Method and apparatus for providing dynamic focussing and beam steering in an ultrasonic apparatus

An acoustic lens having nonuniform cross sectional thickness is disposed in the path of an ultrasonic energy search beam traveling from a transducer probe to an object to be examined. In an alternative embodiment the lens has at least one contoured surface. As the lens undergoes motion in a plane substantially normal to the direction of the search beam, the search beam exhibits a varying focal zone depth and beam path for providing real time ultrasonic scanning of the examined object.

SUMMARY OF THE INVENTION 
The present invention concerns a method and apparatus for providing real 
time dynamic focussing and beam steering of an ultrasonic energy search 
beam. Specifically, an acoustic lens having a nonuniform thickness and 
contoured surface is disposed in the path of the search beam traveling 
from an ultrasonic transducer probe to an object to be examined. The lens 
is made to undergo motion in a plane substantially normal to the direction 
of the search beam thereby continually changing the lens characteristics 
intercepting the search beam in a predetermined manner for changing the 
path and focal zone length of the search beam. 
In ultrasonic pulse-echo testing, for a given transmitter probe and 
ultrasonic frequency, the ratio of the probe diameter to the wavelength of 
the search beam traveling through the test object defines the near field 
focal zone distance and focal zone width. In ultrasonic imaging systems, 
it is desireable to form the zone width as small as possible for obtaining 
optimal lateral resolution while concurrently causing the focal zone to 
occur at different distances from the transducer probe for providing sharp 
images throughout the cross section of the object being examined. 
Moreover, it is advantageous to vary the direction of the search beam 
without moving the transducer probe. To these ends the present invention 
provides an arrangement for varying the focal zone depth in real time 
(so-called real time dynamic focussing) and varying the path of the search 
beam while maintaining the transducer stationary (so-called real time beam 
steering). In the past a trade off between these variables generally 
resulted in a compromise between the focal zone depth and the resolution 
of the imaging system. 
A prior solution to the optimization problem has been to use focussing 
transducer probes in order to increase the depth regions to be examined. 
Such focussing is realized by means of curved radiators, or using plane 
radiators in contact with curved lenses. Depending on the shape of the 
radiator either spherical or cylindrical lenses are used. While the 
desired focal zone width is achieved, varying of the depth is not 
achieved. 
Alternatively, mirrors have been used in a manner similar to focussing in 
optics, see U.S. Pat. No. 3,965,455, issued to M. J. Hurwitz, entitled 
"Focussed Arc Beam Transducer Reflector" as well as rotating mirrors such 
as described in U.S. Pat. No. 3,992,925 issued to J. R. Perilhou, entitled 
"Device for Ultrasonic Scanning". In a further arrangement a transducer 
probe comprising a linear array of juxtaposed elements is energized in a 
predetermined fashion to electronically create the effect of a focussed 
search beam. Such electronic focussing while effective, generally requires 
a control unit or a plurality of delay lines which add to the cost and 
complexity of the test arrangement. In a still further arrangement, 
electronic beam steering and lenses are combined to focus a beam as 
described, for instance, in U.S. Pat. No. 3,936,791, issued to G. Kossoff, 
entitled to "Linear Array Ultrasonic Transducer". 
The concept of varying the focal zone of an ultrasonic search beam in 
ultrasonic testing is described, for instance, in U.S. Pat. No. 3,310,977, 
issued to W. C. McGaughey entitled "Ultrasonic Inspection Apparatus Using 
Variable Focus and Gate." In the patent, a rack and pinion controlled 
piston varies the pressure of a liquid contacting a resilient diaphragm to 
vary the shape of the liquid path coupling the search beam from the 
transducer probe to the diaphragm and hence, to the test object. 
The present invention provides a simplified and inexpensive arrangement for 
varying the focal zone length and directivity of an ultrasonic energy 
search beam. An acoustic lens having nonuniform thickness, for instance in 
the shape of a wedge, is disposed in the path of the search beam. The lens 
is coupled to undergo motion, for instance rotational motion, in a plane 
substantially normal to the path of the search beam for providing a 
varying thickness lens intercepting the search beam at predetermined 
instances of time. Additionally, the lens surface may be contoured for 
providing different refraction characteristics. A lens having 
predetermined surface contours for focussing an ultrasonic search beam is 
described for instance in U.S. Pat. No. 4,044,273, issued to H. Kanda et 
al, entitled "Ultrasonic Transducer." 
The present invention, furthermore, provides a system in which an 
ultrasonic search beam transmitted either from a transducer or an array of 
juxtaposed transducer elements can be steered and the depth of focus 
varied by means of a simple mechanical arrangement. Such an arrangement is 
of value in real time ultrasonic pulse-echo examination of human bodies or 
other workpieces. That is, the heretofore employed complex methods of 
dynamically focussing an ultrasonic search beam in a real time image 
scanning device has been replaced by an acoustic lens coupled to motive 
means. 
A principal object of the present invention is, therefore, the provision of 
an acoustic lens undergoing motion in a plane normal to the path of an 
ultrasonic energy search beam for varying the beam path and/or the focal 
zone depth of the search beam. 
Another object of the invention is the provision of a method and apparatus 
for providing a dynamically focussed search beam for use in real time 
ultrasonic apparatus. 
Another object of the invention is the provision of a method and apparatus 
for varying the beam path and/or the focal zone depth of a search beam by 
rotating an acoustic lens at a constant angular velocity in the path of 
the search beam. 
