CN1468370A - Device for determining and/or monitoring the viscosity of a medium in a container - Google Patents

Abstract

The invention relates to an apparatus for determining and/or monitoring the viscosity of a medium in a container, having a unit which can oscillate (2), having a drive/reception unit (4, 5, 6) and having a control/evaluation unit (8), in which case the unit which can oscillate (2) is arranged at a defined measurement position within the container and/or in which case a unit which can oscillate (2) is fitted such that it is immersed as far as a defined immersion depth in the medium, and in which case the drive/reception unit (4, 5) excites the unit which can oscillate (2) to oscillate and/or in which case the drive/reception unit (4, 6) receives the oscillations from the unit which can oscillate (2). The invention is based on the object of using a vibration detector for determining and/or monitoring the viscosity (eta) of a medium in a container. The object is achieved in that the control/evaluation unit (8) uses the frequency/phase curve (phi g(f)) of the unit (2) which can oscillate to determine the viscosity (eta) of the medium.

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 a vibrating unit, a drive-/receiving unit and a regulating-/computing unit, wherein the vibrating unit is arranged inside the container-determining on the measuring position, or wherein the vibratable unit is arranged so that it is immersed in the medium to a certain immersion depth, wherein the driving-/receiving unit excites the vibration of the vibratable unit, or wherein the driving-/receiving unit receives the vibration of the vibratable unit vibration.

Numerous devices with at least one vibrating element, so-called vibrating sensors, are known for detecting or monitoring the fill level of a medium in a container. In such a vibrating element, usually at least one vibrating rod fixed to the diaphragm is involved. The diaphragm is excited to vibrate by an electro-mechanical transducer, such as a piezoelectric element. Due to the vibration of the diaphragm, the vibrating element fixed on the diaphragm also vibrates. A particularly well-known example of a vibration detector may be cited here as "Liquiphant", which was manufactured and marketed by the woman applicant.

The vibration detector constituted as a material level measuring device makes full use of this effect, that is, the vibration frequency and amplitude depend on the coverage factor of the vibration element at that time: the vibration element can carry out its (resonant) vibration freely and without attenuation in the air, while Once it is partially or completely immersed in the medium, a frequency change and a change in amplitude can be obtained, that is, detuning. With the aid of a predefined frequency change (usually the detection of the fill level measurement frequency), it is then possible to draw precise conclusions as to whether the medium in the container has reached the predefined fill level. In addition, the level measuring device is used in particular as an overfill safety or for the protection of pumps running dry.

In addition, the attenuation of the vibration of the vibrating element is also affected by the density of the medium. Therefore, with a constant coverage factor, there is a functional relationship between the frequency change and the density of the medium, so that vibration detectors are most suitable for determining the level and determining the density.

In practice, for the purpose of monitoring and identifying the fill level or density of the medium in the container, vibrations of the diaphragm are received and converted into electrical response signals by means of at least one piezoelectric element. The electrical response signal is then evaluated by the evaluation electronics. In the case of level determination, the evaluation electronics monitor the vibration frequency and/or the amplitude of the vibrating element and signal 'Sensor covered' or 'Sensor not covered' as long as the measured value is below or above a predefined standard value 'state. Corresponding signals can be transmitted to the operator by optical and/or visual means. Alternatively or additionally a switching process can be initiated; thus, for example, an inlet valve or an outlet valve on the container is opened or closed.

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

The object is achieved in that the regulating/computing unit determines the viscosity of the medium by means of the frequency-phase curve of the oscillatable unit. The basis of the invention is that the damping of the oscillatable unit depends on the viscosity of the medium in contact with it. It is well known that viscosity represents the internal friction of a liquid caused by intermolecular attraction. Viscosity is highly dependent on pressure and temperature parameters.

