Method for monitoring the functionality of a capacitive pressure measuring cell

Abstract

The present invention relates to a method for monitoring a capacitor (C M ) and the reference capacitor (C R ) and a method for the function of a capacitive pressure measuring cell (10) of a temperature element, wherein in an evaluation unit, a reference capacitor (C R ) and the measuring capacitor (C M ) to form a ratio Q of the capacitance values ​​of the measuring capacitor (C ) to obtain the pressure measurement value p. In this context, the method is characterized by the method steps of: - During the verification process, the characteristic line of the ratio Q and the measuring capacitor (C M ) are stored in a lookup table for each pressure and temperature condition; the ratio Q from the lookup table and the measurement capacitor (C M ) are respectively assigned to the pressure measurement value p determined by the temperature element at the temperature detected at that moment; the ratio Q and the measuring capacitor (C M ) with each other; - in the event of significant deviations from the expected characteristic, the evaluation unit is temporarily switched to a safety mode while the gradient of the temperature element is detected and evaluated; - in the event of a significant increase in the gradient of the temperature element, temperature compensation is introduced or an error signal is generated if the gradient of the temperature element remains unchanged.

Description

Technical Field

[0001] The invention relates to a method for monitoring the functionality of a pressure measuring cell of a capacitive pressure sensor. Background Art

[0002] In many areas of industry, capacitive pressure sensors or pressure measuring devices are used for pressure measurement. They usually consist of a ceramic pressure measuring element as a transducer for the process pressure and evaluation electronics for signal processing.

[0003] Capacitive pressure measuring elements consist of a ceramic substrate and a membrane, with a glass solder ring being arranged between substrate and membrane. Due to the influence of pressure, a cavity is formed between substrate and membrane, which allows the membrane to move longitudinally. This cavity is therefore also called a measuring chamber. Electrodes are provided on the underside of the membrane and on the opposite upper side of the substrate, which together form a measuring capacitor. The effect of pressure causes the membrane to deform and thus leads to a change in the capacitance of the measuring capacitor.

[0004] With the help of an evaluation unit, the change in capacitance is detected and converted into a pressure measurement value. Typically, these pressure sensors are used to monitor or control processes. Therefore, they are often connected to a higher-level control unit (PLC).

[0005] A capacitive pressure sensor is known from DE 198 51 506 C1, in which the pressure measurement value is determined by the quotient of two capacitance values ​​of a measuring capacitor and a reference capacitor. Although the pressure measuring unit is not specifically described in this patent specification, the described circuit and method are applicable to a capacitive pressure measuring unit. The particularity of this pressure measuring device is that, regardless of the frequency of the square wave signal, only the amplitude of the square wave signal is relevant for the evaluation of the measurement signal at the output as a measure of the detected measured pressure value.

[0006] EP 0 569 573 B1 discloses a circuit arrangement for a capacitive pressure sensor, in which the quotient method is also used for pressure evaluation.

[0007] The commercial law generally assumes the following pressure dependencies:

[0008] or or

[0009] Among them C M is the capacitance of the measuring capacitor, C R is the capacitance of the reference capacitor and p is the process pressure to be determined. It is also conceivable that C M and C R However, the C given in the denominator MThe example represents the most common form of supporting intrinsic linearization. Therefore, in the following, unless otherwise stated, this embodiment is assumed.

[0010] The reliability of capacitive pressure sensors is becoming increasingly important. For example, publications EP2606330B1 and DE102018118645B3 propose how to reliably detect medium ingress (which is caused by mechanical damage to the pressure measuring cell, in particular mechanical damage to the membrane, or by possible ventilation channels), and DE102018118646B3 knows how the measuring principle of the pressure sensor can be optimized for possible leakage currents in the back side of the measuring cell facing away from the medium to be measured or in the components of the evaluation electronics to eliminate air humidity components that may be introduced by the environment and are prone to condensation.

[0011] It is also known, for example, from DE 10 2011 005 705 B4 that the temperature during the pressure measurement, in particular the temperature of the medium to be measured, can have a considerable influence on the accuracy of the obtained measurement result. For this reason, in addition to the pressure measurement, the temperature is also detected by means of a temperature element arranged on the back of the substrate, so that the temperature dependency of the pressure measurement can be compensated.

