Design And Implementation Of Multi-FPGA Phased Array Ultrasonic Testing System

Phased array ultrasonic can make the beams focus at the desired point by manipulating the delay of high-voltage pulse exciting corresponding elements.In order to deepen the study on features of girth weld defects of pipeline,full control of excitation signal parameters and access to the received echo-signals should be provided.A multi-FPGA phased array ultrasonic testing system was designed with 32 analog channels,which can connect 128 elements and run at a conversion rate of up to 125 MSPS.Some issues such as pulse transmission delay,reception of echo data,digital beamforming,fault localization are researched in this dissertation.Non-destructive testing methods which are appropriate for different depth are summarized and the theory of phased array ultrasonic is described in detail.The delay of each pulse is derived based on focal laws in the case of transducer mounted on wedge.The approach of determining and locating defects through A-scan signal is discussed,and the formula of defect location is obtained.The design of a separate overall hardware solution has the effect of small board area,small debugging work and easy expansion.It includes function board,main control board,and backplane.Functional units such as high-voltage pulse transmitting circuit,high-voltage power supply,frequency-selective amplifier,channel selection,variable gain amplifier,analog-to-digital converter,FPGA,board-to-board connector are introduced detailedly.In the experiment of high-voltage pulse transmission,the reason why the pulse amplitude does not vary with the input voltage is gained.The problem of high-voltage pulse amplitude attenuation was solved by connecting T/R switch between the pulse transmitting circuit and the echo receiving circuit.High-power high-voltage power supply makes all channels work at the same time.Operational circuits are applied in the control port of high-voltage power supply to prevent high-voltage pulse transmitting circuits from being damaged at power-up.Pipelined delay using four clocks with phase difference was adopted,which can realize fractional delay.The pulse transmission delay accuracy is 2.5 ns.LVDS deserializer circuitry in FPGA is used to receive echo data.Digital beamforming can be realized by doing linear interpolation for echo data and saving the data after interpolating according to the delay scheme of pulse.After comparing signal to noise ratios(SNR)before and after filtering by FIR filter designed in DSP Builder,it can be found that SNR after filtering is 9.4 d B higher.A defect detection experiment was made toward the standard test block to locate the defect in thermal welding zone.The location errors and factors influencing positioning error are analyzed.The result shows that the system is capable of detecting defects and the defect position relative error in depth is 1.7%.This error is up to the criterion in industrial non-destructive testing.