Abstract:
The present invention relates to a safety switching device for connecting and safely disconnecting an electrical load, in particular an electrically driven machine. The safety switching device includes first and second electromechanical switching elements whose operating contacts are arranged in series with one another between a first input terminal and an output terminal of the switching device. Furthermore, the switching device has a second input terminal for receiving a switching signal. The switching signal acts on the switch position of the operating contacts of the two switching elements. According to a preferred embodiment, the first switching element has a lower nominal switching capacity than the second switching element.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of copending international patent application PCT/EP00/10788 filed on Nov. 2, 2000 and designating the U.S., which claims priority from German patent application DE 199 54 460.3 filed on Nov. 12, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a safety switching device for connection and safe disconnection of an electrical load, in particular an electrically driven machine, having a first and a second electromechanical switching element, whose operating contacts are arranged in series with one another between a first input terminal and an output terminal of the switching device, and having a second input terminal for a switching signal, which acts on the switch position of the operating contacts of the two switching elements. 
     Safety switching devices of this generic type are primarily used in the industrial field, in order safely disconnect (switch off, shut down) electrically driven machines, such as a press brake or a milling tool. They are particularly used in conjunction with a mechanically operable emergency off button, in order to disconnect the machine quickly and safely in an emergency situation. To this end, the power supply to the machine to be disconnected is routed via the operating contacts of the two electromechanical switching elements which have been mentioned. The power supply to the machine is interrupted as soon as even only one of the two switching elements opens its operating contacts. 
     One known problem with the switching elements that are used is that the opening and closing of a operating contact when voltage is applied to it can result in sparks being formed. Depending on the magnitude of the current which is being carried via the contact, the spark formation is pronounced to a lesser or greater extent. With very heavy currents, an arc is formed between the operating contacts and, as a result of its high temperature, it can lead to the operating contacts being welded to one another. This can lead to the operating contacts remaining firmly stuck to one another, so that it is no longer possible to open the switching element. As the strength of the current to be switched increases, measures are therefore required for arc quenching. The complexity of such measures increases as the strength of the current that is to be switched increases, so that switching elements for heavy and very heavy currents are correspondingly expensive. 
     In safety switching devices of the type mentioned initially, at least two switching elements are used in series, in order to ensure safe disconnection of the power supply even if the operating contacts of one switching element remain stuck to one another as a result of having been welded. In the case of the safety switching device disclosed in DE 197 36 183 C1, by way of example, two safety relays in series are used as switching elements. 
     Until now, two switching elements have always been used in this case which have the same nominal switching capacity with respect to the same load class. The nominal switching capacity here indicates the maximum current that a switching element can switch at a specific voltage and with a specific power factor cos Φ, without being damaged. The load class defines the characteristics of the load to be switched, e.g. whether it is a purely resistive load (load class AC 1) or a somewhat inductive load (load class AC 3). Spark formation is particularly severely pronounced in the latter case. 
     The use of two switching elements with the same nominal switching capacity has the disadvantage that both switching elements are subject to the same relative loads with regard to their respective capacities. This means that both switching elements are subject to the same relative wear, and this also conceals the risk of the possibility of both switching elements failing at the same time, for example by the operating contacts welding in both switching elements in the same switching process. 
     Furthermore, use of two switching elements with the same nominal switching capacity results in the costs always rising by a factor of 2 when switching elements with a relatively high nominal switching capacity are required for switching relatively heavy currents. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to specify an alternative safety switching device which offers particularly high safety with regard to the possible welding of operating contacts when carrying heavy currents, and which is cost-effective at the same time. 
     According to a preferred embodiment of the invention, this object is achieved in that, with the safety switching device mentioned initially, the first switching element has a lower nominal switching capacity than the second switching element. 
     The new safety switching device differs from the previously known safety switching devices in that the two switching elements which are arranged in series with one another have different nominal switching capacities. This applies at least with reference to the same load class. This feature has the advantage that the switching elements used are subjected to relative loads of different severity with respect to their nominal switching capacities. This means that the wear on the two switching elements is different. Furthermore, this reduces the probability of both switching elements being subject to fail at the same time. This results in a particularly high safety margin against unobserved and dangerous welding of the operating contacts. 
     Furthermore, the measure has the advantage that the costs of a switching device for relatively heavy currents no longer rise more than proportionately. In consequence, it is possible to design a safety switching device of the type mentioned initially such that it is capable of switching heavy and very heavy currents already on its own. 
     The stated object is thus completely achieved. 
     In one refinement of the invention, the safety switching device has a timer unit which processes (delays) the switching signal such that, while connecting the load, it acts earlier on the operating contacts of the first switching element, and, while disconnecting the load, it acts later on the operating contacts of the first switching element than on the operating contacts of the second switching element. 
