Signal transmission device for analog current signal

Abstract

The invention relates to a signal transmission circuit (1) for analog current signals in an explosion-proof region, comprising: a transformer (2) having one or more primary coils (4, 4a, 4b) and one or more secondary coils (6, 6a, 6b) coupled in a common magnetic circuit via a transformer yoke (J); a control circuit (10) configured to control a voltage across the one or more secondary coils (6, 6a, 6b) to zero by providing a compensation current for flux compensation, where the control variable corresponds to a measurement voltage that is one of: obtained from a voltage across a measurement coil (8) arranged in the magnetic circuit; the analog current signal to be transmitted corresponds to or depends on the compensation current, and is obtained as a function of the voltage across one of the plurality of secondary coils (6a, 6b) in the magnetic circuit when it is powered off.

Description

Technical Field

[0001] The present invention relates to a signal transmission device for transmitting an analog current signal of a current loop interface having galvanic separation. Background Art

[0002] In process automation, analog signals are often transmitted via current loop interfaces, which set the current to 0 mA or 4 mA to 20 mA depending on the analog signal to be transmitted. This form of variable current based analog signal transmission represents a very robust signaling standard and is common in many areas of process automation.

[0003] However, in explosion-proof areas, it is necessary to provide galvanic isolation between the sensor module in the area of ​​the measuring point and the control unit in the control center. In the field of signal transmission, galvanic isolation is usually achieved by transformers.

[0004] The current signal is transmitted with the help of a transformer whose primary and secondary sides have windings with the same number of turns and opposite winding directions. For an alternating current signal, when the magnetic flux in the core is equal to zero, the primary and secondary sides carry the same current.

[0005] To set the flux in the core to zero, so that a current flow occurs in the secondary that corresponds to the current flow in the primary, the voltage in the secondary is controlled by means of a control current to 0 V. The control current of the control device then corresponds to the primary current, ie the current signal.

[0006] Since the analog current signal to be transmitted may be a direct current signal or a very low frequency signal, it is usually configured to provide DC / AC conversion on the primary side of the transformer and AC / DC conversion on the secondary side of the transformer. Usually, the DC / AC conversion is performed by periodically reversing the polarity, that is, by applying a current signal of periodically reversed polarity to the primary coil of the transformer by means of a primary inverter and by applying a compensation current to the secondary coils by means of a secondary inverter. The primary inverter and the secondary inverter are usually operated in anti-phase.

[0007] However, such an arrangement causes the parasitic characteristics of the semiconductor switches of the inverter and the parasitic ohmic resistance of the copper windings of the coil to distort the signal transmission.

[0008] Therefore, it is preferred to compensate for parasitic influences. Generally, parasitic influences can be compensated by extensive measures, however, since the influences are generally non-linear, a corresponding compensation must be performed individually for each signal transmission circuit. Furthermore, parasitic influences are temperature-dependent, so that temperature compensation must also be provided.

[0009] It is therefore an object of the present invention to provide an improved signal transmission circuit for current signals having a galvanically isolated current loop, in which no additional compensation of parasitic influences is necessary. Summary of the invention

[0010] This object is achieved by a signal transmission circuit according to claim 1 .

[0011] Further embodiments are set forth in the dependent claims.

[0012] According to a first aspect, a signal transmission circuit for simulating a current signal in an explosion-proof area is provided, the signal transmission circuit comprising:

[0013] a transformer having one or more (preferably two) primary coils and one or more (preferably two) secondary coils coupled in a common magnetic circuit via a transformer yoke and having in particular the same number of turns;

[0014] - a control circuit configured to control the voltage across the one or more secondary coils to zero by providing a compensation current for flux compensation, wherein the control variable corresponds to a measured voltage, the measured voltage being one of:

[0015] o a measurement voltage obtained from the voltage across a measurement coil arranged in the magnetic ring; and

[0016] ○ The measured voltage obtained from the voltage across one of the multiple secondary coils in the magnetic coil when the coil is de-energized,

[0017] Therein, the analog current signal to be transmitted corresponds to the compensation current or depends on the compensation current.

