Sorting

The present invention relates to a method of and apparatus for sorting a succession of objects according to shape, including feeding each successive object through a viewing zone and eliminating the object as it passes through the viewing zone, thereafter directing the object into one of at least two paths according to its shape. The objects may be for instance edible products such as peas or sweets, but the invention is in no way limited to edible products.

The general intention of the inventin is to automate the shape sorting of the objects.

According to the invention, each successive object is viewed as it passes through the viewing zone using at least four fixed electronic viewers spaced in one plane around the viewing zone and normally at 90.degree. to the direction of feed of the object, signals being derived from each viewer representative of the edges of the object as viewed at a particular instant by the viewers, the signals being processed electronically to determine in which of at least two shape categories the object falls, and the object being automatically directed into one of at least two paths according to its shape.

Using the invention, good sorting can be obtained. However, it is normally difficult to have a positive sort into acceptables and rejects and one solution is to provide at least three shape categories, namely acceptables, hand-sorts and rejects, the hand-sort category being necessary if the apparatus is unable to discriminate sufficiently; this is acceptable in practice if the percentage of hand-sorts is reasonably low.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic, isometric view of apparatus in accordance with the invention;

FIG. 2 is a block diagram of the electronics of the apparatus;

FIG. 3 is a block diagram of the decision tree of the apparatus; and

FIGS. 4 to 7 represent the functions carried out in the function cards shown in FIG. 2.

FIG. 1 shows a suitable feeder 1 which feeds the objects one by one vertically downwards in rapid succession. The feeder 1 is just shown schematically as suitable feeders are available. The objects should be fed at at least one per second, and preferably more than five per second or ten per second, say twelve per second. The objects are accelerated within the feeder 1, and leave the feeder at a speed of say 1 m/s or preferably 2 m/s. The objects should be unrestrained as they pass through the viewing zone so that the images of the objects are not obscured by any mechanical parts, and thus the objects will be in free flight. Vertical fall is the simplest to arrange, but in theory at least the objects could be projected for instance horizontally through a viewing zone. The path 2 of the objects is indicated. The objects pass through a light curtain 3 which signals their arrival at the shape sorting zone. The light curtain 3 triggers a strobe unit formed by seven illuminators 4. Preferably, the illuminators receive white light from a laser flashlight (flash lamp) by way of fibre optics, and have a lens for forming parallel light. The light can have any suitable wavelength. The length of flash depends upon the speed of the objects, but for a speed of just over 1 m/s, the length can be 15 microseconds.

Diametrically opposite each illuminator 4 there is an electronic viewer 5, the viewers 5 being spaced in one plane around a viewing zone, which plane is at 90.degree. to the direction of feed or path 2 of the objects. In the machine illustrated, there are nine viewers 5, but it may be possible to have as few as four or more suitably five in order to obtain a sensibly efficient sort; seven viewers 5 would be a practical possibility. The angular separation of the profiles which are examined depends on the number of viewers 5. It will be seen that the viewers are spaced through somewhat under 180.degree., the angular distance between each viewer 5 being 180.degree. divided by the number of viewers 5. The viewers 5 are each directed at an illuminator 4, so that a silhouette of the object is generated and the image received is of a dark object against a light field-this gives better resolution and a greater depth of focus. Each viewer 5 views the whole of the profile (i.e., periphery or outline) of the object as presented to the view 5. No viewer is needed from above in view of the arrangement of the viewers 5.

After dropping below the plane of the viewers 5 and illuminators 4, the objects pass through a sorting device comprising a ring of a suitable number of air jet nozzles 6 which direct successive objects into one of a number of paths according to their shapes. The nozzles 6 can be connected to compressed air via solenoid valves (not shown). There are shown a number of bins 7 corresponding to the number of nozzles 6, but there could be a central bin as well. The number of nozzles 6 and bins 7 will depend on the types of categories required for the sort. For instance, six bins 7 are required for the decision tree shown in FIG. 3, which is specifically for sorting diamonds having a maximum weight of 4 carat and and a minimum weight of 1/15 carat, with less than 30% for hand sorting--the acceptables are sawables and the rejects makeables (the latter may need hand sorting to decide if they should be cleaved). The intention is to sort the two "low confidence" bins.