The frequency-phase-curves of the oscillatable units received in media with different viscosities differ significantly from each other - as can be clearly seen with the help of the diagram shown in Figure 1: The less viscous the medium, the lower the frequency-phase-curve The steeper the slope. It has proven to be particularly advantageous if the viscosity of the medium is determined by means of frequency variations occurring in two different phase values. Therefore, it is preferable not to make absolute measurements, but relative ones. As will be explained in more detail below, either the two phase values are adjusted and the associated frequency variation is determined, or a predetermined frequency band is traversed and determined, if at least two predetermined phase values are achieved. With the aid of the frequency corresponding to the phase value, the frequency change is again determined and the viscosity of the medium is determined therefrom.

Figure 2 depicts the relationship between viscosity and frequency variation for different phase shifts. Choose a logarithmic scale. The curves can be described by the following mathematical formula: logη = a logΔf + b, where a is nearly equal for all curves and the curves differ mainly in the constant b. Thus, in the frequency difference-viscosity curve parallel shifting along the frequency difference-axis, different phase shifts are reflected. The advantage of using the measurement frequency variation instead of the absolute frequency measurement is the increased measurement accuracy and—as will be described in more detail below—the automatic elimination of interfering values such as density. The frequency variation at the predetermined phase shift shows a clear dependence on the viscosity. Viscosity can thus be determined by determining the frequency difference in at least two predetermined phase values.

The effect of density is visualized by means of the family of frequency-phase-curves of vibrable elements in media with different densities shown in FIG. 3: Different densities lead to a parallel shift of the frequency-phase-curves along the frequency axis. The higher the density, the lower the vibration frequency at the same phase value. The shape of the curve itself is pretty much the same in all cases. Since according to the invention no absolute value is measured, but rather a relative value, the influence of density changes on the measurement is automatically eliminated.

According to a preferred refinement of the device according to the invention, a piezo drive is used as drive/receiver unit. A piezo drive that can be used in connection with the present invention is known, for example, from EP 0 985 916 A1.

According to an advantageous further development of the device according to the invention, the drive unit excites the vibratable unit to vibrate in a predetermined vibration pattern, wherein the vibration pattern is preferably the basic pattern of the vibratable unit.

According to a preferred configuration of the device according to the invention, it is provided that the regulating/computing unit is assigned to a memory unit in which data reflecting the functional relationship between the frequency and the phase of the vibration of the vibrator unit for different attenuation or different viscosities are stored. The data can relate to characteristic curves, formulas or measured values.

Preferably, the adjustment-/valuation unit adjusts at least two phase values that are sufficiently different from each other; subsequently, the adjustment-/computation unit determines the frequency assigned to the phase value or the corresponding frequency change of the vibration of the vibratable unit, and through the previously determined frequency The change is compared with the stored data to measure the viscosity of the medium.

According to a particularly advantageous embodiment of the device according to the invention, at least two phase values are symmetrical to the [phi]=90° phase value.

In an advantageous embodiment of the device according to the invention, the regulating/computing unit selects the range in which the frequency for determining the viscosity is called in such a way that the functional relationship between phase value and frequency is substantially linear.

According to an optional embodiment of the device of the present invention, the adjustment-/valuation unit adjusts at least two frequencies different from each other; then determine the phase between the transmission signal and the response signal assigned to the vibration frequency of the vibrator; in the last step , the regulating-/computing unit determines the viscosity of the medium by comparing the measured phase value with the stored phase value.

According to a preferred variation of the last listed alternative of the device according to the invention, the adjustment-/computing unit is assigned to a signal generator, which activates the drive unit in such a way that the vibratable unit vibrates continuously using different vibration frequencies, wherein The vibration frequency is within the selected frequency band (→ frequency scan).

In addition, according to a further development of the device according to the invention, the oscillating unit can be designed as a universal detector: thus, the adjustment/valuation unit acts as a limit switch in the first type of operating oscillation and as a viscosity in the second type of operating oscillation. The sensor operates to vibrate the unit. The individual operating oscillation types are predetermined by a program contained in the regulating/evaluating unit.

