| Our country’s forest resources are scarce and the ecology is relatively fragile,so the forestry development is under great pressure.With the rapid development of manufacturing industry,the over exploitation of forest resources leads to the increasing shortage of high-quality wood that can be produced and utilized.It is an effective method to use nondestructive testing related technology to detect the defects of plates so as to judge their internal and external properties.In this paper,a non-contact ultrasonic automatic quality detection system for Chinese fir board is designed.The system takes air as the ultrasonic coupling medium,which can effectively avoid the direct contact between the ultrasonic transducer and the tested sample.The rapid scanning and imaging of Chinese fir board are completed through the independently designed and built cscan platform.This technology effectively solves the problems of contact ultrasonic defect detection.Firstly,this paper describes the principle,advantages,research status at home and abroad and ultrasonic scanning imaging technology of air coupled ultrasonic nondestructive testing technology.The hardware part of the non-contact ultrasonic automatic quality detection system for Chinese fir board is designed,including c-scan platform,ultrasonic signal transmitting module,ultrasonic transmission wave signal receiving module,data acquisition and transmission module,etc.The c scanning platform is composed of two stepper motors and mechanical frame.The stepper motor is controlled by the main controller STM32;The ultrasonic signal transmitting module generates sinusoidal excitation signal through DAC.The excitation signal increases the voltage to 400 V through power amplifier and transformer,and excites the ultrasonic probe to generate ultrasonic signal;The ultrasonic transmission wave signal receiving module processes the weak transmission wave signal received by the receiving probe,including pre signal processing circuit,second-order inverse signal amplification circuit,band-pass filter circuit,etc;The data acquisition and transmission module uses the oscilloscope to capture the signal and transmit it to the upper computer after data acquisition.Then,the software of non-contact ultrasonic automatic quality detection system for Chinese fir board is designed,including host computer interface,c-scan motion control program,serial communication,signal cross-correlation algorithm,imaging algorithm,interpolation algorithm and so on.The main controller communicates with the upper computer through the serial port.When the c-scan motion starts,the ultrasonic probe reaches the specified position,the DAC of STM32 outputs the sinusoidal excitation signal,captures the ultrasonic transmission signal waveform processed by the hardware circuit through the oscilloscope,and transmits the collected data to the upper computer.The transmission time of the ultrasonic signal is obtained by crosscorrelation operation,and the amplitude of the transmission signal is obtained by envelope detection method,Each sampling point is detected in turn,and finally the defect imaging is completed.Then the interpolation algorithm is used to interpolate the original imaging data to make the image of Chinese fir board smooth and intuitively reflect the defect position and shape.Finally,taking the Chinese fir board with a thickness of about 1cm as the test sample,the defects are imaged and detected.The imaging results are compared with the defects of the tested board,which proves that the designed detection system has a high ability to identify the defects.The experimental results show that both amplitude imaging and transmission delay imaging can detect defects,and the effect of amplitude imaging is more obvious.To sum up,the non-contact ultrasonic automatic quality detection system of Chinese fir board designed in this paper can effectively detect the defects in Chinese fir board.The detection effect of this detection system is ideal and is expected to be applied in the actual detection work. |
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