Abstract:
A signal converter circuit includes a sensor connection, two comparator circuits, each having a signal input for an electric connection to the sensor connection, a reference input for an electric connection to a respectively assigned reference signal source and a signal output for a provision of an output signal, with a feedback line being formed between the respective signal output and the respective reference input, and further including two reference signal sources, each of the comparator circuits being configured for comparing a signal level at the signal input to a signal level at the reference input and for outputting a digital output signal as a function of a comparison result between the signal levels, wherein the two reference signal inputs are connected to one another via a coupling line being configured to transmit a presettable fraction of the respective signal level present at one reference input to the other reference input.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a signal converter circuit for processing a sensor signal into an output signal, the circuit comprising a sensor connection for receiving a sensor signal, further comprising two comparator circuits, each having a signal input for an electric connection to the sensor connection, a reference input for an electric connection to a respectively assigned reference signal source and a signal output for a provision of an output signal, with a feedback line being formed between the respective signal output and the respective reference input, and further comprising two reference signal sources, each of which is configured to provide a reference signal, each of the comparator circuits being configured to compare a signal level at the signal input to a signal level at the reference input and for outputting a digital output signal as a function of a comparison result between the signal levels. The invention further relates to a method for evaluating a sensor signal of a safety-oriented device. 
         [0002]    From German Patent Application DE 1 499 505, which is published for opposition, a monitoring device for a redundant circuit arrangement is known, in which between two transmission lines there is in each case connected a comparison element, at the output of which an alarm switching device connected to all comparison elements is provided; this responds if the output signal of at least one comparison element exceeds a preset limit value, wherein an alternating voltage is used for the control signal and a differential amplifier serves as a comparison element, wherein the output of the differential amplifier is connected to a bistable circuit at the output of which the alarm switching device connected to all bistable circuits is switched, and wherein a connecting line with two branches, the first of which branches, which contains a diode, is connected to a first input of the bistable circuit, and the second of which branches, which contains a parallel circuit of a capacitor and a Zener diode, is connected to a second input of the bistable circuit, is located between the differential amplifier and the bistable circuit, and wherein a reverse bias lower than the breakdown voltage of the Zener diode is applied to the diode. 
       SUMMARY OF THE INVENTION 
       [0003]    The invention is based on the problem of providing a signal converter circuit and a method for evaluating a sensor signal of a safety-oriented device, in which an improved identification of different functional states for the safety-oriented device is ensured. 
         [0004]    For a signal converter circuit of the type referred to above, this problem is solved by the following features: the two reference inputs are electrically connected to one another via a coupling line, the coupling line being configured to transmit a presettable fraction of the respective signal level present at one reference input to the other reference input. With the aid of the coupling line, it is accomplished that, at a change of a signal level at the signal output of one of the comparator circuits, which may for example be due to a change of a signal level at the signal input, there occurs a presettable influence on a signal level at the reference input of the other comparator circuit, which basically operates in a redundant manner By a suitable design of the coupling line, this influence is preferably limited to a fraction of the respective signal level, for example to 10 percent. As a result of the preferably bidirectional coupling of the at least two comparator circuits via the coupling line, tolerances of the two comparator circuits can to the greatest extent be eliminated in evaluating the sensor signal. These tolerances are for example due to the properties of the electric and electronic components of the respective comparator circuit and to the properties of any upstream filter arrangements. The coupling of the at least two comparator circuits has the purpose of obtaining, while maintaining an at least singly redundant signal conversion, at least two output signals which are always the same irrespective of the unavoidable tolerances in the respective comparator circuits and on condition that the respective comparator circuits function correctly. In order to maintain redundancy in signal conversion irrespective of the coupling provided, it has to be ensured that the fraction of the signal level provided by one comparator circuit via the coupling line to the respective reference input of the other comparator circuit is greater than the tolerances of the electric and electronic components of the respective comparator circuit but lower than a switching hysteresis of the respective comparator circuit. If these marginal conditions are met, there will always be clear and identical output signals of the comparator circuits irrespective of their unavoidable tolerances. These output signals can be supplied to a downstream comparator which is tasked with evaluating the sensor signal from the incoming output signals and with initiating further measures depending on a result of this evaluation. It may for example be provided that the sensor signal is supplied for safeguarding a machine by a safety-oriented device, in particular an emergency stop button on a machine or a light curtain, and that the machine is stopped at a presettable signal level of the sensor signal in order to avoid damage. Depending on the safety level aimed at, a singly redundant processing of the input signal into two output signals can be achieved with two comparator circuits, or a multiply redundant processing of the input signal into at least three output signals can be achieved with at least three comparator circuits. 
