WO2016150621A1

WO2016150621A1 - Device for determining and/or monitoring at least one process variable

Info

Publication number: WO2016150621A1
Application number: PCT/EP2016/053131
Authority: WO
Inventors: Raphael KUHNEN; Dietmar FRÜHAUF
Original assignee: Endress+Hauser Gmbh+Co. Kg
Priority date: 2015-03-25
Filing date: 2016-02-15
Publication date: 2016-09-29
Also published as: DE102015104536A1

Abstract

A device (1) for determining and/or monitoring at least one process variable of a medium (4) in a container (5), the device comprising: at least one unit (3) that is capable of vibration and has at least one membrane (9) that can be made to mechanically vibrate; two rods (10a, 10b) secured to the membrane (9) perpendicularly to a main surface of the membrane (9); a housing (8), the membrane (9) forming at least one sub-region of a wall of the housing (8) and the two rods (10a, 10b) being directed into the housing interior; at least one drive unit/receiver unit (11) arranged in the end region of the two rods (10a, 10b) that faces away from the membrane (9), the drive unit/receiver unit (11) being designed to cause the unit (3) capable of vibration to mechanically vibrate by means of an electric excitation signal and by means of the two rods (10a, 10b) and to receive the mechanical vibrations of the unit (3) capable of vibration and to convert the vibrations into an electric received signal; and an electronic unit (7) designed to generate an excitation signal from the received signal and to calculate the at least one process variable at least from the received signal.

Description

Device for determining and / or monitoring at least one process variable

The invention relates to a device for determining and / or monitoring at least one process variable of a medium in a container comprising at least one drive / receiving unit, in particular in the form of an electromechanical converter unit. The

Process variable is given for example by the level or the flow of the medium or by its density or viscosity. The medium is for example in a container, a tank, or in a pipeline. In automation technology, a wide variety of field devices are used to determine and / or monitor at least one process variable, in particular a physical or chemical process variable. These are, for example, level gauges, flowmeters, pressure and temperature measuring devices, pH redox potential measuring devices, conductivity measuring devices, etc., which record the corresponding process variables level, flow, pressure, temperature, pH or conductivity, etc. The respective measurement principles are known from a variety of publications.

A field device typically comprises at least one at least partially and at least temporarily come into contact with the process sensor unit and an electronic unit, which serves for example the signal detection, evaluation and / or supply. In the context of the present application, field devices are in principle all measuring devices that are used close to the process and that provide or process process-relevant information, including remote I / Os, radio adapters or generally electronic components, which are arranged at the field level. A variety of such field devices is manufactured and distributed by the Applicant.

In a number of corresponding field devices electromechanical transducer units are used. For example, vibronic sensors such as vibronic level or flow meters are mentioned here, but they are also used in ultrasonic level gauges or flowmeters. To address each genus of field device with an electromechanical transducer unit and its underlying measurement principle separately and in detail, would go beyond the scope of the present application. Therefore, for the sake of simplicity, the following description is limited to where specific field devices are referenced, by way of example to level measuring devices with an oscillatable unit. The oscillatable unit of such, also referred to as vibronic sensor level gauge, for example, a tuning fork, a single rod or a membrane. The oscillatable unit is excited in operation by means of a drive / receiving unit, usually in the form of an electromechanical transducer unit, to mechanical vibrations, which in turn may be, for example, a piezoelectric, electromagnetic or magnetostrictive drive / receiving unit. Corresponding field devices are manufactured by the applicant in great variety and, for example, under the name LIQUIPHANT or

Sold SOLIPHANT. The underlying measurement principles are basically known. The drive / receiving unit excites the mechanically oscillatable unit by means of an electrical pickup signal to mechanical vibrations. Conversely, the drive / receiving unit can receive the mechanical vibrations of the mechanically oscillatable unit and convert it into an electrical reception signal. The drive / receiving unit is either a separate drive unit and a separate receiver unit, or a combined drive / receiver unit.

To stimulate the mechanically oscillatable unit a variety of both analog and digital methods have been developed. In many cases, the drive / receiving unit is part of a feedback electrical resonant circuit, by means of which the excitation of the mechanically oscillatable unit to mechanical vibrations takes place. For example, for a resonant oscillation, the resonant circuit condition, according to which the amplification factor> 1 as well as all the phases occurring in the resonant circuit result in a multiple of 360 °, must be satisfied. This has the consequence that a certain phase shift between the excitation signal and the

Receiving signal must be guaranteed. For this purpose, a variety of solutions have become known. In principle, the adjustment of the phase shift, for example, by

Use of a suitable filter can be made, or also by means of a control loop to a predetermined phase shift, the setpoint regulated. From the

