The Study And Application Of Ratiometric Measurement Principle Capacitive Angular Displacement Sensor

Capacitive angular displacement sensor is the instrument that can convert thecapacitance into the electrical signal according to the mechanical rotating angle. It isbelieved as one of the most hopeful sensors in future because of its non-contactmeasurement, good reliability, high sensitivity, high measurement accuracy, powersaving, compact structure and good abilities of resisting harsh environmentalconditions. The capacitive angular displacement sensor has the disadvantages such asthe non-linearity caused by the electric-field-bending, parasitic capacitance & wiringcapacitance, which influence the accuracy and sensitivity of the sensor, ect. Especiallythe character of the capacitance will be changed with the temperature drift, this limitsthe utilization of the capacitive sensor. Some measures must be taken to eliminate theeffect caused by above shortcomings. It can be compensated by the ratiometricmeasurement principle .So the range of measurement of the sensor is expanded andthe error caused by the ambient temperature error is reduced. It improves theratiometirc measurement principle that the measurement range is up to 180°;it alsooptimizes the time of the measurement that the sensor can response much fasterwithout any hardware added.Besides, MSP430 MCU is applied in the project for MSP430 MCU's highdensity of integration, for example, t has hardware multiplier and AD modules. Thesefunctions will promote the speed of calculation and make the circuit more simple andlower power cost. And the accuracy of the system is higher than before, the error isbelow 1 degree with the uncertainty of 0.43.When the ambient temperature changesfrom –10°C to 55°C, the drift of the measurement is 0.136%/10°C,which is betterthan the foreign sensors' ±0.3%/10k level. In order to use the sensor easier, the function of 4-20mA and the seriescommunication is integrated in the circuit, thus realize the communication with PC. At last the sensor is applied in metal tube floater. It can conclude that the effect isbetter than before from a lot of experiments. From different diameter of the pipe, theup limit varies from 2.5m3/h to 16 m3/h, the accuracy of the metal tube floater is 1.5.