WO2018108401A1

WO2018108401A1 - Vibronic sensor with temperature compensation

Info

Publication number: WO2018108401A1
Application number: PCT/EP2017/078847
Authority: WO
Inventors: Sergej Lopatin; Jörn LANGE; Thomas Vogt
Original assignee: Endress+Hauser SE+Co. KG
Priority date: 2016-12-14
Filing date: 2017-11-10
Publication date: 2018-06-21
Also published as: DE102016124365A1

Abstract

The invention relates to a method for determining and/or monitoring the density (p) and/or the viscosity (v) of a medium (3) in a container (2) by means of a vibronic sensor (1), and a corresponding sensor (1). A vibrateable unit (4) is excited to mechanically vibrate by means of an electrical excitation signal (U_A), and the mechanical vibrations of the mechanically vibrateable unit (4) are received and converted into an electrical receive signal (U_E). In addition, the excitation signal (U_A) is generated from the receiver signal (U_E) in such a way that at least one predeterminable phase shift (Δφ) occurs between the excitation signal (U_A) and the receiver signal (U_E), wherein a frequency (f) of the excitation signal (U_A) is determined from the receiver signal (U_E) with the occurrence of the predeterminable phase shift (Δφ). Furthermore, a damping (D) and/or a damping (D)-dependent variable is/are determined from the receiver signal (U_E) with the occurrence of the predeterminable phase shift (Δφ), and the density (p) and/or the viscosity (v) of the medium is/are determined at least from the damping (D) and/or the damping (D)-dependent variable, and from the frequency (f) of the excitation signal (U_A).

Description

Vibronic sensor with temperature compensation

The invention relates to a device for determining and / or monitoring at least one process variable of a medium in a container and to a method for producing a corresponding device. The device comprises a mechanically oscillatable unit. It is therefore a so-called vibronic sensor.

Vibronic sensors are widely used in process and / or process applications

Automation technology. In the case of level measuring devices, they have at least one mechanically oscillatable unit, such as a tuning fork, a monobloc or a membrane. This is excited in operation by means of a drive / receiving unit, often in the form of an electromechanical transducer unit to mechanical vibrations, which in turn may be, for example, a piezoelectric actuator or an electromagnetic drive. In the case of flowmeters, however, the mechanically oscillatable unit can also be designed as a vibratable tube which is flowed through by the respective medium, for example in a measuring device operating according to the Coriolis principle.

Corresponding field devices are manufactured by the applicant in great variety and distributed in the case of level measuring devices, for example under the name LIQUIPHANT or SOLIPHANT. The underlying measurement principles are in principle made of a variety of

Publications known. The drive / receiver actuation stimulates the mechanically oscillatable unit to generate mechanical vibrations by means of an electrical signal. Conversely, the drive / receiving unit, the mechanical vibrations of the mechanical

receive oscillatable unit and convert it into a received electrical signal. The drive / receiving unit is either a separate drive unit and a separate receiver unit, or a combined drive / receiver unit.

In this case, the drive / receiving unit is in many cases 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 is> 1 and all phases occurring in the resonant circuit are multiples of 360 ° must be satisfied.

To excite and satisfy the resonant circuit condition, a certain phase shift between the excitation signal and the received signal must be ensured. For this reason, a predefinable value for the phase shift, that is to say a setpoint value for the phase shift between the excitation signal and the received signal, is frequently set. For this purpose, the most different solutions, both analog and digital methods known from the prior art become. 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 all-pass filter. The adjustment of the phase shift is also possible by means of a so-called frequency search, such as in DE102009026685A1, DE102009028022A1, and

DE102010030982A1 discloses. The phase shift can also be done by means of a

Phase-locked loop (PLL) can be controlled to a predetermined value. An excitation method based thereon is the subject of DE00102010030982A1.

