Abstract:
A method for manufacturing an annular acoustic transducer array from a plate of transducer material, which enables production of precision aligned arrays at low cost. The circular plate is sawed along at least two lines that are radial to the axis of the plate. At steps along each radial cut, the plate is rotated first in one direction and then in an opposite direction by a predetermined angle such as slightly less than 90°. The cuts result in the forming of several largely ring-shaped lands, each largely ring-shaped land being joined to the other rings of different radii by thin portions of the plate, and each ring being cut into segments. The bridges that join different rings, hold the transducer together until it can be mounted on a lens.

Description:
The Government has rights in this invention pursuant to Contract No. DE-AT03-76SF70030 awarded by the U.S. Department of Energy. 
    
    
     BACKGROUND OF THE INVENTION 
     Annular acoustic arrays generally include a group of concentric rings of a piezoelectric or ferro-electric material which are mounted precisely concentric on a lens. The phase of operation of the different rings can be closely controlled to transmit or receive sound from a particular depth. It is often advantageous to divide the rings into segments so the direction of focus can be shifted sidewardly. 
     The manufacture of an annular phased array is complicated by the difficulties of cutting and machining the brittle piezoelectric materials that are used. The manufacture of such arrays is an active area of investigation. A simple method of fabrication is to apply an electrode to the back of the crystal plate in the pattern desired, to etch away the pattern to a depth just short of the opposite face of the plate. However, the ring elements are still mechanically attached at their front face, which degrades the performance of the resulting transducer. Separate rings have been machined, but it is difficult and time consuming to position the rings precisely concentric on a lens. The difficulty increases where each ring is formed in segments. A low cost method for forming piezoelectric rings, especially in segments, and mounting them precisely concentric on a lens, with each ring or segment being substantially isolated from adjoining rings and segments, would be of considerable value. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide an annular acoustic array which is easily formed and handled for mounting. 
     A further object is to provide an efficient method for forming and mounting an acoustic annular array. 
     In accordance with one embodiment of the present invention, an annular array and method for forming it are provided, which results in a low cost and efficient array. The array includes a plate of acoustic transducer material having a plurality of sets of annular cuts that are all concentric to an axis. All cuts of the same set have the same radius, but their ends are spaced to form thin bridges between them. The regions of the plate between sets of cuts form acoustic transducer rings which are largely isolated from one another except at the thin bridges where their coupling is minimal. 
     The cuts are formed by sawing the plate of acoustic material along a line radial to the axis, and then rotating the plate by no more than about 90° in one direction and then in the opposite direction from the radial cut. After each pair of such cuts is made by turning the plate, the saw is advanced along the radial line to a different radius and the plate is again turned in opposite directions from the cut. 
     The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of an annular acoustic transducer array of the prior art. 
     FIG. 2 is a sectional view of an acoustic transducer of both the prior art and the present invention, shown mounted on a lens and being used to transmit sound. 
     FIG. 3 is a plan view of an annular acoustic transducer array in accordance with the present invention. 
     FIG. 4 is a perspective view showing the manner in which the array of FIG. 3 is manufactured. 
     FIG. 5 is a plan view of an array of another embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 illustrate an annular acoustic transducer array 10 of the prior art, which included a plurality of concentric rings 11-17, of which all but the centermost one are band shaped with holes in their middle. The array is formed of piezoelectric material and is mounted on a lens 20 whose lowermost surface faces an object to be acoustically interrogated. A group of electrodes 22 are coupled to one side of each ring while a common electrode 24 is coupled to the opposite side of the rings. When variable voltages are applied across the thickness of the rings, they radiate acoustic energy, and the phases of the voltages can be controlled so that the acoustic energy has a controlled circular wave front 26 that concentrates sound at a particular location. The array can also be used to detect sound from a particular depth. If the rings are cut into segments and the segments are driven out of phase with respect to one another, the wave front 26 can be directed sidewardly. 
     If the rings are formed as shown in FIG. 1 and attempts are then made to bond them to a supporting surface 27 of a holder formed by the lens 20, considerable time and expense is required to position the rings accurately concentric with axis 28. Applicant avoids this problem by forming the array in a configuration shown at 30 in FIG. 3. The array 30 has nine sets of concentric annular cuts, including an outermost set 32, a radially innermost set 34, and a middle set 36. Each set of cuts such as the middle set 36 includes two cuts 36a, 36b which are both of the same radial distance 38 from the axis 40 of the array. Each cut extends by less than 360° and preferably no more than about 180°, and in fact here each cut extends by a few degrees less than 180°. As a result, the pair of cuts 36a, 36b of the set extend short of each other to form cut ends 42, 44 which are spaced apart to leave a bridge 46 between them. The opposite ends of the cuts 36a, 36b are also spaced to leave another bridge 48 between them. The different sets of cuts such as 36 and the adjacent cut 50 are of different radii with respect to the axis 40, so they leave a land 52 between them. The land 52 forms a portion of the acoustic transducer which generates and/or detects sound waves. 
