Investigation On Eddy Current Sensors With High-Precision And High-Stability

Eddy current sensor is a classic sensor type which is widely used in industrial and laboratory applications.Eddy current seneor has seveval advantages,such as non-contact.high reliability,high bandwidth,high sensitivity,low cost,and high tolerance to environmental contaminants.Eddy current sensor could be divided into two main types according to different target thickness:displacement sensor and thickness sensor.They are both based on eddy current effect and have been developed for tens of years.And there are various sensor models in the market.However,plenty of application requirements need to be satisfied and difficult problems need to be soloved.For eddy current displacement sensor,the biggest challenge is how to meet the requirements in space telescope area,which requires ultrahigh accuracy and stability.Foreign companies now dominate the market of high performance sensors.China has been far behind them,so it is even harder to satisfy the needs of space telescope applications.This paper investigated the key techniques on high-precision and high-stability eddy current sensor.We hope to create the most advanced products to improve China's technologies of high precision displacement measurement.And we think that our prototypes have strong competitiveness in the market of eddy current displacement sensors.For eddy current thickness sensor,the previous technologies have more or less technical or application problems.In fact,there is no technology which can combine all the advanteges such as high measurement speed,high precision,small size and low cost.In this paper,we proposed a novel approach which could meet all these requirements.The main research contents and innovations of this dissertation are as follows:1)The impedance calculation formula of the sensor coil was derived based on transformer equivalent model and circular equivalent model.Finite element simulation software was used to analyze how the following contents affect the sensor performance:the material parameters of the target,the size of the sensor coil and the circuit parameters.Based on theoretical analysis,eddy current sensor has two major branches:displacement sensor and thickness sensor.2)A prototype of displacement sensor was made and it basically achieved the design target.All the major parts of the sensor,such as coil size,measuring bridge,demodulator circuit and AD converter,were analyzed how they influenced the sensor performance.The measuring bridge is the most important part of the sensor which can precisely measure tiny variation of the inductance and resistance,separately.Numerical calculation method based on MATLAB obtained several sensor performances quantitatively,such as sensitivity,linearity,and temperature drift coefficient.It optimized the parameter selection of the measuring bridge.3)A comprehensive analysis of the temperature drift sources was presented.We focused on the optimization of bridge parameters which could significantly affect the temperature drift coefficient.A new temperature compensation method was proposed which used the coil resistance to compensate the displacement measurement.It was proved that resistance and inductance canbe separated through some mathematical transformation of the two output voltages.This method needs no additional temperature sensor and has better performance than using temperature sensor.4)Differential bridge was used instead of wheatstone bridge.Differential bridge has severial advantages,such as good linearity,high sensitivity and immune to common mode disturbance.Numerical calculation was used here to optimize the parameters of differential bridge.The sensor holder fixed on the back of the mirror segments was designed and fabricated.The sensor holder must have ultra-high stability to give full play to the sensor performance.It is also important that the sensor holder can be easily fixed and adjusted.The sensor achieved the design target through comprehensive test.5)We proposed two kinds of eddy current thickness sensor.They have similar measurement theory which uses the slope of the lift-off curve(LOC)in the R-L impedance plane to characterize target thickness.But the implementation is different.The first approach gets the lift-off curve by vibrating the sensor coil(changing the distance between the coil and the target).The second approach is an optimization of the first one.It has a specially designed probe circuit and it uses the phase difference between signals to calculate the slope of the lift-off curve.The sensor has high measurement speed,high precision,small size and low cost.