Both the excitation signal and the received signal are characterized by their frequency ω, amplitude A and / or phase Φ. Accordingly, changes in these quantities are usually used to determine the respective process variable, such as a predetermined level of a medium in a container, or the density and / or viscosity of a medium or the flow of a medium through a pipe. In the case of a vibronic

Level limit switch for liquids, for example, a distinction whether the oscillatory unit is covered by the liquid or vibrates freely. These two states, the free state and the covered state, are for example based on different

Resonance frequencies, ie a frequency shift, distinguished. 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.

As described, for example, in DE10050299A1, the viscosity of a medium can be determined by means of a vibronic sensor on the basis of the frequency-phase curve (0 = g ()). This procedure is based on the dependence of the damping of the oscillatable unit on the viscosity of the respective medium. The rule is that the lower the viscosity, the steeper the fall

Frequency-phase curve. In order to eliminate the influence of density on the measurement, the viscosity is based on one of two different values for the phase

Frequency change determined, ie by means of a relative measurement. For this purpose, either two different phase values can be set and the associated frequency change can be determined, or a predetermined frequency band can be traversed and detected if at least two predetermined phase values are reached. From DE102007043811A1 is also known to close from a change in the natural frequency and / or resonance frequency and / or the phase position to a change in viscosity and / or due to appropriately deposited dependencies

Vibrations of the oscillatory unit of the viscosity of the respective medium to determine the viscosity. In this procedure too, the dependence of the determination of the viscosity on the density of the medium must be taken into account.

For the determination and / or monitoring of the density of a medium, DE10057974A1 has disclosed a method and a device by means of which the influence of at least one disturbing variable, for example the viscosity, on the oscillation frequency of the mechanically oscillatable unit can be determined and compensated accordingly , In DE102006033819A1 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 on the mechanical vibrations of the mechanically oscillatory

sentlichen after the formula

Determined, where S is the density sensitivity of the mechanically oscillatable unit, Fo.vakdie frequency of mechanical vibrations in vacuum at 0 ° C, C and A the linear, or

square temperature coefficient of oscillation frequency Fo.vakder mechanically oscillatory unit, t the process temperature, F-r, p, med the oscillation frequency of the mechanically oscillatable unit in the medium, D the pressure coefficient, and p the pressure of the medium.

In order to be independent of empirical assumptions, DE102015102834A1 has disclosed an analytical measuring principle for determining the density and / or viscosity by means of a vibronic sensor which takes into account interactions between the oscillatable unit and the medium on the basis of a mathematical model. The sensor is operated at two or more different predetermined phase shifts and from the respective

Response signal determines the process variables density and / or viscosity. Based on the prior art, the present invention has the object to expand the scope of a vibronic sensor.

This object is achieved by the method according to the invention as claimed in claim 1 and by the apparatus for carrying out the method according to claim 13. The method is a method for determining and / or monitoring a fill level of a medium by means of a device comprising a sensor unit with a mechanically oscillatable unit,

wherein the oscillatable unit is excited by means of an electrical excitation signal to mechanical vibrations,

wherein the mechanical oscillations of the mechanically oscillatable unit are received and converted into an electrical received signal,

wherein a frequency of the received signal is determined from the received electrical signal, and

being based on the frequency of the received signal

the level is determined.

In contrast to the prior art, the method according to the invention is not a possibility for monitoring the level, but rather a method for the continuous determination of a level of a medium in a container. The oscillatable unit is preferably a single rod or a tuning fork. The fill level can then be determined continuously at least along a predefinable subregion of a longitudinal axis of the oscillatable unit, since the frequency of the mechanically oscillatable unit depends on an immersion depth of the mechanically oscillatable unit into the medium, that is to say on the fill level. In the case of a tuning fork, for example, two oscillating rods are often formed on a membrane, to which in turn in each case a paddle is integrally formed. For the paddle is often chosen a geometric shape of a thin cuboid. For example, in the case of the sensor manufactured and marketed by the applicant under the name LIQUIPHANT, the geometric dimensions of the paddles are for the length L = 40 mm, for the width B = 17 mm and for the thickness D = 1.4 mm.

