Abstract:
A modular safety switching system ( 10 ) with at least one output module ( 14 ) and a central control unit ( 28 ), which is connected to all output modules ( 14 ) via a bus ( 22 ). The output module ( 14 ) receives input data of at least one sensor ( 16 ) or switch ( 16   a ) and has at least one output ( 14   b ) for an actuator. The central control unit ( 28 ) is designed to receive at least a portion of the input data which is evaluated for the transferring of the output data to the output ( 14   b ). The output module ( 14 ) has its own, dedicated output module control unit ( 20 ), which evaluates the input data in collaboration with the central control unit for the purpose of transferring the output data to the output ( 14   b ). A suitable method is also disclosed.

Description:
RELATED APPLICATIONS 
     This application claims the priority of European Patent Application No. 06026381.1 filed Dec. 20, 2006, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a modular safety switching system and a safety switching method, as set forth in the preambles of claims  1  and  21 , respectively. 
     In situations when a hazard signal is present, an associated safety switching system must react flawlessly and in a predetermined way. A typical safety engineering application is to protect a dangerous machine, such as a press or a milling machine, and shut it off immediately when operating personnel approach it in an unauthorized manner. Sensors, for example a light barrier or a light grating, are used to detect such approaches. If the sensor detects a hazard, a downstream circuit must generate a shut-off signal in an absolutely reliable way. 
     In practice a single sensor does not just monitor a single machine. It is typically necessary to monitor a series of hazardous sources. In that case a commensurate number of associated sensors must be configured to define a switching event and to take appropriate measures to eliminate the hazards. In the simplest case it might suffice to react to every switching event with an OR operation. In the case of a machine protected by a light grid behind a protected door which also has an emergency shut-off switch, both the opening of the door and the interruption of the light grid or the actuation of the emergency shut-off switch will trigger a safety measure. However, such a design is not adapted to deal with more complex scenarios. 
     DE 100 200 75 C2 discloses one conventional possibility for configuring a safety switching device. This device provides a series of input and output modules. Each input module receives data from a sensor, and each output module can actuate an actuator, which switches off the hazardous source. The correct association of sensors to an actuator is based on the positions of the modules arranged in a series and unequivocally determines the associated output module. Thus, these positions define the switching rules. With a selection of modules, it can be determined which sensors are connected to an actuator according to the switching rules. This procedure eliminates the need for programming, but is not adapted to solve complex safety control problems. Therefore, this approach is limited to a narrow range of applications for simple machines. 
     In a further development of the conventional configuration the modules are connected to a control module, which includes an association table. All of the modules are connected to one another with a bus. Then logic links for deciding whether an actuator shall or shall not be switched occur in the control module. The drawback with this approach is the needed running time for the data communications. As the complexity increases, more and more computing time is needed just for communicating via the restricted data bus. 
     Such systems have limited utility for time critical applications. In addition, they have to be configured with a high degree of technical complexity and require costly, powerful microcontrollers. Even if the problem of the response time for a specific application can be solved, the solution involves an additional expense or it might be impossible to add an output module to the series of modules. 
     It is also known in modular safety control systems to communicate the status information of the inputs, the outputs and the logic results to the outside. For this, gateways can be used to couple the series of modules to the external electronics with a field bus or another bus. The status information serves to monitor the functionality of the sensors or to make the sensor data available to the downstream electronics for further evaluations. Here, too, there is the problem that the gateways access the process mapping information of a control module, which fails when the capacity of the control module no longer suffices. 
     BRIEF SUMMARY OF THE INVENTION 
     Therefore, the object of the invention is to provide an expandable modular safety switching system, which can variably process even complex arrangements of sensors and actuators. 
     This object is attained with a modular safety switching system and a safety switching method as set forth in claim  1  or claim  21  respectively. The solution has the advantage that even more complex logic links with differing priorities and relationships are possible. Moreover, it is possible to vary the effect on an actuator with respect to time, or to stop a dangerous movement at a machine with respect to time. Time critical applications are solved with a shorter response time. The user can add units without incurring any expense when, for example, the inputs or the outputs no longer suffice because additional sensors or actuators are added. If additional machine parts are to be protected at a later date, they are readily added to the already configured series of modules. 
