Abstract:
The invention relates to a safety switching device for connecting and reliably disconnecting an electrical load in response to a switching event from a safety transmitter. The safety switching device has at least a switching element and an operating mode selector unit for selecting an operating mode in response to a defined input signal. An input port is provided, which is connected to the operating mode selector unit such that the defined input signal can be supplied to the latter. The operating mode selector unit is designed such that it identifies the defined input signal as one from a group of different defined input signals. Responsive thereto, it selects an operating mode.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of copending international patent application PCT/EP01/03381 filed on Mar. 24, 2001 and designating the U.S., which claims priority from German patent application DE 100 16 712.8, filed on Apr. 4, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a safety switching device for connecting and reliably disconnecting an electrical load in response to a switching event of a safety transmitter. The invention particularly relates to a safety switching device having at least one output switching element and one operating mode selector unit for selecting an operating mode in response to a defined input signal. 
     The invention also relates to a method for selecting an operating mode of such a safety switching device. 
     Safety switching devices of the above-mentioned type are generally known. By way of example, the applicant offers various versions of safety switching devices under the name “PNOZ”. Safety switching devices such as these are primarily used in the industrial area, in order to connect and reliably disconnect electrically driven machines, such as a press or a milling tool. They are used in particular in conjunction with a mechanically operable safety transmitter, for example an emergency off button, to disconnect the machine quickly and safely in an emergency situation. For this purpose, the power supply to the machine to be disconnected is passed via make contacts of two electromechanical switching elements. As soon as even only one of the two switching elements opens its make contacts, the power supply to the machine is interrupted. 
     The safety switching device thus carries out the task of safely evaluating the switching event produced by the safety transmitter and, in response thereto, of operating electronic and/or electromechanical switching elements, which then disconnect the power supply to the machine. 
     Since they are used in safety-critical areas, the safety switching devices referred to here require a specific operating approval, from the responsible supervisory authorities, in many countries. For this purpose, the intrinsic fail-safe nature of the devices must be verified in accordance with the Standards that exist, such as the European Standard EN 954-1. This has a considerable disadvantageous effect on the design freedom for the design and development of safety switching devices in comparison to “normal” switching devices. In consequence, the expression “safety switching device” is in this case intended to mean only those devices which have a relevant operating approval or, in the absence of such an approval, at least satisfy the requirements of Safety Category 3 of the above-mentioned Standard EN 954-1. 
     In addition to the emergency off switches which have been mentioned, a large number of other safety transmitters can be used, for example guard door switches, light barriers etc. Owing to the different requirements and alignments of these safety transmitters, the operating mode of the safety switching device needs to be matched to the type of safety transmitter that is used. 
     In this connection, the term “operating mode” relates not only to the selection of specific types of operation of the device, for example whether (constant) steady-state or pulsed output signals are generated, but also to the selection of specific operating parameters, such as delay times of different duration for disconnection. 
     One possible way to match the devices to different requirements has simply been to provide a dedicated safety switching device for each type of safety transmitter. However, this involves increased storage requirements and problems with repair, since only that safety switching device which is respectively matched to the corresponding type of safety transmitter can be used. 
     In order to avoid this situation, the applicant has proposed a safety switching device as it is disclosed in DE 197 07 241 A1, where different safety transmitters from a set of possible safety transmitters can be connected. In this case, a typical combination of input ports and output ports to be used is assigned to each safety transmitter in the set. The way in which a specific safety transmitter is connected is unique for each safety transmitter within the set, by which means, on the basis of the connections made, the safety switching device can identify on start-up what type of safety transmitter is connected, and what function is required in the case of like safety transmitters. The safety switching device automatically selects the operating mode required on the basis of this association. In other words, the safety switching device has a plurality of input ports, and the user has the option to select one out of a number of operating modes by specifically connecting the safety transmitter to one of these input ports. 
     Although this safety switching device has been proven in practice, there is still a desire for improvement. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a safety switching device with high flexibility in terms of operating modes, but which is simpler in design. In particular, the safety switching device is intended to be capable of being produced at lower cost, while having a high flexibility. 
