Explosion protection circuit with impedance adjustment

Description

[0001] The invention relates to a device for monitoring at least one physical or chemical process variable comprising at least one sensor unit and an electronic unit for signal acquisition, evaluation and / or input, wherein the device is suitable for use in potentially explosive atmospheres.

[0002] To operate a field device in a potentially explosive atmosphere, various measures are necessary. These measures aim to prevent sparks that could trigger an explosion or to prevent a spark originating inside an enclosed space from affecting the surrounding environment. In this respect, different protection zones are distinguished, and accordingly, there are different standards for varying environmental conditions and applications. The standard DIN EN 60079-11 defines the necessary safety requirements for intrinsic safety device protection, known by the abbreviation Ex-i.This is a type of ignition protection based on limiting electrical energy within devices and / or connecting cables exposed to a potentially explosive atmosphere to a level below which ignition can be caused by either sparking or heating. Furthermore, various protection levels can be distinguished within this protection class: Ex-ia, Ex-ib, and Ex-ic. These suffixes define the level of fault tolerance of the individual components and connecting cables.

[0003] For explosion protection according to class Ex-i, it must be ensured, in particular, that the electrical quantities current, voltage, and power in a device remain below a specified limit at all times. These limits are selected so that in the event of a fault, e.g., a short circuit, the maximum heat generated is insufficient to produce an ignition spark. This is achieved by keeping the current below the specified limits using resistors, the voltage below the limit using diodes, especially Zener diodes, and the power below the limit using a suitable combination of current- and voltage-limiting components. Such a protective circuit can be obtained, for example, as a pluggable device from Phoenix Contact under the name MCR-PLUGTRAB PT.Regarding field devices, several examples have come to light where explosion protection requirements are met through special circuit arrangements, for example in EP0882955A1, DE29809853U1, or WO2004 / 098014A1. The latter document describes arrangements of series resistors specifically for current and voltage limiting units, while DE29809853U1 provides for the use of Zener diodes. However, the use of diodes has the disadvantage that applications are limited to a specific temperature range.

[0004] US4708022A1 describes a connection point for a flow monitoring assembly with a signal generator and a signal receiver.

[0005] EP0817149A1 describes a circuit for transmitting a measuring current from an intrinsically safe sensor to a non-intrinsically safe environment.

[0006] Furthermore, the aforementioned examples of explosion protection circuits lack impedance matching. This is particularly relevant for field devices where communication between the electronic unit and the sensor element occurs via alternating current or voltage. Typical frequencies range from 10 kHz to 50 MHz. The problem is that the load impedance of the sensor element is often not matched to the cable or electronic unit, leading to standing waves on the cable. As a result, the full power cannot be transmitted. Field devices where the sensor element is actuated via at least one piezoelectric element serve as an example.This includes, among other things, ultrasonic flow meters or vibronic level meters, such as those distributed by the applicant in a wide variety, in the first case under the name Prosonic DDU10, Prosonic Proline P, or Prosonic Flow and in the second case under the name Solifant and Liquifant.

[0007] A typical method for ensuring impedance matching, especially to a piezoelectric element, involves the use of coils or transformers. The characteristics of these components must be matched to the cable, the electronic unit (which in this case usually includes an amplifier), and the piezoelectric element itself.

[0008] The present invention is based on the objective of providing a field device with an explosion protection circuit which enables the operation of the field device in an explosive atmosphere while simultaneously transmitting the signals with minimal power loss.

[0009] This problem is solved according to the invention by a device for monitoring at least one physical or chemical process variable comprising at least one sensor unit and an electronic unit for signal acquisition, evaluation and / or power supply, wherein the sensor unit is operated with alternating current and / or the communication between the sensor unit and the electronic unit takes place with alternating current and / or alternating voltage, and with an intrinsically safe explosion protection circuit which comprises a safety barrier with at least one unit each for current and / or voltage limiting, wherein within the explosion protection circuit a unit configured for impedance matching is provided for lossless signal transmission between the sensor unit and the electronic unit, which unit for impedance matching comprises at least one transformer for galvanic isolation of the sensor unit and the electronic unit. wherein the explosion protection circuit has at least a simple failover capability, wherein a number of parallel branches within the explosion protection circuit (3) determines the failover capability, wherein a simple failover capability is given by two parallel branches, wherein the explosion protection circuit has at least two parallel branches, wherein the voltage limiting unit comprises at least one coil, wherein the current limiting unit comprises at least one resistor connected in series with the coil.

[0010] Impedance matching ensures the most lossless transmission of signals and, consequently, that more energy is available for the sensor unit, since universal matching over a large interval between the impedances of the sensor unit and the electronic unit is possible.

