Method and apparatus for focusing propagating wave paths of a phased array in spherically-bounded materials

[0002] The present invention relates generally to the field of using ultrasonic waves to non-destructively detect structural characteristics of work pieces. More particularly, this invention relates to a method and apparatus for focusing ultrasonic waves that are propagated from a phased array to a spherically-bounded object so that the waves arrive at a pre-selected focal point at the same time and in phase and thereby significantly improve, nondestructively, the detectability of possible flaws in and structural characteristics of the object. Still more particularly, this invention relates to a method and apparatus for non-destructively evaluating spherically bounded objects, such as reactor pressure vessels, ball bearings, and the like by obtaining by iteration of a focal law equation according to the invention which follows a path on the surface of the spherically bounded object.

[0003] The use of ultrasonic waves generally, and phased array mechanisms in particular, to nondestructively detect structural characteristics of work pieces is well known in the art. Generally, the technique consists of transmitting ultrasonic waves toward the work piece through various media such as water or air so that the waves impinge the surface of the workpiece, propagate throughout the internal structure of the work piece, and ultimately reflect back from the work piece. As the waves propagate throughout the work piece, they are reflected and refracted by variations and changes in the medium through which they travel. Defects in the structure of the work piece affect the travel path of the propagating waves. When the propagating waves reflect from the work piece, they are measured and analyzed. The structural characteristics of the work piece, including any defects or flaws, can be detected and reconstructed from the information contained in the reflected waves.

[0004] A phased array mechanism transmits ultrasonic waves from its multiple array elements which are spaced apart from each other. The waves are transmitted in a sequence at slightly different times relative to each other. The transmitted waves thus travel through a coupling material, which is usually water, prior to impinging the surface of the target work piece. Because of the molecular differences between the coupling material and the work piece, a portion of the propagating wave is reflected away from the work piece and a portion is refracted into the work piece. It is the refracted portion of each propagating wave that is used to detect and reconstruct structural defects and flaws of the work piece. The point at which the propagating waves impinge the work piece relative to their respective origination points partially determines the path of the refracted waves as they travel within the work piece.

[0005] In order to have the greatest ability to detect and reconstruct structural characteristics of the work piece, it is desirable for the refracted waves that enter and later exit the work piece to have the greatest possible amplitude. Waves that exit the work piece with a greater relative amplitude provide stronger and more readable signals.

[0006] It is well known that waves which arrive at a particular focal point at the same time and in phase constructively interfere with each other to create a wave with a larger relative amplitude. Accordingly, to improve the detection and reconstruction capabilities of phased arrays, it is desirable to focus all of the waves emitted from the elements of the phased array mechanism to create an internal wave with the greatest possible amplitude. To do so requires that all of the waves be focused and sequenced so that they arrive at a pre-selected focal point at the same time and in phase. However, until this invention, there was no way to determine the preferred refraction point on a spherical work piece or the proper sequence of transmitting pulses from the phased array so that the propagating waves arrived at a selected focal point at the same time and in phase in a work piece having a spherical boundary.

[0007] Accordingly, it is a continuing aim in the art to provide, in the field on nondestructive testing, a method and apparatus for determining the proper sequence of transmitted pulses in a phased array and a method and apparatus for visualizing the propagating wave paths in a spherically bounded material and contoured material that is spherical in the region being evaluated.

[0008] This invention comprises a method and apparatus to focus ultrasonic pulses transmitted from a phased array mechanism into a spherically-shaped work piece to increase the effectiveness of the non-destructive internal examination of the work piece. While this invention can be used with any spherical object, one such possible use is in connection with detecting internal flaws in nuclear reactor pressure vessel heads. To maximize the strength of the ultrasonic pulses when the pulses arrive at a particular selected focal point in the work piece, each pulse transmitted from an element of the phased array should be properly sequenced so that all of the pulses arrive at some selected focal point at the same time and in phase. In one aspect, the invention thus relates to a method and apparatus to determine to where the penetrating waves propagate to in the spherical work piece and how to properly sequence the transmitted pulses from all the phase elements comprising the phased array.

