WO2002031471A2

WO2002031471A2 - Device for determining and/or monitoring the viscosity of a medium in a container

Info

Publication number: WO2002031471A2
Application number: PCT/EP2001/010009
Authority: WO
Inventors: Igor Getman; Sergej Lopatin
Original assignee: Endress + Hauser Gmbh + Co. Kg
Priority date: 2000-10-10
Filing date: 2001-08-30
Publication date: 2002-04-18
Also published as: JP2004511771A; DE10050299A1; EP1325301A2; AU2002210473A1; CN1468370A; WO2002031471A3

Abstract

The invention relates to a device for determining and/or monitoring the viscosity of a medium in a container, comprising a device (2) that can oscillate, a drive/receiving device (4, 5, 6) and a control/evaluation device (8). The device (2) that can oscillate is mounted in a defined measuring position within the container or a device (2) that can oscillate is mounted in such a manner that it dips into the medium to a defined depth of immersion. The drive/receiving device (4, 5) induces oscillation of the device (2) that can oscillate and the drive/receiving device (4, 6) receives the oscillations of the device (2) that can oscillate. The aim of the invention is to use a vibration detector to determine and/or monitor the viscosity (θ) of a medium in a container. To this end, the control/evaluation device (8) determines the viscosity (θ) of the medium on the basis of the frequency-phase curve (f = g(f)) of the device (2) that can oscillate.

Description

Device for determining and / or monitoring the viscosity of a

Medium in a container

The invention relates to a device for determining and / or monitoring the viscosity of a medium in a container with an oscillatable unit, a lifting / receiving unit and a control / evaluation unit, the oscillatable unit being arranged in a defined measuring position within the container or wherein an oscillatable unit is attached such that it is immersed in the medium up to a defined immersion depth, and wherein the drive / receiver unit excites the oscillatable unit to oscillate, or wherein the drive / receiver unit vibrates the oscillatable unit receives.

Devices with at least one oscillating element, so-called vibration detectors, for detecting or monitoring the fill level of a medium in a container have already become known. The vibrating element is usually at least one vibrating rod which is attached to a membrane. The membrane is an electromechanical transducer, for. B. a piezoelectric element excited to vibrate. Due to the vibrations of the membrane, the vibrating element attached to the membrane also carries out vibrations. A very well-known example of a vibration detector is the 'Liquiphant', which is manufactured and sold by the applicant.

Vibration detectors designed as level measuring devices take advantage of the effect that the oscillation frequency and the oscillation amplitude depend on the respective degree of coverage of the oscillation element: While the oscillation element can carry out its (resonance) oscillations freely and undamped in air, it experiences a change in frequency and amplitude , an upset as soon as it is partially or completely immersed in the medium. On the basis of a predetermined change in frequency (usually the frequency is measured for filling level detection), it is consequently possible to draw a clear conclusion that the predetermined filling level of the medium in the container has been reached. Level gauges are used primarily as overfill protection or for the purpose of pump idle protection. In addition, the damping of the vibration of the vibrating element is also influenced by the density of the medium. Therefore, with a constant degree of coverage, there is a functional relationship between the frequency change and the density of the medium, so that vibration detectors are ideally suited for determining both the fill level and the density.

In practice, for the purpose of monitoring and recognizing the fill level or the density of the medium in the container, the vibrations of the membrane are recorded and converted into electrical response signals by means of at least one piezoelectric element. The electrical response signals are then evaluated by evaluation electronics. In the case of level determination, the evaluation electronics monitor the oscillation frequency and / or the oscillation amplitude of the oscillation element and signal the status 'sensor covered' or 'sensor uncovered' as soon as the measured values fall below or exceed a predetermined reference value. A corresponding message to the operating personnel can be made optically and / or acoustically. Alternatively or additionally, a switching operation is triggered; for example, an inlet or outlet valve on the container is opened or closed.

The invention has for its object to use a vibration detector for determining and / or monitoring the viscosity of a medium in a container.

