JP3227846B2 - Foreign matter detection method in molten polyethylene - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting foreign matter in polyethylene flowing in a molten state at a high temperature.

[0002]

2. Description of the Related Art In the case of extrusion coating of an insulator of a crosslinked polyethylene insulated power cable (CV cable) or extrusion molding joint of a CV cable connection portion, if foreign matter is present in polyethylene, insulation breakdown or the like will occur. Cause electrical failure. Therefore, it is necessary to detect the number and size of foreign matter over the entire amount of the molten polyethylene. Further, since a crosslinking agent is mixed in the molten polyethylene, if the temperature of the molten polyethylene is not uniform, a so-called burn phenomenon in which the crosslinking reaction proceeds abnormally occurs. For this reason, the detection of foreign matter is a condition that the measurement can be performed without changing the temperature, flow rate, and the like of the molten polyethylene.

FIG. 6 is an explanatory diagram of an example of a conventional method for detecting foreign matter in molten polyethylene. As shown in the drawing, a part of the pipe of the molten polyethylene 32 is constituted by a glass pipe 31, a laser light is emitted from a light source 33, and the laser light captures scattered light scattered by a foreign substance present in the molten polyethylene 32. This is a method for detecting the presence of foreign matter and its size. In the drawing, 34 is a reflecting mirror, 35 is a lens, 36
Is an optical detector, and 37 is an oscillograph.

[0004] Such a method for detecting foreign matter detects laser light scattered by the foreign matter, and thus can detect minute foreign matter of several μm or less with high sensitivity. However, when the diameter of the foreign matter becomes considerably larger than the wavelength of the laser light. (Up to several tens of μm), the intensity and direction of the scattered light vary depending on the shape and surface state of the foreign matter, so that there is a problem that the measurement error of the outer diameter of the foreign matter increases or the measurement cannot be performed.

FIG. 7 is an explanatory view of another example of a conventional method for detecting foreign matter in molten polyethylene. As shown in the drawing, the shape of the glass pipe is a sheet-like glass pipe 41, and CC
This is a method of detecting the shape by capturing a foreign object image of the molten polyethylene 42 passing through the D camera 43 within the depth of focus as a camera image.

In such a detection method, the focal length of the lens becomes long in order to detect a minute foreign substance, and the focal depth becomes shallow (short) when the magnification is large. Therefore, it is necessary to make the passage through which the molten polyethylene flows wide and thin, and there is a high possibility that the phenomenon of scorching in which the crosslinking reaction proceeds will occur. In addition, since the flow rate of the molten polyethylene is increased, an extremely high-speed image processing is required to perform the entire inspection. Further, since a pipe having a size corresponding to the field of view of the camera is required, it is necessary to keep the temperature of the glass part constant in order to keep the temperature of the molten polyethylene constant.

[0007]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems and provides a method for detecting foreign matter in molten polyethylene having a wide field of view, a high spatial resolution and a large depth of field. The pipe through which the molten polyethylene flows is provided with a window through which the light is transmitted, and from that window, the light from the light source is transmitted through the molten polyethylene, and a shadow generated when the light from the light source is blocked by foreign substances contained in the molten polyethylene. An object of the present invention is to detect the size of a foreign object by detecting a line sensor and calculating the product of the shadow width and the shadow intensity .

[0008]

[Function] A glass pipe is installed in the flow path of polyethylene which is heated to about 120 ° C. and is in a molten state. The light source and the line sensor are placed facing each other across this glass pipe,
The light source intensity of each pixel of the line sensor is monitored. If foreign matter is present in the molten polyethylene, light from the light source is blocked by the foreign matter, and the amount of the blocked light has a correlation with the size of the foreign matter. Furthermore, the shadow of a foreign object becomes blurred (the intensity of the shadow becomes weaker) due to the interference of light when the distance between the foreign object and the sensor increases, but the width of the separation increases, and the product of the intensity of the shadow and the width of the shadow becomes It does not depend on the distance between the foreign object and the sensor. Further, the position of the foreign matter can be known from the pixel of the line sensor where the shadow occurs. The position accuracy and the shadow width measurement accuracy are determined by the number of pixels and the pixel size of the line sensor, and a line sensor having 5,000 pixels of 10 μm can be easily obtained.

