ARRANGEMENT WITH AT LEAST TWO REDUNDANT ANALOGUE INPUT UNITS FOR ONE MEASURING CURRENT

Description

[0001] The invention relates to an arrangement with at least two redundant analog input units for a measuring current, which have parallel-connected analog inputs for the measuring current and each contain a current measuring resistor, a voltage measuring device with a high-impedance voltage measuring input for converting a voltage applied to the voltage measuring input into a digital measured value, a digital output for outputting the digital measured value, and a controllable switching device for switching the analog input between a high-impedance state and a low-impedance state, wherein in the low-impedance state the current measuring resistor is simultaneously connected to the analog input and the high-impedance voltage measuring input, wherein the analog input units are connected to one another via a communication connection and are designed to determine one of the analog input units which switches its analog input to the low-impedance state for measuring and converting the measuring current into a digital measured value,while in every other analog input unit the analog input is switched to the high-impedance state. ,

[0002] Such an arrangement, known from WO 2015 / 128336 A1, has two redundant input units, each of which can be operated selectively as an analog input unit for a measuring current or as a binary input unit for a voltage. To switch between these two operating states, each input unit contains a switching device in which a parallel circuit consisting of a high-ohm resistor and a controllable switch is connected in series with the low-ohm current measuring resistor at the input of the input unit. The voltage measuring input of the voltage measuring device is connected to the current measuring resistor.

[0003] In redundant operation as binary input units, the controllable switch is open on both input units, so that the high-ohm resistor and the low-ohm current measuring resistor form a voltage divider and the voltage measuring device measures a high or low level of the voltage applied to the input of the respective binary input unit.

[0004] In redundant operation as analog input units for a measuring current, the controllable switch is closed in at least one of the two input units, so that the high-ohm resistor is ineffective. If the controllable switches are closed in both input units, the measuring current is divided equally between the two low-ohm current measuring resistors, so that the downstream voltage measuring devices each measure half the measuring current as a voltage drop across their low-ohm current measuring resistor. The respective input unit then performs a computational correction by a factor of 2 before outputting the digital measured value of the measuring current.If the controllable switch in only one of the two input units is closed, the measuring current is distributed in a different ratio between the current measuring resistor in the input unit with the closed switch and the series connection of resistors in the other input unit, so that the downstream voltage measuring devices measure the corresponding portions of the measuring current as voltage drops across their low-ohm current measuring resistors. Here, too, the input unit with the closed switch, for example, makes a mathematical correction for the ratio of the measuring current distribution between the redundant input units before outputting the digital measured value. A desired distribution of the measuring current in a ratio of 97% to 3% is achieved with a resistance ratio of 1 to 31.33 between the current measuring resistor and the high-ohm resistor.However, the precision of the distribution of the measuring current over the specified resistance ratio and thus the precision of the calculated measured value correction is limited by the unavoidable and unknown line resistances between the two analog input units.

[0005] A communication link exists between the two redundant input units of the known arrangement. They use this link to detect whether a redundant partner is present; they determine which of them will perform the measurement task as the master, and if the master fails, they assign the corresponding function to the other input unit.

[0006] WO 03 / 039904 A2 discloses a semiconductor switch for switching, for example, an inductive load. The semiconductor switch is a sense FET that detects a partial current proportional to the load current via a measuring resistor. To increase operational reliability, the partial current is additionally (redundantly) measured by detecting the voltage drop it causes across the measuring resistor using a signal amplifier and converting it into a digital measured value. This can be evaluated with respect to a predetermined upper and / or lower threshold by generating a logical high or logical low signal when the threshold is exceeded or undershot. In addition to the redundant current measurement, two or more sense FETs can also be used to switch the same load to further increase operational reliability.The sense FETs with redundant current measurement can also be arranged in different circuits (driver circuit and freewheeling circuit) of the inductive load. Switching an analog input between a high-impedance state and a low-impedance state is not provided. Based on the arrangement known from WO 2015 / 128336 A1, the invention is based on the object of enabling a redundant analog input for a measurement current that does not require computational correction of the measured values.