For example, DE 102006034105A1 has disclosed the use of an adjustable phase shifter. The additional integration of an amplifier with an adjustable amplification factor for additional regulation of the oscillation amplitude, on the other hand, has been described in DE102007013557A1. DE102005015547A1 suggests the use of an allpass. The adjustment of the phase shift is also possible by means of a so-called frequency search, as disclosed, for example, in DE102009026685A1, DE102009028022A1, and DE102010030982A1. The phase shift can, however, also be regulated to a predefinable value by means of a phase-locked loop (PLL). A based on this

Excitation method is the subject of DE00102010030982A1. Both the start signal and the receive signal are characterized by their frequency, amplitude and / or phase. Changes in these sizes are then usually for

Determining the respective process variable used, such as a predetermined level of a medium in a container, or the density and / or viscosity of a medium. In the case of a vibronic level switch for liquids, for example, a distinction is made as to whether the oscillatable unit is covered by the liquid or vibrates freely. These two states, the free state and the covered state, are differentiated, for example, based on different resonance frequencies, ie a frequency shift, or on the basis of an attenuation of the oscillation amplitude.

The density and / or viscosity in turn can only be determined with such a measuring device if the oscillatable unit is covered by the medium. From DE10050299A1, the

DE102006033819A1 and DE 10200704381 1A1 has become known to determine the viscosity of a medium on the basis of the frequency-phase curve (0 = g (f)). This procedure is based on the dependence of the damping of the oscillatory unit on the viscosity of the respective

Medium. In order to eliminate the influence of the density on the measurement, the viscosity is determined on the basis of a frequency change caused by two different values for the phase, ie by means of a relative measurement. For determining and / or monitoring the density of a medium, however, according to DE10057974A1, the influence of at least one disturbance variable, for example the viscosity, on the oscillation frequency of the mechanically oscillatable unit is determined and compensated. In DE 102006033819A1 is further described to set a predetermined phase shift between the excitation signal and the received signal, in which effects of changes in the viscosity of the medium to the mechanical

Vibrations of the mechanically oscillatable unit are negligible. At this

Phase shift can be an empirical formula for determining the density set up.

The drive / receiving unit is, as already mentioned, usually designed as an electromechanical converter unit. Often, it comprises at least one piezoelectric element in a wide variety of configurations. By utilizing the piezoelectric effect, namely, a high efficiency can be achieved. The term efficiency is understood to mean the efficiency of the conversion of the electrical energy into mechanical energy. Appropriate

Piezoceramic materials based on PZT (lead zirconate titanate) are normally suitable for use at temperatures up to 300 ° C. Although there are piezoceramic materials that maintain their piezoelectric properties even at temperatures above 300 ° C; However, these have the disadvantage that they are significantly less effective than the materials based on PZT. For the

In addition, these high-temperature materials are used in vibronic sensors due to the large differences in the thermal expansion coefficients of metals and ceramic Substances only conditionally suitable. Because of its function as a force transmitter, the at least one piezoelectric element must be non-positively connected to a membrane which is part of the oscillatable unit. In particular, at high temperatures, however, there are increasingly large mechanical stresses, the breakage of the piezoelectric element and thus

accompanied by a total failure of the sensor can result.

An alternative which may be better suited for use at high temperatures is represented by so-called electromagnetic drive / receiving units, as described, for example, in the publications WO 2007/11301 1 and WO 2007/1 14950 A1. The conversion of electrical energy into mechanical energy takes place via a magnetic field. A

corresponding electromechanical transducer unit comprises at least one coil and a permanent magnet. By means of the coil is a magnetic interspersed magnetic

Generated alternating field, and transmitted via the magnet, a periodic force on the oscillatory unit. Usually, the transmission of this periodic force similar to the principle of a plunger, which sits centrally on the membrane. In this way, the drive

/ Receiving unit can be used for a temperature range between -200 ° C and 500 ° C. Often, however, there is no frictional connection between the membrane and the drive / receiving unit, so that the efficiency of the respective field device compared to a piezoelectric drive / receiver unit is reduced.

In addition to the drive / receiving unit used in each case, various electronic components, which are usually integrated as part of an electronics unit in a field device, limit the maximum process temperature at which the respective field device can still be used. To decouple such temperature-sensitive electronic components from a process, a common method in the integration of a so-called

Temperature distance pipe in the structural design of the respective field device. For example, it is a tube, which is part of the housing of the field device, and which is made of a material that is characterized by a high thermal insulation. In this regard, reference is made, for example, to EP2520892A1, in which it is described to design a section of the housing of a measuring device in such a way that, in the presence of a

Temperature difference between an environment of the process connection and the

Electronics unit a low heat flow parallel to a longitudinal axis of the housing to

Electronic unit flows. In order to ensure the most efficient temperature decoupling of the drive / receiving unit from the respective process, it would be desirable, even the drive / receiving unit spatially to separate from the process. However, such a decoupling is not easy to implement, since an efficient transfer of force to the membrane may no longer be possible.

