Optics: measuring and testing – Inspection of flaws or impurities – Having predetermined light transmission regions
Patent
1996-08-26
1998-09-15
Nelms, David C.
Optics: measuring and testing
Inspection of flaws or impurities
Having predetermined light transmission regions
2503418, 318444, 318480, 318DIG2, G01N 2143, H02P 104
Patent
active
058087346
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method and an apparatus for detecting impurities on the surface of a plate transparent to electromagnetic radiation. A particular object of the invention is to provide a method and an apparatus for detecting water and/or solid dirt on the windshield of a vehicle and a demand-based control system based on the method for automatic starting and stopping of a windshield wiper and washer.
In practice a plurality of needs arise for measuring the portion of the area covered by impurities on the surface of a plate. In a great number of such applications the measurement need not be particularly accurate, but rather, simple and reliable function in harsh and varying conditions is required. Typically, a plate surface on which the relative area covered by liquid and/or solid dirt is desired to be measured is the windshield or rear window of a vehicle, working machine or similar equipment. Reliable measurement in such an application permits the automatic control of the windshield wipers and washer equipment.
Several tens of inventions intended to solve the above-described vehicle control application have been filed. However, their commercial use in vehicles has not been implemented. This situation is the result of technical deficiencies in these solutions.
In the prior art four different types of solutions have been proposed based on change of capacitance, refraction and reflection of electromagnetic radiation (mostly infra-red and visible light), reflection of microwaves, and acoustic emissions from the impact of raindrops. Under varying conditions (of humidity and temperature), the capacitive methods fail to achieve sufficient accuracy under conditions of low humidity. A majority of the disclosed inventions are based on the utilization of different types of reflection or refraction of electromagnetic radiation. In fact, this currently seems to offer the only viable solution to achieve a sufficiently sensitive, uncomplicated and reliable embodiment for varying conditions.
Embodiments based on electromagnetic radiation can be categorized into two major classes: in one group the radiation is transmitted from inside the glass via an air gap and the radiation is reflected/backscattered at the water droplets and/or solid dirt deposited on the surface of the glass. In the other group the radiation is transmitted to the inside of the glass via a prism at an angle which makes the radiation undergo total reflection during its propagation in the glass. The inventions of the latter type are mostly based on the principle disclosed in the Swedish Pat. No. 353,497 filed in 1969. According to this invention, radiation transmitted into the glass via a prism propagates undergoing total reflection at alternate surfaces of the glass. A fraction of the radiation is routed out from the glass with the help of another prism after a few incidents of total reflection. If the radiation at the point of total reflection meets a water droplet on the glass surface, it can exit the glass. This causes a reduction in the intensity taken out through the receiving prism, whereby intensity reduction can be used to detect the existence of water droplets on the glass surface.
The method according to cited Swedish patent-has a crucial and essential shortcoming in that the intensity variations of background radiation are excessively large with respect to the measured effect. Resultingly, a detector which is sufficiently sensitive to measure the intensity variations caused by the desired effect will be overloaded (saturated) at extreme levels of background radiation such as, e.g., direct sunlight. In this situation the system functions are inhibited. The same shortcoming applies to almost all inventions employing this principle. An exception to this is described below.
Among other shortcomings, inventions based on the intensity measurement of returning radiation by means of a detector surrounded by free air are hampered by the same problem described above (that is, overloading of a sensitive det
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Nelms David C.
Smith Zandra V.
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