         [0005]    Advantageous further developments of the invention form the subject matter of the dependent claims. 
         [0006]    It is expedient if a series connection of two preferably identical attenuation means from the group consisting of: resistor, capacitor is provided in the coupling line. The attenuation means have the purpose of providing only a presettable fraction of the reference signal present at a reference input to the at least one other reference input, so that an influence between the at least two comparator circuits is limited in such a way that the desired redundancy in the evaluation of the sensor signal is not compromised. 
         [0007]    It is preferably provided that the attenuation means are designed as failsafe electric components in order to reduce the coupling action for the coupling line in case of a fault. In this context, a fault is to be understood as a malfunction of the attenuation means, which may for example be caused by an electric overload of the attenuation means, in particular by external impact of electromagnetic fields, and/or by aging and/or by mechanical loads at the attenuation means. In order to ensure the safety-oriented properties of the signal converter circuit even on the occurrence of such effects, the attenuation means are constructed in such a way that the coupling action between the reference inputs can only be reduced and not increased in case of a fault. In the extreme case, this may have the result that the coupling action between the reference inputs is cancelled completely, because the attenuation means located in the coupling line do not permit any signal transmission because of a fault. In this situation, the signal converter circuit operates in the same way as signal converter circuits of prior art. 
         [0008]    In an advantageous further development of the invention, it is provided that the attenuation means is selected from the group consisting of: MELF resistor, thin-film resistor, capacitor with fail-open characteristics. A MELF (metal electrode leadless faces) resistor is an electric component, designed for surface-mounting in particular, in a usually cylindrical shape, the end faces of which are designed as electric contacts and which has a precisely defined behavior in case of a fault. If there is a fault, a MELF resistor can become high-impedance, but there will be no reduction of its resistance. In a thin-film resistor, the resistance layer is applied to a substrate in such a way that an aging or an electric overload of the resistor results in an increase of its resistance. A capacitor with fail-open characteristics is configured in the same way, so that in case of a fault its capacitance can only decrease and not increase. 
         [0009]    In a further variant of the invention, it is provided that each of the comparator circuits is designed as a Schmitt trigger, in particular with a comparator or operational amplifier, and/or for a redundant processing of the sensor signal. In redundant—if applicable multiply redundant—processing, the input signal is processed with two—if applicable more—comparator circuits in an at least substantially identical way and fed to a downstream signal evaluation device. The signal evaluation device compares the at least two input signals processed into output signals in order to ensure a higher security level for this signal evaluation and the measures to be derived there from, for example the output of a stop signal for a machining unit. 
         [0010]    The problem of the invention is solved by a method comprising the following steps: a provision of a sensor signal of a safety-oriented device to a plurality of signal converter circuits, a comparison of a signal level of the sensor signal to a signal level at a reference input of the respective signal converter circuit, wherein a signal level at the respective reference connection is formed from a reference signal of a respectively assigned reference signal source, an output signal from a signal output of the respective signal converter circuit and a coupling signal from a reference input of a further signal converter circuit, and wherein the coupling signal is provided as a presettable fraction of the signal level by a reference input of the further signal converter circuit, a forwarding of the output signals to a comparator and a provision of a fault signal by the comparator if the output signals consistently lie within a presettable level interval or if the output signals show a deviation greater than a presettable level difference for longer than a presettable test interval. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    An advantageous embodiment of the invention is shown in the drawing, of which: 
           [0012]      FIG. 1  is a block diagram of the signal converter circuit with a redundant arrangement of two comparator circuits which are electrically coupled to one another, and 
           [0013]      FIG. 2  shows a diagrammatic signal course for the comparator circuit according to  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    A signal converter circuit  1  as shown in  FIG. 1  is electrically looped between a sensor  2  and a comparator  3  by way of example. In the illustrated embodiment, the sensor  2  is designed as an emergency stop button, with the aid of which an operator can switch off a machining unit not shown in the drawing in order to reduce or eliminate any potential hazard posed by the machining unit. To facilitate as reliable as possible an evaluation of the sensor signal provided by the sensor  2  and transmitted to the signal converter circuit  1  by the comparator  3 , which is in turn connected in a way not shown in detail to a switching device for influencing the machining unit, the signal converter circuit  1  of the illustrated embodiment has a structure described in greater detail below. 