     It is another object to refine the safety switching device mentioned initially such that a number of operating modes can be selected by using a means with a simple design. 
     Just another object is to provide a safety switching device which capable of operating in one of a plurality of operating modes, with the selection of the operating mode desired being simple to achieve. 
     These and other objects are achieved by a safety switching device having an input, which is connected to the operating mode selector unit so that a selector input signal can be supplied to it, with the operating mode selector unit being designed such that it identifies the selector input signal as one of at least three different defined input signals, and, responsive thereto, selects one of at least three defined operating modes assigned to said different selector input signals. The operating mode selector unit can thus select one of at least three operating modes by evaluating a single signal at a single input. 
     In a method according to the invention the selector input signal is supplied to an operating mode selector unit via one input, with the operating mode selector unit identifying the selector input signal as one of at least three different predefined input signals, and selecting one of at least three predefined operating modes in response thereto. 
     The advantage of this safety switching device is, in particular, its simple design. It is sufficient to provide a single input for supplying the selector input signal. On the one hand, this allows the physical size of the safety switching device to be reduced while, on the other, it leads to cost savings. 
     A further major advantage is that there is no need for complex switching elements, which have to be designed in a redundant manner in order to achieve the required safety, for selecting the various operating modes. 
     In a refinement of the invention, the safety switching device comprises an internal signal generator, which generates the at least three different predefined selector input signals at one output at least. 
     This measure has the advantage that the at least three different defined selector input signals are always available, without any further effort. The safety switching device can thus be used without any additional generator means. Furthermore, it is particularly advantageous that, when a defective, old safety switching device is replaced, the new safety switching device is switched to the correct operating mode just by transferring the previous terminal wiring. This is very simple and convenient, and, furthermore, avoids safety-critical faults during the replacement of a defective device. In addition, this allows a defective device to be replaced more quickly than in the case of conventional selecting elements. 
     In a further refinement of the invention, the operating mode selector unit has a comparator for comparing the input signal with at least three different reference signals. 
     This measure leads to a simple design and, in particular, allows signals which are already present in the safety switching device, for example an operating voltage signal or a 0-volts signal etc., to be used as reference signals. 
     In a further refinement of the invention, a clock generator is provided, which generates a clock signal and provides this as one of the reference signals. The clock generator preferably generates two different clock signals, and provides these as two of the at least three different reference signals. 
     These measures make it possible to supply not only a signal at a constant voltage, that is to say for example at an operating voltage or at 0 volts, as the input signal, but to clock the input signal. This leads to the advantage that the input signal may assume different states, and thus also allows identification of different operating modes (more than two). From the design point of view, it is particularly advantageous to use four different input signals, namely a signal at an operating voltage, at 0 volts, with a first clock cycle or with a second clock cycle. Since the safety switching devices already have clock generator for two different clock signals in order to identify cross connections in connecting lines between a safety transmitter and the safety switching device, no additional complexity is therefore required in order to provide the reference signals. However, it is also feasible for a clock generator to be upgraded so that more than the said two clock signals are generated, thus allowing selection of more than four operating modes. 
     In a further refinement of the invention, at least one output port is provided, which is connected to the clock generator and provides one of the clock signals. 
     This measure has the advantage that a clock signal which has already been generated in the safety switching device for purposes of identifying cross connections can also be used for selecting the operating modes. It is therefore no need for any additional design measures in order to produce appropriate input signals for the operating mode selector unit. 
     In a further refinement of the invention, a control device is provided, which is connected to the operating mode selector unit and selects the desired operating mode in response to at least one output signal from the operating mode selector unit. The operating mode selector unit is preferably part of the control device. 
     It is furthermore preferred for the control device to generate an input signal for a downstream safety switching device, and to provide this at an output. This input signal may be a clock signal from the clock generator, a constant signal at a first voltage level, preferably ground, or a constant signal at a second voltage level, preferably an operating voltage. 
     These measures have the advantage that they make it possible for the control switching device to select the operating mode of the downstream, series-connected safety switching device. The downstream safety switching device can thus be matched to the type of output signal, for example a clock signal or a constant signal, from the upstream safety switching device. 
