The invention relates to a method and an apparatus for sorting objects using a moving conveyor belt and a vision system. The invention particularly provides a method and apparatus for sorting first and second mutually differing types of object which have similar physical characteristics but differing visual characteristics.

Sorting of objects, particularly solid objects, typically takes place on the basis of differences in

physical characteristics. When particle size for instance forms a point of difference, a sieve can be used to separate a first type of object from a second type. When magnetic properties differ, a magnet can be used.

A problem occurs when objects have similar physical characteristics, such as two types of wood material, for instance MDF and wood, or such as two types of stone

material, for instance broken stone and broken concrete. Visual characteristics will be critical in separating such objects. Manual labour will often be employed in practice when these objects have to be separated. An automatic separation, for instance on the basis of camera images, is however much less labour-intensive and will progress much more quickly.

Described in the prior art are apparatuses for

separating objects on the basis of camera images. Such apparatuses are known from US 5,305,894 (McGarvey, 1994). The apparatuses described here comprise a conveyor belt device with a conveyor belt which typically moves with sufficient speed to propel objects reaching the end of the conveyor belt a distance away from the belt and

substantially in line therewith. At the position of the end of the conveyor belt and over the whole width thereof a bank of nozzles is disposed above the ejection path of the objects propelled from the conveyor belt, which nozzles deflect objects downward in forced manner (i.e.

supplementing gravitational force) via directed blasting of a fluid, preferably air, whereby these objects fall close to the conveyor belt. A separation of the objects can in this way be obtained by targeted actuation of the nozzles, wherein a first type is deflected in forced manner by the nozzles and comes to lie close to the conveyor belt, and a second type comes to lie in unforced manner further away from the conveyor belt. Selection of objects which have a defect further takes place in said publication on the basis of colour using a graphics processor, and on the basis of this selection an ejection pattern is placed in a memory queue. This queue is advanced pulse by pulse in the memory on the basis of the pulses from an incremental shaft counter connected to the conveyor belt and, at the end of the queue, sent to a processor which actuates an ejector mechanism in order to eject the objects which have a defect. There is therefore a direct link between the advance of the queue in the memory and the incremental shaft counter.

A drawback of the apparatuses for separating objects via a prior art camera is that only simple graphic analyses can be made of the image because the computation time for the image processing has to be constant and predictable.

This is because this computation time determines the correct further progression of the separating process. Several filters can hereby be applied, the exact computation time of which can be predetermined. In US 5,305,894 use is made of colour filters.

It is an object of the invention to obtain a method for separating objects on a moving conveyor belt via a camera, wherein complex filters with varying computation times can be applied.

The method for sorting first and second mutually differing types of object using a moving conveyor belt comprises for this purpose the following successive steps of:

(a) placing said objects on said conveyor belt;

(b) driving said conveyor belt via a motor;

(c) measuring the displacement of said conveyor belt via a pulse generator by transmitting a pulse from the pulse generator to a controller each time said conveyor belt makes a displacement of Z mm, wherein Z is a real number between 0.1 and 50;

(d) counting said pulses with said controller using a

counter;

(e) forming an image of a predetermined zone of said

conveyor belt with said objects using a camera, optionally supported by a lighting unit;

(f) transmitting said image to an image processing unit, wherein said transmission of said image to said image processing unit takes place after a determined number N of pulses, wherein N is a natural number between 20 and 1000;

(g) analysing said objects on said image with said image processing unit by applying one or more of the

following: shape filters and structure filters (image analysis) ;

(h) selecting said objects of the first type with said

image processing unit on the basis of said image analysis ;

(i) determining position information (X and Y coordinates) of each of said selected objects of the first type with said image processing unit; (j) transmitting said position information (X and Y

coordinates) of each of said selected objects to said controller with said image processing unit;

(k) converting with said controller said position

information (X and Y coordinates) obtained from step (j) to position information (X and Y coordinates) of each of said selected objects on the conveyor belt at the moment said position information (X and Y

coordinates) from step (j) has reached the controller;

(1) selecting and actuating with said controller, on the basis of position information (X and Y coordinates) of each of said selected objects on the conveyor belt from step (k), one or more nozzles of an array of nozzles placed at the end and transversely over the width of said conveyor belt at the moment the selected object comes within range of said one or more nozzles so that the direction of movement of the selected object can be changed by escape of a fluid from the one or more nozzles.

