Research On Proximity Capacitive Sensor Design And ECT Technique

Amongst all types of electrical devices, the capacitor is definitely the oldest. However, the real application of the capacitive sensing technique is developed rapidly until recently. All of these applications, from the technique of touch screen on computers and cellphones to the new technology - electrical capacitance tomography (ECT), have proven that the capacitive sensing technique especially the proximity capacitive sensing technique is potential and powerful. As an important part of sensing technique, the design of sensor which directly decides the measured results of the system is always attractive and fascinating. To find out the rules of capacitive sensor design, plenty of research works are needed to be realized and improved. It is also believed that the optimization of design parameters is important and necessary.The thesis focuses on the problems of tradeoff among parameters in procession of capacitive sensor design. By using simulation software, such as COMSOL and Matlab, and practical test platform (ECT test system), the relations between design parameters and the performance of the designed sensor are presented and the detailed design parameters are listed at the end of the paper. The main research works in the dissertation are as follows:1. A mathematical modeling of capacitive fringe effect under the special conditions is presented based on the operation principle of capacitive sensor. It indicates the exact parameters of sensor design which will affect the sensor performance. For many types of sensing techniques, which have been developed and applied in various field, the background and the categories of them are introduced. Three typical sensing techniques, such as optical, piezoelectric and proximity capacitive sensing techniques are discussed. The working principles, merits and shortcomings of these techniques are analyzed. In addition, other three sensing techniques are introduced as the supplements. By investigating and reading a large number of the references, the basic working principle and application field are summarized and classified.2. The evaluation standards of capacitive sensor performance were established. Firstly, the performances of designed proximity capacitive sensor were concluded into several parts including penetration depth, signal strength, sensitivity of the measurements and reconstruction imaging resolution. Then, by using Finite Element Method (FEM), the relations between general parameters, which include geometry and size of the sensor electrodes, sensor materials, number of the sensor electrodes, gaps between the electrodes and the outer earthed screen, etc.. At the last, the sensor measurement performances are quantitative analyzed.3. Three types of ECT sensor, including square-shaped, octagon-shaped and traditional circular-shaped ECT sensor structures are designed for application of industrial tomography system., and the general parameter optimization standards are concluded and presented. By numerical experiments, the influence of different sensor structures on capacitive sensor measurement is concluded. By using finite element method (FEM),3D modeling of designed capacitive sensors are built. The influences of earthed outer screen, axial driven guards, axial earthed end guards and radial earthed guards on sensor performance is discussed and analyzed. The reconstruction imaging test by using optimized sensor with proper imaging algorithms indicates satisfied results.4. An unknown liquid properties detection system based on multi-electrode of capacitive sensor is presented. Two types of sensor structures including plannar-electrode sensor and traditional cylindrical sensor are designed and compared. The disadvantages and merits of each type of sensor are listed. For an 8-electrode cylindrical capacitive sensor, a large number of simulations and practical test have been completed. An algorithm for system image reconstruction is presented. The image reconstruction results indicate that the 8-electrode cylindrical capacitive sensor is feasible to be used in liquid properties detection, and the same time, it clearly decreases the cost of the system.5. Based on the measurement features of the proximity capacitive sensor, an automatic car wiper control system is presented. Moreover, a new capacitive sensor structure is assumed. The system can adjust the operation speed of car wiper automatically according to the amount of the liquid on the windshield. Three types of capacitive sensor are designed, and 4-leaves-shape sensor is selected to be a data acquisition unit of the system. Sensor design parameters are optimized, and the sensor performance is simulated and practically tested. Since the signal of the sensor derived is very weak, the circuit for signal detection is designed. The software algorithms are developed at the last, and the system errors are analyzed.