Method for compensating temperature shock on capacitive pressure measuring cell

Abstract

The invention relates to a method for compensating for a temperature shock on a capacitive pressure measuring cell (10) comprising a measuring capacitor (CM) and a reference capacitor (CR). In the evaluation unit, the measured pressure value p is obtained by forming a quotient Q from capacitance values from the reference capacitor (CR) and the measuring capacitor (CM), and the measured pressure value pM is obtained using the measuring capacitor (CM), a temperature shock being identified by comparing the measured pressure values p and pM, and monitoring whether a gradient dD of a difference D between the two values exceeds a predefined threshold. The method is characterized by the following method steps: storing a first correction factor k1 in a lookup table in a calibration process; continuously recording and analyzing the difference D; multiplying the identified difference D by a first correction factor k1 and adding it to the measured pressure value p; -outputting the corrected measured pressure value for further processing.

Description

[0001] The invention relates to a method for compensating for temperature shocks on a capacitive pressure measuring cell.

[0002] Capacitive pressure sensors or pressure measuring devices are used to measure pressure in many industrial sectors. They typically have a ceramic pressure measuring cell as the measuring transducer for the process pressure and evaluation electronics for signal processing.

[0003] A capacitive pressure measuring cell consists of a ceramic substrate and a membrane, with a glass solder ring positioned between them. The pressure creates a cavity between the substrate and membrane, enabling the membrane to move longitudinally. This cavity is therefore also called a measuring chamber. Electrodes are located on the underside of the membrane and on the opposing upper side of the substrate; together, these electrodes form a measuring capacitor. The application of pressure causes the membrane to deform, which in turn changes the capacitance of the measuring capacitor.

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

[0005] DE 198 51 506 C1 discloses a capacitive pressure sensor in which the measured pressure value is determined as a function of the quotient of two capacitance values, one from a measuring capacitor and the other from a reference capacitor. Although this patent does not specifically describe a pressure measuring cell, the circuit shown and the method described are suitable for capacitive pressure measuring cells. A special feature of this pressure measuring device is that, for the evaluation of the measurement signal at the output as a measure of the recorded measured pressure value, only the amplitude of the square-wave signal is relevant, not its frequency.

[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 quotient method generally assumes the following pressure dependencies:

[0008] or or 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 to exchange C in the quotient M and C R However, with C in the denominator M The given example of represents the most common form of supporting self-linearization. In the following, unless otherwise stated, this implementation will be assumed.

[0009] Furthermore, it is known, for example from DE 10 2011 005 705 B4, that the temperature prevailing during the pressure measurement (in particular the temperature of the medium to be measured) can have a very significant influence on the accuracy of the measurement result. For this reason, the temperature can also be measured in parallel with the pressure measurement by means of a temperature element arranged on the rear side of the base body, so that the temperature dependency of the pressure measurement can be compensated.

[0010] However, rapid changes in temperature (so-called thermal shocks) present a challenge, as they can lead to tension in the membrane of a pressure measuring cell. The tension in the membrane is caused 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, which faces away from the medium and is thermally connected to the environment.

[0011] Against this background, EP 2 189 774 A1 is based on the discovery that pressure-induced deformation of the membrane differs from deformation of the membrane caused by thermal shock in terms of measurement technology. The method disclosed there for detecting rapid temperature changes is based on the fact that, for the measured value of the measuring capacitance Cm, the measured value of the reference capacitance Cr is compared with an expected value of the reference capacitance Cr, which is derived from the measured value of the measuring capacitance Cm, and a temperature jump is detected if the measured value of the reference capacitance lies outside a tolerance range around the expected value. However, this method assumes that the rapid temperature change is the sole cause of the observed discrepancy between the measured and expected values. However, this is not always the case in practice. For example, if the pressure measuring cell (especially the membrane) is mechanically damaged, a significant effect can occur between the measured and expected values. This can then lead to the erroneous assumption that the active temperature must be compensated for, rather than replacing the pressure measuring cell or ultimately the entire pressure measuring device, as the output pressure measurement value will most likely no longer correspond to the actual pressure conditions.

[0012] EP 2 726 833 B1 also discloses a method in which a value pair of two capacitances is monitored within a predetermined tolerance range to determine whether they correspond to a predetermined functional relationship.

[0013] EP 3 124 937 B1 discloses a method for temperature compensation based on the temperature difference between the membrane and the base of a pressure measuring cell. To measure the temperature, temperature sensors are arranged on the membrane and the base, respectively. However, a disadvantage is the significant delay caused by the natural inertia of the temperature sensors, which means that the actual compensation method also starts with a delay. However, as the applicant is aware from DE 10 2020 122 128 B3, temperature-related errors have the greatest impact on the measurement results after a temperature shock has occurred.

[0014] The object of the present invention is to start temperature compensation very early after a temperature shock and thus significantly reduce temperature-related measurement errors.

[0015] According to the invention, this object is achieved by a method having the features of claim 1. Advantageous embodiments of the invention are described in the dependent claims.

