Method for Monitoring the Function of a Capacitive Pressure Measuring Cell

Abstract

The invention relates to a method for monitoring the function of a capacitive pressure measuring cell (10) comprising a measuring capacitor (C.sub.M) and a reference capacitor (C.sub.R) as well as a temperature element, wherein in an evaluation unit the pressure measurement value p is obtained by forming the quotient Q from the capacitance values of the reference capacitor (C.sub.R) and the measuring capacitor (C.sub.M). The method is characterized by the following method steps: in a matching procedure the characteristic curve of the quotient Q and the capacitance values of the measuring capacitor (C.sub.M) are each stored in a lookup table versus the pressure and at different temperature scenarios; then the corresponding absolute value of the quotient Q and of the capacitance value of the measuring capacitor (C.sub.M) from the lookup table are continuously assigned respectively to the determined pressure measurement value p at the temperature detected at this moment by the temperature element; the behavior of the course of the two absolute values of the quotient Q as well as of the capacitance value of the measuring capacitor (C.sub.M) is compared with each other; in the case of a significant deviation from an expected behavior, the evaluation unit is temporarily switched into a safety mode and meanwhile the gradient of the temperature element is detected and evaluated; in the case of a significant increase of the gradient of the temperature element, a temperature compensation is initiated; or in the case of an unchanged gradient of the temperature element, an error signal is generated.

Claims

1: A method for monitoring the function of a capacitive pressure measuring cell (10) comprising a measuring capacitor (C.sub.M) and a reference capacitor (C.sub.R) as well as a temperature element, wherein in an evaluation unit a pressure measurement value p is obtained by forming a quotient Q from the capacitance values of the reference capacitor (C.sub.R) and the measuring capacitor (C.sub.M), the method comprising the following method steps: in a matching procedure the characteristic curve of the quotient Q and the capacitance values of the measuring capacitor (C.sub.M) are each stored in a lookup table versus the pressure and at different temperature scenarios; then the corresponding absolute value of the quotient Q and of the capacitance value of the measuring capacitor (C.sub.M) from the lookup table are continuously assigned respectively to the determined pressure measurement value p at the temperature detected at this moment by the temperature element; the behavior of the course of the two absolute values of the quotient Q as well as of the capacitance value of the measuring capacitor (C.sub.M) is compared with each other; in the case of a significant deviation from an expected behavior, the evaluation unit is temporarily switched into a safety mode and meanwhile the gradient of the temperature element is detected and evaluated; in the case of a significant increase of the gradient of the temperature element, a temperature compensation is initiated; or in the case of an unchanged gradient of the temperature element, an error signal is generated.

2: The method according to claim 1, wherein during the safety mode the pressure measurement value p is obtained only by the capacitance value of the measuring capacitor (C.sub.M).

3: The method according to claim 2, wherein the pressure measurement values p obtained respectively by the measuring capacitor (C.sub.M) alone and by the quotient Q are compared with one another.

4: The method according to claim 1, wherein in the matching procedure, in addition to the characteristic curves of the quotient Q and the capacitance values of the measuring capacitor (C.sub.M), the charging and discharging times of the measuring capacitor and the reference capacitor (C.sub.M, C.sub.r) versus the pressure and at different temperature scenarios have also been stored in the lookup table, the corresponding absolute value of the charging and discharging time from the lookup table is continuously assigned to the determined pressure measurement value p at the temperature detected at this moment by the temperature element, and the behavior of the course of the absolute values of the quotient Q, of the capacitance value of the measuring capacitor (C.sub.M) and of the charging and discharging time are compared with one another.

Description

[0019] In the following, the invention is explained in more detail by means of exemplary embodiments with reference to the drawings.

[0020] The Figures schematically show:

[0021] FIG. 1 a block diagram of a capacitive pressure measuring device;

[0022] FIG. 2 a schematic sectional view of a capacitive pressure measuring cell;

[0023] FIG. 3 a known evaluation circuit for a capacitive pressure measuring cell according to FIG. 2; and

[0024] FIG. 4 a diagram showing an exemplary curve of quotient Q, the capacitance values of the reference capacitor (C.sub.R) and the measuring capacitor (C.sub.M) and a differentiated temperature signal over time in the case of a temperature shock without external pressure influence.

[0025] In the following description of the preferred embodiments, identical reference symbols denote identical or comparable components.

