WO2011137554A1

WO2011137554A1 - Method and apparatus for measuring the weight of impurities in a mixed volume of fibers and impurities

Info

Publication number: WO2011137554A1
Application number: PCT/CH2011/000107
Authority: WO
Inventors: Youe-Tsyr Chu; Preston S. Baxter; Michael E. Galyon; Hossein M. Ghorashi; Yazhou Liu; Zibo Xu; Xiaoli Yang
Original assignee: Uster Technologies Ag
Priority date: 2010-05-06
Filing date: 2011-05-05
Publication date: 2011-11-10

Abstract

In the method for measuring the weight of impurities in a mixed volume of fibers and impurities the impurities are mechanically separated (101) from the fibers, whereupon some undesired fibers still remain admixed to the impurities due to imperfections of the mechanical separation (101). A total weight of the separated impurities and the undesired fibers is gravimetrically measured (102). An image of the separated impurities and the undesired fibers is created (103). A weight of the undesired fibers is estimated (105) from the image. The estimated weight of the undesired fibers is subtracted (106) from the total weight to yield a corrected weight of the impurities. The mechanical separation and the subsequent electronic correction yield a more accurate weight of the impurities.

Description

METHOD AND APPARATUS FOR MEASURING THE WEIGHT OF IMPURITIES IN A MIXED VOLUME OF FIBERS AND IMPURITIES

FIELD OF THE INVENTION

The present invention relates to the field of fiber processing. More particularly, it relates to a method and an apparatus for measuring the weight of impurities in a mixed volume of fibers and impurities. Its preferred use is the measurement of the impurity content in raw cotton.

BACKGROUND OF THE INVENTION Currently in the textile industry, it is usually necessary to measure the fiber impurity content in raw cotton. The impurity content means the ratio of undesired impurities such as sand, branches and leaves, boll hull, and soft seed skin in the fiber. For example, the impurity content of saw ginned cotton according to the Chinese National Standard is 2.5 %. A raw cotton impurity analyzer is generally used in actual work to measure the raw-cotton impurity content. As the term is used herein, "impurities" refers to any non-primary-fiber material, such as husks, twigs, leaves, dirt, rocks, and any other non-primary- fiber material that might become mixed into the fiber volume. In other publications, the term "trash" is used as a synonym for "impurities". In the case of cotton fibers for example, "impurities" refers to anything that isn't cotton fiber.

A test analysis instrument with a single taker-in cylinder mechanism, e.g. YG041, YG042, and Y101 as described in Chinese National Standard (GB/T0499) "Testing methods for the trash contents of raw cotton", is adopted in all the traditional test methods for raw cotton impurity content. The typical structure of these traditional raw cotton impurity analyzers is the following: first there is a cotton feeding roller, behind which is a taker-in cylinder, along the circumference of which are installed two or more separation knives; then there is an air current channel for stripping and taking away the fibers on the surface of the taker-in cylinder; and below the separation knife is an impurity disk that is used for collecting impurities and can be taken out manually.

The mechanical impurity-separation principle applied in the known instruments is the following: after the raw cotton is rolled up by the cotton feeding roller and brought into contact with the taker-in cylinder. The taker-in cylinder rotates at a high speed and combs the raw cotton. The fibers and the impurities, being loosened after being combed by the sawtooth structure on the surface of the taker-in cylinder, adhere to the surface of the taker-in cylinder under the action of air current, and rotate at a high speed along with the taker-in cylinder. Due to the different shapes, masses and densities of the fibers and the impurities, under the combined action of the centrifugal force and the air current, the fibers adhere to the surface of the taker-in cylinder, while the impurities are floated in the air current layer farther away from the surface of the taker-in cylinder. When passing across the separation knife, the impurities are blocked and fall down to the impurity disk under the action of gravity, while the fibers continue to rotate with the taker-in cylinder. When the fibers continuing to rotate with the taker-in cylinder are brought into the air current channel tangent to the rotating direction of the taker-in cylinder, due to the pressure change resulting from the special shape of the air current channel, the fibers are detached from the surface of the taker-in cylinder, and taken away by the air current.