[0009] The method of the invention includes steps of determining location coordinates for preferred refraction points on the surface of the work piece that will direct impinging waves to arrive at a pre-selected focal point simultaneously. First, a focal point is pre-selected, and, accordingly, the location coordinates for the focal point are known before the method according to the invention is used. Second, the location coordinates for each element of the phased array are also known before the method according to the invention is used. Based upon the coordinates of the elements of the phased array mechanism and the pre-selected focal point, the coordinates for a desired refraction point on the surface of the work piece are calculated for each element of the phased array. In addition, a unique pulse firing time for each element of the phased array is calculated. The unique refraction point and pulse firing time for each array element permit the phased array to transmit the pulses from each element so that they all arrive at the focal point in phase and at the same time.

[0010] Each refraction point is uniquely determined relative to the center of the work piece by calculating two angles, θ_pand φ_p, which together define a unique point on the surface of the work piece. The method of this invention includes an iterative calculation to determine θ_pand φ_p. The values of θ_pand φ_pare referred to as θ_pand φ_pfor each i^thiteration of the method The method according to the invention begins by setting θ_ito an initial value between 0 and 360, preferably to 0. Based upon the initial selected value of θ_i, the angle created by the line extending between the center of the work piece and the particular element of the phased array on the one hand, and the line extending between the center of the work piece and a potential refraction point on the other hand, is calculated. This angle is referred to as β. Then, the angle created by the line extending between the center of the work piece and the potential refraction point on the one hand, and the line extending between the center of the work piece and the pre-selected focal point on the other, is calculated. This angle is referred to as β_f. New values for θ_pand φ_pare calculated based upon the values of β and β_f. The calculation of the new values for θ_pand φ_prequire the analysis of several potential special case situations. The entire method is reiterated using a newly calculated initial value for θ_i. The iteration process ceases when the value of θ_iand the values of θ_pand φ_psatisfy an equation, at which point, the final values for θ_pand φ_pare considered to be the last values of θ_pand φ_p.

[0011] After the preferable refraction point is determined by θ_pand φ_p,the preferable time delay for each array element is calculated based upon the coordinates of the refraction point, the coordinates of the element at issue, and the speed of sound.

[0012] The apparatus for performing the method according to the invention includes a work piece fixed at predetermined spatial location, a phased array, a microprocessor, and appropriate instrumentation.

[0013] FIG. 1 is a perspective view of the environment, including the spherically-shaped boundary, in which the method according to the invention is used according to a first preferred embodiment.

[0014] FIG. 2 is a detailed flowchart showing the steps of the method according to a preferred embodiment of the invention.

[0015] FIG. 3 is a detailed flowchart showing the steps of the method according to the invention associated with calculating values for θ_pand φ_paccording to a preferred embodiment.

[0016] FIG. 4 is a perspective view of the environment, including the spherically-shaped boundary, in which the method according to the invention is used according to a second preferred embodiment.

[0017] FIG. 1 schematically and diagrammatically illustrates one possible environment in which the method and apparatus invention are used. With reference to FIG. 1, a spherically-bounded work piece 10 is surrounded by a coupling material 8. The coupling material 8 is usually water, but could be virtually any type of medium through which ultrasonic pulses can be transmitted and accurately detected. In the first embodiment of the invention, a phased array mechanism 14 is located outside of the spherically-bounded work piece 10, and it is surrounded by the coupling material 8. In a second embodiment of the invention, shown in FIG. 4, the phased array mechanism 14 may be located under the spherically-bounded work piece 10, depending upon the application. Regardless of the particular embodiment, the phased array mechanism 14 includes a plurality of transmitting phase elements. Each element of the phased array 14 is referred to as the “k^thelement” and its physical location is defined by a set of coordinates (x_ek, y_ek, z_ek). The phased array 14 can be a one-dimensional phased array, a two-dimensional phased array, or a three-dimensional phased array, depending on the application.

[0018] Still referring to FIG. 1, the selected focal point 16 is located inside of work piece 10 in the first embodiment of the invention. In the second embodiment, as shown in FIG. 4, the focal point 16 is still located inside of the work piece 10. The coupling medium 8 surround the phased array 14. The selected focal point 16 is defined by a set of coordinates (X_f, y_f, z_f). Based upon the location of the phased array elements 14 and the selected focal point 16, a unique preferred refraction point 12 is disposed on the bottom surface 11 of the work piece 10 for each element of the phased array. The specific locations of each of the refraction points 12 are defined by a set of coordinates, (x_p, y_p, z_p). The location of the center 18 of the work piece 10 is defined by a set of coordinates, (x_f, y_f, z_f).

