Input circuit that can be made redundant, input circuit unit having at least one input circuit, and method for operating the input circuit unit

A method and input circuit unit having a first input circuit and a redundant second input circuit, wherein each input circuit includes an input for feeding a measurement current that can be obtained from a signaling device and that can be switched over and operated in such an input circuit unit, where the measurement current causes a voltage drop across a measurement resistor that can be evaluated as a measured value, where a parallel connection of the first and second input circuits causes division of the measurement current among the first and second input circuits, and where at least one input circuit include a device for compensating the decreased voltage drop across the measurement resistor resulting from the division of the measurement current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. national stage of application No. PCT/EP2015/053865 filed 25 Feb. 2015. Priority is claimed on German Application No. 10 2014 203 429.9 filed 26 Feb. 2014, the content of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a circuit, referred to below as an input circuit, for detecting input signals, and further relates to an input circuit unit having at least one input circuit as well as to a method for operating the input circuit. As an input circuit that can be made redundant, the input circuit makes possible single operation or redundant operation, i.e., operation together with a further input circuit, and thus a redundant or non-redundant detection of the respective input signal.

2. Description of the Related Art

Input circuits and redundant input circuits are known per se. In conventional input circuits, the relevant input signal is detected via inputs permanently designed for a respective measurement range, to which only digital or analog input signals can be fed. To make analog inputs for current measurement redundant, such as 4 to 20 mA, a two-pole input circuit is used.

EP 2 461 221 A2 discloses a control device with a circuit for processing signals of different sensor types. Here, two sensor types which can be connected deliver either analog current signals or PWM signals. In order to provide a reference potential required for evaluating such signals, the circuit comprises a pull-up resistor and a pull-down resistor. Both can be activated by switches which can be actuated by a microcontroller. The measurement current flowing into the circuit is guided via a low-pass filter to two input channels of the microcontroller simultaneously, and the microcontroller evaluates the signals on that input channel which belongs to the previously activated resistor.

US 2012/0280327 A1 and U.S. Pat. No. 6,147,520 disclose conventional circuits in which a single resistor can be bridged by means of a switch connected in parallel in order to adjust a respective overall resistance.

SUMMARY OF THE INVENTION

It is object of the present invention to provide a an input circuit that is able to be made redundant, as well as an input circuit unit with at least one such input circuit.

This and other objects and advantages are achieved in accordance by providing an input circuit having one input for feeding of an input signal obtainable from a signaling device in the form of a measurement current, and having an input resistor and a measurement resistor, where an electrically controllable switch is connected in parallel to the input resistor and where the measurement current invokes a voltage drop able to be evaluated as a measured value across the measurement resistor. The input circuit is able to be operated in accordance with a corresponding operating mode as a binary input circuit or as a current input circuit and is thus able to be switched over. The switch is provided for switching over the input circuit in accordance with the respective operating mode and is accordingly able to be controlled as a function of the operating mode. In its closed state, the switch bridges the input resistor. The input circuit thus becomes low-resistance, so that analog current measurements (operation as current input circuit) are possible. In its open state, the switch has no effect. The input resistor remains active, forms a voltage divider together with the measurement resistor and the potential of the center tap of the voltage divider can be evaluated for recognition of a high or a low level of the input signal (operation as binary circuit). Likewise, for operation of the input circuit as a voltage circuit, the input resistor, as an analog voltage input, causes a restriction of the voltage dropping across the measurement resistor (measurement voltage) and an adaptation to the measurement range circumstances.

In addition, the input circuit is also provided with means, referred to below, as cross coupling for automatic detection of a further redundant input circuit. Because of a redundant input circuit recognized via the cross coupling, an activation of a means for compensating for a reduced voltage drop resulting from a division of the measurement current is able to be effected, i.e., such as an activation of a multiplication that is performed by a microcontroller functioning as means for compensating. In this way, there is an automatic detection of a redundant input circuit and likewise automatic compensation for a falsification of a measured value only determined based on the voltage drop in the respective input circuit associated with the division of the measurement current among the then two input circuits.

The advantage of the inventive input circuit as well as of an input circuit unit with at least one such input circuit lies in the fact that a comparatively simple circuit is provided that can be switched over between analog current measurement and analog or digital voltage measurement by actuating an electrically controllable switch. Thus digital or analog input signals fed in can be detected and evaluated by means of one and the same circuit. By activation of the switch, the input resistor can be short-circuited. The input of the input circuit thus becomes low-resistance overall and, in this way, an analog current measurement can also be undertaken with the input circuit.