[0011] A suitable transformer design also ensures zone separation. This is particularly advantageous for sensor units in contact with the medium, which have a metal housing and cathodic protection on a pipeline carrying the medium. In such an application, cathodic protection means that the potential of the pipeline relative to the ground connection is maintained at a specific value, for example, U=5V, by means of a voltage source to prevent corrosion of the pipeline. The use of an explosion protection circuit with galvanic isolation between the sensor unit and the electronic unit enables independent operation of the circuits in this case. This is often not the case with conventional explosion protection circuits.

[0012] For this purpose, it is advantageous if the transformer is designed in such a way, in particular by ensuring sufficiently large distances and suitable material selection such as lacquers and insulating films, that it guarantees galvanic isolation.

[0013] Such a choice of voltage-limiting components allows the device according to the invention to be used in a wide temperature range; in particular, the possible temperature range is larger than when using diodes.

[0014] Furthermore, it is advantageous if identical components in the current limiting, voltage limiting, and impedance matching units are designed to perform a dual function. This allows, for example, the voltage limiting unit and the impedance matching unit to be combined via the transformer. Additionally, the DC resistance of the coil windings can also limit the current.

[0015] In another preferred embodiment, the sensor unit comprises at least one piezoelectric element. This is particularly the case when the device according to the invention is used for ultrasonic flow meters or for vibronic level measuring devices. The at least one process variable is then, for example, given by the flow rate of a medium through a pipe, the fill level of a medium in a container, or by the density or viscosity of the respective medium.

[0016] It remains advantageous if the explosion protection circuit is permanently installed between the sensor unit and the electronics unit. Alternatively, the explosion protection circuit can also be housed in a separate plug-in adapter, which can be retrofitted between the sensor unit and the electronics unit. In this case, existing devices can be retrofitted with an explosion protection circuit and / or impedance matching.

[0017] The number of parallel branches determines the level of fault tolerance. At least single fault tolerance is ensured if the explosion protection circuit has at least two parallel branches, and at least double fault tolerance is ensured with at least three parallel branches. This corresponds to the Ex-ib standard. With double fault tolerance, which is achieved in the case of three parallel branches, the Ex-ia standard is even met.

[0018] It is advantageous if at least one component within the current and / or voltage limiting units is implemented multiple times. This is particularly relevant in the case of two parallel branches within the explosion protection circuit, as the two branches are then designed equivalently. For example, with single redundancy, two current-limiting resistors and two voltage-limiting coils would be redundantly implemented, while with double redundancy, three coils and three resistors each would be redundantly implemented.

[0019] A further advantage of an unclaimed embodiment arises if at least one of the components is designed to be fail-safe. This can be achieved, for example, by coils with a wire diameter of Ø > 0.05 mm and a minimum breakdown voltage of grade 2 according to IEC, by resistors designed as wire resistors with sufficient temperature resistance, and / or by conductor tracks with a corresponding cross-section, in particular height and width.

[0020] In another particularly preferred embodiment, the explosion protection circuit has a switching function. This switching function allows at least one intrinsic safety unit, in particular the current limiting unit, to be bypassed, especially short-circuited. Thus, in non-hazardous areas, the transformer can be used without current limiting, while in hazardous areas, a version with current limiting and zone separation is provided.

[0021] It is particularly advantageous if the switching function can only be changed with special tools, especially with a key switch, or if it is located in an area of ​​the device that can only be accessed by means of special tools.

[0022] A further advantage of an unclaimed embodiment arises if the areas of the device which may be exposed to an explosive atmosphere are pottable and / or potted.

[0023] It is also advantageous if there is at least one connection between the explosion protection circuit and at least one other component of the device, and if each connection can only be released by means of a tool, or is non-releasable.

[0024] The invention is described in more detail below with reference to several exemplary embodiments and the accompanying figures: These show: Fig. 1 a block diagram of a field device according to the state of the art Fig. 2 a circuit diagram of an explosion protection circuit according to the invention with simple fail-safe operation Fig. 3 a circuit diagram of an explosion protection circuit according to the invention with a switching function according to the invention. Fig. 4a circuit diagram of an explosion protection circuit according to the invention with double fail-safety

[0025] Fig. 1 Figure 1 shows a simplified block diagram of a prior art device 1, such as a field device. The field device could, for example, be a flow meter operating on the ultrasonic principle. The applicant manufactures a wide variety of such field devices and markets them, for example, under the names Prosonic DDU10 or Prosonic Proline P. It is understood, however, that other types of field devices also fall within the scope of the invention. The sensor unit 2 and the electronic unit 4 are indicated, between which an explosion protection circuit 3 according to the invention is integrated. This circuit can either be permanently arranged or housed in a separate plug-in adapter, which is detachably connected between the electronic unit 3 and the sensor unit 2.

[0026] Several variations are possible for the explosion protection circuit, three of which will be shown in detail below. It goes without saying that many other arrangements are conceivable and also fall under the scope of the invention.