The object is achieved in that the control / evaluation unit determines the viscosity of the medium on the basis of the frequency-phase curve of the oscillatable unit. The present invention is based on the fact that the damping of an oscillatable unit depends on the viscosity of the medium with which it is in contact. As is known, viscosity is the internal friction of a liquid, which is caused by attractive forces between the molecules. The viscosity is highly dependent on the parameters of pressure and temperature.

The frequency-phase curves of an oscillatable unit that have been recorded in media with different viscosities differ significantly from one another - as can be clearly seen from the graphs shown in FIG. 1: the lower the viscosity of the medium, the steeper the frequency-phase curve drops. It has proven to be particularly advantageous Determine the viscosity of the medium based on the frequency change that occurs at two different phase values. It is therefore preferred not to perform an absolute measurement, but rather a relative measurement. As will be explained in more detail below, either two phase values are set and the associated frequency change is determined, or a predetermined frequency band is traversed and determined when at least two predetermined phase values are reached. The frequency change and the viscosity of the medium are determined from the frequencies corresponding to the phase values.

2, the viscosity is plotted against the frequency change with different phase shifts. A logarithmic scale was chosen. The curves can be described by the following mathematical formula: log η = a • log Δ f + b, where a is almost the same for all curves, while the curves differ essentially in the constant b. Consequently, different phase shifts are reflected in a parallel shift of the frequency difference viscosity curve along the frequency difference axis. The advantage of measuring the frequency change instead of the absolute frequency measurement lies in an increased measuring accuracy and - as will be described in detail below - in the automatic elimination of disturbance variables, for example the density. The frequency change for a given phase shift shows a clear dependence on the viscosity. Consequently, it is possible to determine the viscosity by determining the frequency difference for at least two predetermined phase values.

The influence of density is visualized on the basis of the frequency-phase curve family shown in FIG. 3 of an oscillatable unit in media with different densities: Different densities lead to a parallel shift of the frequency-phase curve along the frequency axis. The higher the density, the lower the oscillation frequency with the same phase value. The shape of the curves themselves is almost identical in all cases. Since no absolute values but rather relative values are measured according to the invention, the effect that a changing density has on the measurements is automatically eliminated.

According to a preferred development of the device according to the invention, a piezo drive is used as the drive / receiver unit. Piezo drives in In connection with the present invention can be used, for. B. from EP 0 985 916 A1.

According to an advantageous further development of the device according to the invention, it is provided that the drive unit excites the oscillatable unit to oscillate in a predetermined oscillation mode, the oscillation mode preferably being the basic mode of the oscillatable unit.

A preferred embodiment of the device according to the invention proposes that the control evaluation unit is assigned a memory unit in which data are stored which reflect the functional relationship between the frequency and the phase of the oscillations of the oscillatable unit with different damping ratios or with different viscosities. The data can be characteristic curves, formulas or measured values.

The control evaluation unit preferably sets at least two sufficiently different phase values; The control / evaluation unit subsequently determines the frequencies assigned to the phase values or the corresponding frequency change in the vibrations of the oscillatable unit and determines the viscosity of the medium by comparing the previously determined frequency change and the stored data.

According to a particularly favorable embodiment of the device according to the invention, the at least two phase values are symmetrical to the phase value φ = 90 °.

An advantageous embodiment of the device according to the invention provides that the control evaluation unit selects the range in which the frequencies which are used to determine the viscosity are such that the functional relationship between the phase values and the frequencies is essentially linear.

According to an alternative embodiment of the device according to the invention, the control evaluation unit sets at least two frequencies that are different from one another; the phases associated with the frequencies of the vibrations of the oscillatable unit between the transmission and response signals then determined; In a last step, the control / evaluation unit determines the viscosity of the medium by comparing the determined phase values and the stored phase values.

