GB2091415A

GB2091415A - Sorting Objects

Info

Publication number: GB2091415A
Application number: GB8101541A
Authority: GB
Original assignee: Gunsons Sortex Ltd
Current assignee: Gunsons Sortex Ltd
Priority date: 1981-01-19
Filing date: 1981-01-19
Publication date: 1982-07-28
Also published as: JPS57130579A; GB2091415B; JPH0146194B2

Abstract

A sorting machine comprises sensing means (20) for viewing objects to be sorted to determine whether any of them, at least in part, reflects, transmits or emits light in a predetermined part or parts of the spectrum to an undesired extent; feeding means (16, 17) for passing the objects (13) through the shorting zone (14) to be viewed against a background (22); means (24) for separating undesired and desired objects; means (26) for periodically effecting an examination as to whether a predetermined relationship exists between the reflectance, transmission or emission of light in a predetermined part or parts of the spectrum by the background (22) and by the average of the objects (13); and means (32) controlled by means (26) for adjusting the reflectance, transmission or emission of light by the background (22) and/or the radiation incident to the objects to tend to maintain the said predetermined relationship constant. <IMAGE>

Description

SPECIFICATION Method and Machine for Sorting Objects This invention concerns a method and machine for effecting relative separation between desired and undesired objects.

The term "objects" is used in this specification in a wide sense as including agricultural products such as rice, peas and beans, and minerals both in the form of fine particles and in the form of lumps of ore.

Sorting machines commonly view objects to be sorted against a background whose colour is matched to the average colour of the objects being sorted. Such a background provides a compensation for the effects arising from variation in the size of the objects being sorted.

However, the effective colour of both the background and the objects is liable to change.

Thus, for example, the background and the objects may be illuminated by respective fluorescent tubes which age at different rates and/or which have aged unevenly along their lengths in different ways. The ratio between the light outputs of such fluorescent tubes, moreover, may change due to different amounts of dust building up thereon. Furthermore, although the average colour of the objects being sorted normaliy remains substantially constant, nevertheless even where the product being sorted remains the same, occasional changes in the average colour do occur which are normally slow.

If, however, the product being sorted is changed to a similar but different product, such changes in the average colour are more probable.

If, moreover, the sorting machine is a multichannel machine, the objects passing through the various channels are not all necessarily of the same average colour, nor are the backgrounds of the various channels necessarily of the same effective colour for the reasons discussed above.

According, therefore, to the present invention there is provided a method of sorting comprising viewing objects to be sorted to determine whether any of them, at least in part, reflects, transmits or emits light in a predetermined part or parts of the spectrum to an undesired extent or in an undesired ratio, the objects being viewed while passing across a background which is also viewed and whose reflectance, transmission or emission of light in the said part of parts of the spectrum has a predetermined relationship with that of the average of the objects, effecting relative separation between any undesired objects which have been so viewed and the remaining objects, periodically effecting an examination as to whether the said predetermined relationship exists and, if it does not, effecting an adjustment of the reflectance, transmission or emission of light by the background and/or adjusting the incident radiation onto the objects in the said part or parts of the spectrum so as to tend to restore the said predetermined relationship.

By reason of the said examinations and adjustments it is possible to maintain the reflectance, transmission or emission of light from the background at the said predetermined relationship to that of the average of the objects.

As will be appreciated, in most cases the "predetermined relationship" will be equality i.e.

the background will be matched in colour to the average colour of the objects. However, the background could, if desired, be deliverately mismatched in colour with respect to the average colour of the objects provided that this mis-match was kept constant.

