DE3618798A1 - Method for measuring a physical quantity influencing the inherent oscillation of a measurement transducer - Google Patents

Abstract

The invention relates to a device or a method for measuring a physical quantity influencing the inherent oscillation of a measurement transducer either directly or via a flowing medium, such as, for example, pressure, force, temperature, density, acceleration, the measurement transducer being excited into natural oscillations in a defined mode of oscillation via an exciter system with feedback and the frequency of the natural oscillation being picked up via a pick-up system and supplied to a measuring arrangement for evaluation. According to the invention, for eliminating interfering influences due to a change in the influencing variables, such as the measurement transducer is excited into a number of vibration modes, for example in a flowing medium during the measurement of the pressure for compensating for changes in the temperature and the density of the medium, the frequency changes influenced by the physical quantities to be measured or, respectively, interfering are detected separately and supplied to the measuring arrangement in which the dependence of the frequency changes on measuring and/or possible interference variables for each vibration mode is stored, and are evaluated.

Description

The present invention relates to a method for Measure the natural vibrations of a transducer directly or physical influencing via a flowing medium Size, such as B. pressure, force, temperature, density, the acceleration etc.

It is known and is used in particular for measuring air pressure (Altimeter in aircraft) applied the influence of the Pressure on the natural vibration of a transducer, e.g. B. one To determine pressure sensor in the form of a cylinder that the transducer is excited to natural vibrations and the frequency change is evaluated to determine the pressure. The air whose pressure is to be measured is turned on fed to one side of the cylinder wall, while on the other side a defined comparison print, for example there is also pressure 0.

Instead of a pressure sensor designed in the form of a hollow cylinder can anyone else capable of natural vibrations Body in the form of a cuboid, a lamella, a disc etc. made of any elastic material, such as steel, quartz or the like can be used for the same purposes if it is only ensured that the medium is more well known on one side Density, the pressure of which is to be measured can, while on the other hand a more defined and well known Pressure, e.g. B. the pressure 0 is also present.  

In the same way, with such a measuring arrangement at a defined pressure also the density or via the change in density measured the temperature of air or any other fluid will.

Measuring arrangements for the purposes mentioned are different Transducers and with different construction known. A transducer in the form of a cylindrical hollow body is described for example in DE-PS 19 43 925.

In the manufacture of such measuring arrangements and in particular The evaluation of the measured values must have different conditions are fulfilled and interference is eliminated.

First, the transducer itself must be manufactured with high accuracy be, relatively expensive material with a low elasticity temperature coefficient can be used must to largely eliminate the influence of the inherent temperature. In addition, there is especially when measuring the pressure of air the peculiarity that changes in density and thus also Changes in temperature of the air to be measured for a Natural vibration typical frequency change with go.

Conversely, such a measuring arrangement for determining the Density or the temperature of the fluid used, then lead pressure and temperature changes or pressure and Changes in density in the fluid to be measured lead to errors poorly compensated with conventional means and in special can also not be excluded.

With a measuring arrangement for measuring the pressure one becomes accordingly when building the transducer try this to be dimensioned such that, for example, the density of the flowing Medium as little influence on the natural vibration exercises and you will try to keep the temperature as constant as  only possible to keep even temperature changes, which in turn again could lead to errors, largely excluded.

It is readily understood that total elimination the influence of the density of the flowing medium at all is eliminated, while a temperature stabilization only with great effort is possible. Naturally, each of them is one Temperature measurement dependent temperature stabilization extremely sluggish and both when commissioning a measuring arrangement as well as sudden changes in ambient temperature cannot be realized with the necessary accuracy. Considered one that such pressure sensors, for example, as an altimeter to be used in aircraft, then it is easy can be seen that a temperature stabilization because of the large Inertia practically eliminated.

It has already been suggested the temperature in the arrangement to measure and correct the measured values accordingly. A such measure is also inaccurate because the measurement the temperature at a transducer to be executed is difficult to implement and also the measurement at no point guarantee for the temperature distribution of the vibrating transducer and in particular the transducer influencing air there.

In doing so, an additional Temperature sensors, e.g. B. with a resistance thermometer, the Temperature of the arrangement (or of the fluid) is recorded and evaluated.