[0012] However, rapid temperature changes, so-called thermal shocks, can lead to tension in the membrane of the pressure measuring cell, which presents a challenge. The membrane tension is generated by the temperature difference between the medium acting on the membrane of the pressure measuring cell and the base body of the pressure measuring cell facing away from the medium and thermally connected to the environment.

[0013] In this context, EP 2 189 774 A1 is based on the recognition that, for technical measurement reasons, pressure-induced membrane deformations differ from those caused by thermal shocks. The method disclosed therein for detecting rapid temperature changes is based on the fact that for a measuring capacitor C M The measured value will be referenced by the capacitor C r The measured value from the measuring capacitor C M The measured value of the reference capacitance C r The expected value is compared with the measured value of the reference capacitance and a temperature jump is detected when the measured value of the reference capacitance is outside the tolerance range around the expected value. However, this method assumes that the rapid temperature change is the only reason for the detected difference between the measured value and the expected value. In practice, however, this is not always the case. For example, in the case of mechanical damage to the pressure measuring cell, in particular to the membrane, a considerable influence can occur between the measured value and the expected value, but this would lead to the wrong assumption that the acting temperature must be compensated instead of replacing the pressure measuring cell, or ultimately the entire pressure measuring device, since the output pressure measurement value most likely no longer corresponds to the actually existing pressure conditions. Summary of the invention

[0014] The object of the present invention is to overcome this disadvantage.

[0015] First, during the adjustment process, the quotient Q and the measuring capacitor C M The characteristic curves of the capacitance value against pressure and the characteristic curves at different temperatures are stored as expected values ​​in a lookup table. During operation of the pressure measuring unit, the corresponding amount of the quotient Q and the measuring capacitor C are calculated based on the lookup table. M The capacitance value is continuously assigned to the determined pressure measurement value p at the temperature detected by the temperature element at that moment, and the two quotients Q and the amount of the measuring capacitor C M The curve characteristics of the capacitance values ​​are compared with each other. As an example, this comparison can be carried out at intervals of 200 milliseconds. If there is a significant deviation from the expected characteristics, the evaluation unit temporarily switches to a safe mode while detecting and evaluating the gradient of the temperature element. If a significant increase in the gradient of the temperature element is detected, appropriate temperature compensation can be initiated. In this case, the cause of the difference between the measured value and the expected value can be clearly attributed to the temperature jump and this error effect on the output measurement result can be corrected by the corresponding compensation. Otherwise, if the temperature element does not show any significant change in its measurement result despite the difference between the measured value and the expected value, the temperature jump can be excluded as the cause, but it can be assumed that the pressure measuring unit, especially its membrane is damaged. In this case, an error signal is generated, which may have different properties. For example, a simple optical or acoustic warning signal can be conceived, but a corresponding signal received from a high-level control unit (PLC) can also be conceived.

[0016] The invention thus has the advantage that, based on the knowledge that the capacitance value reacts very quickly to the effects of a fault, it is possible to switch to an "alarm state" at an early stage and investigate the corresponding environmental conditions for possible causes. The method according to the invention thus acts as a trigger, which draws attention to the fault condition (preliminary unknown cause), so that only a more detailed observation of the possible causes of the fault can then be started and corresponding targeted countermeasures can be initiated at a very early stage.

[0017] An advantageous further development of the invention provides that during the safety mode, the pressure measurement value p is obtained only by means of the capacitance value of the measuring capacitor. The main purpose of the quotient formed by the capacitance values ​​of the measuring capacitor and the reference capacitor is to compensate for capacitance changes due to changes in the dielectric constant r of the dielectric in the measuring chamber. For the safety mode period, which should be of short duration in any case, this will be negligible, with the advantage that a relatively reliable pressure measurement value p can still be output by using the capacitance value of the measuring capacitor.

[0018] Advantageously, the measured pressure value p obtained by the measuring capacitor alone and the measured pressure value p obtained by the quotient Q are then compared with one another. By comparing these two differently calculated measured values, a plausibility check can be performed (possibly also in parallel), which draws attention to sudden or slow changes in or at the pressure measuring cell.