The invention also comprises a sorting machine comprising viewing means for viewing objects to be sorted in a sorting zone to determine whether any of them, at least in part, reflects, transmits or emits light in a predetermined part of parts of the spectrum to an undesired extent or in an undesired ratio; a background which is in the field of view of the viewing means; feeding means for passing the objects through the sorting zone and past the background so that the objects may be viewed against the background; separating means for effecting relative separation between any undesired objects which have been so viewed and the remaining objects; examination means for periodically effecting an examination as to whether a predetermined relationship exists between the reflectance, transmission or emission of light in a predetermined part or parts of the spectrum by the background and the reflectance, transmission or emission of light in the said part or parts of the spectrum by the average of the objects; and adjustment means, controlled by the examination means, for effecting where necessary an adjustment of the reflectance, transmission or emission of light by the background and/or an adjustment of the incident radiation onto the objects so as to tend to maintain the said predetermined relationship substantailly constant.

Preferably there are means for periodically stopping the feeding means so as to stop the passage of the objects past the background, the examination means being arranged to effect each said examination at least in part during a said stoppage of the objects.

The examination means may comprise means for comparing the average reflectance, transmission or emission of light by the background and objects collectively shortly prior to the said stoppage, with the reflectance, transmission or emission of light by the background alone during the stoppage.

The viewing means are preferably photoelectric viewing means, the examination means comprising an averaging circuit for producing a voltage signal representative of the average electrical output of the viewing means, the examination means also comprising electrical means arranged to receive said voltage signal and also to receive a further voltage signal which is representative of the instantaneous electrical output of the viewing means, said electrical means being arranged to produce a difference signal which is the difference between the two voltage signals and which controls the adjustment means.

The electrical means may comprise a processor which controls the feeding means, the processor being programmed to stop the feeding means at intervals whose length is inversely related to the magnitude of the difference signal.

The electrical means may also control the adjustment means so that, if a difference signal is produced at an examination, the resulting said adjustment is initially insufficient to fully restore the said ,c, edetermined relationship and a further difference signal is therefore produced at the next examination, the electrical means being arranged to determine from the magnitude of these two successive difference signals the necessary signal to be sent to the adjustment means to complete the said adjustment.

There may be means, operative during each said stoppage, for measuring the output of the viewing means and applying, if necessary, a correction factor in said electrical means to maintain the sensitivity of said viewing means substantially constant.

There may also be means, operative during the passage of the objects past the background, for indicating the rate of separation of undesired objects.

The adjustment means may comprise a motor for altering the angular disposition of the background.

There may be at least one light source whose light is directed onto the background or onto the objects, the adjustment means comprising means for varying the light output of the said light source or sources.

The background may, if desired, be provided with light-emitting diodes or with liquid crystal material, the adjustment means being arranged to vary the total light output of the diodes or liquid crystal material.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a diagrammatic side view of a sorting machine according to the present invention, Figure 2 is a diagrammatic view of a part of the sorting machine of Figure 1, Figure 3 is a block diagram of a processor forming part of the structure shown in Figure 1, Figure 4 is a graph illustrating the output of detectors shown in Figures 1 and 2, Figure 5 is a sketch showing the manner in which objects are viewed in the machine, and Figure 6 is a graph illustrating the way in which a correct difference voltage can be derived by the processor for supply to a background brightness controller Referring to the drawings, a sorting machine for sorting rice according to its reflectivity comprises a plurality (e.g. ten) of groups of channels 10, although in order to simplify Figure 2 only two groups 10 are shown thereon. Each group of channels 10 comprises a plurality (e.g.

four) of separate but adjacent substantially vertical channels or chutes 11. Each chute 11 has a substantially flat bottom 12 and is formed to support and to convey simultaneously a plurality (e.g. four) of rice grains 13, which are arranged side by side, to a sorting zone 14.

The groups of channels 10 are arranged in two assemblies of five such groups 10 (and thus of twenty channels). Mounted above each such assembly of five groups 10 is a respective hopper 1 5 for rice, a vibrating table 1 6 which is arranged to receive the rice from the respective hopper 15, and an electromagnetic or other vibrator 1 7 for vibrating the table 1 6 so as to feed the rice grains 13 thereover. Each vibrator 17, when energised, causes rice grains 13 to be fed from the respective hopper 1 5 to the tops of all the respective chutes 11. The rice grains 1 3 fall freely from the bottom of each of the chutes 11 under gravity so as to pass through the sorting zone 14.