[0027] Fig. 2Figure 1 shows a circuit diagram of an explosion protection circuit 3' according to the invention with simple fail-safe operation, which is arranged between the electronics unit 4 and the sensor unit 2. The sensor unit 2 comprises a piezoelectric element 5, which is located within a metallic shield and to which the impedance is matched. The signal-carrying lines are provided by triaxial cables 6, one conductor of which is connected to the metallic housing 6b. A transformer 12 with at least three windings is used for impedance matching. Transformers with two windings are generally used. The third winding here serves not for impedance matching, but for explosion protection. Thus, the transformer 12 has a dual function in the illustrated embodiment.In addition to at least one extra winding, it is advantageous if the winding direction on the side with the piezoelectric element 5 is opposite to that on the side facing the electronic unit 4. This reduces the stored energy in case of a fault.

[0028] The following descriptions refer exclusively to the explosion protection circuit 3'. The two circuit branches on the left each have a coil 7, 7a with a line resistor 9, 9a and a series-connected resistor 8, 8a for current limiting. This redundant design ensures simple fail-safe operation of the explosion protection circuit 3'. A third circuit branch on the side with the piezoelectric element 5 contains another coil 7b and the associated line resistor 8b. By appropriately selecting the inductances and mechanically designing the coils 7, 7a, and 7b, both the power can be adequately transformed and zone separation can be ensured. The resistors 8, 8a, in turn, serve to limit the current. If resistors with too low a resistance are chosen, the current will not be sufficiently limited, and the circuit will only provide impedance matching.If the device is operated in a non-hazardous environment, such a choice of 8.8Ω resistors is advantageous, as more energy can be transferred to the piezoelectric element 5. Explosion protection can only be achieved if sufficiently high-resistance 8.8Ω resistors are used. Therefore, the choice of appropriate components and the intended application are crucial.

[0029] The following section will describe the explosion protection effect of the [product / service] in Fig. 2 The circuit shown will be demonstrated using a concrete example. Specific values ​​will be chosen for the individual components, and two energy analyses will be performed.

[0030] The inductive energy in a coil 7,7a,7b is given by E ind = 1 2 LI 2 .

[0031] Taking into account a standard safety factor of 1.5 for the current, a coil 7b with L=1500µH and E ind =20µJ (maximum permissible energy in the coil: E ind =40µJ, taking into account a safety factor of 2) results in a maximum current of I = 2 × 20 μJ 1500 μH 1 , 5 = 0.163 A and accordingly for the resistance at U=10V (DC in case of fault) R = U I = 10 V 0.163 A = 61.4 Ω

[0032] Depending on the choice of the series resistor 10 of the output stage 4a within the electronic unit 4, additional resistors 8 and 8a are necessary to achieve sufficient current limiting. A typical value for the series resistor of the output stage 4a is R10 = 50 Ω, so the parallel resistance Rs of the two resistors 8 and 8a must be chosen to be at least high enough that Rs = 61.4 Ω - 50 Ω = 11.4 Ω. Furthermore, a second resistor 10a is provided, the internal resistance of the input stage 4b. It is necessary for reflection-free transmission of the response signal and is typically chosen to have the same value as the series resistor 10. When adjusting the AC voltage during operation of the device, the sum of the impedances 7, 7a, 7b and resistances 8 and 8a should ideally be equal to the value of R10a = R10, while R10 corresponds to the characteristic impedance of the triaxial cable 6.The phase of the protection circuit with the connected piezoelectric element 5 should also be φ=0° at the operating frequency. Resistors 8b, 9, and 9a are the DC resistances of the windings.

[0033] A slight modification of the circuit from Fig. 2 The change consists of inserting connection 11. In this case, due to the parallel connection, Rs = 22.8 Ω. Thus, the additional connection 11 increases the resistance Rs and therefore reduces the current. On the other hand, if connection 11 is omitted, less voltage drops across resistors 8.8a, so that more energy is available to the piezoelectric element 5.

[0034] The same applies to the so-called impact test for a mechanical impact of 7J on the piezoelectric element 5. According to the standard, the energy should not exceed a value of 50 µJ without inductance.

[0035] The following applies to capacitive energy E kap = 1 2 CU 2 .

[0036] For a piezoelectric element with C=600pF, a capacitive energy of E kap = 20µJ and taking into account a safety factor of 2.5 for the energy, the voltage at the piezoelectric element is calculated as follows: U piezo = 2 × 20 μJ 600 pF = 258 V

[0037] The maximum voltage should therefore be below 7J in the event of a mechanical impact. U piezo = 258V. This voltage is not reached by using a coil 7b with L=1500µH on the side with the piezo element and the other two coils 8, 8a.

