Abstract:
An input voltage contacted at the input of a contact can be through-connected to an output of the contact or switched off via a movable contactor. In order to determine the switching state of the contact, any drop in the contact voltage between the input and the output is detected. By the detected contact voltage, it can at least be determined whether the input voltage has been through-connected to the output or switched off.

Description:
This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/DE02/04226 which has an International filing date of Nov. 14, 2002, which designated the United States of America and which claims priority on German Patent Application number DE 101 58 316.8 filed Nov. 28, 2001, the entire contents of which are hereby incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention generally relates to a method for determining the switching state of a contact, via which an input voltage, applied to an input of the contact, can be passed on via a moving switching piece to an output of the contact and can be disconnected from it. An evaluation circuit preferably records a contact voltage that is dropped between the input and the output, and the recorded contact voltage is preferably used to determine at least whether an arc is occurring between the switching piece and the input and/or the output. It also generally relates to a corresponding evaluation circuit. 
   BACKGROUND OF THE INVENTION 
   A determination method and an evaluation circuit are known, for example, from U.S. Pat. No. 4,249,223. The evaluation circuit is used in electromagnetic switching devices, in particular in contactors and isolating switches, in order to quench arcs that occur during opening of the contact. 
   U.S. Pat. No. 4,777,479 discloses a determination method for a switching state of a contact, via which an input voltage which is applied to an input of the contact can be passed on via a moving switching piece to an output of the contact, or can be disconnected from it. In this case, the method determines whether the input voltage is or is not being passed on to the output. The recorded nominal state is indicated. 
   The prior, not previously published, DE 100 25 276.1, describes a method for determining the switching state of a contact, via which an input voltage which is applied to an input of the contact can be passed on via a moving switching piece to an output of the contact, or can be disconnected from it. In this case as well, a contact voltage which is dropped between the input and the output is recorded, and the recorded contact voltage is used to determine whether the input voltage has or has not been disconnected from the input. 
   SUMMARY OF THE INVENTION 
   An object of an embodiment of the present invention is to provide a determination method and a corresponding evaluation circuit, by which the contact can be operated more reliably and more safely. 
   An object may be achieved in that the evaluation circuit also determines whether the input voltage is passed on to the output or is disconnected from it. Further, a contact which is arranged upstream of the contact is opened if an arc occurs for a relatively long time. This is because this makes it possible to avoid the arc damaging the contact if it is not itself quenched sufficiently quickly, or is not quenched. 
   It is admittedly already known from the prior, not previously published DE 100 25 276.1 for the contact voltage to be recorded and evaluated. In this prior application, however, the evaluation is used only for recording an arc voltage, that is to say in particular not to distinguish whether the input voltage is passed on to the output or is disconnected from it. 
   The recorded contact voltage is preferably also used to determine whether an arc is occurring between the switching piece and the input and/or between the switching piece and the output. This is because, in particular, it is possible to determine the switching time when a switching state change occurs. The erosion of a contact face can then be deduced from the switching time. Furthermore, it is also possible to record whether an arc has occurred for a lengthy time. In particular, if an arc has occurred for a lengthy time, a contact arranged upstream of the contact can be opened, thus preventing the arc from causing damage to the contact. 
   If the specific switching state is used to determine a signal which is emitted when the signal assumes the same value for a relatively long time, short-term disturbances and transient states during a change in the switching state, in particular, do not influence the emitted signal. 
   The emitted signal may itself correspond to the switching state. However, it is also possible to compare the specific signal state with a nominal state in order to determine the signal. In this case, the signal is a fault signal. 
   The signal may be emitted in a form in which it can be perceived directly by a person, by way of his sensory organs. Examples of signals such as these are acoustic and, in particular, optical signals. Alternatively or additionally, the signal may, however, also be passed on to a control unit which is at a higher level than the contact. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages, features and details of the invention will become evident from the description of illustrated embodiments given hereinbelow and the accompanying drawings, which are given by way of illustration only and thus are not limitative of the present invention, wherein: 
       FIG. 1  shows a contact with an evaluation circuit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As is shown in  FIG. 1 , a load  1  can be connected to a load or input voltage UL via a contact  2 . The contact  2 -has an input  2   a  and an output  2   b . The load voltage UL is permanently applied to the input  2   a . In contrast, it can be disconnected from the output  2   b . The load voltage UL is passed on to or disconnected from the output  2   b  by means of a moving switching piece  2   c.    
   A voltage recording circuit  3  is connected to recording points  4  via recording resistors R 1 . One of the recording points  4  is in this case arranged on the input side and output side, respectively, of the contact  2 . The voltage recording circuit  3  is connected via the recording resistors R 1  to the recording points  4  with a high impedance, but permanently and conductively. 
   The recording resistors R 1  have resistances which are typically in the range above 1 MΩ, for example 3 to 10 MΩ. They preferably have the same resistances. 
   The voltage recording circuit  3  is in the form of an operational amplifier  3  which has an inverting signal input  5 , a non-inverting signal input  6  and a signal output  7 . The operational amplifier  3  is supplied with a supply voltage U 0 . 
   The signal inputs  5 ,  6  are connected to the recording points  4  via the recording resistors R 1  . The inverting signal input  5  is also connected via a capacitor C and a circuit resistor R 2  to the signal output  7 . The capacitor C and the circuit resistor R 2  are arranged parallel with one another, as can be seen in this case. The non-inverting signal input  6  is also connected via a further circuit resistor R 2 ′ and a further capacitor C′ to a reference voltage UR. In principle, the reference voltage UR may be chosen as required. However, it is preferably half the supply voltage U 0 . 
