Abstract:
The invention relates to a contactless safety switch for detecting the shut position of a shutting part in relation to a closeable part, with a switch unit, supplied with current by an electrical supply source on one of the parts and with a trip arranged on the other part and comprising a magnet. In this case, the switch unit comprises a transmission circuit with a clock-signal generator and with a transmission coil, a receiver circuit connected to an evaluation unit and having at least one receiver coil and also a Hall sensor responding to the magnet, and the trip comprises a trip coil which is capable, in the shut position, of being excited inductively by the transmission coil and exciting the receiver coil, while, in the shut position, the evaluation unit triggers a clearance path when the Hall sensor is activated via the magnet.

Description:
TECHNICAL FIELD 
     The invention relates to safety switches as they are used in connection with a corresponding control device for surveying the open and closed positions of a door, a cover, a flap or the like of restricted areas containing revolving machines or other dangerous equipment or production areas to be protected against contamination so that the restricted area is only accessible in the case of no danger for operators or of no danger of contamination, respectively, and the machine or other equipment can only be started when all doors, covers, flaps or the like are closed. More specifically, the invention concerns contactless safety switches, safety switches without mechanical parts to be brought into engagement to actuate the safety switch. 
     BACKGROUND OF THE INVENTION 
     Safety switches or safety sensors serve for detecting the shut position of shutting parts, for example of doors or hinged covers, in relation to closeable parts, for example boundary walls, housing walls and the like provided with corresponding access orifices, and, for safety reasons, cooperate with a special actuator, so that their tripping position cannot be manipulated by means of simple tools or the like. Such safety switches comprise, in general, a switch unit and a trip, in each case one of these components being arranged on the closeable part and the other on the shutable part. In this context, safety switches with mechanical or magnetic, that is to say non-contact coupling between the switch unit and trip are known. 
     As regards mechanical coupling, exact alignment, along with low tolerances, are necessary, with the result that adjustment becomes more difficult. Since mechanical devices do not operate without contact, it is difficult to keep them free of germs, this being important, for example, for areas of use in the food sector. 
     Contactless safety switches based on interconnected Reed contacts are known. However, since Reed contacts weld together under relatively high currents or capacitive loads, the reliability of such safety switches is often inadequate, and, moreover, they require special evaluation units. 
     German Patent 4 112 064 discloses an alarm indicator arrangement with a switch unit which has a transmission circuit with an oscillator and with a transmission coil and a receiver circuit connected to a discriminator circuit and having at least one receiver coil. Here, continuous power transmission, even when signal transmission is taking place, is ensured. However, an arrangement of this kind does not always afford the required safety standard. 
     It is known from German Patent 3 029 543, in the field of signalling and monitoring technology, to use a primary unit in the form of a switch unit which possesses a transmission circuit with an oscillator and with a transmission coil and the receiver circuit connected to an evaluation unit and having at least one receiver coil and which cooperates with a secondary unit, a trip. The secondary unit comprises a receiver coil and a trip coil which is controlled by the transmission coil of the primary unit and which triggers an indicator signal in the shut position. In this case, a first frequency is transmitted from the primary unit to the secondary unit and a second frequency is transmitted in the reverse direction. However, an arrangement of this kind does not always afford the required safety standard, either. 
     German Laid-Open Patent Application 2 033 682 discloses a connecting device for electrical appliances, by means of which device a battery-powered appliance is brought into mutual bearing contact with a mains-powered appliance for signal or power transmission. Here, in each case a permanent magnet in one appliance closes a sealed-armature contact in the other appliance. A special coil with a U-shaped core is provided in each appliance for signal transmission and for power transmission in each case. 
     German Laid-Open Patent Application 4 113 665 discloses an external-field-safe contact arrangement of magnetically actuated switching elements for monitoring the shut state of windows and doors, a permanent magnet being mounted on a window wing or a door and at least two magnetic-field actuated switching elements being mounted on the window frame or door frame. The abovementioned problem of the welding together of contacts of the magnetically actuated switching elements arises here. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a contactless safety switch which has increased reliability. 