However, other geometries for the paddle are conceivable, which in terms of

Measuring behavior of the respective sensor have different advantages, as described for example in the documents WO02 / 079733A1 and DE10204115693A1. It should be noted that the method according to the invention can be used for any geometries of the oscillatable unit, in the case of a tuning fork, in particular for any paddle geometries. In particular, advantageously, the length of the paddle, or the length of the oscillatory unit to a specific application, such as the height of a container or the

Diameter of a tube in which the sensor is used to be adjusted.

The inventive method is advantageously suitable for use in containers with low height expansion, especially containers with heights less than 300mm. Such containers are used, for example, for so-called micro-plants, in which proposed processes can be tested before the construction of a new process plant. Another application is pumps, especially vacuum pumps, with an oil reservoir, the level of which must be continuously monitored during operation of the pump. Many other common measuring principles for detecting a continuous level, such as on the capacitive or conductive measuring method, or microwave level gauges are less suitable for such containers, because from case to case with decreasing container size of the expected measurement error increases, or in principle a Location calibration of the respective sensor to the respective medium is necessary, which in turn is relatively expensive. In the case of microwave and ultrasonic level gauges, for example, there is a so-called

Block distance or a so-called near field area to be considered. In contrast, in the case of a capacitive fill level measuring device, in the case of small measuring ranges Ah along the longitudinal axis L of the container, in particular in the case of small differences in the case of

Dielectric constants come to difficulties.

By means of a tuning fork according to the LIQUIPHANTEN or SOLIPHANTEN described above can be depending on the design and dimensioning of the respective

oscillatory unit over a range of up to about 150mm continuously determine the level. In the case that a larger measuring range is desired, an oscillatable unit of appropriate length can be selected. For a tuning fork, for example, the paddle length can be adapted to the desired application. Thus, according to the invention, typically a certain area is selected along the height of the container (typical containers in the industrial environment usually have heights of up to 3000 mm), for which a continuous level determination is of importance. A continuous one

Level monitoring then takes place only in the measuring range, which corresponds to a partial area along the longitudinal axis of the container instead.

An embodiment of the method according to the invention includes that a predetermined level is determined, wherein the achievement of the predetermined level is signaled and / or output. Thus, not only is a continuous determination of the fill level carried out, but additionally the reaching of a specific limit level in the container is indicated. The limit level is in particular a maximum value or a minimum value for the fill level. It can be provided on the basis of this embodiment, for example, an additional functionality in terms of overflow or no-load protection.

In a further refinement, the fill level is determined on the basis of a mathematical rule and / or on the basis of a reference curve. It is advantageous if the mathematical rule and / or the reference curve indicates a dependence between an immersion depth of the oscillatable unit in the medium or a level of the medium and the frequency of the received signal, or the oscillation frequency of the oscillatable unit.

Furthermore, it is advantageous if the mathematical rule is a polynomial function.

A further preferred embodiment of the method includes determining the density, the temperature, and / or the viscosity of the medium, and / or a variable which is dependent on at least the density, temperature, and / or viscosity of the medium. Advantageously, the density, the temperature and / or the viscosity of the medium, and / or a size which is dependent on at least the density, temperature and / or viscosity of the medium, are then also for the medium

Determination of the level taken into account.

The frequency of the received signal or the oscillation frequency of the mechanically oscillatable unit is basically also dependent on the temperature, in particular the temperature of the medium, the density and the viscosity of the medium. A compensation of the influence of at least one of these variables thus considerably increases the measurement accuracy in determining the fill level according to the method according to the invention. For the determination of the temperature, density, and / or viscosity, various possibilities are conceivable, all of which fall under the present invention: On the one hand, the temperature, density and / or viscosity can be determined using at least one other suitable measuring device at the location of the measuring point. However, it is also possible that the temperature, density and / or viscosity of the medium in a certain

Application are known from the outset. In this case, these quantities can be specified directly and also taken directly into account when determining the fill level.

In a particularly preferred embodiment of the method, the density, the temperature and / or the viscosity, the medium, and / or a quantity dependent on at least the density, temperature and / or viscosity of the medium, at least one change of the mathematical rule and / or or the reference curve.

