Abstract:
A safety switching device for fail-safely switching on and off an electrical load, and to a system comprising at least two safety switching devices which interact in a fail-safe manner via a single-channel. The safety switching device comprises a fail-safe control unit, a first and a second electronic switching element connected with a first and a second output terminal; and at least one input terminal for receiving a first switching signal that causes a switching of said switching elements. Said first and second switching elements each comprise an output which provides depending on the first switching signal an output signal having a first or second potential. A third output terminal connects said safety switching device to a second safety switching device, providing a clocked signal depending on the first switching signal and being monitored by said control unit for performing a cross fault detection.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European Patent Application No. EP10161647 filed on Apr. 30, 2010 titled “Single-Channel Safety Output” and the disclosure of which is incorporated herein. 
       BACKGROUND 
       [0002]    The present invention relates to a safety switching device for fail-safely switching on and off an electrical load. The present invention in particular relates to a safety switching device comprising a fail-safe control unit, a first and second electronic switching element, a first and second output terminal and at least one input terminal for receiving a switching signal that causes switching off said switching elements. 
         [0003]    The invention further relates to a safety switching device system comprising at least two safety switching devices which interact in a fail-safe manner. 
         [0004]    Safety switching devices are, for instance used in industrial production environments in order to reliably and safely disconnect machines, such as a hydraulic press or a milling machine, if this is necessary for the safety of personnel or machinery. Safety switching devices typically have one or more input terminals to which input devices such as emergency stop buttons, two-hand operator buttons, guard door switches or light curtains can be connected. An evaluation and control unit evaluates the input signal from these input devices in a fail-safe manner and produces a switching signal on an output side being a function of same. The evaluation and control unit is also designed to operate in a fail-safe manner, for instance by providing redundancy. The switching signal controls actuators, in particular contactors for disconnecting the machine from a power supply. 
         [0005]    The term “fail-safe” signifies that disconnecting the machine must be ensured under all circumstances, even if a functional fault occurs within the chain comprising the input device, the safety switching device and the actuator. Safety switching devices must therefore be designed such that they ensure that the machine being monitored is always in a safe state, even in the event of the failure of one component, in the case of broken cables or other malfunctions. Accordingly, the term “safety-switching device” in this context refers only to those devices and appliances which comply at least with category 3 of European standard EN 954-1, or comparable requirements. 
         [0006]    In DE 100 11 211 B1, a safety switching device and a system of more than one safety switching device is shown, wherein the safety switching device system comprises two safety switching devices which are connected to each other via two lines. By providing two signals which are clocked in a way that they have different signals, a cross connection between those two lines can be detected by means of an additional feedback signal. With this configuration, a category 4 safety level can be achieved. 
         [0007]    This known solution, however, suffers from the drawback that in order to be able to classify the system as safety category 4, an enhanced expenditure in terminals and additional lines has to be provided. 
         [0008]    There is therefore a need to provide a safety switching device and a system of safety switching devices which allow for a cross-fault detection on the one hand and can be realized in a particularly simple and economic way on the other hand. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention provides a safety switching device that overcomes one or more of the drawbacks above. One aspect of the present invention provides a single-channel connection between a first safety switching device and a second safety switching device which transmits defined clocked pulses and, by monitoring this signal by the first switching device ensures fail safety not via the redundancy of two parallel lines, but via a monitoring of the signal. 
         [0010]    According to another aspect of the present invention, the safety switching device comprises a third output terminal for connecting the first safety switching device to a second safety switching device, the third output terminal providing a clocked signal depending on the first switching signal and being monitored by the control unit for performing the required cross fault detection. A short-circuiting between the two coupling lines cannot occur because only one line is provided and the fault connection to 24 V, 0 V, or other differently clocked lines can be detected. 
         [0011]    According to another advantageous embodiment of the present invention, the signal which is output by the third output terminal is suitable for powering the second safety switching device. This can for instance be achieved by providing an energy storage unit within the second safety switching device, such as a capacitor, which transforms the clocked pulse train of the output signal from the first safety switching device into a constant DC power supply, for instance of 5 V. 
         [0012]    According to another aspect of the present invention, the clocked signal comprises a train of defined pulses in the case that the safety switching device has a valid safety function. In a preferred aspect, this pulse train has the structure of a predetermined code that can be read by the first and/or the second safety switching device. In the case that the safety status indicates a fault, the safety switching device outputs at the third output terminal a constant 0 V signal. 
         [0013]    According to another aspect of the invention, the first safety switching device monitors the output signal and in case of any cross faults, detects the absence of the correct predetermined code signal. Preferably, the second safety device can first monitor the input signal and also recognizes if the expected code signal is not received. The pulsed output signal from the third output terminal is fed into a power supply terminal of the second safety switching device. In the case that the 0 V signal is sent to the second safety switching device, no power supply can be generated from this signal. The second safety switching device either returns to or remains in a safe status. 
         [0014]    By using a pulse code for the clocked signal which differs from all other clocked signals present in the safety switching device and the safety switching device system, according to the present invention a cross fault to other clocked signals can also be detected. In order to achieve redundancy and therefore comply with a high safety standard, the failsafe control unit of the safety switching device may comprise two redundant microprocessors which are operable to monitor the clocked single-channel output signal in a parallel way. 
         [0015]    To the accomplishment of the foregoing and related ends, certain illustrative aspects of the disclosed innovation 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 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 
         [0016]      FIG. 1  shows a schematic diagram of a safety switching device system according to the present invention; 
           [0017]      FIG. 2  shows an example of a pulse code to be transmitted on the fail-safe single-channel connection; and 
           [0018]      FIG. 3  shows a block diagram of a second safety switching device. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    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. 
         [0020]    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. 
