor surface and transmit drive therefrom to the drum via a coupling which ensures the peripheral velocity of the drive pulley and the drum is substantially the same, thereby avoiding substantial relative movement of the stabiliser belt and conveyer surface in the section of the stabiliser belt path which overlays the conveyer surface. The coupling is normally a reversing pulley interposed between the drive pulley and the drum. The conveyer surface is normally the surface of a conveyer belt, but it might take another form, such as the surface of a rotating cylinder. Where the surface is on a rotating cylinder, a stabiliser unit of the invention can be of particular value as there would be less time available for material delivered to the surface to stabilise naturally. 
     The stabilising surface on the drum can be a relatively thick layer of soft but resilient material, such as foam rubber, normally held spaced from the conveyor surface. However, an inflated bag, or a web or mesh, of the kind referred to above might be used to define the operative surface of the drum. The preferred stabilising surface in units of the invention is a stabiliser belt which extend round a drum suspended in a framework over the conveyor surface, and has sufficient length that it moves in a path comprising four primary sections. These are a first section overlaying the conveyor surface; a second section in engagement with a segment of the drum, and third and fourth section extending between the drum surface and respective points of separation of the stabiliser belt from the conveyor surface. A gripping, roller or other means can be included to hold the stabiliser belt against the drum as it is driven thereby ensuring the synchronous movement of the stabiliser belt and the conveyor surface in the overlaying section. 
     In a stabiliser unit according to the invention the drum will normally be of sufficient length to cover substantially the entire width of the conveyor surface with which it is to be used. However, as the drive for the unit is to be transmitted from the conveyor surface itself via the drive pulley and the respective coupling means, these components are disposed at the axial extremities of the drum, normally outside of a pair of flanges between which the stabiliser surface is carried. 
     The drum in stabiliser units according to the invention is preferably mounted in the framework so that it can readily be removed to enable the cleaning of the stabiliser surface. The drum may be mounted on a fixed rotational axis in the framework; for example, with an axle of the drum being received in appropriate slots providing for easy removal. When the stabiliser surface is a loose belt, once the drum is removed, the belt and drum can be readily separated, enabling the stabiliser belt to be replaced and the drum returned to its operative position in the unit in a very short changeover time. In an alternative arrangement, which renders the drum and belt no less easily removable, the drum is supported on either side of its rotational axis by pulleys or rollers, of which those on one side of the axis may constitute part of the drum drive mechanism. On the other side, idler rollers would be mounted in the framework. In this arrangement the actual location of the drum axis can be altered by movement of the supporting rollers, and particularly those which comprise the coupling means between the drive pulley and the drum. 
     The invention will now be described by way of example, and with reference to the accompanying schematic drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows in side elevation sorting apparatus including a stabiliser unit embodying the present invention; 
     FIG. 2 is an enlarged end view, partly in section, of the stabiliser unit shown in FIG. 1, but omitting the belt; and 
     FIG. 3 is an enlarged outline elevation of the stabiliser unit of FIGS. 1 and 2, showing the interrelation between the drum and the pulleys, and the means by which the unit itself can be mounted in a sorting device of the kind shown in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The sorting apparatus shown in FIG. 1 is designed to handle particulate bulk material including moist or frozen produce even when this has a tendency to agglomerate. The apparatus is assembled around a support structure 2 in which a conveyor belt 4 extends nearly the full length thereof. The belt 4 is around 20 inches wide, and carries the material a distance of approximately 50 inches. The material of the conveyer surface will normally be textured. A number of surface patterns can be used, such as dimples and saw-tooth, the latter preferably with the teeth inclined in the direction of travel. 
