Evaluation of sensitivity and linearity for touch-mode capacitive pressure sensor measuring the inner bladder pressure

Recent studies have shown that pressure sensors for monitoring bladder pressure in real time are under way to develop treatments for urinary bladder dysfunction. Therefore, this study is designed to assess the sensitivity and linearity of touch-mode capacitive pressure sensor (TMCPS), which is appropriate for measuring urinary bladder pressure. In addition, this study analyzes TMCPS's ability responding to geometric design parameters. Finite element model consists of diaphragm and insulator plane. The modeling is based on three geometric design variables: size of square diaphragm, thickness of diaphragm and gap distance between diaphragm and insulator. The result from finite element analysis (FEA) based on those design variables can be used to produce capacitance-pressure (C-P) graph and the first order regression curve enables this study to analyze the sensitivity and linearity within 20~100cmH₂O which is the measurable range of urinary bladder pressure. Regarding the effect on the thickness of diaphragm under the gap distance of 4μm corresponding to the square size of 2.6mm, this research shows that remarkable decline in sensitivity from 0.0031pF/cmH₂O to 0.0022pF/cmH₂O occurs as thickness increases from 11μm to 15μm. Even in the case of 5μm gap distance, sensitivity is shown to decrease from 0.0032pF/cmH₂O to 0.0015pF/cmH₂O as thickness increases from 11μm to 15μm. From a linearity point of view, however, it always marks higher value than 0.9 regardless of its thickness and describes relatively small deviation under 3.0mm. The study shows that as the size of diaphragm gets larger, sensitivity increases but linearity decreases. In addition, the thicker the diaphragm is, the less sensitivity the sensors are. However, the change in linearity turns out to be invalid. This research could be helpful in designing a touch-mode capacitive pressure sensor for urinary bladder pressure measurement.