According to a preferred variant of the last-mentioned alternative of the device according to the invention, the control evaluation unit is assigned a signal generator which controls the drive unit in such a way that the oscillatable unit oscillates successively with different oscillation frequencies, the oscillation frequencies being within a selected frequency band (- frequency sweep) ,

Furthermore, a further development of the device according to the invention allows the oscillatable unit to be designed as a universal detector: the control / evaluation unit operates the oscillatable unit in a first operating mode as a limit switch and in a second operating mode as a viscosity sensor. The respective operating mode is specified by a program contained in the control unit.

An input / output unit is preferably provided, via which settings are made on the device or via which information regarding the measured values which the device delivers is provided. At least one bus line is provided for data exchange between the oscillatable unit and a remote control point. The data exchange itself can be carried out using any transmission standard, e.g. B. Profibus PA, Fieldbus Foundation.

The invention is illustrated by the following drawings. It shows:

1: a schematic representation of the frequency-phase curves of an oscillatable unit with different damping coefficients,

2: a diagram which graphically shows the dependence of the viscosity on the frequency change,

3: a diagram which shows the frequency-phase curves at different densities of the medium, 4 shows a block diagram of a first embodiment of the device according to the invention,

5 shows a block diagram of the excitation circuit used in FIG. 4,

6: a graphic representation of a frequency-phase curve with visualization φ of a 'frequency sweep' in two predetermined frequency bands and

7 shows a block diagram of a second embodiment of the device according to the invention.

Fig. 1 shows the representation of three frequency-phase curves of an oscillatable unit 2 in media with different damping coefficients ξ. The turning point of the three curves lies at the resonance frequency fr, which is essentially determined by the rigidity of the membrane and the mass of the vibrating element. As can be seen from FIG. 1, the phase φ between the drive signal and the response signal of the oscillatable unit 2 is 90 ° in the event of resonance. In the case of low damping (damping coefficient ξ1), even slight frequency changes df lead to a phase jump of 180 ° - the phase change takes place abruptly. With larger damping coefficients ξ2, ξ3 the phase change from 0 ° to 180 ° is more or less smooth. The frequency-phase curves show a linear course within a certain frequency or phase range, the slope being dependent on the attenuation by the medium.

2 schematically shows the dependence of the viscosity η on the frequency difference df between the drive signal and the response signal on a logarithmic scale. The family of curves shows the graphs with different phase shifts df (φn- φm) with n, me N, n ≠ m. The frequency change df with a given phase shift df (φn- φm) shows a clear dependence on the viscosity η. It is consequently possible to determine the viscosity η by measuring the frequency difference df with at least two predetermined phase values φ1, φ2 according to a first alternative embodiment of the device 1 according to the invention. The influence of the density p is visualized on the basis of the frequency-phase curves of an oscillatable unit 2 shown in FIG. 3 in media with different densities p: Different densities p lead to a parallel shift of the frequency-phase curve along the frequency axis f , The higher the density p, the lower the oscillation frequency with the same phase value φ. The shape of the curves themselves is almost identical in all cases. Since, according to the invention, no absolute values, but rather relative values (frequency changes or phase changes) are used for the evaluation of the viscosity η, the effect that a changing density p has on the measured values is automatically eliminated.

FIG. 4 shows a block diagram of a first embodiment of the device 1 according to the invention. According to this first embodiment, two predetermined phases φ1, φ2 are successively set between the drive signal and the response signal. The two phase values φ1, φ2 are set via the excitation circuit 9, which will be described in detail below. The frequency values f1, f2 linked to the phase values φ1, φ2 are then determined. Based on the frequency change df = f2 - f1, the viscosity η of the medium is subsequently determined using stored data.

This first method of viscosity determination is very similar to the ^'method as a predetermined filling level can be determined by a vibration detector to achieve. The only difference is basically that only the phase of the natural frequency or the resonance frequency of the oscillatable unit 2 is taken into account in the level measurement, while in the viscosity measurement at least two phase values φ1, φ2 and the corresponding frequencies f1, f2, or respectively corresponding frequency change df = f1-f2 of the oscillatable unit 2 can be considered.

