Safety relay

Details Safety relay Safety relay

1999-08-03

2001-05-22

Fleming, Fritz (Department: 2836)

Electricity: electrical systems and devices

Control circuits for electromagnetic devices

For relays or solenoids

C307S326000

Reexamination Certificate

active

06236553

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a safety relay for supervising the operation and ensuring operational safety of a machine or an industrial process and corresponding safety devices and actuators, comprising at least one input monitored for short circuits and open circuits for connection to said safety devices and actuators, at least one output for connection to said machine or process, one or a plurality of stop relays for maintaining and interrupting, respectively, the connection to said machine or process in response to a momentary state of said safety devices and actuators, and a reset circuit with at least one capacitor and a reset input, the capacitor being arranged to receive and store electrical energy in a first state and to supply this energy in a second state for resetting the safety relay to a normal state of operation.
DESCRIPTION OF THE PRIOR ART
Safety relays of the type described above are used in many industrial applications, for instance for ensuring safe operation of safety devices and actuators, for achieving safe stops for dangerous machines and processes, and for monitoring stop inputs and the internal safety of machines. These safety relays are used today in a variety of different applications, for instance robots, presses, production systems, paper machines, etc. Generally, machines and industrial processes controlled or monitored by a human operator imply a potential risk of personal injury, in case an unforeseen machine fault or incorrect handling takes place. Several different types of safety devices, such as light bars, contact mats, safety gates, hatches, safety strips or emergency stop buttons, are used for obtaining maximum safety. Such safety devices are commonly monitored by a safety relay of the type described above. In summary the purpose of the safety relay is to detect, through its inputs, any changes in the monitored equipment—one example may be a switch, which detects an opened safety gate—and in response thereof cause the stop relays to |de-energize| (i.e., fall), thereby causing an interruption in the supply of power to the apparatus in question.
A variety of different standards and requirements are put on modern safety relays. The present invention is particularly aimed at the field of supervised reset, which means that a fault in a reset button, connection wires or other components comprised in the safety relay must not cause an unintentional reset of the safety relay. The Swedish patent publication SE-C-465 067 discloses a device for production-adapted safety system for a production unit, such as for instance a robot. A safety relay or safety module is provided with double stop relay circuits according to the above. In parallel to the stop relay circuits a reset relay circuit is arranged, said reset relay circuit being provided with a reset means, which must have been closed as well as opened, before the outputs are closed on the double stop relay circuits. The reset relay circuit comprises a capacitor, which is provided with energy in a first state and is arranged to supply energy in a second state to a relay, the latter being arranged to influence the self-energization of the stop relay circuits. Apart from the two main stop relays the device according to SE-C-465 067 comprises two further relays in the reset circuit.
It is most desirable as regards cost as well as performance that safety relays of the type described above may be realized with few and inexpensive components and with minimal space requirements, while all safety requirements are still fulfilled.
A common requirement for safety relays is that a single fault (such as an open circuit or short circuit) in any component must force the safety relay to interrupt the power supply to the machine or process in question. Once the power supply has been interrupted, it must not be resumed, unless the fault has been taken care of. For safety reasons double stop relays are used, which operate independently from each other and which interrupt the power supply when a fault occurs and prevent unauthorized reset according to the above. As regards the reset circuit, all components comprised in the circuit must be actively monitored for malfunction, i.e. an erroneous function of any of the components must not cause the reset circuit to reset the stop relays comprised in the safety relay. Furthermore, both stop relays must have de-energized (i.e. must have interrupted the power supply) before reset is performed. Additionally, both stop relays must energize (i.e., pull) before power supply to the machine or process in question is possible.
Another problem is this regard is that the available voltage is not always sufficient for driving the stop relays, so that they will energize when the safety relay is reset. Such problems may be due to undervoltage or energy losses in long wires.
SUMMARY OF THE INVENTION
The main object of the present invention is to provide a safety relay, the reset circuit of which may be realized with a minimum of costly components (i.e. with a minimum number of relays), where all components comprised in the reset circuit are actively monitored.
Another object of the present invention is to provide an inexpensive and yet secure reset circuit according to the above, which additionally may be used in one-channel safety relays as well as multi-channel safety relays with a constant number of essential components.
Furthermore, some embodiments of the present invention aim at providing a reset circuit, which does not require any relay at all apart from the two main relays (stop relays).
Yet another object of some embodiments is to allow reset for a safety relay, despite a condition of under-voltage or large voltage drops in long wires.
Finally, one embodiment of the present invention aims at providing an automatic reset function for the safety relay.
The objects described above are achieved by a safety relay with features according to the appended patent claims.


REFERENCES:
patent: 3749005 (1973-07-01), Einecker et al.
patent: 5689398 (1997-11-01), Miller et al.
patent: 5825102 (1998-10-01), Babiel et al.
patent: 5912795 (1999-06-01), Wetzel et al.
patent: 0 380 089 A2 (1990-08-01), None
patent: 2-61930 (1990-03-01), None
patent: 465 067 (1991-07-01), None

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