Method and circuit arrangement for operating a vibration sensor with integrated temperature detection

Abstract

A method for operating a vibration sensor (1) with integrated temperature detection, comprising a membrane (5) that can be set into vibration, a piezoelectric drive (3) for setting the membrane (5) into vibration, and for detecting vibrations of the membrane (5), comprising the following steps: - cyclically exciting the piezoelectric drive (3) with a predetermined excitation signal and causing vibration of the membrane (5), subsequently detecting vibrations of the membrane (5) by the piezoelectric drive (3), wherein the excitation and detection steps are carried out continuously, characterized in that - an output voltage of the piezoelectric drive (3) is detected as a received signal (E), - an area of a portion of the received signal (E) caused by the excitation signal (A) is at least approximately determined,- a temperature-dependent capacitance (Cges) of the piezoelectric drive (3) is determined from the value of the area and - the temperature (T) of the piezoelectric drive (3) is determined from the capacitance (Cges).

Description

[0001] The present invention relates to a method for operating a piezoelectrically operated vibration limit switch with integrated temperature detection according to the preamble of patent claim 1 and to a circuit arrangement for temperature detection in such a vibration limit switch according to the preamble of patent claim 10.

[0002] Vibration sensors, used, for example, as vibrating limit switches, are known from the prior art. The vibration sensor comprises a membrane that can be excited to oscillate via a drive, by means of which a mechanical oscillator arranged on the membrane can be excited to oscillate. Depending on the coverage level of the mechanical oscillator with a filling material, as well as the viscosity and density of this filling material, the mechanical oscillator oscillates at a characteristic frequency, which can be detected by the vibration sensor and converted into a measurement signal.

[0003] In Fig. 11 shows a piezoelectrically driven vibration sensor 1 known from the prior art, which can be used in particular as a vibration limit switch.

[0004] The vibration sensor 1 has a membrane 5 that can be excited to vibrate via a drive 3, wherein the drive 3 comprises a plurality of ring-shaped piezo elements 7 with an opening 9 and an electrical contact between the piezo elements 7. Typically, the piezo elements 7 are tensioned against the membrane 5 via a tension screw 19 by a tension bolt 17 arranged on the membrane 5 of the vibration sensor 1, which extends through the opening 9 of the piezo elements 7, so that a vibration of the piezo elements 7 generated via an electrical signal is reliably transmitted to the membrane 5. Typically, an adaptation ceramic 13 and a tension disk 14 are arranged between the piezo elements 7 and the membrane 5, which ensures a targeted introduction of the mechanical vibration into the membrane 5 and electrical insulation of the piezo elements 7 from the typically metallic membrane 5, as well as thermal adaptation.Analogous to the matching ceramic 13 and the clamping disk 14 between the membrane 5 and the piezo elements 7, a further matching ceramic 13 and a clamping disk 14 can also be provided between the clamping screw 19 and the piezo elements 7, so that the clamping screw 19 is also electrically insulated from the piezo elements 7.

[0005] The piezo elements 7 can be insulated from the tension bolt 17 by a sleeve 10 arranged in the opening 9, not shown in detail here.

[0006] Electrical contacting of the piezo elements 7 is effected via electrodes 8 arranged on the surfaces of the piezo elements 7 and contacted from the outside via cables 25.

[0007] On a side of the membrane 5 facing away from the drive 3, two mechanical oscillators 11 designed as paddles are typically arranged, which transmit the oscillation coupled into the membrane 5 to a medium surrounding the mechanical oscillators 11.

[0008] Vibration sensors of the type described above, especially vibrating level switches for liquids, operate according to the principle of resonant frequency shift. The vibrating level switch oscillates at a different resonant frequency and amplitude depending on the coverage, density, temperature, and pressure of the medium. The amplitude of the resonant frequency depends on the viscosity of the medium. The frequency shift depends on the density and temperature of the medium, as well as the prevailing process pressure.

[0009] In order to compensate for the influence of temperature on the resonance frequency, the temperature at the piezo drive and / or the vibration element must be known.

[0010] The influence of temperature on the resonance frequency is known from the material constants of the materials used, especially the membrane and the mechanical oscillators. If the temperature is known, a resonance frequency shift caused by temperature influences can be compensated.

[0011] In the state of the art, a temperature sensor is installed in the drive, Fig. 11 designated 22, e.g. type PT100 installed.

[0012] Since the aforementioned vibration sensors are considered disadvantageous because they require a temperature sensor and thus an additional component with additional cabling, vibration sensors with integrated temperature detection are now available. One such vibration sensor with integrated temperature detection is described, for example, in the applicant's EP 3 312 574 A1.

