Electronic circuit for a field device or a radio adapter

Abstract

An electronic circuit for a field device or a radio adapter for use in a potentially explosive area, comprising: - an interface (5a) for connecting the electronic circuit to a field device interface (15) of the field device or for connecting the electronic circuit to a two-wire line in order to supply energy to the electronic circuit; - an electronic component (6) requiring a higher energy, at least temporarily, than the energy provided via the interface; - an energy storage device (18) configured to store energy transmitted via the interface (5a) and to provide same to the electronic component as required; - an explosion protection unit (21) having at least one voltage limiting unit (21.2) configured to limit a voltage applied across the energy storage device (18) to a maximum value, wherein the explosion protection unit (21) is furthermore configured to activate the voltage limiting unit (21.2) only from a voltage value that is less than 15% below the maximum value.

Description

[0001] Electronic circuit for a field device or radio adapter

[0002] The invention relates to an electronic circuit for a field device or a radio adapter for use in a potentially explosive atmosphere.

[0003] In process automation technology, field devices are often used to record and / or influence process variables. Sensors such as level gauges, flow meters, pressure and temperature gauges, pH and redox potential meters, conductivity meters, etc., are used to record process variables, measuring the corresponding process variables: level, flow, pressure, temperature, pH value, or conductivity. Field devices are essentially all devices that are used close to the process and that provide or process-relevant information.

[0004] Currently, many existing automation systems still use two-wire field devices that are connected to a higher-level unit, such as a PLC control unit, via a two-wire cable. These two-wire field devices are designed so that power is supplied via the two-wire cable. Furthermore, they are designed so that the measured or control values, as the main process variable, are communicated, i.e., transmitted, via the two-wire cable in the form of an analog 4-20 mA signal. In addition to their primary function of generating measured values, modern field devices offer numerous additional functionalities that support the efficient and safe management of the process being monitored. These include functions such as self-monitoring of the field device, saving measured values, generating control signals for actuators, etc.

[0005] Furthermore, so-called radio adapters are increasingly being used in process automation. These adapters are either connected directly to a field device or, if not connected to it, are integrated into the existing wiring, especially a two-wire cable, in order to transmit the recorded process variables wirelessly.

[0006] A large number of such field devices and wireless adapters are manufactured and distributed by Endress + Hauser.

[0007] Due to the two-wire cable supply, field devices and wireless adapters are typically severely limited in terms of power supply. This means that the individual electrical components are typically coordinated in such a way that the total power requirement just does not exceed the maximum power available via the two-wire cable. Due to the limited power available, it is therefore not easily possible to implement high-power electronic components, such as a wireless interface for wireless transmission of process variables.

[0008] To still be able to operate such high-energy electronic components in a field device or wireless adapter, energy storage devices, such as batteries or capacitors, especially supercapacitors or high-capacitance capacitors, are used. The batteries or capacitors can store the energy provided via the two-wire cable and release it again when needed.

[0009] In order to be able to operate the field device or radio adapter in areas where there is a potential risk of explosion, so-called potentially explosive atmospheres, explosion protection measures are usually implemented in the field device or radio adapter. Corresponding protection principles for this have been defined in the IEC EN DIN 60079-ff standard. According to this standard, based on the applicable types of protection, circuitry measures for the electronics of the field device or radio adapter for use in potentially explosive atmospheres are defined, among other things. One of these types of protection is the "intrinsic safety" type of protection (Ex-i marking, IEC EN DIN 60079-11, June 2012 edition).

[0010] The "intrinsic safety" type of protection is based on the principle of current and voltage limitation in an electrical circuit. The energy in the circuit that could potentially ignite an explosive atmosphere is limited in such a way that neither sparks nor excessive heating of the electrical components can ignite the surrounding explosive atmosphere. For this reason, a voltage limiting circuit is usually installed upstream of the aforementioned energy storage devices, which limits the voltage across the energy storage device to a maximum value. Diodes, especially Zener diodes, are typically used for this purpose.

