WO2012079949A1

WO2012079949A1 - Apparatus for determination of at least one process variable and method for pressure measurement using an apparatus of this type

Info

Publication number: WO2012079949A1
Application number: PCT/EP2011/070860
Authority: WO
Inventors: Sascha D'angelico; Martin Urban; Helmut Pfeiffer
Original assignee: Endress+Hauser Gmbh+Co. Kg
Priority date: 2010-12-15
Filing date: 2011-11-23
Publication date: 2012-06-21
Also published as: DE102010063146B4; DE102010063146A1

Abstract

The invention relates to an apparatus for determination of at least one process variable of a medium in a container, having a unit (2) which can oscillate mechanically, having a drive unit (41) which excites the unit (2) which can oscillate to carry out essentially resonant mechanical oscillations, having a receiving unit (42) which is mechanically and electrically connected to the drive unit (41), receives the mechanical oscillations from the unit (2) which can oscillate and converts them to an electrical received signal, and having a control/evaluation unit, which supplies an electrical transmission signal to the drive unit (41), receives the electrical received signal, and uses this to determine the process variable. The invention is distinguished in that the control/evaluation unit (9) is designed to use a coupling voltage, which results from the electrical and mechanical coupling of the drive unit (41) and of the receiving unit (42), to determine the process pressure which exists in an area around the unit (2) which can oscillate. A method is also claimed for pressure measurement using said apparatus.

Description

Device for determining at least one process variable and method for measuring pressure with such a device

The present invention relates to a device for determining and / or monitoring at least one process variable of a medium in a container, with an at least partially tubular housing, with a mechanically oscillatable unit which closes the housing in a first end region, with a drive unit which the oscillatable Unit to substantially resonant mechanical vibrations, with a receiving unit, which is mechanically and electrically coupled to the drive unit, and which receives the mechanical vibrations of the oscillatable unit and converts into a received electrical signal, and with a RegelAAuswerteeinheit, which the drive unit, an electrical transmission signal supplies and which receives the electrical received signal and determines the process variable. The process variable is preferably the fill level of a bulk material or the fill level and / or the density and / or the viscosity of a liquid. The oscillatable unit is in particular a membrane, a tuning fork or a single rod.

In industry, especially in process automation, vibronic measuring devices are frequently used to monitor the level of bulk goods or liquids in containers, but also to determine the density or viscosity of liquids. Such measuring devices usually have an oscillatable unit in the form of a membrane or a fork, which consists of two prongs arranged on a membrane. This oscillating unit is used by a drive unit to oscillate with the

Resonant frequency excited. From the vibration characteristics or their change, i. E. the oscillation frequency and / or the phase position between a transmitted and a received signal, and / or the amplitude, the process variable or a change in the process variable can be determined. Such measuring instruments are in great variety under the name of the applicant

"Liquiphant" for liquids and "Soliphant" for bulk materials produced and distributed. The drive of the oscillatory unit usually consists of one or more, arranged in a stack, piezoelectric elements. The same applies to the receiving unit, which receives the oscillations of the oscillatable unit and converts it into a usable electrical received signal. In principle, a distinction is made between two types of piezoelectric drive / receiving units, the bimorph piezo and the piezo stack. In a Bimorphpiezo is a disc-shaped piezoelectric element in each case at least one drive and

Subdivided receiving segment and attached to the oscillatable unit such that it excites them when applying an electrical AC voltage to the drive segment to vibrate. In contrast, a piezo stack consists of several arranged in a stack piezoelectric disks, wherein the drive unit associated with the discs and the Receiving unit associated discs are arranged in the same stack. The stack is connected to the oscillatable unit via a contact element for targeted power transmission,

for example in the form of a hemisphere, in contact and is for the preparation of the

Force transmission required contact pressure firmly clamped in the housing of the meter. A vibronic measuring device with a drive / receiving unit designed as a piezo stack is described, for example, in the European patent application with the disclosure number EP 1 134 038 A1.

The meter is specified up to a certain process pressure. If this pressure is exceeded during the use of the measuring instrument, individual components, in particular the oscillatable unit, may be damaged. For example, a tuning fork may bend. A membrane can even break when heavily overloaded. In order to avoid the occurrence of such damage, it is desirable to monitor the pressure to which the oscillatory unit is exposed.