The resolution, field of view, and depth of focus of the detection method according to the present invention were examined by the method shown in FIG. In the figure, reference numeral 21 denotes an LE having a wavelength of 850 μm.
D light source, 22 is a PCF optical fiber having a core diameter of 250 μm, 23 is a selfoc lens, and 24 is a 4F as shown in FIG.
Species: 130 μm 25a, 70 μm 25b, 20 μm 25c, 455
A sample having a metal wire of 25 μm 25d, 26 is a line sensor, 27 is a CCD drive circuit, 28 is a computer, 29 is a printer, and as shown in FIG. 3A, the above-mentioned four types of metal wires (25a, 25b, By placing the sample 25 having 25c, 25d) in the optical path and positioning it at A or B in the figure, it was examined how much the detection accuracy of the foreign particle diameter changes depending on the depth of focus. The signal output of the CCD is shown in Fig. 4 (2048 pixels, approx.
(28.6 mm, the distance between the CCD line sensor and the sample is about 10 mm), the pixel corresponding to the position where the metal wire is stretched is captured as a shadow. The results are shown in FIG.

According to FIG. 5, the width and depth (light intensity) of the shadow are about 150 μm or less due to light diffraction, so that the relationship between the width and the depth of the metal wire is not linear. Is 4
It can be seen that there is a good straight line relationship for the metal wire shadow of 55 μm. Further, it was found that this relationship was maintained at the positions of A10 mm and B40 mm where the distance between the line sensor 26 and the sample 25 was changed.

[0011]

FIG. 1 is a circuit diagram of a specific example of a foreign matter detection method according to the present invention, and FIG. 2 is an explanatory diagram of the operation of each part in FIG. The light source 1 uniformly irradiates the flow glass cell 2 through which the molten polyethylene flows with the LED light. The signal output from the CCD line sensor 4 one pixel at a time by the line sensor drive circuit 3 is amplified by the amplifier 5 to a level that can be determined by the comparator 6, and compared with a preset level by the comparator 6. The comparator 6 goes to a high level when there is a foreign substance signal, and the width of the foreign substance shadow can be known by counting the period of this high level with the counter 7.

On the other hand, the signal of the CCD line sensor 4 holds the maximum signal appearing in one frame (time for outputting from the 0th pixel to the 2048th pixel of the CCD) by the peak hold circuit 9 and the shadow depth C Is held. The width of the foreign matter shadow counted by the counter 7 is converted into an analog value b by the D / A converter 8 and multiplied by the multiplier 10 with the shadow depth level C to obtain a signal level proportional to the foreign matter shadow area as an output d. can get.

[0013]

As described above, the method for detecting foreign matter in molten polyethylene according to the present invention has the following effects. (1) The measurement can be performed by capturing the shadow of the foreign matter in the molten polyethylene, and the foreign matter can be detected regardless of the depth of focus of the lens. (2) Since the shadow produced by the foreign matter is detected by the line sensor, the window glass can be short, and the necessity for keeping the temperature of the molten polyethylene due to the provision of the glass portion is eliminated. (3) Since the shadow corresponds to the maximum diameter of the foreign matter in the direction of irradiating the light of the light source, the influence of the surface condition and shape of the foreign matter is small,
The signal processing is small, and it is easy to check the total amount of the molten polyethylene. (4) In order to determine the outer diameter of the foreign substance from the product of the shadow width and the light intensity,
Small errors due to blurring of foreign matter shadows due to light interference,
It is also possible to detect a foreign substance having a small diameter. Therefore, the method of the present invention is extremely effective when used in an extrusion coating step of a CV cable or an extrusion mold joint of a CV cable connection part.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram of a specific example of a method for detecting foreign matter in molten polyethylene according to the present invention.

FIG. 2 is an operation explanatory view of each part in FIG. 1;

FIG. 3A shows the resolution of the foreign matter detection method of the present invention;
FIG. 3 is an explanatory view of an experimental method for investigating a visual field and a depth of focus, and FIG. 3B is an explanatory view of a sample used in the experiment.

4A is an image of a signal output of the CCD line sensor in the experiment of FIG. 3, and FIGS. 4B and 4C are enlarged views of 25c and 25d, respectively.

FIG. 5 is a diagram showing a relationship between a foreign particle diameter and a foreign material shadow in the experiment of FIG. 3;

FIG. 6 is an explanatory diagram of an example of a conventional method for detecting foreign matter in molten polyethylene.

FIG. 7 is an explanatory diagram of another example of a conventional method for detecting foreign matter in molten polyethylene.

[Explanation of symbols]

 Reference Signs List 1 light source 2 molten polyethylene flow glass cell 3 CCD line sensor drive circuit 4 CCD line sensor 5 amplifier 6 comparator 7 counter 8 D / A converter 9 peak hold circuit 10 multiplier

Claims (1)

(57) [Claims] 1. A window through which light is transmitted is provided in a pipe through which molten polyethylene flows, and light from the light source is transmitted through the window to the molten polyethylene, and the light from the light source is blocked by foreign substances contained in the molten polyethylene. A method for detecting foreign matter in molten polyethylene, comprising detecting a shadow with a line sensor and measuring the size of the foreign matter from the product of the width of the shadow and the intensity of the shadow .