In particular, the reference curve is a curve which indicates the change in the frequency of the received signal, or the change in the oscillation frequency of the mechanically oscillatable unit, as a function of the depth of immersion of the mechanically oscillatable unit in the medium. Such a reference curve is usually created under predefinable standard conditions. For example, water is often used as the medium at room temperature (T = 20 ° C) used. For other media with different density and / or viscosity, or for applications at other temperatures, the curve can be adjusted appropriately. This can be done graphically on the one hand. On the other hand, one may follow the course of the reference curve

corresponding mathematical rule, z. B. in the form of an assignment or functional rule, in particular in the form of a polynomial function, determined for the reference curve and adapted accordingly to the medium used in each case.

It is advantageous if at least one correction factor is determined. When determining a correction factor is a particularly simple way of adjusting the

Reference curve to the respective process conditions.

An embodiment of the invention includes determining a first and a second limit for the level of the medium, which first and second thresholds correspond to a minimum and a maximum immersion depth of the oscillatable unit, and wherein the level of the medium between the first and the second Limit is determined continuously. Thus, certain areas along the longitudinal axis of the oscillatable unit can be selected along which, for example, there is a particularly high sensitivity of the frequency change of the received signal as a function of the immersion depth. Alternatively, it may be that a level determination by the application only in a specific section along the longitudinal axis of the oscillatory unit of importance.

A particularly preferred embodiment includes that a condition monitoring is performed. Here are several applications and possibilities conceivable, all of which fall under the present invention. In the case that in addition to a continuous level determination a predetermined

Level is monitored, for example, an overflow or open circuit protection can be realized. Based on the continuous level determination can advantageously be further specified the time course of the level in the container. In this way, for example, be pointed to the loss of medium. This is especially interesting for oil based pumps, especially vacuum pumps. The embodiment thus advantageously permits predictive maintenance.

A particularly preferred embodiment of the method includes that the achievement of the predetermined level of the medium is monitored in a container, wherein a signal is generated depending on the current level, by means of which a rate for a filling and / or emptying process of the container regulated and / or controlled. For example, if a container to be filled up to a certain predetermined level, so can an interval can be defined around the predefinable fill level, within which interval a continuous fill level determination is carried out. On the basis of to a certain

When the fill level is currently present, a control or regulating signal can then be generated, for example, by means of which the filling rate of the medium into the container is regulated or controlled. Based on the information of each currently present level so inflow or outflow rate of the medium can be varied in the container in time. In this way it can be ensured that a predetermined value for the level can be reached very accurately, or it can be prevented that the predetermined level is exceeded slightly. Similar considerations apply to an emptying process of the container to a lower predetermined limit.

The object according to the invention is also achieved by a device for determining and / or monitoring a fill level of a medium comprising a sensor unit with a mechanically oscillatable unit, and an electronics unit, which electronic unit is designed to carry out a method according to the invention.

It should be noted that the embodiments mentioned in connection with the method according to the invention are mutatis mutandis also applicable to the device according to the invention

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 is a schematic drawing of a tuning fork, and

3 shows a reference curve for the dependence of the frequency of the oscillations of a tuning fork as a function of the immersion depth of the tuning fork in water at room temperature, and

4 illustrates the dependence of the immersion depth on the density of the medium.

1 shows a vibronic sensor 1 with a sensor unit 3 comprising an oscillatable unit 4 in the form of a tuning fork, which partially dips into a medium 2 which is located in a container 2a. The oscillatable unit is controlled by means of the