[0016] The present invention is based on the method disclosed by the applicant in DE 10 2020 122 128 B3, according to which the quotient Q and the measuring capacitor C M If the comparison of the two values ​​of the capacitance of the measuring capacitor C deviates from the expected behavior, this comparison leads to switching to an "alarm state" at a very early stage. Specifically, in this comparison, the measured pressure value p formed by the quotient and the capacitance of the measuring capacitor C are monitored. M The resulting pressure value p M Whether the gradient dD of the difference D between the pressure values ​​p and p exceeds a threshold value. M The advantage over a pure differential evaluation is that the gradient ignores gradual changes, such as aging effects or tensions that accumulate or decrease in the pressure measuring cell for various reasons, and only reacts to rapid changes.

[0017] According to the present invention, during the calibration process, a first correction factor k1 is initially stored in a lookup table. This correction factor k1 is determined empirically and depends largely on the structure and geometry of the pressure measuring cell. Corresponding tests have shown that this correction factor k1 is approximately the same for all pressure measuring cells, despite different nominal pressure ranges and correspondingly slightly modified structures, which makes the method easier.

[0018] After activation of the "alarm state", the two measured pressure values ​​p and p are continuously recorded and evaluated. M The difference D between the measured pressure values ​​p and p is preferably stored immediately after activation of the "alarm state" as a deviation in order to compensate for any gradual effects that may have occurred. The permanently determined, if necessary, deviation-corrected difference value D is each multiplied by a correction factor k1 from a lookup table and added to the measured pressure value p. Studies have shown that it is advantageous to continuously record the measured pressure values ​​p and p M The difference D between them is calculated and the factor k1 is then multiplied by the determined difference D, ie weighted, and this calculated value is then added to the measured pressure value p.

[0019] Instead of the actual measured pressure value p, this measured pressure value (now corrected to compensate for temperature influences) is output temporarily for further processing. Temporary means, for example, as long as the measured pressure values ​​p and p M The gradient dD of the difference D between them exceeds the above threshold.

[0020] Alternatively, instead of the first correction factor k1 , a correction curve created during the calibration process and stored in a look-up table can also be used.

[0021] The advantage of the present invention is that the compensation for temperature shocks is initially performed without the involvement of the temperature element, because at this early stage the temperature element cannot yet respond due to its inherent inertia. The influence of temperature-related errors on the measurement result reaches its maximum immediately after the temperature shock occurs.

[0022] In a further development, the pressure measuring cell advantageously has a temperature element, and the gradient dT of this temperature element is recorded and evaluated. This offers the advantageous possibility of performing a plausibility check to determine whether a temperature-related error influence (i.e., a temperature shock) is actually present. The absence of a temperature change recorded by the temperature element triggers an error handling process by generating an error signal, since an error has been detected, the cause of which was initially unknown.

[0023] A further advantageous development provides for switching to a second temperature compensation phase as soon as the difference D no longer exceeds a predefined threshold value. In this second compensation phase, the existing gradient dT of the temperature element is then multiplied by a second correction factor k2 stored in the lookup table and added to the measured pressure value p. This currently corrected measured pressure value is then used instead of the previously corrected measured pressure value for further processing.

[0024] Advantageously, when the temperature gradient dT falls below a predefined threshold, the temperature compensation is terminated and the raw measured pressure value p formed by the quotient Q is output.

[0025] The present invention is explained in more detail below using exemplary embodiments with reference to the accompanying drawings.

[0026] In the attached figure:

[0027] Figure 1 is a schematic cross-sectional view of a capacitive pressure measuring cell,

[0028] Figure 2 is a diagram showing an exemplary course of a temperature-compensated measured pressure value, a quotient Q, a difference D, and a differential temperature signal over time in the absence of a temperature shock due to an external pressure influence.

[0029] In the following description of the preferred embodiments, the same reference numerals denote the same or corresponding components.

[0030] Figure 1 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 essentially consists 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, preferably only in the low-pressure range of up to 50 bar.

[0031] Several electrodes are provided on the substrate 12 and the membrane 14, which 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, and the reference capacitor C R It is formed by a ring electrode R and a membrane electrode ME.

[0032] The process pressure p acts on the membrane 14, which bends more or less depending on the applied pressure, wherein the distance between the membrane electrode ME and the center electrode M changes significantly. This results in a measurement capacitor C M The corresponding change in capacitance of the reference capacitor C R The influence of is low because the distance between the ring electrode R and the membrane electrode ME varies less than the distance between the membrane electrode ME and the center electrode M.

[0033] In the following, no distinction is made between the name of a capacitor and its capacitance value. Therefore, C M and C R Represents the measurement capacitor or reference capacitor, and the capacitance of each capacitor, respectively.

[0034] Figure 2 The diagram shows how the curves of the temperature-compensated measured pressure value, the quotient Q, the difference D and the differential signal of the temperature element change over time without a temperature shock caused by an external pressure. In this case, the measured pressure value p and the reference capacitor C R and the measuring capacitor C M The quotient Q formed by the capacitance value is shown as a dot-dash line, and the measured pressure value p is related to the capacitance value obtained only from the measuring capacitor C. M The measured pressure value p M The difference D between the two is shown as a dashed line. In addition, the differential signal of the temperature element is shown as a dotted line, and the temperature-compensated measured pressure value is shown as a solid line, which is output by the method according to the invention for further processing, for example to a control device.

