Abstract:
The present invention relates to a safety switching device for actuating actuators in a fail-safe manner, and further relates to an emergency shut-off circuit comprising a safety switching device according to the present invention. The safety switching device includes at least one first and second safety input for receiving a first and second input signal, at least one first and second safety output for transmitting a first and second output signal, and a control unit for evaluating said input signals and for generating said output signals. The first and second safety outputs further comprise a feedback loop said that couples the output signals back to the control unit such that the control unit is operable to perform a test routine for testing said output signals.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European Patent Application No. EP 10001716 filed on Feb. 19, 2010 and titled “Safety Switching Device With Universal Signal Input” and the disclosure of which is expressly incorporated herein. 
     BACKGROUND 
     The present invention relates to a safety switching device for actuating actuators in a fail-safe manner, and further relates to an emergency shut-off circuit comprising a safety switching device according to the present invention. 
     Safety switching devices and, in particular, safety relays are apparatuses intended to ensure the safety of humans working in the environment of an industrial process. Safety relays are for instance used to detect the opening of emergency stop switches or other machine lock-out switches, such as interlock switches guarding a gate or limit switches. Furthermore, safety relays are also used for processing the output signals of electro-sensitive protective equipment, such as light curtains or light grids. 
     Generally, all safety devices have to be designed to meet stringent requirements defined in worldwide adapted safety standards. These standards intend to achieve high reliability, which is achieved particularly by applying redundancy, diversity and monitoring principles. Safety relays, for example, provide internal checking or fault conditions, such as jammed, welded or stuck contacts of safety switches. Moreover, safety switches, such as limit switches, which already have redundant normally closed safety contacts for use with dual channel safety relays, are additionally provided with an auxiliary contact for status indication. 
     On the other hand, electro-sensitive protective equipment normally has so-called output signal switching devices, OSSDs, for generating an output signal to be connected to an input of the safety relay. These semiconductor outputs, which in the following will be referred as OSSDs, are provided as safety switching output of protective units, such as light grids or safety laser scanners. When the protective area is violated, the safety sensor switches the OSSDs into an OFF-state. Thus, the switching off of the machine or any endangering state is initiated. As this is generally known, each safety sensor has two parallel OSSD outputs, which are evaluated independently from each other in a two-channel modus. For instance, the terminal of an electro-sensitive protective equipment is connected to a safety relay or a safety controller according to category 3 of EN 954-1 (performance level d according to EN ISO 13849-1) via two OSSD outputs. The safety sensor transmits the status information “protective field free”, which will be evaluated by the safety control device or safety relay. 
     When using the conventional safety switching devices  200  as shown in  FIGS. 6 and 8 , it has to be determined by changing the settings at a configuration unit  201 , whether semiconductor outputs (OSSD) or emergency shut-off circuits, such as protective doors or the like, are to be coupled with the safety inputs S 12 , S 22  of the switching device  200 . This is due to the fact that the semiconductor outputs of the OSSDs perform an inherent self-test regarding any short-circuits between the respective leads. With electro-sensitive protective equipment  110 , consequently, a safety input of a safety relay  200  only has to be equipped for performing a self-test of its own hardware. 
     As shown in  FIG. 6 , a conventional safety device  200  has a configuration unit  201 , for instance, comprising a switch  204 , which selects a different operational mode depending on whether the safety device  200  is connected to a light curtain  110 , having OSSD semiconductor outputs, or is used within an emergency shut-off circuit, as this is shown in  FIG. 8 . In contrast to the present invention, a cross fault monitoring is provided either by the light curtain  110 , or the input terminals S 11 , S 22 , when the configuration is set for the emergency stop operation. The safety relay  200  in the application environment of  FIG. 6  expects a static 24 V signal at the input terminals. 
     On the other hand, when connecting the safety switching device  200  with a safety shut-off circuit, the safety switching device has the task of monitoring the input conductors with respect to any possible cross-circuiting. Known emergency shut-off circuits, for instance, use clocking signals which are transmitted within the emergency shut-off circuit, as this is for instance shown in  FIGS. 8 and 9 . In this conventional arrangement, the terminals S 11  and S 21  output clocking signals of directly opposed polarity which are transmitted to the safety inputs S 12 , S 22  in an unchanged pattern, if no fault condition has occurred. This signal pattern is recognized by the safety device as a safe state. 