In an embodiment said objects are placed on the

conveyor belt in step (a) with an intermediate distance between adjacent said objects. The image processing unit can however be designed such that it is able to separate

overlapping objects in the image from each other and

determine for each object whether it is of the first type or of the second type. The speed of the conveyor belt is preferably set such that a speed is obtained whereby the objects placed on the conveyor belt no longer overlap each other .

In an embodiment the pulse in step (c) is an electrical pulse. This pulse can however also be an optical pulse which is transmitted via for instance a glass fibre cable. In an embodiment the position information in step (i) is a set of X and Y coordinates for each of said selected objects related to the formed image. In another embodiment the position information in step (i) is a set of X and Y coordinates for each of said selected objects related to the conveyor belt. In this latter case the X-coordinates are related to the recorded image, corrected for the fixed distance - in pulses - between the beginning of the recorded image and the end of the conveyor belt. "Beginning of the recorded image" is understood to mean the position of the first scan line of the camera or the position of that side of the recorded image situated closest to the end of the conveyor belt. "End of the conveyor belt" is understood to mean that part of the conveyor belt where the belt returns to its starting position and/or where the objects are propelled from the conveyor belt and/or where the array of nozzles is located. The skilled person is free to choose this value as a function of the best operation of the sorting apparatus, and this value can thus be chosen with some margin.

The conversion in step (k) of said position information (X and Y coordinates) with the controller particularly comprises correcting by said controller the X-coordinate for the processing time of said imaging system and said

transmission to the controller, by reducing the X-coordinate by a number of pulses T which have been generated between the beginning of the recorded image and the moment the X- coordinate is received by said controller.

In an embodiment, said moment of actuation of the selected one or more nozzles by said controller in step (1) is the moment at which the number of pulses generated in said counter as of the X-coordinate being received by said controller in step (k) is equal to the X-coordinate of step (k); and wherein the selected one or more nozzles are those nozzles whose position at the end of said conveyor belt corresponds to the Y-coordinate of the selected object.

According to the invention the synchronization between the passage of an object close to the array of nozzles and the actuation of the nozzles is position-controlled and not time-controlled, and does not depend on the processing speed of the respective processors or even on the position of an object in the image. This is because the ejection of objects by one or more nozzles takes place on the basis of a counter reading which measures a position of the conveyor belt, wherein a predetermined number of pulses is first added in the method in order to correct for the relative distance between conveyor belt and array of nozzles. This

predetermined number of pulses defines a distance in that the quantity of pulses is directly proportional to the displacement of the conveyor belt. Determining of the moment of ejection is hereby separated from the computation time of the image processing. Earlier or later arrival of the results of the image processing will thus have no effect on the X-coordinate which determines the moment of ejection. The actuation of the one or more nozzles is even independent of the sequence of the position of a selected object in the image, this in contrast to the prior art, where the sequence of a selected object in a queue corresponds to the sequence of actuation of the one or more nozzles.

An additional advantage of the invention is that the speed of the conveyor belt, if it is located high enough for objects to be propelled therefrom, can also vary

(acceleration or deceleration) without variation in the synchronization with the nozzles. This while the computation speed is independent of the speed of the conveyor belt. It is even possible to correct for slippage of the conveyor belt, for instance by measuring the passage time of a fixed point on the conveyor belt, for instance via a simple visual detector, and incorporating a change in the passage time, which in the case of slippage will always be increased relative to the normal passage time without slippage, as correction in the counter, whereby the counter can thus be reduced by one or more pulses. It is likewise possible to set a threshold value which indicates when such a slippage correction has to be applied.