[0019] FIG. 1 shows angles θ_pand φ_p, which uniquely determine the refraction point 12. Unique values of θ_pand φ_pcorrespond to each of the individual locations of the phased array elements 14. Angle θ_prepresents the location of refraction point 12 relative to the Y-axis, and angle φ_prepresents the location of refraction point 12 relative to the Z-axis. The value of θ_pcan vary between 0 and 360° and the value of φ_pcan vary between 0 and 180°.

[0020] FIG. 1 also shows angles β, β_f, and β⁰. Angle β represents the angle created by the line extending between the work piece center 18 and the k^thelement of the phased array 14 on the one hand, and the line extending between the work piece center 18 and the refraction point 12 on the other. β_frepresents the angle created by the line extending between the work piece center 18 and the refraction point 12 on the one hand, and the line extending between the work piece center 18 and the selected focal point 16 on the other. Angle β₀represents the sum of the angles β and β_f.

[0021] FIG. 1 also shows line R₀, which represents the radius of the spherical work piece 10. Line R_frepresents the distance between the center 18 of the work piece 10 and the focal point 16. Finally, line R_erepresents the distance between the center 18 of the work piece 10 and the k^thelement of the phased array 14.

[0022] When the phased array 14 is operated, ultrasonic pulses from each of the elements of the phased array 14 are transmitted at various times relative to each other. Because of the relative spacing of the array elements, the waves are transmitted from somewhat different points of origination. The transmitted pulses propagate through the coupling material 8 and impinge upon the surface 11 of the spherical work piece 10, causing part of the wave to be reflected away from the surface 11 and causing a refracted portion of the wave to penetrate into the spherical work piece 10. The penetrating wave is used to detect flaws in the material. The elements comprising the phased array 14 can be omni-directional radiators or directional radiators. If the elements are directional radiators, those elements should be pointed towards the point on the surface 11 of the work piece 10 where the wave refracts into the work piece 10.

[0023] The given coordinates (x_f, y_f, z_f) of the center 18 of the work piece 10 are known prior to beginning the invented method. Similarly, coordinates (x_ek, y_ek, z_ek) of each element of the phased array 14 are known prior to beginning the invented method. These coordinates are fixed values dictated by the respective physical locations of the work piece 10 and the phased array 14. The location of the preferred focal point 16 must be selected and the associated coordinates (x_f, y_f, z_f) determined. The desired focal point 16 is selected either by the user of the method or by a computer algorithm. Once these given coordinate sets are determined, the method according to the invention is employed to determine θ_pand φ_pfor each element of the phased array 14. The unique values of θ_pand φ_pidentify the preferred refraction points 12 for each element of the phased array 14. After the preferred refraction points 12 are calculated, the proper sequencing and pulse firing times for the ultrasonic pulses emitted from the phased array 14 are calculated.

[0024] The basic premise of the method of the invention for locating the preferred refraction point 12 is to determine the unique plane that passes through the center of the sphere 18, the selected focal point 16, and the k^thelement, (x_ek, y_ek,z_ek) of the phased array 14. This plane will also contain the preferred refraction point 12, the location of which is defined by the coordinate set (x_p, y_p, z_p) for the k^thphase element.

[0025] Before the method of the invention can be described in detail, basic mathematical descriptions of the location coordinates of the refraction point 12, the focal point 16 and the phased array 14 are necessary.

[0026] The coordinates of the refraction point 12 for particular values of θ_pand φ_pthat relate to a specific phase element can be described as,

[0027] x_p=R_osin(φ_p)sin(θ_p)+x_r

[0028] y_p=R_osin(φ_p)cos(θ_d)+y_r

[0029] z_p=R_ocos(φ_p)+z_r,

[0030] where, R_ois the known radius 19 of the sphere. These formulas are used to calculate specific coordinate values for each refraction point 12 after the values of θ_pand φ_pfor each phase element are calculated using the method of the invention. The k^thphase element position is defined as,

[0031] x_e=R_esin(φ_e)sin(θ_e)+x_r

[0032] y_e=R_esin(φ_e)cos(θ_e)+y_r

[0033] z_e=R_ecos(φ_e)+z_r,

[0034] where, R_eis the length of the line 32 from the center 18 of the work piece 10 to the k^thelement of the phased array 14. Similarly, the position of the focal point 16 is described as,

[0035] x_f=R_fsin(φ_f)sin(θ_f)+x_r

[0036] y_f=R_fsin(φ_f)cos(θ_f)+y_r

[0037] z_f=R_fcos(φ_f)+z_r

[0038] where, R_fis the length of the line 30 from the center 18 of the work piece 10 to the given focal point 16.