The present invention is therefore based on the object to provide a field device with an electromechanical transducer unit, which is suitable for use at high

Process temperatures is suitable.

This object is achieved by a device for determining and / or monitoring at least one process variable of a medium in a container comprising at least

an oscillatable unit with at least one membrane which can be set into mechanical vibrations,

two perpendicular to a base of the membrane attached to the membrane rods, a housing, wherein the membrane forms at least a portion of a wall of the housing, and wherein the two rods are directed into the housing interior, at least one drive / receiving unit, which faces away in the membrane Endbereich of the two rods is arranged, which drive / receiving unit is adapted to excite the oscillatory unit by means of an electrical excitation signal and by means of the two rods to mechanical vibrations and to receive the mechanical vibrations of the oscillatable unit and convert it into a received electrical signal, and

an electronic unit, which is configured to generate an excitation signal from the received signal, and to determine the at least one process variable at least from the received signal.

The housing and the rods serve for the spatial separation of the drive / receiving unit from the process. As a result, the device according to the invention is optimally suited for use in an extended temperature range, in particular for use at high temperatures

Temperatures, suitable. The direct, in particular non-positive connection of the rods with the membrane ensures despite the spatial separation for a high efficiency of power transmission from the drive / receiving unit to the oscillatory unit. Nevertheless, the structural design of a device according to the invention is comparatively simple.

The drive / receiving unit, in particular an electromechanical converter unit, may be both a separate drive unit and a separate receiving unit, or a combined drive / receiving unit. This can for example be attached to at least the two rods, which is in particular a non-positive connection is. Alternatively, however, it may also be arranged inside the housing such that it does not touch the two rods.

It is advantageous if the drive / receiving unit is designed to set the two rods in mechanical vibrations, wherein the two rods are attached to the membrane such that vibrations of the membrane result from the vibrations of the two rods. Thus, waves propagate along the rods with a wavelength λ predetermined by the drive / receiving unit. For this purpose, the rods are pressed apart by means of the drive / receiving unit frequency properly or pulled together. The two rods behave accordingly like a mechanical resonator. Since the rods are connected to the membrane, in particular non-positively connected, consequently, the oscillatable unit is also set into mechanical vibrations. Conversely, the drive / receiving unit receives the waves which propagate along the two rods from the oscillatable unit and generates an electrical reception signal therefrom.

It is further advantageous if the length L of the rods with respect to the wavelength of the waves propagating along the two rods is Ι_ = ηλ / 2 + K / 4, where n is a natural number. The length of the rods is thus adjusted according to a desired excitation frequency and in view of the respectively necessary temperature decoupling.

In another preferred embodiment, the device according to the invention additionally comprises a fixing element, by means of which fixing element the two rods are mechanically coupled to one another in the end region facing away from the membrane. For this purpose, the two rods and the fixing element, for example, positively connected to each other. The two strands are thus coupled to each other both by means of the membrane and by means of the fixing element. Again, the rods are forced apart by the drive / receiver unit in the correct frequency or contracted such that a wave propagates along the two rods and the oscillatory unit is set into mechanical vibrations.

It is advantageous if the frequency of the excitation signal and / or the length L of the two rods are selected such that a standing wave propagates along the rods. In this way, a particularly high efficiency can be achieved. It is particularly advantageous if the length L of the rods with respect to the wavelength of the propagating along the two rods waves L = nA / 2, where n is a natural number. The length of the rods is thus adjusted according to a desired excitation frequency and in view of the respectively necessary temperature decoupling so that standing waves can propagate. For a refinement of the device according to the invention with a fixing element, in particular a drive / receiving unit which comprises at least one piezoelectric element is suitable.

A particularly preferred embodiment provides that the rods and / or the housing are made of a material which provides good thermal insulation. This increases the degree of thermal insulation. The housing is thus not only the function of protection of the components contained therein such as the rods. Rather, the housing also serves as

Temperature spacer tube. The electronics unit can then either be accommodated in the region of the temperature-distancing tube facing away from the process, or else the housing comprises a separate region within which the electronics unit is arranged. At that portion of the housing, which serves as a temperature spacer tube, in particular further is the

Attached process connection. The exact location of the process connection along the housing results from the particular installation requirements.

It is advantageous if the process variable is given by a level or the flow of the medium in the container, or by the density or viscosity of the medium.