[0011] The disadvantage of such voltage limiting circuits is that they have a leakage current, via which the energy storage device is discharged, especially deep discharged, especially when the electronics are stored for longer periods without operating phases.

[0012] The invention is therefore based on the object of remedying this situation.

[0013] The object is achieved according to the invention by the electronic circuit for a

[0014] Field device or a radio adapter according to patent claim 1, the field device of automation technology according to patent claim 11 and the radio adapter of automation technology according to patent claim 12.

[0015] The electronic circuit according to the invention for a field device or a radio adapter for use in a potentially explosive area comprises: an interface for connecting the electronic circuit to a field device interface of the field device or for connecting the electronic circuit to a two-wire cable in order to supply the electronic circuit with energy; an electronic component which, at least temporarily, requires a higher energy than the energy made available via the interface; an energy store which is connected to the interface and the electronic component and is designed to store energy transmitted via the interface and to make it available to the electronic component when needed;an explosion protection unit with at least one voltage limiting unit, which is designed to limit a voltage applied across the energy storage device to a maximum value, wherein the explosion protection unit is further designed to activate the voltage limiting unit only from a voltage value that is less than 15%, preferably less than 10%, particularly preferably less than 5% below the maximum value;

[0016] According to the invention, an electronic circuit for a field device or a radio adapter is proposed, which is designed for use in a potentially explosive atmosphere in that an explosion-proof unit is connected upstream of an energy storage device, which limits the voltage across the energy storage device to a maximum value in the event of a fault by means of a voltage limiting unit and which is further configured according to the invention to only activate the voltage limiting unit from a voltage value that is less than 15%, preferably less than 10%, particularly preferably less than 5% below the maximum value. Because the voltage limiting unit is only activated shortly before the maximum value to which the voltage across the energy storage device is limited in the event of a fault is reached, the circuit has negligible power loss in the installed state.This makes it possible, for example, for such an electronic circuit to be stored in a warehouse in its installed state for a longer period of time.

[0017] An advantageous embodiment of the electronic circuit provides that the explosion protection unit is further configured to deactivate the voltage limiting unit if the voltage across the energy storage device is not less than 15%, preferably not less than 10%, particularly preferably not less than 5% below the maximum value.

[0018] A further advantageous embodiment of the electronic circuit provides that the explosion-proof unit further comprises a circuit element, preferably connected in series with the voltage limiting unit, and a control circuit. The control circuit and the circuit element are interconnected in such a way and are designed to activate the voltage limiting unit only at a voltage value that is less than 15%, preferably less than 10%, particularly preferably less than 5% below the maximum value. In particular, the embodiment can provide that the circuit element comprises a transistor, preferably a field-effect transistor, particularly preferably a p-channel normally-off MOSFET.

[0019] A further advantageous embodiment of the electronic circuit provides that the control circuit comprises a Schmitt trigger circuit or a comparator circuit.

[0020] A further advantageous embodiment of the electronic circuit provides that the maximum value to which the voltage limiting unit limits the voltage applied across the energy storage device corresponds to a value specified for the energy storage device at which the energy storage device is to be operated in proper operation.

[0021] A further advantageous embodiment of the electronic circuit provides that the maximum value is in the range of 2-10 V, particularly preferably in the range of 3-8 V, most preferably in the range of 3-6 V.

[0022] A further advantageous embodiment of the electronic circuit provides that the voltage limiting unit comprises at least one diode, in particular a Z-diode, with a diode voltage, in particular a Z-voltage, which corresponds to the maximum value.

[0023] A further advantageous embodiment of the electronic circuit provides that the energy storage device comprises a battery or a capacitor, in particular a supercapacitor or a high-capacity capacitor.

[0024] A further advantageous embodiment of the electronic circuit provides that the electronic component comprises a radio module, which is particularly configured to wirelessly transmit a data packet according to a standard of a low-power wide-area network. The invention further relates to a field device in automation technology comprising an electronic circuit according to one of the previously described embodiments, wherein the interface for connecting the electronic circuit is connected to a field device interface of the field device, so that the field device supplies the electronic circuit with power.