     In a further refinement of the invention, an input is provided for supplying a start signal, in order to select one out of a number of start operating modes. In this case, by way of example, possible start operating modes are “automatic start”, “manual start” or “manual monitored start”. 
     This measure allows a further improvement in the flexibility of the safety switching device, since the desired start operating mode can be selected by choosing the start signal which is supplied via the start switch. The start signal, as in the case of the already explained input signal for selection of the operating mode, may thus be a clock signal or a constant signal. 
     In a further refinement of the invention, a selection gate is provided, to which the input signal and the start signal are supplied and which supplies one of the two signals to the operating mode selector unit in response to a control signal. 
     In other words, the already explained operating mode selector unit may also be used for selecting the start operating mode, with the control device sending an appropriate control signal to the selection gate, in order to select the start signal or the input signal. The particular advantage of this measure is design simplification, and hence cost savings. 
     Further advantages and refinements of the invention can be found in the description and the attached drawing. 
     It goes without saying that the features mentioned above and the features which are still to be explained in the following text may 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 
     The invention will now be described in more detail using exemplary embodiments and with reference to the drawing, in which: 
     FIG. 1 shows a schematic block diagram of a safety switching device according to the invention; and 
     FIG. 2 shows a schematic block diagram of one channel of the safety switching device shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a schematic illustration of a safety switching device, which is designated by reference symbol  20 . The safety switching device  20  has a fail-safe evaluation and control unit  22  schematically indicated. This evaluation and control unit is formed from known components, such as those which are also used in the above-mentioned “PNOZ” safety switching device from the applicant. The particular object of this evaluation and control unit is to safely evaluate switching signals that are supplied, and to generate appropriate output signals. 
     The evaluation and control unit  22  in the illustrated exemplary embodiment has two channels, with the two channels respectively being designated by reference symbols  24   a  and  24   b . Other configurations of the evaluation and control unit  22  are, of course, also possible. A more detailed explanation of such an evaluation and control unit  22  can be found, by way of example, in the book “Maschinensicherheit” [Machine Safety], Winfried Gräf, Hüthig Verlag, 1997. 
     The safety switching device  20  also comprises switching elements  30 , which receive control signals from the evaluation and control unit  22 . This is indicated by lines  26  in FIG.  2 . 
     In the simplest case, the switching element  30  is a transistor, whose base receives the control signal and at whose emitter or collector an output signal is tapped off, in which case the output signal from the switching element may be switched to two different potentials. 
     In the present exemplary embodiment, in addition to a transistor  32 , the switching element  30  has two series-connected resistors  34  and an amplifier  36 . The collector of transistor  32  is connected to a first reference potential UB, while the emitter of the transistor  32  is connected to a second potential  0  (ground) via the series-connected resistors  34 . The input side of the amplifier  36  is connected to the node of the two resistors  34 , producing a feedback read signal at its output  37 . 
     The output signal from the switching element  30  is tapped off at the emitter of the transistor  32 , and is available at an output  38 . The output signal from the evaluation and control unit  22  is also passed via an input  39  of the switching element  30  to the base of the transistor  32 . 
     The switching element  30  is designed such that the signal which is generated at the output  38  of the switching element is switched between the first potential and the second potential, in response to the signal at input  39 . In the present exemplary embodiment, the first potential UB is generated at the output  38  when the input signal at the input  39  equals approximately this potential. When the output signal is close to the second potential, the output signal at the output  38  is likewise at the second potential  0 . 
     For reasons of clarity, the switching element  30  is illustrated in highly simplified form in FIG.  1 . However, it will be appreciated that the switching elements  30  are in a corresponding form to the design as just has been described. However, other configurations are also possible in order to achieve the described function of the switching element  30 . 