/ Reception unit 5 excited to mechanical vibrations, and may be, for example, a piezoelectric stack or bimorph drive. It goes without saying, however, that too Other embodiments of a vibronic sensor fall under the invention. Furthermore, an electronic unit 6 is shown, by means of which the signal detection, evaluation and / or supply is carried out. 2 shows an oscillatable unit 4 in the form of a tuning fork, as it is integrated, for example, in the vibronic sensor 1 marketed by the applicant under the name LIQUIPHANT, in a side view. The tuning fork 4 comprises two oscillating rods 8a, 8b integrally formed on a membrane 7, to which end two paddles 9a, 9b are integrally formed [in the side view in FIG. 2, only one oscillation rod 8a is visible with a paddle 9a]. The oscillating rods 8a, 8b together with the paddles 9a, 9b are often referred to as forks. In order to set the mechanically oscillatable unit 4 into mechanical oscillations, a force is impressed onto the membrane 8 by means of a drive / receiving unit 5, which is attached on the side facing away from the oscillating rods 7a, 7b. The drive / reception unit 5 is an electromechanical conversion unit, and includes, for example, a piezoelectric element or an electromagnetic drive [not shown]. Either the drive unit 5 and the receiving unit are constructed as two separate units, or as a combined drive / receiving unit. In the case that the drive / receiving unit 5 comprises a piezoelectric element 9, the force impressed on the diaphragm 7 is generated by the application of a starting signal UA, for example in the form of an electrical alternating voltage. A change in the applied electrical voltage causes a change in the geometric shape of the drive / receiving unit 5, ie a contraction or relaxation within the piezoelectric element such that the application of an electrical AC voltage as a start signal UA TO a vibration of the material fit with the drive / Receiving Unit 5 connected diaphragm 7 causes.

The oscillation frequency of the oscillatable unit 4 is dependent on the immersion depth h in the respective medium h. This dependence is used to determine the respective level. In principle, for example, in the case of an oscillatable unit 4 in the form of a tuning fork, a filling level determination over the entire length L of the paddles 9a, 9b can be made. However, it is useful in many cases, for the level determination a specific

Partial area along the longitudinal axis AL of the paddles 9a, 9b to define for which subsection the sensitivity is particularly high. The criterion for the selection of such a subarea AL can best be explained with reference to the reference curve Rst _d (h) in FIG. 3. In Fig. 3, the change Af of the oscillation frequency f of the oscillatory unit 4 is shown with a paddle length L of 84mm depending on the immersion depth of the oscillatable unit in water at a temperature of 22 ° C. The density of water is PH2O ^« 1, 0 g / cm ³ . As can easily be seen from the curve of this reference curve Rstd (h), the oscillatable unit experiences the greatest frequency change per millimeter immersion depth Af / h in the range between h = 0 mm and h = 60 mm, ie for a penetration depth h of up to h = 60 mm. In this interval, a level determination with an accuracy of about +/- 0.5mm is possible. For immersion depths h> 60mm, the percentage change in the oscillation frequency f of the oscillatable unit 4 is so small that the measurement accuracy in the level determination significantly decreases.

The reference curve Rst _d (h) for the frequency change Af as a function of the immersion depth h can basically be determined by a mathematical rule in the form of a polynomial function,

For example, describe a polynomial function of the 5th degree:

In this case, the frequency change Af is the oscillation frequency f relative to the

fo

Resonant frequency fo of the oscillatable unit 4 in vacuum, and A, B, C, D, E and F.

Parameters that can be determined numerically, for example.

To take account of the influence of the density p for a particular application in a medium 2 having the density PM, for example, a correction factor K can be calculated such that the reference curve RM (II) in the respective medium applies

in which

and where S is the density sensitivity of the mechanically oscillatable unit 4, PM is the density of the medium and po is the density of a reference medium.

To further explain the influence of the density PM on the respective course of the reference curve Rivi (h) for the frequency change Δf as a function of the immersion depth h of the oscillatable unit 4 in a medium 2, three reference curves R _m (h) for three different ones are shown in FIG media

different density PM shown.

The influence of the temperature T and / or viscosity η can be compensated analogously to take account of the influence of the density. On the appropriate regulations is therefore at this Do not resubmit in detail. It should be pointed out that, depending on the medium 2 used, instead of the viscosity η, it may be necessary to take into account the viscoelasticity which is relevant for materials with partially elastic and partially viscous behavior,

for example, in the case of non-Newtonian fluids. The consideration of the viscoelasticity instead of the viscosity also falls under the present invention.