A plurality (e.g. sixteen) of photo-cells or other light-sensitive detectors 20, which constitute photo-electric viewing means for viewing objects 1 3 to be sorted in the sorting zone 14, are provided for each of the groups of four chutes 11.

A respective lens (or lens system) 21 focuses on to each said plurality of detectors 20 images of the rice grains 13 which are falling through the sorting zone 14 as the result of having been delivered to the latter from the respective chutes 11.

A common background 22, which is viewed out of focus by the respective detectors 20, is provided for each of the groups 10 of four chutes 11. Each background 22 is disposed on the side of the groups 10 opposite to that on which the detectors 20 are disposed. Each background 22 is provided to match the average colour of the rice grains 13, i.e. the average reflectivity of the latter in a part of parts of ths spectrum, or is mismatched with respect thereto to a predetermined degree, so that the signals produced by the detectors 20 are not substantially affected by variations in the size of the rice grains.The backgrounds 22 are disposed closely adjacent to each other, but because the backgrounds 22 are out of focus while the rice grains 13 are in focus, each group 10 of four chutes 11 is widely spaced from its adjacent group or groups, e.g. by a distance equal to at least one-third of the width of the said group.

Two fluorescent tubes 23 are provided to direct light onto respective rice grains 1 3 falling through the sorting zone 14 and a fluorescent tube 23a also directs light onto all the backgrounds 22.

Each of the chutes 11 has associated therewith a respective single pneumatic ejector 24. Each ejector 24 is connected to a source (not shown) of compressed air and is provided with an electrically actuated valve (not shown) which is opened and closed under the control of an ejector drive unit 25 which is driven by a processor 26. A plurality of processors 26 are provided, one for each group 10 of four chutes 11. Each processor 26 is in turn controlled by the signals from the respective detectors 20 so that, for example, if a dark or otherwise discoloured rice grain 1 3 is detected by any of the detectors 20, the said valve is opened and a jet of compressed air is directed towards the discoloured rice grain.As a result, the path of movement of the discoloured rice grain is altered so that instead of passing into an "accept" bin 27 it passes into a "reject" bin 28.

As will be seen from Figure 2, the provision of the lens 21 ensures that each of the detectors 20 is in operation responsive to rice grains 1 3 which have been delivered to the sorting zone 14 from a respective portion only of the respective chute and is also responsive to a corresponding portion of the respective background. For example, the detector 20a on the extreme left hand side of each of the plurality of detectors 20 will view only the rice grains 13 falling from a portion 11 a at the extreme right hand edge of the most right hand of the respective chutes 11 and will view a corresponding portion 22a of the respective background 22.Similarly, the detectors 20b, 20c, 20d will respectively view only rice grains from the portions 11 b, 11 It, 1 d of the most right hand of the chutes 11 together with the corresponding portions 22b, 22c, 22dof the respective background 22. Thus if the width of each said portion of a chute 11 is such that it receives only one rice grain 13 at a time, each detector 20 will in operation view only one rice grain 13 at a time and the resolution of the machine, i.e. its ability to detect defective rice grains, will thus be much higher than if each detector 20 were to view more than one rice grain at a time.

The reflectivity of each of the backgrounds 22 may be varied independently of that of the other backgrounds 22. Such variation is desirable in order to be able to compensate both for any variations in the average colour of the rice passing through the various parts of the sorting machine and also for any variations in the effective colour of the backgrounds caused by dust and by ageing of the fluorescent tubes.That is to say, if the average colour of the rice across the whole machine were to slowly change (fast changes do not frequently in practice occur) it would be desirable to make a corresponding change in the effective colour of all the backgrounds 22, while if the average colour of the rice on one side of the machine altered somewhat with respect to that on the opposite side thereof, it would be desirable to make a corresponding change in the relative respective colours of the respective backgrounds.