The object of the invention is to provide a vibronic measuring device with which, in addition to at least one process variable, the process pressure at the location of the

Measuring device can be determined, and specify a method for determining the process pressure with such a meter.

The object is achieved by a measuring device in that the control / evaluation unit is adapted to determine from a coupling voltage, which results from the electrical and mechanical coupling of the drive unit and the receiving unit, the prevailing in an environment of the oscillatory unit process pressure ,

Through the supply lines for voltage supply and discharge, capacitive effects, and the mechanical contact of the parts which form the drive of the oscillatory unit with the parts which receive the mechanical vibrations and convert them into electrical signals, a coupling-related additional signal is produced in the electrical received signal. Due to the mechanical and electrical coupling of the drive unit and the receiving unit, one can also speak of a drive / receiving unit. The coupling-related additional signal is negligible in the case of resonance, but wins with increasing attenuation of the vibrations in weight and can at a generation of the transmission signal by feedback of the

Receive signal causing the system to stop oscillating at the resonant frequency. Even with direct excitation of the oscillatory unit outside the resonance range, the coupling voltage dominates.

This effect is now being used. In that the control / evaluation unit temporarily generates the transmission signal in such a way that the drive unit controlled by it controls the oscillatory Unit can stimulate non-resonant, reduced by mechanical vibrations of the oscillatory unit caused proportion in the received signal so far that the

Coupling voltage dominates. Ideally, the mechanically oscillatable unit no longer vibrates at all. But at least the amplitude of the vibrations is so low that their share of the received signal compared to the coupling-related component is negligible. Since the oscillatable unit and the drive / receiving unit are firmly connected to each other, the process pressure prevailing in the vicinity of the oscillatable unit also acts on the drive / receiving unit. Since the degree of coupling and thus the coupling voltage depends on the pressure acting on the drive / receiving unit pressure in a known manner, this can be determined by measuring the coupling voltage. As an alternative to the non-resonant excitation, the control / evaluation unit determines the coupling voltage

Time points at which the mechanical vibrations due to excessive damping by the medium are substantially prevented or the control / evaluation unit has means for separating the coupling-related signal component of the vibration-related signal component.

In a first embodiment of the solution according to the invention, the drive unit has at least one piezoelectric drive element, the receiving unit has at least one piezoelectric receiving element, and the piezoelectric drive element and the piezoelectric receiving element are arranged in a stack. In other words, it is a piezo stack or stack drive, wherein drive unit and receiving unit are arranged in a common stack. The stack is arranged along a longitudinal axis of the housing between the oscillatable unit and a fastening means in the housing. To produce permanent contact between the drive unit or the receiving unit and the mechanically oscillatable unit in order to ensure power transmission, the piezo stack is introduced into the housing under a certain prestress. This is achieved by the fastening means, which is for example a pressure screw or a yoke. In the case of a diaphragm transducer or a tuning fork is the

Membrane slightly bulged outwards due to the prestressing.

In one embodiment, the drive unit and the receiving unit are designed as Bimorphpiezo. This embodiment is an alternative to that drive and receiving unit are designed as a piezo stack. The coupling and the occurring in the received signal

Coupling voltage are usually lower in the bimorph than in a piezo stack. However, a pressure measurement using Bimorphpiezo is still possible.

In one embodiment, the process variable determined or monitored by the device is the fill level of a bulk material or the fill level and / or the density and / or the viscosity of a liquid. Preferably, the device determines at least the level of a medium. The pressure is an additional determinable process variable.

The object is further achieved by a method for determining and / or monitoring a process pressure with a device according to one of the embodiments described, wherein the drive unit is acted upon by a transmission signal, wherein on hand of the

Receiving signal, at least temporarily, the coupling voltage is measured and wherein from a stored pressure dependence of the coupling voltage of prevailing in the environment of the oscillatory unit process pressure is determined.

During the determination of the actual process variables, the drive unit excites the oscillatable unit to oscillate in the resonance region, i. preferably with the

Resonant frequency, on. This happens for example via a resonant circuit and the specification of a fixed phase shift between the transmission signal and the received signal. Alternatively, the drive unit is subjected to a frequency sweep and the resonant frequency is determined electronically on the basis of the received signal. The received signal here has a low on the mechanical and electrical coupling between drive and receiving unit

attributable portion, and a dominant, due to the mechanical vibrations of the oscillatory unit share. In order to be able to determine the pressure, the proportion of the received signal due to the mechanical vibrations of the oscillatable unit must therefore either be reduced to such an extent that it is negligibly small compared with the coupling-related component, or the two components must be separated from one another.