The evaluation of both output signals, the frequency signal f and the temperature signal, for example a voltage u , enables a back calculation to be made to the pressure p to be measured, the temperature influence being compensated in principle.

The back calculation is carried out using a suitable interpolation algorithm, which uniquely assigns a pressure value p to each frequency value f and each temperature-proportional value u :

p = p ( f , u )

The data required for the interpolation algorithm, d. H. Polynomial coefficients, or support points, are represented by a Calibration measurement determines and is available, for example in a computer Available.

This calibration measurement, the calibration, is used for several Pressure and temperature values the resulting frequencies and Detected voltages of the converter.

Such a method of temperature compensation is already in use some commercially available precision sensors, in particular applied to precision pressure sensors.

The problem with this method is that the temperature sensor cannot be attached directly to the vibrating part of the transducer, because this would undisturbed the natural vibrations and their vibration quality Q could drop to an inadequate value.

The temperature sensor is therefore usually a piece away from the vibrating part and is with this by a sluggish thermal delay line connected. This has to Consequence that between the vibrating membrane and the temperature measuring point a sometimes significant temperature gradient occurs, which falsifies the temperature compensation and a corresponding one Pressure measurement error results.  

With disturbances of the stationary thermal equilibrium, however even with a stationary heat flow along this route pressure measurement errors of up to a few percent were observed.

So there is no satisfactory temperature compensation for as long the temperature sensor is not the temperature distribution of the vibrating Elements recorded itself.

Another suggestion, the aforementioned disruptive influences To compensate for a pressure transducer is in DE-OS 28th 36 286 made. It is stimulated there, two concentrically built Provide pressure cylinders that can be excited to resonate vibrations and the two resulting resonance frequencies against each other to switch in order to influence influences of temperature and density to eliminate or at least mitigate.

Such an arrangement also has the disadvantage that that especially at the start of a measurement is by no means the guarantee it is given that the respective fluid and the two Pressure sensors have exactly the same temperature, so one full compensation of the influence of temperature reached can be.

The object of the invention is to provide a method according to. which by special excitation of the natural vibrations Transducer and by evaluating the frequencies influences of physical quantities that are not currently measured should be eliminated.

This task is accomplished with a method of measuring a Natural vibration of the transducer influencing physical Size solved with the features of claim 1.

Particularly advantageous embodiments of the method are Subject of the subclaims and claim 4 disclosed a measuring arrangement for carrying out the named method,  the sub-claims 5 to 7 advantageous constructive and include circuitry measures.

In the method according to the invention is therefore one and the same Transducers excited to several different vibration modes.

This means that in the case of temperature compensation instead an additional temperature sensor another natural vibration of the transducer and their frequency evaluated becomes.

This additional frequency signal contains the required information about the integral temperature distribution of the vibrating Elements and thus allows full temperature compensation.

A temperature gradient between the vibrating element and the temperature sensor is in principle excluded in this way.

The invention is based on the knowledge that different Vibration modes of a suitable for natural vibrations Oscillators each have orthogonal eigenstates (eigenvectors) are generally independent of each other on external interference react in the form of frequency changes. With a transducer can be accordingly because of the orthogonality with a sufficiently high quality factor, several modes at the same time and stimulate independently.

Based on this knowledge, up to N physical quantities can be detected and evaluated in a conventional manner with one and the same measuring transducer, for example a measuring cylinder that carries out natural vibrations when N modes are excited.

The invention is further preceded by the knowledge that, if one is interested in only one variable, a measured variable can be acquired by exciting n + 1 modes and, at the same time, n disturbance variables can be compensated for. If, for example, a transducer is excited in two modes, the pressure could be measured, for example, by changing the frequency of one of the oscillation modes, and the temperature change and thus the measurement result for the pressure could be completely compensated for by evaluating the frequency change of the other oscillation mode.

If the same cylinder was excited in three modes, it would be an insert possible as a pressure sensor, whereby the measured value is completely temperature and could be compensated for density. With the same measuring device would be in three modes upon excitation of the vibrating cylinder then a density measurement is also possible, which is full temperature and pressure can be compensated.