[0025] The invention further relates to a radio adapter for automation technology comprising an electronic circuit according to one of the previously described embodiments, wherein the interface for connecting the electronic circuit is connected to a two-wire line, so that the electronic circuit is supplied with energy via the two-wire line.

[0026] The invention is explained in more detail with reference to the following drawings. It shows:

[0027] Fig. 1: a schematic representation of a field device to which a radio adapter with an electronic circuit designed according to the invention is connected to a field device interface of the field device, and

[0028] Fig. 2: an electronic circuit designed according to the invention.

[0029] Figure 1 shows a two-wire field device 1, which comprises a preferably metallic housing 2 in which field device electronics 4 are arranged. The field device 1 is designed such that it has connection terminals 13, via which a two-wire line 12 is or can be electrically connected. The field device electronics 4 and thus the field device 1 are connected via the two-wire line 12 to a higher-level unit (not separately shown in Fig. 1) in order to communicate data with the higher-level unit via wire. The measured values, for example as a main process variable, can be communicated via the two-wire line 12 in analog form in the form of a 4-20 mA current signal by setting a corresponding current value of the 4-20 mA current signal by the field device electronics 4. Other data, which may include parameters of the field device, for example, are transmitted in the form of a digital two-wire signal, for example according to the HART standard.

[0030] Furthermore, the field device electronics 4 are also supplied with power via the two-wire cable or the 4-20 mA current signal I2. For this purpose, an operating power is provided to the field device electronics depending on a clamping voltage Uk applied to the connection terminals and the 4-20 mA current signal. The clamping voltage Uk usually has a minimum voltage value of approximately 10 V, resulting in a minimum operating power for the field device electronics of Pmin = 10 V * 4 mA = 40 mW. In principle, however, the clamping voltage can also have a different value, e.g., a value in the range of 10-30 V, which also changes the minimum operating power available to the field device.

[0031] In order to ensure safe operation of the field device 1, the field device electronics 4 are designed such that all functions, in particular the acquisition or setting of measured values, the processing of these and the transmission or setting of a corresponding current value of the 4-20 mA current signal can be carried out by means of the minimum operating power Pmin.

[0032] Furthermore, the field device electronics 4 can be set up at the factory, i.e. by the field device manufacturer, so that part of the minimum operating power is permanently available for a display for showing, for example, measured or set values ​​or other data, such as parameters or configuration values ​​of the field device, etc.

[0033] The display 14, which can be configured either as part of the field device electronics or as a separate unit, can be connected or connectable to the field device electronics via an internal field device interface. The field device can further comprise a display mount or holder for mechanically securing the display. The display mount can, for example, be configured as part of the field device electronics. The display mount can further be arranged in the field device such that a display held therein is located in a housing section with a transparent housing wall, for example, a housing cover.

[0034] Fig. 1 also shows a radio adapter 5 with adapter electronics for connecting to the field device. The radio adapter can, for example, be connected to the internal field device interface 15 provided for the display 14 by means of a cable, so that the field device 1 can be subsequently converted for wireless data transmission, i.e., after it has already been integrated into an automation system. For example, the radio adapter 5 can be attached by a service technician who mechanically removes the cover 2a of the housing 2 to expose the interface, detaches the connecting cable 14b to the display 14 from the interface 15, and then connects the radio adapter 5 to the interface 15 by means of a corresponding connecting cable.

[0035] In order to mechanically fix the radio adapter 5 in the holder 19 actually intended for the display 14, an outer contour of the radio adapter 5 can essentially correspond to an outer contour of the display 14a, so that the holder 19 can accommodate the radio adapter 5. In addition to the embodiment shown in Fig. 1, in which the radio adapter is connected to the field device, a further embodiment can provide that the radio adapter is not connected directly to the field device, but is inserted directly into the two-wire line or connected to it.