     The safety switching device  20  shown in FIG. 1 has two input port or input terminals  41 ,  42 , which are connected to the evaluation and control unit  22 . In the present exemplary embodiment, a two-channel switch  50  is connected to these two input terminals  41 ,  42 , such that the input terminal  41  has a signal S 1  applied to it, and the input terminal  42  has a signal S 2  applied to it, when the switch is closed. In this case, by way of example, the switch  50  is an emergency off switch for a machine. The switch  50  may, of course, also be a guard door monitoring switch. 
     Depending on the desired safety category, the signals S 1  and S 2  are signals at a constant potential, for example an operating voltage or 0 volts, or are connected to clock signals, in order to make it possible to detect cross connections between the two lines to the input terminals  41 ,  42 . 
     In addition to said input terminals, the safety switching device  20  also has output terminals  44 ,  45 , with in each case one output terminal  44 ,  45  being connected to the output  38  of a switching element  30 . Each output terminal  44 ,  45  thus has an associated switching element  30 , in which case the output signal from the switching element  30  may be tapped off externally via the appropriate output terminal  44 ,  45 . 
     The output terminal  44  is connected to an input circuit  46  of a relay  47 , or of a contactor or, in general, to an actuator or a further safety switching device. In the same way, the output terminal  45  is connected to an input circuit  46  of a further relay or contactor  47  or, in general, a further actuator or a further safety switching device. Make contacts  48  of the two relays  47  are in each case connected in series and are used for closing or opening a power supply line to a schematically indicated machine  49 . In the present exemplary embodiment, the machine  49  runs only when both make contacts are closed, that is to say when a signal which is sufficient for activating the input circuits  46  is produced at both output terminals  44 ,  45 . 
     The safety switching device  20  has a further input terminal or input port  43 , to which a start signal can be supplied. This start signal is generated by a start switch  52 , by creating a connection to a signal source S 3 . This signal source S 3  supplies, by way of example, a constant signal (operating voltage, 0 volts) or a clock signal. Two further make contacts  48 ′ of the relay  47  are connected in series with the start switch  52 , the two further make contacts  48 ′ are positively linked to the make contacts  48  but, in contrast to them, are in the form of break contacts. This means that the input terminal  43  is always disconnected from the signal source when at least one of the two make contacts  48  is closed. 
     The basic operation of such a safety switching device  20  is known per se, so that it will be described only briefly. 
     The safety switching device  20  has the task to close the make contacts  48  when a start signal is applied to the input terminal  43 . The machine  49  starts then. If the emergency off switch  50  is pressed in the event of an emergency situation occurring, the evaluation and control unit evaluates this switching event and actuates the switching elements  30  appropriately. The output signals are changed to the second potential (ground), so that the current flow through the two input circuits  46  of the relays  47  is interrupted, with the result that the two make contacts  48  open, and the machine comes to rest. The evaluation and control unit  22  in this case operates safely in the sense of the European Safety Standards, so that, by way of example, welded make contacts  48  of the relay  47  or, for example, a cross connection between the two input terminals  41 ,  42  is identified. Furthermore, a fault in the switching element  30  can be identified via the feedback read signal which is generated at the output  37 . 
     Further explanatory notes relating to this safety switching device  20  as well as to modifications of it and to the capability to couple such safety switching devices in a different way, for example to connect them in series, are disclosed in a copending application from the same applicant, namely PCT/EP01/02561 which was filed on Mar. 6, 2001 designating the U.S. and which claims priority from DE 100 11 211.0, which was filed on 8 Mar. 2000. The exemplary embodiments described there are included here, by reference, for sake of simplicity. 
     As can be seen from FIG. 1, the evaluation and control unit  22  has a first clock transmitter  60  and a second clock transmitter  62 . The clock transmitters  60 ,  62  each generate a clock signal, with the two clock signals having different clock periods and/or phases. The clock signals respectively generated are supplied to the channels  24   a ,  24   b  (in this case, illustrated only for the channel  24   a ) on the one hand, and, on the other, they are provided at the output terminals  64 ,  66 . The signals S 1 , S 2 , which have already been mentioned, can be tapped off at these two output terminals  64 ,  66 , and they can be supplied via switch  50  to the two input terminals  41 ,  42 . When the switch  50  is closed, the signals S 1  and S 2  are evaluated in the respective channel  24   a ,  24   b , for example, by comparing the signals each with the clock signal that is produced by the respective clock transmitter  60  or  62 . If the comparison indicates that there is no match, the switch  50  is either open or there is a cross connection between the lines, causing the machine  49  being switched off, in both cases. 