[0018] In addition, the signal transmission circuit may include a control unit and a first inverter, wherein the first inverter is controlled by the control unit to apply the analog current signal as a cyclic alternating current signal to the primary coil, or alternatively, the first inverter is controlled by the control unit to apply the analog current signal alternately to one of a plurality of primary coils.

[0019] Furthermore, the measuring coil may be arranged in the magnetic circuit, and the signal transmission circuit may further include:

[0020] - a second inverter controlled by the control unit to apply a compensating current with reversed polarity to one or more secondary coils alternately or cyclically,

[0021] Therein, the control unit is configured to cyclically operate the first inverter and the second inverter in the same phase or in anti-phase, in particular at a switching frequency of 10 kHz to 350 kHz.

[0022] As described at the beginning, the signal transmission circuit for current signals in explosion-proof areas can have an inverter, wherein a first primary inverter converts the current signal of the current loop into an alternating current signal and applies the alternating current signal to the primary coil of the transformer. The secondary coil is connected to a second secondary inverter that operates in anti-phase with the first primary inverter, so that the applied current signal acts on the transformer yoke in anti-phase with the current signal applied to the primary.

[0023] The inverter is usually configured as an H-bridge circuit for cyclic pole reversal of the primary coil and / or the secondary coil, or as a two-way circuit for AC energization of the plurality of primary coils and / or one of the plurality of secondary coils, and the inverter is controlled by a suitable control unit. The inverter is used to convert a constant or low-frequency current signal into a periodic alternating current signal and transmit the signal with the aid of a transformer. Galvanic isolation is ensured by the transformer.

[0024] In the case where the inverter is provided as a two-way circuit, the inverter can be implemented by two alternating switches (semiconductor switches) and two coils connected in series. The central node of the two-way circuit can be a source or sink of the signal current and / or the compensation current. The semiconductor switches of the two-way circuit are switched alternately. The alternating switching is performed so that the flow direction in the magnetic circuit is reversed with each switching operation.

[0025] In order to transmit the current signal, flux compensation is usually provided which, by means of a control device, constantly ensures that the voltage across the secondary coil is 0 V. Then - as long as the primary and secondary have the same number of turns - the primary current through the primary coil corresponds to the secondary current of the secondary coil.

[0026] Flux compensation can use the secondary voltage as a control variable by injecting a continuously controlled compensation current so that the secondary voltage is set to 0 volts. However, this causes a current to flow through the semiconductor switch and the secondary coil of the second secondary inverter, which is affected by parasitic effects caused by the voltage drop across the ohmic resistance of the secondary coil and the voltage drop across the on-resistance of the semiconductor switch of the second inverter switched by the switch closure. These parasitic effects are usually also compensated by the control circuit and cause a compensation current that deviates from the primary current signal to be mapped. Therefore, the current mirror from the primary to the secondary is distorted, and it is necessary to compensate for the parasitic effects of the transmitted current signal. In addition, the ohmic resistance of the secondary coil and the on-resistance of the semiconductor switch are highly temperature-dependent, and compensation for this is also complex.

[0027] Furthermore, a third inverter may be provided to perform rectification of the voltage across the measuring coil.

[0028] Therefore, the above signal transmission circuit for transmitting the current signal of the current loop provides an alternative solution for realizing flux compensation by providing the transformer with a third coil (measuring coil), which is connected to the control circuit via a third inverter to provide a measurement voltage as a control variable.

[0029] The third inverter can be controlled synchronously with the second inverter.Like the secondary coil, the measuring coil is inductively coupled to the primary coil via the transformer yoke and is therefore supplied at its connection with a voltage or current that depends on the flux through the transformer yoke.