An input/output unit is preferably provided, via which adjustments are made on the device or via which information is provided regarding the measured values provided by the device. At least one data bus is provided for the purpose of data exchange between the vibratable unit and a remotely located control station. The data exchange itself can take place by means of any desired transmission standard, eg private data bus PA, field data bus basis.

The invention will now be described in detail with the aid of the following figures. in:

Figure 1 shows a schematic diagram of the frequency-phase-curve of the vibrable unit under different attenuation coefficients,

Fig. 2 shows the simple diagram that graphic method reflects the dependence of viscosity on frequency variation,

Figure 3 shows a simplified diagram illustrating frequency-phase-curves for different densities of media,

Figure 4 shows a block diagram of a first embodiment of the device according to the invention,

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

Figure 6 shows a frequency-phase-curve diagram visualized in two pre-specified frequency bands for  using 'frequency sweep', and

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

FIG. 1 shows three frequency-phase-curve representations of an oscillatable unit 2 in a medium with different attenuation coefficients ξ. The turning point of the three curves is in the resonant frequency fr, which is basically determined by the rigidity of the diaphragm and the quality of the vibrating element. As can be seen from FIG. 1, the phase [phi] between the drive signal and the response signal of the vibratable unit 2 is 90[deg.] in the case of resonance. In the case of reduced attenuation (attenuation coefficient ξ1), a small frequency change df already leads to a 180° phase change - the phase change occurs suddenly. When the attenuation coefficient increases (ξ2,ξ3), the phase transition from 0° to 180° is completed more or less smoothly. In certain frequency- or phase-ranges, the frequency-phase-curve varies linearly, wherein the rise depends on the attenuation through the medium.

FIG. 2 shows in a logarithmic scale the dependence of the reflected viscosity η on the frequency difference df between the drive signal and the response signal. The cluster of curves represents graphs for different phase shifts df(n-m), n, m∈N, n≠m. The frequency-dependent df at the predetermined phase shift df (n−m) exhibits a clear dependence on the viscosity η. The viscosity η can thus be determined by measuring the frequency difference df at at least two predetermined phase values [phi]1, [phi]2 according to a first alternative embodiment of the device 1 according to the invention.

The influence of the density ρ is visualized in media with different densities ρ by means of the frequency-phase curves of the oscillatable element 2 shown in FIG. 3: Different densities ρ lead to parallel frequency-phase curves along the frequency axis f move. The higher the density ρ, the lower the vibration frequency under the same phase value . The curve shape itself is pretty much the same in any case. Since according to the invention no absolute value is used, but rather a relative value (frequency or phase change) is preferably used for the viscosity η, the influence of changes in the density ρ on the measured value is automatically canceled out.

A block diagram of a first embodiment of a device 1 according to the invention can be seen in FIG. 4 . According to this first configuration, two consecutive predetermined phases [phi]1, [phi]2 are adjusted between the drive signal and the response signal. The two phase values [phi]1, [phi]2 are set via the driver circuit 9, which will be described in more detail below. Subsequently, the frequency values f1, f2 linked to the phase values [phi]1, [phi]2 are determined. With the aid of the frequency variation df=f2-f1, the viscosity η of the medium is then determined while calling up the stored data.

This first method of determining the viscosity has many similarities to the method of determining whether a predetermined filling level has been reached by means of a vibration detector. The only difference is essentially that only the phase of the natural frequency or resonance frequency of the vibrator unit is taken into account for level measurement, whereas at least the two phase values 1, 2 and the corresponding Frequency f1, f2 or corresponding frequency variation df=f1-f2.

Owing to this high degree of similarity, the oscillating unit 2 can also be designed quite simply as a universal sensor for level-, density- and/or viscosity measurement. The fill level - as already mentioned - is usually determined by monitoring the resonance frequency fr. The viscosity η is preferably determined by setting two different phase values [phi]1, [phi]2 and determining the corresponding frequency or the corresponding frequency change df=f1-f2. The frequency variation df=f1−f2 for the predetermined phase values 1, 2 is functionally dependent on the viscosity η.