On the one hand, the oscillatable unit may be a membrane vibrator. On the other hand, an embodiment provides that at least one vibrating rod is attached to the membrane of the oscillatable unit. Then the oscillatable unit is a single rod or tuning fork. In a preferred embodiment, the drive / receiving unit comprises at least one piezoelectric element. It is therefore a piezoelectric transducer unit, such as a stack or bimorph drive. Alternatively, the drive unit may also be an electromagnetic drive with at least one coil and a magnet. Furthermore, magnetostrictive drive / receiving units are also conceivable. Due to the spatial separation, the drive / receiving unit used in each case must meet no special conditions with regard to the temperature sensitivity. It can rather be optimized in terms of their efficiency in the power transmission to the two rods out.

It is advantageous if the oscillatable unit is arranged in a defined position within the container, such that it dips into the medium to a determinable immersion depth. In this way, the process variables viscosity and / or density can be determined.

In a particularly preferred embodiment, the oscillatable unit is a tuning fork with two oscillating rods, wherein the two rods attached to the membrane and the two oscillating rods attached to the membrane are mutually mirror-symmetrical with respect to the plane perpendicular are arranged opposite to the longitudinal axis through the rods and / or oscillating rods. In each case, a vibrating rod and a rod run essentially along the same imaginary line parallel to their two longitudinal axes. In particular, the two rods and oscillating rods are arranged such that they are at the same distance from the center of the

Base of the membrane are perpendicular to the longitudinal axis of the rods and vibrating rods. This symmetrical arrangement in the case of a vibronic sensor with a tuning fork as a vibrating unit can achieve a particularly high efficiency.

It is advantageous if the two oscillating rods and the membrane has a first

form a mechanical resonator, and when the two rods and the membrane form a second mechanical resonator. Then, the first and the second resonator by means of

Membrane mechanically coupled to each other, wherein the frequency of the exciting signal is selected such that the first and second resonator oscillate in an antisymmetric vibration mode with respect to the plane through the membrane perpendicular to the longitudinal axis of the rods and / or vibrating rods.

The oscillating rods, rods and the membrane thus form a coupled oscillating system, wherein the coupling is determined by the membrane. In such a coupled vibration system, two resonance frequencies occur. If these resonant frequencies are sufficiently close together, the rods and oscillating rods vibrate, ie the first and second mechanical oscillations

Resonator simultaneously with high vibration amplitude. This will be described in detail in connection with FIGS. 3 and 4.

In one embodiment, the length L and / or the rigidity of the two rods are selected such that the oscillation frequency of the first resonator and the oscillation frequency of the second resonator have substantially the same value in the event that the oscillatable unit is not covered by medium. The resonant frequency of the first or second resonator is determined by the length and the geometric configuration of the rods or oscillating rods, and by the nature of the connection with the membrane. Using, for example, a tuning fork, as used in the marketed by the applicant LIQUIPHANTEN or SOLIPHANTEN, the resonant frequency of the oscillatory unit is already established. Then, by properly selecting the length, diameter, wall thickness, and configuration of the connection to the diaphragm, the resonant frequency of the second mechanical resonator including the rods can be adjusted to the resonant frequency of the vibratable unit.

In another embodiment, the length L and / or the rigidity of the two rods are selected such that the oscillation frequency of the first resonator and the oscillation frequency of the second resonator, in the case that the oscillatable unit is covered by a selectable reference medium, substantially the same value exhibit. This allows the device to a adjust certain desired reference medium. The damping of the oscillations of the oscillatory unit by this medium is counteracted, cf. also the following description. The invention and its advantageous embodiments will be described in more detail below with reference to FIGS. 1 to 4. It shows:

1 shows a schematic sketch of a vibronic sensor according to the prior art, FIG. 2 shows a device according to the invention with an oscillatable unit in the form of a

Tuning fork, (a) without fixation element, and (b) with fixation element.

FIG. 3 shows the symmetrical (a) and the antisymmetric (b) oscillation modes of the first and second coupled resonators of the coupled oscillation system of FIG. 2, and FIG

4 shows a diagram of the two resonance frequencies of the first and second resonators of a device according to the invention according to FIG. 2 or FIG. 3.

In Fig. 1, a vibronic level gauge 1 is shown. A sensor unit 2 with a mechanically oscillatable unit 3 in the form of a tuning fork partially immersed in a medium 4, which is located in a container 5. The oscillatable unit 3 is excited by means of the pickup / receiving unit 6, usually an electromechanical transducer unit, to mechanical vibrations, and may for example be a piezoelectric stack or

Bimorphantrieb, but also an electromagnetic or magnetostrictive drive / receiving unit. However, it goes without saying that other embodiments of a vibronic level gauge are possible. Furthermore, an electronic unit 7 is shown, by means of which the signal detection, evaluation and / or supply is carried out.

