CA 02287529 1999-10-25
WO 98/53313 PCT/F198/00420
1
METHOD AND APPARATUS FOR DETERMINING THE PORTION OF WOOD MATERIAL PRESENT IN A
STREAM
OF BARK
The present invention relates to a method and an apparatus for defining the
portion of wood material present in a bark flow coming out from debarking
and for controlling a barking process on the basis of said data for reducing
wood losses in the debarking process.
In debarking, usually a barking process carried out by means of a debarking
drum, the object is to remove bark from the surface of trees so as to achieve
a desired degree of debarking. At the same time, however, there occurs
grinding off and crushing of wood material itself, said wood material repre-
senting wood loss as it becomes part of a bark flow. Naturally, it is desir-
able the wood loss is kept at a minimum, especially since wood is a major
cost factor in the production of pulp and paper. As a rule, the bark flow may
contain wood up to about 10...40 %, which equals wood losses of 2...5 % of
the total quantity of wood material used.
In prior art, the portion or share of wood in a bark flow is measured by
picking up a sample from the bark flow. The sample is handled manually to
separate wood and bark material from each other, followed by drying and
then measuring the relative amount of wood. Drying is necessary in order to
compare dry weights. According to SCAN-standard, the drying takes 16
hours.
The above type measurements have been used mostly for statistics, yet such
measurements have had little significance in terms of process control as the
situation could have changed a lot during the time claimed by the drying.
An object of the invention is to provide a method and an apparatus for
measuring wood loss in a debarking process essentially in real time and,
thus, to provide a possibility of controlling the debarking process for
reducing
wood losses.
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According to the invention, this object is achieved and a method of the
invention is characterized in that a bark flow coming out from debarking is
measured optically for its whiteness and the measuring result is used as a
basis for determining programmatically by means of a data processing unit
the amount of wood material in the bark flow, and that the debarking process
is controlled on the basis of the amount of wood material determined from
the bark flow.
It should be stressed that, in this application, the term whiteness refers not
only to various grey levels but also to colour separation.
The apparatus of the invention is characterized in that a bark flow coming
out from debarking is adapted to be imaged by means of a camera, said
image taken by the camera being adapted to be processed with an image
processing unit which uses different whitenesses of the picture elements or
pixels of the image as a basis for determining programmatically the portion
of wood material in the bark flow, and that the image processing unit is
adapted to produce an output signal for controlling the debarking process.
The real-time measuring of a bark flow in accordance with the invention for
determining the amount of wood material contained in the bark flow offers in
an average wood room a possibility of saving 1...2 % of the total amount of
wood, which represents 5,000...40,000 solid cubic meters of wood annually,
depending on the size of a wood room.
In addition, the invention relates to a method and an apparatus for determin-
ing the portion of wood material present in a bark flow being delivered to a
combustion process and for controlling the combustion process on the basis
of said data for optimizing the combustion process.
Especially in power plants of sawmills, pulp and paper mills, which burn a
mixture of bark and wood material, a problem is the fluctuation of a heat
value of the mixture to be burned. The most important factor effecting to the
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heat value is the moisture of a mixture to be burned, but also the fluctuation
of the
relative portions of e.g. bark and wood material in a mixture to be bumed has
an
effect on the heat value.
In the method of the inventaon, the effect of fluctuation between the relative
portions of bark and wood materiai upon the heat value is resolved in such a
manner that from a bark flow is measured optically its whiteness and the
measuring result is used as a basis for determining programmatically by means
of
a data processing unit the amount of wood material in the bark flow, and that
the
amount of wood material determined from the bark flow is used as a basis for
calculating a heat value for the material present in the bark flow and for
controlling
the burning process as necessitated by said value.
The apparatus of the invention is characterized in that a bark flow is adapted
to be
imaged by means of a camera, said image taken by the camera being adapted to
be processed with an image processing unit which uses different whitenesses of
the picture elements of the image as a basis for determining programmatically
the
portion of wood materiai in the bark flow, and that the image processing unit
is
adapted to produce an output signal for controlling the burning process.