     Since the output modules themselves control actuators assigned to them, the modular construction is promoted. The connections as well as the requisite “intelligence”, namely the logic evaluations, can be assembled modularly. Simple and complex evaluation problems can therefore be arbitrarily combined and adapted to new applications without significant effort or expense for the user. Complex monitoring problems can be solved quickly by means of parallel processing in the output module control units and the central control units. 
     In view of the foregoing, the present invention carries out evaluations needed for controlling the actuator with the shortest possible data paths in order to decrease the response time. More complex evaluations are directly assigned to the causative actuator or its output module. The evaluations are at least carried out in the output module, while the central control unit knows all pending evaluations of the safety switching system and distributes them to the central control unit itself and the output modules for parallel processing. 
     The output module control unit can preferably be configured for the evaluation. This permits a flexible use of the output module and programming for a plurality of tasks. It does not require changing the series of modules. Instead, they can simply be reconfigured. 
     The configuration is advantageously stored in a memory unit of the respective output module or in the central memory unit, which is preferably exchangeable. This significantly simplifies the configuration because it cannot be lost and it can be reinstalled by merely exchanging the memory unit. Each output module can have its own memory unit, and the configuration of the memory unit of only the central control unit, or of only some but not all of the output modules, can be exchanged. 
     The memory unit is preferably connected to the output module control unit or to the central control unit by way of a transponder. This provides a particularly simple and convenient way to exchange the memory unit and, thus, the configuration without contact. 
     The input data is advantageously input data of a monitoring sensor, which can determine the sizes of the detected objects, the locations of the objects, and/or the movements of the objects as time-dependent signal patterns. This makes it is possible to solve more challenging applications, where sensor data for actuating the actuator must be evaluated in a dynamic manner. One example of such an application is monitoring the shut-off of a machine that experiences a stopping delay, or for differentiating between the classes of events and depending on whether or not they require actuation of the actuator. Other examples include evaluation of a security camera or a light grid, where areas can be dynamically excluded from the field of protection. Such exclusions of well-known, authorized interventions can be the tools with known movement patterns or pallets that pass through. 
     The sensor is preferably a monitoring sensor such as a laser scanner or a security camera, a light barrier, a light grating or a laser scanner, an emergency OFF switch or a door switch. The actuator is a shut-off device for a machine or a warning system, and the output module control unit can be configured for evaluating one or more of the sensors and/or actuators. In this manner the series of modules covers a wide spectrum of the more customary components that are to be attached. 
     There is preferably at least one input module which has inputs for the sensors and/or the switches. Each input module is connected to at least one of the output modules with the bus or by way of configurable connections. Input modules can be tailored to the sensor that is to be attached and with its formats can facilitate simple pre-processing steps such as filtering and the like. They can also be used simply to modularly increase the number of output module connections by attaching one or more input modules. 
     The output module preferably has inputs for the sensors and/or the switches so that the output module can react to the inputs without any loss of time. For especially simple applications, the output module together with the central control unit can even replace the entire series of modules. 
     The central control unit preferably forms a control module so that based on its space requirement it can be optically integrated into the series of modules without incurring additional costs. 
     There is preferably a gateway module for connecting the input modules, the output modules and/or the control module to an external bus, in particular a field bus that is connected to an external control. This makes data of the series of modules externally available for testing or display purposes, while the external control unit can perform redundant or additional control tasks. Thus, the series of modules can be integrated in a simple way into a control design. Furthermore, data from the sensor can also be used for evaluations that are not directly related to the actuation of the actuators. This can include, for example, a display of the sensor states. 
     The external control unit is advantageously designed for the functional testing of the modules and the attached sensors. Checking the safe operation of the device is also a requirement of safety engineering. 
     The external control unit is preferably designed for externally actuating all or some of the actuators. In this case the actuation is effective only when the central control unit and/or the output module control unit of an output module assigned to the actuator approves the actuation. At least to some extent, the external control unit takes over the actuation of the actuators during normal operation. The safety switching system does not normally affect actuations needed for normal operation. If the safety switching system has detected a safety state of emergency, it can prevent the actuation because its consent is mandatory. It virtually has the last word in order to guarantee safety, but it has nothing to do with the normal operation. 