In Fig. 2a, a first embodiment of a device 1 according to the invention is shown schematically. In the lower wall of a housing 8, a membrane 9 is introduced. At this side surface, the housing 8 thus closes off with the membrane 9. In this example, the housing 8 is cylindrical and the diaphragm 9 disc-shaped with a circular base A. It goes without saying, however, that other geometries are conceivable and fall within the scope of the present invention. Perpendicular to the base area A of the membrane 9 and extending into the interior of the housing 8, two rods 10a, 10b are fastened to the membrane 9. This is in particular a non-positive connection. The base of the membrane 9 is then in a plane perpendicular to the longitudinal direction of the two rods. For example, the two rods 10a, 10b along an imaginary line or axis through the center of the base A of the membrane 9 symmetrically arranged around this center.

In the end region of the two rods 10a, 10b facing away from the membrane 9, a drive / receiving unit 11 is arranged. This can on the one hand at least be attached to the two rods 10a, 10b. In the example shown here, however, the drive / receiving unit is arranged within the housing 8 such that it does not touch the two rods 10a, 10b. The drive / receive unit 1 1 is an electromechanical converter unit, in particular a piezoelectric converter unit with at least one piezoelectric element, or an electromagnetic converter unit.

In the example shown in FIG. 2a, the housing consists of two partial areas 8a, 8b. The first subarea 8a surrounds at least the two rods 10a, 10b and the drive / receiving unit and serves as a temperature spacer. The length of this temperature spacer tube is in

Substantially adapted to the length of the two rods 10a, 10b. In the second portion 8b, the electronic unit 7 is arranged. The two subregions 8a, 8b are non-positively connected to one another and designed such that signal-conducting cables and the like can be guided from the sensor unit 2 to the electronic unit 7. The connection between the two subregions 8a, 8b may, for example, be a welded, glued or soldered connection. It goes without saying that the housing 8 can also comprise more partial areas or can also be manufactured in one piece. In the central region of the first portion 8a of the housing 8, a process connection 12 is further attached, which is fixedly connected to the housing 8. This can also be an adhesive, solder or welded connection. The exact position of the process connection 12 results in each case from the individual installation situation.

In continuous operation, the drive / receiving unit with a start signal in the form of an AC or AC signal is applied such that the drive / receiving unit, the two rods 10a, 10b in the membrane 9 opposite end portion frequency right apart and / or together, such that the two rods 10a, 10b are vibrated. As a result, a wave propagates along the rods 10a, 10b, which, due to a lever effect, leads to a vibratory movement of the oscillatable unit 3, that is to say in particular the diaphragm 9. The length of the two rods 10a, 10b and the frequency of the start signal taking into account the requirements with respect to

Temperature decoupling coordinated. Depending on the choice of the frequency of the excitation signal and the length of the rods 10a, 10b are then standing waves, which leads to a particularly high efficiency with respect to the power transmission to the diaphragm 9. On the other hand, the driving / receiving unit receives the amplitude of the waves, in particular standing waves, which propagate from the oscillatable unit 3 along the bars 10a, 10b and converts them into an electrical reception signal. The rods 10a, 10b form with the membrane 9 a mechanical resonator.

On the side facing away from the housing 8 of the membrane 9, two oscillating rods 13a, 13b are also fixed in the embodiment of FIG. 2a, which are non-positively connected to the membrane. Accordingly, the oscillatable unit 3 is a

Tuning fork. However, it goes without saying that a single rod or a membrane oscillator can equally be considered as the oscillatable unit 3. The two are preferred

Oscillating rods 13a, 13b and the two rods 10a, 10b attached to the membrane 9 such that in each case a rod 10a, 10b and a vibrating rod 13a, 13b along the same longitudinal axis, that is, the axis perpendicular to the base A through the membrane 9. In this case, the two longitudinal axes intersect the plane parallel to the membrane 9 at the same distance from the center of this surface A. By this symmetrical arrangement, an increased efficiency can be achieved.

An alternative, but very similar embodiment of a device according to the invention is shown in Fig. 2b. In addition to the components described in connection with FIG. 2 a, a fixing element 14 is fastened in the end region of the rods 10 a, 10 b facing away from the membrane 9 in FIG. This may, for example, be disk-shaped and have a round cross-sectional area, or generally have the same base area A 'as that of the membrane 9. The drive / receiving unit 1 1 is arranged in the immediate vicinity of the fixing element 14 on the membrane 9 facing side of the fixing element 14. It should be noted, however, that other arrangements are conceivable. In this example, the drive / receiving unit is attached in particular at least to the two rods 10a, 10b, in particular fastened non-positively. The two rods 10a, 10b are thus coupled together in one of their end regions via the membrane 9 and coupled in the second end region via the fixing element 14. In such an embodiment, it is preferably a drive / receiving unit 1 1 with at least one piezoelectric element.