Because of this high degree of similarity, it is also relatively easy to design an oscillatable unit 2 as a universal sensor for level, density and / or viscosity measurement. As already mentioned, the fill level is usually determined by monitoring the resonance frequency fr. The viscosity η is preferably determined by setting two mutually different phase values φ1, φ2 and the corresponding ones Frequencies or the corresponding frequency change df = f1 - f2 is determined. The frequency change df = f 1 - f with predetermined phase values φ1, φ2 is functionally dependent on the viscosity η.

The oscillatable unit 2 is excited to vibrate via the piezoelectric excitation / reception unit, which in the case shown consists of a disk-shaped piezoelectric element 5, a drive electrode 6 and two reception electrodes 7. Here, the piezoelectric element 5 takes on the function of an interface between the mechanical parts, i.e. the membrane 4 and the vibrating elements 3, and the electronic parts, drive electrode 6 and receiving electrodes 7, of the vibratable unit 2: on the one hand, the piezoelectric element 5 sets an electric drive signal mechanical vibrations around; on the other hand, it converts mechanical vibrations into an electrical response signal. It goes without saying that, instead of a disk-shaped piezoelectric element 5, a so-called stack drive can also be used.

FIG. 5 shows a block diagram of the excitation circuit 9 used in FIG. 4. As can be seen from the block diagram shown in FIG. 5, the excitation circuit 9 has several functions: it picks up the received signal Rx at the receiving electrodes 7. The response signal Rx is passed through the bandpass filter 13. The bandpass filter 13 preferably has a very small bandwidth, so that only the desired frequencies or the desired frequency are or are present at the output of the bandpass filter 13. The filtered response signal Rx is then fed to the amplifier 14 and amplified. In the case shown, two constant phase values φ1, φ2 are set in the phase shifter 15. Via the amplifier 16 and the low-pass filter 17, the response signal is fed back to the drive electrode 6 as the drive signal Tx and excites the oscillatable unit 2 to oscillate with the respectively set phase value φ1; φ2 on.

The response signal Rx passes from the excitation circuit 9 to the microprocessor 10, which for each phase value φ1; φ2 the corresponding frequency f1; f2 determined. The frequency change df = f2-f1 is then determined and compared with corresponding data stored in the memory unit 11. Due to the clear functional relationship between the frequency change df and the viscosity η, the respective viscosity η des Determine the medium. The determined viscosity η of the medium can be brought to the operator's knowledge, for example, via the input / display unit 12. Of course, it is also possible to use the determined viscosity value to control actuators.

According to an alternative embodiment of the device 1 according to the invention, the frequency f is changed within predetermined frequency bands; the oscillatable unit 2 is thus driven with different frequencies (- frequency sweep). Different phase values are assigned to the different frequencies. The continuous traversal of certain frequency ranges is shown graphically in FIG. 6. FIG. 7 shows a block diagram of this second embodiment of the device 1 according to the invention.

In this second embodiment of the device according to the invention, two frequencies f1, f2 are located during the 'frequency sweep', which belong to two predetermined phase values φ1, φ2. Specifically, certain frequency ranges Δf1, Δf2 are run through in continuous steps. As soon as the predetermined phase values φ1, φ2 are measured, the frequencies f1, f2 belonging to the phase values φ1, φ2 are determined. The viscosity η of the medium is then determined on the basis of the frequency difference df = f2 - f1.

The oscillatable unit 2 is excited by a signal generator 19 with drive signals Tx of a predetermined frequency and preferably a predetermined amplitude. The drive signals Tx are fed to a signal adaptation unit 18, which processes the signals in such a way that they can be read by the receiving unit 21. The receiving unit 21 thus receives the response signals Rx of the oscillatable unit 2; a phase meter 22 determines the corresponding phase shift between the drive signal and the response signal. The control unit 20 is responsible for the entire process for determining the frequency change df: it carries out the phase comparison, controls the frequency of the signal generator 19 and finally calculates the corresponding frequency change df. Based on the determined frequency change df, the viscosity η of the medium is subsequently determined in the converter 23. Stored table values, characteristic curves or formulas are used for this. LIST OF REFERENCE NUMBERS