According to a first variant of the method, therefore, the oscillatable unit outside of a resonance range is excited to oscillate and determines the coupling voltage at least on the basis of the height of the received received signal thereupon. Depending on the design of the drive / receiving unit is a certain phase shift between

Coupling voltage and exciting voltage, so that in addition to the height of the received signal and the phase shift is taken into account.

In the first variant of the method, the measuring operation can be divided into two phases: the determination of at least one process variable based on the resonant oscillations and the determination of the process pressure present in the environment of the oscillatable unit. The phases for pressure determination hereby represent only a brief interruption of the phases of the determination of the actual process variables. Actual process variables are those process variables which can be determined or monitored by a conventional measuring device of the type mentioned in the prior art. These are essentially in the case of liquids, the level, in particular limit level, density and / or viscosity and in the case of bulk solids, the level of the bulk material. During the phases for pressure determination, the transmission signal is changed in such a way that the mechanically oscillatable unit no longer carries out any significant oscillations.

For example, the excitation frequency is reduced or increased by a multiple of the resonance frequency. In the event that the excitation frequency is greater than the resonance frequency, the excitation frequency is chosen so that even higher modes are not excited. The transmission signal for non-resonant excitation can be dynamically controlled or fixed. Depending on the viscosity of the resonance range may be relatively wide, so that a sufficiently large distance of the excitation frequency is required. Often form

Drive unit, oscillatable unit, receiving unit and at least part of the control unit a resonant circuit. The resonant excitation is by specifying a specific

Phase shift between the transmission signal and received signal achieved. If a mechanical oscillation of the oscillatable unit is not possible, the oscillating circuit oscillates at a demolition frequency determined by the electronic unit. In one possible embodiment of the first variant of the method, the control / evaluation unit causes the

Abrissfrequenz adjusts and then measured the now the coupling voltage corresponding received signal.

In a second variant of the method, the oscillatable unit is too resonant

Vibrations excited and the coupling voltage of the conditional by the mechanical vibrations of the oscillatory unit fraction in the received signal separated. This can be done for example via a Fourier analysis.

Another option for pressure measurement is the use in highly viscous media. In German Patent Application No. 102010039585.4, which has not yet been disclosed, a method for the electronic compensation of the coupling voltage is described, which enables the measurement of the desired process variables in highly viscous media. Without compensation, the coupling voltage dominates and the resonant circuit oscillates at break-off frequency. Due to a delayed onset of compensation or a short-term switching off of the compensation thus the pressure measurement is possible despite resonant excitation. If the vibrations are not completely damped, so that even a small amount of vibration in the received signal is present, depending on the desired accuracy of the pressure determination still filtering the

Coupling voltage may be required. The following embodiments relate to all possible variants of the method for pressure determination.

According to one embodiment of the method, the process pressure is determined at predeterminable times, preferably periodically. The meter primarily determines the at least one Process variable to be determined. For this purpose, the oscillatable unit is excited to vibrate in its resonance range, preferably at the resonant frequency. In one variant, the measurement is interrupted at selected times and the excitation takes place outside the resonance range, so that the corresponding received signal only contains the coupling-related signal and the pressure can be determined. Switching between the various suggestions is preferably carried out automatically at certain intervals. In another variant, the resonant excitation is not interrupted, but the received signal additionally evaluated with respect to the coupling voltage by these is filtered out. In one embodiment of the method, the specific process pressure is logged at least temporarily. This allows, for example, in case of error, the check whether the

Pressure specifications for the meter have been adhered to. In addition, a temporal course of the process pressure can be read, which can be used for diagnostic purposes in the event of errors or in the event of a failure of the measuring device.

