Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1992-03-20
1994-07-12
Westin, Edward P.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250572, 250563, 356239, 356430, G01N 2188
Patent
active
053291336
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of detecting flaws of an object of examination by using optical and electric (electronic) means and automatically determining the type and nature of each of the detected flaws.
2. Prior Art
Flaws on and/or in a product can seriously damage the performance of the product and significantly reduce its commercial value.
For instance, foreign objects contained in the insulation layers of rubber or plastic of a cable can significantly degrade the insulation breakdown characteristics of the cable.
While the probability with which foreign objects are introduced into insulation layers of rubber or plastic insulated cables has been greatly reduced with the recent development in the field of manufacture of insulated cables, there will still be a long way to go before flawless cables are produced.
Therefore, it is of vital importance to accurately analyze the insulation breakdown characteristics of each cable by examining the type, the size and the number of the flaws of the cable if it is marketed with a reliable degree of quality assurance.
In the case of an extra high voltage power cable, a minute foreign object that can be detected only by microscopic scrutiny using a microscope of a high magnification can seriously affect the insulation breakdown characteristics of the power cable if it is contained in any of its insulation layers. Therefore, a highly reliable inspection system should be established to cope with such problems, involving a large number of specimens to be examined, in order to provide an enhanced level of quality assurance for cables of this category.
The most dangerous foreign object is, of course metal debris. In order to safely eliminate metal debris from marketed cables, the type and nature of foreign objects should be determined before knowing the size and the number of the foreign objects for each type.
It is a common practice for examining the quality of a rubber or plastic insulation layer of an insulated cable to cut out a specimen having a thickness of 0.05 to 2 mm from the layer and observe the specimen through a microscope to visually determine the type, the number and the size of the flaws found in it.
Observation of a specimen proceeds in this technique proceeds in three stages: detecting flaws in the specimen, identifying each of the detected flaws by color and shape and metering the dimensions of each of the flaws (length.times.width.times.height).
Flaws in rubber or plastic insulated cables may normally grouped into three categories: ambers, black foreign objects and voids. Metal debris are regarded as black foreign objects.
When flaws are observed through a microscope that receives rays of light transmitted through the object of examination, each may take any of the following image patterns depending on the depth of observation along the center line of the microscope running through the focal point of the lens system.
Just: signifying that the depth of observation agrees with the focal length of the flaw. Under: indicating that the depth of observation is shallower than the focal length of the flaw. Over: meaning that the depth of observation is deeper than the focal length of the flaw.
These image patterns are summarized in Table 1.
Black foreign objects clearly appear black when the depth of observation is just and blurred and black when the depth of observation is under or over.
Ambers are hardly or not recognizable when the depth of observation is just because they turn totally white and do not make difference with the ambient resin color. They take on a black margin surrounding a white inside area when the depth of observation is over, whereas they are observed as totally black when the depth of observation is under.
Voids shows a black margin with the inside appearing white when the depth of observation is just, although they look just black when the depth of observation is under or over.
The effect of the above described technique of determining flaws by observation greatly depends on
REFERENCES:
patent: 5196716 (1993-03-01), Moriya et al.
Shimada Michihiro
Uesugi Kenji
Davenport T.
The Furukawa Electric Co. Ltd.
Westin Edward P.
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