     Since, in a safety switch with a switch unit and with a trip comprising a magnet, according to the present invention the switch unit comprises a transmission circuit with a clock-signal generator and with a transmission coil, a receiver circuit connected to an evaluation unit and having at least one receiver coil and also a Hall sensor responding to the magnet, and the trip comprises a trip coil which, in the shut position, is capable of being inductively excited by the transmission coil and excites the receiver coil, whilst, in the shut position, the evaluation unit, after comparing the transmitted and received clock signals, triggers a clearance path when the Hall sensor is activated by the magnet, welding together at relatively high currents or capacitive loads cannot occur, so that reliability is increased. 
     Preferably, two signals can be fed to the evaluation unit in the shut position, the evaluation unit triggering the clearance path only when both signals are present. In this case, for the clearance of a shutting part in relation to a closeable part, it is necessary for two conditions to be satisfied. The presence of two signals in the evaluation unit is redundant, because feeding only one signal to the evaluation unit would not mean any loss of information regarding the approach of the trip. The redundant design further increases reliability in terms of the actuation of the safety switch. 
     Further objects, advantages and embodiments of the invention will become apparent from the following description and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic block diagram of a contactless safety switch of a first embodiment of the invention. 
     FIG. 2 shows a schematic block diagram of a contactless safety switch of a second embodiment of the invention. 
     FIG. 3 shows a schematic block diagram of a contactless safety switch of a third embodiment of the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     According to FIG. 1, in a first embodiment of the invention a safety switch possesses a switch unit  1  (for example, a mains supply unit connected to a main supply), accommodated in a housing, and a trip  2 , likewise accommodated in a housing, the said switch unit and the said trip being arranged in each case on a closable part A and a shutting part B. Preferably, the switch unit  1  is arranged on the closeable part A and the trip  2  on the shutting part B. 
     The switch unit  1  is supplied with current by an electrical supply source  3  and comprises a transmission circuit with a clock-signal generator  4  for generating a clock signal and also a transmission coil  5 . The switch unit  1  comprises, furthermore, a receiver circuit with a receiver coil  7  and also a Hall sensor  9 . 
     The trip  2  comprises a magnet  10  which, in the shut position, is arranged so as to be adjacent to the Hall sensor  9 . The magnet  10  may be a permanent magnet. Furthermore, the trip  2  comprises a trip receiver circuit  11  with a trip coil  12  and also a trip transmission circuit  14  with a coil  15 . The coils  12  and  15  are arranged in such a way that, in the shut position, they are essentially adjacent to the transmission coil  5  and to the receiver coil  7  respectively. The trip receiver circuit  11  can thus be inductively coupled to the transmission coil  5  of the transmission circuit via the trip coil  12 , inductive coupling occurring as soon as the transmission coil  5  and trip coil  12  are adjacent to one another. The trip transmission circuit  14  is coupled to the trip receiver circuit  11  via a modifying circuit  17 . 
     During the approach of the trip  2  to the switch unit  1 , the Hall sensor  9  is activated when the magnetic field generated by the magnet  10  at the location of the Hall sensor  9  is sufficient to trigger the Hall sensor  9 . Preferably, the Hall sensor  9  can be balanced via a trimming resistor  18 , so that, by adjusting the switching threshold of the latter, that distance between the switch unit  1  and trip  2  at which the Hall sensor  9  is triggered can be fixed. Moreover, during the approach of the trip  2  to the switch unit  1 , as a result of the inductive coupling with the trip receiver circuit  11  which then commences the transmission coil  5  induces in the latter a voltage which is modified by the modifying circuit  17 . The modifying circuit  17  may comprise, in particular, a frequency divider which divides down the frequency of the clock signal by a predeterminable factor. The trip transmission circuit  14  thus transmits a modified clock signal if the trip receiver circuit  11  receives a clock signal from the transmission circuit. The trip transmission circuit  14 , in turn, can be inductively coupled to the receiver coil  7  of the receiver circuit via the coil  15 , inductive coupling occurring as soon as receiver coil  7  and coil  15  are adjacent to one another. During the approach of the trip  2  to the switch unit  1 , the receiver coil  7  therefore receives the modified clock signal. The receiver coil  7  is excited at the frequency of the modified clock signal. 
