Method for monitoring the operation of a pressure measuring cell of a capacitive pressure sensor

Abstract

The disclosure relates to a method for monitoring the operation of a pressure measuring cell of a capacitive pressure sensor, wherein the pressure measuring cell comprises a pressure-dependent measuring capacitor and a pressure-dependent reference capacitor and the pressure measuring value is obtained as a measuring signal from the capacitance values of the measuring capacitor and the reference capacitor, wherein the measuring signal is supplied to an evaluation unit in the form of an alternating square-wave signal, the pulse height of the signal depending on quotients of the capacitance values of the reference capacitor and the measuring capacitor and the period of the signal being determined by the capacitance value of the measuring capacitor such that, in the nominal pressure range of the pressure sensor, there is a fixed correlation between the pulse height and the period, wherein the pairs of values of pulse height and period (h1, d1), . . . , (hn, dn) are stored as nominal values in an adjustment procedure for determined pressure values p1, . . . , pn, and wherein, for the currently measured pressure value px, with the pair of actual values (h.sub.x-IST, d.sub.x-IST), the pair of nominal values (h.sub.x-SOLL, d.sub.x-SOLL) is determined, and if there is significant deviation between the pair of actual values and the pair of nominal values, an error signal is generated.

Claims

1. A method for monitoring the operation of a pressure measuring cell of a capacitive pressure sensor, wherein the pressure measuring cell comprises a pressure-dependent measuring capacitor and a pressure-dependent reference capacitor and the pressure measurement value (p) is obtained as a measurement signal from the capacitance values of the measuring capacitor and the reference capacitor, wherein the measurement signal is fed to an evaluation unit in the form of an alternating square-wave signal, the pulse height of which depends on the quotient of the capacitance values of the reference capacitor and the measuring capacitor and the period duration of which is determined by the capacitance value of the measuring capacitor so that in the nominal pressure range of the pressure sensor there is a fixed relationship between pulse height and period duration, wherein in a calibration procedure for specific pressure values p.sub.1, . . . , p.sub.n the value pairs of pulse height and period duration (h.sub.1, d.sub.1), . . . , (h.sub.n, d.sub.n) have been stored as target values, wherein the pair of target values (h.sub.x-SOLL, d.sub.x-SOLL) is determined for the currently measured pressure value p.sub.x with the pair of actual values (h.sub.x-IST, d.sub.x-IST) and if there is a deviation between the pair of actual values (h.sub.x-IST, d.sub.x-IST) and the pair of target values (h.sub.x-SOLL, d.sub.x-SOLL) an error signal is generated.

2. The method according to claim 1, wherein the storage of the target values and the comparison with the actual values takes place in a microcontroller.

3. The method according to claim 2, wherein the microcontroller comprises a timer, a memory and a processing unit, characterized by the following method steps: the timer determines the period duration of the alternating square-wave signal and outputs this as a value d.sub.x-IST and forwards it to the processing unit; the currently measured pressure value p.sub.x in the form of the alternating square-wave signal and a trigger signal which the processing unit has generated from the period duration d.sub.x-IST and which precisely defines at which point in time the actual value of the pulse height h.sub.x-IST of the alternating square-wave signal is to be stored, are fed to the memory; the processing unit determines the pair of target values h.sub.x-SOLL, d.sub.x-SOLL for the currently measured pressure value p.sub.x with the pair of actual values h.sub.x-IST, d.sub.x-IST and compares these two value pairs with one another.

4. The method according to claim 3, wherein the pairs of target values (h.sub.x-SOLL, d.sub.x-SOLL) are stored in a lookup table.

