Abstract:
A sensor for safety apparatuses for elevator apparatuses which can move a cab by means of a drive, the sensor including a contact bridge and a contact receptacle for receiving the contact bridge, said contact bridge being arranged in such a way that the closing state of the elevator door can be determined by connection of the contact receptacle and the contact bridge. In order to be able to improve in particular the susceptibility to maintenance, the sensor is in the form of an optical sensor which comprises a transmitter for transmitting an optical signal and a receiver for receiving the optical signal, the transmitter and the receiver being arranged on the contact receptacle and the contact bridge comprising at least one transmission element for transmitting the optical signal.

Description:
FIELD OF THE INVENTION 
     The invention relates to a sensor for safety apparatuses for elevator apparatuses, and a safety apparatus for elevator apparatuses. 
     BACKGROUND OF THE INVENTION 
     The prior art has disclosed conventional safety apparatuses for elevators which use electrical or electromechanical contacts and switches in order to determine the locking or closing state of an elevator door. The intention here is for an elevator cab to only be allowed to travel when all the doors are locked. 
     The object of the invention is to propose a sensor, a safety apparatus and an elevator apparatus in which the susceptibility to the need for maintenance can be improved. 
     SUMMARY OF THE INVENTION 
     By virtue of the measures mentioned hereinafter, advantageous developments and embodiments of the invention are possible. 
     Correspondingly, a sensor according to the invention for safety apparatuses for elevator apparatuses which can move a cab by means of a drive is characterized by the fact that the sensor is in the form of an optical sensor which comprises a transmitter for transmitting an optical signal and a receiver for receiving the optical signal. A particularly advantageous feature of the sensor according to the invention is that it can operate in contactless fashion, i.e. without any wear as well. In addition, the sensor according to the invention therefore does not have any live contact areas, or only has a few live contact areas, and is furthermore safe during fitting. The sensor according to the invention can therefore replace a conventional switch, a so-called interlock switch, from the prior art. 
     In addition, the sensor according to the invention provides the possibility of there being no need for the circuit to be interrupted, in contrast to an electromechanical switch. Advantageously, by virtue of the sensor according to the invention, it is also possible to avoid a defect which can occur, for example in the case of electromechanical sensors and contacts, as a result of contact erosion owing to flashover during opening or closing of the electrical contacts and can ultimately result in a loss of function. Consequently, the susceptibility to the need for maintenance can be improved by the sensor according to the invention. 
     Owing to the fact that the circuit does not need to be interrupted with a sensor according to the invention, in contrast to a switch, improved diagnosis in the case of defects is advantageously possible. 
     According to the invention, a contact bridge and a contact receptacle for receiving the contact bridge are provided which are arranged in such a way that the closing state of the elevator door can be determined by connection of the contact receptacle and the contact bridge. The detection state of the sensor is therefore dependent on the proximity of the contact bridge to the contact receptacle. 
     An elevator itself generally has firstly a cab which can be moved between individual stories or floors. The individual floors each have shaft openings, in the region of which the cab can be moved into a stop position when said cab is intended to travel to the corresponding floor. In this stop position, access to the cab is then enabled. This access can be enabled by virtue of the elevator doors being opened and then closed again and locked prior to continued travel of the elevator. Elevator doors may be shaft doors or cab doors. The shaft doors are fitted or mounted movably on the shaft itself in the region of the shaft opening. The cab doors are in turn fitted or mounted movably on the cab. In general, in each case one cab door is assigned to a shaft door, with the two being arranged so as to overlap one another (so as to overlap one another at least partially) in the stop position. Generally, said doors are also moved at least substantially synchronously. 
     In order that a journey in the cab can be begun or that the cab can continue to travel, it is necessary for all of the doors to be closed and locked. This can be checked by means of corresponding safety apparatuses which can possibly stop the drive by means of an interrupter apparatus. In principle, the interrupting apparatus or interruption circuit can activate the monitoring unit, i.e. controller or regulator, of the motor or the drive, with the result that said monitoring unit stops the drive; it is also conceivable for the interrupting apparatus to directly interrupt the power supply to the drive/motor. 