         [0015]    The signal converter circuit  1  has the task of converting the sensor signal provided by the sensor  2 , which may in particular be an analogue voltage signal or an analogue current signal, into a digital output signal which can adopt two easily distinguished signal states (low, high) and which is fed to the comparator  3  via output lines  5 . 
         [0016]    For this purpose, the signal converter circuit  1  has, by way of example, a singly redundant arrangement of two comparator circuits  6 ,  7 , which are constructed identically merely by way of example. Each of the comparator circuits  6 ,  7  has a signal input  8 , a reference input  9  and a signal output  10 . The signal input  8  is electrically connected to a sensor connection  11 , to which the sensor line  4  is connected. Between the sensor connection  11  and the signal input  8  of the respective comparator circuits  6 ,  7 , there is provided a filter circuit  12 , which is shown only diagrammatically in the illustrated embodiment and in which the sensor signal provided at the sensor connection  11  is processed for the respective comparator circuits  6 ,  7  in a suitable, in particular identical, manner 
         [0017]    In the illustrated embodiment, it is provided that the comparator circuits  6 ,  7  are designed as Schmitt triggers. Each of the comparator circuits  6 ,  7  therefore comprises a comparator or operational amplifier  15 , a reference signal source  16  electrically connected to the reference input  9  and a feedback line  17  formed between the signal output  10  and the reference input  9 . In the illustrated embodiment, the reference signal source  16  is designed as a voltage divider and comprises a first resistor  19  connected to a voltage source  18  and the reference input  9  and a second resistor  20  connected to the reference input  9  and an earth connection  21 . The further description is accordingly based on the assumption that a signal level which is at least substantially influenced by the characteristics if the reference signal source  16  is present at the reference input  9 . 
         [0018]    The reference input  9  is further connected to the signal output  10  via a resistor  22  looped into the feedback line  17 , the resistor  22  being dimensioned such that a presettable fraction of a signal level present at the signal output  10  is fed back to the reference input  9 . 
         [0019]    In addition, the comparator circuits  6 ,  7 , which are connected in parallel in respect of the sensor signal provided at the sensor connection  11  and which are designed for an independent conversion of the sensor signal, are electrically connected to one another by a coupling line  23 . The coupling line  23  runs between the reference inputs  9  of the two comparator circuits  6 ,  7 . In order to limit a mutual influencing of the two comparator circuits  6 ,  7  to a presettable degree, two attenuation means  24  connected in series are looped into the coupling line of the illustrated embodiment. The attenuation means  24  facilitate a bidirectional coupling-in of signals from one reference input  9  to another reference input  9 . Owing to the use of two series-connected attenuation means  24 , a higher security level can be obtained for the signal converter circuit  1  than if only one attenuation means were used. If the security level to be obtained is not critically important, a single attenuation means can be provided. In the illustrated embodiment, the two attenuation means  24  are identically designed as MELF resistors, in which aging and electric overload occurring in case of a fault invariably result, because of internal and/or external voltage and/or current couplings, in an increase of resistance and thus in a reduction of the coupling effect between the reference inputs  9 . 
         [0020]    In this arrangement, the series connection of the attenuation means  24  is matched to the signal levels present at the reference inputs  9  in such a way that the positive feedback or co-influencing of the respective comparator circuits  6 ,  7  does not exceed a presettable coupling level. By coupling the two comparator circuits  6 ,  7 , tolerances can be compensated. These tolerances are due to the electric and electronic components of the respective comparator circuits  6 ,  7  and, if applicable, of the upstream filter circuits  12  and can, without positive feedback, result, irrespective of an identical sensor signal at the respective signal inputs  8  of the two comparator circuits  6 ,  7 , in different output signals at the signal outputs  10  of the two comparator circuits  6 ,  7 . 
         [0021]    The following description of the function of the signal converter circuit  1  shown in  FIG. 1 , which also refers to the signal course shown in  FIG. 2 , is based on the assumption that an identical signal level is present at the signal inputs  8  of the comparator circuits  6 ,  7 . By way of example, it is further assumed that the two comparator circuits  6 ,  7  have a slightly different switching behavior due to component tolerances, so that a switching threshold  31  of the comparator circuit  6  for switching the signal output  10  from a logic high signal level to a logic low signal level is higher than a switching threshold  31  of the comparator circuit  7 . 
