Abstract:
An optical system is disclosed for use in sorting apparatus to monitor light at a viewing station thereof to generate signals indicative of the optical properties of selected items in the product stream being sorted. In the system light received from a single line at the viewing station is split into discrete beams, which are filtered into different wavelength ranges to determine the category of the respective product items. The beams are directed onto a slit through which the light beams must pass on their way to respective lines of light sensors.

Description:
BACKGROUND TO THE INVENTION 
     This invention relates to sorting apparatus. It is particularly concerned with such apparatus which grades product according to colour characteristics, and activates an ejection mechanism based on that grading to remove selected product from the stream. The present invention is directed at an optical system for monitoring light at a viewing station in sorting apparatus in order to grade product passing therethrough. 
     Product can be effectively graded by a colour sorting technique. Various sorting apparatus which grade product according to its ability to reflect light in different wavelength ranges are described in U.S. Pat. Nos. 4,203,522; 4,513,868; 4,630,736; 4,699,273; and 5,538,142, the disclosures of which are incorporated herein by reference. In apparatus disclosed in the &#39;522 and &#39;142 patents for example detectors are responsive to light reflected from a product in different wavelength ranges, and generate signals indicative of different qualities of the product. These signals are compared and analysed, to generate a signal which can activate ejectors to remove the relevant item from the product stream. 
     In some of the apparatus of the kind described above, the reflected light is monitored by optical systems containing CCD arrays with a plurality of lines of sensing elements. Typically a tri-linear array is used; the three lines of elements view different areas of the product and are filtered to respond to particular wavelength ranges. In order that the colour of an area of the product may accurately be determined it is necessary to compare measurements taken on the three lines of elements at different times. This may be achieved if the speed of the product is constant and is known accurately. However, in practice the speed of the product may vary, the product may move across the stream and it may rotate between sensing positions all of which give rise to difficulties in determining the colour. To avoid the problem it is necessary that the lines of arrays all view the same area of the product simultaneously; i.e. their view is co-incident. Previously this has been addressed by others as detailed in U.S. Pat. No. 5,315,384, by building cameras which split a beam of focussed light from a viewing area into a plurality of paths by use of an arrangement of prisms. The selection of colours in the beams is by filters which are cemented together with the prisms and the arrays. The positioning of the components must be very accurate which makes production of these cameras difficult and expensive, and major colour changes cannot be made to a camera. The introduction of multi-linear CCD arrays offered the possibility of simpler assembly and interchangeability of filters if the problem of the absence of co-incident viewing could be addressed. 
     SUMMARY OF THE INVENTION 
     In this invention, light reflected from product at a viewing station in sorting apparatus is monitored by splitting the light received from an area of the product into a plurality of discrete beams, which are then directed onto light sensors, each of which is responsive to light in the visible or infra-red wavelength range required for colour sorting. The beams are directed onto the sensors through a slit which is disposed close to the sensors in such a position that the view of the lines of sensors is co-incident at the viewing station. The light sensors themselves are normally arranged in an array, for example, of charge coupled devices (CCDs), typically a tri-linear array. The beams are filtered into different wavelength ranges by filters positioned in the beams at any position where they follow separate paths. These filters may be changed as required for a particular sorting task without other modifications to the optical assembly being required. 
     Typically, the light received from a product piece in a viewing station can be split into the plurality of beams by means of a prism section, the split beams then being directed towards the light sensors by a lens. The prism section can consist of two prisms, one on either side of a parallel-sided glass plate disposed on the axis of the lens. The deflection angle of each prism will be very small, for example less than 5° and typically less than 1°, with the result that the split beams remain in close proximity as they pass through the lens Filters are disposed in the path of the beams to restrict the light transmitted in each beam to each sensor to the respective wavelength range, and the filters can be disposed on either side of the prism section, relative to the lens. 
     In an alternative light splitting mechanism, a variation of a converging lens system is used. Specifically, the portions of a converging lens system between two or more laterally outer sections thereof are reduced, and these outer sections displaced towards the lens axis. The effect is to simultaneously split received light into a plurality of discrete beams in different wavelength ranges by use of filters, and direct the beams onto respective light sensors. The converging lens system may take the form of a simple bi-convex lens, but other suitable assemblies might equally be used. Once again, the light filters can be disposed at any suitable location between the viewing station and the light sensors. This can include coating on the respective active lens surfaces. 
     Normally, the light emanating from product in the viewing station would be split into beams of light in three discrete wavelength ranges, typically corresponding to those of three specified visible colours, which are known for use in sorting apparatus of this general kind. Alternatively, the wavelength ranges might correspond to those of two specified visible colours, and a third wavelength range in the infra-red. In this configuration, the beams of visible light can be disposed on either side of the beam of infra-red. In one arrangement, the viewing station is illuminated with visible light from the side of which the light sensors are disposed, with light in the third wavelength range being transmitted from the opposite side. The light sensors are thus adapted to monitor reflected light in a visible range, and the light transmitted in the third wavelength range, which may be in the infra-red, being monitored to conduct a “dark” sort and/or monitor the viewing station for the presence or absence of product therefrom, as described in U.S. Pat. No. 5,538,142. 