[0039] Angles φ_eand θ_eare defined as follows:

[0040] φ_e=cos⁻¹[(z_e−z_f)/R_e],

[0041] For the principal values of the inverse tan function,

[0042] if (y_e−y_r)<0 and (x_e−x_r)≧0 then θ_e=180°+tan⁻¹[(x_e−x_r)/(y_e−y_r)]

[0043] if (y_e−y_r)>0 and (x_e−x_r)≧0 then θ_e=tan⁻¹[(x_e−x_r)/(y_e−y_r)]

[0044] if (y_e−y_r)>0 and (x_e−x_r)≦0 then θ_e=360°+tan⁻¹[(x_e−x_r)/(y_e−y_r)]

[0045] if (y_e−y_r)<0 and (x_e−x_r)≦0 then θ_e=180°+tan⁻¹[(x_ex_r)/(y_e−y_r)]

[0046] if (y_e−y_r)=0 and (x_e−x_r)>0 then θ_e=90°

[0047] if (y_e−y_r)=0 and (x_e−x_r)<0 then θ_e=270°

[0048] Similarly the angles, φ_fand θ_fare defined as follows:

[0049] φ_f=cos⁻¹[(z_fz_r)/R_f],

[0050] For the principal values of the inverse tan function,

[0051] if (y_f−y_r)<0 and (x_f−x_r)≧0 then θ_f=180°+tan⁻¹[(x_f−x_r)/(y_f−y_r)]

[0052] if (y_c−y_r)>0 and (x_f−x_r)≧0 then θ_f=tan⁻¹[(x_f−x_r)/(y_f−y_r)]

[0053] if (y_f−y_r)>0 and (x_f−x_r)≦0 then θ_f=360°+tan⁻¹[(x_f−x_r)/(y_f−y_r)]

[0054] if (y_f−y_r)<0 and (x_f−x_r)≦0 then θ_f=180°+tan⁻¹[(x_f−x_r)/(y_f−y_r)]

[0055] if (y_f−y_r)=0 and (x_f−x_r)>0 then θ_f=90°

[0056] if (y_f−y_r)=0 and (x_f−x_r)<0 then θ_f=270°

[0057] FIG. 2 shows a flowchart which sets forth the steps that comprise the method according to the invention for calculating angles θ_pand φ_pfor the pre-selected focal point 16, as well as for calculating the preferred pulse firing times of each phase element. The invented method is an iterative process. After the method is started at step 40, an initial trial value for θ_iis selected at step 42. The value for θ_iused in each pass of the iterative method is referred to as θ_i, where i represents the number of times that the iterative process has been repeated. In the disclosed embodiment, the initial value for θ_i(or θ_o) is 0. After the initial θ_ivalue is chosen, β is calculated at step 44 according to the following formula:

[0058] β=θ_i−sin⁻¹[(R_o/R_e)sin(θ_i)].  (1)

[0059] Then, β_ois calculated at step 45 according to the following formula:

[0060] β_o=cos⁻¹[(R_e²+R_f²−L_f²)/(2R_eR_f)].  (2)

[0061] where, R_eis the length of the line 32 from the center 18 of the spherical work piece 10 to the k^thelement of the phased array 14, R_fis the length of the line 30 from the center 18 of the spherical work piece 10 to the focal point 16, and L_fis the length of the line 31 from the focal point 16 to the k^thelement of the phased array 14. These variables can be defined further as, R_e=[(x_ek−x_r)²+(y_ek−y_f)²+(z_ek−z_r)²]^½, R_f=[(x_f−x_r)²+(y_f−y_r)²+(z_f−z_r)²]^½, and L_f=[(x_ek−x_f)²+(y_ek−y_f)²+(z_ek−z_f)²]^{{fraction (1/2 )}}.

