Modular field device

Abstract

The invention relates to a modular field device (1) for determining a process variable, which despite a plastic housing (1), meets EMC technical requirements and is easy to manufacture. The sensor module (13), which is used to determine the process variable, is arranged on a feed-through in the housing (11). For the purpose of EMC protection, a conductive enclosure (12) is provided, which is fastened in the interior (111) of the housing (11) and surrounds a first interior region (111a). A first end region (121) of the enclosure (12) is aligned with the feed-through (112). The opposite, second end region (122) of the enclosure (12) is closed by a circuit board (14) in such a way that the interior region (111a) is completely electromagnetically shielded from the outside. Accordingly, the evaluation module (15) connected to the sensor module (13), which is used for external communication, is also EMC protected, since it is arranged in the protected interior region (111a) on a surface of the circuit board (14) facing the interior region (111a).

Description

[0001] Modular field device

[0002] The invention relates to a modular field device which, despite its non-metallic housing, meets EMC requirements regarding radiated and conducted interference and is easy to manufacture.

[0003] In process automation technology, field devices are often used to record or influence specific process variables. Depending on the type of field device, it contains specific electronic components to record the respective process variable and implement the corresponding measuring principle. Depending on the design, the respective field device type can be used, for example, to measure a fill level, flow, pressure, temperature, pH value, and / or conductivity. A wide variety of such field device types are manufactured and distributed by the Endress + Hauser Group.

[0004] Due to the diversity of field device types, a modular design is desirable, as it offers numerous advantages for both the manufacturer and the user: Firstly, standardizing individual modules reduces complexity and the associated costs. Secondly, standardizing interfaces allows for a wide variety of variants to be created with just a few module variants, allowing even individual field device types to be adapted to different requirements.

[0005] Specifically, the electronic components of modular field devices are divided into at least two modules: The respective measuring principle for recording the process variable is implemented in the sensor module, while the evaluation module converts the analog or digital raw signal from the sensor module into a standardized analog output signal or digital protocol. A variety of different industry standards, each with specific properties, are available for this purpose, such as "4...20 mA", "HÄRT", "10-Link", "Foundation Fieldbus", "Profibus", "Modbus" or "ETHERNET IP". In general, the term "module" in the context of the invention is understood to mean a separate arrangement or encapsulation of the electronic circuits that are intended for a specific application, e.g. for measurement signal processing or as an interface. Depending on the application, the respective module can therefore contain corresponding analog circuits for generating or processing the process variable.Processing of analog signals. The module can also include digital circuits such as FPGAs, microcontrollers or storage media in conjunction with corresponding programs. The program is designed to carry out the required process steps or apply the necessary mathematical operations. In addition to certain electronic modules, the housing of field devices can also be designed so that it can be used for various field device types. To do this, the housing must be designed in such a way that the sensor module is guaranteed direct contact with the corresponding process, regardless of the measuring principle, in order to be able to determine the process variable. In addition, the housing must accommodate the modules of the respective field device type in an EMC-protected manner ('electromagnetic compatibility'). This means that the modules must be protected from external electromagnetic interference.On the other hand, potentially disruptive electromagnetic radiation from the modules must be prevented. Furthermore, it must be ensured that the modules' operational waste heat is adequately dissipated. Accordingly, the housing can be made of a metal, such as stainless steel. This allows the housing to function as a Faraday cage and requires appropriate grounding. Advantages of a metal housing include its impact resistance, resistance to solvents, and fire safety.

[0006] However, in the majority of applications, a non-metallic housing is advantageous or even necessary. For example, field devices used in corrosive environments, such as coastal locations or in processes involving acidic or alkaline media, are preferably based on a plastic-based housing. In such cases, the electronic modules are protected by an additional, electrically conductive enclosure, which forms a corresponding interior area within the housing that acts as a Faraday cage for the modules. A corresponding field device is shown, for example, in the publication DE 10 2015 107 306 A1.

[0007] However, the electromagnetic shielding and thermal management of non-metallic enclosures with metal frames are not as effective as those of metallic enclosures without technical support measures. The additional technical support measures required for this are sometimes contradictory or contribute to increasing production and development costs for the enclosure.