An input circuit with resistors able to be bridged by means of a switch is likewise known. DE 10 2008 043 199 A disclose a circuit of an I/O module for connection of a field device.

By contrast with the switchable input circuit in accordance with the invention, the I/O module is only intended for analog current measurement and accordingly the I/O module detects a measurement current delivered by the field device. The I/O module also provides a feed current for the field device, which is set as a function of the respective measurement current. In order to minimize the load on a battery of the I/O module intended for supplying the field device, individual resistors provided in a measurement branch of the I/O module can be bridged via a switch connected in parallel and thus immediately switched off. In this way a measurement resistor or a communication resistor is “switched off” when no current measurement or no HART communication is intended.

WO 2006/127373 A likewise discloses a circuit for supplying a field device with an operating voltage. This includes a measurement resistor and an adjustable voltage regulator connected in series thereto and this in its turn contains an amplifier, a Zener diode and two transistors combined into a Darlington circuit. A voltage resulting because of a respective measurement current in each case is amplified by the amplifier. This is used to activate a Zener diode and the Zener Diode is used to activate the Darlington circuit. The Darlington circuit functions as a final control element in the voltage regulator, but, unlike in the case of the switchable input circuit of the invention, does not cause a switchover as a result of different operating modes.

Two transistors combined in a Darlington circuit that, even with only a small control current, bring about a sufficient increase in the current amplification factor, also come into consideration as a particular embodiment of the switchable input circuit. As a result, even with only a small control current that only falsifies the measurement current to an insignificant extent, a secure electrical bridging of the input resistor results.

In a further embodiment of the switchable input circuit, a series circuit of the measurement resistor with an upstream input resistor functioning as a voltage divider connected after the respective input is provided, where the measurement resistor is connected to ground. The fact that the measurement resistor is connected to ground means that there is a tapping off of the measurement current within the respective input circuit without further measures being required. The input circuit can thus be operated as a single-pole input circuit (input circuit with a “single-ended” input). This simplifies the wiring of the respective sensor system to the input circuit or to a plurality of such input circuits, because only one wire has to be attached in each case.

The switchable input circuit described here and below is considered to be an input circuit able to be made redundant for operation together with a further (redundant) switchable input circuit. A combination of two such input circuits is referred to below as an input circuit unit. Similarly, a facility that allows at least two such input circuits to be accommodated and/or connected and thus to an extent function as a carrier unit for at least one input circuit is referred to as an input circuit unit.

The above-mentioned object is thus also achieved with an input circuit unit in accordance with the invention. To this end, there is the input circuit unit is provided with a first part circuit in the form of an input circuit as described here and below and a redundant, second part circuit in the form of a further such input circuit which can be combined with the first part circuit within the input circuit unit. Each part circuit/input circuit comprises an input for feeding a measurement current obtainable from an external signaling device. The measurement current causes a voltage drop across a measurement resistor able to be evaluated as a measured value (input signal), where a parallel connection of two input circuits causes the measurement current to be divided among the first and the second input circuit and thus basically falsifies the measured value obtainable as a result of the measurement current. To compensate for such a falsification, at least one input circuit has means for compensating for the reduced voltage drop across the measurement resistor resulting from the division of the measurement current.

The advantage of the present embodiment of the invention lies in the fact that a comparatively simple input circuit unit is achieved, in which two input circuits of the type described here and below are combined or are able to be combined, but which is also able to be operated with only one input circuit. The two input circuits, i.e., two identical or at least functionally-equivalent input circuits, function as part circuits basically able to be operated independently of one another and evaluate a measurement current that flows into the input circuit because of a current applied to its input or a potential applied to the input.

A falsification resulting from redundant operation of two input circuits because of the measurement current flowing in each input circuit is compensated for, so that a signal output from the respective input circuit and from the input circuit unit overall represents the actual value of the electrical variable output by the signaling device and of the resulting measurement current in each case.

