Optics: measuring and testing – For size of particles
Patent
1995-10-19
1997-11-04
Pham, Hoa Q.
Optics: measuring and testing
For size of particles
356336, G01N 1502
Patent
active
056845839
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an apparatus for detecting foreign matter, if any, in a moving fluid which transmits light, e.g., a moving molten resin which is extruded from an extruder.
BACKGROUND ART
Conventionally, high-voltage cables are used at voltages of 275 kV and 500 kV, for example. These high-voltage cables have a problem that the long-term performance of their insulating plastic covering lowers as the service voltage increases. It is known that the lowering of this performance may be caused by minute foreign matters which penetrate a plastic resin (hereinafter referred to simply as "plastic") during manufacturing processes for the high-voltage cables.
In some apparatuses for manufacturing these high-voltage cables, foreign matters are removed beforehand by means of a mesh screen provided in a passage through which an extruded insulating plastic for covering the cables flows, in order to prevent the foreign matters from penetrating the plastic. Although relatively large foreign matters can be seized by the mesh screen, however, some minute ones may possibly get through fine meshes of the screen. It is necessary, therefore, to inspect in advance the plastic extruded from a plastic extruder for the presence of minute foreign matters therein and remove them.
To attain this, some conventional plastic extruders are furnished with a foreign matter detecting apparatus, such as the one described in Unexamined Japanese Patent Publication (KOKAI) No. 3-284927 (filed Mar. 30, 1990), which is designed to detect foreign matters in a plastic formed by means of the extruder. In this foreign matter detecting apparatus, a minute foreign matter is detected by applying a constricted light beam to the plastic by means of a projector. The transmitted light beam is received by means of a light receiver, and a peak value proportional to the size of the foreign matter in the plastic is obtained from differential waveforms indicative of changes of received light quantities for the foreign matter.
In the foreign matter detecting apparatus of this type, however, the light beam is baffled entirely by a foreign matter of a size larger than its convergence diameter (convergence diameter<foreign matter size), so that the maximum peak value is exceeded. Theoretically, therefore, an accurate foreign matter size cannot be specified in accordance with the differential waveforms.
The foreign matter size may be estimated by measuring the received light quantity reduction time from a voltage waveform obtained before differentiation. This method is impractical, however, since the actual voltage waveform is canceled by noises, so that the foreign matter size can hardly be discriminated.
In the case of a minute foreign matter of a size smaller than the convergence diameter of the light beam (convergence diameter>foreign matter size), there are no differences between the received light quantities, so that even the presence of the foreign matter, not to mention its size, cannot be identified on account of noises. More specifically, in manufacturing a high-voltage cable of 500-kV service voltage, for example, the size of foreign matters which are allowed to penetrate the resin is 50 .mu.m or less with a margin in view. A foreign matter can be detected if half the optical area is baffled. If the foreign matter detection is carried out with the convergence diameter of the light beam adjusted to 70 .mu.m (substantially equal to the product of 50 and the square root of 2), that is, with the convergence diameter not smaller than the foreign matter size, therefore, the foreign matter can be detected within the range of 50 to 70 .mu.m or thereabout. In consideration of the security of the cable performance and feedback to the foreign matter detecting system, however, foreign matters of 70 .mu.m or more must be discriminated.
Foreign matters contained in a molten resin may include minute elongate ones such as fibers. These minute elongate foreign matters are limited in width and move substantially parallel to the flowi
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Abe Fumihiko
Hayashi Hiroshi
Maruki Hiroshi
Noda Hajime
Shinagawa Nobuyuki
Pham Hoa Q.
The Furukawa Electric Co. Ltd.
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