[0008] The invention is therefore based on the object of providing a modular field device with a plastic housing that is efficiently designed with regard to EMC protection and thermal management and can be manufactured with little effort.

[0009] This task is solved by a field device for determining a process variable, which includes the following components:

[0010] An electrically insulating housing, with o an interior, and o a feedthrough that adjoins the interior along a device axis, an electrically conductive enclosure that is fastened in the interior and radially encloses a first interior region of the interior along a partial segment of the device axis, with o a first end region that is aligned towards the feedthrough, and o a second end region that is opposite the first end region in relation to the device axis, a sensor module that is mounted in the feedthrough of the housing in such a way to record the process variable, a printed circuit board that closes off the second end region of the enclosure in such a way that the interior region is electromagnetically shielded, and an evaluation module that is arranged on a surface of the printed circuit board facing the interior region and is in electrical contact with the sensor module in order to transmit the process variable.

[0011] The evaluation module can be designed according to any industrial communication standards, e.g.

[0012] - to communicate with higher-level units according to the digital protocol, "PROFIBUS", "HÄRT", "WirelessHART", "WLAN", "PROFINET", "PROFISAFE", "IO-Link", "MODBUS TCP", "MODBUS RTU", or "ETHERNET / IP", or by means of analog signals according to the standard "Namur IEC 60947-5-6", "4-20 mA" or "0-10 V".

[0013] In principle, the field device according to the invention can be used to measure any type of process variable. Accordingly, the sensor module can be designed to determine, for example, a fill level, temperature, pressure, flow rate, molar concentration, conductivity, viscosity, acceleration, and / or dielectric value as process variables.

[0014] The advantage of the inventive solution is that the circuit board contributes to protecting the modules from electromagnetic interference, thus eliminating the need for additional related auxiliary measures. If the circuit board includes an electrical through-hole connected to the evaluation module in the area of ​​the housing interior that is not EMC-protected, it is particularly advantageous from an EMC perspective if a high-frequency filter tuned to EMC interference is connected to the through-hole to attenuate conducted interference. For this purpose, the high-frequency filter can comprise an LC low-pass filter, in particular at least a 2nd order filter, and / or a current-compensated choke circuit. From an EMC perspective, protection in the interior area can be further increased if the circuit board comprises a predominantly full-surface metal layer.In particular, if this metal layer is made of copper, the synergistic effect of simultaneous heat dissipation from the EMC-protected interior area results. This effect is maximized if the circuit board and the bezel are designed such that the metal layer at the second end region, when attached, forms an electrical contact surface with a thermal resistance of a maximum of 15 Kelvin / Watt, in particular less than 5 Kelvin / Watt. For this purpose, the bezel can be mechanically machined at the contact surface or the second end region, for example by face milling, in order to remove any casting skin that has poor thermal and electrical conductivity, provided the bezel is made of stainless steel, die-cast zinc, or die-cast aluminum. In general, however, it is also conceivable within the scope of the invention for the bezel to be made of a plastic or a ceramic, each with a conductive coating.

[0015] Another advantage of the field device according to the invention is that it can be manufactured with minimal effort. The method for its production comprises the following key steps, although the order of execution can vary: attaching the circuit board to the enclosure so that the evaluation module faces the interior; attaching the sensor module to the feedthrough or to the first end area of ​​the enclosure; connecting the sensor module to the evaluation module; and attaching the enclosure to the interior of the housing.

[0016] The invention is explained in more detail with reference to the following figures. Shown are:

[0017] Fig. 1 : A field device for determining a process variable in a container,

[0018] Fig. 2: an exploded view of the field device housing with inner bezel,

[0019] Fig. 3: a cross-sectional view of the field device according to the invention, and

[0020] Fig. 4: a detailed view of the bezel in the area of ​​the circuit board.