Such compensation is possible because each oscillating transducer changes the natural frequency of its modes with the differential pressure of the resonating medium (gas, air), the density and the temperature in each mode in its own way, the pressure, density and Temperature sensitivity of the individual modes are independent of each other. Using the example of the pressure-frequency characteristics of different eigenmodes of an oscillating cylinder, this is explained in detail below in connection with the drawing ( FIG. 7). So if a transducer is excited in two modes, hereinafter called α mode and β mode, then according to. According to the proposal according to the invention, the two frequency signals f _α and f _β influenced by the medium to be measured are evaluated separately and converted in the measuring arrangement with the aid of the computer to the desired compensated value, the pressure p , the computer having access to the associated calibration data.

The calibration is carried out exactly as explained, except that the two frequency signals f _α and f _β are now used for the back calculation to the pressure.

p = p ( f _α , f _β )

From the above it follows that when excited in three modes also complete elimination of density at one Pressure or temperature measurement is possible.

In the preceding explanations it is assumed that one and the same transducer absolutely simultaneously in the different Modes excited and the frequency changes of each Vibration modes can be removed accordingly at the same time. However, it should be noted that for the purposes of Invention also an immediately successive excitation can be performed at different vibration modes, if only the two or the three vibration modes during one and the same measuring process is constructed and evaluated accordingly will.

So far so far and in the following the method according to the invention using a cylindrical sensor, which is usually is made of steel, has been explained, this is only an example.

It is obvious to the person skilled in the art that the function is independent of the material, the shape and the clamping or storage of the vibrating element.

The following are the general statements above deepened in connection with the attached drawings.

The following explains in detail using these drawings:  

Fig. 1 to 6 definition of the vibration modes;

Fig. 7 pressure-frequency characteristics of different vibration modes;

Fig. 8 is the evaluation of the measurement results on the basis of a block diagram of an arrangement;

FIGS. 9 to 10 of the basic structural design of a hollow cylinder-pressure sensor as a transducer for the purposes of the invention, when excited in two modes.

On the basis of the schematic diagrams shown in FIGS. 1 to 6, the terms of the vibration modes that are also used below are to be defined first.

For this purpose, Fig. 1 2 initially shows schematically a vibration cylinder in longitudinal section and FIGS. The associated cross-section. This vibration cylinder has a length ε and a diameter d .

Because of the high sensitivity to pressure, cylindrical Membranes prefer the excited radial bending modes.

This creates longitudinal, corresponding to the cylindrical symmetry the cylinder axis and over the circumference antinodes that are separated from each other by nodes.

The number of node lines parallel to the axis and along the circumference is used to designate the radial modes. The number of half-waves over the cylinder axis is usually designated by m and half the number of circumferential nodes by n .

A certain radial mode is then identified by the pair of values ( m , n ), e.g. B. ( 3, 4 ) for a mode with m = 3 and n = 4. A section through the mode shape with m = 1 or n = 2 or 4 shown in FIGS. 3 to 5.

FIGS. 6a and b show a 3-dim. Representation of the modes ( 1 , 4 ) ( Fig. 6a) or ( 2, 4 ) ( Fig. 6b).

Excitation systems for the generation of different vibration modes are well known. The suggestion is made generally electromagnetic. It should be noted that also capacitive, piezo-electric or even acoustic Suggestions and corresponding vibrations are definitely possible are. Any vibration condition, for example bending, stretching or torsional vibrations but also surface or density waves are excited.

Fig. 7 shows the pressure-frequency characteristics of different vibration modes of one and the same cylinder with certain geometric dimensions.

The square of the Apply frequency over pressure. It's easy to see that each mode has its own pressure-frequency characteristic and thus has its own pressure sensitivity.

But it should be pointed out again that with others Geometric dimensions of the cylinder result in curves that not only in the slope, but also in their relative Distinguish location from those shown.