     However, the safety switching device  200  according to  FIGS. 6 and 8  must either have configuration means for choosing the settings in accordance with the field of application, or must have a plurality of different inputs, each configured for a different kind of application. Such configuration, however, is costly and also enhances the expenditure for installing a safety system. 
     The present invention therefore aims at overcoming the above-identified problems. In particular, an object underlying the present invention is to provide a safety switching device and an emergency shut-off circuit, comprising such a safety device, which can be used universally within different safety circuits without the necessity of setting a different configuration depending on the respective application field. 
     SUMMARY OF THE INVENTION 
     The present invention is based on the idea that the clocking safety outputs S 11  and S 21  which form the output signal for an emergency shut-off circuit, output the same pulse pattern as a conventional OSSD signal. Consequently, the safety inputs S 12 , S 22  always receive the same signal, irrespective of the kind of sensor that is connected with the inputs of the safety switching device. No changing of any settings is required. 
     According to the present invention, the safety outputs S 11  and S 21  are monitored with respect to their proper function because the input terminals do not perform any cross-circuiting monitoring. 
     According to the present invention, the safety outputs S 11  and S 21  are switched off for a short period, each at a different instant. The status of the safety output terminals S 11  and S 21  are fed back to the controller of the safety device. In case of a short-circuit or a contact to 24 Volts or 0 Volts, this fault condition is detected and the safety device switches the safety outputs into a predefined secure state. 
     To the accomplishment of the foregoing and related ends, certain illustrative aspects of the disclosed invention are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles disclosed herein can be employed, as it is intended to include all such aspects and their equivalents. Other advantages and novel features will become apparent from the following detailed description, when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of a safety device according to the present invention, when applied in an emergency shut-off circuit; 
         FIG. 2  shows a signal pattern at the output terminals of the safety device of  FIG. 1 ; 
         FIG. 3  shows a schematic diagram of the inventive safety device when being connected with a light curtain; 
         FIG. 4  shows the signals which are input from the light curtain to the safety input terminals of the safety device; 
         FIG. 5  shows a circuit diagram of a safety input; 
         FIG. 6  shows a schematic diagram of a known safety device, when being connected with a light curtain; 
         FIG. 7  shows the signal output by the light curtain shown in  FIG. 6 ; 
         FIG. 8  shows a schematic diagram of a known safety device, when being connected in an emergency shut-off circuit; and 
         FIG. 9  shows the clocked signals at the output terminals of the conventional safety device of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The innovation is now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding thereof. It may be evident, however, that the innovation can be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate a description thereof. 
     As used in this application, the terms “component”, “system”, “equipment”, “interface”, “network” and/or the like are intended to refer to a computer related entity, either hardware a combination of hardware and software, software or software in execution. For example, a component can be, but is not limited to being, a process running on a processor, or a processor, a hard-disk drive, multiple storage drives (of optical and/or magnetic storage medium), an object, an executable, a thread of execution, a program and/or a computer, an industrial controller, a relay, a sensor and/or a variable frequency drive. By way of illustration, both an application running on a server and a server can be a component. One or more components can reside within a process and/or thread of execution, and a component can be localized on one computer and/or distributed between two or more computers. 
     In addition to the foregoing, it should be appreciated that the present invention can be implemented as a method, apparatus, or article of manufacture using typical programming and/or engineering techniques to produce software, firmware, hardware, or any suitable combination thereof to control a computing device, such as a variable frequency drive and controller, to implement the disclosed subject matter. The term “article of manufacture” as used herein is intended to encompass a computer program accessible from any suitable computer-readable device, media, or a carrier generated by such media/device. For example, computer readable media can include but are not limited to magnetic storage devices (e.g., hard disk, floppy disk, magnetic strips . . . ), optical disks (e.g., compact disk (CD), digital versatile disk (DVD) . . . ), smart cards, and flash memory devices (e.g., card, stick, key drive . . . ). Additionally it should be appreciated that a carrier wave generated by a transmitter can be employed to carry computer-readable electronic data such as those used in transmitting and receiving electronic mail or in accessing a network such as the Internet or a local area network (LAN). Of course, those skilled in the art will recognize many modifications may be made to this configuration without departing from the scope or spirit of the present invention. 
     Moreover, the word “exemplary” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the word exemplary is intended to present concepts in a concrete fashion. As used in this application, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or”. That is, unless specified otherwise, or clear from context, “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, if X employs A; X employs B; or X employs both A and B, then “X employs A or B” is satisfied under any of the foregoing instances. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from context to be directed to a singular form. 
