Explosion Protection Circuit with Impedance Matching

Abstract

An apparatus for monitoring at least one physical or chemical process variable, comprising at least one sensor unit and an electronics unit for signal registration, evaluation and/or feeding, wherein the sensor unit is operated with alternating electrical current and/or communication between the electronics unit and the sensor unit occurs with alternating electrical current and/or alternating voltage. An explosion protection circuit with intrinsic safety, which includes a safety barrier, which has at least one unit for electrical current- and/or voltage limiting, is provided within the explosion protection circuit a unit for impedance matching, which unit for impedance matching includes at least one transformer.

Claims

1-16. (canceled)

17. An apparatus for monitoring at least one physical or chemical process variable,comprising: at least one transformer; at least one sensor unit; an electronics unit for signal registration, evaluation and/or feeding; and an explosion protection circuit, wherein: said at least one sensor unit is operated with alternating electrical current and/or communication between said electronics unit and said at least one sensor unit occurs with alternating electrical current and/or alternating voltage; said explosion protection circuit with intrinsic safety, which includes a safety barrier, which has at least one unit for electrical current and/or voltage limiting, and, there is provided within said explosion protection circuit a unit for impedance matching, which unit for impedance matching includes said at least one transformer.

18. The apparatus as claimed in claim 17, wherein: said unit for impedance matching is so embodied that said at least one sensor unit and said electronics unit are galvanically isolated from one another.

19. The apparatus as claimed in claim 18, wherein: said at least one transformer is so embodied, especially by assuring sufficiently large separations and suitable choice of materials, such as lacquers and insulating films, that the galvanic isolation is assured.

20. The apparatus as claimed in claim 17, wherein: said unit for electrical current limiting includes at least one resistor.

21. The apparatus as claimed in claim 17, wherein: said unit for voltage limiting includes at least one coil.

22. The apparatus as claimed in claim 17, wherein: respectively equal components in the units for electrical current limiting, voltage limiting and impedance matching are designed in such a manner that they have a double function.

23. The apparatus as claimed in claim 17, wherein: said at least one sensor unit has at least one piezoelement.

24. The apparatus as claimed in claim 17, wherein: said explosion protection circuit is arranged fixedly between said at least one sensor unit and said electronics unit.

25. The apparatus as claimed in claim 17, wherein: said explosion protection circuit is arranged in a separate plug adapter, which plug adapter is retrofittably secured between said at least one sensor unit and said electronics unit.

26. The apparatus as claimed in claim 17, wherein: the number of parallel branches within said explosion protection circuit determines failure safety, especially there is at least one single failure safety, when said explosion protection circuit has at least two parallel branches, and at least one double failure safety in the case of at least three parallel branches.

27. The apparatus as claimed in claim 17, wherein: at least one component, which is provided within the units for electrical current- and/or voltage limiting, is embodied multiple times.

28. The apparatus as claimed in claim 17, wherein: at least one of the components is designed failure safely.

29. The apparatus as claimed in claim 17, wherein: said explosion protection circuit includes a switch function, and, with said switch function, at least one unit for assuring intrinsic safety, especially said unit for electrical current limiting, can be shunted, especially short circuited.

30. The apparatus as claimed in claim 29, wherein: said switch function can be changed only with a special tool, especially with a key-operated switch, or said switch function is located in a region of the apparatus, which is accessible only by means of a special tool.

31. The apparatus as claimed in claim 17, wherein: the regions of the apparatus, which are possibly exposed to an explosive atmosphere, are pottable and/or potted.

32. The apparatus as claimed in claim 17, wherein: between said explosion protection circuit and at least one additional component of the apparatus at least one connection is present, and each connection is releasable, or non-releasable, only by a tool.

Description

[0022] The invention will now be described in greater detail by means of the appended drawing based on a number of examples of embodiments. The figures of the drawing show as follows:

[0023] FIG. 1 a block diagram of a field device according to the state of the art;

[0024] FIG. 2 a circuit diagram of an explosion protection circuit of the invention with single failure safety;

[0025] FIG. 3 a circuit diagram of an explosion protection circuit of the invention with a switch function of the invention; and

[0026] FIG. 4 a circuit diagram of an explosion protection circuit of the invention with double failure safety.