[0013] The vibration sensor described therein comprises a piezo stack drive with at least two piezo elements made of different piezoelectric materials connected in series, with a temperature being determined by detecting the capacitance of at least one piezo element. Such temperature determination is possible because the piezoelectric materials used between the electrodes for exciting the piezo elements act like the dielectric of a plate capacitor, and their relative permittivity in the 33 direction (and possibly in other directions) exhibits a temperature dependence, which also allows temperature determination through a capacitance measurement.

[0014] The dielectric constant ε or the relative dielectric constant ε r is direction dependent. ε rdescribes the ratio of the dielectric constant ε of the piezoelectric material and the dielectric constant ε0, where the dielectric constant ε is a measure of the polarizability of the piezoelectric material in the electric field. The dependence of the relative dielectric constant ε r of the direction of the electric field and the dielectric displacement is also indicated by corresponding indices. ε 33 describes the dielectric constant ε in the polarization direction when an electric field is also applied in the polarization direction.

[0015] To determine directions in relation to the piezo elements used, as shown in Fig. 10, axes 1, 2, and 3 are introduced, which are aligned analogously to the X, Y, and Z axes of the Cartesian coordinate system. In addition, the numbers 4, 5, and 6 indicate the directions of rotation around the respective axes, also as in Fig. 7. Axis 3 is aligned along the polarization direction of the respective piezoelectric element. Since the piezoelectric material is anisotropic, the corresponding physical quantities are described by tensors and indexed according to the axis numbering above.

[0016] When using correspondingly different materials, it is advantageously described in the prior art if a first relative permittivity of the first piezoelectric material and a second relative permittivity of the second piezoelectric material have different temperature dependences. In particular, it is advantageous if the first relative permittivity in the 33 direction and the second relative permittivity in the 33 direction have different temperature dependences.

[0017] If in the present application a dielectric constant is mentioned without explicit indexing or direction indication, in these cases the dielectric constant in the 33 direction is meant, ie parallel to the polarization of the piezo element and in a direction perpendicular to the membrane of the vibration sensor.

[0018] To ensure the best possible temperature detection using a capacitance measurement of the drive as a whole or of an individual piezoelectric element, it is described in the prior art as advantageous if the second relative permittivity in the temperature range to be measured exhibits a relative change of more than + / - 10%, preferably more than + / - 15%, in particular in a range from -10% to +30%, preferably in a range from -10% to +50%. A correspondingly large change in the permittivity of the piezoelectric element results in a correspondingly large change in the capacitance of a plate capacitor formed with the piezoelectric elements, so that a temperature determination can be carried out with sufficient accuracy.

[0019] Preferably, the relative dielectric constant in the temperature range has a change of at least 30%, preferably at least 50%, in particular at least 80%, whereby this means a change in amount over the entire temperature range.

[0020] In order to achieve sufficient drive power across all temperatures when using a suitable piezoelectric material, it is useful or may even be necessary for the drive to have at least a second piezo element that has a coupling factor that is stable across the entire temperature range.

[0021] The coupling factor k is a measure of the extent of the piezoelectric effect. It describes the ability of a piezoelectric material to convert absorbed electrical energy into mechanical energy and vice versa. The coupling factor is calculated from the square root of the ratio of stored mechanical energy to the total absorbed energy. Under dynamic conditions, the coupling factor k depends on the corresponding vibration mode of the piezoelectric body. The coupling factor of the longitudinal vibration k is particularly relevant for piezoelectric drives of vibration sensors of this type. 33 , ie the extent to which a piezo element converts an electrical voltage applied in the direction of polarization into an extension in the direction of axis 3, ie in the direction of polarization.

[0022] The coupling factor in the 33 direction is defined as follows: k332=Wmech.3Welektr.3=0.5⋅d332⋅U32⋅b⋅h / (S33E⋅1)0.5⋅U32⋅ε33T⋅b⋅h / 1=d332s33E⋅ε33T.

[0023] The formula components used have the following meaning: K 33 - Coupling factor in 33 direction W mech.,3 - mechanical work in 33-direction W elektr.,3 - electrical energy in 33-direction U3 - Electrode voltage in 33 direction b - Piezo width in 22 direction h - Piezo height in 11 direction l - Piezo length in 33 direction d 33 - Piezoelectric charge constant in the 33-direction s 33 E - Elastic compliance constant in 33-direction ε 33 T - Relative permittivity in 33-direction

[0024] To ensure a sufficiently large coupling factor for the entire drive across the entire temperature range, it may be advantageous for the drive to have at least two piezo elements made of the first piezoelectric material. The coupling factors of piezo elements mechanically connected in series add up to form the overall coupling factor of the drive.

[0025] It is advantageous if the coupling factor of the drive is at least 20, preferably at least 25, more preferably at least 35 over the entire temperature range, since in this way a sufficient mechanical excitation of the membrane can be achieved with a given preload of the drive.