The figure shows an example of the one already mentioned Fact that different vibration modes one to natural vibrations suitable oscillator for external interference in general react independently in the form of frequency changes. This applies equally to the density and thus also temperature characteristics of different modes on and the same oscillator, especially for a vibrating one  Cylinder. There are e.g. B. cylinder modes with relatively high pressure sensitivity but comparatively small sensitivity to density or temperature and vice versa. With the help of such for A computer can then use a specific system to determine curves immediately the corrected pressure, temperature or density values determine, two physical signals from two frequency signals Sizes and three physical signals from three frequency signals Sizes can be calculated. In practical operation must then a computer only has this calibration data available use the program entered in ihme to determine the actual to determine undistorted values and, if necessary display.

An arrangement for carrying out the method then, as is clearly shown in FIG. 8, initially only requires a transducer 8 , for example in the form of a cylindrical pressure sensor, to which the flowing medium with the pressure to be measured is fed from below. It is assumed that a disturbance variable 7 acts on the cylinder in the form of temperature changes.

According to the invention, the cylinder is excited in two modes, the frequencies f _α and f _{β then being} taken off and fed to the microprocessor 9 . The microprocessor 9 has access to the calibration data, which are stored, for example, in an EPROM 10 and, depending on the frequency that has been removed and digitized accordingly, calls this data to determine the adjusted pressure value. The actual pressure p _corr is then output accordingly and displayed in a manner known per se.

The excitation of the transducer to natural vibrations in different Vibration modes can be done by the person skilled in the art without difficulty will be realized.  

For reasons of completeness, an arrangement for exciting a cylindrical pressure sensor to the vibration modes m = 1, n = 4 and m = 2, n = 4 is explained with reference to FIGS. 9 and 10.

The two vibration modes are shown in FIGS. 6a and 6b.

The Figure 9 shows. Only schematically illustrates the structure of the printing cylinder without structural details and without playback of individual parts that are important for the understanding of the invention without meaning, and Figs. 10a to 10c the structure of the excitation and reading systems.

In the embodiment according to FIG. 9, a pressure cylinder 1 is firmly clamped in a holder 2 , which has a through opening 3 for the air, the pressure of which is to be measured. At the top, the pressure cylinder is sealed airtight by a cover 4 . The operation of such a pressure cylinder is known. The air, the pressure of which is to be measured, is conducted into the interior, while a defined atmosphere, e.g. B. the pressure is 0.

This pressure difference creates an additional tension in the cylinder wall, which is a corresponding change in cylinder frequency entails.

This change in frequency changes to the pressure value to be measured closed.

Two excitation systems, 11 and 14 , and two pickup systems 12 and 13 are arranged inside the cylinder.

The modes m = 1, n = 4 and m = 2, n = 4 are selected in accordance with the symmetry of the sensor arrangement 12 and 13 . Modes with m = 3, 4, 5. . . and n = 8, 12, 16,. . . require much larger excitation forces and are therefore strongly damped.

The two selected modes, ( 1, 4 ) and ( 2, 4 ) are distinguished by sum or. Forming the difference between the two of the sensors 12 and 13 induce voltages supplied u ₁₂ and u. ₁₃ The result of the in-phase combination of 12 and 13 , u _α = u ₁₂ + u ₁₃ corresponds to the mode ( 1 , 4 ) ( FIG. 6a) while the antiphase combination, u _β = u ₁₂ - - u ₁₃ the mode ( 2, 4 ) ( Fig. 6b).

The movement amplitudes obtained in this way, u _a and u _β , are amplified, filtered if necessary, and supplied to the two excitation systems 11 and 14 according to the phase.

Standard circuits for such a tracking are known.

The arrangement of the excitation systems 11 and 14 also contributes to the selectivity.

The formation of sums or differences is not a prerequisite for the simultaneous excitation of two (or more) modes is reduced but the constructive effort for their geometric Selection.

In FIGS. 10a to 10d, the basic structure of the excitation system is now 11 (Fig. 10a, 10b) and the picker 12 (10d Fig. 10c, Fig.) Is reproduced.

In this case, in a longitudinal section, Fig. 10a shows the structure of the excitation system 11, using 111 with a coil 112, a permanent magnet, pole pieces 113 and 114, the coil housing is designated.

Fig. 10b shows the section corresponding to the dotted line in Fig. 10a.