Both in the exemplary embodiment according to FIG. 2 a and also in the case of FIG. 2 b there is a coupled resonator system in which the two oscillating rods 13 a, 13 b of the oscillatable unit 3 with the diaphragm 9 have a first mechanical resonator 15 and the two rods 10 a, 10 b form a second mechanical resonator 16 with the membrane 9 and optionally the fixing element 14. The mechanical coupling of the two resonators 15, 16 takes place essentially via the membrane 9, via which the coupling is also adjustable. For example, the coupling can be influenced by the thickness or the material of the membrane 9 be, but also by the respective connection with the oscillating rods 13a, 13b or rods 10a, 10b. In such a coupled resonator system, two vibration modes occur with two different resonance frequencies (F1, F2), which are illustrated in FIGS. 3 and 4. The two vibration modes are symmetrical - and one

antisymmetric vibration mode, as illustrated in Fig. 3 with reference to a device according to the invention shown in FIG. 2b. For an exemplary embodiment according to FIG. 2a, an equivalent situation exists. In the case of the symmetrical oscillation mode (FIG. 3 a), the first resonator 15 and the second resonator 16 oscillate in mirror symmetry with respect to one another, parallel to the plane

Base area A of the membrane 9. The rods 10a, 10b move in the membrane 9

towards the end region facing away from one another, the two oscillating rods 13a, 13b also move toward one another in the region facing away from the membrane 9. In the antisymmetric

On the other hand, the oscillation modes (FIG. 3 b) move the rods 10 a, 10 b towards one another in the end region facing away from the membrane 9 when the two oscillating rods 13 a, 13 b

move away from each other. Thus, the two rods 10a, 10b and the two oscillating rods 13a, 13b remain aligned in the immediate vicinity of the diaphragm 9 along a common longitudinal axis, which is not the case for the symmetrical vibration mode. The antisymmetric oscillation mode thus corresponds to the natural oscillatory motion of an oscillatable unit 3, such as the vibration forks used in a LIQUIPHANTEN or SOLIPHANTEN. In contrast, in the symmetric vibration mode, the membrane 9 remains largely unmoved. If the resonance frequencies F1, F2 of the two oscillation modes are sufficiently close to one another, the oscillating rods 13a, 13b and the two rods 10a, 10b oscillate in the event that the oscillatable unit 3 is not in contact with medium 4, simultaneously with maximum amplitude relative to a specific one Control Power Even if the first 15 and the second resonator 16 are designed such that both have the same resonant frequency (F1 = F2) as a single system, the coupling of the two resonators 15, 16 by means of the diaphragm 9 will produce two resonance frequencies (Fl? ^) or vibration modes, wherein the distance between the two resonance frequencies F1, F2 is determined by the coupling.

4 shows a diagram in which the frequencies of the two resonators 15, 16

are applied against each other. F1 denotes the frequency of the first resonator 15 and F2 the frequency of the second resonator 16. While the frequency F1 changes upon immersion of the oscillatable unit 3 into a medium 4, the frequency F2 of the second resonator remains at Essentially constant. Due to the coupling through the membrane 9, however, the two hatching lines R1 of the first 22 and R2 of the second resonator 16 result, the width of the hatching indicating the oscillation amplitude of the respective resonator 15, 16. If, for example, the first resonator 15, that is to say the oscillatable unit 6, oscillates at a frequency of F1 = 700 Hz, the oscillation movement takes place with a comparatively small oscillation

Oscillation amplitude. It is assumed that the oscillation at F1 = 700 Hz corresponds to a vibration of the oscillatable unit 3 in the case of partial immersion in a specific medium 4. If the second resonator 23 simultaneously oscillates at F2 = 1000 Hz, then this points

Oscillation on a comparatively large vibration amplitude. If now the oscillatable unit 3 is slowly pulled out of the medium 4, both the frequency F1 of the first resonator 15 and its oscillation amplitude R1 increase. For the sake of simplicity, the medium attenuation diminishing due to the extraction of the oscillatable unit 3 from the medium 4 is neglected for this consideration. As a consequence, the tuning of the two resonators 15, 16 improves and more energy can be transmitted from the rods 10a, 10b to the oscillating rods 13a, 13b. To the same extent, however, the oscillation amplitude R2 of the second resonator 16 decreases.

