EP3008432A1

EP3008432A1 - Method for calibrating or adjusting any oscillatable unit

Info

Publication number: EP3008432A1
Application number: EP14726927.8A
Authority: EP
Inventors: Tobias Brengartner; Gerd BECHTEL; Sascha D'angelico
Original assignee: Endress and Hauser SE and Co KG
Current assignee: Endress and Hauser SE and Co KG
Priority date: 2013-06-13
Filing date: 2014-05-19
Publication date: 2016-04-20
Also published as: WO2014198493A1; DE102013106172A1; CN105324647A; CN105324647B; US10078005B2; US20160138964A1

Abstract

The invention relates to a method for calibrating or for adjusting any oscillatable unit by means of a mathematical model (model) which describes the oscillatable unit, wherein the oscillatable unit interacts at least intermittently with a medium located in a container. According to the invention the oscillatable unit is stimulated to oscillation by means of a real input signal; the real output signal of the oscillatable unit is ascertained; the real output signal is digitised and a real output sequence (yu(k)) is generated; the real input signal is digitised and a digital input sequence (u(k)) is generated; the digital input sequence (u(k)) is supplied to a function block (model) which in interaction with the medium provides the mathematical model (model) of the oscillatable unit defined by at least two sensor-specific variables (G1, G2); a virtual output sequence (ym(k)) is generated by means of the mathematical model (model); the virtual output sequence (ym(k)) is compared to the real output sequence (yu(k)); in the event of a deviation (e(k)) the sensor-specific variables (G1, G2) of the mathematical model (model) are adaptively modified, until the deviation between the virtual output sequence (ym(k)) and the real output sequence (yu(k)) of the oscillatable unit lies within a predetermined tolerance range.

Description

METHOD FOR CALIBRATING OR FOR ADJUSTING A

VIBRANT UNIT

The invention relates to a method for calibrating or adjusting any oscillatable unit used in process automation. The oscillatable unit interacts at least temporarily with a medium in a container and is used in particular to determine or monitor at least one sensor and / or system-specific parameter. Here, the process-specific parameters provide information about the process conditions that prevail in the process in which the oscillatable unit is arranged. The sensor-specific variables relate to factors influencing the behavior of the sensor

Characterize oscillatory unit in the medium. These sizes are

in particular geometric parameters and / or they relate to the material properties and / or the mass ratios of the oscillatory unit.

The use of vibronic sensors for the determination of physical quantities is used in automation technology - in process and process engineering

Factory automation technology - widely used. The oscillatable element of a vibronic sensor is connected with a membrane in a material and / or non-positive manner and can be configured as a tuning fork of any type or as a single rod. The diaphragm and the oscillatory element connected to the diaphragm, ie the oscillatable unit, are excited to oscillate via a transmitter / receiver unit. The transmitting / receiving unit is usually at least one

piezoelectric or electromechanical element. In addition, so-called membrane transducers have become known in which the oscillatory element consists only of a membrane.

Usually, a vibronic sensor is connected via analogue electronics

Vibrations excited, the analog electronics together with the oscillatory unit form the analog resonant circuit. Corresponding vibronic sensors or vibronic measuring devices are named by the applicant

LIQUIPHANT and SOLIPHANT are offered and distributed in various designs.

Vibronic sensors make it possible to detect a process-specific parameter, such as the level of a liquid or a solid in a container. Usually, the sensor is operated at the resonant frequency of the oscillatable unit to detect a predetermined level (limit level). By detecting the frequency change at a set phase of usually 90 ° It can be detected whether the oscillating unit is in contact with the medium or whether it oscillates freely.

In addition, it has become known to determine or monitor further process-specific parameters by evaluating the vibration behavior of vibronic sensors in a medium. These process-specific parameters are, in particular, the density and the viscosity, but also the temperature of the medium. For the purpose of determining the density of a liquid medium, the

Phase difference (often simply referred to as phase) between the

Input signal and the output signal set to 45 ° or -135 °. When this phase difference is adjusted, a change in frequency is clearly due to a change in the density of the medium, since influence by the viscosity of the fluid medium can be ruled out. In WO 02/031471 A2 a device for measuring viscosity is described. From EP 2 041 529 B1 an apparatus for determining the density of a liquid medium has become known.