[0021] To provide a basic understanding of the invention, Fig. 1 shows a process container 3 of a process plant, which container is used, for example, for chemical or biological reactions or for storing a filling material 2. Depending on the type of filling material 2 and the area of ​​application, the container 3 can be up to more than 100 m high. Depending on the process, a process variable within the container 3 can be, for example, a fill level, a limit level, a temperature, a pH value or a conductivity. In order to be able to determine the specific process variable, a field device 1 with a correspondingly designed sensor module 13 is attached to the container 3 in a defined installation position. For this purpose, the housing 11 of the field device 1 is fastened or aligned to an opening in the container 3 via a connection adapter 17 in such a way that the sensor module 13 has access to the interior of the container via a feedthrough 112 in the housing 11 and can thus record the process variable.

[0022] In the case of the fill level as a process variable, the sensor module 13 can, for example, be based on the FMCW radar principle. For pressure measurement, the sensor module 13 can in turn comprise a membrane whose pressure-dependent deflection is detected capacitively or resistively. In order to be able to measure the temperature in the container 3 as a process variable, it is known in the art to design the sensor module 13, for example, with a temperature-dependent resistor such as a Pt100. Depending on the implemented measuring principle, the sensor module 13 initially generates the measured value corresponding to the process variable as an analog measurement signal, e.g. in the form of a direct current, a direct voltage, or an alternating voltage signal, which is further processed either directly in the sensor module 13 or alternatively in the evaluation module 15, for example, amplified, filtered, scaled, and / or converted into a digital signal.

[0023] Except for the sensor module 13, all other modules 15 of the field device 1 are located completely outside the container 3 in the housing 11. The housing 11 is made of a plastic such as ABS, PBT, PEEK, or PP for corrosion and weather resistance. Contrary to the illustration in Fig. 1, the field device 1 can also be installed on a pipeline section with appropriate design to measure, for example, a flow rate as a process variable.

[0024] As can be seen from Fig. 1 and Fig. 3, the field device 1 is connected to a higher-level unit 4, such as a local process control system or a decentralized server system, via an evaluation module 15 accommodated in the housing 11. Via this, the field device 1 can transmit the current measured value of the process variable using an appropriate standard, such as "4-20 mA", "PROFIBUS", "HÄRT", "WLAN" or "ETHERNET / IP", for example, to control inflows or outflows at the container s. For this purpose, the evaluation module 15 within the field device 1 is electrically connected to the sensor module 13. In order to further process the measurement signal from the sensor module 13, e.g. by normalizing the measurement signal with respect to a reference value known through calibration, and / or by value and time discretization of the measurement signal, the measured value is preferably converted into a digital value.Depending on the design of the sensor module 13, it is conceivable that the sensor module 13 pre-processes the measurement signal in this regard, at least to a certain extent, before the measured value of the process variable is transmitted to the evaluation module 15.

[0025] In addition to the processed measured value of the process variable, other information about the general operating status of the field device 1 can also be communicated between the higher-level unit 4 and the evaluation module 15. It goes without saying that the data transmission can be bidirectional, so that, in principle, data such as software updates or calibration data can also be transmitted to the field device 1 via the evaluation module 15. The modular design of the evaluation module 15 has the advantage that it can be used in more than just one specific field device type.

[0026] Since the housing 11 of the field device 1 is electrically insulating, an electrically conductive enclosure 12 is arranged in the interior 111 of the housing 11, forming a separate inner region 111a that protects against high-frequency electromagnetic interference. For this purpose, the enclosure 12 can be made, for example, from stainless steel, die-cast zinc, die-cast aluminum, or a plastic with a conductive coating. In Fig. 2, the enclosure 12 and the housing 11 are shown as an exploded view with respect to a defined device axis a. Accordingly, the EMC-protected inner region 111a of the interior 111 is formed by the enclosure 12 radially enclosing it along a partial segment of the device axis a. As a result, the enclosure 12 is divided into an approximately circular, first end region 121, which is aligned towards the feedthrough 112.The corresponding second end region 122 of the frame 12 is opposite the first end region 121 with respect to the device axis a.

