A computer-controlled ultrasonic C-scan instrumentation system that was developed primarily for use in finding defects inside solid material specimens can also be used to obtain three-dimensional profiles of the top surfaces of such specimens. In some applications, ultrasonic profiling by use of this system might be an attractive alternative to profiling by use of calipers, linear variable-differential transformers, coordinate-measuring machines, laser profilometers, and other instruments.
The present ultrasonic instrumentation system was described in "Apparatus for Advanced Ultrasonic C-Scan Imaging" (GSC-13524),NASA Tech Briefs, Vol. 21, No. 4 (April 1997), page 34. To recapitulate: The system includes an ultrasonic transducer that is scanned in a horizontal (x,y) plane and that is connected to an electronic pulser/receiver, a dual timing gate, a peak detector, and a universal timer. At each position x,y along the scan, the computer estimates the depth of any feature that reflects ultrasound, using the known or assumed speed of sound and the measured round-trip travel time (also called "time of flight," or "TOF" for short) of the ultrasonic signal. The peak detector operates with a time gate chosen so that its output indicates the amplitude of the signal reflected from the feature of interest atx,y. Once the scan has been completed, the computer processes the x,y scanning-position data and the associated depth and amplitude data into a single three-dimensional-appearing plot that shows both depth and amplitude as functions of x and y.
The figure illustrates the use of the system for profiling. An object to be profiled is placed on the bottom of a tank of water with the surface of interest facing upward. The transducer is immersed in the water so that its x,y scanning plane lies at a convenient height above the object. For the purpose of profiling, the TOF to measure is the round-trip travel time for ultrasound that originates at the transducer and that is reflected from the top surface of the object back to the transducer. Then the local height of the object is given by
Z(x,y) = (Vw/2)[TOF0 - TOF(x,y)],
whereVw is the speed of sound in water, TOF0is the round-trip travel time observed when the object is not present or when the transducer is not over the object and is aimed at the surface on which the specimen sits, and TOF(x,y) is the round-trip travel time for position x,y.
This work was done by E. James Chern of Goddard Space Flight Center. No further documentation is available. GSC-13911