According to a first aspect of the invention, there is provided a method for
defining
the amount of wood material present in a bark flow coming out from a debarking
process and for controiling the debarking process on the basis of said amount
of
wood material for reducing wood losses in the debarking process, in which
method
the bark flow is measured optically for its whiteness and a measuring result
is
used as a basis for determining programmatically by means of a data processing
unit the amount of wood material in the bark flow, and the debarking process
being controlled on the basis of the amount of wood material determined from
the
bark flow, wherein an image taken of the bark flow is analyzed for whitenesses
and the method for defining the amount of wood material comprises the steps
of:
dividing the image into picture elements;
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defining the whiteness of each picture element of the image in terms of at
least a three=way split numerical scale, whose end points are m and n, one end
point being applied to represent bark and the other to represent wood;
setting predetermined threshold values a and b wherein a<b, m<a<n, and
b<n;
setting the value of a picture element to m, when the picture element has a
whiteness within the range from m to a;
setting the value of a picture element to n, when the picture element has a
whiteness within the range of from b to n;
maintaining the value of a picture element when the picture element has a
whiteness within the range from a to b;
processing the image in image sections by means of a mean-value filter,
each said image section oonsisting of a pnsdetermined number of picture
elements, the mean-value filter defining a whiteness mean value for each
picture
element of a particular image section and the obtained mean values are used as
a
basis for classifying all picture elements of the particular image section
with the
same value m or n or with an unchanged value, if within the range from a to b;
repeating the foregoing steps until all picture elements are classified or
until
a predetermined number of iterations is achieved;
calculating the ratio of a number of picture elements representing wood to
the total number of picture elements in an image for expressing the amount of
wood present in the bark flow.
According to a second aspect of the invention, there is provided a method
for defining the amount of wood material in a bark flow being infed to a
combustion process and for controlling and optimizing the combustion process
on
the basis of said amount of wood material, in which method bark flow is
measured
optically for its whiteness and a measuring result is used as a basis for
defining
programmatically by a data processing unit the amount of wood material in the
bark flow, and the amount of wood material defined from the bark flow is used
as
a basis for calculating a heat value for the materia) in the bark flow and for
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controiling the combustion process as necessitated by said heat value, wherein
an
image taken of the bark flow is analyzed for whitenesses and the method for
defining the amount of wood materiai comprises the steps of;
dividing the Image into picture elements;
defining the whiteness of each picture element of the image in terms of at
least a three-way split numerical scale, whose end points are m and n, one end
point being applied to represent bark and the other to represent wood;
setting predetermined threshold values a and b wherein a<b, m<a<n. and
b<n;
setting the value of a picture element to m, when the picture element has a
whiteness within the range from m to a;
setting the value of a picture element to n, when the picture element has a
whiteness within the range of from b to n;
maintaining the value of a picture element when the picture element has a
whiteness within the range from a to b;
employing a mean value filter to process the image in image sections, each
said image section consisting of a predetemnined number of picture elements,
the
mean-value filter defining a whiteness mean value for each picture element of
a
particular image section and the obtained mean values are used as a basis for
classifying all picture elements of the particular image section with the same
value
m or n or with an unchanged value, if within the range from a to b;
repeating the fonrgoing steps until all picture elements are classified or
until
a predetermined number of iterations is achieved;
calculating the ratio of a number of picture elements representing wood to the
total
number of picture elements in an image for expressing the amount of wood
present in the bark flow.
Preferred evolutions of the invention are set forth in the non-independent
claims.
The invention will now be described in more detail with reference made to the
accompanying drawing, in which.
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Fig. 1 shows schematically a wood room equipped with an apparatus according to
one embodiment of the invention.
Fig. 2 shows an original black-and-white image taken of a stream of bark and
shaping of the Image after an iteration process.
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In fig. 1, reference numeral 1 designates a rotatable debarking drum, the
trees to be barked being delivered therein through one end thereof by means
of a infeed conveyor 2 and the debarked trees coming out of the other end
on a discharge conveyor 3.