[0007] According to the invention, which is defined by the features of claim 1, the object is achieved in that in the arrangement of the type specified at the outset in each analogue input unit, the high-impedance voltage measuring input of the voltage measuring device is directly connected to the analogue input, and each of the at least two analogue input units is designed to compare the digital measured value currently generated by it with a threshold value, both in the high-impedance and in the low-impedance state of its analogue input, to output an error message if the digital measured value falls below the threshold value, and, if its analogue input is in the low-impedance state, to cause another of the analogue input units to switch its analogue input to the low-impedance state for measuring and converting the measuring current into a digital measured value.

[0008] Since the voltage measurement input of the voltage measuring device is directly connected to the analog input of each analog input unit, all redundant analog input units measure practically the same voltage, namely the voltage drop across the current measuring resistor of the one analog input unit whose analog input is switched to the low-ohmic state. In contrast to the arrangement known from WO 2015 / 128336 A1 mentioned above, there is no voltage division, so that the resistance ratio between the high-ohmic resistor and the current measuring resistor is irrelevant and can therefore not only be selected to be higher, but in practice is also selected to be significantly higher, with a factor greater than 1000. Thus, no division of the measuring current to the redundant analog input unit takes place, or only a division that is negligible within the scope of the measurement accuracy.

[0009] Since, as explained above, all redundant analog input units detect at least approximately the same voltage drop, they can monitor themselves by converting the voltage applied to its analog input into a digital measured value and comparing this value with a threshold value, regardless of whether its analog input is switched to the low-impedance or high-impedance state. In the event of a voltage or current interruption at the analog input of an analog input unit, the voltage detected by the voltage measuring input of the voltage measuring device drops below the threshold value to zero. The cause of such an interruption can be a fault, e.g. a cable break, but also that the current-measuring analog input unit is removed from a rack, for example for calibration or as a result of a module replacement.This is now automatically detected and causes the affected analog input unit to output an error message instead of a measured value. This can be a value that lies outside the valid measuring range or information, e.g. a bit, that marks the measured value as incorrect. If the affected analog input unit is the current-measuring analog input unit whose analog input is switched to the low-impedance state, it sends a detection signal to the communication connection to cause one or the other redundant analog input unit to switch its analog input to the low-impedance state and take over the current measuring function. The analog input unit previously measuring current also preferentially switches its analog input to the high-impedance state.

[0010] Ideally, all redundant analog input units measure the same voltage drop, so that in principle each analog input unit can convert the voltage it measures into a digital measured value for the measuring current and output it via its digital output. However, since in practice only the current measuring resistors are designed as precision resistors and non-negligible line resistances may exist between the analog inputs of the various analog input units, it can no longer be assumed without further ado that the measured values ​​generated by the analog input unit(s) with the high-impedance analog inputs meet the specified requirements when very high measurement resolution requirements are combined with equally high measurement accuracy.For this case, the analog input units are designed to receive and output the digital measured value generated by the current-measuring analog input unit with the low-impedance analog input via the communication connection when their analog input is in the high-impedance state instead of the self-generated measured value.

[0011] The fact that the analog input units monitor themselves is particularly advantageous when, for reasons of redundancy in the transmission of the measuring current, the redundant analog input units are connected via separate lines to a measuring current source (e.g., a 4-20 mA measuring transducer) that supplies the measuring current. In this case, each of the separate lines is monitored for wire breaks.

[0012] As already explained above, an essential feature of the invention is that, for each analog input unit, the voltage measurement input of the voltage measuring device is directly connected to the analog input. This can be achieved by the switching device comprising a series circuit consisting of the current measuring resistor and a controllable switch, which is connected in parallel with a high-ohm resistor to the analog input and the voltage measurement input. Alternatively, the switching device comprises a parallel circuit consisting of a high-ohm resistor and a controllable switch, which is connected in series with the current measuring resistor to the analog input and the voltage measurement input.

[0013] For further explanation of the invention, reference is made below to the figures of the drawing; in detail, each shows a schematic representation: Fig. 1 shows a first embodiment of the arrangement according to the invention and Fig. 2 shows a second embodiment of the arrangement according to the invention.