[0026] It may be advantageous if the first piezo element has a Curie temperature of at least 300°C, preferably at least 320°C.

[0027] With state-of-the-art vibration sensors with integrated temperature measurement, capacitance determination and thus temperature measurement are only possible when the sensor is at rest, i.e., when the sensor's mechanical vibration has completely subsided. To perform a capacitance / temperature measurement, excitation of the vibration sensor is interrupted, waiting until the sensor's mechanical vibration has completely subsided, then a temperature measurement is performed, and then the vibration sensor is excited again to vibrate, so that point level monitoring is possible again. With these sensors, the decay of the mechanical vibration typically takes between 1 and 5 seconds.A renewed vibration excitation, which allows point level monitoring to resume, takes between 0.1 and 1 second. Therefore, if the measurement time for the capacitance / temperature measurement is negligible, a worst-case scenario would require an interruption of 6 seconds for temperature determination, during which point level monitoring is not possible. In safety-critical applications, this duration is considered significantly excessive and represents a significant competitive disadvantage compared to systems with a separate temperature sensor.

[0028] Further prior art is known from DE 10 2011 090 014 A1 and DE 10 2010 030 791 A1.

[0029] It is the object of the present invention to provide a method for operating a vibration sensor with integrated temperature detection and a circuit arrangement for temperature determination which overcome these disadvantages of the prior art, in particular enabling temperature measurement without interrupting the mechanical oscillation of the vibration sensor.

[0030] These objects are achieved by a method having the features of patent claim 1 and a circuit arrangement having the features of patent claim 10.

[0031] A method according to the invention for operating a vibration sensor with integrated temperature detection with a membrane that can be set into vibration, a piezoelectric drive for setting the membrane into vibration and for detecting vibrations of the membrane, enables a cyclic excitation of the piezoelectric drive with a predetermined excitation signal and causing an vibration of the membrane, subsequent detection of vibrations of the membrane by the piezoelectric drive, wherein the steps of excitation and detection are carried out continuously.

[0032] The method according to the invention is characterized in that an output voltage of the piezoelectric drive is detected as a received signal, an area of a portion of the received signal caused by the excitation signal is determined at least approximately, wherein a temperature-dependent capacitance of the piezoelectric drive is determined from the value of the area and the temperature of the piezoelectric drive is determined from the capacitance.

[0033] By determining the area of the area portion in the received signal caused by the excitation signal, which can be clearly identified as a signal peak in the received signal, a charging process of the plate capacitor formed by the electrodes for contacting the piezo element and the piezoelectric material lying as a dielectric between these electrodes can be understood.

[0034] Since the excitation signal is known, i.e. in particular the signal shape and amplitude or voltage, and the dimensions of the structure used are also known, the capacitance of the plate capacitor can be determined from the area of the signal peak caused in the received signal by a rising or falling edge of the excitation signal, and since the dimensions of the capacitor can be assumed to be constant, the temperature can be determined from a known temperature dependence of the permittivity (dielectric constant) of the piezoelectric material.

[0035] This can be done purely mathematically if the temperature dependence of the permittivity of the piezoelectric material is known by formula, or via empirically determined relationships.

[0036] The formula for calculating the capacitance of a plate capacitor is C=ε0×εr×A / d

[0037] Where ε0 is the dielectric constant, ε ris the relative dielectric constant of the piezo material, A is the area of the electrode and d is the distance between the electrodes.

[0038] From the received signal, the capacity can be determined via the relationship ∫0tU(t)dt=U0⋅R⋅C This means that it can be determined from the voltage curve generated during the charging process of the capacitor, the time t during which the capacitor is being charged, and the voltage U0 applied to the capacitor. The voltage curve can be determined from the received signal; the remaining quantities are known due to the known signal shape.

[0039] Because the capacitance of the above-mentioned plate capacitor or a value that is in a known or at least empirically determinable relationship to it can be determined during ongoing operation of the vibration sensor, a temperature determination can be carried out without interrupting the level measurement.

[0040] In the present application, a vibration sensor with integrated temperature detection is understood to mean a vibration sensor with a membrane that can be set into vibration, a piezoelectric drive for setting the membrane into vibration and for detecting vibrations of the membrane, in which temperature detection is carried out by determining a capacitance of at least one piezo element.

[0041] The mechanical design of the vibration sensor can correspond to the design described in the state of the art. However, a design is also conceivable in which the drive is manufactured and clamped separately and is attached to the membrane via, for example, a thread between a collar arranged on the membrane and the drive.

[0042] Vibration sensors with temperature detection, in which, for example, a temperature sensor is incorporated into the drive, or a thermoelectric voltage is measured on a supply line to the drive via another line, are not considered vibration sensors with integrated temperature detection within the meaning of the present application.