     If appropriate, shields are mounted at the transmission coil  5  and receiver coil  7  and/or at the trip receiver circuit and trip transmission circuit  11 ,  14 , so that undesirable coupling, for example between the trip transmission circuit  14  and transmission coil  5 , is avoided. Alternatively, a correspondingly small clock signal may also be selected, or a sufficiently long distance is provided between the transmission coil  5  and receiver coil  7  or between the trip receiver circuit  11  and trip transmission circuit  14 . 
     The receiver circuit also contains a bandpass filter  19 . The bandpass filter  19  is designed in such a way that it lets through only the clock signal modified by the modifying circuit  17  and filters out other external signals. The said bandpass filter is connected, via an amplifier  20  for amplifying the modified clock signal, to a pulse-shaping and counting stage  21 , in which the modified clock signal is conditioned and added on. 
     Moreover, the clock-signal generator  4  is connected directly to a further pulse-shaping and counting stage  22 . The pulse-shaping and counting stage  22  thus receives the non-modified clock signal generated by the clock-signal generator  4 . Connected to the outputs of the pulse-shaping and counting stages  21 ,  22  is a common comparator stage  23 , the output of which is led to one channel of a relay stage  24 . A further channel of the relay stage  24  is connected to the Hall sensor  9 , a clearance signal being fed from the Hall sensor  9  to this channel when the Hall sensor is activated as a result of the approach of the magnet  10  belonging to the trip  2 . The comparator stage  23  evaluates the counts received by the pulse-shaping and counting stage  21 ,  22 , the comparator stage  23  being activated by the first arriving signal from the counter stage  22  and interrogating the pulse-shaping and counting stages  21 ,  22  after at least one suitable selected time interval or count. 
     If a frequency divider is used as the modifying circuit  17 , the comparator stage  23  may be designed in such a way that it interrogates the counts of the pulse-shaping and counting stages  21 ,  22  after suitably selected different time intervals or counts, so that the pulse-shaping and counting stage  21 , which receives the modified clock signal at a frequency lower by a specific factor than that of the clock signal generated by the clock-signal generator  4 , is interrogated only after a timespan longer by the same factor or a count lower by the same factor than the pulse-shaping and counting stage  22 . The comparator stage  23  transmits an output signal to the relay stage  24  only when the two counts received are in a predetermined ratio to one another. 
     Alternatively, the counts of the pulse-shaping and counting stages  21 ,  22  may also be interrogated simultaneously by the relay stage  24 , in which case the comparator stage  23  is designed in such a way that it divides the counts of the pulse-shaping and counting stages  21 ,  22  by one another. The said comparator stage transmits an output signal to the relay stage  24  only when the quotient determined corresponds to the factor of the frequency divider. The pulse-shaping and counting stages  21 ,  22  may also be designed in such a way that the different frequency of the signal generated by the clock-signal generator  4  and modified is balanced again by means of different modulation. The comparator stage  23  then interrogates the count after equal, suitably selected timespans and transmits an output signal to the relay stage  24  only when the said counts are identical. 
     The relay stage  24  is interconnected with a clearance path  25 . The relay stage  24  is preferably a safety relay stage which comprises two safety relays. 
     The clearance path  25  is enabled via the relay stage  24  only if the two channels of the relay stage  24  were previously inactive and a signal is subsequently present in each case at both channels of the relay stage  24 . This presupposes both that the comparator stage  23  generates an output signal and that the Hall sensor  9  is activated. The safety switch consequently has a redundant design. 