For this purpose each of the backgrounds 22 may be mounted on a pivot 30 and may be pivotable by a respective motor 31. Such pivotal movements of a background 22, which may be effected by a background brightness controller 32 which is controlled by the respective processor 26, will alter the amount of light from the fluorescent tube 23a which it reflects towards the respective detectors 20 and will therefore alter its effective colour.

Alternatively, or additionally, for each of the backgrounds 22, one or more light sources 33 may be provided whose light output falls onto the respective background 22. The light source, or one of the light sources, 33 may be turned on.or off, or its position with respect to the respective background may be adjusted or its light output may be varied by a respective control 34 connected to the respective background brightness controller 32. Alternatively, or additionally, the light from the light source or sources 33 may fall onto the respective background 22 after passing through a respective rotatable optical wedge 35 whose angular disposition is controlled by the respective background brightness controller 32.

Yet another possibility is that each background 22 is provided with an assembly of light-emitting diodes (not shown) or with liquid crystal material (not shown), the respective background brightness controller 32 being arranged to vary the total light output of the said assembly of diodes or liquid crystal material.

Alternatively or additionality it is possible to adjust the relative positions and/or the relative light output of the fluorescent tubes 23, 23a.

Each group of four detectors 20 simultaneously views both the objects 13 and the unobstructed portion of the respective background 22 across which the objects 1 3 pass in free fall except when, as described below, the feed is stopped, when the detectors 20 will view only the respective background. When, therefore, the feed is present, the electrical outputs of these detectors 20 will be representative of the reflectivity of the objects 1 3 and respective background 22 in a predetermined part or parts of the spectrum, and when the feed is absent, the outputs of the detectors 20 will be representative of the reflectivity of only the respective background 22 in the said part of parts of the spectrum.

The four outputs 20a, 20b, 20c, 20dfrom the four adjacent detectors 20 associated with each respective chute 11 are passed to respective d.c.

coupled pre-amplifiers 36. Each pre-amplifier 36 has an output 37, the four outputs 37 being passed to the respective processor 26.

Each processor 26 comprises a digital/analog, analog/digital interface 42 (Figure 3) which receives the respective four outputs 37. Each processor 26 has in addition an averaging circuit comprising a low pass filter 41 which receives one of the respective four outputs 37. A d.c.

output 43 of the low pass filter 41 is also received by the digital/analog, analog/digital interface 42.

The outputs 37 may also be a.c. coupled into the analog/digital, digital/analog interface 42. Each processor 26 has, in addition to the interface 42, a micro-processor unit 44, a program memory 45, a random access memory 46, and an input-output device 47 interconnected by a bus system 50.

The input-output device 47 of each processor 26 has an output lead 51 extending to the respective vibrator 17, an output lead 52 extending to the respective background brightness controller 32, and an output lead 49 extending to the respective ejector 24.

The program memory 45 of each processor 26 is programmed so that signals are periodically sent to the respective input-output device 47 to cause the latter to produce an output on the output lead 51. When this occurs, the respective vibrator 17 is stopped for a predetermined period, e.g. 5 seconds, and is then started again.The voltage signal produced by each low pass filter 41 is a signal representing the average value of the output from the respective detector 20 and thus, immediately before the stoppage, of the reflectance of the light both by the unobstructed portions of the respective background 22 and by the objects 13 passing over the latter, whereas the voltage signal produced from the outputs 37 is representative of the instantaneous value of the output from the respective detectors 20 and thus, during the stoppage, of the reflectance of the light by the respective background 22 only. The two said voltage signals are compared at the interface 42 of each processor 26 and a respective difference voltage signal is produced thereby which is the difference between the two said voltage signals.If the colour of the respective background has been selected to correspond exactly to the average colour of the objects, then the difference voltage will be zero if there has been no relative change of colour between that of the background and that of the objects. If, on the other hand, the background has been deliberately mis-matched in colour with respect to the average colour of the objects, then the differencs voltage should have a predetermined value, and in this case the difference between the actual and predetermined values of the difference voltage can be used to effect the subsequent control.