     The corresponding sensor is therefore designed to check whether the corresponding door of an elevator or a shaft is open or closed and locked. In the present case, it is particularly advantageous for the sensor to have a similar design to a plug-type connection, with the result that a contact bridge can engage in a contact shaft. In addition, this measure provides the possibility of an apparatus which is mechanically very stable. In principle, a sensor according to the invention can be designed in such a way that the contact bridge is accommodated in the shaft of the contact receptacle with play or in interlocking fashion. 
     In addition, according to the invention, the contact bridge is designed in such a way that it comprises at least one transmission element for transmitting an optical signal. This advantageously makes it possible in particular to achieve a so-called failsafe circuit. Only when the contact bridge has reached a particular position by corresponding connection to the contact receptacle when the door is closed, is it possible for corresponding enabling for travel to be issued. In the case of simply a light barrier, this would in principle not be the case: the transmission element can be designed in such a way that the transmission of the optical signal takes place in a particular way which can only be manipulated with considerable difficulty and can also not readily be realized by accident. In the case of a simple light barrier, for example, which would be interrupted when the door is closed, this would mean that the drive would also be enabled when, for example, a corresponding object, a fly or the like interrupts the light barrier. 
     A further option is to arrange the transmitter or the receiver on the contact receptacle. The transmission of light by means of the transmission element can then only take place via the contact bridge. This design enables a particularly compact construction. 
     One possibility consists in designing the transmission element as a reflective surface, with this reflective surface then reflecting the optical signal or the light and only in this way guiding it onto the corresponding receiver. The reflective surface can be arranged in a notch in the contact bridge, for example. However, it is also conceivable for the transmission element to be an optical medium. It is conceivable, for example, for the light refraction to be utilized in the transition from the air into this optical medium and the light beam is therefore directed in a certain direction, with the result that only then is it guided either onto the receiver or not onto the receiver. 
     In one embodiment of the invention, a fiberoptic conductor can be provided as optical medium. The optical signal is transmitted when the light from said signal is coupled into the fiberoptic conductor, propagates through the fiberoptic conductor and passes into the receiver via the fiberoptic conductor. 
     It is particularly advantageous to design the transmitter as a light-emitting diode and/or the receiver as a photodiode. Particularly cheap standard electronic components can be used; this results in the possibility of particular cost savings. 
     In order that no parasitic light passes accidentally from the transmitter into the receiver, an isolating element, in particular an isolating web for optically isolating the transmitter and the receiver can also be provided. This once again reduces in principle the possibility of errors occurring as a result of an incorrect interpretation of the signals. In addition, a diffuser can moreover also be provided, said diffuser distributing parasitic light diffusely. It is also conceivable for the receiver to be set during the detection to a certain threshold value as regards the intensity of the incident light, with the result that, in the case of a certain amount of parasitic light which possibly enters the receiver, a corresponding sequential signal which should only be resolved when light enters the receiver via the transmission element is nevertheless not triggered. 
     In order to be able to once again increase the safety of the apparatus, one embodiment can be configured such that the probability of accidental reflection, by means of which the signal passes from the transmitter and to the receiver in the contact receptacle, is as low as possible. One possibility for this consists in that the signal from the transmitter cannot be passed to the receiver by means of a single reflection, i.e. a special optical path is required. In particular, the transmitter and the receiver are arranged in such a way that the signal from the transmitter needs to be passed to the receiver by means of at least two reflections. A combination of reflections and coupling into a fiberoptic conductor is also possible, with the result that the light is reflected and then needs to propagate through a fiberoptic conductor over a certain distance. Special coupling-in, for example through a lens, may also be required. It is also conceivable for the sensor to be shielded in such a way that the reflection in practice cannot be accidental, such as, for example, in the case of the concept of a shaft with a lug to be inserted therein. In addition, there is the possibility of setting the threshold value in the receiver to be so high that only a corresponding reflection by a reflection unit results in the production of a signal by means of which the threshold can be exceeded. 
     In particular, the contact bridge is designed to pass the signal from the transmitter to the receiver by means of 4 reflections. 