[0043] Determining the enclosed area of the measurement signal over the duration of a rising or falling edge means determining the area enclosed by the measurement signal, which lies on the time axis between the point in time beginning with the rise or fall of the excitation signal until the end of the rise or fall. In this application, a rise or fall, or a rising or falling edge, of the excitation signal is understood to mean a gradient of 5 V per 100 µs.

[0044] The area of a portion of the measurement curve caused by the excitation signal is understood to be the area enclosed by the measurement curve which is caused by the excitation signal in addition to the portion generated by a mechanical vibration of the membrane.

[0045] In a simple embodiment of the method, the area of the portion of the received signal caused by the excitation signal can be determined by determining the area enclosed by the received signal over the duration of a rising or falling edge of the excitation signal.

[0046] Because the excitation signal is generated, for example, using a signal generator, it is known when the excitation signal exhibits rising or falling edges that are applied to the drive. The duration of these rising or falling edges can be used to determine the duration and time for the area determination in the received signal.

[0047] The area can be determined, for example, by approximating the signal peak by inserting or rewriting polygons or by forming upper and / or lower sums. Depending on the temperature dependence of the permittivity and thus the temperature dependence of the capacitance change, it may be sufficient to approximate the size of the area of the signal peak. For example, the area of the signal peak can be approximated by the area of an isosceles triangle with a base defined by the duration of the rising or falling edge of the excitation signal and a height determined by a maximum voltage in the received signal.

[0048] However, other approaches are also possible and conceivable.

[0049] A variant of area determination that is also easy to implement in terms of circuitry is to determine the area by integrating the received signal over the duration of the rising or falling edge of the excitation signal. The integration can be performed numerically using a microcontroller or circuit-wise using an integrator. Alternatively, an appropriately designed ASIC (application-specific integrated circuit) or an appropriately designed FPGA (field-programmable gate array) can be used as application-specific circuits.

[0050] Improved temperature determination is possible if the integral is corrected for the area portion caused by the mechanical vibration of the membrane. Since the integral in the Cartesian coordinate system describes an area lying under a function over a specific section of the x-axis, integrating the received signal also determines an area portion caused by the mechanical vibration of the sensor. To obtain only the portion of the received signal caused by the excitation signal, the integral must be corrected for the area portion due to the mechanical vibration. This area portion can be described by the integral over the mechanical vibration: ∫0tsin(2π⋅fres⋅Φ)dt

[0051] Where f resthe mechanical resonance frequency of the membrane and mechanical oscillator and Φ the phase shift of the mechanical oscillation relative to the excitation signal.

[0052] It should be noted at this point that in the above-mentioned area determination, DC components of the received signal, e.g. a DC voltage component of the received signal, must be compensated.

[0053] The portion of the received signal caused by the mechanical vibration of the membrane is usually sinusoidal. Due to the position of the rising or falling edges of the superimposed excitation signal near the inflection points of the sinusoidal signal, the area portion of the received signal caused by the mechanical vibration of the vibration sensor can be approximated, for example, by a triangle. More precisely, the area portion can be calculated by integrating the sine function caused by the mechanical vibration of the membrane over the duration of the rising or falling edge of the excitation signal.

[0054] To reduce the computational or circuitry effort, temperature determination can only be carried out every hundredth, every fiftieth or every tenth cyclic excitation of the membrane.

[0055] Furthermore, it may be useful to perform a temperature determination only when the amplitude of the received signal deviates from the amplitude of the excitation signal by 10 mV or less. This ensures that the vibration sensor is in a steady state and prevents incorrect measurements due to changes in the mechanical vibration of the vibration sensor.

[0056] A circuit arrangement according to the invention for temperature determination in a vibration sensor with integrated temperature detection with a membrane that can be set into vibration, a piezoelectric drive that can be acted upon by an excitation signal for setting the membrane into vibration and for detecting vibrations of the membrane, with a circuit for detecting an output voltage of the piezoelectric drive as a received signal, is characterized by a unit for determining the area of a portion of the received signal caused by the excitation signal, wherein a temperature-dependent capacitance of the piezoelectric drive is determined from the value of the area and the temperature of the piezoelectric drive is determined from the capacitance.

[0057] The step of determining the capacitance of the piezoelectric drive can also be designed in such a way that the capacitance is not determined according to its exact value, but only the temperature corresponding to the area is output given a known temperature behavior of the permittivity of the piezoelectric material.

[0058] In order not to distort the received signal by further signal processing units, it can be fed to an amplifier connected in parallel with the further signal processing for limit level determination, which can be connected upstream of the unit for area determination.

