US-12618735-B2 - Method for monitoring the operation of a capacitive pressure measuring cell

Abstract

A method for monitoring the operation of a capacitive pressure measuring cell which has a measuring capacitor (C M ) and a reference capacitor (C R ) is disclosed, where an internal excitation voltage U E0 is applied in the form of an alternating square-wave signal, and the pressure measured value p is obtained from the capacitance values of the measuring capacitor (C M ) and the reference capacitor (C R ). The excitation voltage U E0 is converted into a voltage signal U COM by integration, with additional voltage values being acquired over time such that the value of the voltage signal U COM is indicative of an external influence of either an electromagnetic and/or low-frequency nature or a resistive interfering influence caused by moisture.

Inventors

• Manfred Maurus

Assignees

• IFM ELECTRONIC GMBH

Dates

Publication Date

20260505

Application Date

20230805

Priority Date

20220818

Claims (8)

1. 1 . A method for monitoring the operation of a capacitive pressure measuring cell ( 10 ) which has a measuring capacitor (C M ) and a reference capacitor (CR), wherein an internal excitation voltage Ugo is applied in the form of an alternating square-wave signal, and a pressure measured value p is obtained from the capacitance values of the measuring capacitor (C M ) and the reference capacitor (C R ); wherein the measuring capacitor (C M ) converts the excitation voltage U E0 into a rising or falling voltage signal U COM by integration; wherein the voltage signal U COM is supplied to a comparator-oscillator (SG), resulting in the excitation voltage U E0 being generated; and wherein the corresponding voltage values U 1 , U 2 are acquired from the voltage signal U COM during the falling and/or rising signal characteristic in at least two defined times t 1 , t 2 and a linear equation U=f(t) is determined on the basis of the two value pairs t 1 ; U 1 and t 2 ;U 2 ; wherein at least one time t 3 is defined between the time t 2 and the switching point of the voltage signal U COM that is set in the comparator-oscillator (SG), the associated voltage value U 3 is calculated using the linear equation U=f(t) within the falling or rising signal characteristic at this time t 3 and this voltage value U 3 is compared with the actual value of the voltage signal U COM at the time t 3 in terms of absolute value; wherein if in a first case the value of the voltage signal U COM is greater than the calculated voltage value U 3 , an external influence of electromagnetic nature is present and wherein if in a second case the value of the voltage signal U COM is less than the calculated voltage value U 3 , a resistive interfering influence caused by moisture may be present.
2. 2 . The method according to claim 1 , wherein the first case and the second case are counted in a time interval in which the triangular-waveform voltage signal U COM has covered more than one hundred periods.
3. 3 . The method according to claim 1 , wherein the excitation voltage U E0 has a varying frequency.
4. 4 . The method according to claim 3 , wherein the varying frequency of the excitation voltage U E0 is produced by changing at least one threshold of the comparator-oscillator (SG).
5. 5 . The method according to claim 1 , wherein the time t 1 correlates to the switching time of the voltage signal U COM .
6. 6 . The method according to claim 1 , wherein the time t 2 is in the first half of the falling or rising signal characteristic.
7. 7 . The method according to claim 1 , wherein the time t 2 is in the middle of the falling or rising signal characteristic.
8. 8 . The method according to claim 1 , wherein further times t 4 , t 5 are defined between the time to and the switching point of the voltage signal U COM that is set in the comparator-oscillator (SG), the respectively associated voltage value U 4 , U 5 is calculated using the linear equation U=f(t) within the falling or rising signal characteristic at these times t 4 , t 5 and the respective voltage value U 4 , U 5 is compared with the respective actual value of the voltage signal U COM at the respective time t 4 , t 5 in terms of absolute value; wherein if the value of the voltage signal U COM is greater than the respectively calculated voltage value U 4 , U 5 an external influence of electromagnetic and/or low-frequency nature is present and if the value of the voltage signal U COM is less than the respectively calculated voltage value U 4 , U 5 , a resistive interfering influence caused by moisture may be present.