     According to FIG. 2, in a second embodiment of the invention the transmission circuit of the switch unit  1  comprises a switch  26  for closing and breaking the transmission circuit, said switch being capable of being switched on via the Hall sensor  9 . During the approach of the trip  2  to the switch unit  1 , the switch  26  is therefore switched on via the Hall sensor  9  when the magnetic field generated by the magnet  10  at the location of the Hall sensor  9  is sufficient for triggering the latter. Here, too, the Hall sensor  9  can preferably be balanced via a trimming resistor  18 , so that it is possible, by adjusting the switching threshold of the latter, to fix that distance between the switch unit  1  and trip  2  at which the Hall sensor  9  is triggered. When the Hall sensor  9  is triggered, the switch  26  is switched on and the transmission circuit is consequently closed. Only then is the clock signal generated by the clock-signal generator  4  enabled, whereupon, in a similar way to the embodiment illustrated in FIG. 1, the transmission coil  5  induces a voltage in the trip receiver circuit  11  as a result of the inductive coupling with the latter. 
     The trip  2  and the evaluation unit of the safety switch are otherwise designed in exactly the same way as in the first embodiment of the invention. According to FIG. 2, however, in the second embodiment of the invention the clock signal generated by the clock-signal generator  4  is not fed continuously to the pulse-shaping and counting stage  22 , but only in the event of clearance as a result of the activation of the Hall sensor  9 . Inductive coupling between the transmission coil  5  and the transmission circuit and the trip coil  12  of the trip receiver circuit  11  likewise occurs only when transmission coil  5  and trip coil  12  are adjacent to one another and the transmission circuit is closed as a result of the activation of the Hall sensor  9  by the magnet  10 . During the approach of the trip  2  to the switch unit  1  with transmission circuit closed, a clock signal modified by the modifying circuit  17  belonging to the trip  2  is fed to the pulse-shaping and counting stage  21  in a similar way to the first embodiment of the invention. The pulse-shaping and counting stages  21 ,  22  are connected to the common comparator stage  23 , the output of which is connected to the relay stage  24 , in a similar way to the first embodiment of the invention. The clearance path  25  interconnected with the relay stage  24  is enabled by the relay stage  24  only when the comparator stage  23  receives the enabled clock signal and the clock signal modified by the modifying circuit  17  in a predetermined frequency ratio. The feed of an enabled signal and of a modified signal is redundant, because the transmission of the enabled signal only would not mean any loss of information as regards the approach of the trip  2 . In this embodiment, too, the redundant design thus results in increased reliability in terms of the actuation of the safety switch. 
     According to FIG. 3, in a third embodiment of the invention similar to the second embodiment, a switch  26  is likewise provided, which, when the Hall sensor  9  is triggered by the magnet  10 , enables the clock signal generated by the clock-signal generator  4 . In a similar way to the first two embodiments, the transmission circuit comprises a transmission coil  5  which, here, has a magnetizable U-shaped core  27 . However, the receiver circuit comprises two receiver coils  7  which likewise each have a magnetizable U-shaped core  27  and which are each followed by an amplifier stage  20 . The outputs of the amplifier stages  20  are in each case connected directly to a channel of the relay stage  24 . 
     The trip  2  comprises, apart from the magnet  10  arranged so as to be adjacent to the Hall sensor  9  in the shut position, only the trip coil  12  which, in the shut position, is arranged so as to be adjacent to the transmission coil  5  and to the two receiver coils  7 . The trip coil  12  likewise has a U-shaped magnetizable core  27 . 
     In the shut state, therefore, inductive coupling occurs both between the transmission coil  5  and the trip coil  12  and between the trip coil  12  and the two receiver coils  7 . When the clock signal generated by the clock-signal generator  4  is enabled, the transmission coil  5  therefore induces, in the trip coil  12 , a voltage which in turn generates a voltage in the two receiver coils  7 . The signal received by the receiver coils  7  is transmitted via the amplifier stage  20  to the two channels of the relay stage  24 . The clearance path  25  interconnected with the relay stage  24  is enabled, in a similar way to the first two embodiments, only when a signal is present in each case at the two channels of the relay stage  24 . In a similar way to the first two embodiments, the feed of two signals from the two receiver coils  7  to the relay stage  24  is redundant, because the transmission of only one signal would not mean any loss of information as regards the approach of the trip  2 . 
     Although the invention has been described in some detail by way of illustration and example, for purposes of clarity and understanding, it will be obvious that certain changes and modifications may be practiced within the scope of the invention.