This invention relates to a method for the detection of defective light transmitters and/or light receivers at a light grid and to a light grid for carrying out this method.
With such light grids the light transmitters, which are normally arranged in a row, are sequentially switched on one after the other in a continually cyclically repeating manner. A light receiver is associated with each light transmitter and is synchronously activated with the associated light transmitter, so that each light receiver normally can receive light from only a specific light transmitter. The synchronization of the light transmitters and light receivers can take place over a connecting line between the light transmitter row and the light receiver row (DE 24 09 113 B2) and also through a suitable coding of the light transmitter pulses to which the light receivers are matched. A synchronization of light transmitters and light receivers without an electrical connecting line between the light transmitter and light receiver rows is also possible by providing a synchronization pause between the light pulses of the last and first light transmitters of the light transmitter row (DE-OS 38 03 033).
Such light grids are generally used to secure areas which are to be protected, such as for example the opening of an elevator entrance at the closing edges of the elevator door, in order, in the case of an obstruction or encroachment, for example by the hand of the user, to bring about a stopping or reversal of the closing movement of the door. If a light transmitter and/or a light receiver fails with such a light grid then the evaluation circuit evaluates this as an encroachment and transmits a corresponding alarm signal which is generally a switch off or reversal signal and/or a light signal and/or an acoustic signal.
Frequently a light grid is however still operable when only a few light transmitters and/or light receivers have failed; nevertheless, in all known light grids, the failure of only a single light transmitter and/or light receiver requires that the defective component be exchanged before the light grid is operable again. In this way long periods of disuse must be tolerated, for example with elevators, in particular when the failure of a component takes place on a weekend and several days can pass before the arrival of a servicing specialist.