In an embodiment of the input circuit unit, a microcontroller functions as a way to compensate for the reduced voltage drop resulting from the dividing up of the measurement current, where via the microcontroller a computational correction of the measured value, i.e., a computational correction in the form of a multiplication of the measured value by a predetermined or predeterminable factor, is able to be performed. A multiplication of a measured value by a factor is a measure easily able to be performed with a microcontroller. The respective factor quantitatively describes, as a predetermined or predeterminable factor, the division of the measurement current among the two input circuits. For a division of the measurement current, in which for example 97% of the measurement current flows into the first input circuit and the remaining 3% into the second input circuit, compensating for the associated falsification of the measured value can easily be achieved by the measured value being multiplied on the side of the first input circuit by a factor of 1.03 (=1+3%). The computational compensation on the side of the second input circuit, for the assumed division, can be achieved with a factor of 32.33 (=97%/3%). In accordance with the scheme illustrated, other factors are produced for other divisions of the measurement current.

It is also an object of the invention to provide a method for operating an input circuit unit as described here and below. With the operating method, in the case of a parallel connection of a first and a second input circuit, a division of the measurement current among the first and the second input circuit is compensated for.

In one embodiment of the method, a parallel circuit of a first and a second input circuit is detected automatically and as the result of such an automatic detection, in order to compensate for the division of the measurement current, a computational correction of the measured value obtainable because of the voltage drop across the measurement resistor is performed, i.e., a computational correction in the form of a multiplication of the measured value by a predetermined or predeterminable factor.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1shows a schematic diagram of a circuit unit for redundant signal detection, i.e., to detect input signals. The circuit unit functions accordingly as input circuit unit10. The input circuit unit10comprises a first part circuit12, which can be used for redundant signal detection, together with an identical or at least functionally-equivalent, complementary second part circuit14. The part circuit or each part circuit12,14of the input circuit unit10functions within the input circuit unit10as input circuit12,14, so that the designations input circuit12, input circuit14and part circuit12,14have the same meaning.

In an input circuit unit10with two such part circuits12,14, to make a distinction, a first part circuit/input circuit12is referred to below as the master circuit12(or master12for short) and a second complementary part circuit/input circuit14is accordingly referred to as the slave circuit14(or slave14for short). The input circuit unit10comprising one part circuit12,14or both part circuits12,14, to distinguish it from the or from each part circuit/input circuit12,14that it contains, is thus referred to below as the overall circuit10.

The slave circuit14is optional within the overall circuit10and both the first part circuit12and also the second part circuit14can assume the functionality of the master circuit12. With part circuits12,14that are modular, plug-in or able to be releasably connected to the overall circuit10in any other suitable manner, the functionality of one of the two part circuits12,14, as master12or slave14, is produced by the circumstances as to which of the two part circuits12,14has first become active in the overall circuit10.

In a manner known per se, a signal source referred to below as a signaling device16, is connected to the input circuit unit10and thus to at least one part circuit12,14included therein. Such a signaling device or any other measurement transducer usually involves a signal source located in a technical process to be controlled or monitored, such as a power station, i.e., an analog or digital sensor basically known per se. By means of such a signaling device16(and in practice with a plurality of such signaling devices16), the state of the respective technical process is detected in a manner known per se.

In the schematic depicted inFIG. 1, it can be seen that the signaling device16is connected to the input18of the first part circuit12and to the input20of the second part circuit14. This can be achieved by corresponding attachment of the or of each signal line22coming from the signaling device16or via a bridge24within the overall circuit10.

With a signal from the signaling device16, a measurement current IMflows into the respective part circuit12,14. By means of a switching element included in the part circuit12,14and referred to below for short as switch S1, the function of the respective input18,20as a binary input or as an analog input for current signals can be set. The switch S1is able to be activated by a control device, such as a microcontroller26and a control current able to be output by the controller, included in the part circuit12,14. For the measurement, the part circuit12,14includes a voltage divider with an input resistor REand a measurement resistor RM, connected to ground and thus functioning as a pull-down resistor. Here, the switch S1is connected in parallel to the input resistor RE.

For operation as a current input, the switch S1is closed. This leads to the input18,20of the part circuit12,14being low-resistance. For operation of the part circuit12,14as current input, a measurement current IMis detected in the usual value range, such as 0 to 20 mA or 4 to 20 mA. In the other cases, i.e., when the part circuit12,14is functioning as a binary input or as an analog voltage input, the switch S1is open.

To obtain the voltage drop across the measurement resistor RMreferred to below, a measured value UMis detected, for example, by a measurement system28comprising a multiplexer, a measurement amplifier and an analog-to-digital converter (ADC) and is read in by the microcontroller26and evaluated as the respective input signal. If the part circuit12,14is operated as binary input, a digitized signal output by the measurement system28is evaluated by the microcontroller26in relation to a predetermined or predeterminable upper and/or lower threshold value and, if the threshold value is exceeded or not reached, then a logical high or logical low signal is created.

