Design Of Nondestructive Testing System For Standing Tree Based On Electrical Impedance Tomography

The development of tree nondestructive testing technology is the key to realize the sustainable utilization of wood resources and improve the utilization rate.Currently,the commonly used nondestructive testing methods of tree,such as stress wave method,ultrasonic method and X-ray method,will cause damage to the tree,and these methods are expensive,the equipment is too large,not suitable for outdoor testing of living wood.Electrical Impedance Tomography(EIT)is a non-destructive,high efficiency and high precision detection method.The principle is to place a number of electrodes uniformly around the object,inject AC excitation signal,and then measure the corresponding boundary voltage.The measured voltage data is selected by an appropriate algorithm for image reconstruction,and the electrical conductivity distribution inside the biological tissue is calculated,so as to judge the internal structure of the biological tissue.This paper studies how to use the theory of electrical impedance tomography to locate and identify the holes in the internal structure of living wood.A set of electrical impedance tomography system with 16 electrodes is designed to detect and identify the holes in the internal structure of living wood.The main work contents are as follows:(1)The excitation source of the electrical impedance tomography system is designed and realized,including signal source and current source.USB-6361 data acquisition card is controlled by Lab VIEW to generate sinusoidal AC voltage signal with peak-peak value of 4 V.After the signal source is connected with an amplifier circuit with adjustable gain of 1 ～ 2 times,the voltage signal with peak-peak value of 4 V is amplified to peak-peak value of 8 V.A bridge Howland circuit is selected as the current source,and a capacitor is connected in parallel with the input end and the negative feedback end of the operational amplifier to form the hystere-lead capacitance compensation,ensuring the current source has high output impedance,good stability and bandwidth.(2)The excitation electrode,the measuring electrode of the multiplexing circuit and signal conditioning circuit are designed.The multiplexing circuit adopts the adjacent excitation-adjacent measurement mode.Signal conditioning circuits include voltage follower,instrument amplification,programmable gain amplification,phase-sensitive demodulation,and low-pass filtering circuits.(3)The main control program based on Lab VIEW virtual instrument is designed,including three modules: signal source,boundary voltage signal acquisition and switch of multiple power selection circuit.(4)System performance test,including current source stability test and channel consistency test.After the test was completed,the physical data collection experiment was carried out.In the experiment,the felled fresh oak tree was selected as the test object,and the hole defects existing in the standing tree were simulated by artificial drilling.The excitation current signals with peak-to-peak values of 4 m A and 8 m A were injected respectively,and the boundary voltage signals of healthy,single target holes at different positions and double target holes with different sizes were measured.Then the sensitive matrix algorithm is used to reconstruct the image in Matlab.The experimental results show that the EIT system can realize the detection and location of the hole in the stand,meet the design requirements,and has a good resolution.