     Bulk material is fed to the conveyer 4 from a delivery mechanism such as a vibratory feeder 6 which agitates and breaks up agglomerated material prior to discharging it onto the conveyor belt at delivery section 8. The conveyer belt 4 moves at a speed, to the left as shown, sufficient to project the material from the downstream end 10 thereof in a free trajectory to a chute 12 from which it is delivered to a suitable collection receptacle (not shown). The product is sorted just as it leaves the end 10 of the conveyer, where it passes through a scanning zone 14 in which it is illuminated and scanned for light reflected from particles in the product stream through a gap 16 using arrays of detectors mounted in a computer housing 28 on the support structure 2. Two arrays are used, each monitoring around 10 inches of the product stream width. In order to provide a background against which the reflected light is monitored, a rotating roller 20 is illuminated by a light source 22, which light is reflected off the roller surface into the scanning zone 14. 
     Downstream from the scanning zone 14 is disposed an array 18 of ejectors which are activated in response to signals from the scanning system to eject by pneumatic means a product in the stream which has been deemed unacceptable as a consequence of analysis of light reflected therefrom in the scanning zone 14. Typically, there are forty ejectors arranged across the product stream width at half inch spacings. The ejectors discharge unacceptable product downwards out of the product stream, into a suitable receptacle (not shown). The operation of the apparatus shown in FIG. 1 is controlled by a computer in the housing 28. 
     These scanning and ejector mechanisms can be of any suitable kind commonly used in colour sorting apparatus. Suitable sorting techniques using optical scanning systems and pneumatic ejection systems as described herein are more fully disclosed in U.S. Pat. Nos. 4,203,522; 4,513,868 and 4,699,273, the disclosures whereof are hereby incorporated by reference. 
     Material leaving the vibratory feeder 6 will normally have only a small linear speed, typically around 1/2 meter per second, and must be rapidly accelerated to reach a speed sufficient to project the product into the trajectory described above. A typical linear conveyor speed for this purpose is around 3 meters per second. This acceleration, as the material transfers from the feeder 6 to the conveyor belt 4, leads to a relatively unstable product on the conveyor. 
     In order to stabilise material on the conveyor between the delivery section 8 and the discharge end 14 of the conveyor, a stabiliser unit comprising a framework 24 is mounted over the conveyor belt 4. The unit includes a belt 26 which overlays a section of the conveyor belt path, and moves with the conveyor belt over this section, to hold material against the conveyor belt, stopping movement thereof, with the result that downstream of the stabiliser unit material on the conveyor belt 4 is substantially stationary relative to the conveyor belt. 
     FIG. 2 shows a front elevation of the stabiliser unit in FIG. 1 and as can be seen, the framework 24 supports a drum 30 for carrying the stabiliser belt 26 (not shown in this Figure) between two flanges 32. At either end of the drum, outside of the flanges 32, a simple gear train is mounted on the framework 24, which has the effect of driving the periphery of the drum 30 at substantially the same speed and in the same direction as the conveyor belt 4 over which it is disposed. This gear train comprises a drive pulley 34 for driving engagement with the conveyor belt 4, and a reversing pulley 36 which couples the periphery of the pulley 34 to the periphery of the drum 30. The operation of the stabiliser unit will now be described in more detail with reference to FIG. 3. 
     The stabiliser belt 26 is only loosely carried on the drum 30, and rests by virtue of its own weight on the conveyor belt 4. FIG. 3 illustrates a situation in which the conveyor belt 4 is in motion, to the left as shown, thus drawing the stabiliser belt 26 with it. The stabiliser belt is normally composed of food quality approved inert material such as silicon rubber, of sufficient thickness and flexibility to stabilise product on the belt. 
     In order to ensure that the stabiliser belt 30 is driven at a speed substantially the same as that of the conveyor belt 4, the movement of the conveyor belt 4 is coupled directly to the periphery of the drum 30 via two pulleys 34 and 36. The periphery of drive pulley 34 makes direct contact with the conveyor belt 4 and thus, the periphery of the reversing pulley 36 also moves at the same speed as the conveyor belt 4. This movement is imparted to the peripheral surface of the drum 30, with the result that the periphery of the drum 30 will move in synchronism with the conveyor belt 4. 