     The safety switching device  20  which is shown in FIG. 1 has a further input terminal, which is identified by the reference symbol  70 . This input terminal  70  is connected to the evaluation and control unit  22 , where it is connected not only to channel  24   a  but also to channel  24   b . The input terminal  70  is used for supplying an input signal E 1  which defines the operating mode in which the safety switching device  20  is intended to operate. For sake of simplicity, the function of this signal is explained with reference to channel  24   a  only in the following description. 
     It will be appreciated that exactly the same function is also carried out by channel  24   b , in order to achieve a two-channel design, i.e. the operating mode for both channels is selected via the input signal. 
     Dashed lines in FIG. 1 indicate the source of the signal E 1 . One option is to connect the input terminal  70  to the output terminal  64  via a wire link D 1  so that the signal E 1  is supplied from the clock transmitter  60 . The second option is to connect the input terminal  70  to the output terminal  66  via a wire link D 2 , so that the second clock transmitter  62  supplies the signal E 1 . A third option is a wire link D 3  between the input terminal  70  and, for example, a terminal  75  to which the operating voltage is applied. A fourth option, which is indicated by dashed lines in FIG. 1, is to connect a wire link D 4  between a terminal  76 , which is at 0 volts, and the corresponding input terminal  70 . This option may also be achieved, for example, by no signal being applied to the input terminal  70 . 
     In this way, it is possible to use a single wire link to supply one of four signals, which are already present in the safety switching device  20 , to the input terminal  70  and hence to the channel  24   a  and to the channel  24   b  of the evaluation and control unit  22 . As a consequence, it is also possible to select four different operating modes using a single input signal E 1 . 
     It has already been mentioned that a start signal S 3  can be supplied to the safety switching device  20  via the input terminal  43 . As in the case of the input signal E 1 , this start signal S 3  may be tapped off from the output terminals  64 ,  66  or  75  (a 0V signal cannot be identified due to internal circuitry), so that a total of three different start signals are possible without needing to add any further signal generator to the safety switching device  20 . In fact, means which are already available are used for additional functions. 
     In schematic illustration and representative for the two channels  24   a  and  24   b , FIG. 2 shows a block diagram of channel  24   a  shown in FIG.  1 . This channel  24   a  has a control device  80 , which essentially carries out the safe evaluation of the signal S 1  supplied via the input terminal  41 . Signal S 1  is transmitted via an electrical connection  81 . A signal which actuates the downstream switching element  30  is generated at the output  26  of the control device  80 , as a function of the signal S 1 . 
     A comparator unit  85  is provided for evaluation of the input signal E 1 , and the comparator unit has the task of determining the type of input signal, i.e. the task of stating which of the possible input signal types (four in the present exemplary embodiment) is being applied. One option to achieve this task is to compare the input signal E 1  with each of the possible signal types, i.e. with the clock signals from the two clock transmitters  60 ,  62 , with operating voltage UB, and with voltage 0. Four comparators  87 . 1 ,  87 . 2 ,  87 . 3  and  87 . 4  are provided for this purpose. Each of these comparators  87  is supplied with the signal E 1  on the one hand, and with one of the above-mentioned signals on the other. Thus, by way of example, one input of the comparator  87 . 4  is connected to the clock transmitter  62 , one input of the comparator  87 . 3  is connected to the other clock transmitter  60 , and one input of the comparator  87 . 2  is connected to the operating voltage UB. The four comparators  87 . 1  to  87 . 4  generate a total of four output signals A 1  to A 4 , which are supplied to the control device  80  for further evaluation. The control device uses these output signals A 1  to A 4  to decide which of the four possible operating modes is desired, and it can also select this operating mode in an appropriate manner then. The four output signals A 1  to A 4  may also be combined to form a two-bit signal, so that only two signals need to be transmitted to the control device  80 . Furthermore, it is also feasible that the function of the comparator unit  85  is carried out by the control device  80 . This can be done by appropriate configuration of the control device  80 , preferably as a microcontroller. 