As can be seen from the examples given above, analogue electronics have the disadvantage of being relatively inflexible. In particular, the analog electronics to each sensor or sensor type depending on its vibration characteristics and further depending on the application - so whether the sensor for the

Level, density or viscosity measurement to be used - be adjusted. One solution that circumvents the aforementioned disadvantages is not

previously published DE 10 2012 1 13 045.0, filed on 21.12.2012, the applicant described. The content of DE 10 2012 1 13 045.0 is explicitly attributable to the disclosure content of the present patent application.

The portfolio of vibronic sensors is very varied. Examples include the Applicant's products, which are offered and sold under the names LIQUIPHANT and SOLIPHANT. It can be stated that the known as well as the future sensors are relatively strong in terms of their geometry

differ. In the solution presented in DE 10 2012 1 13 045.0, for the purpose of analytical determination of the interaction between the oscillatory unit designed as a vibrating fork and the fluid medium, the two forks of the tuning fork are approximated using a mathematical model. In this particular case, the two forks are mathematically approximated as ideal elliptical cylinders.

In order to transfer the method known from DE 10 2012 1 13 045.0 to oscillatable units with a deviating geometry, it is necessary to modify the mathematical model accordingly. It should be noted that the mathematical description of the interaction or the interaction between the oscillatable unit and the medium is relatively complex. There are also two other aspects:

Due to the complex geometry of the oscillatable unit, there are always minor differences between the model description and the real description

Deviations.

the geometries of the oscillatory units of a sensor type, e.g.

LIQUIPHANT T or LIQUIPHANT M are commonly used e.g. not 100% consistent due to manufacturing tolerances.

Both aspects have an unfavorable effect on the desired high measuring accuracy of the vibronic sensors.

The invention has for its object to propose a method with which the measurement accuracy of a vibronic sensor or measuring device can be improved.

The object is achieved by a method for calibrating or aligning any oscillatable unit with a mathematical model describing the oscillatable unit, wherein the oscillatable unit interacts at least temporarily with a medium located in a container and for determining or

Monitoring of at least one process- and / or system-specific parameters in the automation technology is used,

wherein the oscillatable unit is excited to vibrate via a real input signal;

wherein the real output of the oscillatable unit is detected;

wherein the real output signal is digitized and a real output sequence is generated; wherein the real input signal is digitized and a digital input sequence is generated; wherein the digital input sequence is supplied to a functional block which provides the mathematical model of the oscillatory unit defined by at least two sensor-specific magnitudes in interaction with the medium;

wherein a virtual output sequence is generated via the mathematical model;

wherein the virtual output sequence is compared with the real output sequence;

wherein in the case of a deviation, the sensor-specific magnitudes of the mathematical

Model adaptively changed until the deviation between the virtual

Output sequence and the real output sequence of the oscillatory unit is within a predetermined tolerance.

The differential equation of a vibronic sensor results - as derived in DE 10 2012 1 13 045.90 - as an equation of the second order: υ _ε * ω = φ + 2 * Ο * ω ₀ * φ + ω * φ

With the natural angular frequency ω ₀ and the Lehr's damping measure D, the following formulas result:

C (70

Wn = ^■

m _s + m _F

d _s + d _F

D = ^■

2 * (m ₅ + m _F ) * ω ₀

The total force F _F acting on the body by the fluid medium results from the summation of the pressure force F _D and the frictional force F _R. The direction of the individual forces should be positive:

F _F = F _D + F _R

you

F _F = m _F ^■ - + d _F ^■ u

dt m _F is to be interpreted as additionally coupling mass and d _F as additional acting damping:

G1, G2 are parameters that depend exclusively on the geometry of the oscillatory body.

