AU670347B2 - Sorting translucent objects - Google Patents

Description

P/00/011 AUSTRALIA Regulation 3.2

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

NOTICE

1. The specification should describe the Invention In full and the best method of performing it known to the applicant.

2. The specification should be typed on as many sheets of good quality A4 International size paper as are necessary and inserted inside this form.

3. The claims defining the invention must start on a new page. If there is insufficient space on this form for the claims, use separate sheets of paper.

The words "The claims defining the invention are as follows' should appear before claim 1. After the claims the date and the name of the applicant should appear in block letters.

S 4. This form must be accompanied by a true and exact copy of the description, claims and drawings (if any) and an additional copy of the claims.

(see Pamphlets explaining formal requirements of specifications and drawings) TO BE COMPLETED BY APPLICANT iame of Applicant: A.PPLIED SORTING TECHNOLOGIES PTY LTD ctual nventor(s):. ALBERT PETER HAWKINS A ctu a l Inv e nto r(s) Address for Service: IM..CLO.S.E. ULLE.E I.C .1 Invention Title: Invention Title: F. OBJECTS Details of Associated Provisional Applications: Nos: The following statement Is a full description of this invention, including the best method of performing it known to me:- SORTING TRANSLUCENT OBJECTS High speed automatic inspection and sorting machines have been in common use in a wide range of industries to upgrade a particulate stream fed to such machines, where generally each individual particle is inspected for presence (or absence) of an important leature.

Mechanical and/or pneumatic means are then used to remove exceptional particles from the normal flow stream, these exceptional particles differing in features from the normal majority of particles.

In the case of translucent objects, it is known that small inclusions of foreign or undesirable opaque matter may be present inside such objects. Depending on the degree of scatter and absorption, it may or may not be possible to see or otherwise optically identify such contaminants for purposes of removing contaminated particles and thereby leaving a purer product stream. It may also be the case that other potential sensing techniques (such as x-ray scanning) may also be unable to be used to reliably distinguish such contaminated from uncontaminated objects.

The invention described below allows such separations to be made by use of optical arrangements not previously used in such sorting machines. This is achieved by suppressing light scatter from the internal volume of the translucent object, whilst at the same time enhancing the image-forming process to distinguish the small opaque imbedded inclusions.

The scatter is suppressed by confocal beam control techniques whereby an incoming converging beam is fccussed at the same point as the objective lens of the receiving photosensitive element and light stop, as well as by correct choice of laser wavelength.

A practical example will help clarify and explain the invention. This example relates to the requirement to separate, on a production processing line, cherries some of which have not been properly pitted (ie have not had all or part of the pip removed from the fruit by a previous automatic operation). The invention is not limited to this application, and is expected to be widely applicable in the sorting field.

A number of optical and other techniques have been used in the past in an attempt to provide the processed food manufacturer with cherries totally free of such undesirable inclusions. These range from visual inspection and picking by teams of human sorters, to attempts to automate by use of automatic image processing on backlit or shadowed fruit.

However nothing has been demonstrated until now with the required accuracy for the confectionary industry. Other non optical methods have been tried and largely rejected (eg scanning ultrasound, CT scanning, nuclear magnetic resonance) as being either technically unsuccessful, or have required disproportionate capital expenditure, with the result that no S. current method of separation for this task exists.

A general embodiment of this invention as it relates to the cherry-pit problem is discussed below.

The invention described here relies on a beam (see fig. 1) of coherent light from a suitable laser appropriately expanded by beam expander to form a large cross section parallel beam refocussed by objective lens o a focal plane in approximately the centre of the translucent object In the transmitted light version of the setup, a second similar objective lens (11) focussed on the same focal plane refocussed an image on a 2 dimensional photodetector such as a CCD array In order to inspect the whole object, the incoming focussed beam requires to be scanned in two dimensions by eg rocking mirror (shown scanning in one of the two directions only flor convenience of drawing). In this arrangement, the light detector array (10) is scanned in positional synchronism with the deflected laser beam, such that only the light originating at the focal position in the object is read by the sensor. Light scattered in the volume of the object would be smeared across the whole sensor and would be ignored (except for the very small amount that originates from the small focal volume at any instant in time).