In an embodiment said first type of object is Medium- Density Fibreboard (MDF) and said second type of object is wood. Medium-Density Fibreboard (MDF) is pressed board with a medium-hard density, comprising dried wood fibres mutually connected by means of resin. Wood is understood here to mean wood from a natural source, for instance pinewood, oak, cedar, fir, ash and so on. The visual difference between MDF and wood is more complex than merely a colour difference. Surface texture and/or shape have to be analysed via

respective structure filters and/or shape filters in order to be able to make a good selection. The shape filter is for instance based on one or more of anisometry, strucfactor and circularity. The structure filter is based for instance on a region growing algorithm.

In an embodiment said image is built up by scanning a line at each pulse via a line scan camera and joining the scanned lines together to form an image which is

subsequently transmitted. Line scan cameras are known from the prior art and are known for their high operating speed. The recording of an image also becomes position-controlled by coupling this camera to the pulse generator. Changing the speed of the conveyor belt will hereby have no effect on the image. Because each image line represents a pulse, it will further be easy to express the distance of an object on the image in number of pulses. A further advantage is that, because each line is recorded at one and the same position on the conveyor belt, the image will be very homogenous, whereby less account need be taken of ambient factors in the image processing.

In an embodiment a lighting unit is disposed for the purpose of illuminating said predetermined zone at an angle which differs at least 25° from the viewing angle of the camera, so that the surface relief of objects is more readily visible. Surface texture is an important element particularly for the purpose of distinguishing MDF and wood. Placing the light source at an angle relative to the camera allows the camera to observe shadows where there are height differences in the surface. These shadows can assist the image processing in obtaining a better defined view of the surface texture.

In an embodiment the conveyor belt has a length between said predetermined zone and said end of the conveyor belt which amounts to at least three times Z times N mm. Z is the number of mm per pulse, while N represents the number of pulses per image. Z times N is thus the distance visible in one camera image.

The invention further relates to a sorting apparatus for sorting first and second types of object using a moving conveyor belt, comprising:

a conveyor belt;

a feed unit provided for placing objects spaced apart on the conveyor belt;

- an array of nozzles placed at the end and transversely over the width of the conveyor belt;

a motor for driving the conveyor belt; a pulse generator coupled to the conveyor belt in order to measure the displacement of the conveyor belt and to transmit a pulse to a controller every Z mm, wherein Z is a real number between 0.1 and 50;

a camera placed above a predetermined zone of the conveyor belt and provided to form an image of said zone, and wherein the camera is operatively coupled to the pulse generator in order to transmit an image to an image processing unit after a determined number N of pulses, wherein N is a natural number between 20 and 1000;

an image processing unit provided for receiving and analysing images from the camera in order to select objects of the first type in the images and determine position information (X and Y coordinates) of each of the selected objects of the first type;

a controller provided for receiving pulses from a pulse generator, wherein the controller comprises a counter for the purpose of counting the pulses, provided for the purpose of conversion by said controller of said position information (X and Y coordinates) obtained from step (j) to position information (X and Y

coordinates) of each of said selected objects on the conveyor belt at the moment that said position

information (X and Y coordinates) from step (j) has reached the controller; and provided for the purpose of selecting and actuating said array of nozzles on the basis of position information received from the image processing unit.

The invention will now be further described on the basis of an exemplary embodiment shown in the drawing.

In the drawing: Figure la is a top view of a sorting apparatus

according to the invention.

Figure la is a side view of a sorting apparatus

according to the invention.

Figure 2 is an overview of the computation of the X and

Y coordinates of an object on the conveyor belt.

In the drawing the same or similar elements are given the same reference numeral .

