Modeling, design and characterization of a 3D-printed capacitive shear stress sensor

This bachelor assignment discusses the modeling, design, fabrication, and characterization of a 3D-printed shear stress sensor. The design is optimized for application in measurements on the human foot, which is interesting for health-related applications. More specifically, shear stress monitoring could help to prevent the development of diabetic foot ulcers.

This application imposes some requirements and objectives. Measurement range, the accuracy of peak measurements, and the sensor dimensions are the most crucial points of attention. Based on these requirements and objectives, capacitive sensing and Fused Deposition Modelling-printing are selected as the most promising sensing and printing principles.

Theoretical models are developed to study the electrostatic (capacitive) and mechanical behavior of sensing structures. The electrostatic model introduces an approximation for structures of parallel wires. The mechanical model is based upon a hyperelastic beam model. Finite Element Methods are used for the verification of the models. These models are used to optimize the design of the sensor, which consists of 3 sets of 3 electrodes in a beam structure. These are used to measure differential capacitance changes using capacitance to digital converter.

The first results show that it is possible to measure shear stresses. The smallest fabricated sensor with dimensions of 8.75 x 8.75 x 11 mm has a sensitivity of 0.9 pF/MPa in the desired measurement range of 0 to 140 kPa. The fabricated sensors show a linear displacement-capacitance relation. The mechanical behavior of the sensor shows some hysteresis.

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