As shown in FIG. 2, each viewer 5 is connected to a channel capture board 11 which normalises (white made true white, black made true black) by selecting a voltage threshold between black and white, and digitises the signal, thereby providing digital (video data) signals representative of the edges of the object as viewed by the viewer 5, and tracks round the edge or boundary. The data signals are then fed to a computer 12.

The computer 12 incorporates a channel scanner 13 which scans each channel (from each viewer 5) in turn, a general purpose function card 14, a number of special function cards 15, a memory 16 and a head processor 17. The head processor 17 controls the admission of compressed air to the nozzles 6, to open one of the valves. In the arrangement illustrated, separate function cards 14, 15 are used, and these are hand wired. Apart from the general purpose function card 14, each card is specifically for one function. It would be possible to programme these functions using normal software, but the function cards are preferred. The function cards can be changed, for instance if a large amount of objects having a certain peculiarity must be sorted. The general purpose function card 14 is programmable, so that it can be programmed for any modifications; it runs more slowly than the cards 15 but is more flexible.

The decision tree is shown in FIG. 3. The general function card 14 is not represented. For each decision, the average value of the parameter (sphericity, symmetry and convex hull deviance) is determined for all the channels, i.e. views, before combining the parameters (if required) and making the decision.

At 15a, a signal is derived representative of any optical edge breakthrough, and edges are joined on either side of the breakthrough. Edge breakthrough occurs when the objects are translucent or transparent, caused by refraction or internal reflection. There is a highly irregular reentrant, represented schematically in FIG. 4. The edge (boundary) is traced and the rate of changge in direction of each incremental length of the edge (boundary) is determined. The rate of change of direction in a normal reentrant is much lower than in a breakthrough. The beginning and end of the zone of large rates of change of direction are determined, and are electronially joined up. If desired, the shorest distance between the beginning and end can be determined--if this is low (say less than 1% of the total edge length as detected), breakthrough is present. Another possibility is to determine the length of the detected edge between the beginning and end of the high frequence profile and compare it to the length of the remainder of the edge--if the high frequency length is great, breakthrough is present.

At 15b, signals are derived representative of the approximation of the object to a spherical shape (blockiness) and representative of the approximation of the object to symmetry, as illustrated in FIGS. 5 and 6. In order to determine the blockiness, the area of the image is determined and the area is divided by the square of the length of the edge. In order to determine the symmetry, the centroid 21 is determined, the image is divided into two parts along a line passing through the centroid 21, one part is rotated about 180.degree. to superimpose it on the other part, and the mismatch area 22 is compared with the overlapped area 23. The line passing through the centroid 21 may be taken as the horizontal line, thus determining whether there is symmetry about a horizontal plane for that particular channel or view; only one line through the centroid 21 is needed due to the 180.degree. rotation, in order to obtain a good approximation to a determination of axial symmetry about the centroid. The signals representative of blockiness and symmetry are added. Objects having low values are directed to bin 7a (high confidence rejects).

At 15c, the sphericity and symmetry are again determined, and also a signal is derived representative of reentrants in the image. The latter signal can be determined by determining the convex hull deviance, i.e. the difference between the length of the edge and the length of the line which extends around the edge but extends, like an elastic band would, straight across any reentrant 24 (see FIG. 7). In more detail, a line of polygonal (say hexagonal) shape is placed around the image and is then shrunk on to the edge of the image by not being permitted to go within the minimum distance between any two points. The signals of blockiness, symmetry and inverse convex hull deviance are combined, and objects having a high value are directed to bin 7f (high confidence acceptables).

At 15d, the inverse convex hull deviance is combined with the standard deviation of blockiness and symmetry. Objects having a low value are directed to bin 7a (high confidence rejects).

At 15e, the overall blockiness and symmetry signals are again combined, and low values are directed towards bin 7b (medium confidence rejects).

At 15f, the signals of overall blockiness, symmetry and inverse convex hull deviance are combined. Low values are directed to bin 7c (low confidence rejects), high values are directed to bin 7e (medium confidence acceptables) and the remainder are directed to bin 7d (low confidence acceptables). It will be appreciated that the limiting values in 15b and 15e are different, as are the limiting values in 15c and 15f, thus changing the confidence of the sort.

As an alternative to the electronic viewers 5, it would be possible to use strobed area sensors.