During the above-mentioned impurity separation process, under the action of machinery and air current, a small amount of fibers may inevitably be detached from the surface of the taker-in cylinder and fall onto the impurity disk. In the traditional impurity analytical apparatus, the weight content of impurities in the raw cotton can be obtained through manual picking and weighing of the fibers admixed to the impurities. This manual picking method will not only waste a great deal of manpower and time, but also its results show personal differences resulting from the personal picking, causing a deviation in the measured value.

The publication EP-0'533'079 A2 gives an example of an aeromechanical separation of impurities from fibers, as applied in the fiber-testing system USTER^® AFIS PRO 2 from Uster Technologies AG, Uster, Switzerland. The weight of the mixed volume of fibers and impurities is measured by weighing on scales. Then the mixed volume is formed into a sliver, and the sliver is delivered to a first pinned separator wheel. A second pinned separator wheel is located below the first separator wheel. The separator wheels have each a radius of about 32 mm and rotate at very high speeds of 7000-8000 rpm (i.e., 117-133 s^"1). Due to the large centrifugal forces generated at such high rotational speeds, the impurities are centrifuged from the surfaces of the separator wheels into a counterflow of air. The counterflow air returns fibers back to the separator wheels, but is overcome by the impurities. The thus separated impurities are optically sensed by an optical sensor. A computer receives the weight data from the scales and the output signal from the optical sensor. It calculates the weight of the impurities from the accumulated projected area of the impurities. The fibers may be processed in the same way as the impurities. This method also suffers from the drawback that the mechanical separation may be incomplete. In summary, the conventional analysis instruments and methods have shortcomings such as low efficiency and great labor load; besides, two analysis cycles on the test sample are generally required in the test analysis process, with the lower analysis efficiency; moreover, the separated impurities usually contain effective cotton fibers, which results in a deviation in the test result and requires manual picking, thus still resulting in a personal difference in the test result. CONTENTS OF THE INVENTION

A purpose of the present invention is to provide an apparatus and a method for measuring the weight of impurities in a mixed volume of fibers and impurities, which not only increase efficiency and accuracy of the measurement to a great extent, but also reduce the labor load.

The above problem is solved by the method and the apparatus as defined in the

independent claims. Preferred embodiments are defined in the dependent claims.

Any mechanical separation of the impurities from the fibers in the mixed volume is potentially imperfect, since some undesired fibers will still remain admixed to the impurities after separation. Therefore, the invention proposes to mechanically separate the impurities from the fibers, to weigh the separated impurities and the undesired fibers, and to subsequently correct the measured weigh by means of image processing. In this manner, the weight can be corrected by electronic means. This yields a more accurate weight of the impurities.

In the inventive method for measuring the weight of impurities in a mixed volume of fibers and impurities, the impurities are mechanically separated from the fibers, whereupon some undesired fibers still remain admixed to the impurities due to imperfections of the mechanical separation. A total weight of the separated impurities and the undesired fibers is gravimetrically measured. An image of the separated impurities and the undesired fibers is created. A weight of the undesired fibers is estimated from the image. The estimated weight of the undesired fibers is subtracted from the total weight to yield a corrected weight of the impurities.

In a preferred embodiment, an air current is provided for the mechanical separation. The mixed volume is fed onto a surface of a rotating primary taker-in cylinder located in the air current. The impurities are mechanically striped off from the fibers on the primary taker-in cylinder. Part of the mixed volume is transferred from the primary taker-in cylinder to a secondary taker-in cylinder located in the air current. The impurities are separated from the fibers on the secondary taker-in cylinder. The impurities separated on the primary taker-in cylinder and the secondary taker-in cylinder are collected.

The separation of the impurities from the fibers on the primary taker-in cylinder and the secondary taker-in cylinder may make use of the action of centrifugal force, gravity, the air current and mechanical stripping.