[0027] In the embodiment shown in Fig. 2 and Fig. 3, the enclosure 12 is grounded via a radially tapered grounding terminal 123, which can be contacted via a corresponding recess in the housing 11. Alternatively, it is also conceivable to ground the enclosure 12 or the modules 13, 15 via the cable entry 16, for example, via a cable shield or piping.

[0028] In terms of manufacturing technology, during assembly, the bezel 12 is inserted along the device axis a from an opposite opening in the housing 11 in the direction of the feedthrough 112 into the interior 111 and fastened there, for example by means of a plug connection. For this purpose, the interior 111 of the housing 11 has a fundamentally cylindrical geometry corresponding to the bezel 12 along the device axis a. In the assembled state, the end region 122 of the bezel 12 which faces away from the sensor module 13 is closed off by a printed circuit board 14 arranged orthogonally to the device axis a, as can be seen from the sectional view of the field device 1 in Fig. 3. According to the invention, the evaluation module 15 is arranged on a surface of the printed circuit board 14 facing the interior region 111 a. As a result, the evaluation module 15, in cooperation with the bezel 12, is completely protected from an EMC perspective. As described in connection with Fig.2, the printed circuit board 14 can be fastened to the frame 12 via a screw connection 143, which in the embodiment shown comprises four screws, corresponding internal threads in the frame 12 and corresponding drill holes in the printed circuit board 14.

[0029] In the embodiment shown in Fig. 3, the evaluation module 15 or the corresponding surface of the circuit board 14 is additionally encapsulated by a soft potting compound or a corresponding first potting cup 146. The sensor module 13 is also encapsulated in a second potting cup 131 using soft potting compound and, in the embodiment shown there, is mechanically attached to the first potting cup 146 of the evaluation module 15 such that the sensor module 13 is suitably positioned or aligned in the feedthrough 112 in order to detect the process variable. Alternatively or additionally, the sensor module 13 can also be attached to the first end region 121 of the enclosure 12.

[0030] Fig. 3 also schematically shows the electrical contact between the sensor module 13 and the evaluation module 15 through the soft encapsulation. This allows the measurement signal from the sensor module 13, which represents the current measured value of the process variable, to be transmitted to the evaluation unit 15 for further processing.

[0031] Not explicitly shown in Fig. 3 is that a mechanical barrier can also be used in the connection adapter 17 to the container interior to seal the field device 1 from the container interior, or vice versa. In this case, the barrier must be adapted to the respective measuring principle implemented: In the case of radar-based level measurement, the barrier must be transparent to radar signals. In the case of temperature measurement, sufficient thermal conductivity is required. For pH measurement, the barrier must be correspondingly ion-conductive, etc.

[0032] Since the field device 1 is attached to the container s via the connection adapter 17 as shown in Fig. 1 and Fig. 3, the housing 11, together with the components 12, 13, 14, 15 located in the interior 111, must be attached to the connection adapter 17 as securely as possible, for example by means of an M48 threaded connection 18. For this purpose, the housing 11 can be provided with a corresponding internal thread in the area of ​​the feedthrough 112. For this purpose, in the exemplary embodiment shown, the connection adapter 17 is provided with a corresponding external thread at its end area facing the housing - in relation to the device axis a - at the level of the feedthrough 112, so that the resulting threaded connection 18 is again aligned along the device axis a.

[0033] Alternatively, the internal thread of the threaded connection 18 can not be embedded in the plastic of the feedthrough 112, but rather as a component of the bezel 12 in the area of ​​the feedthrough 112. Since the bezel 12 is in turn mechanically fastened in the interior 111 of the housing 11, the housing 11 is in this case indirectly mechanically connected to the connection adapter 17 via the bezel 12. As a result, the maximum possible number of screwing cycles of the threaded connection 18 is not limited by the plastic of the housing 11. This is particularly noticeable when the housing 11 has to be unscrewed periodically, for example for service and maintenance work. In both fastening variants, the sensor module (13) is designed such that it closes the first end area (121) of the bezel (12) using high frequency technology and thus protects against radiated EMC interference.This can be achieved, for example, by making the connection adapter (17) from metal and forming an electrical contact with the frame (12).

