Capacitive Strain Sensors For Measurement Of Large Strain For Structural Health Monitoring
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Structural health monitoring (SHM) is increasingly used to replace time based management, with a more efficient Condition Based Maintenance (CBM). The traditional scheme of scheduling down time for aircraft inspection or periodic evaluation of civil infrastructure costs millions of dollars in maintenance due to either downtime or lack of timely maintenance. SHM involves periodic, real time sampling and analysis of signals, from onboard sensors like accelerometer, pressure, strain or acoustic sensors to determine the condition of the structure. This can be used to carry out immediate repairs as well as predict failure. We report the development of a new high sensitivity capacitive strain sensor which can measure large strains. This can be used to measure large strains to detect failure as well as small variations to efficiently analyze and predict failure. Two different topologies are explored. The first comb based structure is a modification of a previously reported structure used for measuring small strains. Despite the high sensitivity achieved, this topology suffers from some intrinsic limitations. This is addressed by the second parallel plate based topology. This retains advantages of the comb topology like high sensitivity and differential gain while overcoming limitations of maximum strain and also simplifies fabrication. A simple design approach has been used to realize both devices. Approximate equations are first used to establish course guideline for the design and then finite element method (FEM) based simulation is performed in CoventorWare© to optimize the devices. Several devices are designed in both topologies and optimum designs are selected from these. Since the device material will be subjected to large strains the material selection and fabrication process is developed simultaneously with the design. Considering readout requirements all sensors were designed for rest capacitance between 0.5pF to 1pF. The dimensions of the plate sensor, measuring similar strain (2%), were almost 60% smaller at 1500μm X 950μm as compared to the comb structure at 2450μm X 1350μm. The comb based topology demonstrated higher sensitivity with a gauge facto of 81 for 1% strain sensor and 51 for the 2% strain sensor, but could not be designed to measure higher strains. The plate structure had a gauge factor of 57, 38, 38 and 19 for 0.5%, 1%, 2% and 3% strain sensors respectively. A fabrication process is also outlined with emphasis on design specific requirements. In case of fabrication he plate structure would require a nominal aspect ratio of 2 as compared to 10 in case of the comb structure.