Fig. 10c shows a longitudinal section through the pickup assembly 12 and
Fig. 10d is a top view. Here, the numeral in the last place are analogous to the numerals in Figs. 10a and 10b selected, that is, by 121 is again the coil winding 122, a permanent magnet, pole shoes 123 and 124 of the coil support respectively.

The invention has been made relatively concrete using a hollow cylinder excitable for natural vibration explained.

The preceding general explanations show the person skilled in the art but that the same compensation principle in all vibratable Forms such as strings, beams, membranes or cuboids can be applied by, for example, a membrane Vibration modes of different symmetry are pressed.

The body made of steel, which can be excited to vibrate naturally, Quartz, glass or another highly elastic material.

That it can also be recommended for the customer systems different types for mutual decoupling to apply, for example a system works electromagnetically, the other capacitive or piezo-electric or in Borderline case, even acoustically, has already been mentioned. Are quite different types of vibration, such as bending vibrations, Expansion vibrations, impression vibrations or even Surface vibrations can be used at the same time because, as stated, these vibrations with sufficient quality of the vibration system do not influence each other.  

For the sake of completeness, it should be pointed out again that the selectivity of a transducer / exciter system alone depends on its symmetry and that the sum- or difference formation of the signals from subsystems only serves the constructive effort to excite two or reduce three modes.

Claims (7)

1. A method for measuring a physical quantity influencing the natural vibration of a transducer directly or via a flowing medium, such as, for. B. pressure, force, temperature, density, acceleration, wherein the transducer is excited via a feedback excitation system to natural vibrations in a defined vibration mode and the frequency of the natural vibration is removed via a transducer system and fed to a measuring arrangement for evaluation, characterized in that for the elimination of Interfering influences by changing the influencing variables, such as in the flowing medium when measuring the pressure to compensate for changes in temperature and the density of the medium of the transducers, are excited to form a plurality of vibration modes that separate the frequency changes influenced by the physical variables to be measured or disturbing detected and the measuring arrangement, in which the dependence of the frequency changes on measurement and / or possible disturbance variables for each vibration mode is stored, supplied and evaluated. 2. The method according to claim 1, characterized in that the transducer simultaneously to different vibration modes is excited. 3. The method according to claim 1, characterized in that the transducer the different vibration modes in Multiplex operation in short consecutive periods be pushed open.   4. Measuring arrangement for performing the method according to one of claims 1 to 3 for measuring physical quantities of a flowing medium, such as pressure, temperature or density by means of a transducer which can be excited to natural vibrations, the body which can be excited to natural vibrations, e.g. B. in the form of a membrane, a cuboid or a hollow cylinder, on one side the medium whose properties are to be measured is supplied, and on the other side a comparison medium with known properties or vacuum is present, characterized in that at least two of them Decoupled excitation / pickup systems ( 11, 12, 13, 14 ) are provided for the simultaneous and independent excitation of vibrations of different modes, that an evaluation system is also provided, which for evaluating the signal values supplied by the pickup system by means of calibration data ( 10 ) accessible by the evaluation system a computer ( 9 ) is assigned to individual vibration modes. 5. Measuring arrangement according to claim 4, characterized in that a cylindrical hollow body made of elastic material is provided as the transducer, the interior of which is sealed against the surrounding space, and that two pairs of excitation / pickup coils ( 11, 12, 13, 14 ) for excitation are provided in two different modes, the coil axes of the excitation coils ( 11, 14 ) on which the pickup coils ( 12, 13 ) are perpendicular for the purpose of electromagnetic decoupling. 6. Measuring arrangement according to claim 4 or 5, characterized in that for the purpose of simultaneous excitation of two cylinder modes for the geometric selection of the modes to be excited special pole shoe shapes ( 10 a - 10 d ) and a certain spatial arrangement ( Fig. 9) of the excitation / Transducer systems ( 11, 12, 13, 14 ) are provided. 7. Measuring arrangement according to claim 4 to 6, characterized in that the geometric selection of the two modes to be excited by the sum or difference formation of the signals of the two sensor systems ( 12 and 13 ) and the two excitation systems ( 11 and 14 ) is effected.