     Moreover, it is also conceivable in principle for the contact receptacle to comprise transmission elements for transmitting the optical signal, for example reflective surfaces or optical media such as fiberoptic conductors, for example. It is conceivable for a subsection of the propagation path of the optical signal from the transmitter to the receiver to be over a reflective surface or through a fiberoptic conductor in the contact receptacle. It is also conceivable for the fiberoptic conductor in the contact receptacle or in the contact bridge to be shifted, by virtue of the contact bridge being received, in such a way that transmission of the light is enabled. 
     A connection can be produced so as to be particularly robust, for example, in which the contact receptacle comprises a shaft and the contact bridge comprises a tongue-shaped lug, which engages in the shaft during connection of contact bridge and contact receptacle. It is particularly advantageous here also that corresponding coding can be performed, i.e. the contact bridge, in a similar way to a key, needs to be provided with a particular design in order for it to be able to enter the contact receptacle. In particular, this can increase the safety of this apparatus, in particular when the contact receptacle shaft is designed in such a way that it is not possible for a hand to gain access. 
     In a particularly preferred embodiment of the invention, at least two transmission elements are provided which are arranged in series in the movement direction of the contact bridge, which means that, when the door is locked, the contact bridge dips correspondingly into the contact receptacle and is initially visible for the optical signal and the optical light beam of one of the transmission elements (namely the first transmission element in the movement direction). As the movement progresses, the next transmission element then becomes visible, while the previous transmission element is pushed out of the optical path. It is thus possible for a plurality of optical signals to occur with a temporal offset. In addition, it is conceivable to design an associated evaluation unit or electronics unit in such a way that the occurrence of the corresponding signals is determined as a function of time, for example. It is thus possible for conclusions to be drawn in respect of the speed of the locking process. This also makes it possible to draw a conclusion in respect of the functional and maintenance state of the locking device of the doors. In principle, the locking process and not the door lock is moreover monitored. Depending on the way in which the corresponding transmission elements are arranged and how many of the transmission elements are arranged, the precision of such a determination can possibly be increased. 
     Furthermore, in one embodiment of the invention, the sensor can comprise an electronics unit for the evaluation of the receiver, said electronics unit being designed to interpret the evaluation of the receiver in one of the switching states and/or into an electrical signal. This means that the electronics unit is designed to generate an electrical signal, namely depending on what is detected by the receiver. Depending on the detection by the receiver, it is necessary to produce or possibly maintain a switching state. A switching state can be changed, for example, by producing an electrical signal depending on the detection by the receiver. 
     Since, moreover, the mechanical closing state is detected purely optically, this means that it is not absolutely necessary for a mechanical contact or a mechanical opening state to be produced in order to produce an electrical signal. It is conceivable, for example, for the optical signal to enable the receiver, for example a photodiode, and therefore for it to be possible for a conduction state to be reached (in contrast to an interruption). As a result, an interpretation of the switching state of the sensor is performed electronically to a certain extent. However, the electronics unit can also additionally be designed to enable a connection to further electronics. For example, it can also be designed to enable a connection to a bus. This design makes it possible in particular to improve the relatively low susceptibility to the need for maintenance even further since mechanical contacts and sensors are substantially avoided. It is also particularly advantageous that it is merely necessary for the contact bridge to enter the contact receptacle as the mechanical contact closure. 
     In one development of the invention, the electronics unit is for communication with a switching unit, in particular for transmission of switching states and/or identification signals. The switching unit is a component part which can be used to open or close a line by virtue of a switching operation, in a similar way to in the case of a relay or contactor. However, the switching operation is triggered when a corresponding signal or a corresponding item of information is passed on to the switching unit from the sensors. In particular, it is advantageous that the line between the switching unit and the sensor no longer needs to be interrupted, as is often the case in the case of a contactor/relay, for example. 
     The electronics unit can in particular be arranged in or on the contact receptacle in which the transmitter and receiver are also arranged. The contact receptacle can be arranged, for example statically, in the elevator apparatus, while the contact bridge is arranged on a moving part and merely represents the “key” in order to enable signal transmission in the contact receptacle. 
