Improved capacitive sensor for combined angular and linear displacement sensing

This paper presents the design and development of a non-contact capacitive sensor suitable to measure both angular and linear displacements. The sensor is composed of two parts: a rotating shaft and three fixed pairs of electrodes. The shaft has a semi-cylindrical rectangular slot at its middle. Two pairs of electrodes are designed for the measurement of angular displacement and the remaining pair, for linear measurement. A suitable signal conditioning circuit is developed to obtain the change in capacitance between the electrodes and the shaft-slot combination. A function is defined to calculate the absolute angular and linear positions of the shaft. A prototype of the sensor, capable to measure 360° for angular measurement and ±45 mm for linear measurement, was built and tested in the laboratory. The measurement results show that the resolution of the prototype is 0.15° for angular displacement and 41μ m for the linear displacement. The linearity error in measurement was found to be less than 0.9% for both angular and linear displacements.