In the event that the oscillatory system is approximated by two elliptical prongs, the following equations result:

m _F = 2 ^■ l ^■ b ^■ X ^■ ^{2 ρη} . hl 1 ^■ p ^■ a 2 · π

ω

d _F = 2 ^■ l ^■ b ^■ X ^■ ^ 2ωρη

The stiffness of the oscillatory unit is given by c (T), m _s is the mass, d _{s is} the damping of the freely oscillating oscillatable unit, D is the Lehrsche

Attenuation and ω ₀ the natural frequency. The sensor-specific variables, in particular the geometric parameters, can be determined via a parameter estimation method. The following is information for defining the natural frequency and the resonance frequency:

The resonance frequency ω _γ is always the frequency at which the maximum amplitude occurs.

The natural frequency ω _ά is the frequency with which an oscillatable unit freely oscillates.

In the undamped case, there is a phase of 90 ° between transmission and

Receive signal when the oscillatory unit is excited at the natural frequency. In this special case, the natural frequency is designated by ω ₀ .

The natural and resonant frequencies also differ from each other in the damped case.

For vibronic sensors, the excitation always takes place with the natural frequency of the undamped vibrating unit, in which a phase of 90 ° is established. The natural frequency can be determined with the following formula: In the undamped - as well as in the muted case - wO can be calculated with the above formula. In these cases, one obtains the frequency at which a phase of 90 ° occurs. In the damped case, however, this corresponds neither to the resonance frequency ω _γ nor to the natural frequency ω _{ά of} the damped system. These are defined as follows, where c is the stiffness, m is the mass, ω _{0 is} the unattenuated natural frequency, ω _{ά is} the damped natural frequency, ω _{γ is} the resonance frequency, and D is the learning curve

Damping is.

After completion of the invention, preferably automatically running

Calibration method, the process-specific parameters, in particular the fluid parameters density p, viscosity η and temperature T can be determined via a parameter estimation method in a downstream adjustment. While in the implementation of the method described in DE 10 2012 1 13 045.0 for

Determination and / or monitoring of process parameters the sensor-specific

Parameter GG ₂ are kept constant, be in the inventive solution In particular, the above-mentioned process-specific parameters kept constant. Subsequently, according to the invention, the sensor-specific parameters are varied until the deviation between the virtual output sequence and the real output sequence of the oscillatable unit is within a predetermined tolerance range or preferably approaches zero. For this, the oscillatable unit or the vibronic sensor must be operated in a defined medium at a constant temperature.

The process according to the invention is preferably preceded by the process described in DE 10 2012 1 13 045.0. The model comparison according to the invention makes it possible to adapt at least two sensor-specific variables, which significantly influence the interaction between the oscillatable unit and the medium, adaptively to the real behavior of the oscillatable unit. In this case, sensor-specific quantities are to be understood as meaning, in addition to the geometrical parameters already mentioned several times, variables which determine the interacting behavior of the sensor with the medium, such as the material properties of the oscillatable unit or sensors

Mass ratios on the oscillatory unit.

The method according to the invention has the distinct advantage that an existing model, e.g. the model, which is described in detail in DE 10 2012 1 13 045.0 for two elliptical forks, can be adapted to any and a wide variety of sensor-specific variables or geometrical parameters, without having to invest effort in the rewriting of the mathematical model. While in DE 10 2012 1 13 045.0 e.g. the geometric parameters are kept constant, the geometric parameters or otherwise

sensor-specific variables in the inventive solution adaptively changed until the model behavior and the behavior of the real oscillatory unit match highly accurately. This adaptive approach process runs automatically. According to the invention it is possible for the first time - regardless of the actual

Embodiment of the oscillatory unit - to always achieve a high degree of correspondence between any oscillatable unit and the mathematical model used.

As already indicated above, the oscillatable unit is associated with a sensor or a measuring device of automation technology, which is used in particular to determine the fill level or the limit level, the density or the viscosity of the medium in the container.

An advantageous development of the method according to the invention suggests that the oscillatable unit is operated at a natural frequency. However, it is completely sufficient if the oscillation frequency of the oscillatory unit is in the vicinity of the resonance frequency of the mechanical oscillatory system. This does not have to be identical to the natural frequency. According to a preferred embodiment of the solution according to the invention, the adjustment or the calibration of the oscillatable unit in the medium takes place under defined process or system-specific conditions. In particular, the temperature of the calibration medium is kept constant. The same applies to the viscosity and the density of the calibration medium. In addition, care must be taken that the

medium contacting part of the oscillatory unit during the adjustment is always in contact with the medium up to a defined immersion depth.