[0027] FIG. 1 shows a simplified block diagram of an apparatus 1 according to state of the art, such as, for example, a field device. The field device can be, for example, a flow measuring device working according to the ultrasonic principle. Such field devices are produced by the applicant in great variety and sold, for example, under the designation, Prosonic DDU10 or Prosonic Proline P. Other types of field devices also fall within the scope of the invention. As shown, apparatus 1 includes the sensor unit 2 and the electronics unit 4, between which is integrated an explosion protection circuit 3 of the invention. The explosion protection circuit 3 can be arranged fixedly, or provided in a separate plug adapter, which is placed releasably between the electronics unit 3 and the sensor unit 2.

[0028] Options for the explosion protection circuit include different variants, of which three different examples will be explained in detail below. Of course, many other arrangements can be provided, which also fall within the scope of the invention.

[0029] FIG. 2 shows a circuit diagram of an explosion protection circuit 3′ of the invention having single failure safety and arranged between the electronics unit 4 and the sensor unit 2. Sensor unit 2 includes a piezoelement 5, which is located within a metal shielding and to which the impedance is matched. The signal carrying lines are provided by triaxial cable 6, of which one conductor 6b is connected with the respective metal housing. Used for impedance matching is a transformer 12 having at least three windings. As a rule, transformers are embodied with two windings. The third winding serves here correspondingly not for impedance matching, but, instead, for explosion protection. Thus, the transformer 12 in the illustrated example of an embodiment has a double function. Besides the at least one additional winding, it is advantageous, when the winding handedness on the side with the piezoelement 5 is opposite that on the side toward the electronics unit 4. This acts in the case of malfunction to reduce the stored energy.

[0030] The following descriptions refer exclusively to the explosion protection circuit 3′. The two circuit branches on the left side have each a coil 7,7a with each a line resistance 9,9a and each a series resistor 8,8a for electrical current limiting. This redundant construction provides a single failure safety of the explosion protection circuit 3′. In a third circuit branch on the side with the piezoelement 5 is another coil 7b and the associated line resistance 8b. By suitable choice of the inductances and mechanical embodiment of the coils 7,7a,7b, both the power can be suitably transformed, as well as also a zone isolation assured. The resistances 8,8a, in turn, serve for electrical current limiting. If one selects ohmic resistances, which are too low, then the electrical current is not sufficiently limited and the circuit assures only an impedance matching.

[0031] If the apparatus is operated in a non-explosion-endangered environment, such a choice of resistors 8,8a is advantageous, since more energy can be transmitted to the piezoelement 5. An explosion protection effect can only be achieved, when sufficiently high-valued ohmic resistances 8,8a are applied. It depends thus on the choice of the corresponding components and the desired application.

[0032] In the following, the explosion protective action of the circuit shown in FIG. 2 will be demonstrated based on a concrete example. In this regard, particular values are selected for the individual components and two energy considerations performed.

[0033] The inductive energy in a coil 7,7a,7b is given by

[00001]$E_{\mathrm{ind}}=\frac{1}{2}.Math.{\mathrm{LI}}^2.$

[0034] If one takes into consideration a standard safety factor of 1.5 for the electrical current, in the case of a coil 7b with L=1500 μH and E.sub.ind=20 μJ (maximum allowed energy in the coil: E.sub.ind=40 μJ, taking into consideration a safety factor of 2) there results a maximum electrical current of

[00002]$I=\frac{\sqrt{\frac{2\times 20.Math..Math.\mathrm{\mu J}}{1500.Math..Math.\mathrm{\mu H}}}}{1.5}=0.163.Math..Math.A$

[0035] and correspondingly for the resistor in the case of U=10V (DC in the case of malfunction)

[00003]$R=\frac{U}{I}=\frac{10.Math..Math.V}{0.163.Math..Math.A}=61.4.Math.\Omega$

[0036] Depending on choice of the series resistor 10 of the output stage 4a within the electronics unit 4, thus additional resistances 8,8a are necessary, in order to achieve a sufficient electrical current limiting. A typical value for the limiting resistor of the output stage 4a amounts to R.sub.10=50Ω, so that the parallel circuit R.sub.s of the two resistances 8,8a would be selected at least high enough that R.sub.s=61.4Ω−50Ω=11.4Ω.

