Digital Signal Processing in Acoustical Focused Image Holography
This paper presents a new concept of signal processing in acoustical focused image holography, named “digital accustical holography”. Presented here is a method based on time coincidence measurement between digitized reflected pulses and digital clock pulses for phase detection instead of heterodyne phase detection used in conventional acoustical holography. This digital phase detection permits the use of a wide band (spike shaped) transmitted pulse which increases the depth resolution remarkably. Furthermore, fringe representation corresponding to the deviation in the propagation path length of less than half a wave length can be easily achieved by changing the clock pulse frequency independently of the carrier frequency of the transmitted waves.
A prototype signal processor based on the above mentioned method was produced and its function was tested by imaging artificial defects fabricated in a 10 mm thick stainless steel plate and located at a distance of 5mm from the weld. A focus type transducer was used and its resonance frequency was 3 MHz. Images of artificial defects can be distinguished clearly from images of the weld by the use of a wide band transmitted pulse. The height of each defect is evaluated by counting the number of fringes appearing in the focused image hologram. The deviation of the estimated height from the true value is about 30% for the defect of 1 mm height with 6 MHz clock pulses, while the corresponding deviation is about 50% by the conventional focused image acoustical holography.
KeywordsQuartz Welding Acoustics
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