In a preferred embodiment, the primary taker-in cylinder has a diameter of 20-30 cm, preferably 25 cm, and the secondary taker-in cylinder has a diameter of 10-20 cm, preferably 16 cm. The primary taker-in cylinder rotates at a rotational speed of

1300-1700 rpm (21.7-28.3 s^"1), and preferably 1500 rpm (25.0 s^"1). The secondary taker-in cylinder rotates at a rotational speed of 900-1200 rpm (15.0-20.0 s^"1), and preferably 1050 rpm (17.5 s^"1). The primary taker-in cylinder has a surface linear velocity of

15-25 m/s, preferably 19.7 m/s, and the secondary taker-in cylinder has a surface linear velocity of 5-12 m/s, preferably 8.7 m/s. The centrifugal acceleration on the surface of the

2 2

primary taker-in cylinder is 1860-4740 m/s , preferably 3090 m/s , and the centrifugal acceleration on the surface of the secondary taker-in cylinder is 444-1580 m/s , preferably 967 m/s². These preferred centrifugal accelerations are clearly lower than those on the surfaces of the separator wheels as described in EP-0'533'079 A2, where mechanical stripping devices are not used. The surface of the primary taker-in cylinder and/or the secondary taker-in cylinder preferably bears a serrated structure or sawtooth structure. The primary taker-in cylinder and the secondary taker-in cylinder may have the same rotational direction. In an advantageous embodiment, the air current below the taker-in cylinders has essentially a horizontal direction. Such a horizontal air current acts like a sheet of air that carries away fibers detached from the taker-in cylinders. The impurities, which are heavier than the fibers, fall through this sheet of air under the action of gravity.

The inventive apparatus for measuring the weight of impurities in a mixed volume of fibers and impurities comprises a separation device for mechanically separating the impurities from the fibers, a gravimetric scale for measuring a total weight of the separated impurities and undesired fibers remaining admixed to the impurities, and a sensor for creating an image of the separated impurities and the undesired fibers. The apparatus further comprises a processor adapted for detecting undesired fibers within the image, estimating a weight of the undesired fibers from the image, and subtracting the estimated weight of the undesired fibers from the total weight to yield a corrected weight of the impurities. In a preferred embodiment, the separation device comprises an air current channel, a fiber feeding device located at a front end of the air current channel, a primary taker-in cylinder located in the air current channel behind the fiber feeding device, and at least one stationary stripping device located near the surface of the primary taker-in cylinder. A secondary taker-in cylinder is located in the air current channel behind the primary taker-in cylinder, surfaces of the primary taker-in cylinder and the secondary taker-in cylinder being adjacent to each other. An impurity collecting apparatus is located below the primary taker-in cylinder and the secondary taker-in cylinder, the impurity collecting apparatus being connected to the gravimetric scale. The primary taker-in cylinder and the secondary taker-in cylinder should be mutually arranged such that part of the mixed volume is transferrable from the primary taker-in cylinder to the secondary taker-in cylinder. In a preferred embodiment, the minimum distance between the surfaces of the taker-in cylinders is between 0.1 and 1 mm, preferably 0.25 mm. The primary taker-in cylinder has a diameter of 20-30 cm, preferably 25 cm, and the secondary taker-in cylinder has a diameter of 10-20 cm, preferably 16 cm.

In a preferred embodiment, the separation device comprises a drive mechanism for the primary taker-in cylinder, which drive mechanism is adapted for driving the primary taker-in cylinder at a rotational speed of 1300-1700 rpm (21.7-28.3 s^"1), and preferably 1500 rpm (25.0 s^"1). Likewise, the separation device comprises a drive mechanism for the secondary taker-in cylinder, which drive mechanism is adapted for driving the secondary taker-in cylinder at a rotational speed of 900-1200 rpm (15.0-20.0 s^"1), and preferably 1050 rpm (17.5 s^"1). The primary taker-in cylinder and/or the secondary taker-in cylinder may have a width in axial direction of 30-70 cm, preferably 50 cm. The surface of the primary taker-in cylinder and/or the secondary taker-in cylinder may bear a serrated structure or sawtooth structure. Such serrated surfaces are more aggressive than the pinned surfaces known from the prior art, and thus more effectively separate the impurities from the fibers. A potential damaging of the fibers is irrelevant in the present application. The height of the serrated structure may be 1 -4 mm, and preferably 2.5 mm.

The apparatus according to the invention is thus able to process 30 grams of sample per minute, whereas the apparatus according to EP-0'533'079 A2 processes only 0.25 grams per minute. The high processing capacity makes the apparatus according to the invention suitable for high-volume fiber processing.

At least one additional stationary stripping device may be located near the surface of the secondary taker-in cylinder. The distance between the at least one stripping device and the surface of the respective taker-in cylinder is between 0.1 mm and 1 mm, and preferably between 0.2 and 0.6 mm.