[0038] A second example of a 3" explosion protection circuit is in Fig. 3 shown. Fig. 3 differs from Fig. 2This is achieved simply by adding a switch 13. This switch allows the resistors 8, 8a connected in series with the coils 7, 7a to be bypassed, so that the device can be operated either with or without explosion protection. This eliminates the need to choose between high- or low-resistance resistors 8, 8a, as these can be bypassed at any time using the switching function.

[0039] One final example is in Fig. 4 This is shown. It is an explosion protection circuit 3‴ with double fail-safety. In this example, this is achieved by a third circuit branch 14 on the side facing away from the piezoelectric element 5. In each of the circuit branches, a resistor 8, 8a, 8c is connected in series with a coil 7, 7a, 7c, and a connection between the three circuit branches 11a is added.

[0040] An alternative measure is to ensure a sufficiently large distance between the circuit containing the piezoelectric element 5 and the circuit containing the three switching branches. This allows higher voltages to be used on the piezoelectric element during operation with a low risk of flashover.

[0041] It goes without saying that the essential functional units of the three examples shown can be freely combined. This applies to the choice of the type of fail-safe, the integration of additional switches such as switching function 13 in Fig. 3 , or the additional connection 11,11a. However, with regard to the individual components, the resonant circuits must then be adapted to the respective application.

[0042] In a fixed arrangement of the explosion protection circuit, it can, for example, be housed together with the circuit containing the piezoelectric element 5 in a potted enclosure. However, other arrangements are of course also conceivable. Reference symbol list

[0043] 1 Field device 2 Sensor unit 3 Explosion protection circuit 4 Electronic unit 4a, 4b Input stage and output stage of the electronic unit 5 Piezoelectric element 6 Triaxial cable 6a Connection of a conductor of the triaxial cable to the metallic housing 7, 7a, 7b Coils 8, 8a-f Resistors 9, 9a, 9b Lead resistances of the coils 10, 10a Series resistors of the input and output stages 11, 11a Connection 12 Transformer 13 Switching function or switch 14 Fourth switching branch

Claims

1. Device for monitoring at least one physical or chemical process variable, comprising at least one sensor unit (2) and an electronics unit (4) for signal acquisition, evaluation and / or supply, wherein the sensor unit (2) is operated with alternating current and / or the communication between the electronics unit (4) and the sensor unit (2) takes place via alternating current and / or alternating voltage, and with an explosion protection circuit (3) with intrinsic safety, which comprises a safety barrier with at least one unit for current limitation and voltage limitation, wherein the explosion protection circuit (3) is arranged between the sensor unit (2) and the electronics unit (4), wherein within the explosion protection circuit (3) a unit for impedance matching is provided for lossless signal transmission between the sensor unit and the electronics unit, which unit for impedance matching comprises at least one transformer (12) for galvanic isolation between the sensor unit and the electronics unit, characterized in that the explosion protection circuit (3) has at least simple fault tolerance, wherein a number of parallel branches within the explosion protection circuit (3) determines the fault tolerance, with simple fault tolerance being provided by two parallel branches, wherein the explosion protection circuit (3) has at least two parallel branches, wherein each branch comprises a coil (7, 7a, 7b, 7c) as the voltage limiting unit, and wherein each branch comprises a resistor (8, 8a) connected in series with the coil as the current limiting unit.

2. Device according to claim 1, characterized in that the impedance matching unit is designed such that the sensor unit (2) and the electronics unit (4) are galvanically isolated from each other.

3. Device according to claim 1 or 2, characterized in that the transformer (12) is designed, in particular by ensuring sufficiently large distances and suitable material selection such as varnishes and insulation films, to ensure galvanic isolation.

4. Device according to any of the preceding claims, characterized in that at least one component in the units for current limitation, voltage limitation and impedance matching is designed to have a dual function.

5. Device according to any of the preceding claims, characterized in that the sensor unit comprises at least one piezo element (5).

6. Device according to any of the preceding claims, characterized in that the explosion protection circuit (3) is permanently arranged between the sensor unit (2) and the electronics unit (4).

7. Device according to any of claims 1 to 6, characterized in that the explosion protection circuit (3) is arranged in a separate plug-in adapter, which is retrofittable and mounted between the sensor unit (2) and the electronics unit (4).

8. Device according to any of the preceding claims, characterized in that at least one component provided within the units for current and / or voltage limitation is implemented in multiple instances.

9. Device according to any of the preceding claims, characterized in that the explosion protection circuit (3) comprises a switching function (13), and that with the switching function (13) at least one unit for ensuring intrinsic safety, in particular the current limiting unit, can be bypassed, in particular short-circuited.

10. Device according to claim 9, characterized in that the switching function (13) can only be changed with a special tool, in particular with a key switch, or that it is located in a part of the device that is only accessible using a special tool.