Method for compensating for a temperature shock on a capacitive pressure measuring cell

Abstract

Method for compensating a temperature shock on a capacitive pressure measuring cell (10) which includes a measuring capacitor (C M ) and a reference capacitor (C R ) exhibits a pressure measurement p in an evaluation unit by forming the quotient Q from the capacitance values ​​of the reference capacitor (C R ) and the measuring capacitor (C M ) as well as a pressure measurement p M using the measuring capacitor (C M ) is won, the temperature shock is detected by the pressure readings p and p M are compared with each other and the gradient dD of the difference value D of both values ​​is monitored with regard to exceeding a predetermined threshold, characterized by the following process steps: - In a matching procedure, a first correction factor k1 has been stored in a lookup table; - continuous recording and evaluation of the difference value D; - the determined difference value D is multiplied by the first correction factor k1 and added to the pressure measurement value p; - Output of this corrected pressure measurement for further processing.

Description

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

[0002] Capacitive pressure sensors or pressure gauges are used for pressure measurement in many industrial sectors. They often feature a ceramic pressure measuring cell, which acts as a transducer for the process pressure, and evaluation electronics for signal processing.

[0003] Capacitive pressure sensors consist of a ceramic base body and a diaphragm, with a glass solder ring arranged between the base body and the diaphragm. The resulting cavity between the base body and the diaphragm allows the diaphragm to move longitudinally under pressure. This cavity is therefore also referred to as the measuring chamber. Electrodes are provided on the underside of the diaphragm and on the opposite upper side of the base body, which together form a measuring capacitor. The application of pressure causes a deformation of the diaphragm, resulting in a change in the capacitance of the measuring capacitor.

[0004] With the help of an evaluation unit, the capacitance change is recorded and converted into a pressure measurement. These pressure sensors are typically used to monitor or control processes. They are therefore often connected to higher-level control units (PLCs).

[0005] DE 198 51 506 C1 discloses a capacitive pressure sensor in which the measured pressure value is determined from the quotient of two capacitance values, a measuring capacitor and 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. The special feature of this pressure measuring device is that for the evaluation of the measurement signal at the output, as a measure of the measured pressure value, only the amplitude of the square-wave signal is relevant, regardless of its frequency.

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

[0007] Quotient methods usually assume the following pressure dependencies: p~CRCM or p~CRCM−1 or p~CM−CRCM−CR, where CM 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 use C M and C R in the quotient. The given example with C M However, in the denominator, this is the most common form in favor of self-linearization. In the following, this embodiment is therefore assumed unless otherwise stated.

[0008] Furthermore, it is well known, for example from DE 10 2011 005 705 B4, that the temperature prevailing during pressure measurement, especially that of the medium being measured, can have a significant influence on the accuracy of the measured results. For this reason, a temperature element located on the back of the base body measures the temperature in parallel with the pressure measurement, allowing the temperature dependence of the pressure measurement to be compensated.

[0009] However, a rapid temperature change, a so-called thermal shock, presents a challenge, as it can cause stress in the pressure cell's diaphragm. The stress in the diaphragm results from a temperature difference between the medium acting on the pressure cell's diaphragm and the pressure cell's base body, which faces away from the medium and is thermally connected to the environment.

[0010] Against this background, EP 2 189 774 A1 is based on the finding that pressure-induced deformation of the membrane differs from thermal shock-induced deformation in terms of measurement technology. The method disclosed therein for detecting rapid temperature changes is based on comparing the measured values ​​of the measuring capacitance Cm with the measured values ​​of the reference capacitance Cr with expected values ​​of the reference capacitance Cr, which result from the measured values ​​of the measuring capacitance Cm, and whereby a temperature jump is detected if the measured value of the reference capacitance lies outside a tolerance range around an expected value. However, this method assumes that a rapid temperature change is the sole cause of the detected discrepancy between the measured values ​​and the expected values. However, this is not always the case in practice.For example, in the event of mechanical damage to the pressure measuring cell, particularly the membrane, a comparable effect would occur between the measured and expected values, which would then lead to the erroneous assumption that one would have to compensate for an influencing temperature instead of replacing the pressure measuring cell or ultimately the entire pressure measuring device, since the output pressure measured values ​​would very likely no longer correspond to the actual pressure conditions.

[0011] EP 2 726 833 B1 also discloses a method in which the value pairs of the two capacitances are monitored within a predetermined tolerance range to determine whether they correspond to the relationship of a predetermined function.

[0012] EP 3 124 937 B1 discloses a method for temperature compensation based on the temperature difference between the diaphragm and the base body of the pressure measuring cell. A temperature sensor is arranged on the diaphragm and the base body to measure the temperature. However, the disadvantage here is the enormous delay, which is due to the natural inertia of temperature sensors. Consequently, the actual compensation process also starts with a corresponding delay. However, the temperature-related error influence on the measurement result is greatest immediately after the occurrence of a temperature shock, as is known from DE 10 2020 122 128 B3 of the applicant.