Description

CROSS REFERENCE TO RELATED PATENT APPLICATIONS This application claims priority to German Patent Application 10 2022 120 883.4 filed on Aug. 18, 2022 entitled “Verfahren zur Funktionsüberwachung einer kapazitiven Druckmesszelle” (Method For Monitoring The Operation of a Capacitive Pressure Measuring Cell) by Manfred Maurus, the entire disclosure of which is incorporated herein by reference in its entirety. BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to pressure sensors, and more specifically to a method for monitoring the operation of a pressure measuring cell of a capacitive pressure sensor. 2. Description of Related Art Capacitive pressure sensors, or pressure measuring devices, are used in many areas of industry for pressure measurement. They frequently have a ceramic pressure measuring cell as a transducer for the process pressure, and evaluation electronics for signal processing. Capacitive pressure measuring cells have a ceramic base body and a membrane, with a glass solder ring arranged between the base body and the membrane. The resultant cavity between the base body and the membrane facilitates longitudinal mobility of the membrane due to a pressure influence. This cavity is therefore also referred to as a measurement chamber. On the underside of the membrane and on the opposite top side of the base body, there is provision for respective electrodes, which together form a measuring capacitor. The action of pressure leads to a deformation of the membrane, resulting in a change of capacitance in the measuring capacitor. An evaluation unit is used to record the change of capacitance and to convert it into a pressure measured value. These pressure sensors are generally used for monitoring or controlling processes. They are therefore frequently connected to superordinate control units (PLCs). German Patent DE 198 51 506 C1 discloses a capacitive pressure sensor in which the pressure measured value is determined from the quotient of two capacitance values, relating to a measuring capacitor and a reference capacitor. Although this patent specification does not specifically describe a pressure measuring cell, the circuit shown and the method described are suitable for capacitive pressure measuring cells. The particular feature of this pressure measuring device is that only the amplitude of the square-wave signal, regardless of the frequency thereof, is relevant for evaluating the measurement signal at the output, as a measure of the recorded pressure measured value. European Patent EP 0 569 573 B1 discloses a circuit arrangement for a capacitive pressure sensor that likewise involves the use of a quotient method for pressure evaluation. Quotient methods are generally based on the following pressure dependencies: p∼CRCM⁢ and/or⁢ p∼CRCM-1⁢ or⁢ p∼CM-CRCM+CR, where CM denotes the capacitance of the measuring capacitor, CR denotes the capacitance of the reference capacitor and p denotes the process pressure to be determined. There is also conceivably the option of interchanging CM and CR in the quotient. However, the indicated example with CM in the denominator is the most common form in support of self-linearization. This embodiment is therefore assumed below, unless indicated otherwise. Reliability in capacitive pressure sensors is becoming more and more important. A desirable aim is to optimize the measurement principle in pressure sensors with regard to potential leakage currents on the back of the measuring cell—the side facing away from the medium to be measured—or in parts of the evaluation electronics for the purposes of eliminating humidity portions that may have been introduced by the environment and have a tendency to condense. German Patent DE 103 33 154 A1 and German Patent DE 10 2014 201 529 A1 disclose monitoring the operation of capacitive pressure sensors. German Patent DE 197 08 330 C1 and European Patent EP 2 738 535 A1 provide further general background information. German Patent DE 10 2018 118 645 B3 and German Patent DE 10 2018 118 646 B3 each disclose a method for monitoring the operation of a pressure measuring cell of a capacitive pressure sensor. German Patent DE 10 2018 118 645 B3 is based on the knowledge that there is a fixed relationship between pulse height, or amplitude, and period duration, or frequency, in the nominal pressure range of the pressure sensor and that the square-wave signal formed from the above-described quotients of the measuring capacitor CM and the reference capacitor CR changes significantly when medium enters the measuring chamber—as a result of damage to the measuring membrane or ingress via the ventilation channel. German Patent DE 10 2018 118 646 B3 discloses where moisture on the back of the measuring cell, facing away from the medium to be measured, or in parts of the evaluation electronics and the leakage currents resulting therefrom bring about a change in the measurement signal present in the form of