     In order to improve the flexibility and to reduce the physical complexity, it is possible for the comparator unit  85  also to be used to determine the type of input signal S 3 . In order to supply only one of the two signals E 1  or S 3  to the comparator unit  85 , a changeover switch  89  is schematically indicated, which can be switched via a control signal from the control device  80  (not shown in FIG.  2 ). Three different start operating modes can thus also be selected by the appropriate selection of the signal S 3 . 
     As has already been indicated in the introduction, an operating mode comprises, for example, passing clock signals to the input terminals  41  and  42  so that a cross connection between the two supply lines can be identified. Another operating mode is for the switching element  30  to be actuated by the respective channel  24   a ,  24   b  such that a clock signal is generated at the output terminals  44 ,  45 . This is advantageous when a safety switching device  20  as shown in FIG. 1 is intended to be connected in series with another safety switching device  20 . A detailed description of such a series circuit is given in copending international patent application PCT/EP01/02561 which was filed on Mar. 6, 2001 designating the U.S. and which claims priority from DE 100 11 211.0 filed on Mar. 8, 2000. Reference is made to this corresponding description, for sake of simplicity. 
     A further operating mode can be selected when the safety switching device  20  is supplied with clock signals from an upstream safety switching device and also has to supply clock signals to a downstream safety switching device. A fourth operating mode can be selected when the safety switching device  20  receives clock signals from an upstream safety switching device  20  but has to generate a constant signal, i.e. not a clock signal, at the output terminals  44 ,  45 . This would result in four possible operating modes, which could be selected by connecting the input terminal  70  to output terminal  64 , to output terminal  66 , to output terminal  75 , or to output terminal  76  (or, alternatively, by omitting the connection). 
     Furthermore, however, other operating modes are also feasible. It is preferred for the input signal E 1  not to have to be produced just for this purpose but, instead, to already exist in the safety switching device  20 , where it needs only to be supplied to the output terminal  70  via a simple wire link. Reliability against short circuits etc. is advantageously achieved by the terminals, which are to be connected via the wire links D 1  to D 4 , not being located immediately alongside one another on the safety switching device  20 . 
     Usually, the signal E 1  is not evaluated until a check has been carried out to determine whether there are any wiring faults in conjunction with the switch  50 , and this is achieved, for example, via the different clock signals. If no wiring faults are present, the control device  80  reads the result of the comparison supplied from the comparator unit  85 , and then selects the desired operating mode. 
     FIG. 2 also shows that the control device  80  supplies a further output signal P, which is produced at an output terminal  77  to be seen in FIG.  1 . This signal P may, by way of example, form the input signal E 1  for a downstream safety switching device when a number of safety switching devices are connected in series, so that no wire links are required on this safety switching device to connect the terminal  70  to one of the terminals  64 ,  66 ,  75  or  76 . It is thus possible for the control device  80  to define the operating modes of the downstream safety switching device  20 . 
     In a further exemplary embodiment, which is not illustrated in any more detail here, an operating parameter, for example a delay time for disconnection, is selected via the input signal E 1 . In a preferred application of such an exemplary embodiment, two functionally identical input terminals  70  are provided, so that the safety switching device can in this case be supplied with two input signals E 1 . Each of the two input signals E 1  may be either 24 volts, 0 volts, a first clock signal or a second clock signal. Thus, overall, in this exemplary embodiment,  16  different delay times can be selected without any complex and expensive multi-position switches being required for this purpose. The various delay times are preferably associated with the two input signals such that the application of the two 0 volt signals results in selection of the shortest possible delay time. This means that the shortest delay time, which is also the safest with regard to emergency situations, is also selected when the input signals E 1  are not present, or disappear, as a result of a fault. This embodiment is particularly fast when it is necessary to replace the device, since there is no need to operate any selecting elements for the delay time.