Capacitive Pressure Sensor Self-Monitoring via Auxiliary Capacitor
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Solution Overview
Problem
Capacitive pressure sensors using the quotient method struggle to reliably detect medium ingress or membrane rupture due to changes in the dielectric constant, especially when the medium's dielectric constant is close to that of air, as seen in oil, which can lead to undetected faults.
Innovation Solution
A method involving an auxiliary capacitor with a constant capacity, independent of pressure, is used to obtain a control pressure measuring value, allowing for comparison with the actual pressure measuring value to infer the operability of the pressure measuring cell, utilizing a quotient or difference formation to detect defects, and an evaluation circuit with a comparator unit to generate a diagnosis signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the quotient method is used for pressure evaluation, then self-linearization is achieved, but medium ingress cannot be detected when the dielectric constant of the incoming medium is similar to air
Solution Approach 1:
The evaluation is segmented into two independent quotient evaluations: one for actual pressure measurement (using measuring capacitor CM and reference capacitor CR) and another for fault detection (using auxiliary capacitor CZ and reference capacitor CR). This segmentation allows the system to simultaneously achieve self-linearization for pressure measurement and reliable fault detection, as the auxiliary capacitor branch independently monitors dielectric constant changes without being affected by pressure-induced capacity changes in the measuring capacitor.
2Measurement precision
If the measuring capacitor capacity changes due to dielectric constant changes, then the pressure measurement is affected, but the quotient formation masks the fault detection
Solution Approach 1:
The auxiliary capacitor CZ acts as an intermediary element that is electrically connected to the same evaluation circuit but physically isolated from the pressure-measuring membrane assembly. This intermediary capacitor provides a reference path that responds to dielectric constant changes (indicating medium ingress) without being subjected to pressure-induced capacity changes, thereby enabling fault detection that is masked in the direct quotient formation of the measuring capacitor.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables secure and reliable detection of medium ingress and membrane ruptures, ensuring the capacitive pressure sensor's accuracy and operational integrity by differentiating between intact and defective states through distinct quotient comparisons.
Implementation Method 1
Capacitive pressure sensors are used to measure pressure in many industrial sectors. They frequently feature a ceramic pressure measuring cell as transducer for the processing pressure
Implementation Method 2
The possibility of exchanging CM and CR in the quotient is also conceivable. The disclosed example with CM in the denominator represents, however, the most common form for the benefit of the self-linearization
Data Source
AI summary
A method for monitoring the operation of a pressure measuring cell (10) of a capacitive pressure sensor (1), wherein the pressure measuring cell (10) has a measuring capacitor (CM) and a reference capacitor (CR) and the pressure measuring value (p) is obtained from the capacity values of the measuring capacitor (CM) and the reference capacitor (CR), characterized in that a control pressure measuring value (p′) is obtained with an auxiliary capacitor (CZ) arranged outside the pressure measuring cell (10) and the operability of the pressure measuring cell (10) is inferred by comparing the pressure measuring value (p) to the control pressure measuring value (p′) is provided.