[0014] The same reference symbols have the same meaning in the different figures.

[0015] Fig. 1 shows two redundantly operating analog input units 1, 2, which are connected in parallel to a measuring current transmitter 7 via separate lines 5, 6 with their analog inputs 3, 4. The measuring current transmitter 7 is, for example, a measuring transducer that generates a measuring current I between 4 and 20 mA that is proportional to a measured variable.

[0016] The analog input unit 1 contains a voltage measuring device 8 consisting of a differential amplifier 9 and a downstream analog / digital converter 10, which is connected with its voltage measuring input 11 directly to the analog input 3; i.e., the analog input 3 is the voltage measuring input 11. The analog input 3 can be switched between a high-impedance state and a low-impedance state by means of a controllable switching device 12. The switching device 12 consists of a parallel circuit comprising a high-impedance resistor 13 and a controllable switch 14, which is connected in series with a low-impedance current measuring resistor 15 to the analog input 3 or the voltage measuring input 11. The current measuring resistor 15 is designed as a precision resistor and has a resistance value of, for example, 250 Ω. The high-impedance resistor 13 has a resistance value of, for example, 1 MΩ.When the switch 14 is closed, i.e. when the analog input 3 is in the low-impedance state, the voltage measuring device 8 detects the voltage drop generated by the measuring current I across the current measuring resistor 15 and converts this into a digital measured value 16 for the detected measured variable. This is output by a control device (CPU) 17 via a digital output 18 as an interface to a higher-level controller (not shown here). The control device 17 also controls the switching device 12. The second analog input unit 2 is identical in construction to the analog input unit 1 and contains a voltage measuring device 19, the high-impedance voltage measuring input 20 of which coincides with the analog input 4 and which consists of a differential amplifier 21 and a downstream analog / digital converter 22. At the analog input 4 orThe voltage measuring input 20 is connected to a switching device 23 with a low-ohm current measuring resistor 24 in series with a parallel circuit consisting of a high-ohm resistor 25 and a controllable switch 26. This is controlled by a control device 27, which also outputs a digital measured value via a digital output 28.

[0017] The control devices 17, 27 of the two redundant analog input units 1, 2 communicate with each other via a communication connection 29, e.g., RS-485, and ensure that the analog input 3 of only one of the two analog input units 1, 2, here, for example, the analog input unit 1, is switched to the low-impedance state. In the other and, if applicable, each additional redundant analog input unit 2, the switch 26 is open and, accordingly, the analog input 4 is switched to the high-impedance state. While the analog input unit 1 measures the measuring current I and converts it into the digital measured value 16 and outputs it, the analog input unit 2 only monitors the voltage at its analog input 4, for which purpose the voltage is also converted into a digital measured value 30. Due to its very high resistance, the resistor 25 does not impair the current measurement in the analog input unit 1, or only impairs it to a negligible extent.The voltage measured by the voltage measuring device 19 of the analog input unit 2 corresponds to the voltage drop across the current measuring resistor 15 in the analog input unit 1 plus the voltage drop across the line 5 caused by the measuring current I. Since the latter voltage drop is not negligible with regard to the measuring accuracy, the analog input unit 2 accepts the digital measured value 16 generated by the current-measuring analog input unit 1 and communicated via the communication connection 29 and outputs it redundantly via its digital output 28 instead of its own measured value 30.

[0018] In both analog input units 1, 2, the respective control device 17, 27 compares the generated digital measured value 16, 30 with a predefined threshold value. If line 6 is interrupted, the voltage at analog input 4 or voltage measurement input 20 drops to zero, causing digital measured value 30 to fall below the predefined threshold value. In this case, an error message is output instead of the measured value 16 received from the current-measuring analog input unit 1 via the communication connection 29, or the output measured value 16 is marked as faulty.

[0019] Conversely, if a wire break is detected in the current-measuring analog input unit 1, an error message is also output via the digital output 18. Opening switch 14 switches analog input 3 to the high-impedance state, and the redundant analog input unit 2, by outputting a detection / command signal 31 via the communication link 29, causes it to switch its analog input 4 to low-impedance, assume the current measurement function, and transmit the generated measured values ​​30 to the analog input unit 1 via the communication link 29.

