Influence factor analysis of pulsed eddy current testing system

Nearly a decade developed under pulsed eddy current technology can be used to the surface and the surface crack quantitative detection,due to pulsed eddy current frequency spectrum is wide，so Using a single scan the technology can detect the tested workpiece with different depth of a crack。

At present in the field of many structural nondestructive evaluation has been more widely used,    

Before adopting pulsed eddy current technology,it Should first analyze the factors which influence the ability of detection respectively ,In order to reasonably adjust various parameters in the process of testing,Make the system reach the best working condition,The author respectively analyze from the size of the drive coil, excitation frequency, excitation pulse duty ratio and voltage 。

Pulsed eddy current testing system 

Pulsed eddy current testing system consist of pulse signal generating circuit, sensor and data acquisition circuit

Pulsed generating circuit used for producing drive square signal ，In the design of detection system,Square wave signal voltage is 5 ~ 10 v is adjustable,Adjustable frequency 10 hz ~ 100 KHZ,duty cycle is 0.1-0.9,changeable,Probe includes coil excitation coil and testing coil,excitation coil adopts rectangular structure,The center of the testing coil is located in the bottom of the drive coil,

Used for detecting on magnetic vertical component produced by crack disturbance 。Data acquisition circuit adopts 12 PCI data acquisition card。Sampling frequency is 100khz。

The influence of system parameters on the detection performance 

Excitation coil size

In experiments,adopting two rectangular exciting coil with different sizes to analyze,experiments show ,no matter detection on to surface flaw or under surface flaw,using a small incentive coil is bigger than using the big excitation coil of eddy current response signal peak quantity changed,in a word,small coil has high sensitivity than big coil，this is because small excitation coil impedance is small,so under same excitation voltage ,it can generates bigger current and magnetic field,and produce stronger eddy current on the surface and close to surface，so it has higher flaw detection ability。

The influence of excitation pulse frequency

When doing comparison experiment Of pulse frequency ,according to experiment results,Choose small coil with high sensitivity as excitation coil, pulsed duty cycle is 0.5,voltage is 10v,frequency is from100Hz to 1KHz,punch advance is 100Hz。

Concluded from experiment result,when excitation frequency is changing from 100Hz to 400Hz,for surface flaw and under surface flaw,with increasing of excitation frequency,change quantity of eddy current response signal peak value is apparently increased。For surface flaw,with frequency increasing,pulse eddy current testing ability is improving ;for under surface flaw is due to flaw depth is small adopted in experiment。According to skin effect principle,in practical testing process,it should according to the position that flaw will be existed,reasonably choose excitation frequency,so as to up to better testing result。

the effects of duty cycle in Excitation pulse

· When doing duty cycle experiment,Choose small coil with high sensitivity as excitation coil,other parameters are set as follows:excitation pulse frequency is 100hz,voltage is 10v,duty cycle is changing from 0.1 to 0.9,each time step 0.1.

· Analyzing on pulse Fourier,small duty cycle pulse frequency spectrum energy distribution is even，therefore,under high frequency ,it can also have higher energy，this is good for testing surface flaw。But when duty cycle is bigger pulse,its frequency spectrum energy is mainly collected in low frequency，this is good for under surface deeper flaw detection。For deeper under surface flaw,can properly improve pulse duty cycle，improve excitation pulse energy，so as to up to better testing effect。

· Effect of Excitation pulse voltage 

· When doing excitation voltage experiment,choose excitation coil with little dimension。Frequency of Excitation pulse is 100hz,duty cycle is 0.5,excitation voltage is even change from 5-10v。

· From experiment result we can see,with excitation pulse voltage is increasing,regardless of surface flaw or under surface flaw,change quantity of peak value is gradually increasing， this due to excitation voltage increasing,the magnetic field intensity of pulse eddy current testing system produced is improving,so it is good to test flaw,but in practical testing process,excitation current should not be too big,or coil is easy to reach saturation condition 。

Conclusion

The choices of Pulse eddy current parameter has big effect on properties of testing system ，reasonably choose parameter of pulse eddy current testing system has important meaning on improving testing sensitivity。The author analyzes from excitation coil dimension,excitation pulse frequency,duty cycle and voltage affect pulse eddy current testing system working property,giving a choose principle of pulse parameter in practical testing 。However ,due to in practical testing process,detected object is complex and testing requirements are different,therefore,when choose parameter ,must  take every aspect affecting fastors into consideration ,make system up to optimization testing results。