The fiber feeding device preferably includes a fiber feeding roller and a fiber feeding plate. Thanks to the invention, the mixed volume does not need to be painstakingly separated in some time-consuming or labor-consuming process. Nor does the weight need to be compromised by the weight of undesired fibers. Thus, a corrected weight that accurately represents the impurities can be quickly, easily, and automatically generated. After the preferred double impurity removal with the primary taker-in cylinder and the secondary taker-in cylinder, the impurities are removed from the cotton sample more completely compared to the prior-art single taker-in cylinder structure, making the subsequent impurity measurement value more accurate. Therefore, efficiency and accuracy of the measurement is increased significantly.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention is further described below in detail with reference to the drawings.

Figure 1 is a functional block diagram of the apparatus according to the invention.

Figure 2 shows functional block diagram of the impurity separation section of the

apparatus according to the invention.

Figure 3 shows a schematic front view of part of a taker-in cylinder included in the

apparatus according to the invention.

Figure 4 shows functional block diagram of the weight-determining section of the

apparatus according to the invention.

Figure 5 shows a flow chart of an embodiment of the method according to the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

As can be seen in Figure 1, the apparatus according to the invention comprises a fiber feeding device comprising a fiber feeding roller 1 and a fiber feeding plate 2, the fiber feeding roller 1 feeding the raw cotton sample (not shown) that needs an impurity test. The raw cotton sample, gripped by the fiber feeding roller 1 and the fiber feeding plate 2, is combed by a primary taker-in cylinder 5 and a secondary taker-in cylinder 6. The mechanical separation of the impurities from the fibers is described in more detail below with reference to Figures 2 and 3.

An impurity disk 8 is positioned below the taker-in cylinders 5, 6. The impurities that are combed out fall downwards to the impurity disk 8. The impurity disk 8 is big enough such that all the impurities separated from the taker-in cylinders 5, 6 are collected on the impurity disk 8. An electronic scale 9 is positioned below the impurity disk 8 and connected with it. The impurities, falling to the impurity disk 8 when passing across the taker-in cylinders 5, 6, are weighed automatically by the electronic scale 9 after sample completion. In most cases the separation of the impurities from the fibers is imperfect, so that some undesired fibers are still admixed to the impurities on the impurity disk 8. Therefore, the weight measured by the electronic scale 9 is higher than the actual weight of the impurities. The invention proposes to correct the weight, as described in the following. A digital camera 12 takes images of the impurities and undesired fibers on the impurity disk 8. The digital camera 12 and the electronic scale 9 are both connected to a processor 13. The processor 13 analyzes the image provided by the digital camera 12 and estimates the weight of the undesired fibers admixed to the impurities by means of image processing. Then it corrects the measured weight by subtracting from it the estimated weight of the undesired fibers. The weight correction is described in more detail below with reference to Figures 4 and 5.

Figure 2 shows in more detail the mechanical impurity separation section of the apparatus according to the present invention. It includes an air current channel which comprises an air current guide 7. The fiber feeding device 1 , 2 and the taker-in cylinders 5, 6 are arranged in the air current. The directions of the air current at various locations are indicated by arrows. The air current below the taker-in cylinders 5, 6 has an essentially horizontal direction. The black dots shown in Figure 2 indicate the impurities 11 that are combed out, some of the impurities 11 falling downwards to the impurity disk 8 under the combined action of gravity and centrifugal force.

Primary separation knives 3.1, 3.2 are positioned in a stationary manner along and near the surface of the primary taker-in cylinder 5. The above-mentioned impurities adhering to the taker-in cylinder 5, when passing across the primary separation knives 3.1 , 3.2, are blocked by the separation knives 3.1, 3.2 and fall down to the impurity disk 8. Thus, the separation knives 3.1, 3.2 act as stripping devices that stripe off or comb out the impurities. There are one or more such separation knives 3.1, 3.2, the amount being determined as required. The linear surface velocity v (see Figure 3) of the primary taker-in cylinder 5 according to the invention is significantly higher, e.g., nearly twice as high, than that of the taker-in cylinder in a traditional analytical apparatus. It is within the range of 15-25 m/s, preferably

17.7-21.7 m/s, and most preferably 19.7 m/s.

