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Getting the shape right

02/16/2012

In recent years, optical sorters have revolutionized the sorting of bulk commodities. The latest models from Buhler Sortex function not only as colour sorters, but if required, act also as shape sorters or as colour and shape sorters.

The first optical sorting machines sorted bulk commodities on the basis of colour patterns. As a consequence, the first machines were called “colour sorters.” Colour sorting covers the majority of all sorting applications to this day. Now “colour sorting” refers to sorting by monochromatic cameras (greyscale) and colour cameras as well as the use of light in the visible and infrared wavelengths.

New applications
In the course of the development of optical sorters, fields of application gradually emerged in which the capability to shape-sort products is essential. The term “shape sorting” refers to the detection of product defects either on the basis of shape alone or the combination of shape and colour.
One of the main drivers for the introduction of shape sorting technology was long the so-called “killer application” of detecting stalks on fine green beans. Once shape sorting had been identified as the optimal solution for sorting green beans, the principle was adapted to other applications in the fruit and vegetable sectors. Finally, shape sorting also became an important function in the optical sorting of dry commodities such as coffee, nuts, and pulses.

Object separation
The transfer of shape sorting to dry commodities brought new challenges, which required further enhancements to the technology. For example, dry products tend to be sorted at comparatively greater throughputs per width of machine, resulting in more clumping of the product stream. One of the main improvements in shape recognition was the addition of an image processing technique known as “object separation.” It enables sorting by shape of individual items even when some of these touch other product pieces.

Sorting by shape and color
Today, shape sorting is one of Buhler Sortex’s core technologies and is available on the company’s entire range of bi-chromatic optical sorters.
Shape sorting is an important add-on to colour sorting. In most applications, the optical sorter sorts simultaneously by both colour and shape. During shape sorting, each object is classified as either good or defective based upon its silhouette. This silhouette is the two-dimensional shape of the object. For example, an optical sorter “sees” a round object such as a pea as a circle, not a sphere.

Two points to check
There are two points to check in preparation for shape sorting on an optical sorter. The first is the feed quality of the product stream. The flow of the product past the cameras of the sorter must be as uniform as possible, ideally with each product item separated from the rest. In practice, however, some product will always be touching other product. If the product items are approximately round in shape, the technique of object separation can be used to separate touching product. The second point to check is that the silhouette of the product is a true image. Misleading results can occur if the glass of the machine is not kept sufficiently clean or if there is an excessive amount of water in the product flow.

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