At the intersection point 17, the first 15 and the second resonator 16 are matched to one another. Nevertheless, due to the coupling through the membrane 9, two different

Resonant frequencies F1 and F2 on. Since no assignment of the resonances to the oscillating rods 13a, 13b or rods 10a, 10b is possible in this area, this area is not provided with hatching. If the frequency F2 of the first resonator 15 continues to increase, the result is a mirror-symmetric behavior for the two oscillation modes of the first 15 and second 16 resonators.

In order to achieve the most efficient transfer of energy from the electromechanical transducer unit 4 to the oscillating rods 13a, 13b, ie the highest possible efficiency, should not too large a distance between the resonant frequencies (F1, F2) of the first 15 and second 16th

Resonator arise. On the other hand, the resonance frequency should be F2 of the second

Resonator 16 but not in the dynamic range for the resonance frequency F1 of the

oscillatory unit 3 are so that no double assignment of a frequency can be done. With the dynamic range is meant that interval of resonance frequencies F1, with which the oscillatable unit 3 can vibrate in contact with different media 4 and in the case of different immersion depths into the respective medium 4. It follows that the resonant frequency F2 of the second resonator 16 is to be chosen to be just above the highest frequency F1 of the dynamic range of a particular vibration mode of the oscillatable unit 3. At the same time, it is necessary to optimize the rigidity and mass of the rods 10a, 10b so that a the greatest possible leverage exists. If, for example, a LIQUIPHANT tuning fork is used, without contact with the medium F1 to be measured is 1000 Hz. Then, the second resonator 16 is tuned, for example, to a frequency of F2 1100 Hz, so that the frequency F2 of the second resonator 16 drops to about 950 Hz due to the coupling. When immersed in a medium to be measured, the frequency F1 of the first resonator 15 decreases while the frequency F2 of the second resonator 16 remains substantially constant.

For example, the adaptation of the resonance frequencies F1 and F2 can be carried out in such a way that they are matched to one another without contact of the oscillatable unit 3 with a medium 4. In this case, the frequencies F1 and F2 at least partially submerge the oscillatable unit in a medium 4 from the intersection point 17 away. On the other hand, the adaptation of the resonance frequencies F1 and F2 can also be carried out in such a way that, in the case of a specific immersion depth of the oscillatable unit 3, they are matched to one another in a selectable reference medium 4. In this case, the type of adaptation of the two resonators 15, 16 counteracts the damping by the reference medium.

To determine the respective process variable, a change in the resonant frequency of the oscillatable unit 3 for the determination of the respective process variable is frequently used for media 4 in the form of liquids. As mentioned in connection with FIG. 4, the current one depends

Resonance frequency F1 of the oscillatory unit 3, ie the first resonator from the respective medium 4, and from the immersion depth of the oscillatory unit 3 in the medium 4 from. Similar considerations also apply to the vibration amplitudes R1 and R2 of the two

Resonators 15, 16. If the medium 4 is a bulk material, usually the amplitude damping of the oscillatable unit 3 is used to determine the at least one process variable. Upon contact of the oscillatory unit 3 with the bulk material is the

Vibration amplitude R1 of the oscillatable unit 3, so the first resonator 15 attenuated. As a result, the standing wave which propagates along the bars 10a, 10b is deprived of energy, resulting in a decrease in the amplitude of the received signal.

In summary, the device according to the invention allows the use of a field device with an electromechanical transducer unit in an extended temperature range, in particular for use at high temperatures. The maximum permissible process temperature is essentially determined only by the material properties of the oscillatable unit 3 and by the length and the material of the housing 8, or the temperature spacer tube. The length of the rods 10a, 10b and the housing 8 can be extended in multiples of half the wavelength of the standing wave and adapted to the respective present temperature requirements become. In this case, no special temperature requirements for the drive / receiving unit must be met.

LIST OF REFERENCE NUMBERS

1 Vibronic sensor

2 sensor unit

3 Oscillatory unit

4 medium

5 container

6 drive / receiver unit

7 electronics unit

8 housing

8a, 8b first, second portion of the housing

9 membrane

10a, 10b rods

1 1 drive / receiver unit

12 process connection

13a, 13b oscillating rods of the oscillatory unit

14 fixing element

15 first resonator

16 second resonator

17 crossing point

F1 frequency of the first resonator

F2 frequency of the second resonator

R1 oscillation amplitude of the first resonator

R2 oscillation amplitude of the second resonator

A base area of the membrane

L length of the rods

λ Wavelength of the waves propagating along the rods

Claims

claims

Device (1) for determining and / or monitoring at least one process variable of a medium (4) in a container (5) comprising at least

