Method for monitoring the function of a capacitive pressure measurement cell

A method for monitoring unwanted leakage currents of a capacitive pressure measurement cell to detect resistive disturbing influences caused by leakage currents, wherein the pressure measurement cell comprises a measuring capacitor (CM) and a reference capacitor (CR) to which an internal excitation voltage (UE0) is applied in the form of an alternating square-wave signal, and a pressure measuring value (p) is obtained from capacitance values of the measuring capacitor (CM) and of the reference capacitor (CR),

• wherein an excitation voltage (UE0) is converted by means of the measuring capacitor (CM) by integration into a rising or falling voltage signal (UCOM), and
• wherein the voltage signal (UCOM) is fed to a comparator oscillator (SG), as a result of which the excitation voltage (UE0) is generated,
• wherein using an evaluation circuit comprising a microcontroller configured to determine a belly-like progression of voltage values (U1, U2) indicative of an error state, corresponding voltage values (U1, U2) are detected from the voltage signal (UCOM) during falling and/or rising signal progression at at least two defined time (t1, t2), and a linear equation U=f(t) is determined based on two pairs of values (t1, U1) and (t2, U2),
• wherein using the evaluation circuit, within a falling or rising signal progression with aid of the linear equation U=f(t) a time point (tx) is calculated, at which a voltage value (Ux) set in the comparator oscillator (SG) as the threshold value or switchover point is achieved,
• wherein using the evaluation circuit either the time point (tx) is compared with an actual switchover time point of the comparator oscillator (SG) and wherein the evaluation circuit is further configured to generate an error signal when in an event of a significant deviation, or a fictitious switchover point of the comparator oscillator (SG) is defined by the time point (tx) and a fictitious working frequency is calculated therefrom and wherein the evaluation circuit is further configured to generate an error signal when in an event of a significant deviation of this fictitious working frequency from the actual working frequency of the comparator oscillator (SG).