Assuming, however, as would usually be the case, that the colour of the background is intended to correspond to the average of the objects, the difference signal is processed in the respective processor 26 to provide outputs to the respective background brightness controller 32 and, after a delay, to the respective vibrator 1 7.

The background brightness controller 32, on receiving such output, will if necessary cause the respective motor 31 to adjust the angular disposition of the respective background 22 and/or will rotate the respective wedge 35 and/or will adjust the light output of the respective light source or sources 33 by way of the respective control 34, in which case the amount of light reflected by the respective background 22 will be increased or reduced as necessary to maintain the correspondence between the colour of the background and that of the average of the objects.

The outputs 51 to each vibrator 17, which are used to effect a temporary stoppage of the latter and thus a temporary stoppage of the feed to the twenty respective chutes 11, is delayed for a period of time inversely related to the magnitude of the difference signal. Thus if the largest of the difference signals from the respective processors 26 is small, the vibrator 17 will be stopped at long intervals, whereas if the said largest difference signal is large, the vibrator 1 7 will be stopped at short intervals.

Reference will now be made to Figure 4 which is a graph illustrating the output from a detector 20 when the brightness of the respective background does not correspond to the average brightness of the objects being viewed, Vp is the average voltage produced by the detector 20 when the objects are present, i.e. during the feed, and VN is the voltage produced by the detector 20 when no objects are present, i.e. during a said stoppage. As will be seen, VP+VN- That is to say, when no objects 13 are viewed by the detector, the voltage VN produced by the latter will be due to the reflectivity of the background 22 only.When however an object 13 whose reflectivity is greater or less than that of the background 22 is viewed by the detector, pulses 53, 54 will be respectively produced The voltage Vp will depend both upon the average reflectivity of the objects 13 in the part or parts of the spectrum to which the detector 20 is responsive and upon the average amount of the objects which are within the field of view of the detector 20. Thus Figure 5 illustrates a field of view of the detector 20 which is constituted by a rectangular area 55 having a length li and a breadth 4. Objects I 3a, 1 3b, 1 3c are shown as passing through the field of view 55 and as having respectively areas A1, A2 and A3 within the field of view 55.

If, therefore, the brightness of the background 22 is intended to be the same as that of the average of the objects 13, then the brightness of the background 22 when no objects 13 are being fed therepast needs to be altered from its original level VN to a new level VN2 where VN2=VN+(VPVN)F, and the area of the field of view 55 F= the average area of the objects in the field of view = 12/A1+A2+A3 F is a "load" factor which is dependent mainly upon the average amount of the objects 13 in the field of view 55 for a given type of objects 1 3. The factor F will depend also on the throughout, but for a given constant throughout, the factor F may be treated as fixed and may be used in the processor 26 to set the brightness of the background 22 to the correct level for balanced conditions.

In programming the processor 26, however, it is desirable to select a factor F which is less than the actual factor F needed to effect the correct setting of the background. In this way, during an initial stoppage, the adjustment of the background which results from the production of the said difference signal is insufficient to fully restore the brightness of the background to correspond exactly to that of the average of the objects 13, but the background will be subsequently further adjusted during a subsequent stoppage or stoppages to the correct extent. If, on the other hand, the chosen factor F were to be too high, there would be a danger that the background would be constantly adjusted backwards and forwards about a mean position at each said stoppage.