an oscillatable unit (3) with at least one membrane (9) which can be set into mechanical vibrations,

two rods (10a, 10b) attached to the membrane (9) perpendicular to a base of the membrane (9),

a housing (8), wherein the membrane (9) forms at least a portion of a wall of the housing (8), and wherein the two rods (10a, 10b) are directed into the housing interior,

at least one drive / receiving unit (1 1), which in the membrane (9)

remote end portion of the two rods (10a, 10b) is arranged, which drive / receiving unit (1 1) is designed to oscillate the unit (3) by means of an electrical pickup signal and by means of the two rods (10a, 1 0b) to mechanical vibrations to stimulate and to receive the mechanical vibrations of the oscillatable unit (3) and to convert it into a received electrical signal, and

an electronic unit (7), which is designed to generate an excitation signal from the received signal, and to determine the at least one process variable at least from the received signal.

Device according to claim 1,

wherein the drive / receiving unit (1 1) is adapted to set the two rods (10 a, 10 b) in mechanical vibrations, and wherein the two rods (10 a, 10 b) are attached to the membrane (9), that from the vibrations of the two rods vibrations (10a, 10b) of the membrane (9) result.

Device according to claim 2,

wherein the length (L) of the rods (10a, 10b) with respect to the wavelength (λ) of the waves propagating along the two rods (10a, 10b) is Ι_ = ηλ / 2 + K / 4, where n is a natural number is.

Device according to claim 1,

additionally comprising a fixing element (14), by means of which fixing element (14) the two rods (10a, 10b) in the membrane (9) facing away from the end portion are mechanically coupled together.

5. Apparatus according to claim 4,

wherein the excitation signal and / or the length (L) of the two rods (10a, 10b) are chosen such that vibrations of the rods (10a, 10b) result in the propagation of standing waves along the rods (10a, 10b).

6. Apparatus according to claim 4 or 5,

wherein the length (L) of the rods with respect to the wavelength (λ) of the waves propagating along the two rods (10a, 10b) is L = nA / 2, where n is a natural number.

7. Device according to at least one of the preceding claims,

wherein the rods (10a, 10b) and / or the housing (8) are made of a material which provides good thermal insulation.

8. Device according to at least one of the preceding claims,

wherein the process variable is given by a predetermined level or the flow of the medium (4) in the container (5), or by the density or the viscosity of the medium (4).

9. Device according to at least one of the preceding claims,

wherein on the membrane (9) of the oscillatable unit (3) at least one vibrating rod (13a, 13b) is fixed.

10. Device according to at least one of the preceding claims,

wherein the drive / receiving unit (11) comprises at least one piezoelectric element, or wherein the drive / receiving unit (1 1) is an electromagnetic drive with at least one coil and a magnet.

1 1. A device according to at least one of the preceding claims,

wherein the oscillatable unit (3) is arranged at a defined position within the container (5), such that it dives into the medium (4) to a determinable immersion depth.

12. Device according to at least one of the preceding claims,

wherein the oscillatable unit (3) is a tuning fork with two oscillating rods (13a, 13b), and wherein the two rods (10a, 10b) attached to the membrane (9) and the two oscillating rods (13a, 13b) attached to the membrane (9) 13b) mirror-symmetrical to each other relative to the plane perpendicular to the longitudinal axis by the rods (10a, 10b) and / or oscillating rods (13a, 13b) are arranged opposite one another.

13. Device according to at least one of the preceding claims,

wherein the two oscillating rods (13a, 13b) and the membrane (9) have a first mechanical

Form resonator (15),

wherein the two rods (10a, 10b) and the membrane (9) form a second mechanical resonator (16),

wherein the first (15) and the second resonator (16) are mechanically coupled together by means of the membrane (15), and

wherein the frequency of the excitation signal is selected such that the first (15) and second resonator (16) in an antisymmetric vibration mode with respect to the plane through the membrane (9) perpendicular to the longitudinal axis of the rods (10a, 10b) and / or oscillating rods ( 13a, 13b) swing.

14. Device according to at least one of the preceding claims,

wherein the length L and / or the rigidity of the two rods (10a, 10b) are selected such that the oscillation frequency (F1) of the first resonator (15) and the oscillation frequency (16) of the second resonator (F2) in the case that oscillatory unit (3) is not covered by medium (4), have substantially the same value.

15. Device according to at least one of the preceding claims,

wherein the length L and / or the rigidity of the two rods (10a, 10b) are selected such that the oscillation frequency (F1) of the first resonator (15) and the oscillation frequency (F2) of the second resonator (16) in the case that oscillatable unit (3) is covered by a selectable reference medium (4), have substantially the same value.