[0035] The temperature shock begins at the point where the signals for the quotient Q, the difference D, and the compensated measured pressure value suddenly swing downward or upward. This shows how delayed the temperature element's reaction to temperature influences is. However, this strong temperature change is immediately "detected" in the capacitance values ​​of the measuring and reference capacitors, with the reference capacitor exhibiting a significantly stronger signal deflection than the measuring capacitor. This phenomenon is already known from the initial descriptions in EP 2 189 774 B1 and DE 10 2020 122 128 B3.

[0036] because Figure 2No external pressure effects are shown, so the target measured pressure value (i.e., quotient Q) should actually be constant on the abscissa (i.e., zero line). This is clearly not the case and illustrates the dramatic effect of temperature changes caused by shock.

[0037] The following also becomes apparent when considering the signal curve against the backdrop of an ideal line whose abscissa represents the measured pressure values. On the one hand, when comparing the (uncompensated) quotient Q with the measured pressure values ​​compensated by the method according to the invention, a significantly lower signal deflection can be detected, and thus, due to the method according to the invention, the absolute measurement error immediately after the temperature shock is also correspondingly significantly lower. On the other hand, it can be seen that the measured pressure values ​​compensated by the method according to the invention return to the ideal line very quickly and therefore correctly assume the value zero, while the uncompensated quotient values ​​are still subject to measurement errors until the end of the diagram.

[0038] The method according to the invention is triggered by exceeding the measured pressure value p and the pressure value p M The difference D between the gradient dD and the predefined threshold value of the measured pressure value p is determined by the pressure from the reference capacitor C R and the measuring capacitor C M The quotient Q of the capacitance value is formed, the pressure value p M Only from the measurement capacitor C M If this threshold is exceeded, an "alarm state" is activated and the compensation method according to the present invention is initiated. In this case, it is also advantageous to more closely observe the signal curve of the temperature element, advantageously located on the pressure measuring cell 10, to see whether the hypothetical temperature shock is confirmed by a significant increase in the gradient dT of the temperature element. If this is not the case, this plausibility check can initially generate an error signal and search for another cause of the error.

[0039] Once the temperature element confirms the temperature shock, the second temperature compensation phase can be activated. The switching point will preferably be at the point where the difference D no longer exceeds the predefined threshold. In this second compensation phase, instead of the capacitance C M and C R The difference D is calculated using the gradient dT of the temperature element. When the temperature gradient dT falls below a predefined threshold, the compensation method can be terminated and the measured pressure value p formed by the quotient Q can be output again.

[0040] Reference Signs List

[0041] 10 Pressure measuring unit

[0042] 12 Matrix

[0043] 14 membrane

[0044] 16 Glass solder ring

[0045] 18 ventilation channels

[0046] 19 chambers

[0047] C M Measuring capacitors

[0048] C R Reference capacitor

[0049] Q Quotient

[0050] p is the measured pressure value, formed from the quotient Q

[0051] p M The pressure value is measured by the measuring capacitor C M form

[0052] D Measured pressure value p and measured pressure value p M The difference between

[0053] M Center electrode

[0054] R Ring Electrode

[0055] ME membrane electrode

Claims

1. A method for compensating for temperature shocks on a capacitive pressure measuring cell (10) having a measuring capacitor (C M ) and the reference capacitor (C R ), and in the evaluation unit the pressure value p is measured by the reference capacitor (C R ) and the measuring capacitor (C M ) is obtained by forming a quotient Q of the capacitance value, and measuring the pressure value p M The measurement capacitor (C M ) to obtain, The measured pressure values ​​p and p are M The temperature shock is detected by comparing the two values ​​with each other and the gradient dD of the difference D of the two values ​​is monitored as to whether a predefined threshold value is exceeded, characterized in that The following steps: - During the calibration process, the first correction factor k1 is stored in a lookup table; - continuous recording and evaluation of said difference D; - multiplying the determined difference D by the first correction factor k1 and adding it to the measured pressure value p; - Output of the corrected measured pressure value for further processing.

2. The method according to claim 1, The pressure measuring cell (10) has a temperature element, and the gradient dT of the temperature element is recorded and evaluated.

3. The method according to claim 2, A plausibility check is performed in this case by detecting the temperature shock if the gradient dT of the temperature element increases significantly and by generating an error signal if the gradient of the temperature element does not change.

4. The method according to any one of claims 2 or 3, As soon as the difference D no longer exceeds a predefined threshold, a switch is made to a second temperature compensation phase, in which the existing gradient dT of the temperature element is then multiplied by a stored second correction factor k2 and added to the measured pressure value p, and the corrected measured pressure value is then output for further processing.

5. The method according to any one of claims 2 to 4, When the temperature gradient dT drops below a predefined threshold, the compensation is terminated and the original measured pressure value p formed by the quotient Q is output.

6. The method according to any one of the preceding claims, The current difference D is stored as the deviation when the temperature shock is detected.

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