     In a particularly preferred development of the invention, precisely two terminals are provided which are used firstly for power supply and secondly for communication with the electronics unit. The same line which is also used for power supply is therefore used for the communication. This measure enables a particularly compact and inexpensive design. In addition, this means that no additional lines or terminals need to be laid during retrofitting, when a conventional sensor is replaced by a sensor according to the invention, for example. This measure enables in particular good retrofittability. 
     In addition, in one embodiment of the invention, the communication can take place via modulation of its internal resistance of the sensor. In the circuit with the switching unit, the voltage and/or the current intensity can thus be modulated depending on the circuitry. This modulation then carries the information which is intended to be transmitted during the communication. For example, a circuit which comprises sensors connected in series and a switching unit (likewise connected in series) is conceivable. If the resistance of a sensor in the case of sensors connected in series is changed, the current intensity changes. If, for example, a constant current source is used for the circuit, a change in the resistance has the effect that the voltage needs to be increased in order to compensate for the resulting reduction in the current intensity which is initially caused by the lower resistance. The modulation can therefore act as an information carrier. The changes in the current intensity or voltage can be measured and can be interpreted as information. 
     In particular, the sensor is intended to be used in a series circuit. In the case of a series circuit, it is particularly advantageous if the sensor has a low transfer resistance. The resistance of a sensor can be, for example, in the range of from greater than 0 ohm to 100 ohms, in particular in the range of from greater than 0 ohm to 20 ohms, preferably than 10 ohms, in particular less than 5 ohms. Precisely in the case of a series circuit, it is advantageous to design the transfer resistance to be as low as possible, in particular lower than 5 ohms, in order that the voltage drop across the sensor is not excessively high. 
     Correspondingly, in addition a safety apparatus for elevator apparatuses which can move a cab by means of a drive, with an interrupting apparatus for interrupting the drive and with at least one sensor, which can be switched between at least two switching states depending on the closing state of the elevator door, and the interrupting apparatus being designed to interrupt and/or continue the drive depending on the switching state of the at least one sensor, is characterized by the fact that a sensor according to the invention is used. The interrupting apparatus serves the purpose of interrupting the drive, with the interruption being dependent on the nature of the switching states of the corresponding sensor, i.e. on whether all of the doors are actually locked. This measure makes it possible to correspondingly improve the susceptibility to the need for maintenance and to increase the safety of the elevator. 
     The interrupting unit can, for example, also be designed as a switching unit which has a communication unit in order to be able to perform a switching operation in the case of a corresponding signal of the sensor. 
     If a plurality of doors is provided, travel can only be begun or continued when all of the doors are locked. 
     Correspondingly, it is expedient if the corresponding sensors which are each assigned to a door are connected in series. 
     In order to be able to establish which sensor is affected, for example in the case of a door being blocked or in the event of a defect, it is advantageous to provide an indicator apparatus which indicates the switching state of the individual sensors and at the same time enables assignment to the individual sensors. 
     In addition, the invention claims an elevator apparatus which comprises a safety apparatus in accordance with the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention are illustrated in the drawings and will be explained in more detail below with further details and advantages being given. 
       Specifically, in the drawings: 
         FIG. 1  shows a sensor comprising a contact bridge with reflective strips and a contact receptacle in accordance with the invention, 
         FIG. 2  shows a contact receptacle in accordance with the invention, 
         FIG. 3  shows a contact bridge with reflective strips in accordance with the invention, 
         FIG. 4  shows a sensor comprising a contact bridge with a fiberoptic conductor and a contact receptacle in accordance with the invention, 
         FIG. 5  shows a contact receptacle in accordance with the invention, as in  FIG. 2 , 
         FIG. 6  shows a contact bridge with a fiberoptic conductor in accordance with the invention, 
         FIG. 7  shows the connection (temporal sequence) of the contact bridge and contact receptacle in accordance with the invention, 
         FIG. 8  shows a sensor with reflective strips in accordance with the invention, 
         FIG. 9  shows a safety apparatus with sensors in accordance with the invention, 
         FIG. 10  shows a safety apparatus with a safety circuit in accordance with the invention, 
         FIG. 11  shows a safety apparatus with a bus in accordance with the invention, 
         FIG. 12  shows a safety apparatus with a bus and an integrated contactor in the switching unit in accordance with the invention, 
         FIG. 13  shows a circuit diagram for an elevator in accordance with the invention, 
         FIG. 14  shows a sensor with fiberoptic conductors in accordance with the invention, 
         FIG. 15  shows a perspective view of the sensor shown in  FIG. 14 , and 
         FIG. 16  shows a schematic illustration illustrating the way in which communication with individual sensors takes place in a safety apparatus in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a sensor  1  with a contact receptacle  2  (shaft) and a contact bridge  3 , the contact bridge having reflective strips  9 , which reflect light emitted from a transmitter of the contact receptacle  2  in the direction of a receiver of the contact receptacle  2 . 