For carrying out the method according to the invention, it is conceivable, for example, for a reference curve R (h) to be predefinable in the course of the production of the respective sensor

Standard conditions, such as water at room temperature, created and in one

Electronic unit 6 of the respective sensor 1 is stored. Alternatively, the reference curve Rstd (h) can also be stored in the form of a mathematical rule. For a specific application, or for a specific use in a particular medium, the reference curve Rst _d (h) can be suitably adapted to the respective medium R _m (h), for example by determining a suitable correction factor K.

LIST OF REFERENCE NUMBERS

1 Vibronic sensor

2 medium

2a container

3 sensor unit

4 Oscillatory unit

5 Electromechanical transducer unit

6 electronics unit

7 membrane

8a, 8b oscillating rods

9a, 9b paddle

UA start signal

UE received signal

ΔΦ specifiable phase shift

PM density of the medium

HM viscosity of the medium

T temperature of the medium

h immersion depth

L Length of the paddle or longitudinal axis of the oscillatable unit

AL partial area along the longitudinal axis of the paddles

Af frequency change

Rst _d (h) Reference curve under standard conditions

Rm (h) Reference curve for a given medium

A-F parameter

K correction factor

ki, k2 parameters

Claims

claims

Method for determining and / or monitoring a fill level of a medium (2) by means of a device (1) comprising a sensor unit (3) with a mechanically oscillatable unit (3),

wherein the oscillatable unit (4) is excited to mechanical vibrations by means of an electrical excitation signal (UA),

wherein the mechanical vibrations of the mechanically oscillatable unit (4) are received and converted into a received electrical signal (UE), wherein from the received electrical signal (UE) a frequency (f) of the

Receiving signal (UE) is determined, and

where based on the frequency (f) of the received signal (UE)

the level is determined.

Method according to claim 1,

wherein a predetermined level is determined, and wherein the achievement of the predetermined level is signaled and / or output.

Method according to claim 1 or 2,

wherein the level is determined based on a mathematical rule and / or on the basis of a reference curve (Rstd (h), Rivi (h)).

Method according to claim 3,

wherein the mathematical rule and / or the reference curve (Rstd (h), Rivi (h)) has a dependence between an immersion depth (h) of the oscillatable unit (4) in the medium (2) or a level of the medium (2) and the Frequency (f) of the received signal (UE) indicates.

Method according to claim 3 or 4,

where the mathematical rule (R _s td (h), Rivi (h)) is a polynomial function.

Method according to at least one of the preceding claims,

wherein the density (p), the temperature (T) and / or the viscosity (η) of the medium (2), and / or one of at least the density (p), the temperature (T) and / or the viscosity (η ) of the medium (2) dependent variable is / are determined.

7. The method according to claim 6,

wherein the density (p), the temperature (T) and / or the viscosity (η) of the medium (2), and / or one of at least the density (p), the temperature (T) and / or the viscosity (η ) of the medium (2) dependent variable for the determination of the level is / are taken into account.

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

wherein, for consideration of the density (p), the temperature (T) and / or the viscosity (η) of the medium (2), and / or one of at least the density (p), the temperature (T) and / or the viscosity (η) of the medium (2) dependent variable at least one change of the mathematical rule and / or the reference curve (Rstd (h), RM (II)) is made.

9. The method according to at least claim 8,

wherein at least one correction factor (K) is determined.

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

wherein a first and a second limit value for the level of the medium (2) are determined, which first and second limit value correspond to a minimum or a maximum immersion depth (h) of the oscillatable unit (4), and wherein the level of the medium (2) is continuously determined between the first and second limits.

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

wherein a condition monitoring is performed.

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

wherein the achievement of the predetermined level of the medium (2) in a container (2a) is monitored, wherein a signal is generated depending on the current level, by means of which a rate for a filling and / or emptying process of the container (2a) and / or controlled.

13. Device (1) for determining and / or monitoring a fill level of a medium (2) comprising a sensor unit (3) with a mechanically oscillatable unit (4), and an electronics unit (6), which electronic unit (6) is designed to to carry out the method according to at least one of the preceding claims.