[0030] However, the control circuit is configured to energize the secondary coil so that the flux through the transformer yoke is compensated to zero. The control circuit is based on the measuring coil voltage rectified by the third inverter and measured in the power-off state, and therefore the parasitic effects caused by the ohmic resistance do not have an effect on the flux compensation of the control circuit. Therefore, there is no need to compensate for parasitic effects, and the calibration of the signal transmission circuit can be omitted where appropriate. In addition, since the flux through the measuring coil is controlled to zero, the temperature effect is compensated.

[0031] In the case of a two-way circuit for controlling a plurality of secondary coils in a magnetic circuit according to a further alternative, the secondary coils are alternately connected to the current path by the control circuit, wherein the respective other coils remain de-energized. The respective de-energized secondary coils can be used to provide a measurement voltage.

[0032] In this aspect, the control circuit may have an operational amplifier that compensates for a voltage deviation of the measured voltage from 0 volt by providing a variable compensation current through the second inverter and the secondary coil to control the measured voltage to 0 volt, wherein the compensation current corresponds to the transmitted current signal.

[0033] Furthermore, the control circuit can have an amplifier semiconductor switch, in particular a bipolar transistor or a field effect transistor, which is controlled by a signal at the output of the operational amplifier, wherein the amplifier semiconductor switch controls the level of the compensation current as a function of the signal at the output of the operational amplifier.

[0034] In order to filter higher frequency transients after switching of the inverter, a low-pass filter can be provided which smoothes the control variable.

[0035] Furthermore, the control unit may be configured to operate the third inverter with a phase offset of a cycle duration between 0% and 50% relative to operation of the second inverter.

[0036] Furthermore, the switching timing of the third inverter for the measuring coil can be delayed relative to the control of the second inverter so that at least a portion of the transient process after switching the semiconductor switch of the second inverter is suppressed, wherein the switching delay is between 0% and 50% of the period duration of the cyclic operation of the inverter.

[0037] Furthermore, the energy in the leakage inductance can have a significant effect on the transmission of the current signal. This effect usually manifests itself as a transient process, which can be harmlessly eliminated by appropriate switching measures. These measures can be attenuators, such as RC combinations or selective suppression.

[0038] In addition, the inverters can each be implemented by an H-bridge circuit, each of which is implemented by two inverter circuits. The center nodes of the inverter circuits are connected to each other via corresponding coils. The semiconductor switches are switched alternately. Alternating switching occurs so that the current direction in the coils arranged between the center nodes of the semiconductor switches connected in series is reversed with each switching operation.

[0039] In particular, the cyclic operation of the inverter may be provided without switching gaps or with overlaps in the conduction phases of the semiconductor switches of at least one of the inverter circuits.

[0040] Typically, switching gaps are provided for the switching of such inverters to avoid short-circuit current phases through the inverter circuit or simultaneous conduction of switches in the dual-circuit circuit when switching delays occur. These switching gaps may cause the compensation current to exceed the current value of the current signal. However, since the current signal is applied continuously, the energy introduced during the switching gap is added to the average value of the current signal, and therefore must be compensated with the corresponding compensation current. In order to avoid this effect, a compensation current can be provided to operate the inverter or dual-circuit circuit without a switching gap, and if necessary, the overlap of the conduction phase of the semiconductor switch (when the semiconductor switch is closed) is allowed or permitted, for example, to reach 5% of the cycle duration during which the semiconductor switch of the inverter branch of the secondary coil is briefly closed, which has no effect on the transmission of the current signal because the voltage across the secondary coil is controlled to 0 volts anyway. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The embodiments are described in more detail below with reference to the accompanying drawings, in which:

[0042] Figure 1 shows a schematic representation of a signal transmission circuit according to an embodiment;

[0043] Figure 2 shows a schematic representation of a signal transmission circuit according to another embodiment; and

[0044] Figure 3 A schematic representation of a signal transmission circuit according to yet another embodiment is shown.

