JP2004511771A - A device for measuring and / or monitoring the viscosity of a medium in a container - Google Patents

Abstract

The invention comprises a vibrable unit (2), a drive / receiver unit (4, 5, 6) and a control / evaluation unit (8), wherein said vibrable unit (2) is defined inside the container. A unit which is arranged in the measuring position, or in which the vibration unit (2) is installed so as to be immersed in the medium to a defined immersion depth, and the drive / receiver units (4, 5) can vibrate It relates to a device for measuring and / or monitoring the viscosity of a medium in a container, which excites (2) or in which the drive / receiver unit (4, 6) receives the vibration of a vibrable unit (2). It is an object of the present invention to use a vibration detector for measuring and / or monitoring the viscosity (η) of a medium in a container. This problem is solved by the control / evaluation unit (8) measuring the viscosity (η) of the medium with the frequency / phase curve (φ = g (f)) of the vibrable unit (2).

Description

[0001]
The invention comprises a vibrable unit, a drive / receiver unit and a control / evaluation unit, wherein said vibrable unit is arranged in a defined measuring position inside the container, or the vibrating unit is defined The medium in the container is installed to be immersed in the medium to the immersion depth, and the driving / receiving unit excites the vibrating unit or the driving / receiving unit receives the vibration of the vibrating unit. And a device for measuring and / or monitoring the viscosity.
[0002]
Devices having at least one vibration element, a so-called vibration detector, for detecting or monitoring the filling surface of a medium in a container are already known. The vibrating element is usually at least one vibrating rod fixed to the membrane. The membrane is excited via an electromechanical transducer, for example a piezoelectric element. Based on the vibration of the membrane, the vibrating element fixed to the membrane also vibrates. The best known example of a vibration detector is "Liquiphant" manufactured and sold by the applicant.
[0003]
A vibration detector configured as a filling surface measuring device takes advantage of the effect that the vibration frequency and the vibration amplitude depend on the degree to which the vibration element is covered in each case. The vibrating element can resonate freely and undamped in air, but undergoes frequency and amplitude changes, ie detuning, as soon as it is partially or completely immersed in the medium. Thus, with a given frequency change (usually the frequency is measured for filling surface recognition), a clear conclusion can be drawn on the predetermined filling surface of the medium in the container. The filling level measuring device is generally used as an overfill safety device or for the purpose of protecting the pump from running dry.
[0004]
Further, the damping of the vibration of the vibrating element is affected by the density of the medium. Therefore, the vibration detector is suitable for both the filling surface measurement and the density measurement, since a functional relationship between the frequency change and the density of the medium occurs at a certain degree of coverage.
[0005]
In practice, the vibration of the membrane is employed for monitoring and recognizing the filling surface or density of the medium in the container and is converted into an electrical response signal by at least one piezoelectric element. The electrical response signal is subsequently evaluated by evaluation electronics. In the case of a filling surface measurement, the evaluation electronics monitors the vibration frequency and / or the vibration amplitude of the vibrating element and, as soon as the measured value falls below or exceeds a predetermined reference value, the state “sensor covered”. Covered "or" sensor uncovered ". A corresponding alarm to the operator is provided in an optical and / or acoustic manner. Alternatively or additionally, a switching process is initiated, whereby a supply or discharge valve for the container is opened and closed.
[0006]
It is an object of the present invention to use a vibration detector for measuring and / or monitoring the viscosity of a medium in a container.
[0007]
The object is achieved in that the control / evaluation unit measures the viscosity of the medium according to the frequency / phase curve of the vibrable unit. The invention is based on the fact that the damping of the vibrable unit depends on the viscosity of the medium in contact. Viscosity, as is well known, is understood to be the internal friction of a liquid caused by intermolecular tensile forces. Viscosity is highly dependent on the parameters pressure and temperature.