[0034] As shown in Fig. 3, the printed circuit board 14 comprises, on the surface facing away from the interior region 111a of the enclosure 12, electrical connection terminals 144, 144' for contacting the evaluation module 15 with the higher-level unit 4 or for supplying power to the field device 1. In the embodiment shown, the corresponding cabling to the higher-level unit 4 is carried out from the housing 11 via a cable entry 16, unless a wireless transmission protocol is implemented.

[0035] For electrical contacting of the connection terminals 144, 144' with the evaluation unit 15, an electrical through-hole 141 is provided in the printed circuit board 14.

[0036] The via 141 is shown in detail in the sectional view of the circuit board 14 in Fig. 4: It can be seen that the vertically straight via 141 connects the corresponding signal-carrying conductor layers on both surfaces of the circuit board 14. Corresponding, horizontally running signal ground layers are also connected to each other by vias.

[0037] In the version of the circuit board 14 shown in Fig. 4, the evaluation module 15 and the connection terminals 144, 144' are not directly connected to each other via the through-hole 141, but rather additionally via a high-frequency filter 161, 162, 163, 164. This filter serves to filter out any high-frequency electromagnetic interference in order to protect the interior area 111a from an EMC perspective. The high-frequency filter 161, 162, 163, 164 is designed to be permeable to the transmission frequencies at which the transmission standards operate. Since the transmission frequencies in the case of "HÄRT", "Profibus" and comparable industrial transmission standards are below 10 kHz, it is therefore advantageous within the scope of the invention to design the high-frequency filter 161, 162, 163, 164 as a low-pass filter.To achieve a sufficient slope of at least 12 dB per octave, it is further advantageous to design the high-frequency filter 161, 162, 163, 164 as at least a second-order low-pass filter. Not shown in Fig. 4 is that the high-frequency filter 161, 162, 163, 164 can optionally be expanded with a common-mode choke (better known in English as a "common-mode choke") to suppress common-mode interference that may be present via the terminals 144, 144'.

[0038] In the embodiment shown in Fig. 4, the high-frequency filter 161, 162, 163, 164 is designed as a fourth-order LC low-pass filter. A first capacitor 161 and a first inductor 163 of the LC low-pass filter are arranged on the surface of the circuit board 14 facing away from the inner region 111a of the enclosure 12. A second capacitor 162 and a second inductor 164 of the LC low-pass filter are arranged on the opposite surface of the circuit board 4, which faces the inner region 111a. Accordingly, the invention does not strictly prescribe on which surface of the circuit board 14 the high-frequency filter 161, 162, 163, 164 is to be arranged. The components of the high-frequency filter 161, 162, 163, 164 can be designed, for example, as SMD components. As an alternative to being arranged on the circuit board surface, the components 161, 162, 163, 164 can also be embedded in the circuit board 12.

[0039] The EMC protection effect of the circuit board 14 shown in Fig. 4 is further enhanced by a separate copper layer 142, which is formed over the entire surface except for recesses for feedthroughs 141. In principle, this function can also be achieved by using another metal, such as gold, instead of copper. Since metals with good electrical conductivity generally also have good thermal conductivity, the synergistic effect is achieved that the copper layer 142 can simultaneously serve to dissipate heat from the components arranged on the circuit board 14.For this purpose, the circuit board 14 and the enclosure 12 are to be designed such that the copper layer 142 at the second end region 122, when attached, not only forms an electrical connection to the enclosure 12, so that the circuit board 14 is grounded, thereby shielding interference radiation and dissipating conducted interference. In the embodiment shown in Fig. 4, the signal ground layers and the copper layer 142 are connected to one another via an electrical capacitance to dissipate conducted interference without eliminating the galvanic isolation.

[0040] It is advantageous if the copper layer 142 and the enclosure 12, through appropriate design, also form a sufficient thermal contact surface with a low thermal resistance of approximately 4 Kelvin / watt. For this purpose, as shown in Fig. 4, the copper layer 142 can also be designed such that it expands in the circuit board 14 in a defined lateral area around the screw connection 143 to several or all layers of the circuit board, as shown in Fig. 4.