[0026] FIG. 1 shows a block diagram of a typical capacitive pressure measuring device used to measure a process pressure p (e.g. of oil, milk, water, etc.). The pressure measuring device 1 is designed as a two-wire device and essentially comprises a pressure measuring cell 10 and 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 20 is digitized and processed further. The microcontroller μC provides the evaluation result as a digital or analog output signal, for example to a PLC. For power supply, the pressure measuring device 1 is connected to a voltage supply line (12-36V).

[0027] FIG. 2 shows a schematic diagram of a typical capacitive pressure measuring cell 10, as used manifold in capacitive pressure measuring devices. The pressure measuring cell 10 essentially consists of a base body 12 and a membrane 14, which are connected to each other via a glass solder ring 16. The base body 12 and the membrane 14 delimit a cavity 19 which—preferably only for low pressure ranges up to 50 bar—is connected to the rear side of the pressure cell 10 via a vent channel 18.

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

[0029] The process pressure p acts on the membrane 14, which deflects to a greater or lesser extent in accordance with the applied pressure, wherein essentially the distance between the membrane electrode ME and the center electrode M changes. This leads to a corresponding change in capacitance of the measuring capacitor C.sub.M. The influence on the reference capacitor C.sub.R is smaller, since 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 designation of the capacitor and its capacitance value. C.sub.M and C.sub.R therefore denote both the measuring and the reference capacitor itself as well as their respective capacitance.

[0031] In FIG. 3, a known evaluation circuit 30 for the pressure measuring cell 10 is shown in more detail. The measuring capacitor C.sub.M is arranged together with a resistor R.sub.1 in an integrating branch IZ and the reference capacitor C.sub.R is arranged together with a resistor R.sub.2 in a differentiating branch DZ. A square-wave voltage U.sub.E0, which preferably alternates symmetrically around 0 volts, is applied to the input of the integrating branch IZ. The input voltage U.sub.E0 is converted via the resistor R.sub.1 and the measuring capacitor C.sub.M with the use of an operational amplifier OP1, which operates as an integrator, into a linearly rising or falling voltage signal (depending on the polarity of the input voltage), which is output at the output COM of the integrating branch IZ. Here, the measuring point P1 is virtually grounded by the operational amplifier OP1.

[0032] The output COM is connected to a threshold comparator SG, which drives a square-wave generator RG. As soon as the voltage signal at the output COM exceeds or falls below a threshold value, the comparator SG changes its output signal, whereupon the square-wave generator RG respectively inverts its output voltage.

[0033] The differentiating branch DZ further consists of an operational amplifier OP2, a voltage divider with the two resistors R.sub.5 and R.sub.6 and a feedback resistor R.sub.7. The output of the operational amplifier OP2 is connected to a sample-and-hold circuit S&H. The measuring voltage U.sub.Mess is provided at the output of the sample-and-hold circuit S&H, from which the process pressure p acting on the pressure measuring cell 10 is obtained.

[0034] The function of this measuring circuit is explained in more detail below. The operational amplifier OP1 ensures that the connection point P1 between the resistor R.sub.1 and the measuring capacitor C.sub.M is kept virtually at ground. This causes a constant current I.sub.1 to flow across resistor R.sub.1, which charges the measuring capacitor C.sub.M until the square-wave voltage U.sub.E0 changes its sign.

[0035] FIG. 3 shows that for the case R.sub.1=R.sub.2 and C.sub.M=C.sub.R, the measuring point P2 in the differentiating branch DZ is at the same potential as the measuring point P1, i.e. at ground level, even if the connection between the measuring point P2 and the operational amplifier OP2 were not present. This is true not only in this particular case, but whenever the time constants R.sub.1*C.sub.M and R.sub.2*C.sub.R are equal to each other. During zero offset adjustment, this condition is set accordingly via the variable resistors R.sub.1 or R.sub.2. If the capacitance of the measuring capacitor C.sub.M changes due to the effect of pressure, the condition of equality of the time constants in the integrating branch IZ and in the differentiating branch DZ is no longer given and the potential at the measuring point P2 would deviate from the value zero. However, this change is directly counteracted by the operational amplifier OP2, since the operational amplifier OP2 continues to hold the connection point P2 virtually at ground level. Therefore, a square wave voltage U.sub.R is present at the output of the operational amplifier OP2, the amplitude of which depends on the quotient of the two time constants. It can easily be shown that the amplitude is directly proportional to the process pressure p˜C.sub.R/C.sub.M−1, wherein the dependence is essentially linear. The amplitude can be adjusted via the voltage divider formed by the two resistors R.sub.5 and R.sub.6.