Furthermore, it is provided that the sensor-specific variables determined during the adjustment or the calibration, in particular the geometric parameters, are transmitted to a memory of the sensor or of the measuring device. As already mentioned above, the determined sensor-specific variables or the geometrical parameters of the previously calibrated or calibrated oscillatable unit will subsequently be used in the regular measuring operation of the measuring device or the sensor for determining the process and / or system-specific parameters. A preferred method, as has to be done, is described in detail in the already cited several times DE 10 2012 1 13 045.0.

Both for determining the sensor-specific quantities or the geometric parameters and for determining the process and / or system-specific parameters, a description of the oscillatable unit in a mathematical model describing the oscillatable unit as a linear or non-linear system

State space or used as a transfer function. In particular, the mathematical model in which the oscillatable unit is described as a linear or nonlinear system is extended by the quantities to be determined

Parameter estimation method to use. The usable mathematical

Model describes the relationship between the input signal and the output signal through transfer functions or transfer matrices. When

Parameter estimation method for mathematical models in the form of a

Transfer functions are preferred: the least squares method, the generalized LS method, the RLS method, the auxiliary variable method, or the maximum likelihood method. Preferred is as

Parameter estimation methods for mathematical models in state space using the Extended Kalman Filter, the Unscented Kalman Filter, or a subspace method. The invention will be explained in more detail with reference to the following figures. It shows:

1 is a block diagram illustrating the method according to the invention, FIG. 2 is a flow chart showing the method steps of the invention

Method and the method presented in DE 10 2012 1 13 045.0 illustrated, and

FIG. 2 a shows an enlarged representation of the individual method steps of the method according to the invention shown in FIG. 2.

1 shows a block diagram which visualizes the method according to the invention for the determination and / or monitoring of at least one sensor-specific variable, in particular of two geometric parameters. To carry out the

According to the invention, the oscillatable unit is at least temporarily in contact with a medium or fluid in a container. The oscillatable unit in contact with the medium or fluid is in Fig. 1 with the term

Labeled "process".

The oscillatable unit is excited via an analog input signal to vibrate. The real output signal is determined as an output signal of the oscillatable unit and then digitized, so that a real output sequence yu (k) is generated. In the case shown, a disturbance n (k) is taken into account, so that the real output sequence yp (k) results.

Parallel to this, the real input signal is digitized; it becomes a digital one

Input sequence u (k) generated. The digital input sequence u (k) is fed to a function block - here referred to as model - which provides at least one mathematical model of the oscillatory system in interaction with the medium, defined by a plurality of process and / or system-specific parameters. The mathematical model generates a virtual output sequence ym (k). Subsequently, the virtual output sequence ym (k) is compared with the real output sequence yu (k) or yp (k). In the case of a deviation e (k), at least one of the process- or system-specific parameters-in the method of DE 10 2012 1 13 045.0-or the sensor-specific variables-in the method according to the invention-of the mathematical model is adaptively changed until the deviation e (k) between the virtual

Output signal ym (k) and the real output signal yu (k) or yp (k) of the oscillatory unit is within a predetermined tolerance.

Subsequently, the adaptively determined parameters G1, G2; η, p, T provided.

2 shows a flowchart which describes the program steps 1 -3 of the method according to the invention - see also the enlargement in FIG. 2 a - and the program steps 10, 20, 30, 40, 50 of the method described in DE 10 2012 1 13 045.0 Clarified procedure.

The method according to the invention for the calibration or comparison of any oscillatable unit with a vibratory unit descriptive

mathematical model is started under program item 1. At program point 2, the calibration or adjustment starts, the sensor is initialized. Subsequently, at program point 3, the sensor-specific quantities, here the geometrical parameters G1, G2, are automatically transmitted via e.g. one of the previously mentioned

Parameter estimation method determined.