1 device according to the invention

2 oscillating unit

3 vibrating element

4 membrane

5 Piezoelectric material

6 excitation electrode

7 receiver electrode

8 control evaluation unit

9 excitation circuit

10 microprocessor

11 storage unit

12 display unit

13 bandpass filters

14 amplifiers

15 phase shifters

16 amplifiers

17 low pass filter

18 signal matching unit

19 signal generator

20 control unit

21 signal receivers

22 phase meter

23 converter

24 bus line

25 inspection body

Claims

claims

1.Device for determining and / or monitoring the viscosity of a medium in a container with an oscillatable unit, a drive / receiver unit and a control evaluation unit, the oscillatable unit being arranged in a defined measuring position within the container or an oscillatable unit so it is appropriate that it is immersed in the medium up to a defined immersion depth, and the drive / receiving unit excites the oscillatable unit to vibrate or the drive receiving unit receives the vibrations of the oscillatable unit, characterized in that the control evaluation unit (8) uses the frequency-phase curve (φ = g (f)) of the oscillatable unit (2) determines the viscosity (η) of the medium.

2. Device according to claim 1, characterized in that the drive unit (5, 6, 7) excites the oscillatable unit (2) to vibrate in a predetermined oscillation mode, the oscillation mode preferably being the basic mode of the oscillatable unit (2) is.

3. Apparatus according to claim 1 or 2, characterized in that the drive unit (5, 6, 7) is a piezo drive which is in contact with the membrane (4) on which the at least one oscillating element ( 3) is attached.

4. Apparatus according to claim 1, 2 or 3, characterized in that the control evaluation unit (8) is assigned a memory unit (11) in which data are stored which show the functional relationship between the frequency (f) and the phase (φ) reflect the vibrations of the oscillatable unit (2) with different damping (ξ) or with different viscosities (η).

5. The device according to claim 4, characterized in that the RegelVAuswerteinheit (8) sets at least two sufficiently different phase values (φ1, q> 2) that the RegelVAuswerteinheit (8) assigned to the phases (φ1, φ2)

Frequencies (f 1, f2) or the corresponding frequency change (df) of the

Determines vibrations of the oscillatable unit (2) and that the control evaluation unit (8) by comparing the determined

Frequency change (df) with stored data determines the viscosity (η) of the medium.

6. The device according to claim 5, characterized in that the at least two phase values (φ1, φ2) are symmetrical to the phase value φ = 90 °.

7. The device according to claim 5 or 6, characterized in that the RegelVAuswerteinheit (8) selects the range in which the frequencies (f), which are used to determine the viscosity (η), so that the functional relationship between the Phase values (φ) and the frequencies (f) is essentially linear.

8. The device according to claim 4, characterized in that the RegelVAuswerteinheit (8) sets at least two different frequencies (f1, f2), that the RegelVAuswerteinheit (8) associated with the frequencies (f1, f2) of the vibrations of the oscillatable unit (2) Determines phase values (φ1, φ2) and that the control evaluation unit (8) determines the viscosity (η) of the medium by comparing the determined phase values (φ1, φ2) with stored data.

9. The device according to claim 8, characterized in that the RegelVAuswerteinheit (8) is assigned a signal generator (19) which controls the drive unit (6) so that the vibratable unit (2) successively vibrates with different vibration frequencies, the Vibration frequencies lie within at least one selected frequency band.

10. The device according to one or more of the preceding claims, characterized in that the RegelVAuswerteinheit (8) operates the oscillatable unit (2) in a first operating mode as a limit switch and in a second operating mode as a viscosity sensor.

11. The device according to claim 1, 5, 8 or 10, characterized in that an input / output unit (12) is provided, via which settings are made on the device (1) or via the information relating to the measured values provided by the device becomes.

12. The apparatus according to claim 4, characterized in that at least one bus line (24) is provided, via which the control / evaluation unit (8) communicates with a remotely located control point (25).