[0020] The Fig. 2 The embodiment shown differs from the one shown in Fig. 1in that, on the one hand, the two analog input units 1, 2 are connected directly in parallel at their analog inputs 3, 4 and are connected to the measuring current transmitter 7 via a line 32, and, on the other hand, the switching devices 12, 23 each comprise a series circuit consisting of the current measuring resistor 15, 24 and the controllable switch 14, 26, which, together with the high-ohmic resistor 13, 25, are connected in parallel to the analog input 3, 4 and the voltage measuring input 11, 20. Both modifications are independent of one another and can each be implemented independently.

Claims

1. Assembly with at least two redundant analogue input units (1, 2) for a measurement current (I), having - parallel-connected analogue entry points (3, 4) for the measurement current (I), and containing in each case - a current-measuring resistor (15, 24), - a voltage-measuring device (8, 19) with a high-ohmic voltage-measuring entry point (11, 20), for converting a voltage that is applied to the voltage-measuring entry point (11, 20) into a digital measurement value (16), - a digital output (18, 28) for outputting the digital measurement value (16), and - a controllable switching device (12, 23) for switching the analogue entry point (3, 4) between a high-ohmic state and a low-ohmic state, which switching device (12, 23) consists of a parallel circuit comprising a high-ohmic resistor (13, 25) and a controllable switch (14, 26), wherein in the low-ohmic state the current-measuring resistor (15, 24) is situated at both the analogue entry point (3, 4) and the high-ohmic voltage-measuring entry point (11, 20), wherein - the analogue input units (1, 2) are connected together via a communication connection (29) and are designed to select one of the analogue input units (for example 1), which switches its analogue entry point (3) into the low-ohmic state in order to measure and convert the measurement current (I) into a digital measurement value (16) while the analogue entry point (4) in each remaining analogue input unit (2) is switched into the high-ohmic state, wherein - in each analogue input unit (1, 2), the high-ohmic voltage-measuring entry point (11, 20) of the voltage-measuring device (8, 19) is directly connected to the analogue entry point (3, 4) in one switch (14, 26) of said switches (14, 26) is open and - each of the at least two analogue input units (1, 2) is designed to compare, in both the high-ohmic and low-ohmic state of its analogue entry point (3, 4), its currently generated digital measurement value (16, 30) with a threshold value, to output an error message if the digital measurement value (16, 30) falls below the threshold value and, if its analogue entry point (for example 3) is in the low-ohmic state, to cause another of the analogue input units (for example 2) to switch its analogue entry point (4) into the low-ohmic state in order to measure and convert the measurement current (I) into a digital measurement value, characterised in that each analogue input unit (for example 2) is further designed, when in the high-ohmic state of its analogue entry point (4), instead of the measurement value (30) that was generated locally, to output the digital measurement value (16) that was generated by the analogue input unit (1) having the low-ohmic analogue entry point (3) and received therefrom via the communication connection (29).

2. Assembly according to claim 1, characterised in that each analogue input unit (for example 1) is further designed, when in the low-ohmic state of its analogue entry point (3), to switch this into the high-ohmic state if the digital measurement value (16, 30) falls below the threshold value.

3. Assembly according to claim 1 or 2, characterised in that the switching device (12, 23) comprises a series circuit consisting of the current-measuring resistor (15, 24) and a controllable switch (14, 26), wherein said series circuit is situated in parallel with a high-ohmic resistor (13, 25) at the analogue entry point (3, 4) and the voltage-measuring entry point (11, 20).

4. Assembly according to one of the preceding claims, characterised in that the redundant analogue input units (1, 2) are connected via separate lines (5, 6) to a measurement current sensor (7) supplying the measurement current (I).

5. Assembly according to one of claims 1 to 3, characterised in that the redundant analogue input units (1, 2) are wired in parallel at their analogue entry points (3, 4) and connected via a line (32) to a measurement current sensor (7) supplying the measurement current (I).