According to the present invention, behind the primary taker-in cylinder 5 is positioned the secondary taker-in cylinder 6, whose surface is near but not in direct contact with the surface of the primary taker-in cylinder 5. The secondary taker-in cylinder 6 rotates more slowly than the primary taker-in cylinder 5; its linear surface velocity v is within 5-15 m/s, preferably 7.5-9.9 m/s, and most preferably 8.7 m/s. The secondary taker-in cylinder 6 has the same rotational direction as the primary taker-in cylinder 5. In the region where the surfaces of the taker-in cylinders 5, 6 have minimum distance, the surface speed vectors of the taker-in cylinders 5, 6 are opposed to each other and the relative linear surface velocity equals the sum of the two velocities. The fibers are transferred from the primary taker-in cylinder 5 to the secondary taker-in cylinder 6. The cotton fibers, after being combed, are attached to the surface of the primary taker-in cylinder 5 and move with it and, when passing the region where the surfaces of the taker-in cylinders 5, 6 have minimum distance, are combed again by the secondary taker-in cylinder 6. Thus, impurities not combed out by the separation knives 3.1 , 3.2 are combed out by the secondary taker-in cylinder 6. In addition, a secondary separation knife 4 may be assigned to the secondary taker-in cylinder 6; the secondary separation knife 4 and the secondary taker-in cylinder 6 cooperate as described for the primary separation knives 3.1, 3.2 and the primary taker-in cylinder 5 in order to stripe off the remaining impurities. As mentioned above, the impurities 11 fall to the impurity disk 8 under the action of gravity and centrifugal force. Thus, the invention, after double impurity removal with the primary taker-in cylinder 5 and the secondary taker-in cylinder 6, removes the impurities from the cotton sample more completely compared to the prior-art single taker-in cylinder structure, making the subsequent impurity measurement value closer to the actual value. The cotton fibers, on the other hand, continue to rotate with the taker-in cylinders 5, 6. When the air current is tangent to the surface-velocity vector of the respective taker-in cylinder 5, 6, they experience a pressure drop. The fibers are then detached from the surface of the respective taker-in cylinder 5, 6, and taken away by the air current.

The secondary taker-in cylinder 6 can be designed to have the same structure as the primary taker-in cylinder 5. For example, two or more separation knives, the amount being determined as required, can be positioned along the surface of the secondary taker-in cylinder 6.

The primary taker-in cylinder 5 has a diameter 2r (see Figure 3) of 20-30 cm, preferably 25 cm, and the secondary taker-in cylinder 6 has a diameter 2r of 10-20 cm, preferably

16 cm. The widths in axial direction of the taker-in cylinders 5, 6 are 30-70 cm, preferably 50 cm. The minimum distance between the surfaces of the taker-in cylinders 5, 6 is between 0.1 and 1 mm, preferably 0.25 mm. The distance between each of the separation knives 3.1 , 3.2, 4 and the surface of the primary taker-in cylinder is between 0.1 and 1 mm, and preferably between 0.2 mm and 0.6 mm.

Figure 3 shows a schematic front view, not to scale, of part of the taker-in cylinder 5 or 6. The radius r of the taker-in cylinder 5, 6 is preferably in the range between 5 and 15 cm. The surface of the taker-in cylinder 5, 6 bears a serrated structure 10 built up, e.g., of a sequence of saw teeth equally distributed along the circumference of the taker-in cylinder 5, 6. The height h of the serrated structure is preferably in the range between 1 and 4 mm, i.e., the ratio of the height h and the radius r is in the range between 0.7 % and 8 %. The serrated structure 10 extends over essentially the whole width of the taker-in cylinder 5, 6. This may be realized by a serrated band that wraps the lateral area of the cylinder 5, 6 in the form of a helical curve. The angular speed co of the taker-in cylinder 5, 6 is preferably in the range between 94.3 rad/s and 178 rad/s. The surface velocity v can be calculated according to the formula v = cor , and the centrifugal acceleration a is given by the formula a = co²r .