Referring to Figure 6, let it be assumed that the background was originally at a setting represented by the point X such that, when the feed of the objects 13 therepast is stopped, the voltage produced by the detector 20 in the absence of the objects is VN and the said difference voltage is V. Assume now that, as explained above, an insufficient adjustment of the background was made as a result of underestimation of the factor F so that the background was adjusted to the setting represented by the point Y. Consequently assume that at the next stoppage it was found that the background was not fully adjusted and that the voltage produced by the detector 20 in the absence of the objects was VN1 and the said difference voltage was V1.Then from similar triangles it may be determined that the correct factor F at a setting Z representing the correct setting of the background would be V,,-V, F V and VAN1 VAN F V-V1 Thus since VN1Z NN V and V1 have all been previously measured, an accurate value of F may be determined. Hence to get to the correct setting Z from the setting Y, account is taken of the fact that VN2=VNi +(V1 x F)

Each processor 26 is thus programmed to make the above calculations which thus involve re-calculating the factor F at each stoppage.

During each stoppage, each processor 26 is programmed to examine the output of the respective d.c. coupled pre-amplifier 36 to determine whether the sensitivity of the detectors 20 has latered and to apply a correction factor in the processor 26 if necessary to maintain the effective sensitivity of the detectors 20 substantially constant.

During the passage of the objects 1 3 past the backgrounds 22 and the sorting of the objects, each processor 26 is programmed to measure the average rate of separation of the undesired objects and to send a signal on an output line 60 to a respective indicator 61.

In a further embodiment, which is particularly applicable to a multi-channel sorting machine, the information on the average rate of separation from each processor 26 is sent to a central processor. The latter is arranged to compute the mean value of the average rates of separation of each channel and compare each individual rate of separation with the mean value to determine the difference between the individual rate of a particular channel and the mean value. Indication means (not shown) are provided to indicate to a machine operator which channels are above or below the mean value.

Although the invention has been described with reference to the reflection of light by the objects 1 3 and by the backgrounds 22, the invention is also applicable to sorting machines which examine the transmission of light through the objects 13 or the emission of light from the latter, e.g. under fluorescence. Similarly, the light from the backgrounds 22 could be transmitted or emitted thereby.

Moreover, although the invention has been described with reference to monochromatic sorting, the invention is also applicable to bichromatic sorting in which the examination of the objects 1 3 is effected to determine whether the ratio of the light therefrom in certain parts of the spectrum is outside certain limits.

In the description above reference has been made to the adjustment of the reflectance transmission or emission of light being effected by an adjustment relating to the backgrounds, e.g.

by altering their reflectivity. However, it is possible instead (or additionally) to achieve the required adjustment by adjusting the incident radiation onto the objects. For example, each channel could have respective light sources (not shown) which directed light onto the respective objects and whose light was prevented from being directed onto objects in other channels, means being provided for varying the light output of said light sources.

Claims

1. A method of sorting comprising viewing objects to be sorted to determine whether any of them, at least in part, reflects, transmits or emits light in a predetermined part of parts of the spectrum to an undesired extent or in an undesired ratio, the objects being viewed while passing across a background which is also viewed and whose reflectance, transmission or emission of light in the said part of parts of the spectrum has a predetermined relationship with that of the average of the objects, effecting relative separation between any undesired objects which have been viewed and the remaining objects, periodically effecting an examination as to whether the said predetermined relationship exists and, if it does not, effecting an adjustment of the reflectance, transmission or emission of light by the background and/or adjusting the incident radiation onto the objects in the said part of parts of the spectrum so as to tend to restore the said predetermined relationship.

2. A method as claimed in claim 1 in which the passage of the objects past the background is periodically stopped temporarily, and each said examination occurs at least in part during the stoppage of the objects.

3. A method as claimed in claim 2 in which each examination comprises comparing the average reflectance, transmission or emission of light by the background and objects collectively shortly prior to the respective stoppage with the reflectance, transmission or emission of light by the background alone during the respective stoppage.

4. A method as claimed in any preceding claim in which if a result of a said examination a said adjustment appears necessary, an initial adjustment is first effected which is insufficient to fully restore the said predetermined relationship, a further said examination occurs at the next stoppage, and the results of the two examinations are used to caiculate the required final adjustment, whereby the said predetermined relationship is then fully restored.