[0019] Another advantageous development is that during the matching process, the charging and discharging times of the measuring capacitor and the reference capacitor as a function of the pressure and the charging and discharging times under different temperature conditions have also been stored in a lookup table, the corresponding charging and discharging time amounts from the lookup table are continuously assigned to the pressure measurement value p determined by the temperature element at the temperature detected at that time, and the process characteristics of the quotient Q, the capacitance value of the measuring capacitor and the amount of charging and discharging time are compared with one another. The entire process thus prolongs the charging and discharging time of the measuring and reference capacitors, i.e. the time period consumed for charging the capacitance to a certain level and discharging it, respectively. In this way, in addition to this temperature influence and mechanical damage to the pressure measuring cell, external frequency influences can also be detected, for example, if the measuring device is operated in the vicinity of a frequency converter, coupling of the frequency of an external signal source can occur, so that a resonance is formed with the fixed operating or operating frequency of the pressure measuring device.

[0020] In the following, the invention is explained in more detail by means of exemplary embodiments with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] These figures schematically show:

[0022] Figure 1 is a block diagram of a capacitive pressure measurement device;

[0023] Figure 2 is a schematic cross-sectional view of a capacitive pressure measuring cell;

[0024] Figure 3 is based on Figure 2 A known evaluation circuit for a capacitive pressure measuring cell; and

[0025] Figure 4 It shows the relationship between the quotient Q, the reference capacitor (C R ) and the measuring capacitor (C M ) and an exemplary curve of the capacitance value and the differential temperature signal changing over time. DETAILED DESCRIPTION

[0026] In the following description of preferred embodiments, the same reference numerals denote the same or comparable components.

[0027] Figure 1The block diagram of a typical capacitive pressure measuring device for measuring process pressure p (e.g. oil, milk, water, etc.) is shown. The pressure measuring device 1 is designed as a two-wire device and essentially comprises a pressure measuring unit 10 and an evaluation electronics 20. The evaluation electronics 20 comprises an analog evaluation circuit 30 and a microcontroller μC, wherein the analog output signal of the evaluation circuit 30 is digitized and further processed. The microcontroller μC provides the evaluation result as a digital or analog output signal to, for example, a PLC. For power supply, the pressure measuring device 1 is connected to a voltage supply line (12V-36V).

[0028] Figure 2 A schematic diagram of a typical capacitive pressure measuring cell 10, as widely used in capacitive pressure measuring devices, is shown. The pressure measuring cell 10 consists essentially of a base body 12 and a membrane 14, which are connected to one another via a glass solder ring 16. The base body 12 and the membrane 14 define a cavity 19, which is connected to the rear side of the pressure measuring cell 10 via a ventilation channel 18, and which is preferably used only for the low pressure range of up to 50 bar.

[0029] A plurality of electrodes are provided on both the substrate 12 and the membrane 14, and the plurality of electrodes form a reference capacitor C R and the measuring capacitor C M .Measure the capacitor C M Formed by the membrane electrode ME and the center electrode M, the reference capacitor C R It is formed by a ring electrode R and a membrane electrode ME.

[0030] The process pressure p acts on the membrane 14, which deflects more or less depending on the applied pressure, wherein the distance between the membrane electrode ME and the center electrode M changes substantially. This causes the measuring capacitor C M The capacitance of the reference capacitor C R The influence of is small because the distance between the ring electrode R and the membrane electrode ME changes less than the distance between the membrane electrode ME and the central electrode M.

[0031] In the following, no distinction is made between the names of capacitors and their capacitance values. Therefore, C M and C R Represents both the measured capacitance and the reference capacitance themselves and their corresponding capacitance.

[0032] exist Figure 3 , which shows a known evaluation circuit 30 for a pressure measuring cell 10 in more detail. The measuring capacitor C M With resistor R 1 Arranged together in the integrator branch IZ, the reference capacitor C R With resistor R 2 Arranged together in the micro branch circuit DZ. Square wave voltage UE0 Applied to the input of the integrator branch IZ, the square wave voltage preferably alternates symmetrically around 0 volts. Using an operational amplifier OP1 operating as an integrator, the resistor R 1 and the measuring capacitor C M The input voltage U E0 The voltage is converted into a linearly rising or falling voltage signal (depending on the polarity of the input voltage), which is output at the output terminal COM of the integration branch IZ. Here, the measurement point P1 is actually grounded by the operational amplifier OP1.