[0045] Like reference numerals refer to elements having similar or comparable functions. DETAILED DESCRIPTION

[0046] Figure 1 The circuit diagram of a signal transmission circuit 1 for transmitting a current signal is shown. The current signal I can be a current signal of a current loop between 0 mA or 4 mA and 20 mA, which is common in process automation. The current signal I corresponds to an analog signal through which an analog variable can be transmitted. In an explosion-proof area, it is necessary to electrically isolate the line for transmitting the current signal from the explosion-proof area.

[0047] Thus, the signal transmission circuit 1 is intended to transmit a current signal by means of a transformer 2. The transformer 2 is provided with a primary coil 4 and a secondary coil 6, which are coupled to each other in a magnetic circuit via a transformer yoke J. The primary coil 4 and the secondary coil 6 preferably have the same number of turns and are wound in opposite directions.

[0048] Since the current signal is usually applied continuously and changes only at a low frequency, a first inverter 3 is provided for the primary coil 4 and a second inverter 5 is provided for the secondary coil 6. These inverters are used to convert the low frequency current signal into an alternating current signal to ensure that the signal information is transmitted via the transformer 2.

[0049] The inverters 3, 5 are configured as H-bridge circuits and have two inverter paths including semiconductor switches 31, 51 connected in series, whose center nodes M are connected to each other via the respective coils 4, 6. By alternately switching the semiconductor switches 31, 51, the polarity of the interconnection of the respective coils 4, 6 can be periodically reversed so that corresponding signal transmission via the transformer 2 is possible.

[0050] The first inverter 3 and the second inverter 5 are usually operated in phase or in anti-phase so that the second inverter 5 applies the same current signal to the transformer 2 as the first inverter 3. The semiconductor switches 31, 51 are controlled by means of a control unit 9 or the like at a predetermined switching frequency between 10 kHz and 350 kHz.

[0051] A control circuit 10 is provided to control the voltage across the secondary winding 8. To this end, the control circuit 10 has an operational amplifier 11 and an amplifier semiconductor switch 12, in particular a bipolar transistor or a field effect transistor, which is controlled by a signal at the output of the operational amplifier 11. To this end, the output of the operational amplifier 11 controls the control terminal (base terminal or gate terminal) of the amplifier semiconductor switch 12.

[0052] The amplifier semiconductor switch 12 is connected between a supply voltage source 14 and the second inverter 5 having the secondary winding 6 so that the current through the second inverter 5 can be variably set. The supply voltage source 14 is at a high supply potential V 高 With low supply potential V 低 A constant supply voltage is provided between 1 and 2. The current through the supply voltage source 14 represents the transmitted current signal.

[0053] The non-inverting input of the operational amplifier 11 is connected to the low supply potential V 低 , low supply potential V 低 Indicates the reference potential or the potential of 0 V. The inverting input of the operational amplifier 11 is connected to the flux-dependent measurement voltage.

[0054] The measurement voltage is detected via a third coil, the measurement coil 8, which is arranged in the magnetic circuit of the transformer 2. The third inverter 7 is assigned to the measurement coil 8 to rectify the voltage induced in the measurement coil 8 and provide it as the measurement voltage. To do this, the third inverter 7 is synchronously controlled with the second inverter 5.

[0055] The third inverter 7 switches between a low supply potential (reference potential) and the inverting input of the operational amplifier 11 .

[0056] The amplifier semiconductor switch 12 switches between the supply voltage source 14 and the second inverter 5 with the secondary coil 6, so that the output of the operational amplifier 11 controls the current flowing through the secondary coil 6 according to the voltage difference across the measuring coil 8. If a magnetic flux appears in the transformer yoke, the measured voltage corresponding to the rectified voltage across the measuring coil 8 changes to a non-zero value, and a control intervention is performed via the operational amplifier 11 and the amplifier semiconductor switch 12 by injecting a current into the secondary coil 6 so that the voltage across the measuring coil 8 is controlled to zero. To do this, the required current substantially corresponds to the current signal of the primary side of the transformer 2.