[0036] The adapter electronics further comprises a current limiting circuit 16 configured to limit a current I2 flowing via the interface 15 to a value, preferably requested by the field device 1, so that only maximum operating power is provided to the radio adapter 5 via the interface. The current limiting circuit 16 can be configured to adjust the value to which the current I2 flowing via the interface 15 is limited, in particular to optimize it depending on the terminal voltage Uk applied to the connection terminals 13 and the 4-20 mA current signal.

[0037] The adapter electronics further comprises an adjustable output voltage boost converter 17, which is connected at an input 17a to an output 16b of the current limiting circuit 16. The output voltage boost converter 17 is configured to convert an input voltage applied to its input or the output of the current limiting circuit to an output voltage that is higher than the input voltage. For example, the output voltage boost converter 17 can be configured to convert an input voltage to a higher output voltage. In the described embodiment, the output voltage boost converter 17 converts from approximately 3 V to approximately 3.9 V, which is provided at an output of the output voltage boost converter 17b.

[0038] A charge controller 20 is connected to the output 17b of the output voltage boost converter 17 via one input, which charge controller is at least configured to regulate the charging of the energy storage device. For this purpose, the charge controller 20 can have an operational amplifier N1 and a metal oxide semiconductor field-effect transistor (MOSFET) V1. The charge controller 20 can be designed in particular to charge the energy storage device 18 with the maximum possible power. This can be achieved, for example, by monitoring the input of the output voltage boost converter 17a, e.g., by the operational amplifier N1, for a possible voltage drop, and in the event of such a voltage drop, the charging or the charging current is regulated accordingly, e.g., by the MOSFET. In this way, the energy storage device can be charged at the maximum possible charging point.

[0039] To enable the wireless adapter to be operated in the potentially explosive atmospheres mentioned above, the adapter electronics also include a so-called explosion-protection unit 21, which is designed to limit the voltage to a maximum value for an energy storage device (to be described in more detail later). The maximum value to which the explosion-protection unit limits the voltage is determined or selected depending on the downstream energy storage device in order to take into account the principle of voltage limitation laid down in the standard IEC EN DIN 60079-11, June 2012 edition, for the "intrinsic safety" type of protection.

[0040] The aforementioned rechargeable energy storage device 18 is connected to an input 18a at the output of the explosion-proof unit 21, so that the output voltage provided by the boost converter 17 charges the energy storage device 18. The energy storage device 18 can, for example, comprise one or more capacitors. Due to their high storage capacity combined with their small size, supercapacitors, such as a hybrid pulse capacitor (HPC) or a hybrid layer capacitor (HLC), have proven particularly preferred. Alternatively, the energy storage device can also comprise one or more batteries.

[0041] The adapter electronics further comprises an electronic component 6, which, for proper operation, at least temporarily requires a power level that exceeds the operating power provided by the two-wire cable. To accommodate this higher power requirement, the electronic component is connected to the output of the energy storage device 18b. As shown in Fig. 1, such an electronic component can be, for example, a radio module that is at least configured to wirelessly transmit data packets. The radio module 6 can, in particular, be configured to transmit the data packets in accordance with the LTE Cat NB1 standard of 3GPP Release 13, published in June 2016.

[0042] In order to ensure sufficient energy is available to transmit a single data packet, the adapter electronics can additionally be configured to monitor the charge state or charge level of the energy storage device 18 and, depending on the charge state, provide the radio module 6 with the energy required to transmit the data packet. For this purpose, the adapter electronics can provide a logic unit 10, for example, a microprocessor and / or a Schmitt trigger circuit.