[0008]
The frequency / phase curves of the vibrable units recorded in media having different viscosities are clearly different from one another, as can be clearly seen from the graph shown in FIG. The lower the viscosity of the medium, the steeper the frequency / phase curve falls. It has proven to be particularly advantageous to measure the viscosity of the medium by means of frequency changes occurring at two different phase values. Therefore, it is advantageous to perform a relative measurement, rather than an absolute measurement. This can be done by setting two phase values and measuring the associated frequency changes, as described in more detail below, or by passing two predetermined frequency bands and at least two predetermined phase bands. Check when the value is achieved. Next, the frequency change and the viscosity of the medium are measured by the frequency corresponding to the phase value.
[0009]
FIG. 2 plots the viscosity versus frequency change at different phase shifts. Log scale was selected. The curve has the following formula:
logη = a · logΔf + b
Where a is the same for all curves, while the curves differ substantially by a constant b. Thus, different phase shifts are reflected in parallel shifts 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 the increased measurement accuracy and, as will be described in more detail below, the automatic rejection of disturbance quantities, for example density. A frequency change at a given phase shift shows a clear dependence on viscosity. Thus, it is possible to measure viscosity by measuring the frequency difference between at least two predetermined phase values.
[0010]
The effect of density is visualized by the family of frequency / phase curves of vibrable units in media with different densities, shown in FIG. Different densities result in a parallel shift of the frequency / phase curve 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 almost identical in all cases. Since the present invention measures relative values rather than absolute values, the effect of varying density on the measurements is automatically eliminated.
[0011]
According to an advantageous embodiment of the device according to the invention, a piezoelectric drive is used as the drive / receiver unit. A piezoelectric drive that can be used in connection with the present invention is known, for example, from EP 0985916 A1.
[0012]
According to an advantageous embodiment of the device according to the invention, the drive unit excites the vibrating unit in a predetermined vibration mode, which vibration mode is preferably the fundamental mode of the vibrating unit.
[0013]
According to an advantageous embodiment of the device according to the invention, the control / evaluation unit has stored therein data reflecting a functional relationship between the frequency and the phase of the vibration of the unit that can vibrate under different damping conditions and different viscosities. A storage unit is assigned. The data may be a characteristic curve, shape or measurement.
[0014]
Advantageously, the control / evaluation unit sets at least two mutually distinct phase values. Subsequently, the control / evaluation unit measures the change in the frequency assigned to the phase value of the vibration of the vibrable unit and / or the corresponding frequency change and measures the medium by comparing the predetermined frequency change with the stored data. Is calculated.
[0015]
According to a particularly preferred embodiment of the device according to the invention, the at least two phase values are symmetric with respect to the phase value φ = 90 °.
[0016]
According to a particularly advantageous embodiment of the device according to the invention, the control / evaluation unit determines the range in which the frequency used for measuring the viscosity is present, in which the functional relationship between the phase value and the frequency is substantially linear. Choose to be relevant.
[0017]
According to a preferred embodiment of the device according to the invention, the control / evaluation unit sets at least two different frequencies. Subsequently, the phase assigned to the frequency of oscillation of the oscillating unit between the transmission and the response signal is calculated, and in the last step the control / evaluation unit compares the calculated phase value with the stored phase value. Determines the viscosity of the medium.
[0018]
According to a last-mentioned preferred embodiment of the device according to the invention, a signal generator is assigned to the control / evaluation unit, the signal generator comprising a drive unit and an oscillating unit. The vibration is controlled so as to gradually vibrate at different vibration frequencies (in this case, the vibration frequency is within the range of the selected frequency band) (→ frequency sweep).
[0019]
Furthermore, according to an advantageous embodiment of the device according to the invention, it is possible to configure the oscillating unit as a universal detector. The control / evaluation unit thereby activates the oscillating unit as a limit switch in a first operating mode and as a viscosity sensor in a second operating mode. Each operating mode is set by a program contained in the control / evaluation unit.