[0013] Further methods for temperature compensation are known from DE 10 2018 106 563 A1, in which the determination of the shock-induced capacitance change, i.e. change in the reference capacitance compared to the measuring capacitance under standard conditions and shock conditions, and output of the compensated pressure change is disclosed, as well as from DE 10 2009 027 899 A1, from which the compensation with the gradient of the temperature signal depending on the sign is known.

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

[0015] The object is achieved according to the invention by a method having the features of claim 1. Advantageous embodiments of the invention are specified in the subclaims.

[0016] The invention is based on the method disclosed in DE 10 2020 122 128 B3 of the applicant, according to which by comparing the two amounts of the quotient Q and the capacitance value of the measuring capacitor C M is switched to a kind of "alarm state" at a very early stage if this comparison deviates from an expected behavior. Specifically, this comparison determines the gradient dD of the difference value D between the pressure measurement value p formed by the quotient and the pressure measured by the measuring capacitor C M formed pressure value p M monitored for exceeding a threshold value. Advantageously, the pressure measured values ​​p and p M previously linearized.

[0017] According to the invention, a first correction factor k1 is initially stored in a lookup table in a calibration procedure. This correction factor k1 was determined empirically and depends significantly on the design and geometry of the pressure measuring cell. Corresponding tests have shown that this correction factor k1 is approximately identical across all pressure measuring cells, despite different nominal pressure ranges and correspondingly slightly modified designs, which significantly simplifies the process.

[0018] After activation of the “alarm state”, the difference D between the two pressure measurements p and p Mcontinue to be continuously recorded and evaluated. Preferably, immediately after activation of the "alarm state," the difference D is stored as a bias in order to compensate for any creeping effects that may have occurred. The permanently determined, and if necessary advantageously bias-corrected, difference values ​​D are each multiplied by the correction factor k1 from the lookup table and added to the pressure measurement value p. This corrected pressure measurement value, now compensated for the temperature influence, is temporarily output for further processing instead of the actual pressure measurement value p. Temporary means, for example, as long as the gradient dD of the difference value D between the pressure measurement values ​​p and p M exceeds the above threshold.

[0019] Alternatively, instead of the first correction factor k1, a correction curve created in a calibration procedure and stored in a lookup table can be used.

[0020] The advantage of the invention is that the temperature shock compensation initially occurs without the involvement of a temperature element, since at this early stage a temperature element cannot yet respond due to its natural inertia. The temperature-related error influence on the measurement result is particularly greatest immediately after the onset of a temperature shock.

[0021] 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 provides 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 detected by the temperature element would trigger error handling by generating an error signal, since an error has been detected whose cause is initially unknown.

[0022] A further advantageous development provides for switching to a second temperature compensation stage as soon as the difference value D no longer exceeds a predetermined threshold. In this second compensation stage, 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 pressure measurement value p. This currently corrected pressure measurement value is then used for further processing instead of the previously corrected pressure measurement value.

[0023] Advantageously, the temperature compensation is terminated and the original pressure measurement value p formed by the quotient Q is output when the temperature gradient dT falls below a predetermined threshold value.

[0024] The invention is explained in more detail below using exemplary embodiments with reference to the drawings.

[0025] They show schematically: Fig. 1 a schematic sectional view of a capacitive pressure measuring cell, Fig. 2 a diagram showing an example of a course of the temperature-compensated pressure measurement, the quotient Q, the difference value D and a differentiated temperature signal over time in the case of a temperature shock without external pressure influence.

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

[0027] Fig. Figure 1 shows a schematic representation of a typical capacitive pressure measuring cell 10, as used in a variety of capacitive pressure measuring devices. The pressure measuring cell 10 essentially consists of a base body 12 and a diaphragm 14, which are connected to each other via a glass solder ring 16. The base body 12 and the diaphragm 14 define a cavity 19, which – preferably only for low pressure ranges up to 50 bar – is connected to the rear of the pressure measuring cell 10 via a vent channel 18.

[0028] Both on the base body 12 and on the membrane 14, several electrodes are provided, which form a reference capacitor C R and a measuring capacitor C M The measuring capacitor C M is formed by the membrane electrode ME and the center electrode M, the reference capacitor C R through the ring electrode R and the membrane electrode ME.

[0029] The process pressure p acts on the membrane 14, which bends more or less depending on the pressure applied, whereby essentially the distance between the membrane electrode ME and the center electrode M changes. This leads to a corresponding change in the capacitance of the measuring capacitor C M . The influence on the reference capacitor C R is lower 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 center electrode M.

[0030] In the following, no distinction is made between the capacitor's designation and its capacitance value. C M and C R therefore refer to both the measuring or reference capacitor itself and its respective capacitance.