Consequently if a fragment of opaque contaminant was present at or near the focal point (plane) it would produce a very low response at the corresponding picture element on tihe sensor thus increasing the contrast between object and scatter. Such an arrangement may be likened in general terms to a confocal optical system, in biological microscopy terms.

It is apparent that the choice of objective lens ii) is important in that we want to make the depth of focus in the object small to maximise sharpness and minimise influence of scatter, but at the same time we want in a practical sorting inspection station to be able to cover desirably the whole volume of the object in a single complete x,y scan. Depending on the size/possible location of the opaque contamination inside the object, it might be necessary to use several x,y scans at different focal planes within the object to give complete volume coverage. This could be done by either moving the object backwards and forwards along the optical axis, or moving the lenses (6,11) together whilst keeping the object substantially stationary.

Alternatively, the object could be temporarily placed on a rotating table with the focal plane positioned across the centre of rotation, so that every volume element in the object is covered A small number of planes would then be x,y scanned as the object is rotated by 360 degrees.

In any of the 3 approaches to full volume coverage (ie, weaker focus, in/out translation or object rotation) 1 or more x,y image fields would be produced which would be subject to rapid computer image processing to extract the opaque contamination feature (if present in a given object). The processor would then give appropriate directions to mechanical separation means, well known in the sorting field.

It may be preferable, depending on the optical nature of object and contaminant, to use a reflection arrangement as opposed to the transmission one described above. This would be appropriate if the contaminant had a high inherent diffuse reflectivity, and has the additional advantage that the 180 degree scatter component is considerably lower than the forward scatter component. However the specular reflection from the front surface of the object would need to be avoided by the sensor. This would be done by either immersion in an index-matching liquid, or by use of crossed polarisation. In Fig 1, a reflection station is shown consisting of beam splitting mirror objective lens (12) and 2-d optical imaging sensor

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In general, differences can be exploited to enhance the imbedded feature contrast by such careful choice of laser wavelength. In the specific case of glaced cherries treated with anthrocyanin dye, the optimum wavelength of 575nm yields an almost 8-fold increase in contrast over the wavelength of maximum transmission (650-700nm). This is not an intuitive outcome. The overall transmission, and hence signal strength, at 568nm is appreciatively lower that at the 650-700nm peak, and hence in order to ensure satisfactory signal levels, a laser with high power is required. The krypton argon laser emitting at 568nm in the case of glace cherry sorting is easily able to provide such power. The graphs shown in Fig. 2 illustrate the gains in contrast to be made by choosing such a wavelength which gives a high ratio of transmission to scatter, even though this is a long way from the wavelength region of highest transparency.

Further tests have established that in the case of detection of cherry pits, it is not necessary to impose a confocal regime in 2 dimensions. It was found that a slit aperture at right angles to direction of movement of a sample through transparent sucrose-containing transport tube would provide sufficient signal to scatter backgi md signal to detect fragments of pits down to below 2mm in cherries above 25mm diameter. This could also be done without shifting the focal plane of the initial focussing lens. Such a practical arrangement is shown in figure 3 below.

Referring to figure 3, a linearly polarised beam from a krypton argon laser at 568nm (14) is reflected via a plane mirror (15) and expanded through a beam expander A lens (17) of around 90mm focal length focuses the beam via a rotating polygon (18) onto the centre of transport cell through which cherrie c (20) are forced under pressure in a sucrose solution.

A photomultiplier detector (25) receives light from the sample via polariser (21) which removes light not impinging on the cherry and hence reduces the chance of overloading the photomultiplier with light when the scanned beam does not strike the cherry. Light from the cherry (20) is gathered by objective lens (22) of typical focal length 65mnm, with slit aperture (23) defining the field of view of the photomultiplier. A field lens (24) is used between the objective lens and the photomultiplier to give flexibility of system magnification. In this arrangement, the only scattered light that the detector receives is that scattered in a very narrow vertical plane through the cherry, corresponding to the field of view of the slit aperture held in place by the objective lens.

The laser beam focused on the cherry target is scanned to provide vertical coverage, and the horizontal coverage is provided by the movement of the cherry through the transport tube by the pumped sucrose solution. In a full sorting system, a means of diverting cherries f, by the detection system to have pit inclusions would be diverted into a separate stream. This diversion is not shown as it can be accomplished with existing techniques well known in the food industry.

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