[0041] List of reference symbols

[0042] 1 field device

[0043] 2 Filling material

[0044] 3 containers

[0045] 4 Superior unit

[0046] 11 housings

[0047] 12 border

[0048] 13 Sensor module

[0049] 14 circuit board

[0050] 15 Evaluation module

[0051] 16 Cable entry

[0052] 17 connection adapters

[0053] 18 threaded connection

[0054] 111 Interior of the housing

[0055] 111 a Interior

[0056] 112 Implementation

[0057] 121 First end area

[0058] 122 Second end area

[0059] 123 Earth connection

[0060] 131 Second casting cup

[0061] 141 Electrical through-hole plating

[0062] 142 metal layers

[0063] 143 screw connection

[0064] 144, 144' connections

[0065] 145 Insulating layer

[0066] 146 First casting cup

[0067] 147 capacity

[0068] 161 , 162 Capacitors of the high-frequency filter

[0069] 163, 164 Inductances of the high-frequency filter a device axis

Claims

Patent claims 1 . Field device for determining a process variable, comprising: An electrically insulating housing (11), with o an interior space (111), and o a feedthrough (112) which adjoins the interior space (111) along a device axis (a), an electrically conductive enclosure (12) which is fastened in the interior space (111) and radially encloses a first interior region (111a) of the interior space (111) along a partial segment of the device axis (a), with o a first end region (121) which is aligned towards the feedthrough (112), and o a second end region (122) which is opposite the first end region (121) with respect to the device axis (a), a sensor module (13) which is mounted in the feedthrough (112) in such a way as to detect the process variable, a printed circuit board (14) which closes off the second end region (122) of the enclosure (12) in such a way that the interior region (111a) is electromagnetically shielded, and an evaluation module (15),which is arranged on a surface of the circuit board (14) facing the inner region (111a) and is electrically contacted with the sensor module (13) in order to transmit the process variable., 2. Field device according to claim 1, wherein the circuit board (14) comprises an electrical via (141) connected to the evaluation module (15), and wherein a high-frequency filter (16) is connected to the via (141).

3. Field device according to claim 2, wherein the high-frequency filter comprises an LC low-pass filter (161, 162, 163, 164), in particular at least 2nd order and / or a current-compensated choke circuit.

4. Field device according to at least one of the preceding claims, wherein the printed circuit board (14) comprises a predominantly full-surface metal layer (142), in particular made of copper.

5. Field device according to claim 4, wherein the circuit board (14) and the enclosure (12) are designed such that the metal layer (142) at the second end region (122) forms an electrical contact surface with a thermal resistance of a maximum of 15 Kelvin / Watt, in particular less than 5 Kelvin / Watt, in the attached state.

6. Field device according to at least one of the preceding claims, wherein the evaluation module (15) is designed - according to the digital protocol, "PROFIBUS", "HÄRT", "WirelessHART", "WLAN", "PROFINET", "PROFISAFE", "IO-Link", "MODBUS TCP", "MODBUS RTU", or "ETHERNET / IP", or by means of analog signals according to the standard "Namur IEC 60947-5-6", "4- 20 mA” or “0-70 V” to communicate with a higher-level unit (4).

7. Field device according to at least one of the preceding claims, wherein the sensor module (13) is designed to determine a fill level, a temperature, a pressure, a flow rate, a molar concentration, a conductivity, a viscosity, an acceleration value and / or a dielectric value as a process variable.

8. Field device according to at least one of the preceding claims, wherein the enclosure (12) is made of stainless steel, die-cast zinc, die-cast aluminum or a plastic with a conductive coating.

9. Method for manufacturing the field device (1) according to one of the preceding claims, comprising the following method steps: Attaching the circuit board (14) to the frame (12) so that the evaluation module (15) is directed towards the inner area (111 a), Attaching the sensor module (13) to the bushing (112) or to the first end region (121) of the frame (12), Connecting the sensor module (13) to the evaluation module (15) and fastening the frame (12) in the interior (111) of the housing (11).

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