         [0021]    In addition to the foregoing, it should be appreciated that the claimed subject matter 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 claimed subject matter. 
         [0022]    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. 
         [0023]    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. 
         [0024]    Referring now to the drawings,  FIG. 1  schematically shows a safety switching device system  100  according to a first embodiment in the most basic construction. The system comprises a first safety switching device  102  and a second safety switching device  104 . 
         [0025]    The first switching device  102  may for instance be an emergency stop device having safe semiconductor outputs at the terminals  14  and  24 . In order to allow for an enhanced number of actuating outputs, for instance in the case that several machines or machine elements have to be switched off in the case of an emergency, there may be the requirement of adding to the first safety switching device  102  either an additional safety switching device of the same kind or a contact expansion unit representing a second safety switching device  104 . 
         [0026]    These contact expansion units  104  supply additional output current paths as this is generally known, but do not posses logic units of their own for processing the signals apart from a logic for monitoring the integrity of the signals. 
         [0027]    Conventional systems, such as the one shown in DE 100 11 211 B4, use two separate connections in order to comply with the highest safety standard, category 4, as IL3. The disadvantage of such a dual channel solution can firstly be seen in the fact that an additional line and an additional terminal has to be provided, and that secondly cross faults between these two lines can occur. 
         [0028]    According to the present invention, the safety switching device  102  comprises a fail-safe single-channel output L 11 . This single-channel output L 11  can be connected via a connection line  106  to an input A 1  of the contact expansion device  104 . 
         [0029]    The required safety is ensured by transmitting a pulsed output signal on the single-channel line  106  which can be monitored internally by the safety switching device  102 . By monitoring the power strain of the clocked signal on line  106 , short circuits to 24 V and 0 V potential and connections to lines which have a differently clocked signal can be detected. Consequently, an essential requirement for fulfilling the standard category 4 can be met. 
         [0030]    The contact expansion device  104  which is connected to the fail-safe output L 11 , receives electric power from the safety output L 11  and at the same time, the status of the safety function. 
         [0031]    Although in  FIG. 1  only two safety switching devices  102 ,  104  are shown, it is clear for a person skilled in the art that an arbitrary number of devices can be connected via the inventive single-channel output. 
         [0032]      FIG. 2  shows an example of the signal  108  that is output by the inventive single-channel safety output terminal L 11 . The signal  108  may for instance be equal to a constant 0 V signal in the case that the safety function is “not true”, that is, indicates the occurrence of a fault condition. 
         [0033]    On the other hand, if the safety output is “true”, the signal  108  has a well-defined pulse pattern as shown in  FIG. 2 . This pulse pattern which represents a code can be recognized by an internal monitoring unit (not shown in the figures) of the first safety switching device  102 . As this is usually done by conventional devices, this monitoring is done in a two-channel manner by two independent microprocessors. 
         [0034]    The contact expansion device  104 , on the other hand, receives the signal  108  and processes same as will be shown with reference to  FIG. 3 . As can be derived from this circuit diagram, the second safety switching device which receives the signal at terminal A 1  splits the received signal and firstly, uses it for feeding a power storage capacitor CS and secondly, inputs the signal into the microcontrollers μC 1  and μC 2 . 
         [0035]    A DC/DC converter  110  which is connected to the storage capacitor CS generates a supply power having a supply voltage of 5 V from the input 24 V pulse train signal. 
         [0036]    The input A 2  of the second safety switching device  102  receives the 0 V ground potential which is also input into the first switching device  102 . 
         [0037]    As soon as the safety function of the first safety switching device  102  is “true”, the clocked output L 11  produces a clocked signal  108 . Consequently, connected second switching device  104  is provided with the supply voltage via the diode D 1 , the storing capacitor CS and the voltage controller  110 , representing the necessary operating voltage. The capacitor CS may for instance be an electrolyte capacitor. Any other storing means may of course also be used here. When powered by the output of the DC-DC converter  110 , the microcontrollers μC 1  and μC 2  start up. If these microprocessors now additionally detect a valid pulse code at their I/O terminals, the driver units  112 ,  114  are controlled to trigger the relays  116  and  118  for providing a switched-on status of the safety outputs  14  and  24 . 
         [0038]    Consequently, A 1  must receive a voltage different from 0 V which additionally must have a valid pulse pattern in order to cause the safety switching device  104  to output a valid output signal. With the inventive safety device topology, it is possible to fulfill the requirements of category 4 of the safety standards, although only a single-channel connection is employed. 
         [0039]    This high safety category is achieved on the side of the second safety switching device due to the predetermined dynamics of the signal and the two-channel evaluation of this signal by using both microprocessors. On the other hand, the high safety standard on the side of the sending first safety switching device  102  is achieved by means of a two channel feedback of the output signal generated at terminal L 11  whereby short circuiting to 24 V, 0 V, or to other clocked signals can be detected. 
         [0040]    Furthermore, by feeding back this output signal to both processors in a parallel way, a plausibility check can be performed in order to monitor the fault free function of the microprocessors. 
         [0041]    However, in case of a fault condition, the first safety switching device  102  outputs a constant 0 V signal at terminal L 11  because the safety function of this device is no longer “true”. Then also the safety outputs of the subsequent safety switching device  104  are switched off because, firstly, no valid clocked signal pattern is supplied to input terminal A 1  and, secondly, because the input terminal A 1  no longer receives a supply voltage which could supply energy to the microprocessors and the relays. As can be seen in  FIG. 3 , the relays  116 ,  118  can also be powered by the output from the DC/DC converter  110 . 
         [0042]    Of course, the single-channel safety output according to the present invention can also be used for connecting to any other safety input of different safety switching devices. The system is particularly suitable for connecting a safety switching device with one or more a delayed expansion modules.