Determination of Rough Surface Scattering by Ultrasound TDS Measurements
Andreas Grebe
MS Thesis, Electrical and Computer Engineering, Worcester Polytechnic Institute, May 1996.
SUMMARY
This thesis deals with the implementation and evaluation of an ultrasound measurement system to determine the backscattered amplitude from different rough surfaces versus frequency and incident angle as well as for different transducer geometries. This system is based upon the Time Delay Spectrometry (TDS) concept, which utilizes a linear sweep as excitation signal and can deliver an improved signal-to-noise ratio (SNR) in comparison to conventional pulse-echo measurements. The drawback is that two transducers are required instead of one transducer for pulse-echo measurements.
COMPLETE ABSTRACT
The characterization of rough surfaces is of great importance for both material studies and in medical applications. It is preferable that the measurements take place in a non-contact, non-destructive way. This thesis deals with the implementation and evaluation of an ultrasound measurement system to determine the backscattered amplitude from different rough surfaces versus frequency and incident angle as well as for different transducer geometries. This system is based upon the Time Delay Spectrometry (TDS) concept, which utilizes a linear sweep as excitation signal and can deliver an improved signal-to-noise ratio (SNR) in comparison to conventional pulse-echo measurements. The drawback is that two transducers are required instead of one transducer for pulse-echo measurements.
The TDS measurements are performed in the reflective environment of a water-filled scanning tank. Range discrimination is based on the ultrasound propagation delay along the round trip path between transmitting/receiving transducers and a given reflecting surface. Since different delays translate into different shifts in instantaneous frequencies between the transmitted and received signal, TDS is actually a frequency-gating technique.
The complete TDS measurement system which was implemented in this thesis work includes a spectrum analyzer as central analog signal processing unit and a stepper motor. The system is automated by computer software control. The measurement result at a given incident angle is the magnitude frequency response of the ultrasound backscattered from the rough surface. It is obtained as the mean of uncorrelated measurements at different surface regions. The individual measurements are possible by lateral movements of the transducers by means of the stepper motor.
Using the TDS measurement system, the magnitude response in the lower MHz frequency range was obtained for seven random surfaces and one periodic surface over a range of incident angles from 0 to 30 degree with three different transducer types, two planar piston transducers and one annular array transducer. The RMS height of the random surfaces covers a range of 0.02 mm to 0.19 mm and the correlation length a range of 0.16 mm to 0.34 mm. These roughness parameters are obtained independently by profilometric laser scannings.
The magnitude frequency response seems adequate for rough surface characterization since its shape and its variation over a range of incident angles are strongly dependent on the roughness. For the case of random surfaces, the correlation between the magnitude frequency response and the roughness parameters RMS height and correlation length was examined empirically. Furthermore, the results of different transducer geometries were compared.
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