Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1996-10-16
1999-05-04
Bacares, Rafael
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359194, H04B 1006
Patent
active
059009608
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention pertains to a circuit arrangement for receiving light signals, which are emitted over an open optical path, i.e., in ambient light. The circuit is used to receive light signals which are sent by evaluating the intensity for recognizing changes occurring over a limited time in the structure of a medium which is located in the transmission path. Changes of this kind may be caused by, for example, water drops located in the air of the optical transmission path or on a glass pane which is arranged in the transmission path.
An arrangement for measuring or recognizing the wetting of a wall or plate transparent to a certain radiation was proposed in German Utility Model No. G 93 09 837.5. This arrangement can be used, for example, to detect water drops located as precipitate on the windshield of a motor vehicle.
This device uses an optical arrangement with two light-emitting diodes, between which a light sensor is located, and which are arranged behind the windshield of a motor vehicle. The light-emitting diodes emit push-pull-pulsed light. The light sensor receives part of the light of the light-emitting diodes reflected from the windshield. The windshield reflects the rays emitted in the push-pull mode approximately equally in the dry state, so that the received light from the two light-emitting diodes acts like constant light in the circuit arrangement. The reflection condition, and consequently the signal ratio, change irregularly at short time intervals upon the impact of rain drops. The received light component of one light-emitting diode predominates, and an alternating current is generated in the light sensor in the cycle of the pulse frequency of the light that is emitted by the light-emitting diodes. These changes are evaluated.
Such a device, which is operated in an optically unshielded space, has the problem that intensely varying, undefinable ambient light is present, and this light continuously reaches the light sensor as an interfering light. This light is usually constant light, for example, the light of the sun or of incandescent lamps. However, it may additionally also be received as chopped light, such as the light of gas discharge lamps or a constant light influenced by the travel motion of the motor vehicle. The intensity of the ambient light may change in rapid succession in an undefinable manner during the travel of the motor vehicle, so that no reliable information can be obtained from this light on the nature of the transmission path, for example, the presence of rain drops. If the light sensor is equipped with optical filtering means, this parasitic light is then received at varying intensity and is converted into a corresponding electrical signal component. The converted electrical signal, therefore, always consists of an equisignal component, whose value may vary within a broad range, and a superimposed alternating signal component, which represents the evaluable wanted signal. The level of the equisignal component may be a multiple of the amplitude of the wanted signal. At high intensity, parasitic light may overmodulate the light sensor and/or the receiving amplifier. In this case, the wanted signal cannot be sufficiently separated from the equisignal component and evaluated. Measures are therefore necessary to eliminate the parasitic light.
To support these measures, the light is scanned at a frequency of about 40 kHz. Only insignificant parasitic light components are present in this frequency range.
A light receiver comprises, in general, a light sensor, an amplifier, means for suppressing the effects of parasitic light, and a demodulator for recovering the signal from the converter photocurrent. This signal is evaluated by a synchronous demodulator, and it generates a signal which corresponds, for example, to the frequency of defects in the optical transmission path. Photodiodes are usually used as light sensors.
Such a circuit arrangement has been known from the book "Linear Circuits Data Book 1992," Volume 1, Operational Amplifi
REFERENCES:
patent: 4805237 (1989-02-01), Donald et al.
patent: 5359446 (1994-10-01), Johnson et al.
"Linear Circuits Data Book 1992," vol. 1, Operational Amplifiers, Texas Instruments, Figure 47.
Schmith, R.G. et al.: Optical Detector Package, The Bell System Technical Journal, vol. 57, No. 6, Jul./Aug., 1978, pp. 1809-1822.
Schmidt, Wolfgang; Feustel, Ortwin: Optoelektronik kurz und bundig, Vogel-Verlag, 1975, pp. 53-60, 71-74.
Bacares Rafael
ITT Automobive Europe GmbH
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