List of reference numerals

Figure 1 shows the sorting apparatus according to the invention for separating two types of object 4, in

particular MDF and wood. A conveyor belt device, comprising a conveyor belt 1 running over at least a starting roller 2a and an end roller 2b, is driven by a drive motor 3 which is connected here to the starting roller in order to minimize slippage. Present on the conveyor belt are objects of a first type and a second type to be separated. The end roller is preferably as small as possible in order to improve the ejection of the objects. This is because it has been

established that the smallest possible end roller markedly increases the ejection accuracy. Conveyor belt 1 is further connected via the end roller to a pulse generator 10 which measures the displacement of conveyor belt 1. Pulse

generator 11 is preferably connected to the roller with the least slippage, in this exemplary embodiment the roller which is not driven by the motor, i.e. the end roller. - Objects are laid on the conveyor belt using a feed device 8. This can be a manual feed, but is preferably a mechanical feed device; connected for instance to a container with objects, or a feed device connected for instance to a wood chipper. A vibrating chute is preferably used in order to ensure that the objects are separated and come to lie adjacently of each other and not on top of each other on the conveyor belt. This latter is also realized by maintaining a sufficiently high speed of the conveyor belt, for instance 3 m/sec. A camera 6 is placed above conveyor belt 1, over a predetermined zone thereof. This camera 6 is provided for the purpose of forming an image of said zone. In the shown embodiment the camera is a line scan camera. The placing of the camera can be determined experimentally. It is important that the camera can form an image of the full width of the conveyor belt. The camera hangs for instance about 1 metre above the conveyor belt and makes a recording of a narrow strip of the conveyor belt, the scan line 7. The line scan camera is connected to an image processing unit 13. Placed adjacently of the camera is a lighting unit 9. This consists in this exemplary embodiment of an LED bar extending over the whole width of the conveyor belt. The lighting ensures that the shapes and colours to be distinguished are

accentuated. Another type of lighting can of course also be used, for instance halogen lamps, depending on the application and the filters used to analyse the objects.

The image processing unit transmits the position information of each object to a controller 14, which in turn actuates the array of nozzles but also receives pulse information from pulse generator 10. At the end of the conveyor belt and over the full width thereof the array of nozzles 5 is arranged above the ejection path of the objects propelled from the conveyor belt, wherein each nozzle can be separately activated. Through activation of a nozzle a fluid, here air, is blasted with force onto the object which is propelled from the conveyor belt ("ejection") . The objects which are ejected fall close to end roller 2b of the conveyor belt into a first collecting device 11, for

instance a container or conveyor belt; the objects which are not ejected fall further from the conveyor belt into a second collecting device 12, for instance a container or conveyor belt. Operation

When the conveyor belt is started up with the objects situated for instance at the beginning of the conveyor belt, the pulse generator begins to generate pulses. The first pulse is thus generated at moment t = 0. At a determined moment t = ti the belt passes under the line camera and the forming of an image begins by the line camera, which

generates a first image line at pulse Po, for instance after 10,000 pulses, so at pulse 10,001. An image line is

preferably generated at each pulse. The image line length is then preferably equal to a displacement of Z mm so as to be able to form a complete image without gaps. A typical image line length is for instance 1 mm, which then corresponds - at a conveyor belt speed of 3 m/sec - to a pulse frequency and to the recording speed of a single image line of 1/3000 sec. This lies well within the capabilities of a typical image line camera.

After a fixed number of pulses which can be set, for instance 100, the complete image is formed and transmitted to the image processing device. The size of the image thus amounts to 100 image lines. This transmission may require a determined amount of time. Once the image has been

transmitted, the conveyor belt in the example has already advanced 40 pulses and the selected object is located 597 pulses from the nozzles. The image processing unit then determines the X and Y coordinates of each object on the basis of their position in the image and - via appropriate filters - it is determined for each object whether or not it has to be selected and ejected by one or more nozzles. The X and Y coordinates of each object to be ejected are stored in a table in the image processing system. Shown in the example is an object with the X-coordinate 37. This object is located during the forming of the image at a distance of 637 pulses from the nozzles, i.e. at a distance (X-coordinate) which is determined by an X-coordinate of the image plus a fixed number of pulses, here 600, this being the offset value which indicates the distance in pulses between the first image line and the array of nozzles.