   The circuit resistors R 2 , R 2 ′ are considerably smaller than the recording resistors R 1 . Their resistances are typically in the range below 10 kΩ, for example 1 to 5 kΩ. They preferably have the same resistances as one another. The capacitors C, C′ preferably have a relatively small capacitance, for example 10 to 470 nF. 
   The inverting signal input  5  is connected to the reference voltage UR via two back-to-back parallel-connected diodes  8 . Furthermore, the non-inverting signal input  6  is also connected to the reference voltage UR via two further back-to-back parallel-connected diodes  9 . The diodes  8 ,  9  are preferably identical to one another. 
   Owing to the circuitry of the operational amplifier  3 , the reference voltage UR is produced at the signal output  7  of the operational amplifier  3  when the input voltage UL is passed on. If, in contrast, the input voltage UL is disconnected from the output  2   b , the entire input voltage UL is dropped across the contact  2 . This results in the signal that is emitted at the signal output  7  being shifted considerably upwards or downwards (depending on the mathematical sign of the input voltage UL). 
   If, in contrast, an arc occurs between the switching piece  2   c  and the input  2   a , or between the switching piece  2   c  and the output  2   b , then only an arc voltage in the order of magnitude of a few volts is dropped across the contact  2 . In this case, the majority of the load voltage UL is thus still passed to the load  1 . 
   In order to distinguish between these three states—input voltage UL passed on to the output  2   b , input voltage UL disconnected from the output  2   b  and the occurrence of an arc—the signal output  7  is followed by two window comparators  10 ,  11 . 
   The first window comparator  10  is supplied with two comparison voltages U 1 , U 2 . The two comparison voltages U 1 , U 2  are slightly above and below the reference voltage UR, respectively. The window comparator  10  produces a positive output signal when the voltage which is emitted at the signal output  7  of the operational amplifier  3  is within the voltage window defined by the comparison voltages U 1 , U 2 . Otherwise, it produces a null signal. The comparison voltages U 1 , U 2  are in this case chosen such that the window comparator  10  produces a positive output signal only when the input voltage UL is passed on to the output  2   b  without an arc. The window comparator  10  produces a null signal, in contrast, even when only the arc voltage is dropped. 
   The method of operation of the window comparator  11  corresponds to that of the window comparator  10 . However, it is supplied with comparison voltages U 3 , U 4  which are further above and below the reference voltage UR, respectively, than the comparison voltages U 1 , U 2 . The window comparator  11  therefore produces a null signal only when the contact  2  is completely open, that is to say when the load voltage UL is completely disconnected from the output  2   b . If, in contrast, the arc voltage is dropped across the contact  2  or the contact  2  is completely closed, it produces a positive output signal. 
   The output signals from the two window comparators  10 ,  11  are supplied to a state determination circuit  12 . The state determination circuit  12  then uses the output signals, supplied from the window comparators  10 ,  11 , to determine the switching state of the contact  2 . If both window comparators  10 ,  11  produce a positive output signal, the input voltage UL is passed on to the output  2   b . If both window comparators  10 ,  11  produce a null signal, the input voltage UL is disconnected from the output  2   b . If the window comparator  10  produces a null signal but in contrast the window comparator  11  produces a positive output signal, an arc is occurring between the switching piece  2   c  and the input  2   a  and/or between the switching piece  2   c  and the output  2   b.    
   The state determination circuit  12  uses the switching state as determined in this way to determine a signal. Furthermore, it transmits a drive signal to a timer  13  whenever the output signals that are transmitted from the window comparators  10 ,  11  change, that is to say when the signal is redetermined. The timer  13  is in consequence set to 0 and is started. When the timer  13  reaches a time limit T, it transmits a trigger signal back to the state determination circuit  12 . The timing out of the timer  13 , that is to say the reaching of the time limit T. indicates that the switching state as determined by the state determination circuit  12  is the same for a relatively long time. As such, the signal has assumed the same value for a relatively long time. In this case, the determined signal is emitted from the state determination circuit  12 . 
   Firstly, the state determination circuit  12  drives a signal transmitter  14 , for example a light-emitting diode  14 . If the light-emitting diode  14  illuminates, then this corresponds to the switching state in which the input voltage UL is passed on to the output  2   b . If the light-emitting diode  14  is switched off, the input voltage UL is disconnected from the output  2   b . If the light-emitting diode  14  blinks, then an arc is occurring. The signal is thus emitted from the state determination circuit in a form in which it can be perceived directly by a person by way of his sensory organs. 
   In addition to this, the signal can be passed on from the state determination circuit  12  to a control unit  15 . The control unit  15  is a higher-level control unit  15  by which, in particular, the contact  2  can be controlled. According to the example explained above, the state determination circuit  12  itself emits the switching state as a signal. However, it is also feasible for the higher-level control unit  15  to transmit a nominal state for the contact  2  to the state determination circuit  12 . 
   In this case, for example, the state determination circuit  12  can compare the switching state of the contact  2  as determined by itself with the nominal switching state, and can use this to determine a fault signal as the signal. In this case as well, the signal can once again be transmitted to the signal transmitter  14  and to the higher-level control unit  15 . If the light-emitting diode  14  is blinking, this may, for example, indicate a fault situation, lack of a drive for correct operation of the light-emitting diode  14 , that is to say with the nominal switching state matching the specific switching state. 
   The evaluation circuit according to an embodiment of the invention makes it possible to directly determine the switching state of the contact in a simple, cost-effective, safe and reliable manner. The power loss that occurs in the evaluation circuit is in this case negligible. 
   Exemplary embodiments being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.