         [0022]    Owing to the configuration of the a switching threshold  31  of the comparator circuits  6 ,  7  as Schmitt triggers, the comparator circuits  6 ,  7  further have a switching behavior liable to hysteresis, so that a change-over from a first signal level for the output signal to a second signal level for the output signal always requires a presettable signal difference between a chronologically earlier and a chronologically later signal level at the signal input. 
         [0023]    Owing to the configuration of the a switching threshold  31  of the comparator circuits  6 ,  7  as Schmitt triggers, the comparator circuits  6 ,  7  are configured for comparing the sensor signal level present at the respective signal input  8  to a reference signal level present at the respective reference input. 
         [0024]    As has been explained with reference to  FIG. 1 , the reference inputs  9  of the two comparator circuits  6 ,  7  are electrically connected to one another via the coupling line  23 . In addition, owing to their configuration as Schmitt triggers, a feedback of the output signal level  33 ,  34  to the respective reference input  9  via the respective feedback lines  17  is provided for each of the comparator circuits  6 ,  7 . The result is a reference signal level (not shown) at the respective reference input as a combined signal level from the voltage level made available by the reference signal source  16  together with the output signal provided via the feedback line  17  and the reference signal at the other reference input  9 , which is provided by the respective other reference input  9  and attenuated with the aid of the attenuation means  24 . 
         [0025]    The following description of an exemplary switchover process is based on the assumption that the sensor signal is at a high voltage level up to a point in time t 1  and after the point in time t 1  drops to a lower voltage level as a result of a manual actuation of the sensor  2 . As a result of shortcomings in the signal transmission between the sensor  2  and the comparator circuits  6 ,  7 , the sensor signal is changed from the point in time t 1  by the actuation of the sensor  2  not suddenly, but rather in accordance with the signal curve of the sensor signal level  30  according to  FIG. 2 . 
         [0026]    At the point in time t 2 , the sensor signal level  30  reaches the switching threshold  31  of the first comparator circuit  6 , so that at this point the output signal level  33  of the first comparator circuit  6  is switched from low to high. Without any coupling of the reference inputs of the two comparator circuits  6 ,  7 , the output signal level  34  of the second comparator circuit  7  would only be switched from low to high at a point in time t 4 , because the sensor signal level  30  does not undershoot the switching threshold  32  of the second comparator circuit  7  before this point in time. 
         [0027]    Depending on the design of the comparator  3 , the time difference between the changes of the output signal levels of the two comparator circuits  6 ,  7  would be evaluated as a fault or result in a time-delayed response of the comparator  3  with the disadvantage of a delayed reaction, which may for example be the shutdown of the machining unit not shown in the drawing. 
         [0028]    If, on the other hand, the comparator circuits  6 ,  7  are connected to one another in terms of their reference inputs  9  via the coupling line  23  as shown in  FIG. 1 , the switchover of the comparator circuit  6  at the point in time t 2  leads to a rise in the output signal level  33  provided at the signal output of this comparator circuit  6  in the illustrated embodiment. This output signal level  33  is made available to the comparator  3  on the one hand and, via the feedback line  17 , to the reference input  9  of the comparator circuit  6  on the other hand. In addition, this output signal level is made available by the reference input  9  of the comparator circuit  6  to the reference input  9  of the comparator circuit  7  via the coupling line  23 . 
         [0029]    Owing to the tolerance-related difference in its switching threshold  32  compared to the comparator circuit  6 , the comparator circuit  7  does not yet perform a switchover operation at the point in time t 2  before the arrival of the attenuated feedback signal of the comparator circuit  6  because of the difference between the signal level at the signal input  8  and the signal level at the reference input  9 . However, the signal difference required for the switchover of the comparator circuit  7  is already reached at the point in time t 3  as a result of the contribution of the, however small, proportion of the reference signal level of the comparator circuit  6  made available to the comparator circuit  7  via the coupling line  23 , so that—compared to the variant without coupling via the coupling line  23 —there is an earlier switchover of the comparator circuit  7  from a low level to a high level. 
         [0030]    By way of example, it may be provided that a coupling action of the signal levels of the reference signals transmitted bidirectionally via the coupling line  23  does not exceed a proportion of, for example, 10 percent in order to eliminate an undesirable complete positive coupling between the redundantly operating comparator circuits  6 ,  7 . 
         [0031]    In the same way, the two comparator circuits  6 ,  7  can have switching thresholds (not shown) for switching the respective output signal level from high to low, so that, owing to the proportional coupling of the reference signal levels, there will be a time-coordinated switchover of the two comparator circuits  6 ,  7  even at a rise of the sensor signal level  30 .