[0037] Furthermore, a second resistor 10a is provided, the internal resistance of the input stage 4b. It is necessary for a reflection free transmission of the response signal. Typically selected for it is the same value that the limiting resistor 10 has. In the case of a matching of the alternating voltage during operation of the device, the sum of the impedances 7,7a,7b and resistances 8,8a should in the ideal case equal the value of R.sub.10a=R.sub.10, while R.sub.10, in turn, corresponds to the wave resistance of the triaxial cable 6. Likewise, at the working frequency, the phase of the protection circuit with connected piezoelement 5 should be φ=0°. In the case of the resistors 8b,9 and 9a, these are the ohmic resistances of the windings.

[0038] A slight modification of the circuit of FIG. 2 is the provision of the connection 11. In this case, there results, due to the parallel circuit, R.sub.s=22.8Ω. Thus, the additional connection 11 provides an increase of the resistance R.sub.s and, thus, a reduction of the electrical current. On the other hand, when the connection 11 is omitted, less voltage drops on the resistors 8,8a, so that more energy is available to the piezoelement 5.

[0039] Somewhat the same holds for the so-called shock test for a mechanical impact with 7 J on the piezoelement 5. The energy without inductance should according to standard not exceed a value of 50 μJ.

[0040] In the Case of the Capacitive Energy

[00004]$E_{\mathrm{kap}}=\frac{1}{2}.Math.{\mathrm{CU}}^2.$

[0041] For a piezoelement with C=600 pF, a capacitive energy of E.sub.kap=20 μJ as well as taking into consideration a safety factor of 2.5 for the energy, there results for the voltage on the piezoelement

[00005]$U_{\mathrm{piezo}}=\sqrt{\frac{2\times 20.Math..Math.\mathrm{\mu J}}{600.Math..Math.\mathrm{pF}}}=258.Math..Math.V$

[0042] The maximum voltage would, thus, in the case of a mechanical impact with 7 J, lie below U.sub.piezo=258V. By applying a coil 7b with L=1500 μH on the side with the piezoelement as well as the other two coils 8,8a, this voltage is not achieved.

[0043] A second example of an explosion protection circuit 3″ is shown in FIG. 3. FIG. 3 differs from FIG. 2 only by the adding of a switch 13. This enables shunting of the resistances 8,8a connected in series with the coils 7,7a, so that the apparatus can be operated selectively with or without explosion protection action. This makes the deciding between high, respectively low, ohm resistors 8,8a unnecessary since, they can, at any time, be bypassed by means of the switch function.

[0044] A last example is shown in FIG. 4. In such case, of concern is an explosion protection circuit 3′″ with double failure safety. This is achieved in the present example by a third circuit branch 14 on the side away from the piezoelement 5. In each of the circuit branches, a resistor 8,8a,8c is connected in series with a respective coil 7,7a,7c. Additionally, a connection 11a of the three circuit branches is added.

[0045] An alternative measure is to provide a sufficiently large separation between the circuit containing the piezoelement 5 and the circuit containing the three circuit branches. Then, higher voltages on the piezoelement can be used with little risk of a flashover during operation.

[0046] Of course, the essential functional units of the three illustrated examples are freely combinable with one another. This relates to the choice of the type of failure safety, the integration of additional switches, such as the switch function 13 in FIG. 3, and even the additional connection 11,11a.

[0047] With reference to the individual components, the oscillatory circuits must then, however, be matched to the particular application.

[0048] In the case of a fixed arrangement of the explosion protection circuit, this can be arranged, for example, together with the circuit containing the piezoelement 5 in a potted housing. However, also other arrangements are, of course, implementable.

LIST OF REFERENCE CHARACTERS

[0049] 1 field device [0050] 2 sensor unit [0051] 3 explosion protection circuit [0052] 4 electronics unit [0053] 4a, 4b input stage and output stage of the electronics unit [0054] 5 piezoelement [0055] 6 triaxial cable [0056] 6b connection of a conductor of the triaxial cable with the metal housing [0057] 7,7a,7b coils [0058] 8,8a-f resistors [0059] 9,9a,9b line resistances of the coils [0060] 10,10a series resistors of the input and output stages [0061] 11,11a connection [0062] 12 transformer [0063] 13 switch function, respectively switch [0064] 14 fourth circuit branch