     In turn,  FIG. 2  shows the corresponding contact receptacle  2  with a transmitter  4  and a receiver  5 , with an isolating web  6  being arranged between said transmitter and said receiver, to be precise in a front view, a side view and a plan view. Fitting apparatuses or fitting aids are indicated by the reference symbol  7 . The contact receptacle  2  has additional electrical terminals, via which the sensor  1  can be connected to the rest of the sensor apparatus or to the safety circuit. 
       FIG. 3  shows a contact bridge in different views, to be precise in a front view, a side view and a plan view. Said contact bridge also comprises corresponding fitting aids  8 . Slots are incorporated into the contact bridge  3  as transmission elements  9 , said slots each having reflective surfaces. In total there are three reflection units  9   a ,  9   b ,  9   c , with the result that dynamic contact detection is enabled to a certain extent since first the reflection unit  9   a , then the reflection unit  9   b  and finally  9   c  enter the contact receptacle  2  or the optical path when the contact bridge  3  is inserted and therefore dynamic measurement of the signal with temporal dependence is possible. 
       FIG. 4  shows a sensor  1 ′ with a contact receptacle  2  (shaft) and a contact bridge  3 ′, the contact bridge having a fiberoptic conductor; the light emitted by a transmitter of the contact receptacle  2  passes into the fiberoptic conductor inlet  4 ′, propagates through the fiberoptic conductor and emerges from the fiberoptic conductor outlet  5 ′ again, with the result that it passes to the receiver of the contact receptacle  2 . 
     In turn,  FIG. 5  shows the corresponding contact receptacle  2 , as has already been described in relation to  FIG. 2 , said contact receptacle also being suitable for a sensor  1 ′ with a fiberoptic conductor. 
       FIG. 6  shows a contact bridge  3 ′ in different views, to be precise in a front view, a side view and a plan view. Said contact bridge also comprises corresponding fitting aids  8 . A fiberoptic conductor is incorporated into the contact bridge  3 ′ as transmission element L and the light signal transmitted by the contact receptacle can propagate through said fiberoptic conductor. The figure also shows the light inlet  4 ′ and the light outlet  5 ′. 
       FIG. 7  shows the contact bridge  3  (with reflective strips) entering the contact receptacle  2  in this way, with the contact bridge not yet being connected to the contact receptacle  2  in situation A. In situation B, the reflection unit  9   a  has just entered in the region of the optical path and transmits the light path from the transmitter to the receiver. In situation C, the contact bridge  3  is positioned at this time such that the optical signal is interrupted since the contact bridge  3 , in terms of its height, is precisely between the reflection units  9   b  and  9   c  and the optical path is therefore interrupted. Only in situation D is the contact bridge, which has been completely inserted into the contact receptacle  2 , in such a position that the optical path is not interrupted and light can pass from the receiver  4  into the detector/photodiode via the reflection element  9   c . The reflection units  9 , and also other transmission units such as optical media, can have different forms and provide characteristic reflections or light transmissions, with the result that these can each be identified, if appropriate, by means of the receiver or the electronics unit as well. 
       FIG. 8  shows a similar illustration in which the contact bridge  3  enters the contact receptacle  2 . 