     The output module control unit can preferably generate and direct an immediately effective shut-off signal to an assigned actuator. Conversely, a switch-on signal of the output module control unit for the assigned actuator may not become effective until after an enabling signal from the central control unit and/or the external control unit. The shut-off signal is therefore very quickly sent along the shortest path in order to immediately react to a hazard. However, the output module alone cannot decide whether the problem that triggered a hazard has been eliminated. In such a case the output module waits for a signal from a higher ranking control unit. A typical example is an enabling of the system following a previous shut-off, which quite often is not authorized by operating personnel, but instead by a person responsible for the safety by using a password or a key. 
     The output module control unit is preferably designed for a diversity of or redundant evaluations that are assigned to the central control unit. This helps to accelerate the evaluation and simplifies the construction of the output module control units. A diversity of and/or redundant evaluations reduce possible errors and thereby satisfies the requirements of safety engineering. 
     The input module, the output module, the central control unit and the bus are made fail-safe in an advantageous way by employing a two-channel design. This feature is a requirement to prevent the failure of the series of modules and enhances safety. The two-channel design constitutes a robust implementation. 
     The modules are disposed in housings of the same type, each with a plug and a socket for connecting them. Such a housing is especially well adapted for modular constructions and permits a simple expansion or rearrangement at any time. 
     In one embodiment, the output module control unit and/or the central control unit can be configured with a rotary switch or an operating element. They can be mounted in the housing so that said housing can be manufactured in a compact and simple way while making the operating elements readily accessible for the user. 
     The output module and/or the central control unit advantageously includes a connection for a mobile telephone, a computer or a PDA for configuring the output module control unit and/or the central control unit with the help of a graphics surface. This allows the user to flexibly design and adapt the series of modules for special applications without the need for an extensive knowledge of their internal construction. 
     The connection is preferably a permanent connection, like a serial interface or a wireless connection, in particular by way of Bluetooth, wLAN, GSM, UMTS or infrared. These are tried and tested connections, which provide an inexpensive and reliable permanent connection or an especially user friendly wireless connection. 
     The central control unit and/or the output module control unit preferably include a web server. Web servers are available as complete solutions and offer a convenient and flexible option for the configuration with access via a conventional web browser, instead of proprietary configuration software. 
     The inventive method can be improved with analogous features set forth illustratively but not restrictively in the following dependent claims which provide similar advantages. 
     The invention is described in more detail below with respect to other features and advantages and with reference to the embodiments shown in the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a first embodiment of the safety switching system constructed in accordance with the invention; 
         FIG. 2   a  is a block diagram of an input module with configured modular communication outputs; 
         FIG. 2   b  is a block diagram of an input module with a connection to an internal bus; 
         FIG. 3   a  is a block diagram of an output module with configured modular communication inputs; 
         FIG. 3   b  is a block diagram of an output module with a connection of the sensors by way of an internal bus; 
         FIG. 3   c  is a block diagram of an output module with dedicated sensor inputs; and 
         FIG. 4  is a block diagram of a second embodiment of the safety switching system constructed in accordance with the invention with a gateway module for attachment to a field bus. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of a safety switching system  10  constructed in accordance with the invention is shown in  FIG. 1 . In this case the safety switching system  10  is constructed, for example, of four input modules  12  and three output modules  14 . 
     Each input module  12  has at least one input with one or more channels and/or with an available connection for one or more sensors  16 . The sensor can be a light barrier, a light grid, a laser scanner, a security camera, or a switch such as a door position or an emergency shut-off switch, for example. The input module  12  will be explained in more detail below in conjunction with  FIG. 2 . 
     Correspondingly each output module  14  has at least one output with one or more channels for an actuator  18 . Actuator  18  can be a robot, a motor, a power line or a machine, such as a milling machine or a press. The phrase “an actuation of the actuator  18 ” is not limited to its normal operating mode and includes special activations of the safety switching system  10  for moving the actuator into a safe state. It can involve a simple switching off as well as the generation of warning signals or a controlled movement into a safe park position. When the actuator  18  is not actuated, no stoppage is involved and the operation continues undisturbed. Output module  14  is described in greater detail below with reference to  FIG. 3 . This understanding does not prevent other actuations initiated by or via output module  14  to actuator  18 , as is described in more detail below with reference to the additional external control unit shown in  FIG. 4 . 