     This feature has the advantage that the first switching element is not switched on load during normal operation. In consequence, no sparks or arcs can be formed between its operating contacts, so that the wear on the first switching element is reduced considerably, and welding is also precluded. The first switching element thus has a long life despite its relatively low nominal switching capacity while, at the same time, the safety switching device can be designed overall for switching heavy currents. If the second switching element, which is always switched on load, were to fail as a result of the operating contacts being welded, it is sufficient that the previously “protected” first switching element can carry out one successful switching process on load, in which its operating contacts are opened. 
     In this refinement of the invention, as well, the two switching elements thus have different “life expectancies”, with the “daily load” in this case being applied to the stronger, second switching element. However, simultaneous failure of the two switching elements is in consequence once again virtually precluded. Furthermore, this refinement has the advantage that the first switching element may even be designed to have a lower nominal switching capacity than in comparable switching devices, in some circumstances. This is because the first switching element essentially needs to be able to carry out only one successful switching process on load. If its operating contacts are damaged in this switching process, this is irrelevant, since the safety switching device must be replaced in any case, owing to the defect in the second switching element. The first, very low-cost switching element thus acts as a type of fuse in this case, which can be damaged when it operates. The safety switching device in this refinement is, however, actually highly cost-effective for switching heavy and very heavy currents, since it requires only one switching element having the necessary very high nominal switching capacity. 
     In a further refinement of the invention, at least the first and the second switching element are surrounded by a common, tightly closed enclosure, from which the first input terminal and the output terminal are passed out. 
     The tightly closed enclosure is a compact enclosure which surrounds the two switching elements such that the user has no access to them. This avoids damage in the safety-relevant operating circuit of the safety switching device. The reliability and safety of the switching device with regard to faults in the installation and with regard to manipulation are thus considerably improved. The advantage of said feature becomes particularly clear in comparison to the previously practiced solutions, in which contactors, which had to be installed individually, were used in addition to the known safety switching devices in order to switch very heavy currents. In contrast to this, the said feature provides a single, compact component, which is simple to install. 
     In a further refinement of the invention, the first and the second switching element are arranged on a common component mount. 
     This feature again has the advantage that the safety and reliability of the switching device are improved, since faulty wiring is actually prevented at the production stage. Furthermore, this feature also improves the compactness and capability to use the safety switching device in modular form. 
     In a further refinement of the invention, the first and the second switching element each have at least one auxiliary contact, which is positive-guided by the respective operating contact in a mechanical way. 
     Positive-guided means that the switch position of the auxiliary contacts is necessarily or inevitably coupled to the switch position of the operating contacts, so that the switch position of the auxiliary contacts always makes it possible to reliably determine the switch position of the operating contacts without having to access the operating circuit of the switching elements. By means of such a positive-guidance it is possible to obtain a reliable statement on the switch position of the operating contacts of the two switching elements. Due to the feature mentioned, safety of the switching device is further improved, since a safe disconnection of the power supply can easily be checked, just from the position of the auxiliary contacts. 
     In a further refinement of the invention, the first switching element is a relay. 
     As is usual in the conventional specialist technology, the term “relay” relates here to an electromechanical switching element that is capable of switching low to medium current levels. In particular, such a relay has only a single pair of contacts as operating contact. The measure provides the advantage that such relays are available at low cost as standard components, so that their use reduces the costs of the safety switching device overall. This applies in particular in combination with the already described refinement, in which the first switching element is used in the manner of a fuse. 
     In a further refinement of the invention, the second switching element is a contactor. 
     According to German Industry Standard DIN 57 660, Part 103, a contactor is, to be precise, a switching element with only one rest position, which is not operated by hand, and which can connect, convey and disconnect currents under normal circuit conditions including operation overloads. In practice, the primary difference between contactors and simple relays is that the current path in the main circuit passes via at least two mutually isolated pairs of operating contacts, so that a contactor intrinsically has redundancy with respect to the operating contacts. In contrast, a simple relay has only one pair of contacts in the main circuit. In addition, contactors have integrated measures for quenching sparks and arcs. 
     The measure has the advantage that, by virtue of its nature, a contactor is very robust even when switching operations occur frequently. Particularly when used in combination with the refinement of the invention mentioned initially, the life of the safety switching device is considerably increased accordingly. Furthermore, the measure has the advantage that the main circuit of the safety switching device is closed only when it is active, since a contactor naturally trips back to its open rest position when no switching signal is present. In consequence, the safety of the switching device is further improved when using a contactor. 
     In a further refinement of the invention, the safety switching device is configured as a contact enhancing unit for connection to a preceding switching device. 