The present invention can be applied to the impurity measurement in raw cotton and other fiber products. The embodiment discussed above has a primary taker-in cylinder 5 and a secondary taker-in cylinder 6. Depending on the actual application, based on the

conception of the present invention, a third taker-in cylinder, a fourth taker-in cylinder and so on can be provided, with their surfaces consecutively near to each other and their structure being similar to that according to the embodiment discussed above. The total number N of taker-in cylinders is a positive integer bigger than or equal to 2. The fibers, after being combed by the primary taker-in cylinder 5, can be combed again by the secondary taker-in cylinder 6 according to the invention, which can comb out more impurities that are not combed out during the first impurity removal process. For example, the apparatus according to the above embodiment of the invention can complete the impurity weight content analysis of a 30-gram raw cotton sample within one minute. Its efficiency is increased by a factor of 3.5 compared with the traditional raw cotton impurity content analysis instruments. Meanwhile, with the introduction of a camera system, the analysis accuracy of raw cotton impurity content is increased to a great extent, and the labor load reduced at the same time.

In Figure 4, there is depicted a functional block diagram of a weight-determining section of the apparatus according to the invention. The impurity disk 8 receives the volume in which impurities 11 are to be weighed. In the example as depicted, the volume is comprised of components 11, 14 and 15. For example, the volume might include impurities 11 , an unknown object 14, and fibers 15. The electronic scale 9 measures the total weight of the volume, and provides the total weight to the processor 13 for further analysis. The camera 12 records an image of the volume on the impurity disk 8 within a field of view 16, and provides the image to the processor 13 for further analysis. The processor 13 implements the algorithm as described below, and determines the corrected weight, as desired.

With reference now to Figure 5, there is depicted a flow-chart of a method according to the invention. As given in block 101 , the impurities 11 are mechanically separated from the fibers 15, albeit incompletely, so that some undesired fibers 15 are admixed to the impurities 11. The separated impurities 11 and the remaining undesired fibers 15 are weighed, as given in block 102. This weight can be accomplished in a variety of different ways. For example, the separated impurities 11 and the remaining undesired fibers 15 can be directly weighed with a gravimetric device like a scale or a scale 9. Whatever method is used, this initial weight of the mixed volume is designated herein as the total weight. An image is then created of the volume on the impurity disk 8, as given in block 103. In some embodiments, the volume is scattered across a surface, such that all components of the mixed volume can be readily seen from one direction, such as from above the volume. In this manner, the individual components of the mixed volume are not hidden, one by another, from the view-point of the camera 12. In some embodiments a single optical visible-light image from a single camera 12 at a single location is used to create the image of the volume. In other embodiments, multiple images from multiple sensors at multiple orientations are created, and in some embodiments wavelengths other than visible wavelengths are used to create the image or images. In still other embodiments,

three-dimensional or quasi-three-dimensional imaging techniques such as tomography are applied. Other combinations of properties such as these are also contemplated.

Once the image has been obtained, as given in block 103, an algorithm is performed using the image as an input. The algorithm discriminates the various components of the image, as given in block 104. By "discriminates" it is meant that the various components 11 , 14, 15 of the volume as depicted in the image are identified as to classification. For instance, those portions of the image that represent fibers 15 are identified as one classification, and those portions of the image that represent impurities 11 are identified as another

classification.

The algorithm can be adapted so as to identify more than two classes of components 11, 14, 15 within the volume, as desired. Various threshold levels can be set as desired so as to make the determination as to how a given portion of the image should be classified.

Because in some embodiments the volume does not completely cover the surface upon which is it disposed, the algorithm can be set, in those embodiments, to exclude from classification those portions of the surface that are visible in the image, as desired.

Once the image has been classified, the weight of at least those classes of material that do not relate to impurities 11 is estimated, as given in block 105, such as by the algorithm. In some embodiments^ the weights of all of the classes of material within the volume are estimated, or the weights of some variable number of the classes are estimated. This can be accomplished by, for example, determining from the image the total volume of fibers 15 within the volume, and then multiplying that total volume by a presumed or measured fiber density value. A variety of different algorithms for determining the weight of the fibers 15 could be used in different embodiments. These determined weights are designated as the component weights.