[0059] The amplifier can have a gain of 1, i.e., be designed as an impedance converter. This ensures that the area measurement unit is circuit-wise decoupled from the tuning fork, thus eliminating any potential interference with the received signal.

[0060] The area determination unit can have a Schmitt trigger to start and stop the area determination, to which the received signal is fed. If the received signal exceeds a certain value, the area determination is started; if the received signal falls below a certain value, the area determination is stopped again. By using a Schmitt trigger to start and stop the area determination, the processor load of a microcontroller that is supposed to perform the temperature determination can be easily reduced through circuitry measures, since the start and stop signal for the area determination is not calculated by the microcontroller but generated by a simple circuit component.

[0061] Furthermore, the area determination can also be implemented largely in circuitry by incorporating an integrator into the area determination unit. An integrator can be easily implemented as a circuit component, which can further reduce the processor load on the microcontroller.

[0062] It should be noted at this point that both the starting and stopping of the area determination, as well as the integration and correction of the integral, can take place in a microcontroller. The received signal is fed to the microcontroller via an analog-to-digital converter. All computational operations can then be performed in the microcontroller.

[0063] The present invention is explained in detail below using exemplary embodiments and with reference to the attached figures.

[0064] They show: Fig. 1 a longitudinal section through a vibration sensor according to the present application, Fig. 2 a simplified electrical equivalent circuit diagram of the drive of the vibration sensor from Fig. 1, Fig. 3 exemplary signal curves for the excitation signal and a reception signal generated by it, Fig. 4 an enlarged section of the received signal according to Fig. 3, Fig. 5 a possible circuit arrangement for operating a vibration sensor with integrated temperature measurement, Fig. 6 a possible circuit for the circuit implementation of an integration, Fig. 7 the dependence of the capacitance of an exemplary piezoelectric drive on temperature, Fig. 8 the dependence of the resonance frequency of a mechanical oscillator on temperature, Fig. 9 the dependence of the measured temperature on the immersion depth of the sensor without compensation of the mechanical vibration, Fig. 10 a coordinate system as used to designate the directions of polarization, coupling factor and relative dielectric constant (already discussed) and Fig. 11 a vibration sensor according to the state of the art (already discussed).

[0065] Fig. Figure 1 shows a longitudinal section through a vibration sensor 1 according to the present application. A housing and sensor electronics are not shown for the sake of clarity.

[0066] The vibration sensor 1 shown essentially consists of a drive 3, which is screwed to a membrane 5 by means of a tension bolt 17 and a clamping screw 19. On a side of the membrane 5 opposite the drive 3, mechanical oscillators 11 are arranged, for example in the form of paddles arranged on the membrane 5 and aligned parallel to one another.

[0067] In the present embodiment, the central component of the drive 3 is a stack of two piezo elements 7, which will be referred to below as the first piezo element 71 and the second piezo element 72 to distinguish them. The piezo elements 7 are annular in shape and thus enclose the tension bolt 17 in the circumferential direction. For the centered alignment of the piezo elements 7 and for their electrical insulation from the metallic tension bolt 17, an insulating sleeve 10 is arranged between the tension bolt 17 and the piezo elements 7. The piezo elements 7 are electrically contacted via electrodes 8, with an excitation signal A being conducted via cables 25 running in the rear direction. The electrodes 8 each contact a surface metallization of the piezo elements 7 (not shown in detail in the present figure).

[0068] In the present application, the front side is understood to mean an orientation in the direction of the membrane 5, and the rear side is understood to mean an orientation facing away from the membrane 5.

[0069] A matching ceramic 13 is arranged on the front and rear sides of the piezo elements 7 for electrical insulation. Clamping discs 14 are arranged on the front and rear sides of the respective matching ceramics 13. These clamping discs allow the drive to rest on the diaphragm 5 at the front and to be screwed to the tension bolt 17 at the rear by means of a clamping screw 19, thus preloading it toward the diaphragm 5.

[0070] The first piezo element 71 and the second piezo element 72 are in the Fig. 1, the first piezo element 71 and the second piezo element 72 are stacked one above the other and thus mechanically connected in series. This means that changes in length of the first piezo element 71 and the second piezo element 72 occurring in the direction of a longitudinal axis L of the tension bolt 17 add up within the drive 3, thus allowing an increased stroke to be achieved.

[0071] In a mechanical series circuit, as in Fig. 1, the coupling factors of the stacked piezo elements 7 add up to a total coupling factor of the drive 3.

[0072] In Fig. 2 is an electrical equivalent circuit diagram of the piezo elements 7 from Fig. 1. From this equivalent circuit diagram, it can be seen that the first piezo element 71 and the second piezo element 72 are electrically connected in parallel.