5. A method for monitoring the operation of a pressure measuring cell of a capacitive pressure sensor, wherein the pressure measuring cell comprises a pressure-dependent measuring capacitor and a pressure-dependent reference capacitor and the pressure measurement value (p) is obtained as a measurement signal from the capacitance values of the measuring capacitor and the reference capacitor, wherein the measurement signal is fed to an evaluation unit in the form of an alternating square-wave signal, the pulse height of which depends on the quotient of the capacitance values of the reference capacitor and the measuring capacitor and the period duration of which is determined by the capacitance value of the measuring capacitor so that in the nominal pressure range of the pressure sensor there is a fixed relationship between pulse height and period duration, wherein the pressure sensor comprises a microcontroller with a timer, a memory and a processing unit, characterized by the following method steps: defining and storing the functional relationship between the pulse height h.sub.x and the period duration d.sub.x in the form of a polynomial in the memory; the timer determines the period duration d.sub.x of the alternating square-wave signal and forwards it as the value d.sub.x-IST to the processing unit; determining the period duration d.sub.x-SOLL associated with a measured pressure value p.sub.x with the pulse height h.sub.x-IST based on the polynomial; comparing the value for the period duration d.sub.x-SOLL determined based on the polynomial with the actual, measured period duration d.sub.x-IST and generating an error signal in the case of a deviation between d.sub.x-IST and d.sub.x-SOLL.

Description

DRAWINGS

(1) The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

(2) The disclosure is explained below in detail based on exemplary embodiments with reference to the drawings.

(3) The drawings schematically show:

(4) FIG. 1 a block diagram of a capacitive pressure measuring device;

(5) FIG. 2 a schematic sectional view of a capacitive pressure measuring cell;

(6) FIG. 3 a known evaluation circuit for a capacitive pressure measuring cell according to FIG. 2; and

(7) FIG. 4 the evaluation circuit of FIG. 3, supplemented by a microcontroller for carrying out the method according to the disclosure.

(8) In the following description of the preferred embodiments, the same reference symbols designate the same or comparable components.

DETAILED DESCRIPTION

(9) Example embodiments will now be described more fully with reference to the accompanying drawings.

(10) FIG. 1 shows a block diagram of a typical capacitive pressure measuring device which is used to measure a process pressure p (for example of oil, milk, water, etc.). The pressure measuring device 1 is designed as a two-wire device and essentially consists of a pressure measuring cell 10 and evaluation electronics 20. The evaluation electronics 20 comprises an analog evaluation circuit 30 and a microcontroller μC in which the analog output signal of the evaluation circuit 20 is digitized and further processed. The microcontroller μC provides the evaluation result e.g. to a PLC as a digital or analog output signal. The pressure measuring device 1 is connected to a voltage supply line (12-36 V) for energy supply.

(11) FIG. 2 shows a schematic representation of a typical capacitive pressure measuring cell 10 as it is used in a variety of capacitive pressure measuring devices. The pressure measuring cell 10 consists essentially of a base body 12 and a diaphragm 14, which are connected to one another via a glass solder ring 16. The base body 12 and the diaphragm 14 delimit a cavity 19, which—preferably only at low pressure ranges up to 50 bar—is connected to the rear side of the pressure measuring cell 10 via a venting channel 18.

(12) Both on the base body 12 and on the diaphragm 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 diaphragm electrode ME and the center electrode M, the reference capacitor C.sub.R is formed by the ring electrode R and the diaphragm electrode ME.

(13) The process pressure p acts on the diaphragm 14, which bends more or less according to the applied pressure, wherein essentially the distance between the diaphragm electrode ME and the center electrode M changes. This leads to a corresponding change in the capacitance of the measuring capacitor C.sub.M. The influence on the reference capacitor C.sub.R is less because the distance between the ring electrode R and the diaphragm electrode ME changes less than the distance between the diaphragm electrode ME and the center electrode M.

(14) In the following, it is not differentiated between the designation of the capacitor and its capacity value. C.sub.M and C.sub.R therefore designate both the measuring or reference capacitor per se, as well as its capacitance.

(15) A known evaluation circuit 30 for the pressure measuring cell 10 is shown in more detail in FIG. 3. 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 at 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 works as an integrator, into a linearly increasing or decreasing voltage signal (depending on the polarity of the input voltage) that is output at the output COM of the integrating branch IZ. Here, the measuring point P1 is virtually at ground due to the operational amplifier OP1.

(16) The output COM is connected to a threshold value 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.

