Minimum variance ultrasonic imaging applied to an in situ sparse guided wave array

Abstract

Ultrasonic guided wave imaging with a sparse, or spatially distributed, array can detect and localize damage over large areas. Conventional delay-and-sum images from such an array typically have a relatively high noise floor, however, and contain artifacts that often cannot be discriminated from damage. Considered here is minimum variance distortionless response (MVDR) imaging, which is a variation of delay-and-sum imaging whereby weighting coefficients are adaptively computed at each pixel location. Utilization of MVDR significantly improves image quality compared with delay-and-sum imaging, and additional improvements are obtained from incorporation of a priori scattering information in the MVDR method, use of phase information, and instantaneous windowing. Simulated data from a through-hole scatterer are used to illustrate performance improvements, and a performance metric is proposed that allows for quantitative comparisons of images from a known scatterer. Experimental results from a through-hole scatterer are also provided that illustrate imaging efficacy.

Analysis of Distributed Sparse Array Configurations for Guided Wave Imaging Applications

“…Hall and Michaels [6] demonstrated that for high SNR environments, such as that considered here, the integration interval in Eq. (4) can be reduced to a single time-sample corresponding to the peak of the transmitted signal.…”

“…This paper considers two elliptical imaging algorithms that calculate each pixel value independently, specifically conventional delayand-sum imaging [4,5] and MVDR imaging [6,7]. These imaging algorithms are briefly summarized here to orient the reader.…”

“…Diagonal loading is used to regularize the matrix inversion of Eq. (7), as in Hall and Michaels [6], with 0.1 times the squared norm of…”

“…All images generated from guided wave imaging algorithms, including conventional delay-and-sum imaging [4,5] and MVDR imaging [6,7], include artifacts that result from energy that has reflected from both structural boundaries and internal scatterers. These imaging artifacts can severely degrade imaging performance and therefore the ability of an array to detect and localize damage.…”

Basic research on machinery fault diagnostics: Past, present, and future trends

“…Lin et al [72,73] investigated different parameters affecting the guidedwave inspection resolution and developed the pulse compression method for carbon fiber reinforced plastic laminates. Hall et al [74,75] proposed the minimum variance ultrasonic imaging method, which adaptively determines the weighting coefficients at each pixel based on traditional delay-and-sum imaging, and better imaging performance was achieved in a composite plate, such as fewer artifacts and robustness to multiple wave modes. Levine and Michaels [76] proposed a Lamb wave propagation model-based imaging method via sparse reconstruction to locate damage position.…”