Design of ultrastable and high resolution eddy-current displacement sensor system

This paper presents a comprehensive analysis and discussion of how to design a high resolution and low thermal drift eddy current sensor (ECS). A simple transformer based equivalent model was presented to explain the influence of target conductivity and working frequency to the ECS´s sensitivity and stability. The relationships between the ECS´s performances and target conditions were also obtained by finite element analysis (FEA). These theoretical and FEA results prove that ECS working at higher frequency and with better conductive target can achieve better performances. A new high performance bridge based synchronous demodulation circuit was designed to precisely measure tiny variations of the sensor coil´s inductance and resistance, separately. An ECS prototype was manufactured and tested. The results shows that The total noise of this ECS from 0.1 Hz to 100 Hz was only 70 pm_rms, and the temperature drift is less than 3 nm/°C by using both the inductance and resistance signal. This new ECS system with sub-nanometer resolution and ultrahigh stability could be widely used in all kinds of advanced applications, even in harsh environments.