Precision displacement measurement system based on high sensitivity inductive sensing method

Abstract

An inductive sensing system is widely used to measure the displacement. The inductive displacement measurement system has a high ratio of range to resolution, reliability, and durability. Furthermore, it is easy to reduce its size and its production cost is low. However, conventional inductive displacement measurement systems have low sensitivity and thus their resolution is low. A variable air-gap type inductive system was introduced to have high sensitivity. However, it has a schematic nonlinearity, and its guaranteed linear measurement range is too small compared with the maximum measurement range. A newly designed inductive precision displacement measurement system has been proposed to have high sensitivity and high linearity. The proposed system consists of driving coils, position-detecting coils, movable cores, and magnetic blocks. Mn-Zn ferrite is adopted as a magnetic material for its good magnetic and electric characteristics. A symmetric and closed-loop form with narrow air gap is adopted to generate strong flux and to reduce the flux leakage. The flux is divided into two parts according to the overlap-area sizes in two cores, which induces the variation in inductance to be linearly proportional to the input displacement and therefore its linearity is enhanced. The important elements that affect the system characteristics are turn ratio, air-gap dimension, initial overlap length, excitation frequency, LC resonance, load resistance, geometric dimension and so forth. The system performance has been experimented and its characteristics have been evaluated by permeance modeling based on a magnetic circuit. The appropriate turn ratio and air-gap sizes are selected to generate high sensitivity and stable responses, considering LC resonance. The obtained results show that the sensitivity was $2200 - 2800 mV\cdotV^{-1}mm^{-1}$ with a linearity error below ±0.10% over a ±200㎛ range. The excitation frequencies were 2.5k, 5k, and 10kHz. The designed sensi...