[0073] As from Fig. As can be seen in Figure 2, the first piezo element 71 forms a first capacitance C1 and the second piezo element 72 forms a second capacitance C2. Due to the electrical parallel connection, the capacitances C1 and C2 add up to a total capacitance C ges of the drive 3.

[0074] As can be seen from the equivalent circuit diagram, the piezo elements 7 are to be regarded as a dielectric within plate capacitors, so that a change in the relative dielectric constant ε r of the piezoelectric material of the respective piezoelement 71, 72 leads to a change in the capacitance C1, C2 of the associated plate capacitor. In this way, by determining the total capacitance C ges of the drive 3 as shown below, a temperature T of the piezo elements will be determined with sufficient accuracy.

[0075] The piezoelectric drive 3 is actuated by an excitation signal A, which is supplied via the cable 25. Due to the cyclic excitation signal A and the mechanical structure described above, the membrane 5 and the mechanical oscillator 11 arranged on the membrane 5 are caused to oscillate at its resonant frequency f res excited, which can be detected again by the piezoelectric drive 3 and tapped as received signal E. During operation of the vibration sensor 1 shown, a signal component of the mechanical vibration in the received signal E is superimposed by signal components that are caused by the cyclical excitation of the drive 3 or the membrane 5 by the excitation signal A.

[0076] Fig. 3 shows exemplary signal waveforms for the excitation signal A and the reception signal E.

[0077] In Fig. 3 above shows the excitation signal A. The excitation signal A basically has the form of a periodic square wave signal, but rising edges F1 and falling edges F2 are not realized by a voltage jump in the form of a step function, but rather by 1 / 4 of a sine function.

[0078] As can be seen from the lower part of Fig. 3, the received signal E is a superposition of a sinusoidal oscillation, due to the oscillation of the membrane 5 and the mechanical oscillator 11 arranged on the membrane 5, and signal components which arise at rising edges F1 or falling edges F2 of the excitation signal A. These signal components, which can be clearly seen as signal peaks S, arise because the piezoelectric drive 3 acts like a capacitor consisting of a capacitance C, namely the capacitance C gesof the piezoelectric drive and a resistor R acts as a CR high-pass filter and allows the high frequency components of the edges F1, F2 to pass through. In Fig. 3 is a duration D of a rising edge.

[0079] The excitation is not carried out with a pure square wave signal, but with a 1 / 4 sine wave signal, which is output by a microcontroller 110, which is used to control the vibration sensor 1, via a digital-to-analog converter (DA converter). Since the value of the resistor R and the excitation signal A output by the DA converter DA are known both in terms of the signal shape and the maximum voltage Umax, and also the maximum voltage U max is constant, the area of the signal peak S is proportional to the capacitance C ges of the piezoelectric drive 3.

[0080] Thus, during an excitation pulse of the excitation signal A, a relationship to the capacitance C can be determined by determining the area of the signal peak S, which is solved here by an integration. ges be calculated. This capacity C ges in turn has a material-dependent, known behavior over the temperature T, so that the size of the capacity C ges The temperature T can be determined from the temperature-dependent permittivity ε of the piezo material. The temperature can be determined mathematically by determining the area enclosed by the signal peak S, calculating the capacitance C ges and from this, a calculation of the temperature T via the permittivity ε. Alternatively, a corresponding temperature T can be stored for each surface, e.g., in a corresponding lookup table in a memory of the microcontroller 110.

[0081] Fig. 4 shows an enlarged section of the received signal E from Fig. 3 in the area of a signal peak S.

[0082] In order to determine the temperature T even more accurately, it is necessary to correct the integral over the signal peak S by the portion of the mechanical oscillation that is superimposed on the signal peak S. This correction can be carried out, for example, as in Fig. 4, by approximating a correction area K as the area of a triangle. For this approximation, an initial value W1 of the received signal E is determined at the beginning and an end value W2 at the end of a rising edge F1 of the excitation signal A, and the area of a triangle with a base of the length D of the rising edge F1 is calculated. The approximate correction area is therefore determined as follows: K=1 / 2×D×(W2−W1)

[0083] As an alternative to an approximation of the correction surface K, this can also be determined by integrating the signal component of the received signal E caused by the mechanical vibration.

[0084] The procedure for operating the example in Fig. 1 with integrated temperature detection enables temperature determination during ongoing measurement operation without having to interrupt a vibration measurement. According to the present method, the piezoelectric drive 3 of the vibration sensor 1 is cyclically excited with a predetermined excitation signal A, thereby causing vibration of the membrane 5 and the mechanical oscillator 11 arranged on the membrane 5. The arrangement of the membrane 5 and the mechanical oscillator 11 is thereby caused to oscillate at its mechanical resonance frequency f reswhich, as shown above, depends on the coverage state of the mechanical oscillator 11 and the temperature T.