After the weight of at least one component of the volume has been estimated, the corrected weight of the impurities is determined, as given in bock 106, such as by subtracting one or more of the component weights from the total weight. For example, the component weight of the fibers 15 can be subtracted from the total weight, yielding a corrected weight of impurities 11. It is appreciated that some of the steps of the embodiment of the method as described above do not need to be performed in the order as described above or depicted in Figure 5. For example, measuring the total weight of the mixed volume, as represented in block 102, does not need to be accomplished prior to imaging the mixed volume and estimating the component weight or weights, as given in blocks 103-105. However, the steps of measuring the total weight 102 and estimating at least one component weight 105 do need to be accomplished prior to determining the corrected weight 106. In some embodiments, these steps of measuring the total weight 102 and estimating at least one component weight 105 are accomplished substantially simultaneously. The present invention is not limited to the embodiment discussed above. The description of the embodiment above is only for describing and explaining the technical solution involved in the invention. An obvious transformation and substitution based on the present invention should also be thought to be within the scope of protection of the invention. The embodiment above is used to indicate the most preferred embodiment of the invention, so as to enable those skilled in the art to achieve the purpose of the present invention by using various embodiments and various substitute methods.

LIST OF REFERENCE SIGNS

1 Fiber feeding roller

2 Fiber feeding plate

3.1, 3.2 Primary separation knives

4 Secondary separation knife

5 Primary taker-in cylinder

6 Secondary taker-in cylinder

7 Air current guide

8 Impurity disk

9 Electronic scale

10 Serrated structure

11 Impurities

12 Sensor

13 Processor

14 Unknown object

15 Fiber

16 Field of view 101 Mechanical separation

102 Total weight measurement

103 Image creation

104 Image discrimination

105 Fiber weight estimation

106 Weight correction h Height of the serrated structure

r Radius of the taker-in cylinder

v Surface linear velocity of the taker-in cylinder ω Rotational speed of the taker-in cylinder

Claims

1. A method for measuring the weight of impurities in a mixed volume of fibers and impurities, comprising the steps of:

mechanically separating (101) the impurities from the fibers, some undesired fibers

(15) still remaining admixed to the impurities (11) due to imperfections of the mechanical separation (101),

gravimetrically measuring (102) a total weight of the separated impurities (11) and the undesired fibers (15),

creating an image (103) of the separated impurities (11) and the undesired fibers

(15),

estimating a weight (105) of the undesired fibers (15) from the image, and subtracting the estimated weight (106) of the undesired fibers (15) from the total weight to yield a corrected weight of the impurities (11).

2. The method according to claim 1 , wherein the mechanical separation (101)

comprises the steps of

providing an air current,

feeding the mixed volume onto a surface of a rotating primary taker-in cylinder (5) located in the air current,

mechanically striping off the impurities (11) from the primary taker-in cylinder (5), transferring part of the mixed volume from the primary taker-in cylinder (5) to a secondary taker-in cylinder (6) located in the air current,

separating the impurities (11) from the fibers on the secondary taker-in cylinder (6), and

collecting impurities (11) separated on the primary taker-in cylinder (5) and the secondary taker-in cylinder (6).

3. The method according to claim 2, wherein the primary taker-in cylinder (5) has a diameter (2r) of 20-30 cm, preferably 25 cm, and the secondary taker-in cylinder (6) has a diameter (2r) of 10-20 cm, preferably 16 cm.

4. The method according to claim 2 or 3, wherein the primary taker-in cylinder (5) rotates at a rotational speed (ω/(2π)) of 1300-1700 rpm (21.7-28.3 s^"1), and preferably 1500 rpm (25.0 s^"1), and the secondary taker-in cylinder (6) rotates at a rotational speed (ω/(2π)) of 900-1200 rpm (15.0-20.0 s^"1), and preferably 1050 rpm (17.5 s^"1).

5. The method according to any of the claims 2-4, wherein the primary taker-in cylinder (5) has a surface linear velocity (v) of 15-25 m/s, preferably 19.7 m/s, and the secondary taker-in cylinder (6) has a surface linear velocity (v) of 5-12 m/s, preferably 8.7 m/s.

6. The method according to any of the claims 2-5, wherein the centrifugal acceleration on the surface of the primary taker-in cylinder (5) is 1860-4740 m/s , preferably 3090 m/s², and the centrifugal acceleration on the surface of the secondary taker-in cylinder (6) is 444-1580 m/s², preferably 967 m/s².

