Ultrasonic Gauging and Wavelet Image Processing for Wear and Erosion Mapping

A Computerized Ultrasonic Gauging System (CUGS) has been developed to generate very precise topographical maps of the outer and inner surfaces of tubes during various stages of manufacture and life-cycle of the parts. Measurements of the tube dimensions are obtained with a resolution of 2.5 μm (10−4 inches) and accuracies of the order of 10 μm (4×10−4 inches) or better. A typical output of CUGS is an ultrasonic image, in which the horizontal and vertical axes represent the axial and angular position of the part, respectively[1]. The system is currently utilized to gauge tubes as long as 7.5 m (25 feet) resulting in the acquisition of more than 15,000,000 data points, which require up to 60 Mbytes of disk memory on a computer. A Wavelet Transform Image decomposition technique was developed for compression and processing of the ultrasonic images, resulting in a great decrease in time and resources needed to perform such operations. Waveletbased image analysis has two distinct characteristics: multiresolution and high spatial localization[2]. Multiresolution refers to the possibility of obtaining representations of the same image with different resolutions. The high spatial localization properties of the filters used for the wavelet decomposition can also be utilized for the enhancement of features such as erosion pattern without the loss of localization, a problem commonly encountered in Fourier analysis. In the application here discussed, CUGS is utilized to map the wear of the internal surface of steel tubes, before and after exposure to extreme environments involving temperature, pressure, corrosive gases and mechanical forces