[0020]
Advantageously, an input / output unit is provided, via which an adjustment of the device takes place or information about the measurements provided by the device is provided. At least one bus line is provided for data exchange between the oscillating unit and the remotely located control position. The data exchange itself can be performed according to any transmission standard (for example, Profibus PA, Fieldbus Foundation).
[0021]
Next, the present invention will be described in detail with reference to the drawings.
[0022]
FIG. 1 shows three frequency / phase curves of a vibrable unit 2 in a medium having different damping coefficients ξ. The turning point of the three curves is the resonance frequency fr substantially determined by the stiffness of the membrane and the mass of the vibrating member. As is clear from FIG. 1, the phase φ between the drive signal and the response signal of the vibrable unit 2 is in the case of resonance at 90 °. In the case of a low attenuation (attenuation coefficient ξ1), a slight phase change df already causes a rapid phase change of 180 ° (the phase change is performed rapidly). In the case of larger attenuation coefficients ξ2 and ξ3, the phase change from 0 ° to 180 ° is performed to a greater or lesser extent. In a certain frequency and phase range, the frequency / phase curve shows a linear course, where the slope depends on the attenuation by the medium.
[0023]
FIG. 2 illustrates the dependence of the viscosity η on the frequency difference df between the drive signal and the response signal using a logarithmic scale. The curve family is a graph at different phase shifts df (φn−φm), where n, m∈N, n ≠ m. The frequency change df at a given phase shift df (φn−φm) shows a clear dependence on the viscosity η. Thus, according to a first alternative embodiment of the device 1 according to the invention, it is possible to determine the viscosity η by measuring the frequency difference df at least two predetermined phase values φ1, φ2.
[0024]
The effect of the density ρ is visualized on the basis of the frequency / phase curve shown in FIG. 3 for a vibrable unit 2 in media having different densities ρ. Different densities ρ cause a parallel shift of the frequency / phase curve along the frequency axis f. The higher the density φ, the smaller the vibration difference at the same phase value φ. The shape of the curve itself is almost identical in all cases. According to the invention, the relative value (frequency change or phase change), rather than the absolute value, is used for the evaluation of the viscosity η, so that the effect of the changing density ρ on the measured value is automatically eliminated. You.
[0025]
FIG. 4 shows a block diagram of a first embodiment of the device 1 according to the invention. According to the first embodiment, two predetermined phases φ1 and φ2 are progressively set between the driving vibration and the response signal. The setting of the two phase values φ1 and φ2 is performed via an excitation circuit 9 which will be described in more detail below. Subsequently, the frequency values f1, f2 combined with the phase values φ1, φ2 are determined. Subsequently, the viscosity η of the medium is calculated by using the stored data according to the frequency change df = f2−f1. This first mode for viscosity measurement has many similarities to the method by which it is possible to confirm that a predetermined filling height has been achieved using a vibration detector. The only difference is that, in principle, only the natural or resonant frequency of the oscillating unit 2 is taken into account in the case of a filling surface measurement, whereas at least two of the oscillating units 2 in the case of a viscosity measurement. Two phase values φ1, φ2 and different frequencies f1, f2 and / or corresponding frequency changes df = f2-f1 are taken into account.
[0026]
Due to this high degree of similarity, it is also relatively easy to configure the vibrable unit 2 as a universal sensor for measuring the filling surface, density and / or viscosity. The filling surface is usually calculated by monitoring the resonance frequency fr, as described above. The measurement of the viscosity η is preferably carried out by setting two different phase values φ1, φ2 and calculating the corresponding frequency and / or the corresponding frequency change df = f2-f1. The frequency change df = f2−f1 at the predetermined phase values φ1 and φ2 functionally depends on the viscosity η.