     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 illustrates diagrammatically the operation of sorting apparatus embodying the invention: 
     FIG. 2 shows, not to scale, an optical system according to the invention; 
     FIG. 3 shows, not to scale, an alternative optical system of the invention, and 
     FIG. 4 shows, not to scale, a further alternative optical system of the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is an illustration of sorting apparatus including a conveyor  2  to which product is fed from a hopper  4  down a chute  6 . The conveyor belt is driven such that its upper level moves from right to left as shown at a speed (for example, 3 meters per second) sufficient to eject material in a product stream  8  to receptacle  10 . During its passage from the end of the conveyor  2  to the receptacle  10 , the material is kept in the product stream a solely by its own momentum. Ejectors  12  extend over the width of the product stream, and are operable to remove items from specific zones of the product stream  8  by high-pressure air jets, directed towards the reject receptacle  14 . Typically, the width of the product stream is around 500 mm, with forty ejectors equally spaced thereover. The ejectors are instructed by a computer or a microprocessor  16 , which itself receives input data from the optical scanning system  18 , described below. 
     Reference  20  indicates a viewing station where product in the product stream  8  is scanned. The station is illuminated by visible light on one side from the sources  22 , and with radiation from a further source  24  on the other side. The source  24  can be of visible light, but may alternatively be of light in the infra-red range, as will be described below. 
     Light reflected from product in the product stream as it passes through the viewing station  20  is monitored by an array of sensors  26  in the form of charge-coupled devices (CCDs) sensitive to light in different wavelengths. In its passage to the sensors  26 , the light is split into discrete beams at a prism section  28 , and the resultant three beams are filtered to restrict the transmitted light to the appropriate wavelength range before being directed by a lens  30  through a slit  32  to the sensors  26 . The CCDs are arranged in a tri-linear sensor array which, with the slit  28 , extend the viewing to the entire lateral dimension of the product stream. 
     By monitoring the reflected light in the visible wavelength ranges, and the transmitted light in the third wavelength range, not only can product in the stream be graded, but it is also possible to register the presence or absence of product from the viewing station. Signal generated by the sensors  26  are sent to the computer  16 , which in turn instructs the ejectors to remove selected product from the stream. In this respect, the analysis of the light received and the operation of the ejectors is similar to that described in our U.S. Pat. Nos. 4,699,273 and 5,538,142, referred to above. 
     FIG. 2 shows in a little more detail the optical scanning system described above with reference to FIG.  1 . As can be seen, light emanating from the viewing station  20  passes to a prism section  34  where it is split into three discrete beams. The prism section  34  comprises two glass prisms with a parallel sided glass plate therebetween. The angle of each prism is normally less than 5°, typically less than 1°. The central beam  36  is not substantially deflected, but the beams to either side thereof refract as they pass through the upper and lower prism sections shown before being redirected by the lens  38  onto the array of sensors located behind an aperture plate  40 . As indicated above, the drawing is not to scale, and it should be noticed that the ratio of the distance between the product piece in the viewing station and the lens on the one hand to the spacing of the lens from the sensor array on the other, is typically around 20 to 1. 
     In the arrangement shown in FIG. 3 the optical system of FIG. 2 has been revised and refined primarily to avoid the use of a separate prism section. This has been accomplished by the use of an adapted converging lens system  42  in which two laterally outer sections  44  of a biconvex or achromatic lens are displaced towards each other and a remaining central section at the lens axis. This results in the creation of what is essentially a prism arrangement, but which also has a focussing effect to redirect the refracted beams to the array of sensors  26 . In other respects though, the optical systems of FIGS. 2 and 3 operate in essentially the same way. 
     FIG. 4 shows an arrangement similar to that of FIG. 3, but with the biconvex or achromatic lens replaced by two plano convex lenses  48 . The optical effect of this arrangement is the same as that of the arrangement in FIG.  3 . As in the embodiment of FIG. 3, chordal sections of each lens  48  have been removed, and the laterally outer sections  50  (upper and lower as shown) displaced towards the remaining central section  52 . 
     FIGS. 2 to  4  show the disposition of filters  46  in the path of the light in transmission from the viewing station  20 . In FIG. 2 the filters are between the prism section  34  and the lens  38 . In FIG. 3 they are located in front of the lens system  42 . The filters may be disposed between the lens  38 , or lens system  42  respectively, and the sensor array, or in the embodiment of FIG. 2, in front of the prism section  34 . Alternatively, filter media can be coated onto active surfaces of the lens or lens system to achieve the same effect. The filters determine the wavelength range of light in each beam and where one of the beams is to carry light in the infra-red range, it is preferred that this beam is disposed between the beams of visible light. 
     The embodiments described above are given by way of example only, and illustrate ways the invention can be put into effect. Variations can be made, and alternative equipment can be used without departing from the spirit and scope of the invention claimed.