For Stress Testing Of Rail Temperature Structure Of The Capacitive Displacement Sensor

The structure scheme of the capacitive displacement sensor that tests temperaturestress of seamless railroad is described. Based on traditional measure technique andcalibrated length technique of rail, the sensor is designed in draft. The free rail and thefixed rail are installed by parallel. When the environmental temperature changes, thefixed rail can not flex because of the resistances of fasteners and rail bed. And thetemperature stress is produced. At the same time, the free rail can flex and change itslength by the release of rail's temperature stress. Accordingly, the difference betweenthe instantaneous natural length of the free rail and the fixed rail, which is tested bythe capacitive displacement sensor, reflects the magnitude of temperature stress. In thetest method, the moving electrode is installed on the free rail and the stable electrodeis installed on the fixed rail. The displacement of the two electrodes is the differencebetween the instantaneous natural length of the free rail and the fixed rail. When theenvironmental temperature is changed, the length of fixed rail is not changed, but thefree rail can flex and its length is changed. The free rail pushes the moving electrode,and the distance between the electrodes is changed. As a result, the capacitance ischanged. Thereby, the temperature stress is tested through the change of thecapacitance.The temperature strain is only tens of microns in a convenience distance. Inorder to improve the sensor's sensitivity and reduce the difficulty in circuit's design,the displacement amplifier is designed which is double flexible parallel mechanism.The mechanism can reduce the transferring error, the installing error, and the sensor'svolume. With regard to the displacement amplifier's design, on one hand, it'samplificatory multiple and the movement are analyzed, and the parameters of flexiblehinge are designed. On the other hand, the model of FEM for the structure of theflexible hinges amplifier (HIA) is set up, and the HIA is analyzed and optimized.The model of FEM for electric field is set up and analyzed. To decreaseinfluence by the field changes caused by the spacer, the electrode of capacitance isimproved by applying the guard electrodes. The electric field with guard electrodescompare with the electric field without guard electrodes. The parameters of thecapacitance are optimized.Finally, the affect of the sensor's sensitivity and linearity by the HIA isanalyzed and the sensor is optimized.