An embodiment of the method includes setting a threshold for the process pressure, and that the determined process pressure is logged, at least if it is above the threshold, and / or that an alarm is generated when the determined process pressure exceeds the threshold. The threshold is set according to the device specification, i. it corresponds to an upper limit for a pressure at which

it is ensured that the components of the measuring device are not damaged. If this threshold is exceeded, damage is possible. In one variant is therefore to

Error diagnosis exceeding the threshold, preferably the time and / or duration of the crossing, and the amount by which the threshold was exceeded, are stored. In a development, in addition to or as an alternative to logging, an alarm signal is generated as soon as the threshold is exceeded. The operator of the measuring device is hereby informed about leaving the permissible pressure range and can, for example, carry out a timely check of the measuring device. In one embodiment, with regard to predictive maintenance, the interval of - for example, in the context of SIL - mandatory reviews shortened accordingly.

According to one embodiment of the method is for determining the process pressure a

Temperature compensation performed. The coupling voltage is usually not only dependent on the pressure, but also on the temperature. The temperature in the surroundings of the drive / receiving unit is therefore preferably determined. On the basis of the measured temperature, an assignment of stored value pairs for coupling voltage and pressure or adaptation of a stored formula for determining the pressure to be assigned to a measured coupling voltage is then carried out, for example. The invention will be explained in more detail with reference to the following figures.

Fig. 1 shows the structure of a vibronic measuring device with a Stapelpiezo; 2 shows a diagram of the relationship between pressure and coupling voltage.

In Fig. 1, a measuring device 1 for determining the level, the density, and / or the viscosity of a liquid is shown. The oscillatable unit 2 in the form of a vibration fork applied to a diaphragm 21 closes the housing 3 at the end. In the housing 3, the drive / receiving unit 41, 42 is introduced in the form of an enclosed by the drive housing 7 insert. The drive / receiving unit 41, 42 is

disc-shaped piezoelectric elements which are arranged in a stack, wherein at least one piezoelectric element of the drive unit 41 and at least one

piezoelectric element of the receiving unit 42 is assigned. Drive unit 41 and receiving unit 42 preferably consist of a plurality of piezoelectric disks for better application of force or signal pick-up, the drive units of the

Empfangspiezos are preferably separated by a cutting disc. For example, the bottom of the piezoelectric element 21 as seen from the membrane 21 are the

Receiving unit 42 and the rest of the drive unit 41 assigned.

To produce an electrical contact, the piezoelectric disks are metallized or metallic plates are arranged between them. These are with electric

Contact lines 8 are connected, which are preferably arranged on a flexible printed circuit board, and via which the drive unit 41 is supplied to a transmission signal and the received signal is derived from the receiving unit 42. Via a contact element 52, the drive / receiving unit 41, 42 is in contact with the diaphragm 21 of the oscillatable unit 2. To produce a contact pressure, a fastening means 6, for example in the form of a screw, mounted in the housing 3, that it over the Contact element 51 exerts pressure on the drive / receiving unit 41, 42. Preferably, the membrane 21 is under the fastening means 6 under a certain bias, so that at any time, for example, even with temperature fluctuations, there is a contact between the drive / receiving unit 41, 42 or contact element 52 and oscillatable unit 2. The contact elements 51, 52 are preferably designed so that they allow optimum power transmission in the axial direction. For example, this is achieved by end-hemispherical contact elements 51, 52. The axial direction corresponds to a longitudinal axis A of the housing 3, which is tubular at least in the region of the drive / receiving unit 41, 42.

The suggestively outlined control / evaluation unit 9 is located in a housing section, not shown here above the fastening means. For example, the Control / evaluation unit 9 electrical components, which form a resonant circuit with the drive / receiving unit 41, 42 and the oscillatable unit 2, and a microcontroller. This sets, for example, the electrical components such that a certain

Phase shift between the received signal and the transmission signal is generated. To excite resonant vibrations in air, these are for example 90 °. The determination of fill level, density and viscosity with a measuring device 1 of the type described is well known to the person skilled in the art and will not be described further here.

For pressure measurement according to the invention, for example, the microcontroller temporarily alters the phase shift and assigns the value of the subsequently recorded one

Receive signal on the basis of a stored for the determination of pressure in a memory

Context or a table to a pressure. If necessary, compares the

Microcontroller the pressure with a legal threshold and / or stores the value. The membrane 21 is clamped with an edge region end in the housing 3. The

Drive / receiving unit 41, 42 presses from the inside against the center of the membrane 21. A pressurization from the outside therefore causes a concave deformation of the area lying between the center and edge region of the membrane 21. The center and the edge region of the membrane 21 are hardly deflected.