     The input modules  12  are connected to predetermined output modules  14 . This association depends on the application, because each actuator  18  is to be actuated as a function of the states of specific sensors  16 . 
     For example, an input module  12  can be connected to an attached monitoring sensor  16 , and a second input module  12  can be connected to an attached door position switch  16  on an output module  14 , which can place a dangerous machine  18  into a harmless state. This feature would be used in a situation in which the dangerous machine  18  is protected by a door that is to be kept closed and by a monitoring sensor. In such a case each input module  12  can also be connected to multiple output modules  14 . For example, the response of the door position switch  16  can be used to switch off the dangerous machine  18  and simultaneously trigger a warning signal via another output module or a second output of the same output module  14 . 
     Alternatively the input modules  12  can also be connected to a bus  22 , to which they transmit their data. In this case each output module  14  determines whether the data is relevant for a particular output module  14 . Irrespective of whether the data is exchanged over linked connections or over bus  22 , the correct association is configured by the user. In simple cases this can be done with rotary switches on modules  12 ,  14 . More convenient and flexible is programming with a programming tool, preferably with a graphics user surface, which is described in greater detail below. 
     Each output module  14  has a dedicated output module control unit  20  (identified in  FIG. 1  as “logic”). The output module control unit  20  evaluates the data from the sensors  16  in order to decide whether the associated actuator  18  is to be actuated. The type of evaluation can be configured. In some cases a simple switching logic is sufficient, for example when the response from just one sensor  16  is sufficient to activate the actuator  18 . 
     However, the majority of the cases are significantly more complex and require that output module control units  20  have sufficient computing capacity. It may therefore be necessary to take into account the feedback from actuator  18  which is monitored by a dedicated sensor  16  and its input module  14 . 
     Another example of complex tasks for output module control unit  20  is the monitoring of motions during which a robot or any other machine is to observe a predetermined motion pattern. If the motion is uncontrolled or too fast, the safety switching system  10  must be able to react immediately. Another case is a monitoring sensor  16  that should not react to only a detection of an object, but that classifies the object according to its size, location or movement. This makes it possible to allow specific penetrations of objects such as known motion patterns of tools or machines, the passage of a pallet, ignoring very slow movements or various manipulations of close-by objects with respect to objects that are farther away from the operating personnel. Other examples are the so-called “muting/blanking” of light barriers or light grids, the monitoring of cam shaft gears of presses or of the speed and/or stoppage of motors. Such monitoring tasks can also be performed with a security camera that delivers large amounts of data that must be evaluated in real time. The complexity increases when the camera has a range resolution capacity and generates a three-dimensional image. To this end not only a stereo camera, but also a triangulating camera or a camera that determines the running time of light, can be used. 
     To implement this more sophisticated type of evaluation, the output module control unit  20  can be configured to be application specific, so that it solves such complex safety logic function together. Such application-specific solutions can be pre-configured at the plant or they can be offered to the user for configuring the evaluation. The output module control unit  20  is therefore not only able to perform simple logic links, but can also detect more complex, time-dependent signal patterns at the modular communications inputs, can classify the signal patterns and can compare them. 
     To this end in one embodiment of the invention each output module  14  has a memory module (not illustrated in the Figure). The configuration for the evaluation is stored in the memory module. By simply exchanging the memory module, the output module control unit  20  can be adjusted to perform a completely new task. The data exchange with the memory module can take place via a plugged contact or contactless via a transponder. In an alternative embodiment just one part of the output modules  14 , or only a central control unit  28 , have such a memory module. In this case the configuration is communicated to the appropriate output modules  14  by way of bus  22 . 
     In addition to input modules  12  and output modules  14 , safety switching system  10  includes a central control module  28 . Control module  28  is connected via data bus  22  to the inputs of the input modules  12  and the output module control units  20 . The output module control units take over all of the evaluations that relate to the associated sensors  16  and the actuators  18  of their output module  14 . The control module  28  can give “master” commands, such as an emergency shut-off, to all output modules  14 . However, it can also support via bus  22  output module control units  20  for redundant or distributed relieved loads evaluations and assign tasks to the output module control unit. If, for example, the output module control unit  20  of a given output module  14  is overburdened, individual tasks can be assigned to the central control unit  28 , or another output module  14 , which then return the results. 