     As an alternative to this feature, it is possible to design the safety switching device as an intrinsically fully functional unit. In contrast, said feature provides the advantage that the safety switching device is required, as a modular connection appliance, only where there is actually a need to switch heavy and very heavy currents. Furthermore, numerous switching devices which have been developed on a customer-specific basis for low and medium currents can be upgraded easily and cost-effectively in this way for switching heavy and very heavy currents. This refinement of the inventive safety switching device can thus be manufactured in considerably greater quantities, thus once again allowing the costs to be reduced overall. 
     It goes without saying that the features mentioned above as well as those which are still to be explained in the following text can be used not only in the respectively stated combination but also in other combinations or on their own, without departing from the scope of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are illustrated in the drawing, and will be explained in more detail in the following description. In the drawing: 
     FIG. 1 shows a schematic illustration of a safety switching device according to one embodiment of the invention in the form of a safe contact enhancing unit, and 
     FIG. 2 shows the switching sequence, in time, for the first and the second switching element according to a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In FIG. 1, a safety switching device which is configured as a safe contact enhancing unit, is designated by reference number  10 . 
     The switching device  10  is installed in a compact, tightly closed enclosure  12 , from which a number of input terminals and output terminals are passed out. In FIG. 1, only those components of the switching device  10  that are relevant with respect to the invention are schematically illustrated. Further components of switching devices of this type, which are known per se, such as readiness indications for instance, have been omitted for sake of clarity. 
     The switching device  10  has a first switching element  14  and a second switching element  16 , whose respective operating contacts  18  and  20  are arranged in series with one another. In the present case, each of the two switching elements  14 ,  16  has three sets of operating contacts  18  and  20 , respectively, which are positive-guided one to another. Each of the two switching elements  14 ,  16  is thus able to switch three phases of a power supply  22 . Furthermore, each of the two switching elements  14 ,  16  has an auxiliary contact  24 ,  26 , which is likewise positive-guide by the respective operating contacts  18  and  20 . The auxiliary contacts  24 ,  26  of the two switching elements  14 ,  16  are likewise connected in series with one another. It is thus possible by means of a current which is passed via the auxiliary contacts  24 ,  26  (not illustrated) to check the switch position of the operating contacts  18 ,  20  of the switching elements  14 ,  16  without having to access the main circuit of the switching elements  14 ,  16  directly. 
     The two switching elements  14 ,  16  are rigidly arranged on a common component mount  28  within the enclosure  12 . The first switching element  14  is a relay, whose operating contacts  18  each have only one pair of contacts. For load class AC 3, it has a nominal switching capacity of 8 A. The second switching element  16  is a contactor, whose nominal switching capacity for load class AC 3 is 16 A. 
     The operating contacts  18 ,  20  of the two switching elements  14 ,  16  each form a current path which connects first input terminals  30  of the switching device  10  to output terminals  32 . During the installation of the switching device  10 , the individual phases of the power supply  22  are connected to the input terminals  30 . In contrast, the output terminals  32  are connected to the electrical load which is in turn to be connected and disconnected by means of the switching device  10 . By way of example, a motor  34  is shown here as an electrical load. 
     The switching device  10  furthermore has an input circuit, which has a timer unit  36 . Timer unit  36  is driven via a second input terminal  38  and an output terminal  40  with a switching signal, which acts on the switch position of the operating contacts  18 ,  20  in the manner explained in the following text. The timer unit  36  in this case delays the switching sequence of the operating contacts  18 ,  20  in the manner illustrated in FIG.  2 . 
     Starting from the second input terminal  38 , the timer unit  36  first has a diode  42  arranged in the forward direction, and whose cathode is connected to a series circuit formed from a resistor  44  and a reverse-biased zener diode  46 . The anode of zener diode  46  is connected to an input connection of a control circuit for the second switching element  16 . A reverse-biased diode  48  is connected in parallel with the second switching element  16 . The output connection of the control circuit of the second switching element  16  is connected to the collector of a transistor  50 , whose emitter is connected to the output terminal  40 . The base of transistor  50  is connected via a resistor  52  to the anode of the zener diode  46 , and to the input terminal of the control circuit for the second switching element  16 . 
     A diode  54 , whose cathode is connected to the cathode of diode  42 , is connected in parallel with resistor  44 . The anode of diode  54  is connected to the cathode of zener diode  46 . Furthermore, the anode of diode  54  is connected to the output terminal  40  via a capacitor  56 . 
     On the cathode side, the diodes  42  and  54  are connected to the input terminal of the control circuit for the first switching element  14 . On the output side, the control circuit for the first switching element  14  is connected to the output terminal  40 . A reverse-biased diode  58  is connected in parallel with the first switching element  14 . Finally, the input terminal  38  is also connected directly to the input side of the control circuit for the second switching element  16 . 
     The switching device  10  of this exemplary embodiment is used as a contact enhancing unit, which can be connected via the second input terminal  38  and the output terminal  40  to a preceding switching device, which is not illustrated here. This exemplary embodiment has been chosen for sake of simplicity, since the circuitry of a contact enhancing unit is constructed in a comparatively simple and clear manner. However, the invention can equally well be used with a complete safety switching device, to which an emergency off button just needs to be connected for operation. 
