Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1997-01-29
1998-08-04
Le, Que
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250214R, 340555, G01V 904
Patent
active
057897409
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention concerns a method and a device for eliminating interference signals in a light barrier.
Such a light barrier is known from the German Patent DE-PS 43 23 910. The transmitter periodically emits light pulses with a defined pulse length and a subsequent transmitting break. A counter is connected following the receiver, in which the light pulses that arrive at the receiver are counted. The counter is connected to a multi-channel evaluation unit. Each channel has one output, which is directed to a logic unit.
Depending on the number of recorded light pulses, signal changes take place at the channel outputs for defined time intervals, which are processed in the logic unit such that a signal train with a defined pulse-break-ratio is present at its output. This signal train can be assigned clearly to the series of light pulses emitted by the transmitter. The pulse-break-ratio of the signal train is changed when internal or external interferences appear, which makes it possible to detect an interference.
Such a multi-channel evaluation of received signals is particularly suited for detecting internally occurring component errors with high certainty. This is of importance particularly if such light barriers are to be used in the field of personal security, where extremely high safety requirements must be met. The evaluation unit, which ensures the required safety, it correspondingly expensive.
SUMMARY OF THE INVENTION
It is the object of the invention to ensure a trouble-free operation of a light barrier of the aforementioned type.
The features in claim 1 are provided as solution. Advantageous embodiments and suitable modifications of the invention are described in the claims 2-8.
The invention at hand is based on the idea that the evaluation of the received signals is synchronous to the light pulses emitted by the transmitter.
The transmitter and the receiver of light barriers represent separate systems, which are arranged at a distance from each other. There is no connection via feed lines between these systems. A direct synchronization of transmitter and receiver via such a feed line is therefore on principle not possible.
In accordance with the invention, an evaluation unit is therefore connected to the receiver, which records the received signals present at the receiver output only during activation intervals that are limited in time. With trouble-free operation and clear beam path for the light barrier, the opening and closing of one of the activation intervals occurs synchronous to the light pulses periodically emitted by the transmitter, wherein the transmitting frequency is known and is preset as parameter in the evaluation unit. If individual transmitting light pulses are missing, for example when an object enters the beam path of the light barrier or if short interference pulses appear, this operational mode is interrupted and a second activation interval is opened, for which the maximum length corresponds to the period length of the transmitting light pulses.
The second activation interval is connected as soon as a light pulse is recorded in the evaluation unit within this scanning interval. If no light pulse is recorded, then the second activation interval is continuously opened and closed until the next light pulse is recorded. Following that, the first activation interval is opened periodically, in phase with the reception of the light pulses.
All interference pulses that appear asynchronous can be filtered out in a simple way through the synchronous operation of the light barrier. The synchronous operation can be interrupted only by briefly appearing interference pulses, which are recorded almost simultaneously with the light pulses emitted periodically by the transmitter.
However, by configuring the activation intervals in accordance with the invention, a new synchronization occurs quickly, which restores the error-free operation of the light barrier.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in the following with the aid of the dra
REFERENCES:
patent: 4549086 (1985-10-01), Herzer
patent: 4642455 (1987-02-01), Rodi et al.
patent: 5057683 (1991-10-01), Fukuyama
patent: 5250801 (1993-10-01), Grozinger et al.
Patent Abstracts of Japan, vol. 9, No. 225, Sep. 11, 1985 & JP-A-60 084013 (Tateishi Denki KK), May 13, 1985.
Patent Abstracts of Japan, vol. 9, No. 225, Sep. 11, 1985 & JP-A-60 084012 (Tateishi Denki KK), May 13, 1985.
Le Que
Leuze electronic GmbH & Co.
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