     Control module  28  makes possible a hierarchical control in addition to strictly modular evaluations in the individual output module control units  20 . Depending on how time critical and complex the tasks are, the evaluation tasks can then be divided between the control module  28  and the output module control units  20 . For this, evaluation rules, evaluations and partial evaluations can be exchanged between output module control units  20  and control module  28 . In addition, control module  28  can fulfill other tasks, such as monitoring the functionality of modules  12 ,  14  with their attached sensors  16  and actuators  18 . 
     The modular communication between the input modules  12  and the output modules  14  can take place, as described above, by means of a configurable association of inputs and outputs. As an alternative, however, all of the modules  12 ,  14  can be connected to an internal bus  22 . In such a case the evaluation rules of the output module control unit  20  determine the associations of input modules  12  and output modules  14  and of sensors  16  and actuators  18 . Data selected in this way is forwarded via bus  22  from an input module  12  to the associated output modules  14 . 
     Output module control units  20  of output modules  14  can exchange the evaluation rules via bus  22  and without detouring to control module  28  for their mutual configuration. Logic results, such as intermediate evaluation results, can also be transmitted. In this way a distributed or a redundant evaluation for expediting and/or enhancing security become possible. A sensor  16 , such as a security camera, which requires an especially complex evaluation, can be evaluated with multiple output modules  14  and/or their output module control units  20  with the resulting increase in computing power. 
     In one embodiment of the invention each module  12 ,  14 ,  28  is placed in a housing of a common type. The housing can be an IP20 housing system, in which the individual modules are connected together with a rear wall bus as the data bus  22 . Alternatively, and depending on the demands placed on the housing, the housing can be conformed to a higher protection class, such as IP65. Each housing is provided with a plug and a socket to interconnect them. In this way a series of modules of the desired construction and length can be plugged together in a simple manner. 
     Because of its modular construction the safety switching system  10  is readily adapted for new or changing applications. Depending on how many outputs, inputs, logic functions and/or evaluations are required, additional modules  12 ,  14  can be added to safety switching system  10 . 
     A two-step configuration is necessary in order to adapt to a given application. In a first step it is determined to which output modules  14  the input modules  12  are connected and/or which sensors  16  are to be considered when deciding which actuators  18  are to be switched. In a second step it is determined which evaluation rules each output module  14  uses for the evaluation. For less complicated configurations, rotary switches or other operating elements can be provided on the output modules  14 . 
     As an alternative, a programming tool with a graphics user surface is provided, especially for more complex configurations. The user surface offers graphical displays for a series of components, like the input modules  12  or the output modules  14 , which can be assembled and connected into the desired series on the monitor screen. The evaluation rules for the output module control unit  20  are provided for selection on the user surface. They can be linked in the desired manner on the screen and assigned to the output module  14 . 
     The connection to the user surface can be made by attaching a computer, a mobile telephone, a PDA or a similar device on which the programming tool with the user surface runs. The connection can be a permanent connection by way of a serial interface, like RS232 or USB. Alternatively, a wireless connection with Bluetooth, infrared or wLAN can be established. For a simple and user friendly operation, the desired application, that is, the choice and distribution of the specific evaluation rules among the various output modules  14 , can be automated with the programming tools. In such a case the user specifies evaluation goals on a relatively high abstraction level, and it is not necessarily seen which modules  12 ,  14  are needed, how they are to be arranged and how to be configured. 
     Depending on the experience of the user, the specific implementation in the output module control units  20  as well as the specific allocation to a specific output module  14  can also be performed incrementally and in detail based on experience. 
     In a preferred embodiment of the programming tool, a dedicated web server is implemented in each output module control unit  20 . In such a case the programming tool can be retrieved from a conventional web browser with a computer, a mobile telephone, or a PDA. Hence, a specially configured maintenance and service device is no longer necessary because the specific properties of the programming tool can be implemented in the output module control unit  20  and its web server. 
       FIGS. 2   a  and  2   b  show an input module  12 . The same reference numerals refer to the same features here and in what follows. The sensors  16  can be connected to inputs  12   a . The sensor can be a real sensor  16  or a switch  16   a . Examples thereof were given above. The illustrated input module  12  has four inputs  12   a , of which two are used. Of course, embodiments with different numbers of inputs  12   a  can be used. 