     Furthermore, the terms to “infer” or “inference”, as used herein, refer generally to the process of reasoning about or inferring states of the system, environment, and/or user from a set of observations as captured via events and/or data. Inference can be employed to identify a specific context or action, or can generate a probability distribution over states, for example. The inference can be probabilistic—that is, the computation of a probability distribution over states of interest based on a consideration of data and events. Inference can also refer to techniques employed for composing higher-level events from a set of events and/or data. Such inference results in the construction of new events or actions from a set of observed events and/or stored event data, whether or not the events are correlated in close temporal proximity, and whether the events and data come from one or several event and data sources. 
     Referring to the drawings,  FIG. 1  depicts a safety switching device  100  according to the present invention. In the particular arrangement of  FIG. 1 , the safety device  100  is connected with an emergency shut-off switch to form an emergency shut-off circuit. The emergency shut-off switch  102  comprises two sets of contacts which are mechanically linked for a dual channel operation of the safety device  100 . 
     The safety device  100  is for instance a two-channel safety relay with four external terminals, S 11 , S 12 , S 21  and S 22 , but may of course also have a large number of additional terminals, as this is well-known in the art. 
     Terminals S 11  and S 21  represent the safety signal outputs and terminals S 12  and S 22  are the signal inputs of the safety device  100  and serve to be connected to other safety devices, such as the emergency stop switch  102 . The emergency stop switch  102  comprises two sets of normally closed contacts, which are mechanically linked to one another. The output terminal S 11  is connected to +24 Volt DC and the output terminal S 21  is connected to ground. Accordingly, both poles of a signal voltage of 24 Volts DC are available at the signal output terminals S 11  and S 21 . 
     The input terminal S 12  is connected via the magnet coil of a first contactor (not shown) to ground and input terminal S 22  is connected via the magnet coil of a second contactor (not shown) to +24 Volts DC. The two contactors are used to operate safety outputs (not shown) of the safety relay  100 . In order to perform a cross fault monitoring in the circuit arrangement of  FIG. 1 , the output terminals S 11  and S 21  output a pulse train pattern as shown in  FIG. 2 , which is exactly the same as the one that is generated by the OSSD of an electro-sensitive protective equipment  110 , for instance a light curtain. 
     In order to monitor the status of terminals S 11  and S 21 , according to the present invention, a feedback connection  104 ,  106 , is provided at each output. A control unit  108  comprising at least one safety processor evaluates the measured signals and generates corresponding output signals. In case of a cross fault or a short-circuiting towards 24 Volts or 0 Volt, this fault condition is detected and the control unit  108  assigns a safe value to the output signals. 
     On the other hand, the pulse trains transmitted by the outputs S 11  and S 21  are passed through the emergency shut-off switch  102  and are received unchanged at the input terminals S 12 , S 22  for the case that neither a fault condition has occurred nor the emergency switch has been actuated. Otherwise, the safety device does not detect the expected values, when monitoring the signals at the terminals S 12  and S 22  and the control unit  108  of the safety device  100  initiates the safe status of the signals at the output terminals S 11  and S 21 . 
     Of course, the control unit  108  will advantageously also be constructed in a redundant way, as this is known to a person skilled in the art. For instance, the control unit  108  comprises two safety processors which monitor each other&#39;s proper functioning. Furthermore, the safety device according to the present invention also can be used in connection with safety shut-down mats. 
     According to the present invention, the input terminals S 12 , S 22  of the safety device  100  always expect an input signal as the one that is normally generated by the OSSDs of an electro-sensitive protective equipment  110 . Thus, as shown in  FIG. 3 , the safety device according to the present invention can also be coupled to a light curtain  110 , without changing any configurations. The input terminals S 12 , S 22  again receive the same signal in this case, not from the output terminals S 11  and S 21  of the safety switching device, but from the semiconductor outputs of the light curtain  110 , as this is shown in  FIG. 4 . 
       FIG. 5  shows a circuit diagram of a safety signal input S 12 , which is able to switch off the input signal and test the hardware down to the safety processors  108 . 
     By leaving the safety outputs or the safety device  100  at a 24 Volt static potential and by providing a regular testing with a pulse pattern for responding to conventional OSSD outputs, the safety device according to the present invention can be used for all signal generating devices, such as emergency shut-off circuits and electro-sensitive protective equipment as well as switching mats without the necessity of changing any configurations. The state of the outputs is monitored by the safety processors  108  and therefore a cross fault detection can be performed.