A further object of the invention is the provision of a method and 
apparatus for providing steering of an ultrasonic search beam. 
Further and still other objects of the present invention will become more 
clearly apparent when the following description is read in conjunction 
with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the figures and FIG. 1 in particular, there is shown a 
tank 10 and an ultrasonic transducer probe 12 disposed therein for 
transmitting an ultrasonic search beam 14 through a coupling medium 16, 
such as water, toward an object W to be examined. The frequency of the 
transmitted search beam is typically in the range between 0.5 and 10 MHz. 
The probe 12 is preferably energized in a manner for providing cyclically 
pulsed search beams such as are used in pulse-echo ultrasonic testing, 
although continuous wave search beams may also be used in practising the 
invention. Interposed in the path between the transducer 12 and the test 
object is a plate 18. The plate 18, an acoustic lens, is dimensioned to 
exhibit nonuniform thickness for refracting the search beam 14 in a 
predetermined manner for instance first along path 20, then path 22 and 
finally path 24 as the plate 18 is rotated by motor 28 about shaft 26 in 
the direction of arrow 30 as seen in the plan view in FIG. 2. 
Alternatively, the plate 18 may be dimensioned to change the focal zone 
length of the search beam. For instance, the search beam wave front may 
first follow the contour of the solid lines 44 in FIG. 3 for creating a 
focal zone at region 46, then follow the contour of dotted lines 48 for 
creating a focal zone at region 50 as the plate 18 undergoes rotation. 
Rotation of the plate 18, a disk, causes a change of the search beam path 
or of the focal zone depth or both, thus providing for an examination of 
the workpiece W at varying locations. 
As the plate 18, in the form of a scanning disk, undergoes rotational 
motion in a plane normal to the path of the search beam, preferably at a 
uniform speed, the search beam 14 encounters a steadily changing lens 
thickness, thus causing the beam to exhibit a repetitive pattern of 
varying beam path and focal zone depth. 
In an alternative embodiment a plate 18' may have a contoured groove in the 
top surface, at a location for intercepting the search beam 14. 
Alternatively, the plate 18' may have contoured grooves in the bottom 
surface as shown in FIG. 4b or contoured grooves in both the top and 
bottom surfaces as shown in FIG. 4c. In a still further modification, not 
shown, the lens may exhibit varying thickness as well as have one or more 
contoured grooves. By proper contouring of the surface of plate 18' which 
acts as an acoustic lens, the search beam 14 is refracted to scan the 
object to be examined in a predetermined manner. By synchronizing the 
transmission of the search beam 14, the rotational speed of the motor 28 
and the sweep signal on a cathode ray tube, all as known in the art, a 
real time scanning of the object to be examined can be performed and 
displayed on the cathode ray tube 36 of an ultrasonic nondestructive test 
apparatus 38. 
Sensing means 32, for instance, a cam and switch, a shaft encoder, or a 
light reflecting arrangement as shown in U.S. Pat. No. 4,034,744 issued to 
P. R. Goldberg, entitled "Ultrasonic Scanning System With Video Recorder" 
coupled to the rotating shaft 26 can provide a trigger signal to the 
pulser 34 comprising a portion of the ultrasonic nondestructive test 
apparatus 38 for energizing the probe 12 when the plate 18 is at a desired 
position to scan the test object. By rotating the plate 18 at a 
predetermined speed the path of the refracted search beam and the focal 
zone depth as a function of time is known and used to control the 
deflection of the beam on the screen of a cathode ray tube 36 to produce a 
real time image of the test object. 
It will be understood that the plate may undergo elliptical as well as 
circular rotational motion. Moreover, the plate 18 may exhibit linear 
motion to affect the dynamic focussing and beam steering effects. It is 
possible for the transducer 12, which may be a single transducer element 
or an array of juxtaposed elements, the construction of both as is well 
known in the art, to undergo motion, preferably linear translatory motion, 
concurrently with the plate 18 undergoing motion as shown in FIG. 6. 
In FIG. 6, the arrangement per FIG. 1 is modified to include motive means 
52 such as a cam and cam follower, a reciprocating rod or the like, 
causing the shaft 26 and hence lens 18 to undergo bodily linear 
translating or elliptical motion depending upon the elements in motive 
means 52 as is well known to those skilled in the art. 
As the disk undergoes motion the effect of an infinite sequence of lenses 
each of different contour intercepting the search beam in a plane 
substantially normal to the search beam path is manifest. 
In a further modification shown in FIG. 5, the plate 18" includes one or 
more receiver probes 40 (three shown) for receiving back-scattered echo 
signals and reflected signals from the workpiece. The received echo 
signals are converted by the receiver probe into electrical signals and 
conducted via slip rings or the like to the ultrasonic nondestructive test 
apparatus 38. By analyzing the scatter responsive signals in a manner such 
as described in U.S. Pat. No. 3,996,791 by L. Niklas et al, entitled 
"Ultrasonic Test Method and Apparatus Utilizing Scattered Signals" flaw 
characterization of the defect can be provided. It will be apparent that 
other combinations of transmitter and receiver probes may be disposed in 
or in relation with plate 18" to provide flaw characterization analysis. 
While a preferred embodiment and several modifications of the present 
invention have been described and illustrated, it will be apparent to 
those skilled in the art that further variations and modifications can be 
made without deviating from the broad principle of the invention which 
shall be limited solely by the scope of the appended claims.