Method for operating an absolute, or relative, pressure sensor having a capacitive transducer

1. A method for operating a pressure sensor, wherein the pressure sensor includes a measuring membrane, at least one platform and a capacitive transducer having at least a first pressure dependent capacitance and a second pressure dependent capacitance, wherein the measuring membrane divides a volume pressure-tightly into a first volume portion and second volume portion, the second volume portion is enclosed in a measuring chamber between the measuring membrane and the platform, a deflection of the measuring membrane depends on a pressure measurement variable p, which is a difference between a first pressure p 1 in the first volume portion and a second pressure p 2 in the second volume portion in the measuring chamber, the first capacitance and the second capacitance are measured, in each case, between an electrode on the measuring membrane and a counter electrode having an essentially pressure independent position, a respectively current value of the pressure measurement variable p is ascertained as a function of the first capacitance and of the second capacitance, for an intact pressure sensor in thermal equilibrium, the second capacitance is representable as a predetermined function of at least the first capacitance the method comprises the steps of:

registering value pairs of the first capacitance and the second capacitance;

testing whether the registered value pairs of the first capacitance and the second capacitance actually correspond within a predetermined tolerance range to the relationship of the predetermined function; and

determining a change and/or damage of the sensor, when this is not the case for a time period, which lasts longer than a time limit value wherein:

the time limit value is a predetermined limit value, especially a time constant, or a multiple of this time constant, which specifies the time after which an equilibrium state of the pressure sensor after a temperature jump is reached.

2. The method as claimed in claim 1 , wherein:

the time limit value is a function of the deviation of the values of the first capacitance and the second capacitance from the relationship of the function.

3. The method as claimed in claim 2 , wherein:

the time constant is a monotonically falling function of the deviation.

4. The method as claimed in claim 1 , wherein:

an intact pressure sensor has a first intact operating state and at least a second intact operating state;

for an intact pressure sensor in the first operating state in thermal equilibrium, the second capacitance is representable as a first predetermined function of the first capacitance and, in given cases, the temperature;

for an intact pressure sensor in the second operating state in thermal equilibrium, the second capacitance is representable as a second predetermined function of the first capacitance and, in given cases, the temperature;

the method further includes:

testing whether registered values of the first capacitance and the second capacitance actually correspond within a predetermined tolerance range to the relationship of one of the predetermined functions;

in case this is so, ascertaining the pressure measurement variable based on a transfer function, which is associated with the operating state, which corresponds to that predetermined function, whose relationship the registered values of the first and the second capacitance satisfy; and

determining damage of the sensor, in case this is not so for a time period, which lasts longer than a time limit value.

5. The method as claimed in claim 4 , wherein:

the pressure sensor transfers irreversibly from the first intact operating state into the second intact operating state.

6. The method as claimed in claim 5 , wherein:

the method further includes the steps of:

checking, whether the sensor is located in the first operating state, and when this is not the case;

checking whether the sensor is located in the second operating state; and in case this is not the case for a time period, which lasts longer than a time limit value;

determining that the sensor has been damaged.

7. The method as claimed in claim 1 , wherein:

the capacitive transducer includes a differential capacitor having at least a first capacitor, whose first capacitance has a first transfer function, and a second capacitor, whose second capacitance has a second transfer function; and

the transfer functions differ in their pressure dependencies.

8. The method as claimed in claim 7 , wherein:

the two capacitances are equal, when the pressure measurement variable has the value zero.

9. The method as claimed in claim 7 , wherein:

the differential capacitor comprises a circular disk shaped measuring electrode and an annular reference electrode, especially a capacitance equal, annular reference electrode, which surrounds the measuring electrode;

the measuring electrode and the reference electrode are arranged especially on a platform surface facing the measuring membrane; and

the measuring membrane has a circular disk shaped membrane electrode, which extends preferably at least to the outer edge of the annular reference electrode.

10. The method as claimed in claim 9 , wherein:

the pressure measurement variable p is given as:

p=p (( C p −C r )/ C r ),

wherein C p refers to the capacitance between the measuring electrode and the membrane electrode, and C r refers to the capacitance between the reference electrode and the membrane electrode.

11. The method as claimed in claim 1 , wherein:

the sensor comprises a ceramic, circular disk shaped measuring membrane and a ceramic circularly plate shaped platform, and wherein the measuring membrane is connected along a peripheral joint pressure-tightly with the platform to form the measuring chamber pressure.

12. The method as claimed in claim 1 , wherein:

the pressure sensor is arranged in a metal housing having a sealed housing opening, through which the measuring membrane is contactable with a media pressure;

the housing has an annular sealing surface, which surrounds the housing opening;

a sealing ring is clamped between the measuring membrane and the sealing surface; and

the pressure sensor is supported on the rear-side of the platform facing away from the measuring membrane with a clamping apparatus, in order to hold the pressure sensor clamped against the sealing ring.

13. The method as claimed in claim 12 , wherein:

the sealing ring comprises an elastomer;

the sealing ring changes its elastic properties irreversibly, when it is exposed to a temperature, which exceeds a material dependent limit value, whereby clamping forces acting on the measuring membrane and the platform change, whereby the relationship between the capacitances is changed; and

based on value pairs c p , c r , it is ascertained, which function c r (c p ) is suitable for describing a current relationship between the capacitances.

14. The method as claimed in claim 13 , wherein:

based on the ascertained suitable function c r (c p ), a function p(c p , c r , T) is selected for calculating the pressure measurement variable, and a value for the pressure measurement variable is calculated by means of the selected function p(c p , c r , T).