The measuring device 1 shown here is a so-called tuning fork for liquids. However, the invention is equally applicable in vibrating forks for bulk materials or so-called membrane vibrators with a membrane without vibrating rods as a vibratory unit. Furthermore, the drive / receiving unit 41, 42 also applied to a connected to the diaphragm 21 bolts Ringpiezos as essential elements of

Drive / receiving unit 41, 42 have. In yet another variant, the drive unit 41, 42 is designed as a bimorph piezo, i. Drive unit 41 and

Receiving unit 42 are configured as segments of a single piezoelectric element. In addition, the pressure determination by means of coupling voltage can also be used in so-called rods for level measurement in bulk solids. Such measuring instruments are manufactured and marketed by the applicant under the name "Soliphant T." A single rod has a closed tube as a vibratory unit, wherein a compensation unit for avoiding the transmission of vibrations to the clamping is usually mounted in the tube Drive / receiving unit of a single rod consists

for example, two piezoelectric disks with two segments

opposite polarization, as described in EP 0 1573 282 A1. FIG. 2 shows a measurement curve which shows the value U ^* cos (cp) of the phase-weighted

Coupling voltage U depending on the pressure acting on the drive / receiving unit 41, 42 pressure p shows. The pressure acting on the driving / receiving unit 41, 42 corresponds to the pressure which prevails outside the measuring device 1 in the vicinity of the oscillatable unit 2, in addition to a constant pressure caused by a bias voltage. For

Bimorph piezos are ambient pressure and pressure acting on the bimorph piezo due to the absence of bias substantially identical. Such a measurement curve is obtained, for example, in non-resonant excitation of the oscillatable unit 2 by receiving the received signal and simultaneously increasing the process pressure.

The coupling voltage is dependent on the pressure which prevails in the vicinity of the oscillatable unit 2 and thus acts on the drive / receiving unit 41, 42 via the diaphragm 21. As the pressure increases, the coupling voltage increases, with the slope decreasing for greater pressures. In the case of a piezo stack according to FIG. 1, the pressure acts via the contact element on the drive / receiving unit 41, 42 in the form of an axial force. An external pressure on the membrane 21 has on the built-in piezo stack thus the same effect as the

Preload. An external force due to the process pressure is thus measurable as well as the bias voltage via the coupling voltage. The coupling voltage has a mechanical and an electrical component, which are structurally in phase or in antiphase. In the illustrated example, the electrical component has a phase shift of 180 ° and the mechanical

Component a phase shift of 0 °. If no pressure is exerted on the oscillatable unit 2 or on the drive / receiving unit 41, 42, the electrical component predominates, so that the phase-weighted voltage signal is negative in this case. Does that rise?

mechanical component by increasing force, the phase-weighted coupling signal assumes correspondingly positive values as soon as the mechanical component predominates. In other embodiments, in which both components are positive, the phase-weighted coupling signal assumes only positive values from the beginning, so that the evaluation of the amplitude of the coupling signal for pressure determination is sufficient. Both embodiments have in common that with increasing force from the outside by increasing pressure the

Coupling voltage continues to increase. This relationship between coupling voltage and pressure is stored as a table, comparison curve, or in the form of a simulated formula in the control / evaluation unit 9. To compensate for the offset caused by the bias offset in the coupling voltage is preferably a production-side adjustment

performed, so that the recorded measured values can be clearly assigned to a corresponding pressure. Since the coupling voltage is superimposed on the voltage signal determined by the process variable during the measuring operation, the coupling voltage is preferably not measured simultaneously for determining the actual process variables such as fill level, density, and / or viscosity, but in a separate measuring phase. However, the actual measurement must be interrupted only very briefly, so that monitoring of the actual process variables by the pressure measurement is not disturbed. To measure the coupling voltage, the oscillatable unit 2 is outside the resonance range, preferably with the

Antiresonant frequency or the frequency of breakup, stimulated. In this case, the oscillatable unit 2 performs no oscillations and the received signal consists exclusively of the coupling signal.