     In addition, the input module  12  has a number of module communication outputs  12   b . As an example, in the embodiment shown in  FIG. 2   a , two of the four module communication outputs  12   b  are connected to output modules  14  arranged on the left, and another module communication output  12   b  is connected to output module  14  on the right.  FIG. 2   b  shows an alternative embodiment, in which the module communication outputs  12   b  are connected to the data bus  22 . Then the assignment to the output modules  14  is determined by the corresponding output module control units  20 . 
     The inputs  12   a  can also be connected to the data bus  22 . In that case all data from sensors  16  and not just data of interest for actuating the actuators  18  is available outside input module  12 . This can be used, for example, to check the functionality of the sensors  16  or to indicate their status data. 
       FIGS. 3   a - c  show an output module  14 . Module communication inputs  14   a  on output module  14  are provided for connection to input modules  12 . In the embodiment shown in  FIG. 3   a , on the left of output module  14  are connections  14   c  to two input modules  12  and on the right a connection  14   d  to an additional input module  12 . The output module control unit  20  itself is attached to data bus  22  for communications with other output modules  14  and the central control unit  28 . In the embodiment of  FIG. 3   b , the connection to input modules  12  is also over data bus  22 , instead of over the direct modular communication inputs  14   a.    
     Output module control unit  20  receives data from the associated input modules  12  (or it selects the necessary data, which are received over the data bus  22 ) over module communication inputs  14   a . Output module control unit  20  is configured in accordance with the requirements for safety products by means of a two-channel microcontroller structure, which performs not only the input and output processing but also the logistical calculations and, if desired, supports data communication with the data bus  22 . Since the output module control unit  20  performs only evaluation tasks of its one output module  14 , inexpensive microcontrollers can be selected. Exceptions are output modules for especially complex evaluations, such as security cameras, or the provision of a “computing buffer” for the distributed evaluations together with other output modules and/or the central control unit  28 . A single-channel safety architecture with a more efficient microcontroller, which performs a variety of internal or redundant or other safety evaluations, can also be used. The appropriate safety requirements for the control category are set forth in the EN 954-1 or ISO 13849 (performance level) standards. The safety levels that can be attained and the other safety requirements required for an application are set forth in the EN 61508 and EN 62061 standards. 
     The output module control unit  20  determines in accordance with its evaluation rules and association whether actuator  18  should be actuated via output  14   b . This is indicated by a switch  18   a . The output module  14  is shown with an actuator  18  and its assigned output  14   b . It is of course also possible to connect multiple actuators  18  to an output module  14 , which is taken into consideration by output module control unit  20 . 
       FIG. 3   c  shows an embodiment of an output module  14  that is directly connected to a sensor  16  or a switch  16   a  via dedicated inputs  14   e . This permits output module  14  to simultaneously take over the tasks of input module  12 . For especially simple applications, output module  14  together with control module  28  can replace the entire row of modules. In a mixed arrangement, module communication inputs  14   a  can be next to the direct inputs  14   e  for the sensors. For specific applications the embodiment of  FIG. 3   c  allows one to dispense with one of the input modules  12 . 
     For failure-safe requirements of safety engineering, the inputs and the outputs of the modules  12 ,  14  are designed fail-safe. One robust and simple implementation of this are two-channel inputs and outputs. In an alternative embodiment, the entire system operates as a one-channel system. In such a case high reliability is guaranteed by means of redundancy or a special layout of the output module control unit  20  and/or the central control unit  28 . The switching safety requirements are known and can again be gathered from the earlier mentioned standards such as EN 954-1. If a data bus  22  is used, the bus should be correspondingly reliably designed. 
       FIG. 4  shows a further embodiment of the present invention. The difference between this embodiment and the embodiment of  FIG. 1  is the additional gateway module  24 . The safety switching system  10  can be connected to an external field bus  26  by way of the gateway module  24 . The internal data bus  22  is then coupled to field bus  26  by way of gateway module  24  for direct data access to all output modules  14  and/or their output module control units  20 , all input modules  12  and/or the data from their attached sensors  16  and control module  28 . Alternatively the data bus  22  can also be a field bus. This embodiment simplifies a distributed, decentralized system. The field bus  26  also facilitates communicating the conditions of safety switching system  10  and of all attached sensors  16  and actuators  18  to the outside for evaluating and displaying the condition externally. 