[0031] Fig. Figure 2 shows a diagram illustrating how the temperature-compensated pressure measurement, the quotient Q, the differential value D, and the differentiated signal of the temperature element could look over time in the case of a temperature shock without external pressure influence. The quotient Q, which corresponds to the pressure measurement p and is derived from the capacitance values ​​of the reference capacitor C R and the measuring capacitor C M is formed, dash-dotted and the difference value D between the pressure measured value p and the only one from the measuring capacitor C M obtained pressure measurement p M shown in dashed lines. Furthermore, the differentiated signal of the temperature element is dotted, and the temperature-compensated pressure measurement value output using the method according to the invention for further processing, for example, to a control device, is shown as a solid line.

[0032] The temperature shock begins at the point where the signals of the quotient Q, the difference D, and the compensated pressure measurement abruptly swing upwards or downwards. The significant delay with which the temperature element reacts to the temperature influence can be seen. In contrast, this strong temperature change is immediately "noted" in the capacitance values ​​of the measuring and reference capacitors, with the reference capacitor showing a significantly stronger signal swing than the measuring capacitor. This phenomenon is already known from EP 2 189 774 B1 cited above and DE 10 2020 122 128 B3.

[0033] Since it is in Fig. Since Figure 2 is a representation without external pressure influence, the target pressure measurement, i.e., the quotient Q, should actually be constant on the abscissa, i.e., the zero line. This is clearly not the case, highlighting the enormous influence of the temperature change triggered by the shock.

[0034] If one considers the signal curves against the background that the abscissa represents the ideal line for a pressure measurement, the following is also noticeable. Firstly, when comparing the (uncompensated) quotient Q with the pressure measurement value compensated by the inventive method, the significantly lower signal deflection is evident, which means that the absolute measurement error immediately after the temperature shock caused by the inventive method is also significantly lower. Secondly, it can be seen that the pressure measurement value compensated by the inventive method returns to the ideal line very quickly and thus correctly assumes the value zero, whereas the uncompensated quotient value is still subject to a measurement error until the end of the diagram.

[0035] The trigger for the method according to the invention is the exceeding of a predetermined threshold value of the gradient dD of the difference value D between the pressure measurement value p, which is determined by the quotient Q from the capacitance values ​​of the reference capacitor C R and the measuring capacitor C M is formed, and the pressure value p M , which consists only of the measuring capacitor C M is obtained. If this threshold is exceeded, an "alarm state" is activated and the compensation method according to the invention is started. It is also advantageous to more closely observe the signal curve of a temperature element, advantageously located on the pressure measuring cell 10, to determine whether the assumed 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.

[0036] As soon as the temperature element confirms the temperature shock, a switchover to a second temperature compensation stage can be made. The switchover point would preferably be at the point when the difference value D no longer exceeds a predetermined threshold. In this second compensation stage, instead of the value applied to the capacitances C M and C R The gradient dT of the temperature element is used to calculate the difference value D. The compensation process can be terminated and the measured pressure value p formed by the quotient Q can be output again if the temperature gradient dT falls below a specified threshold. List of reference symbols 10 pressure measuring cell 12 basic bodies 14 Membran 16 Glass solder ring 18 Ventilation duct 19 Cavity C M measuring capacitor C R Reference capacitor Q quotient p Pressure measurement value, formed by the quotient Q p M Pressure measurement value, through the measuring capacitor C M educated D Difference between pressure measured value p and pressure measured value p M M center electrode R ring electrode ME membrane electrode

Claims

[1] Method for compensating a temperature shock on a capacitive pressure measuring cell (10) which has a measuring capacitor (C M ) and a reference capacitor (C R ) wherein in an evaluation unit a pressure measurement value p is determined by forming the quotient Q from the capacitance values ​​of the reference capacitor (C R ) and the measuring capacitor (C M ) and a pressure measurement value p M using the measuring capacitor (C M ) is obtained, where the temperature shock is detected by comparing the pressure values ​​p and p M are compared with each other and the gradient dD of the difference value D of both values ​​is monitored with regard to exceeding a predetermined threshold value, characterized by following procedural steps: - in a comparison procedure, a first correction factor k1 has been stored in a lookup table; - continuous recording and evaluation of the difference value D; - the determined difference value D is multiplied by the first correction factor k1 and added to the measured pressure value p; - Output of this corrected pressure measurement value for further processing. [2] Method according to claim 1, wherein the pressure measuring cell (10) has a temperature element and the gradient dT of the temperature element is detected and evaluated. [3] Method according to claim 2, wherein a plausibility check is carried out by confirming the temperature shock in case of a significant increase in the gradient dT of the temperature element and generating an error signal in case of an unchanged gradient of the temperature element. [4] Method according to one of claims 2 or 3, wherein a change is made to a second temperature compensation stage as soon as the difference value D no longer exceeds a predetermined threshold value, wherein the existing gradient dT of the temperature element is then multiplied by a second stored correction factor k2 and added to the pressure measurement value p and then this corrected pressure measurement value is output for further processing. [5] Method according to one of claims 2 to 4, wherein the compensation is terminated and the original pressure measurement value p formed by the quotient Q is output when the temperature gradient dT falls below a predetermined threshold value. [6] Method according to one of the preceding claims, wherein at the time of detection of the temperature shock the current difference D is stored as bias.

Images