[0043] Fig. 2 shows an explosion-protection unit 21 designed according to the invention in detail. This comprises a circuit element 21.1 and a voltage-limiting unit 21.2 arranged in series with the circuit element 21.1. The explosion-protection unit 21 is connected in parallel to the energy storage device 18, and the voltage-limiting unit 21.2 is designed and configured to limit a voltage applied across the energy storage device 18 to the maximum value. The maximum value to which the voltage is limited depends on a voltage value specified by the manufacturer for the specifically used energy storage device 18, at which the energy storage device 18 may be properly operated without sustaining damage, according to the manufacturer. This specified value is typically in the range of a few volts. The value is preferably in the range of 2-10 V, more preferably in the range of 3-8 V, and most preferably in the range of 3-6 V.For example, the voltage value specified by the manufacturer at which the energy storage device 18 may be properly operated. This value depends on the type of energy storage device. For example, if the energy storage device comprises a battery, it may be approximately 3.9 V, or possibly slightly higher at 4.2 V. If the energy storage device has a capacitor, the value may be up to 5.5 V. The voltage limiting unit 21.2 comprises a Zener diode whose Z voltage corresponds to the maximum value to which the voltage is to be limited. For example, the explosion-proof unit, as shown in Fig. 2, may have a 3.9 V Zener diode.

[0044] In the embodiment shown in Fig. 2, the circuit element 21.1 comprises a p-channel normally-off MOSFET. In this case, the MOSFET is connected with its source to the energy storage device 18 and with its drain to a cathode of the Zener diode. The gate of the MOSFET is connected to a control circuit 21.3, which is configured to control the MOSFET such that it only activates the voltage limiting unit 21.2 when the voltage across the energy storage device 18 is less than 15%, preferably less than 10%, particularly preferably less than 5% below the maximum value. Activated here means that the MOSFET is switched on, so that in the event of an Ex fault, the Zener diode limits the voltage to the Z voltage, in this case 3.9 V. For example, the control circuit can be set up in such a way that it controls the MOSFET in such a way that it activates the voltage limiting unit 21 .1 when the voltage of approximately 3.8 V is reached, ieOnly above this value does the MOSFET 21.1 become conductive. The control circuit is further configured to control the MOSFET 21.1 in such a way that, as long as the voltage above which the voltage limiting unit 21.2 is activated is not reached, the voltage limiting unit is not activated or deactivated, i.e. the MOSFET 21.1 is switched to non-conductive. In the present exemplary embodiment, the control circuit switches the MOSFET 21.1 to non-conductive at a voltage applied across the energy storage device of less than 3.8 V, so that the voltage limiting unit 21.2 is not activated, i.e. no current flows through the voltage limiting unit 21.2 with the exception of a small leakage current caused by the MOSFET 21.1.

[0045] The control circuit 21.3 can comprise a Schmitt trigger circuit or a comparator circuit 21.33, which are connected via a voltage divider 21.31 connected to a first input and a reference (possibly also an internal reference) 21.32 connected to a second input such that the Schmitt trigger circuit or the comparator circuit at an output controls the gate of the MOSFET 21.1 such that it switches on from a voltage value that is less than 15%, preferably less than 10%, particularly preferably less than 5% below the maximum value.