(17) The differentiating branch DZ moreover 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. At the output of the sample and hold circuit S&H the measurement voltage U.sub.Mess is provided, from which the process pressure p is obtained which acts on the pressure measuring cell 10.

(18) Hereinafter, the function of this measuring circuit is explained in more detail. 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. As a result, a constant current I.sub.1 flows through the resistor R.sub.1, which charges the measuring capacitor C.sub.M until the square-wave voltage U.sub.E0 changes its sign.

(19) It can be seen from FIG. 3 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 even then at the same potential as the measuring point P1, i.e. at ground level, if the connection between the measuring point P2 and the operational amplifier OP2 would not exist. This applies not only in this special 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 the zero point calibration, this state is set accordingly via the variable resistors R.sub.1 and R.sub.2, respectively. If the capacitance of the measuring capacitor C.sub.M changes due to the action of pressure, the condition of the 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 immediately counteracted by the operational amplifier OP2 because the operational amplifier OP2 continues to hold the connection point P2 virtually at ground. At the output of the operational amplifier OP2 therefore a square-wave voltage UR is provided, 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, which is formed by the two resistors R.sub.5 and R.sub.6.

(20) The positive and negative amplitudes A+ and A− of the square-wave signal are added in terms of amount via a sample and hold circuit S&H, wherein the amount A is output as measurement 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 present at the output of the square-wave generator RG, is set dependent on the measurement voltage U.sub.Mess in order to achieve 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 can advantageously be adjusted.

(21) The positive operating voltage V+ is typically at +2.5 V and the negative operating voltage V− at −2.5 V.

(22) FIG. 4 shows in principle the evaluation circuit known from FIG. 3, which, however, is supplemented by a microcontroller μC. In this microcontroller μC on the one hand, the comparator-oscillator SG from FIG. 3 is integrated and, on the other hand, it comprises the units necessary for carrying out the method according to the disclosure: a timer 60, a memory 40 and a CPU 50 as a processing unit. The elements disposed outside of the microcontroller μC are essentially identical and therefore also denoted identically. In order to avoid repetitions, in the following, only the elements essential to the disclosure will be discussed.

(23) On the one hand, the output signal of the threshold value comparator SG is fed back in order to drive the square-wave generator RG, which is already known from FIG. 3. On the other hand, this signal is fed to the timer 60. In the timer 60 the time period behavior of the triangular signal is recorded, in particular with regard to the reaching of the set threshold values. The period duration which is fed to the CPU 50 is derived therefrom. In the present case, the period duration of this triangular signal is identical to that of the actual measurement signal UR, so that the detection of the period duration is particularly advantageous in this way.

(24) Furthermore, the microcontroller μC comprises a memory 40, which is initially supplied with the currently measured pressure value p.sub.x in the form of the voltage signal UR known from FIG. 3. At the same time, and not shown in detail for reasons of illustration, the pressure value p.sub.x is also sent to the output switch_out or analog_out of the microcontroller μC in order to output the measured pressure values as a switching or analog signal. The sample and hold circuit S&H known from FIG. 3 as part of the evaluation circuit shown there is then also integrated into the microcontroller μC and simulated there functionally identical.

(25) Moreover, a trigger signal, which the CPU 50 generates from the period duration, is also fed to the memory 40. This trigger signal precisely defines the point in time at which the actual value of the pulse height h.sub.x-IST of the voltage signal UR is to be stored. This point in time is advantageously exactly in the middle of a positive square-wave pulse in terms of time.

(26) In a calibration procedure that took place before the application of the pressure sensor, for specified pressure values p.sub.1, . . . , p.sub.n the value pairs of pulse height and period duration (h.sub.1, d.sub.1), . . . , (h.sub.n, d.sub.n) have been stored, too, as target values in the memory 40, in particular in a look-up table.

(27) In the CPU 50, the pair of target values h.sub.x-SOLL, d.sub.x-SOLL is determined for the currently measured pressure value p.sub.x with the pair of actual values h.sub.x-IST, d.sub.x-IST, and if there is a significant deviation between the pair of actual values and the pair of target values, an error signal is generated, which is output at the output diag_out.

(28) The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are inter-changeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.