After starting the method, it is decided at program point 10 whether the medium properties or the process-specific parameters p, η, T are determined directly or indirectly via the parameter estimation method. If the direct path is selected, the parameter estimation of the process-specific parameters p, η, T takes place under the program point 20.

If the indirect route is selected, then under program item 30 a

Parameter estimation of the Lehr's damping measure D and the natural angular frequency ω ₀ made. Subsequently, the program point 40 is used to calculate the medium properties or the process-specific parameters p, η, T. The required process-specific parameters p, η, T are output at program point 50.

Claims

claims

A method of calibrating or aligning any oscillatable unit with a mathematical model describing the vibratable unit, wherein the oscillatable unit interacts at least temporarily with a medium in a container and determines or monitors at least one process and / or system specific parameter (p, η, T) is used in automation technology,

wherein the oscillatable unit is excited to vibrate via a real input signal;

wherein the real output of the oscillatable unit is detected;

wherein the real output signal is digitized and a real output sequence (yu (k)) is generated;

wherein the real input signal is digitized and a digital input sequence (u (k)) is generated;

the digital input sequence (u (k)) being supplied to a function block (model) which provides the mathematical model (model) of the vibratable unit in interaction with the medium defined by at least two sensor-specific quantities (G1, G2);

wherein a virtual output sequence (ym (k)) is generated via the mathematical model (model);

wherein the virtual output sequence (ym (k)) is compared with the real output sequence (yu (k));

wherein, in the event of a deviation (e (k)), the sensor-specific quantities (G1, G2) of the mathematical model (model) are adaptively changed until the deviation between the virtual output sequence (ym (k)) and the real output sequence (yu (k )) of the oscillatable unit is within a predetermined tolerance range.

2. The method according to claim 1,

wherein the oscillatable unit is associated with a sensor or meter which is used to determine the level, density (p) or viscosity (η) of the medium in the container.

3. The method according to claim 1 or 2,

wherein the sensor-specific variables are in particular geometric

Parameter (G1, G2) that describe the geometry of the oscillatory unit.

4. The method according to claim 1, 2 or 3,

wherein the oscillatable unit is operated at a natural frequency.

5. The method according to claim 1, 2, 3 or 4,

wherein the adjustment or the calibration of the oscillatory unit in the medium is carried out under defined process or system-specific conditions.

6. The method according to claim 5,

wherein the viscosity (η) and the density (p) of the medium are kept at least approximately constant during the calibration or the calibration and wherein the medium-contacting part of the oscillatable unit is in contact with the medium up to a defined immersion depth.

7. The method according to one or more of claims 1-6,

wherein the sensor-specific variables or the geometric parameters (G1, G2) ascertained during the calibration or the calibration are transmitted to a memory of the sensor or of the measuring device.

8. The method according to one or more of the preceding claims,

wherein the sensor-specific variables or the geometric parameters (G1, G2) of the previously calibrated or calibrated oscillatable unit are used for the determination of the process and / or system-specific parameters (p, η, T) during measurement operation of the sensor or the measuring device ,

9. The method according to claim 1 or 8,

wherein for determining the sensor-specific quantities or the geometric parameters (G1, G2) and for determining the process and / or system-specific parameters (p, η, T) as a mathematical model (model), the oscillatable unit as a linear or non-linear Describes a description of the oscillatable unit in a state space or as a transfer function.

10. The method according to claim 9,

wherein, as an adaptation algorithm, a parameter estimation method is applied to the mathematical model (model), in which the oscillatable unit is described as a linear or non-linear system and which determines the relationship between the input signal and the

Output signal by transfer functions or transmission matrices is used to determine the unknown parameters.

1 1. A method according to claim 10,

preferred parameter estimation method being the least squares method, the generalized LS method, the RLS method, the auxiliary variable method or the maximum likelihood method is used for mathematical models in the form of a transfer function.

12. The method according to claim 9,

the preferred parameter estimation method being the extended Kalman filter,

Unscented Kalman Filter or a subspace method, used for mathematical models in state space.