[0036] Via a sample-and-hold circuit S&H the positive and negative amplitude A+ and A− of the square wave signal are added in terms of absolute value, wherein the absolute value A is output as measuring voltage U.sub.Mess at the output of the operational amplifier OP3 and forwarded to the microcontroller μC (not shown). However, it could also be output directly as an analog value. The amplitude of the input voltage U.sub.E0, which is provided at the output of the square wave generator RG, is adjusted in dependence of the measuring voltage U.sub.Mess in order to achieve a better linearity. To this end, a voltage divider consisting of resistors R.sub.20 and R.sub.10 is provided. This voltage divider is connected to a reference voltage VREF and is advantageously adjustable.

[0037] The positive operating voltage V+ is typically +2.5 V and the negative operating voltage V− is typically −2.5 V.

[0038] FIG. 4 shows a diagram of how in the case of a temperature shock without external pressure influence the curves of quotient Q, the capacitance values of the reference capacitor C.sub.R and the measuring capacitor C.sub.M as well as the differentiated signal of the temperature element could appear over time as an example. Here, the quotient Q is dash-dotted, the measuring capacitor C.sub.mess is dashed, the differentiated signal of the temperature element is dotted and the reference capacitor C.sub.ref is shown as a continuous line.

[0039] The temperature shock starts at the beginning of the second box (in x-direction). It can be seen with which clear delay the temperature element reacts to the temperature influence. On the other hand, this strong temperature change is “noticed” immediately in the capacitance values of the measuring and the reference capacitor, wherein the reference capacitor shows a significantly stronger signal amplitude compared to the measuring capacitor. This phenomenon is already known from EP 2 189 774 B1 cited at the beginning.

[0040] Normally, the values of the measuring capacitor and those of the quotient should behave almost identical, as can be seen from the end of the third box in the x-direction. However, due to the temperature shock, there is a clear discrepancy between the two values, which already appears immediately at the beginning of the temperature shock. According to the invention, this discrepancy is used as a trigger to switch the entire unit into a safety mode, i.e. to a kind of “alarm state”, during which the corresponding environmental conditions can be investigated with regard to a possible cause. By observing the temperature element, it would be possible to determine very quickly in the present example that a temperature shock is actually present, and by means of an appropriate compensation procedure, the error influence on the pressure measurement value could be subtracted. However, the very first moment after the temperature shock is decisive, when the temperature element has not yet reacted at all and thus naturally no temperature compensation can yet be initiated. This is where the advantage of the method according to the invention becomes apparent, since it is already possible to switch into a safety mode at this early point in time, since an exceptional situation must be present in any case.

[0041] As already described, it can be seen that the value of the measuring capacitor was falsified by the temperature shock to a much lesser extent than the value of the reference capacitor and thus also the quotient Q formed from both values. This then leads to the fact that in the very first moment, when still no compensation of the temperature error takes place, the capacitance value of the measuring capacitor can be output as pressure measurement value p, in order to keep at least the degree of error as small as possible.

[0042] In the case of a mechanical damage to the pressure measuring cell, in particular the membrane, a similar curve of the values of quotient Q and measuring capacitance would result at the beginning, however, in this case the signal of the temperature element would not show such an amplitude as in FIG. 4. If then a switch into the safety mode described above occurs, it would be possible to quickly determine by observing the temperature element that in this case no temperature shock is present and that instead a different cause of error has to be searched for. For example, an investigation with respect to a mechanical damage could then be initiated by means of the method described in EP 2 606 330 81, in which by use of an additional capacitor, whose capacitance is independent of the membrane pressure, a control pressure measurement value is determined and compared with the actual pressure measurement value p.

LIST OF REFERENCE SYMBOLS

[0043] 1 Pressure measuring device [0044] 10 Pressure measuring cell [0045] 12 Base body [0046] 14 Membrane [0047] 16 Glass solder ring [0048] 18 Vent channel [0049] 19 Cavity [0050] 20 Evaluation electronics [0051] 30 Evaluation circuit [0052] C.sub.M, C.sub.mess Measuring capacitor [0053] C.sub.R, C.sub.ref Reference capacitor [0054] Q Quotient [0055] p Pressure measurement value [0056] M Center electrode [0057] R Ring electrode [0058] ME Membrane electrode [0059] IZ Integrating branch [0060] DZ Differentiating branch [0061] SG Threshold comparator [0062] RG Square wave generator