7. The method according to any of the claims 2-6, wherein the surface of the primary taker-in cylinder (5) and/or the secondary taker-in cylinder (6) bears a serrated structure (10).

The method according to any of the claims 2-7, wherein the primary taker-in cylinder (5) and the secondary taker-in cylinder (6) have the same rotational direction.

The method according to any of the claims 2-8, wherein the air current below the taker-in cylinders (5, 6) has essentially a horizontal direction.

An apparatus for measuring the weight of impurities in a mixed volume of fibers and impurities, comprising

a separation device for mechanically separating the impurities from the fibers, a gravimetric scale (9) for measuring a total weight of the separated impurities (11) and undesired fibers (15) remaining admixed to the impurities (11),

a sensor (12) for creating an image of the separated impurities (11) and the undesired fibers (15), and

a processor (13) adapted for

detecting undesired fibers (15) within the image,

estimating a weight of the undesired fibers (15) from the image, and

subtracting the estimated weight of the undesired fibers (15) from the total weight to yield a corrected weight of the impurities (11).

11. The apparatus according to claim 10, wherein the separation device comprises

an air current channel,

a fiber feeding device (1, 2) located at a front end of the air current channel, a primary taker-in cylinder (5) located in the air current channel behind the fiber feeding device (1, 2),

at least one stationary stripping device (3.1, 3.2) is located near the surface of the primary taker-in cylinder (5),

a secondary taker-in cylinder (6) located in the air current channel behind the primary taker-in cylinder (5), surfaces of the primary taker-in cylinder (5) and the secondary taker-in cylinder (6) being adjacent to each other, and

an impurity collecting apparatus (8) located below the primary taker-in cylinder (5) and the secondary taker-in cylinder (6), the impurity collecting apparatus (8) being connected to the gravimetric scale.

12. The apparatus according to claim 11, wherein the primary taker-in cylinder (5) and the secondary taker-in cylinder (6) are mutually arranged such that part of the mixed volume is transferrable from the primary taker-in cylinder (5) to the secondary taker-in cylinder (6).

The apparatus according to claim 11 or 12, wherein the minimum distance between the surfaces of the taker-in cylinders (5, 6) is between 0.1 and 1 mm, preferably 0.25 mm.

The apparatus according to any of the claims 11-13, wherein the primary taker-in cylinder (5) has a diameter (2r) of 20-30 cm, preferably 25 cm, and the secondary taker-in cylinder (6) has a diameter (2r) of 10-20 cm, preferably 16 cm.

The apparatus according to any of the claims 11-14, wherein the separation device comprises a drive mechanism for the primary taker-in cylinder (5), which drive mechanism is adapted for driving the primary taker-in cylinder (5) at a rotational speed (ω/(2π)) of 1300-1700 rpm (21.7-28.3 s^"1), and preferably 1500 rpm (25.0 s^"1) and the separation device comprises a drive mechanism for the secondary taker-in cylinder (6), which drive mechanism is adapted for driving the secondary taker-in cylinder (6) at a rotational speed (ω/(2π)) of 900-1200 rpm (15.0-20.0 s^"1), and preferably 1050 rpm (17.5 s^"1).

The apparatus according to any of the claims 11-15, wherein the primary taker-in cylinder (5) and/or the secondary taker-in cylinder (6) has a width in axial direction of 30-70 cm, preferably 50 cm.

The apparatus according to any of the claims 11-16, wherein the surface of the primary taker-in cylinder (5) and/or the secondary taker-in cylinder (6) bears a serrated structure (10).

18. The apparatus according to claim 17, wherein the height (h) of the serrated structure (10) is 1-4 mm, preferably 2.5 mm.

19. The apparatus according to any of the claims 11-18, wherein at least one additional stationary stripping device (4) is located near the surface of the secondary taker-in cylinder (6).

The apparatus according any of the claims 11 -19, wherein the distance between the at least one stripping device (3.1, 3.2, 4) and the surface of the respective taker-in cylinder (5, 6) is between 0.1 mm and 1 mm, and preferably between 0.2 mm and 0.6 mm.

The apparatus according to any of the claims 11-20, wherein the fiber feeding device (1 , 2) includes a fiber feeding roller (1) and a fiber feeding plate (2).