The pressure measurement is less the exact pressure determination for pressure monitoring in the process, as the monitoring of the meter 1. Measuring devices 1 of the type mentioned are approved for a specific pressure range. If this area is left when using the measuring device 1, damage is for example the oscillatable unit 2 or

Piezo drive possible. The pressure measurement can be used to monitor the application area and to generate an alarm signal when leaving the approved area. Furthermore, it can be traced in the course of a repair whether the measuring device 1 was used outside the specification.

In addition to the pressure, the coupling voltage is also dependent on the temperature. Therefore, the temperature at the location of the measuring device 1, in particular at the location of the drive / receiving unit 41, 42 is advantageously determined. For example, a thermocouple or a

Resistor sensor adjacent to the drive / receive unit 41, 42 arranged or the temperature information is, as in the not yet disclosed German

Patent application with the file reference 102010030791.2 described on hand of a

Useful signal, which when excited with an out of range

Excitation signal was obtained or decoupled in continuous operation determined. Particularly advantageous is the integration of a thermocouple in the piezo stack or the formation of a thermocouple by means of the membrane 21 and the contact element 52 which contacts the membrane 21. The temperature compensation is preferably carried out by a set of voltage-pressure dependencies for different temperatures in the control / evaluation unit 9 are stored, the temperature is measured, and this

Temperature corresponding stored dependency to the pressure determination is used. Alternatively, the temperature enters into a formula for calculating the pressure as a parameter, which is determined by the temperature measurement. LIST OF REFERENCE NUMBERS

1 measuring device

2 Oscillatory unit

21 membrane

3 housing

41 drive unit

42 receiving unit

51 contact element

52 contact element

6 fasteners

7 drive housing

8 electrical contact lines

9 control / evaluation unit

A longitudinal axis

Claims

claims

Device for determining and / or monitoring at least one process variable of a medium in a container, having an at least partially tubular housing (3), with a mechanically oscillatable unit (2) which closes the housing (3) in a first end region, with a drive unit (41), which excites the oscillatable unit (2) to substantially resonant mechanical oscillations, with a receiving unit (42), which is mechanically and electrically coupled to the drive unit (41) and which controls the mechanical oscillations of the oscillatable unit (2). receives and converts into an electrical received signal, and with a control / evaluation unit (9) which supplies an electrical transmission signal to the drive unit (41) and which receives the electrical received signal and determines therefrom the process variable,

characterized,

in that the control / evaluation unit (9) is set up from a coupling voltage, which results from the electrical and mechanical coupling of the drive unit (41) and the receiving unit (42), in an environment of the oscillatable unit (2) To determine process pressure.

Device according to claim 1,

characterized,

the drive unit (41) has at least one piezoelectric drive element, in that the receiving unit (42) has at least one piezoelectric receiving element,

and that the piezoelectric drive element and the piezoelectric

Receiving element are arranged in a stack.

Device according to claim 1,

characterized,

in that the drive unit (41) and the receiving unit (42) are designed as bimorph piezo.

Device according to one of the preceding claims,

characterized,

that the process variable is the fill level of a bulk material or the fill level and / or the density and / or the viscosity of a liquid.

5. Method for determining and / or monitoring a process pressure with a

Device according to one of claims 1-4, wherein the drive unit (41) with a Transmitted signal is applied, wherein at least temporarily the coupling voltage is measured on the basis of the received signal and wherein from a stored pressure dependence of the coupling voltage of the prevailing in the environment of the oscillatable unit (2) process pressure is determined.

Method according to claim 5,

characterized,

that the oscillatable unit (2) is excited to oscillate outside of a resonance range, and that the coupling voltage is determined at least by reference to the magnitude of the subsequently received received signal,

or

that the oscillatable unit (2) is excited to resonant oscillations and the coupling voltage of the by the mechanical vibrations of the

oscillatory unit (2) conditional share is separated in the received signal.

Method according to one of claims 5 or 6,

characterized,

the process pressure is determined at predefinable times, preferably periodically.

Method according to one of claims 5-7,

characterized,

that the specific process pressure is at least temporarily logged.

Method according to one of claims 5-8,

characterized,

that a threshold for the process pressure is set,

and that the particular process pressure is logged, at least if it is above the threshold,

and / or that an alarm is generated when the particular process pressure exceeds the threshold.

10. The method according to any one of claims 5-9,

characterized,

that a temperature compensation is performed to determine the process pressure.