When the image processing system has generated the table with coordinates, this table is transmitted to the controller, in this case a PLC (Programmable Logical

Controller), which is commercially available. At the moment that the table is ready however, the conveyor belt has again advanced a number of pulses further, i.e. 20 pulses in the example, this corresponding to a processing time of the image processing unit of 20/3000 sec. The table is then transmitted to the controller, which may also take a determined amount of time, in the example 20 pulses. The calculated X-position (637) is then corrected for the processing times (20 + 20 pulses) for respectively the image processing unit and the transmission to the controller, which can compare the position of the first image line in the recorded image, in the example at pulse P ₀ , to the actual counter reading in the controller Pi at which the X-position is received by said controller, wherein the overall

correction is equal to Pi minus Po, in the example 80 pulses. The object is now located at X-coordinate 557, this being the result of 637 - 80.

The controller then monitors the X-coordinate of each object in the table until the number of pulses generated in the counter from the moment the controller receives the X- position is equal to the corrected X-position of the object in question.

When this latter is the case, a nozzle is activated in order to eject the selected object. The duration of the activation, i.e. the period of time for which a fluid escapes, can be determined, for instance as being directly proportional to the size of the selected object and

expressed as number of pulses.

After a determined number of pulses, the counter can optionally be reset to zero.

A particular case is an object present in two adjacent images. This object also has to be recognized with the same filters and if these filters select, among other criteria, according to shape, the shape recognition would not be correct (e.g. recognition of round shapes between angular shapes, wherein a round shape over two images would be recognized in a single image as an angular shape) . Known in the prior art is a method for dealing with such a case by forming two images overlapping each other in a zone, superimposing the images and calculating the position of each object from the superimposed images.

In the present invention another method is followed, wherein the image processing analyses not only the most recently transmitted image but also the most recent image but one so as to thus also analyse and select objects of the first type visible at the boundary between two images. If an object is located at the underside of a first image, the coordinates are not calculated. Instead, in a second

subsequent and adjacent image, where an object is also located on the upper side of the image, this object is selected from the image and combined to form a single object having a shape corresponding to the actual shape of the object on the conveyor belt. The object is then processed by the image processing unit and the position information is determined .

Example

A sorting apparatus according to the invention for the purpose of separating wood and MDF was constructed with the following components.

Conveyor belt and array of nozzles - of own

construction - the bar consisting of 30 nozzles over a length of 84 cm.

- Camera: E2V-camera: AVIIVA TM UC2 GE Color Line Scan

Camera.

- Lens: L 50ir 1.9/50 (F-mount)

- LED-bar: 6 x LED bar of TPL of the type: Bar 6 LEDs XBAR XBAR6-xxx

- PC system for image processing: Siemens box PC:

6ES7647-6CG50-1AA0

- PLC: Siemens, 6ES7318-3EL01-0AB0 : PLC 319-3 PN/DP Fast counter card for counting pulses: Simatic S7-300, Counter Module FM350-2 6ES7350-2AH01-0AE0

- Encoder from Sick: art: 1036724 DFS60B-S4PM10000 Basic programmable encoder, face mount.

Settings: The pulse generator generates a signal every mm; the image consists of 300 image lines (N=300) and is thus 300 mm in size. There is only 100 ms to perform all processes per image. The filters are thus chosen such that they operate within this narrow time frame.

100 kg of mixed waste (pieces of MDF/wood = 50/50% by weight, dimensions between 3 and 12 cm per piece) was then separated into two fractions using the sorting apparatus according to the invention, at a speed of the conveyor belt of 2.2 m/sec. The waste was placed manually and at random on the belt in a continuous flow. The MDF was selected and ejected from the flow and thus came to ground close to the conveyor belt (zone 11 in Figure 1). Following separation, the two separated fractions were analysed. The MDF fraction was substantially pure, and amounted to 46.2% by weight of the total amount of waste. The wood fraction amounted to 53.8% by weight of the total amount of waste, but also comprised 7.6% by weight MDF.