     The method of operation of the timer unit  36 , and hence of the switching device  10 , will be explained in the following text. 
     When a positive voltage signal is present between the second input terminal  38  and the output terminal  40 , diode  42  is forward-biased. In consequence, a current flows from the second input terminal  38  via diode  42  and through the control circuit for the first switching element  14  to the output terminal  40 . In consequence, the first switching element  14  is activated, that is to say the operating contacts  18  are closed. At the same time, the positive guidance results in the auxiliary contact  24  being opened. Furthermore, the current also flows from the second input terminal  38  via resistor  44  to capacitor  56 , which is charged in consequence. As soon as the voltage across capacitor  56  exceeds the breakdown voltage of the zener diode  46 , this zener diode  46  carries current, and, in consequence, a base current flows via resistor  52  through transistor  50 . In consequence, transistor  50  is in turn switched on, so that a current can now flow through the control circuit for the second switching element  16 . As a consequence of this, the second switching element  16  is also activated, that is to say the operating contacts  20  are closed, and the auxiliary contact  26  is opened. In this state, the current paths between the first input terminals  30  and the output terminals  32  are closed, so that motor  34  is supplied with power. 
     For the following description, it is assumed that the timer unit  36  has been connected to voltage for a sufficiently long period to allow the capacitor  56  to be charged. If the voltage between the second input terminal  38  and the output terminal  40  is now removed, the second switching element  16  drops back to its passive state. In consequence, the operating contacts  20  are opened at the same time, and the auxiliary contact  26  is closed. As a consequence of this, the power supply to the motor  34  is interrupted abruptly. 
     Furthermore, owing to the charged capacitor  56 , diode  54  is forward-biased, and capacitor  56  is discharged via the control circuit for the first switching element  14 . This first switching element  14  is thus still held in its active state for a certain time, that is to say the operating contacts  18  still remain closed for a certain time period. As soon as the voltage across capacitor  56  falls below the tripping voltage of the first switching element  14 , the operating contacts  18  also trip, so that the power supply to the motor  34  is interrupted at the latest at this time, even if one or more operating contacts  20  in the second switching element  16  were still to stick to one another. Furthermore, the current path via the two auxiliary contacts  24 ,  26  is now closed, thus allowing a reliable conclusion to be drawn that the motor  34  has been disconnected, on the basis of the positive guidance. 
     The diodes  48  and  58 , which are arranged in parallel with the two switching elements  14 ,  16 , are used in a manner known per se for supplementary spark quenching. 
     The timer unit  36  ensures that the operating contacts  18  of the first switching element  14  are always closed earlier, during connection of the power supply for the motor  34 , than the operating contacts  20  of the second switching element  16 . Conversely, the operating contacts  20  of the second switching element  16  are always opened earlier during disconnection of the motor  34 , than the operating contacts  18  of the first switching element. 
     These timings are shown in FIG. 2 in the form of three timing diagrams in which U s  denotes the switching signal between the second input terminal  38  and the output terminal  40 . As can be seen from the illustration, the pull-in voltage U 1  for the operating contacts  18  of the first switching element  14  occurs at a time advanced by a time interval T 1  before the pull-in voltage U 2  for the operating contacts  20  of the second switching element  16 . Conversely, the pull-in voltage U 2  for the second switching element  16  falls by a time interval T 2  earlier than the pull-in voltage U 1  for the first switching element  14 . In addition to the two time intervals T 1  and T 2 , FIG. 2 also shows delays T v , which are dependent on the switching times, between the switching signal U s  and the pull-in voltages U 1  and U 2 . 
     It will be appreciated that the term “connection” for the purposes of the present invention denotes a voltage rise from a magnitude below the tripping voltage of the two switching elements  14 ,  16  to a magnitude above the pull-in voltage of the two switching elements  14 ,  16  within a time which is short in comparison to T 1 . Conversely, the term “disconnection” denotes a drop in voltage from above the holding voltage of the switching elements  14 ,  16  to a value below the tripping voltage of the switching elements  14 ,  16  within a time period which is short in comparison with the time interval T 2 . In fact, the switching signals shown in FIG. 2 do not have infinitely steep rising and falling flanks. 
     In a further exemplary embodiment of the invention, which is not illustrated here, the safety switching device  10  is a fully functional stand-alone appliance which, in addition to the components described already, has its own voltage supply. The switching device of this exemplary embodiment uses the voltage supply to produce a voltage signal, by means of which the switch position of a passive emergency off button can be checked. The operating contacts of the two switching elements  14 ,  16  are then driven via a circuit, which corresponds to the timer unit  36 , as a function of a switching signal obtained from this.