[0046] List of reference symbols

[0047] 1 field device

[0048] 2 Housing of the field device

[0049] 2a Cover of the housing

[0050] 4 Field device electronics

[0051] 5 field device adapters

[0052] 5a Interface of the field device adapter

[0053] 6 radio module

[0054] 7 Housing of the field device adapter

[0055] 9 radio antenna

[0056] 10 Logic unit

[0057] 12 two-wire cable

[0058] 13 Connection terminal

[0059] 14 Display

[0060] 14a Outer contour of the display

[0061] 14b Display connection cable

[0062] 15 Field device internal interface

[0063] 16 Current limiting circuit

[0064] 16a Input of the current limiting circuit

[0065] 16b Output of the current limiting circuit

[0066] 17 Output voltage boost converter

[0067] 17a Input of the output voltage boost converter

[0068] 17b Output of the output voltage boost converter

[0069] 17c Feedback input

[0070] 18 energy storage

[0071] 18a Input of the energy storage

[0072] 18b Output of the energy storage

[0073] 19 Display mount or holder

[0074] 20 charge controllers

[0075] 21 Ex-protection unit

[0076] 21.1 Circuit element

[0077] 21.2 Voltage limiting unit

[0078] 21.3 Control circuit

[0079] 21.31 Voltage divider

[0080] 21.32 Voltage reference

[0081] 21.33 Schmitt trigger circuit or comparator circuit

[0082] I2 4-20 mA current signal

[0083] Pmin minimum operating power

[0084] Uk terminal voltage R1 , R2 resistors of the voltage divider

[0085] D1 diode in the upper part of the voltage divider

[0086] D2, D3, D4 diodes of the explosion protection unit

Claims

Patent claims 1 . An electronic circuit for a field device or a radio adapter for use in a potentially explosive atmosphere, comprising: an interface (5a) for connecting the electronic circuit to a field device interface (15) of the field device or for connecting the electronic circuit to a two-wire cable in order to supply the electronic circuit with energy; an electronic component (6) which at least temporarily requires a higher energy than the energy provided via the interface; an energy storage device (18) connected to the interface and the electronic component and configured to store energy transmitted via the interface (5a) and to make it available to the electronic component when required; an explosion-proof unit (21) with at least one voltage limiting unit (21.2), which is designed to limit a voltage across the energy storage device (18) to a maximum value, wherein the explosion protection unit (21) is further designed to control the voltage limiting unit (21 .2) is to be activated only from a voltage value which is less than 15%, preferably less than 10%, particularly preferably less than 5% below the maximum value.

2. Electronic circuit according to claim 1, wherein the explosion protection unit (21) is further configured to deactivate the voltage limiting unit if the voltage across the energy storage device is not less than 15%, preferably not less than 10%, particularly preferably not less than 5% below the maximum value.

3. Electronic circuit according to one or more of the preceding claims, wherein the explosion protection unit (21) further comprises a circuit element (21.1) preferably connected in series to the voltage limiting unit (21.2) and a control circuit (22.3), wherein the control circuit (22.3) and the circuit element (21.1) are interconnected in such a way and are designed to activate the voltage limiting unit (21.2) only from a voltage value which is less than 15%, preferably less than 10%, particularly preferably less than 5% below the maximum value.

4. Electronic circuit according to the preceding claim, wherein the circuit element (21.1) comprises a transistor, preferably a field effect transistor, particularly preferably a p-channel normally-off MOSFET.

5. Electronic circuit according to one of claims 3 or 4, wherein the control circuit (21.3) comprises a Schmitt trigger circuit or a comparator circuit.

6. Electronic circuit according to one or more of the preceding claims, wherein the maximum value to which the voltage limiting unit (21.2) limits the voltage applied across the energy storage device (18) corresponds to a value predetermined for the energy storage device (18) at which the energy storage device (18) is to be operated in proper operation.

7. Electronic circuit according to one or more of the preceding claims, wherein the maximum value is in the range of 2-10 V, particularly preferably in the range of 3-8 V, most preferably in the range of 3-6 V.

8. Electronic circuit according to one or more of the preceding claims, wherein the voltage limiting unit (21.2) comprises at least one diode, in particular a Z-diode, with a diode voltage, in particular a Z-voltage, which corresponds to the maximum value.

9. Electronic circuit according to one or more of the preceding claims, wherein the energy storage device (18) comprises a battery or a capacitor, in particular a supercapacitor or a high-capacity capacitor.

10. Electronic circuit according to one or more of the preceding claims, wherein the electronic component (6) comprises a radio module which is in particular configured to wirelessly transmit a data packet according to a standard of a Low Power Wide Area Network. 1 1. Field device of automation technology comprising an electronic circuit according to claim 1, wherein the interface (5a) for connecting the electronic circuit is connected to a field device interface (15) of the field device (1), so that the field device (1) supplies the electronic circuit with energy.

12. Radio adapter of automation technology comprising an electronic circuit according to claim 1, wherein the interface (5a) for connecting the electronic circuit is connected to a two-wire line (12) so that the electronic circuit is supplied with energy via the two-wire line.

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