Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1992-05-18
1993-12-28
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250572, G01N 2188
Patent
active
052742440
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and to apparatus for continuously indicating the presence of bark on such material as logs and chips, and/or for determining the extent to which such material has been barked.
In the manufacture of wood-based paper pulp, it is extremely important, both with respect to product quality and production economy, to ensure that bark has been removed from the wood with sufficient thoroughness. Depending on the final product desired, wood which is delivered to the refinery or to the digester may be allowed to have a maximum bark percentage of 0.3-2%. The separation of bark from such material should not, however, be undertaken too strenuously, since this will result in the removal of wood together with the bark. This loss of good wood can have a very noticeable effect on the production economy of the mill concerned. Thus, in the manufacture of paper pulp, it is extremely important to debark the wood or the chips to an optimum degree. This optimum barking degree also varies from product to product and between the various manufacturing processes applied.
In order to achieve this optimum barking degree, it is not only necessary to adapt the barking process with respect to the product and the process concerned, but also to the nature of the wood starting material. Mutually different treatment times and treatment intensities are required when debarking different types wood, e.g. wood which has been felled or logged at different times of the year, and also with respect to the length of time the wood has been stored, the dryness of the wood, and whether or not the wood is frozen or thawed. The wood used in a pulp mill is seldom of uniform quality, due to availability and variations throughout the year, these variations often being unknown, and, consequently, it is often impossible to achieve an optimum degree of debarking. It is therefore desirable to be able to determine the barking result continuously and in a rational manner, so that corrections can be made in the bark separating process.
One significant drawback in this respect is that hitherto there has been no objective method for determining the degree of barking of wood or chips rapidly. The only quick method used hitherto involves inspecting the wood or chips visually, with subjective assessment of the extent to which the wood or chips has been barked. This method, however, is not reliable and continuous inspection of the wood material is also very expensive and extremely tiring for the persons involved. The objective methods applied hitherto have been manual. One method is to separate bark from the wood or the chips and then dry and weigh the two constituents individually, such as to enable a measurement of the degree of barking to be obtained. Alternatively, a measurement is taken of the total area of bark remaining on a log and this measurement compared with the total mantle area of the log. Since these objective methods are highly work intensive and expensive, they are only applied on singular occasions, for instance when testing new installations or plants in order to ensure that guarantees are fulfilled, or in the case of special process studies.
SE-400 381 teaches a method and apparatus for the optical detection of faults in sawn or planed timber.
The apparatus includes, for instance, a light source and a light detector, each of which is located on a respective side of an opaque screen having a thickness of about 2 mm. The light detector sends signals to a comparator which produces an output signal when light that falls on the detector produces an electric signal which exceeds a threshhold value. A fault, for instance in the form of twigs, blue-stained wood, and certain types of decay, is indicated in dependence on the intensity of the light reflected. This apparatus can only be used on sawn or planed timber, where the surface is so smooth and regular as to enable the opaque screen to be connected to the surface of the timber, and where product requirements include relatively well-defined requirements on the optica
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patent: 4245913 (1981-01-01), Sarlos
patent: 4266675 (1981-05-01), Barwise et al.
patent: 4482250 (1984-11-01), Hirvonen et al.
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Johansson Lennart
Ringstrom Leif
Nelms David C.
Shami K.
STFI
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