     The array 30 with ten lands such as 52 form therein (all but the innermost land are band-shaped) can be readily mounted on a lens with all of the lands maintained precisely concentric. The bridges such as 46, 48 on opposite sides of the axis 40 lie on radial lines such as 49 and maintain the lands concentric and hold them together (provided care is used) until the array can be bonded to the lens. Where the bridges 46 have an average length which is no more than about ten percent of the length of the sets of cuts, they introduce substantially negligent coupling between adjacent lands. Thus, by forming an array similar to that of FIG. 1, but with at least two narrow lands on opposite sides of the axis connecting those lands of different radii, the array operates with high efficiency and yet can be easily handled during the process of mounting it on a lens with all of the lands precisely concentric. 
     Applicant manufactures the array 30 by sawing the cuts such as 36a, 36b so they each extend from opposite sides of a largely radially-extending cut 54 or 56. Applicant cuts along a largely radial line such as 58, from the outer edge 60 of the plate 62 of the transducer material of which the array is made. At every location where an annular cut such as 32 is to be formed, applicant rotates the plate 62 around the axis 40, first in one direction 64 up to a cut end, and then in the opposite direction 66 to the other end of the cut. Then the plate 62 is rotated back so the saw tool or saw is on the line 58, and the saw is then advanced along the line 58 to a radius where the next cut such as 68 is to be made ad the process is repeated. Thereafter, the saw is withdrawn, and the plate is turned 180° and the largely radial cut 56 and annular cuts such as 36b are made. 
     The saw is a cutting element such as an abrasive wire which moves largely perpendicular to the surface of the plate 62. The wire can move continuously in one direction, although it is more commonly reciprocated up and down. Such saws cut rapidly and are of relatively low cost. FIG. 4 shows a setup which can be used. The workpiece or plate 62 of transducer material is attached to a turntable platform 74 which can rotate in the directions 64, 66 about the axis 40. A wire saw 76 which is coated with industrial diamonds moves up and down. The workpiece 62, which can be a disc of lithium niobate, is attached to the turntable as by a coating of wax. The turntable platform is mounted on a turning mechanism and holder 80 which is, in turn, mounted on a linearly moveable bed 82. As the saw 76 reciprocates, the bed is moved in the direction of arrow 84, and then the platform 74 is rotated. After the cuts are formed, the array formed from the plate of transducer material is bonded to a lens. 
     If the largely radial cuts shown in FIG. 3 at 54 and 56 are not wanted, it is possible to drill holes along a cut such as 32 and to then thread the cutting wire through the hole and then proceed to form the cut. However, the largely radial cuts 54, 56 make cutting much simpler and faster, and also have the advantage of dividing each circular land such as 52 into two land portions such as 52a and 52b. As discussed earlier in connection with FIG. 2, such segmentation of a ring or land has the advantage that one land portion can be advanced in phase with respect to the other, to steer the acoustic beam or direction of sensing laterally. Where such division of the land is not desired, the two land portions 52a and 52b can be electrically connected together, as by extending an electrode which extends on the upper face of land 52a, so that it contacts the electrode on the upper face of the land 52b. 
     FIG. 5 illustrates another array 90 which is somewhat similar to that of FIG. 3, except that the array 90 includes four largely radial cuts 91-94. Each annular cut such as cut 100 extends by slightly less than 90° about the axis 102 of the array. Each land such as 104 which is formed between a pair of cuts of different radii, such as 100, 106, is divided into four sectors 104a, 104b, 104c, and 104d. This allows steering of acoustic energy or detection, in different lateral positions. 
     In making a cut with the saw, such as 76 in FIG. 4, the saw is fed into the crystal transducer material at a rate less than a safe cutting rate at which the saw could be fed without danger of breakage of the substrate. Such slow cutting is made to avoid appreciable bending of the saw 76, which could cause it to wander slightly to the side, so it would not cut in a precisely straight or annular direction. It is possible to have more than the four largely radial lines 91-94 of FIG. 5, but this can result in bridges 108 that are of considerable width compared to the lengths of the cuts. In such a case, the bridges such as 108 can be removed as by the use of air abrasive jets just at the bridges. 
     Applicant has constructed arrays of the type shown in FIG. 3. One array, used for transmitting acoustic energy of a frequency of about 5 megahertz, is formed of a thick crystal of lithium niobate having a thickness of about one millimeter and an outer diameter of about two centimeters. Each cut such as 36 has a width 110 of about one-fourth millimeter, and the lands such as 52 between adjacent cuts have a width 112 of about one-half millimeter. The bridges such as 48 have a width 114 of about one millimeter. For thinner crystals, the width of the bridges should be thicker to avoid breakage, though all of this depends upon the degree of gentleness of the handling of the array until it can be bonded to the lens. The lands can be of different widths. 
     Thus, the invention provides an annular acoustic transducer array and a method for producing it, which results in low cost arrays with transducer lands or rings held precisely concentric. The array has a plurality of concentric annular cuts, each set of cuts having a different radius to form lands between adjacent sets of cuts, and each set of cuts having at least two cuts with their ends spaced to leave bridges between them. The bridges hold the array together until it can be mounted on a lens. The array can be formed by sawing, by sawing largely along a largely radial line and sawing annular cuts on either side of the line. 
     Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.