Calibration of ultrasonic hardware for total focusing method imaging


In Non-Destructive Evaluation (NDE), phased array calibration typically pertains to its beam steering, focussing ability and defect detection capability. This paper demonstrates a robust methodology for ultrasonic hardware calibration that aims to investigate hardware integrity rather than beam integrity. To achieve this, information was extracted from the back wall reflection from a planar interface. Using a 128 element linear phased array coupled to two different Phased Array Controllers (PACs), the pulse-echo response of the back wall reflection was recorded for each element. This was repeated five times under identical experimental conditions to provide an understanding of the reliability of measured ultrasonic data and an understanding of the variation between individual array elements and PACs. Post-processing of the pulse-echo signals was used to infer the bandwidth, pulse length, sensitivity and peak time for each element. Following this, using the measured peak time and the empirical time-of-flight (TOF) to the back wall, a relative Delay Factor (DF) was established for each element. The DF was incorporated into the Total Focussing Method (TFM) [1] to offset the TOF estimated for each image pixel. The DF was applied to a typical steel NDE calibration block, where the Array Performance Indicator (API) [1] was used to spatially describe the point spread function of a point reflector. Upon application of the DF, the API of a 3 mm side-drilled hole was enhanced by 56 %. In addition, it was found that the DF matrix was different depending on the PAC used to drive the array. Therefore, the calibration of ultrasonic hardware should be considered in terms of the coupled array-PAC system.