[0057] In order to eliminate the influence of transient processes, the control signal for controlling the third inverter 7 may be delayed by 50% relative to the control of the second inverter 5 .

[0058] Alternatively, or in addition, a low-pass filter may be provided between the third inverter 7 and the inverting input of the operational amplifier, which low-pass filter passes only the DC component of the measurement voltage to the operational amplifier 11 .

[0059] Furthermore, the leakage inductance can be eliminated in a suitable manner. A suitable means for the neutralization of the leakage inductance when the voltage across the measuring coil 8 is detected is an attenuator with an RC combination or a sampling delay.

[0060] Typically, an inverter constructed in the form of an H-bridge circuit is operated with a switching gap to avoid a short circuit of the inverter path via the bridge circuit. However, this results in an increase in the compensation current through the amplifier semiconductor switch 12. Since the energy transfer via the transformer 2 is prevented by means of flux compensation, the second inverter 5 and the third inverter 7 can be operated without a switching gap, and in particular, a switching overlap can be provided to briefly allow a short circuit path via at least one of the inverter paths in the inverter path. The overlap can be between 0% and 5%. Since the voltage across the secondary coil 6 and the measuring coil 8 is controlled to 0V, a short circuit of one of the inverter paths of the inverters 5 and 7 does not cause a significant current flow.

[0061] Figure 2 Another embodiment of the signal transmission circuit is shown. Figure 1 Compared with the embodiment of the present invention, the inverter 3', 5' is not configured as an H-bridge circuit, but is configured as a two-way circuit controlled by a control unit 9 to switch one of the two primary coils 4a, 4b and one of the two secondary coils 6a, 6b into the current path and disconnect the corresponding other coil from the current path. The two primary coils 4a, 4b and the two secondary coils 6a, 6b are arranged in the same magnetic circuit by means of a transformer yoke. Each of the coils is connected in series with a semiconductor switch and can therefore be separated from the current path.

[0062] The inverters 3', 5' are controlled by means of Figure 1 The embodiment of the measurement coil 8 and the third inverter 7 is similar. It can also be arranged that one of the inverters is formed as an H-bridge circuit for controlling only one coil, and the other inverter in the inverter is formed as a two-way circuit for controlling two coils.

[0063] Figure 3 Another embodiment of the signal transmission circuit is shown. Figure 2 As in the embodiment of the present invention, the second inverter 5' is configured as a two-way circuit controlled by the control unit 9. The first inverter 3 may be configured as an H-bridge circuit for controlling only one primary coil or as a two-way circuit for controlling two primary coils 4a, 4b.

[0064] The node between the secondary coils 6a, 6b and the corresponding semiconductor switches of the second inverter 5' is connected to the inverting input of the operational amplifier 11 via an additional semiconductor switch 52. The inverter is controlled by the control unit 9 in the manner described above. The additional semiconductor switch 52 is controlled by the control unit 9 to connect the de-energized secondary coils 6a, 6b to the operational amplifier and to disconnect the other coils from the operational amplifier.

Claims

1. A signal transmission circuit (1) for simulating current signals in an explosion-proof area, comprising: - a transformer (2) having one or more primary coils (4, 4a, 4b) and one or more secondary coils (6, 6a, 6b) coupled in a common magnetic circuit via a transformer yoke (J); - a control circuit (10) configured to control the voltage across the one or more secondary coils (6, 6a, 6b) to zero by providing a compensation current for flux compensation, wherein the control variable corresponds to a measured voltage, the measured voltage being one of: o obtained from the voltage across a measuring coil (8) arranged in the magnetic circuit; and ○ obtained based on the voltage across one of the plurality of secondary coils (6a, 6b) in the magnetic circuit when the secondary coil is de-energized, Therein, the analog current signal to be transmitted corresponds to the compensation current or depends on the compensation current.

