A Capacitive Sensor for the Measurement of Departure From the Vertical Movement

A capacitive sensor is proposed to measure the departure from the vertical movement. It has a special structure consisting of a long silica filament static plate and a cylindrical-surface moving plate. Now it is adopted to measure the horizontal displacement of the movable coil moving along the vertical direction in a joule balance. For the consideration of avoiding the error caused by the transfer process of direction reference, the sensor is designed to have a direct vertical direction reference. The measurement errors caused by the imperfection of the capacitive sensor plates are discussed. In addition, the characteristics of the sensor, such as sensitivity, resolution, and short-time stability, have been analyzed through experiments. The experimental results show that the resolution of the sensor is better than 0.05 μm, and the short-time stability is about 0.1 μm peak to peak, which is better than that of the alcohol mirror. The combined uncertainty of the capacitive sensor obtained through analysis and experiments is 1.68 μm.