     The design of stabiliser units according to the invention provides for the velocities of the conveyor surface and that of the stabiliser surface or belt to be substantially equal. However, within this term a degree of inequality is permissible. For example for some products there could be some benefit in controlling the stabiliser surface to move slightly more slowly than the conveyor surface so that the stabiliser surface has a slight dragging effect. This can be of especial use where particles of the product have agglomerated. 
     As noted above, the stabiliser belt 26 is carried only loosely on the drum 30. This enables its length at any particular instant in its use as described herein to be divided into four discrete sections. A first section 38 is substantially flat, and directly overlays a corresponding section of the conveyor belt 4. A second section is in engagement with a segment of the drum surface which drives it, and third and fourth sections 40 and 42 are defined between the drum surface and the respective points of convergence and separation of the stabiliser belt 26 and the conveyor belt 4. The length of the linear section 38 is not normally critical, and this arrangement of the stabiliser belt on the drum 30 thus enables the drum to be operationally raised or lowered relative to the conveyor belt 4 as desired within tolerable limits. The spacing of the drum 30 over the conveyor belt 4 can be set relative to the dimensions of the material being stabilised. A typical spacing could be around one inch for material to be stabilised having a mean diameter of say 1/4 inch, for example frozen peas. However, the stabiliser unit can be equally effective with the drum at a much greater spacing over the belt than the size of the material on the belt requires. In some circumstances such a greater spacing is especially useful if agglomerated particles are to be broken up with minimal damage. 
     The stabiliser unit illustrated is designed with a mechanism for altering the spacing between the framework 24 and the conveyor belt 4, and thereby adjust the pressure between the drive pulley 34 and the belt 4. On either side of the conveyor belt 4 there is a fixed rail 44. In order to accommodate the stabiliser unit, a section of each side rail is cut away as shown in FIG. 3 to accommodate the ends of the drum 30, and brackets 46 of the framework 24, between which the drum 30 is suspended, are pivotally attached to the rails 44 by means of a shaft 48 that extends between the brackets 46 and the rails 44. To allow each bracket 46 to be fixed at a given level, a screw clamp 50 passes through a slot 52 in each bracket, and engages a correspondingly threaded hole in the respective rail 44. Thus, to increase the pressure between the drive pulley 34 and the belt 4, each screw 50 may be loosened, the framework pressed downward, and the screws re-tightened. 
     The spacing of the drum 30 over the conveyor belt 4 can be altered by adjusting the mounting of the drum 30 in the framework 24. The drum 30 could of course be mounted in fixed bearings in the brackets 46, but in order to provide maximum scope for adjustment, a system is illustrated in FIG. 3 in which the drum 30 is supported on pulleys engaging its peripheral surface on either side of and below its axis, one of which pulleys is the reversing pulley 36 discussed above. An idler pulley 54 is mounted in each bracket 46 on the opposite side of the drum axis to the reversing pulley 36, and the distance of the reversing pulley 36 from the idle pulley 54 is made adjustable to raise or lower the drum 30. It is desirable in any event to arrange for the reversing pulley 36 to be resiliently urged against the drive pulley 34 and the drum 30 to ensure good driving engagement therebetween, and a resilient mechanism is indicated schematically at 58. The resilient force so generated is of course balanced by the weight of the drum 30, the stabiliser belt 26, and of course the movement of the conveyor belt 4 which is in frictional engagement with the stabiliser belt 26 over the overlaying section 38. 
     In order to ensure that the stabiliser belt 26 is driven at substantially the same speed as the periphery of the drum 30, and therefore as the conveyor belt 4, a gripping roller or rollers 60 may he used. The roller or rollers 60 are mounted at the distal ends of arms 62 pivotally mounted on the shaft 48. The arms 62 are contiuously urges by spring means such as torque springs (not shown) in an anti-clockwise direction as shown, about the shaft 48 to urge the roller or rollers 60 against the stabiliser belt 26 to preserve and maintain contact between the belt 26 and the drum 30. 
     The embodiment of the invention described above is given by way of example only, and numerous variations can be made within the spirit and scope of the invention disclosed herein.