Reactive sub-nanometer displacement sensors: Advantages and limitations

In this paper we present the results of an investigation on the performance of eddy-current and capacitive sensors for measuring very small displacements in the sub-nanometer range. The term “reactive sensor” is used as a generic name for inductive sensors (L) using the magnetic field, and capacitive sensors (C) using the electric field, to convert displacement into electrical signals. The need for an accurate displacement/position measurement in such extremely small scales as nanometers and picometers has increased significantly over the last few years. Application examples can be found in the high-tech industry, metrology, and space equipment. This study is based on our recent development results, as well as on the latest reports found in the literature. The goal of the paper is to analyze the commonalities between these two types of sensors, as well as the main performance differences and limitations which define the preferred choice for a specific application. The comparative assessment is done based on both theoretical analysis and experimental results. The main performance criteria used are: sensitivity, resolution, compactness, long-term stability, thermal drift, power efficiency. To the best of our knowledge, such a systematic comparison has not been done yet.