[0085] To determine the temperature, an output voltage of the piezoelectric drive 3 is recorded as the received signal E, and an area of a portion of the received signal E caused by the excitation signal A is determined at least approximately. The area portion of the received signal caused by the excitation signal A is clearly identifiable as the signal peak S.

[0086] The area fraction of the signal peak S is proportional to the capacitance C ges of the piezoelectric drive 3 and thus enables a determination of the temperature-dependent capacitance C ges of the piezoelectric drive. From the capacitance C ges The temperature T of the piezoelectric drive 3 is then determined using the known temperature dependence of the permittivity ε of the piezo material.

[0087] The temperature T determined in this way can then be used for temperature compensation of the frequency determination for the point level measurement.

[0088] Fig. 5 shows a possible circuit arrangement 100 for operating a vibration sensor 1 with integrated temperature measurement.

[0089] The piezoelectric drive 3 of the vibration sensor 1 is connected on the input side to the microcontroller 110 for controlling the level measurement. The microcontroller 110 supplies the excitation signal A to the vibration sensor 1 via a digital-to-analog converter DA. The excitation signal A is generated by a digital controller within the microcontroller 110 and output via direct memory access (DMA) via the digital-to-analog converter DA. The excitation signal A is thus applied to the vibration sensor 1, as shown in Fig. 3 shown above.

[0090] On the output side of the vibration sensor 1 is the received signal E, as shown in Fig. 3 shown below. On the one hand, the received signal E is fed to the digital controller for frequency determination via a series circuit comprising a switch S1, a bandpass filter BP and a first analog-to-digital converter AD1. A series circuit comprising an amplifier 111 and a second analog-to-digital converter AD2 is connected in parallel to the first analog-to-digital converter DA1. A Schmitt trigger is in turn connected in parallel to the second analog-to-digital converter AD2. If the amplitude is too low, the received signal is amplified via the amplifier 111 so that it can still be measured even with large signal attenuation. The Schmitt trigger in turn converts the signal into a square wave signal, thus enabling highly accurate frequency measurement.

[0091] The received signal E, which can be tapped off at the output side of the vibration sensor 1, is in this case fed to a third analog-to-digital converter AD3 via an impedance converter 112, i.e., an amplifier with a gain factor of 1. In parallel, the signal tapped off at the output side of the impedance converter 112 is fed to a capacitance measurement device 114. The output signal of the capacitance measurement 114 is also fed to the third analog-to-digital converter AD3. The third analog-to-digital converter AD3 is connected on the output side to the microcontroller 110. In parallel to the capacitance measurement device 114, the output signal of the impedance converter 112 is fed to a Schmitt trigger, which generates a start signal and a stop signal for the capacitance determination outside of the microcontroller 110. In this way, the load on the microcontroller can be reduced, since the start and stop signals for the capacitance measurement can be generated using circuitry.

[0092] Within the microcontroller 110, the value of the capacitance measurement 114 is corrected by the correction area K on the basis of the A / D converted received signal E, as shown in Fig. 5. The temperature is then determined based on the corrected capacitance value. Based on the corrected capacitance value, the temperature is determined and temperature compensation is performed for the digital control and a switching command generated by the microcontroller 110.

[0093] A possible circuit for the circuit implementation of the capacitance measurement device 114 is shown in Fig. 6 shown.

[0094] According to the present embodiment, the area under the signal peak S is determined by means of an integration. In Fig. Figure 6 shows an analog integrator that can be fed via a switch S2 with the received signal present at the output of the impedance converter 112. The integrator 114 is constructed from an operational amplifier OPi with a capacitance Ci in negative feedback. The operational amplifier OPi is connected to a series resistor R v and a reset branch connected in parallel with the negative feedback consisting of a reset resistor R res and a switch S res wired to activate the reset of the integrator.

[0095] Fig. 7 shows the dependence of a change in the capacity C ges of an exemplary piezoelectric drive 1 from the temperature T. How Fig. As can be seen from Figure 7, with a suitable choice of the materials of the piezo elements 71, 72 of the piezoelectric drive 3, there is an approximately linear dependence of the capacitance change of the piezoelectric drive 3 on the temperature T.

[0096] In Fig. 8 is the dependence of the resonance frequency f res of the mechanical oscillator 11 on the temperature T. Here, too, an approximately linear relationship can be observed. With increasing temperature T, the resonance frequency f res of the mechanical oscillator 11 due to the material properties.

[0097] Knowing the temperature T, a suitable compensation can be carried out so that always, i.e. especially independent of temperature, a correct switching command from the resonance frequency f res of the mechanical oscillator 11.