5. A method as claimed in any preceding claim in which the reflectance of light by the background is adjusted by varying its angular disposition.

6. A method as claimed in any preceding claim in which the reflectance of light by the background or the adjustment of the incident radiation onto the objects is effected by varying the light output of a light source whose light is directed onto the background or objects respectively.

7. A sorting machine comprising viewing means for viewing objects to be sorted in a sorting zone to determine whether any of them, at least in part, reflects, transmits or emits light in a predetermined part of parts of the spectrum to an undesired extent or in an undesired ratio; a background which is in the field of view of the viewing means; feeding means for passing the objects through the sorting zone and past the background so that the objects may be viewed against the background; separating means for effecting relative separation between any undesired objects which have been so viewed and the remaining objects; examination means for periodically effecting an examination as to whether a predetermined relationship exists between the reflectance, transmission or emission of light in a predetermined part or parts of the spectrum by the background and the reflectance, transmission or emission of light in the said part or parts of the spectrum by the average of the objects; and adjustment means, controlled by the examination means, for effecting where necessary an adjustment of the reflectance, transmission or emission of light by the background and/or an adjustment of the incident radiation onto the objects so as to tend to maintain the said predetermined relationship substantially constant.

8. A sorting machine as claimed in claim 7 comprising means for periodically stopping the feeding means so as to stop the passage of the objects past the background, the examination means being arranged to effect each said examination at least in part during a said stoppage of the objects.

9. A sorting machine as claimed in claim 8 in which the examination means comprises means for comparing the average reflectance, transmission or emission of light by the background and objects collectively shortly prior to the said stoppage, with reflectance, transmission or emission of light by the background alone during the stoppage.

10. A sorting machine as claimed in claim 9 in which the viewing means are photo-electric viewing means, the examination means comprising an averaging circuit for producing a voltage signal representative of the average electrical output of the viewing means, the examination means also comprising electrical means arranged to receive said voltage signal and also to receive a further voltage signal which is representative of the instantaneous electrical output of the viewing means, said electrical means being arranged to produce a difference signal which is the difference between the two voltage signals and which controls the adjustment means.

11. A shorting machine as claimed in claim 10 in which the electrical means comprise a processor which controls the feeding means, the processor being programmed to stop the feeding means at intervals whose length is inversely related to the magnitude of the difference signal.

12. A sorting machine as claimed in claim 10 or 11 in which the electrical means controls the adjustment means so that, if a difference signal is produced at an examination, the resulting said adjustment is initially insufficient to fully restore the said predetermined relationship and a further difference signal is therefore produced at the next examination, the electrical means being arranged to determine from the magnitude of these two successive difference signals the necessary signal to be sent to the adjustment means to complete the said adjustment.

13. A sorting machine as claimed in any of claims 8-12 in which there are means, operative during each said stoppage, for measuring the output of the viewing means and applying, if necessary, a correction factor in said electrical means to maintain the sensitivity of said viewing means substantially constant.

14. A sorting machine as claimed in any of claims 8-13 in which there are means, operative during the passage of the objects past the background, for indicating the rate of separation of undesired objects.

15. A sorting machine as claimed in any of claims 7-1 4 in which the adjustment means comprises a motor for altering the angular disposition of the background.

16. A sorting machine as claimed in any of claims 7-1 5 in which there is at least one light source whose light is directed onto the background or onto the objects, the adjustment means comprising means for varying the light output of the said light source or sources.

17. A sorting machine as claimed in any of claims 7-16 in which the background is provided with light-emitting diodes or with liquid crystal material, the adjustment means being arranged to vary the total light output of the diodes or liquid crystal material.

1 8. A sorting machine substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

1 9. A method of sorting substantially as hereinbefore described with reference to the accompanying drawings.