     In turn,  FIG. 9  shows a safety apparatus with a plurality of optical sensors  10 , which are all connected in series. Furthermore, a series of further electromechanical normally closed switches  11  is provided which can otherwise be used in connection with an elevator. In addition, a voltage source  13  is provided. All of these switches or sensors  11  and  10  are connected in series and are connected to a switching unit  12 . This circuit comprising a series circuit comprising the switches  11 , the sensors  10  and the switching unit  12  forms a safety circuit. If one of the switches  11  is interrupted, the entire circuit is interrupted, and the switching unit  12  disconnects the motor M, which represents the drive for the elevator cab. The switches  11  can be normally closed switches of the known type. If one of the sensors  10  detects that the elevator has not been locked properly, for example, said sensor transmits a corresponding signal via the circuit, and this signal is received by the communication unit of the switching unit  12 , with the result that said unit can disconnect the motor M. Correspondingly, the switching unit  12  partially takes over the function of your relay; in addition, switching operations of the switching unit are also dependent on signals from the sensors, however. The switching unit  12  therefore does not only respond to line interruptions. 
       FIG. 10  shows a safety apparatus with a safety device, namely a (second) safety circuit  14 , with corresponding optical sensors  10 . This safety circuit is connected to the first safety circuit  16  via a switching unit  12 ′, said first safety circuit in turn having further sensors  11 . The switching unit  12 ′ is similar to the switching unit  12  and has the same mode of operation; in this case, in contrast to the switching unit  12  shown in  FIG. 9 , however, the voltage source is also integrated in the switching unit  12 ′. A contactor/relay  15 , which can in turn disconnect a drive M, is located in the first safety circuit  16 . The contactor  15  is merely designed to disconnect the motor M in the event of a line interruption in the circuit  16 . If one of the sensors  10  is interrupted interrupted optically, the switching unit  12 ′ is also interrupted, and therefore the line in the first safety circuit  16 . The contactor  15  disconnects the motor M. Instead of the conventional normally closed switches, the sensors according to the invention are combined in a dedicated safety circuit  14  and are connected to the original, first safety circuit  16  via the switching unit  12 ′. The safety circuit  16  can in this case partly use the wiring of the original safety apparatus. 
     In addition,  FIG. 10  illustrates how retrofitting of a conventional apparatus can be performed by virtue of the original first safety circuit  16  being capped at the points U and the second safety circuit  14  with the switching unit  12 ′ being used correspondingly. It is then only necessary for a relatively long cable K to be drawn in. 
       FIG. 11  shows a corresponding apparatus in which, instead of a second safety circuit, a bus  20  is arranged as the safety device. The corresponding sensors  21  have an electronics unit which enable a connection to the corresponding bus  20 . The bus is likewise connected to a switching unit  25 , with the result that when one of the optical sensors  21  is interrupted, said sensor in turn transmits a signal to the switching unit  25  which in turn interrupts the first safety circuit  26 . Owing to the interrupted line of the safety circuit  26 , the motor M is disconnected by the contactor  15 . The switching unit  25  can form the master in the bus, for example, while the sensors  21  have a slave configuration. 
       FIG. 12  shows a similar apparatus to that shown in  FIG. 8 , but in this case the contactor  15  is additionally integrated in the switching unit  27 , with the contactor disconnecting the motor, if appropriate. 
       FIG. 13  shows an exemplary circuit diagram  30  for an elevator in accordance with the invention. 
       FIG. 14  shows a sensor  41  in a plan view and in a side view with a contact receptacle  42  and a contact bridge  43 , in which a fiberoptic conductor  44  is arranged. In this case, the contact bridge  43  is overall in the form of a fiberoptic conductor  44 , i.e. consists of the corresponding optical medium. The contact receptacle  42  comprises a transmitter  45  and a receiver  46  for transmitting/receiving optical signals. The optical signal transmitted by the transmitter  45  can propagate through the fiberoptic conductor  44 , as soon as the contact, receptacle  42  has received the contact bridge  43 , and therefore passes into the receiver  46 . The contact bridge  43  (or the fiberoptic conductor  44 ) is in the form of a U and, when it is plugged into the contact receptacle  42 , engages only with both limbs in the two shafts of the contact receptacle  42 . The fiberoptic conductor  44  correspondingly likewise has a U-shaped design.  FIG. 15  in turn shows the sensor  41  in a perspective view. 