[0098] In Fig. 9 shows the dependence of the determined temperature T as a function of an immersion depth of the mechanical oscillator 11 into a measuring medium, if no adjustment of the determined area by the correction area K takes place.

[0099] Depending on the immersion depth, the temperature T determined without taking the correction area K into account varies by several degrees Celsius. This illustration clearly shows that taking the correction area into account when determining capacitance and temperature is necessary in the present embodiment. However, with a different combination of piezoelectric materials for piezo elements 71, 72, determining the area without correction may also be sufficient. List of reference symbols 1 vibration sensor 3 Drive 5 Membran 7 piezo elements 8 electrodes 9 Opening 10 sleeves 11 mechanical oscillators 13 Adaptive ceramics 14 clamping disc 17 tension bolts 19 Clamping screw 22 Temperature sensor 25 cables 71 first piezo element 72 second piezo element 110 microcontrollers 111 amplifiers 112 impedance converters 114 Area determination facility OP operational amplifier ε0 dielectric constant ε r1 first relative dielectric constant ε r2 second relative dielectric constant k1 first coupling factor k2 Second coupling factor k ges Total coupling factor A excitation signal E reception signal S signal tip t time f frequency f res Resonance frequency C1 first capacity C2 second capacity C ges Total capacity C i capacity R resistance R v Ballast resistor L Longitudinal axis U max Maximum voltage AD1 first analog-to-digital converter AD2 second analog-to-digital converter AD3 third analog-to-digital converter DA digital-to-analog converter T Temperature W1 initial value W2 final value F1 rising edge F2 falling edge

Claims

[1] Method for operating a vibration sensor (1) with integrated temperature detection with a membrane (5) that can be set into vibration, a piezoelectric drive (3) for setting the membrane (5) into vibration and for detecting vibrations of the membrane (5), with the following steps: - cyclically exciting the piezoelectric drive (3) with a predetermined excitation signal and causing an oscillation of the membrane (5), then detecting oscillations of the membrane (5) by the piezoelectric drive (3), wherein the excitation and detection steps are carried out continuously, characterized by , that - an output voltage of the piezoelectric drive (3) is detected as a received signal (E), - an area of a portion of the received signal (E) caused by the excitation signal (A) is determined at least approximately, - from the value of the area a temperature-dependent capacity (C ges) of the piezoelectric drive (3) is determined and - from the capacity (C ges ) the temperature (T) of the piezoelectric drive (3) is determined. [2] Method according to claim 1, characterized by that the area is determined by determining the area enclosed by the received signal (E) over a duration (t) of a rising edge (F1) or falling edge (F2) of the excitation signal (A). [3] Method according to claim 2, characterized by that the area is determined by approximating the received signal (E) by inscribing or redescribing polygons or by forming upper and / or lower sums. [4] Method according to claim 2, characterized by , that the area by integrating the received signal (E) over the duration (t) of the rising or falling edge (F1, F2) of the excitation signal (A). [5] Method according to claim 4, characterized bythat the integral is corrected by a correction area (K) caused by the mechanical vibration of the membrane (5). [6] Method according to claim 5, characterized by that the correction area (K) is approximated by a triangle. [7] Method according to claim 5, characterized by that the correction area (K) is calculated by integrating the sine function caused by the mechanical vibration of the membrane (5). [8] Method according to one of the preceding claims, characterized by that a temperature determination is only carried out every hundredth, every fiftieth or every tenth cyclic excitation of the membrane (5). [9] Method according to one of claims 1 to 7, characterized by that a temperature determination is only carried out if the amplitude of the received signal (E) deviates from the amplitude of the excitation signal (A) by 10 mV or less. [10] Circuit arrangement (100) for temperature determination in a vibration sensor (1) with integrated temperature detection with a membrane (5) that can be set in vibration, a piezoelectric drive (3) that can be acted upon by an excitation signal for setting the membrane (5) in vibration and for detecting vibrations of the membrane (5), with a circuit for detecting an output voltage of the piezoelectric drive (3) as a received signal (E), characterized by - a unit for determining the area of a portion of the received signal (E) caused by the excitation signal, wherein - from the value of the area a temperature-dependent capacity (C ges ) of the piezoelectric drive (3) is determined and - from the capacity (C ges ) a temperature (T) of the piezoelectric drive (3) is determined. [11] Circuit arrangement (100) according to claim 10, characterized bythat an amplifier (112) is connected upstream of the area determination unit. [12] Circuit arrangement (100) according to claim 11, characterized by that the amplifier (112) is designed as an impedance converter. [13] Circuit arrangement (100) according to claim 10 or 11, characterized by that the area determination unit (114) has a Schmitt trigger which starts and stops the area determination. [14] Circuit arrangement (100) according to one of claims 10 to 13, characterized by that the area determination unit (114) has an integrator.

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