[0027]
The vibrable unit 2 is excited via a piezoelectric excitation / vibration unit including a disk-shaped piezoelectric element 5, a drive electrode 6 and two reception electrodes 7 in the illustrated case. In this case, the piezoelectric element 5 functions as a mechanical part of the vibrable unit 2, that is, an interface between the membrane 4 and the vibration element 3, an electric part, and the drive electrode 6 and the reception electrode 7. The piezoelectric element 5 converts an electric drive signal into a mechanical signal on the one hand, and converts a mechanical signal into an electric response signal on the other hand. Of course, it is obvious that a so-called stack drive can be used instead of the disk-shaped piezoelectric element 5.
[0028]
FIG. 5 shows a block diagram of the excitation circuit 9 used in FIG. The excitation circuit 9 has a plurality of functions as apparent from the block diagram shown in FIG. The circuit picks up the reception signal Rx at the reception electrode 7. The response signal Rx is guided through the band pass filter 13. The bandpass filter 13 advantageously has a very small bandwidth, so that only one or more desired frequencies are present at the output of the bandpass filter 13. Subsequently, the filtered response signal Rx is supplied to the amplifier 14 and amplified. In the case shown, two fixed phase values φ1 and 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 signal 16 as the drive signal Tx, and the oscillating unit 2 is excited with the set phase values φ1 and φ2, respectively.
[0029]
From the excitation circuit 9, the response signal Rx reaches the microprocessor 10, which calculates the frequencies f1, f2 corresponding to the respective phase values φ1, φ2. Subsequently, the frequency difference df = f2-f1 is determined and compared with the corresponding data stored in the storage unit 11. Based on the apparent functional relationship between the frequency change df and the viscosity η, the respective viscosity η of the medium can be calculated. The calculated viscosity η of the medium can be recognized by the operator via the input / display unit 12, for example. Of course, the calculated viscosity value can also be used to activate the control element.
[0030]
According to an alternative embodiment of the device 1 according to the invention, the frequency f is varied within a predetermined frequency band. Therefore, the vibrable unit 2 is driven at a different frequency (→ frequency sweep). Different phase values are assigned to different frequencies. The continuous course of a certain frequency band is shown graphically in FIG. FIG. 7 shows a block diagram of the device 1 according to the invention.
[0031]
In a second embodiment of the device according to the invention, two frequencies f1, f2 belonging to the previously fixed phase values φ1, φ2 are localized during the frequency sweep. In particular, certain frequency ranges f1, f2 for this purpose are swept in successive steps. As soon as the previously fixed phase values φ1, φ2 are measured, the frequencies f1, f2 belonging to the phase values φ1, φ2 are calculated. Subsequently, the viscosity η of the medium is determined based on the frequency difference df = f2-f1.
[0032]
The oscillating unit 2 is excited by a signal generator 19 with a drive signal Tx of a predetermined frequency and preferably of a predetermined amplitude. The drive signal Tx is supplied to a signal matching unit 18, which processes the signal so that it can be read by the receiving unit 21. Accordingly, the receiving unit 21 receives the response signal Rx of the oscillating unit 2 and the phase meter 22 determines a corresponding phase shift between the drive signal and the response signal. The control unit 20 is responsive to all processes for calculating the frequency change df. This performs a phase comparison, controls the frequency of the signal generator 19 and finally calculates the corresponding frequency difference df. Subsequently, the viscosity η of the medium in the converter 23 is determined based on the calculated frequency change df. For this purpose, stored table values, characteristic curves or equations are used.
[Brief description of the drawings]
FIG.
FIG. 3 shows a frequency / phase curve of a vibrable unit at different damping coefficients.
FIG. 2
5 is a graph showing the dependence of viscosity on frequency change.
FIG. 3
FIG. 4 shows frequency / phase curves at different densities of the medium.
FIG. 4
1 is a block diagram of one embodiment of the device according to the present invention.
FIG. 5
FIG. 5 is a block diagram of an excitation circuit used in FIG.
FIG. 6
5 is a graph showing a frequency / phase curve visualizing a frequency sweep at two predetermined frequencies.
FIG. 7
FIG. 4 is a block diagram of a second embodiment of the device according to the present invention.
