Study On System Of Ultrasonic Beamforming Based On FPGA

The results of ultrasonic testing signal dependent on the ultrasonic transducer and its excitation signal. Therefore, except the ultrasonic probe itself, the type and characteristics of the excitation signal have a significant effect on ultrasonic testing. Since the different types of detected work-pieces, defects and the varying depth of detection, different detecting waveforms are usually required. To achieve the integration of ultrasonic testing technology would require ultrasonic testing system could emit pulses with different frequencies, different pulse widths, and different number to meet the needs of detect demands. So, in this paper, a digital ultrasonic beam former with several ultrasonic waveforms (Single pulse, sine burst, frequency modulation signal, sine wave modulated by the time-domain window) was developed to meet different requirements for transmitting pulses under each major ultrasonic testing method.For the design of the ultrasonic beam former, the response characteristics and the equivalent model of the ultrasonic probe were researched, and compared the response in the time domain and frequency domain using MATLAB simulation, and analyzed the required pulse to achieve the best detection in different test materials or under different detecting methods.According to the needs of waveform analyzed, an ultrasonic beam former designed has four functional blocks:a common pulse switching module, a sine burst generating module, a frequency modulated signal generating module and a time-domain window modulating sine wave generating module. To meet the system’s requirements, the direct digital synthesis (DDS) technology was used to develop the digital ultrasonic beam former based on the field programmable gate array (FPGA), since both the frequency and phase of DDS are controllable, and the output of DDS can be any arbitrary waveform. In addition, a microcontroller MSP430 was added in the actual circuit to simplify the control circuit, and the communication between MCU and FPGA was kept by the UART serial communication circuit.Meanwhile, there were some tests to the ultrasonic beam former which mainly include two aspects:one is testing the basic function of the ultrasonic beam former whose system error was analyzed; the other is doing several ultrasonic testing experiments on the basis of the ultrasonic beam former.The results indicate that the ultrasonic beam former can output four types of waveforms including single pulse, sine burst, frequency modulation signal, sine wave modulated by the time-domain window, of which output range of the frequency is from 0 Hz to 10 MHz, and the resolution of the Frequency Modulation is less than 1Hz. Beside, its adjustable range of pulse repetition frequency is from 0 Hz to 10 kHz, the type of time domain window can be choose, and the number of pulse burst can be controlled, as well.In addition,the founds by researching and comparing these results are as follows.The vertical resolution of broadband pulse, such as square pulse is better than narrowband pulse as modulation signal, but its echo signal will oscillating longer. So, when it comes to detect fine cracks or minor defects, a single high-frequency pulse is more appropriate.Increase the number of pulses can enhance the echo signal’s energy. When detecting deep defects or a work piece with large attenuation, a multi-pulse excitation signal can be used such as the sine burst. When detecting the work pieces with a long testing length and a wide testing range, it is better to use a sinusoidal modulation signal whose directivity is better, in which the sine signal modulated by Harming Window is best.