The Design, Preparation And Characteristics Research Of Large Strain Sensors

As one of the most important parameters of structural safety evaluation, strain reflects the inherent characteristics of system structures. In certain structure measurement with large strain and even super-large strain, the conventional detection method can not meet the requirements. Therefore, it is urgent to develop the miniaturization large strain sensor with the high precision and good repeatability and stability. In this dissertation, the various structure large strain sensor, such as the capacitance with the variable area, the moiré fringe grating and composite materials resistance types, were designed and fabricated based on the different large strain testing principles, aiming at the testing application requirement of large strain for some skin materials. And the design and simulation research of the signal readout circuit were made. It was innovative to combine the pull-out structure with the resistance test method, and the slide resistance type large strain sensor was developed, which meet the application requirements. The main results are as follows:1. Based on the basic principle of variable area type capacitance sensor, the miniaturization large strain sensor(the dimension was 25 mm×15 mm ×15 mm) was designed and fabricated with the pull-out single and double capacitance structure, and the alumina and chromium- gold alloy was used as the capacitance plate and electrode material, respectively. The large strain sensing characteristics were tested, analyzed and compared. The influence of the structural parameters such as capacitance plate width and spacing on the large strain sensing property was simulated and researched with the finite element analysis. The results indicated that the larger of plate width and the smaller spacing result in the higher sensitivity of devices. The characteristic test of single capacitance structure large strain sensor results showed that, the sensor had the good linear characteristic(1.60% FS) and repeatability(0.89% FS), high sensitivity(19.06V/ε) and less hysteresis(1.17% FS), however, the resolution could not meet the requirement, which should be better than 0.5%FS. On this basis, the double capacitance structure large strain sensor was designed using double symmetrical fixed and movable plates for the first time. The structural stability was improved, and the error caused by the environmental strain was compensated. Meanwhile, the initial capacitance value of the sensor was increased with the double capacitance structure and mica sheet as the medium. The signal readout circuit was designed using the difference method, and the measurement accuracy was improved, meanwhile, the influence of the zero capacitance and stray capacitance of the measured capacitance was counteracted. The results indicated that the linearity(0.68% FS), sensitivity(20.39V/ε), repeatability(0.17% FS) and hysteresis(0.27% FS) were all superior to those of single capacitor structure, and the resolution can meet the application demands. However, the zero output drifting of the sensor occurred with the temperature, this is because the environment temperature had some effect on the capacitance structure and the dielectric constant of medium.2. Based on the principle of moiré-fringe measuring displacement, the pull-out grating large strain sensor was preliminary designed and prepared, and the large strain characteristics of sensors were tested and analyzed. Based on the requirements of the grating moiré fringe system, the SFH4680 gallium arsenide near-infrared solid light-emitting diode was adopted as the light source, and the M1045 photodiode array was used as the photoelectric detector. The split phase grating was the indicating grating, which was fabricated using the chromeplate process, and the lattice spacing was 25 lines per millimeter. The assembly and commissioning process of sensors were investigated. The sensor signal readout circuit was composed of the differential amplifier circuit, the distinguishing circuit and the count and display circuit. The test results showed that the fabricated sensor could measure the large strain within 0-0.2ε, and the good linear characteristic between the pulse output and the large strain input was obtained(0.82% FS), the sensitivity was high(627 pulses/ε), and the hysteresis was less(0.80%FS), but the repeatability was a little poor(1.94% FS), and the resolution could not meet the application requirements. The characteristics of sensors were analyzed from the views of structure design, preparation process and circuit design, and the improvement solutions were put forward briefly.3. The polymer/conducting filler composites strain sensitive materials were prepared by the physical blending technology, and the effect of the component and proportion of composite materials on the large strain sensing properties was investigated. The microstructures of composite materials were characterized by scanning electron microscopy(SEM), and the sensing mechanism model was established. The test results showed that the silicon disulfide/carbon black and polydimethylsiloxane(PDMS)/multi-walled carbon nanotubes(MWNTs) composite sensitive materials could all detect the large strain within 0-0.2ε. When the volume fraction of carbon black nano particle was 30%, the composite material sensor showed the best linear characteristic(R2 value was 0.9735) and the largest sensitivity(the strain coefficient was 2.9). The “aggregate-chain” strain sensitive model was established from the point of view of conductive path theory. The optimal large strain sensing property was obtained when the content of MWNTs was 9 wt % in the PDMS/MWNTs composites(the R2 value was 0.9982, and the strain coefficient was 3.1), and the sensing model of the sensor was established based on the tunnel theory. The large strain properties of PDMS composite material filled with MWNTs and graphite were preliminary explored. The resistance change of the composite materials exhibited the nonlinear characteristics with the large strain, which was ascribed to the mutual complementation of the remote conductive network of MWNTs and short-range conductive network of graphite, and the more stable conductive pathways and network was built, therefore, the linear increase of resistance was retarded.4. The slide resistance type large strain sensor was designed and fabricated for the first time through combining the pull-out structure with the composite materials resistance test method. The resistance of sensor was altered by changing the contact point between the electric brush and the composite materials resistor stripe under the action of large strain. The signal readout circuit was designed by the single Wheatstone bridge measurement method, and the nonlinearity error of the sensitive device was amended using the positive feedback voltage linearization method. The large strain characteristics test results showed that the sensor had good linear characteristic(0.83% FS) and repeatability(0.32%FS), high sensitivity(20.21V/ε) and less hysteresis(0.25%FS), and the resolution was superior to 0.5%. Meanwhile, the effect of ambient temperature on the zero output of the sensor was less(temperature drift coefficient was 0.027%FS/℃), and the sensor fully meet the application requirements for the some skin materials with miniaturization and high performance.