Lift-off Effect for Capacitive Imaging Sensors
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Updated: Jan 31, 2026

The Calibration and Use of Capacitance Sensors to Monitor Stem Water Content in Trees
Published on: December 27, 2017
Xiaokang Yin1, Chen Li2, Zhen Li3
1Centre for Offshore Engineering and Safety Technology, China University of Petroleum (East China), Qingdao 266580, China. xiaokang.yin@upc.edu.cn.
Abstract:
Capacitive Imaging (CI) sensors are capable of non-destructively detecting both surface and hidden defects in dielectric materials and characterizing conducting surfaces through a relatively thick insulation layer. However, the complex Measurement Sensitivity Distribution (MSD) of CI sensors render the sensor capacitance variation with lift-off highly non-linear, which may lead to misinterpretation of defect indications. This work systematically studied the lift-off effect using both Finite Element (FE) analysis and experimental approaches. Sensor MSD was used as a tool to predict the imaging performance. Normalized Variation Ratio (NVR) was introduced and used to characterise sensor responses due to defects for a CI sensor. Both the FE analysis and experiments suggest that the lift-off effect for a CI sensor is specimen type and condition dependent. For a given defect, the NVR may vary non-monotonically with increased lift-offs. A case study on a glass-fibre composite/aluminium hybrid structure with multiple artificial defects demonstrated the feasibility of defects discrimination using multiple CI scans with increased lift-offs.
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