[0033] The output terminal COM is connected to a threshold comparator SG, which drives a square wave generator RG. Once the voltage signal at the output terminal COM exceeds or falls below the threshold, the comparator SG changes its output signal, whereupon the square wave generator RG inverts its output voltage accordingly.

[0034] The micro-branch circuit DZ also includes an operational amplifier OP2, a resistor R 5 and R 6 The voltage divider and feedback resistor R 7 The output terminal of the operational amplifier OP2 is connected to the sample-and-hold circuit S&H. A measurement voltage U is provided at the output terminal of the sample-and-hold circuit S&H. Mess , from which the process pressure p acting on the pressure measuring cell 10 is obtained.

[0035] The function of this measurement circuit is explained in detail below. Operational amplifier OP1 ensures that resistor R 1 and the measuring capacitor C M The connection point P1 between them is actually kept at ground. This results in a constant current I1 across the resistor R 1 flow, which has an impact on the measuring capacitor C M Charge until the square wave voltage UE0 changes its sign.

[0036] Figure 3 It is shown that for R 1 =R 2 and C M =C R In the case of the microbranch DZ, the measuring point P2 in the microbranch DZ is at the same potential as the measuring point P1, i.e. at ground level, even though the connection between the measuring point P2 and the operational amplifier OP2 does not exist. This is true not only in this special case but also as long as the time constant R 1 *C M and R 2 *C R During zero bias adjustment, this condition is established through the variable resistor R 1 or R 2 Make the corresponding settings. If the capacitor C is measured MIf the capacitance of the input circuit changes due to the pressure, the condition of equal time constants in the integral branch IZ and the micro-branch DZ no longer holds and the potential of the measuring point P2 will deviate from the zero value. However, this change is directly offset by the operational amplifier OP2, because the operational amplifier OP2 continues to keep the connection point P2 at practically ground level. Therefore, the square wave voltage U R appears at the output of operational amplifier OP2, and its amplitude depends on the quotient of the two time constants. It can be easily seen that the amplitude is related to the process pressure p~C R / C M -1, where the correlation is essentially linear. 5 and R 6 The voltage divider formed is used to adjust the amplitude.

[0037] By means of the sample-and-hold circuit S&H, the positive and negative amplitudes A+ and A- of the square wave signal are added in absolute value, where the absolute value A is used as the measured voltage U Mess The output of the operational amplifier OP3 is output and forwarded to the microcontroller μC (not shown). However, it can also be output directly as an analog value. The amplitude of the input voltage UE0 provided at the output of the square wave generator RG is dependent on the measured voltage U Mess To achieve better linearity, a resistor R 20 and R 10 This voltage divider is connected to a reference voltage VREF and is advantageously adjustable.

[0038] The positive operating voltage V+ is typically +2.5V, and the negative operating voltage V- is typically -2.5V.

[0039] Figure 4 shows how the quotient Q, the reference capacitor C, is affected by a temperature shock without external pressure. R and the measuring capacitor C M Here, the quotient Q is represented by a dotted line, and the measured capacitor C mess The temperature element is represented by a dotted line, the differential signal of the temperature element is represented by a dotted line, and the reference capacitor Cref is represented by a solid line.

[0040] The temperature shock starts at the beginning of the second frame (in the x direction). A clear delay in the reaction of the temperature element to the temperature influence can be seen. On the other hand, this strong temperature change is immediately "noticed" in the capacitance values ​​of the measuring capacitor and the reference capacitor, wherein the reference capacitor shows a significantly stronger signal amplitude than the measuring capacitor. This phenomenon is already known from EP 2 189 774 B1 cited above.

[0041] Normally, the value of the measured capacitance and the value of the quotient should appear almost identical, which can be seen at the end of the third frame in the x-direction. However, due to the temperature shock, there is a clear difference between these two values, which already appears immediately at the beginning of the temperature shock. According to the invention, this difference is used as a trigger to switch the entire unit into a safe mode, i.e. into a kind of "alarm state", during which the corresponding environmental conditions can be investigated for possible causes. By means of observing the temperature element, it is possible to determine very quickly in this example that a temperature shock actually exists, and by means of a suitable compensation process, the error influence on the pressure measurement value can be subtracted. However, the first moment after the temperature shock is decisive, when the temperature element has not yet reacted and, therefore, naturally, the temperature compensation cannot yet be started. This is where the advantage of the method according to the invention becomes clear, since an abnormal situation must be present in any case, so it is already possible to switch to a safe mode at this early point in time.