The vibratable unit 2 is excited to vibrate by means of a piezo-electric exciter/receiver unit consisting, in the example shown, of a disc-shaped piezoelectric element 5 , a drive electrode 6 and two receive electrodes 7 . In this case, the piezo element 5 assumes the function of the interface between the mechanical part of the vibratable unit 2, namely the diaphragm 4 and the vibrating element 3, and the electrical part, namely the drive electrode 6 and the reception electrode 7: the piezo element 5 On the one hand, the electrical drive signal is converted into mechanical vibration, and on the other hand, the mechanical vibration is converted into an electrical response signal. It goes without saying that a so-called group drive can also be used instead of the disc-shaped piezoelectric element 5 .

A block diagram of the drive circuit 9 used in FIG. 4 is shown in FIG. 5 . The excitation circuit 9—as the block diagram shown in FIG. 5 indicates—has several functions: it intercepts the receive signal Rx at the receive electrode 7 . The response signal Rx is fed through a bandpass filter 13 . The bandwidth of the bandpass filter 13 is preferably so small that only the desired multifrequency or the desired single frequency is present at the output of the bandpass filter 13 . The filtered response signal Rx is then transmitted to amplifier 14 for amplification. In the phase shifter 15 two constant phase values [phi]1, [phi]2 are adjusted in the example shown. Via the amplifier 16 and the low-pass filter 17, the response signal returns as the drive signal Tx to the drive electrode 6 and excites the vibratable unit 2 to vibrate with the respectively adjusted phase values [phi]1; [phi]2.

The response signal Rx passes from the excitation circuit 9 to the microprocessor 10, which determines the corresponding frequency f1; f2 for each phase value [phi]1; [phi]2. The frequency change df=f2−f1 is then determined and compared with the corresponding data stored in the memory unit 11 . Due to the clear functional correlation between the frequency-dependent df and the viscosity η, the viscosity η of the medium at that time can be determined. The determined viscosity η of the medium can be communicated to the operator, for example, via the input/output unit 12 . It goes without saying that the determined viscosity values can also be used to control actuators.

According to an optional configuration of the device 1 according to the invention, the frequency f is varied within a predetermined frequency band; the vibratable unit 2 is thus driven with different frequencies (→ frequency sweep). Different frequencies are assigned different phase values. The diagram in Figure 6 shows certain frequency ranges for continuous operation. A block diagram of a second embodiment of a device 1 according to the invention can be seen in FIG. 7 .

In this second configuration of the device according to the invention, the positions of two frequencies f1, f2 which belong to two predetermined fixed phase values [phi]1, [phi]2 are determined during the 'frequency sweep'. For this purpose, a certain frequency range Δf1, Δf2 is in particular run continuously in successive steps. As soon as a predetermined fixed phase value [phi]1, [phi]2 is measured, the frequency f1, f2 to which the phase value [phi]1, [phi]2 is assigned is determined. From the frequency difference df=f2-f1, the viscosity η of the medium is then determined.

The vibratable unit 2 is excited by a signal generator 19 with a drive signal Tx of a predetermined frequency and preferably a predetermined amplitude. The drive signal Tx is supplied to a signal matching unit 18 which arranges the signal in such a way that it can be read out by a receiving unit 21 . The receiving unit 21 thus receives the response signal Rx of the vibratable unit 2. The phase measurer 22 determines the corresponding phase shift between the drive signal and the response signal, respectively. The control unit 20 is responsible for the whole process of determining the frequency variation df: performing phase comparison, controlling the frequency of the signal generator 19 and finally calculating the corresponding frequency variation df. Using the determined frequency variation df, the viscosity η of the medium is subsequently determined in the frequency converter 23 . For this purpose stored table values, characteristic curves or formulas are called up.