     In a further embodiment of the invention the tasks or portions of the tasks of the control module  26  are performed by an external control unit  30  connected to field bus  26 . This is primarily useful for controlling tasks that do not concern safety and relate to the normal operation of sensors  16  and actuators  18 . The external control  30  can continuously control, for example, the operation of a motor as the actuator  18  which is not relevant to the safety switching system  10 . However, as soon as an output module  14  associated with an actuator  18 , or the central control unit  28 , detects a hazard, the actuation from the external control unit  30  must remain ineffective. The actuation of the external circuit  30  therefore requires approval from output module  14  and/or central control unit  28 . Such approval can come, for example, from a switch, a logic link or a required approval signal. In that event the communication takes place between output module  14 , central control unit  28  and external control unit  30  over field bus  26  and gateway module  24  and/or internally via data bus  22 . 
     Data from the sensors  16  and actuators  18  can also be sent over these data paths to external control unit  30  for displaying the condition of the system, to configure the system or to test the functionality of the system and/or its components. 
     For an output module  14  to react very quickly to a hazard, in a preferred embodiment the output module itself switches an associated actuator  18  without any feedback from central control unit  28  or external control unit  30  because they normally do not deal with safety relevant controls. The extremely short switching path enables very fast reactions with shortest response times. However, upon startup or returning to an operating state following such a shut-off event, output module  14  cannot itself decide whether the operation may resume. The event that triggered the shut-off may have affected additional units and must have been deemed eliminated prior to resumption by at least one higher level unit such as central control unit  28  or external control unit  30 . It is also possible that a release comes from a person who is responsible for safety. As a result, a switch-on command from output module  14  has an effect on actuator  18  only after the higher level unit has generated a release signal. This can be achieved with an AND link of a first switch-on signal from output module  14  and a second switch-on signal from the higher level unit. 
     The fast direct shut-off in accordance with the last described embodiment can be advantageously combined with the above-described output module  14  that has the inputs  14   e  for sensors  16  or switch  16   a , because all of the switching paths, including those for receiving the relevant data, are then special and ensure an especially short response time. 
     The distribution of the individual logistic tasks among the output modules  14  results in small logistic units (logic evaluation modules) capable of solving even complex system problems of low technical complexity while ensuring low production costs. This results in high granularity that at any time allows use of those components that are optimal, namely sensors  16  and actuators  18 , to make use of the advantage and efficiency of the programmable modular safety switching system  10  with its simple restructuring and expansion options. 
     The present invention reduces the high costs of conventional prior art modular safety switching systems for minor tasks to a reasonable level. Thus, with just one modular system the user can solve the safety tasks of his wide machine spectrum. The spectrum extends from simple safety functions, such as a simple emergency shut-off, to complex, intertwined safety functions, such as access protection with laser scanners or stoppage monitoring of motors. It is not necessary to install a plethora of technologies, train personnel or service the technologies, which reduces personnel costs, avoids application errors and reduces the inventory. The safety switching system  10  can handle at optimal costs applications ranging from safety relays all the way to freely programmable control systems. 
     Another advantage attained with the present invention is the reduced response time of the system. Time critical input signals can be processed directly and optimally with respect to time by output module  14  and can be assigned to actuators  18 , which are directly attached. The evaluation, which output module control unit  20  has to perform, is tailored to actuators  18  and can be executed leaner and faster than a central control unit alone can. In addition, the response time is not extended when safety switching system  10  is expanded to include additional modules  12 ,  14 . 
     Due to parallel processing during the evaluation, the complexity of the system is divided between central control unit  28  and output module control unit  20 . But it is also possible to differentiate on the basis of the underlying sensor  16 . A security camera requires much local computing capacity, which is provided by one or more output module control unit(s)  20  and central control unit  28 . To a large extent time critical operations remain in the immediate vicinity of output module control unit  20 . On the other hand, there are “master” evaluations, like a general emergency shut-off, which must be assigned to the central control unit  28 . In this manner the evaluation tasks are advantageously distributed for parallel processing. 
     Even if features of the invention have been described in connection with one embodiment, the invention nevertheless includes other combinations of the described features that are possible. This is particularly true for the modules designed for exchangeability.