       FIG. 16  shows a schematic illustration of the communication in the safety circuit  14  between the controller  57  of the switching unit and the individual sensors  10  or microcontrollers μC thereof. The communication from the controller  57  to the individual sensors takes place via current modulation, while, conversely, that from the sensor  10  to the controller  57  takes place via voltage modulation. 
     It is generally necessary for notable current or voltage changes or modulations to take place since, owing to the long cable lengths occurring in an elevator system, the change would otherwise be unnoticeable. For example, current changes in the region of a factor of 3 are conceivable. 
     The units  50 ,  51  each correspond to a sensor. Variable resistors are denoted by the reference symbols  52 ,  53 . Each sensor is assigned a variable resistor. The resistance can be changed in different ways: it is conceivable for resistors to be added into this circuit in parallel with other resistors, as a result of which the total resistance is correspondingly reduced. However, it is also conceivable for the resistance to be influenced by means of the circuitry used, for example by individual transistors being switched off. The change in resistance can be influenced optically, for example by means of phototransistors, photodiodes, optocouplers or the like. 
     The circuit comprises constant current sources  54 ,  55 , which are each designed to match their voltage in the event of a change in the resistance in the circuit in such a way that a constant current flows. A change in the resistance (communication: controller  57  to sensor  10 ) regulates the constant current source  54  to a constant current intensity, with the result that the voltage measured via the voltmeter  56  changes. 
     If a further constant current source  55  is added into the circuit, the current intensity can also be modulated, i.e. the voltage does not remain constant (communication: sensor to controller). The change in the voltage applied to the circuit can be determined by the voltmeter  58 . 
     The states of the individual sensors or other data relating to the sensors can therefore be output via an output  60 . The relay  59  is controlled corresponding to the sensors via the microcontroller  57 . 
       FIG. 16  illustrates a switching unit  12 ″, as is also illustrated, for example, in  FIG. 9  as switching unit  12  or in  FIG. 10  as switching unit  12 ′. The switching unit  12 ′ also comprises a voltage source. The switching unit  12  from  FIG. 9  in particular also comprises the function of a relay, which can disconnect the motor M in the event of a line interruption as well. The switching unit  12  is connected to a (second) safety circuit  14  in  FIG. 16 . 
     LIST OF REFERENCE SYMBOLS 
     
         
           1  Sensor 
           1 ′ Sensor 
           2  Contact receptacle 
           3  Contact bridge 
           3 ′ Contact bridge 
           4  Transmitter 
           4 ′ Fiberoptic conductor inlet 
           5  Receiver 
           5 ′ Fiberoptic conductor outlet 
           6  Isolating web 
           7  Fitting unit 
           8  Fitting unit 
           9  Reflective surface 
           9   a  Reflective surface 
           9   b  Reflective surface 
           9   c  Reflective surface 
           10  Optical sensor 
           11  Electromechanical normally closed switch 
           12  Switching unit 
           12 ′ Switching unit (with voltage source) 
           12 ″ Switching unit 
           13  Voltage source 
           14  Second safety circuit 
           15  Contactor/relay 
           16  First safety circuit 
           20  Bus 
           21  Optical sensor with electronics unit 
           25  Switching unit 
           26  Safety circuit 
           27  Switching unit with integrated contactor 
           30  Circuit diagram 
           41  Sensor 
           42  Contact receptacle 
           43  Contact bridge 
           44  Fiberoptic conductor 
           45  Transmitter 
           46  Receiver 
           50  Communication unit 
           51  Communication unit 
           52  Variable resistor 
           53  Variable resistor 
           54  Constant current source 
           55  Constant current source 
           56  Voltmeter 
           57  Microcontroller of switching unit 
           58  Voltmeter 
           59  Relay 
           60  Output 
         A View at first time 
         B View at second time 
         C View at third time 
         D View at fourth time 
         K Cable/electrical line 
         L Fiberoptic conductor 
         M Drive motor 
         μC Microcontroller of a sensor 
         U Interruption