Rotation of the drum 1 causes rolling and hitting of the logs against each
other, the bark being removed from the surface thereof and discharging from
the drum 1 through bark slots (not shown) present in drum shell onto a
conveyor belt 4 arranged below the drum 1. Depending on the process
settings, however, the trees release at the same time more or less of the
actual wood material, said wood material representing wood losses as it
discharges through the bark slots along with barks.
In order to photograph a food flow falling onto the conveyor belt 4 set below
the drum 1 and moving thereon in the direction of an arrow 5, a camera 6
and necessary lighting units 7 have been mounted above the discharge end
of the conveyor belt 4.
The camera 6 is connected to an image processing unit 8, which uses
various whitenesses in the picture elements of an image taken by the cam-
era 6 as a basis for determining programmatically the amount of wood
material in the bark flow.
The image processing unit 8 is adapted to produce an output signal 9 for
controlling the debarking process. Most preferably, the output signal 9 is
adapted to automatically control the debarking process.
The measurement of a stream of bark is essentially performed as a real-
time measurement and preferably by carrying out the measurement from a
moving bark flow. Naturally, it is also possible to pick up samples and
photograph those in a stationary position.
The image processing proceeds as follows.
CA 02287529 1999-10-25
WO 98/53313 PCT/F198/00420
The preset threshold values for whitenesses are used as a basis for mapping
large pieces of wood from small pieces of bark and the background (con-
veyor belt).
5 For example, if the whiteness varies e.g. between 0 and 256, wherein, in
terms of whiteness, zero represents black and 256 white, the preset thresh-
old values, on the basis of experiments, are set for certain species of wood
for example at 120 and 200, the whiteness 200-256 representing wood and
0-120 representing bark.
2. The whitenesses of picture elements identified as bark or wood are set at
the minimum or maximum, i.e. bark at the value of 0 and wood at the value of
256.
3. The new image, composed at the preceding stage, is analyzed by means
of a mean-value filter, which studies the entire image in small sections and
calculates an average whiteness from each section. The average or mean
value is used as a basis for determining an image section either as wood,
bark or unidentified to wait for the next cycle. This sequence separates small
wood pieces and large bark pieces.
If the average falls on a range determined as wood (200-256) or as bark
(0-120), such ranges are presented as values bark = 0 and wood = 256.
Sequences 1-3 are repeated until all pixels are determined or until reaching
a preset number of iterations.
The operation of this system is based on the fact that the areas determined
either as bark or wood influence the averages obtained from the mean-value
filter over the next iteration cycle, each cycle thus providing a more
complete
result.
Fig. 2a depicts an example of an original image taken of a bark flow.
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Fig. 2b shows an original image corresponding to fig. 2a and processed by
means
of the Image processing unit 8, following the first iteration cycle. On the
basis of
whitenesses, it is possible to calculate in terms of the relative numbers of
picture
elements determined as wood, bark, or undefined that the amount of wood is
12%, the amount of bark 33, 1%, with more than a half, or 54.8%, consisting of
an
area still to be defined.
In fig. 2c, the original image (fig. 2a) is illustrated in the same shape as
fig. 2b
after the completion of 20 iteration cycles. On the basis of whitenesses, it
is
possible to calculate that the amount of wood is 27;4%, the amount of bark
70;5%,
the amount of a still undefined area being no more than 2;1q6.
A method and apparatus for reducing wood losses in a debarking process are
descriibed herein above, with the apparatus being depicted In fig. 1. However,
the
basic concept of the invention can also be applied, for example, in an
arrangement where bark flow is fed from a conveyor belt to a combustion
process
to be bumed. In such an arrangement, any bark flow present on the conveyor
belt
4, as shown in fig. 1, and moving therealong is analyzed as described in
connection with fig. 1, but the amount of wood material is used to calculate a
heat
value of the material for control of the combustion prooess, rather than for
control
of a debarking process.