[Explanation of symbols]
1 Apparatus according to the present invention, 2 Vibrating unit, 3 Vibrating element, 4 Membrane, 5 Piezoelectric material, 6 Excitation electrode, 7 Receiving electrode, 8 Control / evaluation unit, 9 Excitation circuit, 10 Microprocessor, 11 Storage unit, 12 Display unit, 13 band pass filter, 14 amplifier, 15 phase shifter, 16 amplifier, 17 low pass filter, 18 signal matching unit, 19 signal generator, 20 control unit, 21 receiver, 22 phase meter, 23 converter, 24 bus line , 25 control position

Claims (12)

A vibrating unit, a driving / receiving unit and a control / evaluation unit, wherein the vibrating unit is arranged in a defined measuring position inside the container, or the vibrating unit is moved to a defined immersion depth Measuring the viscosity of the medium in the container, which is installed so that it is immersed in the medium and the drive / receiver unit excites the vibrable unit or the drive / receiver unit receives the vibration of the vibrable unit And / or a monitoring device, characterized in that the control / evaluation unit (8) measures the viscosity (η) of the medium according to the frequency / phase curve (φ = g (f)) of the vibrable unit (2). A device for measuring and / or monitoring the viscosity of a medium in a container. 2. The drive unit (5, 6, 7) excites the oscillating unit (2) in a predetermined oscillating mode, wherein the oscillating mode is preferably the fundamental mode of the oscillating unit (2). The described device. 3. The device according to claim 1, wherein the drive / receiver unit (5, 6, 7) is a piezo drive in contact with a membrane (4) fixed to at least one vibrating element (3). A storage unit (11) is assigned to the control / evaluation unit (8), in which a vibration of the unit (2) that can vibrate at different damping rates (ξ) or different viscosities (η) is stored. Apparatus according to claim 1, 2 or 3, wherein data reflecting a functional relationship between frequency (f) and phase (φ) is stored. The control / evaluation unit (8) sets two phase values (φ1, φ2) sufficiently different from each other,
The control / evaluation unit (8) measures the frequencies (f1, f2) or the corresponding frequency changes (df) assigned to the vibration phases (φ1, φ2) of the vibrable unit (2) and controls / evaluates Apparatus according to claim 4, wherein the unit (8) calculates the viscosity (η) of the medium by comparing the calculated frequency change (df) with the stored data. 6. The device according to claim 5, wherein at least two phase values (.phi.1, .phi.2) are symmetric with respect to the phase value .phi. = 90. The control / evaluation unit (8) determines the range in which the frequency (f) to be employed for measuring the viscosity (η) is present, such that the functional relationship between the phase value (φ) and the frequency (f) is substantially linear. The device according to claim 5 or 6, wherein the device is selected to be: The control / evaluation unit (8) sets at least two different frequencies (f1, f2);
The control / evaluation unit (8) measures the phase values (φ1, φ2) assigned to the vibration frequencies (f1, f2) of the vibrable unit (2), and the control / evaluation unit (8) is calculated. 5. The apparatus according to claim 4, wherein the viscosity ([eta]) of the medium is calculated by comparing the phase values ([phi] 1, [phi] 2) with the stored data. A signal generator (19) is assigned to the control / evaluation unit (8), the signal generator oscillating the drive unit (6) and the oscillating unit (2) oscillating at progressively different oscillation frequencies. 9. The apparatus according to claim 8, wherein the vibration frequency is within at least one selected frequency band. 10. The control / evaluation unit (8) operates the vibrable unit (2) as a limit switch in a first operating mode and as a viscosity sensor in a second operating mode. Equipment. 9. An input / output unit (12), via which an adjustment of the device (1) is made or information about the measurements supplied to the device is provided. Or the apparatus according to 10. 5. The device according to claim 4, wherein at least one bus line (24) is provided, via which the control / evaluation unit (8) communicates with a remotely located monitoring position (25).