[0042] As already described, it can be seen that the influence of temperature shocks on the value of the measuring capacitor is much smaller than on the value of the reference capacitor, and therefore the quotient Q formed by the two values ​​is also much smaller. This then leads to the fact that at the first moment, when no compensation of the temperature error has yet taken place, the capacitance value of the measuring capacitor can be output as the pressure measurement value p in order to at least keep the error degree as small as possible.

[0043] In the event of mechanical damage to the pressure measuring element, in particular the membrane, a similar curve of the quotient Q value and the measured capacitance value will initially result, however, in such a case the signal of the temperature element will not show the same Figure 4 Such amplitudes are shown in . If the safety mode described above is then switched to, it is possible to quickly determine by observing the temperature element that there is no temperature shock in this case, but that a different error cause must be sought. For example, the investigation of mechanical damage can be started with the method described in EP 2 606 330 B1, in which a control pressure measurement value is determined and compared with the actual pressure measurement value p by using an additional capacitor, whose capacitance is independent of the membrane pressure.

[0044] Reference numerals list

[0045] 1. Pressure measurement equipment

[0046] 10 Pressure measurement unit

[0047] 12 Matrix

[0048] 14 Membrane

[0049] 16 Glass welding ring

[0050] 18 ventilation channels

[0051] 19 Cavity

[0052] 20 Evaluating electronic devices

[0053] 30 Evaluation Circuit

[0054] C M , C mess Measuring capacitors

[0055] C R , Cref reference capacitor

[0056] Q Quotient

[0057] p Pressure measurement value

[0058] M Center electrode

[0059] R Ring Electrode

[0060] ME membrane electrode

[0061] IZ Ji branch road

[0062] DZ Micro Branch Road

[0063] SG Threshold Comparator

[0064] RG Square Wave Generator

Claims

1. A method for monitoring the function of a capacitive pressure measuring cell, the capacitive pressure measuring cell comprising a measuring capacitor (C M ) and the reference capacitor (C R ) and temperature elements, in, In the evaluation unit, the reference capacitor (C R ) and the capacitance value of the measuring capacitor (C M ) to form the quotient Q to obtain the pressure measurement value p, The method is characterized by the following steps: During the matching process, the characteristic curve of the quotient Q and the measuring capacitor (C M ) are stored in a lookup table corresponding to pressure and different temperature conditions; The quotient Q from the lookup table and the measurement capacitor (C M ) are respectively and continuously assigned to the pressure measurement value p determined by the temperature element at the temperature detected at that moment; The process characteristic of the measured value of the quotient Q and the measuring capacitor (C M ) are compared with each other in terms of process characteristics of the measured values ​​of the capacitance values; In the event of a significant deviation from the expected behavior, temporarily switching the evaluation unit to a safety mode and simultaneously detecting and evaluating the gradient of the temperature element; In the event that the gradient of the temperature element increases significantly, initiating temperature compensation; or In case the gradient of the temperature element is constant, an error signal is generated.

2. Method for monitoring the function of a capacitive pressure measuring cell according to claim 1, in, During the safety mode, only the measuring capacitor (C M ) to obtain the pressure measurement value p.

3. Method for monitoring the function of a capacitive pressure measuring cell according to claim 2, in, The measuring capacitor (C M ) The pressure measurement value p obtained separately and the pressure measurement value p obtained by the quotient Q are compared with each other.

4. Method for monitoring the function of a capacitive pressure measuring cell according to any of the preceding claims, in, In the matching process, in addition to the characteristic curve of the quotient Q and the measuring capacitor (C M ) capacitance value, the measurement capacitor (C M ) and the reference capacitor (C r ) are also stored in the look-up table with respect to the charging and discharging times for different pressure and temperature conditions, the corresponding values ​​of the charging and discharging times from the look-up table being successively assigned to the pressure measurement value p determined by the temperature element at the temperature detected at the moment, and the quotient Q, the measuring capacitor (C M ) The process characteristics of the capacitance value, and the values ​​of the charging time and the discharging time are compared with each other.

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