Reference Symbol Table

1 according to the device of the present invention

2 vibrating units

3 vibrating elements

4 Diaphragms

5 Piezoelectric materials

6 excitation electrodes

7 receiving electrodes

8 Adjustment-/valuation unit

9 excitation circuit

10 microprocessors

11 memory unit

12 display unit

13 bandpass filter

14 amplifiers

15 phase shifter

16 amplifiers

17 low pass filter

18 signal matching unit

19 signal generator

20 control unit

21 signal receiver

22 Phase Meter

23 Inverter

24 data bus

25 control position

Claims (12)

1. one kind is used for device definite and/or monitoring container medium viscosity, but have vibration unit, drive-/receiving element and adjusting-/the evaluation unit, wherein, but vibration unit is arranged on the measuring position that internal tank one determines, or wherein but vibration unit is provided with like this, make it immerse an immersion depth of determining in the medium, wherein, drive-/but receiving element excites the vibration of vibration unit, or wherein drive-/but receiving element receives the vibration of vibration unit, it is characterized in that, regulate-/evaluation unit (8) but determine the viscosity (η) of medium by the frequency-phase place-curve (=g (f)) of vibration unit (2).

2. by the described device of claim 1, it is characterized in that, driver element (5,6,7) but excite vibration unit (2) vibration with the mode of vibration of predesignating, wherein, but mode of vibration is preferably the basic mode of vibration unit (2).

3. by claim 1 or 2 described devices, it is characterized in that, drive-/receiving element (5,6,7) is piezoelectricity-driving, it contacts with diaphragm (4), fixes at least one vibrating elements (3) on the diaphragm.

4. by claim 1,2 or 3 described devices, it is characterized in that, regulate-/evaluation unit (8) be assigned to a memory cell (11), under the inside storage reflection differential declines (ξ) situation or under different viscosities (η) situation, but the data of funtcional relationship between the frequency (f) of vibration unit (2) vibration and the phase place ().

5. by the described device of claim 4, it is characterized in that, regulate-/evaluation unit (8) regulate two enough different each other phase values ( 1, and 2) at least; Regulate-/evaluation unit (8) but determine vibration unit (2) frequency vibration, that distribute to phase place ( 1, and 2) (f1, f2) or corresponding become frequently (df); Regulate-/evaluation unit (8) by becoming the viscosity (η) that (df) and the data of storage compare the mensuration medium to the frequency of measuring.

6. by the described device of claim 5, it is characterized in that at least two phase values ( 1, and 2) and =90 ° phase value symmetry.

7. by claim 5 or 6 described devices, it is characterized in that, regulate-/evaluation unit (8) such residing zone of frequency (f) of selecting to be used for determining viscosity (η), make the funtcional relationship between phase value () and the frequency (f) be essentially linear.

8. by the described device of claim 4, it is characterized in that, regulate-/evaluation unit (8) regulate at least two different frequencies (f1, f2); Regulate-/evaluation unit (8) but determine to distribute to vibration unit (2) shake (f1, phase value f2) ( 1, and 2) frequently; Regulate-/evaluation unit (8) compare the viscosity (η) of measuring medium by the data to the phase value ( 1, and 2) measured and storage.

9. by the described device of claim 8, it is characterized in that, regulate-/evaluation unit (8) be assigned to a signal generator (19), it starts driver element (6) like this, but make vibration unit (2) utilize the different frequency vibrations that shakes continuously, wherein, shaking is at least one selected frequency band frequently.

10. by one of aforementioned claim or many described devices, it is characterized in that, regulate-/evaluation unit (8) in first operation type of oscillation as limit switch, but in second operation type of oscillation as viscosity sensor operation vibration unit (2).

11. by claim 1,5,8 or 10 described devices is characterized in that, have input-/output unit (12), carry out adjusting on the device (1) by it, perhaps the information by the measured value aspect that its generator provided.

12. by the described device of claim 4, it is characterized in that having a data bus (24) at least, by its regulate-/evaluation unit (8) communicate with a remote control position (25) that is provided with.

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