2. The signal transmission circuit (1) according to claim 1, further comprising: - a control unit (9), - a first inverter (3, 3') controlled by the control unit (9) to apply the analog current signal as a circulating alternating current signal to the one or more primary coils (4, 4a, 4b).

3. The signal transmission circuit (1) according to claim 1, further comprising: - a control unit (9), - a first inverter (3, 3'), the first inverter (3, 3') being controlled by the control unit (9) to alternately apply the analog current signal to one of the plurality of primary coils (4a, 4b).

4. The signal transmission circuit (1) according to claim 2 or 3, wherein: The measuring coil (8) is arranged in the magnetic circuit, and the signal transmission circuit (1) further comprises: - a second inverter (5, 5'), which is controlled by the control unit (9) to cyclically reverse the polarity of the compensation current or cyclically apply the compensation current alternately to the one or more secondary coils (6, 6a, 6b), The control unit (9) is configured to cyclically operate the first inverter (3, 3') and the second inverter (5, 5') in phase or in anti-phase, in particular at a switching frequency between 10 kHz and 350 kHz.

5. The signal transmission circuit (1) according to claim 4, wherein: A third inverter (7) is provided to rectify the voltage across the measuring coil (8).

6. The signal transmission circuit (1) according to claim 5, wherein: The control unit (9) is configured to operate the third inverter (7) in phase with the second inverter (5, 5') or in synchronization with the second inverter (5, 5').

7. The signal transmission circuit (1) according to claim 5, wherein: The control unit (9) is configured to operate the third inverter (7) with a phase shift of between 0% and 50% of the cycle duration relative to the operation of the second inverter (5, 5').

8. The signal transmission circuit (1) according to claim 7, wherein: The switching timing of the third inverter (7) of the measuring coil (8) is delayed relative to the control of the second inverter (5, 5'), so that at least a part of the transient process after switching the semiconductor switch (51) of the second inverter (5, 5') is suppressed, wherein the switching delay is between 0% and 50% of the period of the cyclic operation of the inverter.

9. The signal transmission circuit (1) according to one of claims 1 to 3, wherein: A plurality of secondary coils (6a, 6b) are arranged in the magnetic circuit, and the plurality of secondary coils (6a, 6b) are alternately switched into the current path by the control circuit with the aid of a second inverter (5'), wherein the corresponding de-energized secondary coils (6a, 6b) are used to provide the measurement voltage.

10. The signal transmission circuit (1) according to one of claims 1 to 9, wherein: One or more primary coils (4, 4a, 4b) and one or more secondary coils (6, 6a, 6b) carrying current simultaneously have the same number of turns.

11. The signal transmission circuit (1) according to one of claims 1 to 10, wherein: The control circuit (10) has an operational amplifier (11), which controls the measured voltage to 0 volt by equalizing the voltage deviation between the measured voltage and 0 volt, wherein the voltage deviation between the measured voltage and 0 volt is equalized by providing a variable compensation current via the second inverter (5, 5') and the secondary coil (6, 6a, 6b), wherein the compensation current corresponds to the transmitted current signal.

12. The signal transmission circuit (1) according to claim 11, wherein: The control circuit (10) comprises an amplifier semiconductor switch (12) controlled by a signal at the output end of the operational amplifier (11), wherein the amplifier semiconductor switch (12) is particularly a bipolar transistor or a field effect transistor, wherein the amplifier semiconductor switch (12) controls the level of the compensation current according to the signal at the output end of the operational amplifier (11).

13. The signal transmission circuit (1) according to one of claims 1 to 11, wherein: The inverters (3, 5) are each configured as an H-bridge circuit including an inverter circuit or as a dual-path circuit including at least two semiconductor switches (31, 51), wherein a cyclic operation of the inverters (3, 5) is provided without switching gaps or overlaps in the conduction phases of the semiconductor switches (3, 5) of the dual-path circuit or at least one of the inverter circuits.

14. The signal transmission circuit (1) according to one of claims 1 to 12, wherein: The measured voltage is low-pass filtered.

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