The master property is assigned via a cross coupling30to the respective redundancy partner, i.e., from the first part circuit12to the second part circuit14and vice versa. By means of the cross coupling30, it is also recognizable for a part circuit12,14whether a further part circuit12,14is present as a redundancy partner. If a redundancy partner is present, the measurement current IMis divided up and a small extent flows via the input20of the slave circuit14. On the part of the master circuit12, this is able to be corrected computationally and, in the presently described embodiment, a correction is provided in the form of a multiplication by a fixed factor. The microcontroller26is considered as a way to provide such a correction, and for correction the microcontroller26accordingly causes the read-in input signal to be multiplied by the respective correction factor. If a redundancy partner is not plugged in, the measurement current IMflows entirely in the master12and accordingly no correction is necessary.

When inputs18,20functioning as current input are made redundant, only the switch S1of the part circuit12functioning as the master12is closed. Here, significant proportion of the measurement current IMflows across the measurement resistor RMof the master circuit12and creates the voltage drop UM. When inputs18,20functioning as binary input (voltage input) are made redundant, both switches S1, i.e., the switch S1of the part circuit functioning as the master12and the switch S1of the part circuit14functioning as the slave14are open.

FIG. 2shows a schematic of a particular embodiment as a realization of one of the part circuits12,14or two redundant part circuits12,14. The switch S1(FIG. 1) is realized in this embodiment in the form of two transistors T1, T2in a Darlington circuit. The switch S1or each of the switch elements/transistors T1, T2functioning as switch S1is activated via a control signal (USt) output by the microcontroller26. Specifically, when the switch S1is realized in the form of two transistors T1, T2combined into a Darlington circuit or a Darlington transistor, only a negligibly small falsification of the measurement current IMis produced by the electronic activation of the switch S1.

In the embodiment shown inFIG. 2, the control signal UStgenerated by the microcontroller is given to a series transistor T3functioning as an inverting stage.

The series transistor T3is connected between ground and, via a pull-up resistor (RP), an operating potential. The Darlington circuit is supplied with power from the operating voltage potential. For a control signal UStfrom the microcontroller26with a HIGH level the series transistor T3becomes conductive and the switch S1is open. For a control signal UStfrom the microcontroller26with a LOW level the series transistor T3is open and the Darlington circuit is connected to the operating voltage potential, so that the switch S1closes and the input resistor REis short circuited. The series resistor RVis high-resistance, so that only a low control current flows, which only increases the measurement current IMinsignificantly.

Individual advantages of the switchable input circuit unit10described here lie in the fact that a single connecting terminal is sufficient as an input18,20, because when the circuit is made redundant the measurement current IMcan be conveyed via a bridge24(seeFIG. 1) within the circuit to the slave circuit14. The measurement current IM, in redundant operation, but just as much in individual operation as well, is tapped off within the circuit to ground and thus does not require any further measures within the input circuit unit10or the respective part circuit12,14. The respective input type, i.e., “binary”, “current analog” or “voltage analog” is able to be selected by the microcontroller26via software or predetermined or predeterminable parameters. The switchover is performed electronically via the microcontroller26. Even with the input type “current analog” making the circuit redundant is possible by direct parallel connection of two parallel part circuits12,14. External additional measures, such as bridge settings within the input circuit unit10, are not necessary. Binary values can also be measured as an analog voltage. The state is determined via a predetermined or predeterminable comparison threshold. Overload protection of the input circuit unit10can be achieved by a clocked opening and closing of the switch S1of the part circuit or of each part circuit12,14.

A part circuit12,14of the type described here is also able to be operated autonomously as input circuit12,14, i.e., as an input circuit12,14able to be switched over by a corresponding activation of the electronically controllable switch S1contained therein. Depending on the switch position of the switch S1, an analog current measurement or an analog or digital voltage measurement is performed via the input circuit12,14.

FIG. 3is a flowchart of a method for operating an input circuit unit (10). The method comprises detecting a parallel connection of the first and the second input circuit (12,14), as indicated in step310. Next, a compensation of the division of a measuring current (IM) among first and the second input circuits (12,14) is performed, as indicated in step320.