[0062] After β and β_oare calculated, β_fis calculated at step 46 of FIG. 2 according to the following formula:

[0063] β_f=β_o−β.  (3)

[0064] Once β and β_fare calculated in steps 44 and 46 for the initial value of θ_i, the following equations (4) and (5) are solved for new values of angles θ_pand φ_pin step 48:

[0065] cos(β)=sin(φ_e)sin(θ_e)sin(φ_p)sin(θ_p)+sin(φ_e)cos(θ_e)sin(φ_p)cos(θ_p)+cos(φ_e)cos(φ_p)  (4)

[0066] and

[0067] cos(β_f)=sin(φ_f)sin(θ_f)sin(φ_p)sin(θ_p)+sin(φ_f)cos(θ_f)sin(φ_p)cos(_74p)+cos(φ_f)cos(φ_p),  (5)

[0068] where, (φ_eθ_e), and (φ_f,θ_f) are the angles that give the position of the k^thelement of phased array 14 and focal point 16, respectively, with respect to the center 18 of the spherical work piece 10. The method for solving these equations for the new values of θ_pand φ_pis shown in detail in FIG. 3, which illustrates in detail the substeps that comprise step 48 in FIG. 2.

[0069] Referring to FIG. 3, there are several special cases of solutions to equations (4) and (5). Each one of these special cases must be reviewed and evaluated prior to solving equations (4) and (5) according to the general case. If one of the special cases apply, then the values for θ_pand φ_pwill have pre-defined values. If none of the special cases apply, then the values of θ_pand φ_pwill be determined according to the general case. Each of the special cases and the general case, along with their associated solutions, are as follows:

[0070] Special case 1 (steps 50, 52)

[0071] IF φ_e=0 and φ_f=0

[0072] THEN φ_p=0 and θ_pis arbitrary (any angle will satisfy)

[0073] Special case 2 (steps 54, 56)

[0074] IF φ_e=0, φ_f≠0, and φ_f≠180°

[0075] THEN φ_p=β≠0, and θ_p=θ_f

[0076] Special case 3 (steps 58, 60)

[0077] IF φ_e≠0, φ_f=0 and φ_e≠180°

[0078] THEN φ_p=β_f≠0, and θ_p=θ_e

[0079] Special case 4 (steps 62, 64)

[0080] IF φ_e=180° and φ_f180°

[0081] THEN φ_p=180° and θ_pis arbitrary (any angle will satisfy)

[0082] Special case 5 (steps 66, 68)

[0083] IF φ_e=180°, φ_f≠180° and φ_f≠0°

[0084] THEN φ_p=180°−β, β≠0° and θ_p=θ_f

[0085] Special case 6 (steps 70, 72)

[0086] IF φ_f=180°,φ_e≠180°, and φ_e≠0°

[0087] THEN φ_p=180°−β_f, and θ_p=θ_e

[0088] Special case 7 (steps 74, 76)

[0089] IF φ_e=90° and φ_f=90°

[0090] THEN φ_p=90°, and

[0091] if θ_f>θ_ethen θ_p=β+θ_eor θ_p=θ_f−β_f

[0092] if θ_f<θ_ethen θ_p=θ_e−β or θ_p=β_f+θ_f

[0093] Special case 8 (steps 78, 80)

[0094] IF θ_e=θ_fand φ_e=φ_f≠0

[0095] THEN φ_p=φ_e=φ_fand θ_p=θ_e=θ_f

[0096] Special case 9 (steps 82, 84)

[0097] IF θ_e=θ_f, φ_e≠φ_fand φ_p≠0

[0098] THEN φ_p=180°−cos⁻¹[{cos(β)sin(φ_f)−cos(β_f)sin(φ_e)}/{sin(φ_e−φ_f)}] and θ_p=θ_e=θ_f

[0099] General case (steps 86, 88)

[0100] IF φ_e≠0, φ_r≠0, φ_e≠180°, φ_f≠180°, and θ_e≠θ_f

[0101] THEN φ_p=cos⁻¹[−B/A]

[0102] where,

[0103] B=cos(β)sin(φ_f)[sin (φ_e)cos(φ_f)cos(θ_f−θ_e)−cos(φ_e)sin(φ_f)]+cos(β_f)sin(φ_e)[sin(φ_f)cos(φ_e)−cos(φ_f)sin(φ_e)]

[0104] and,

[0105] A=cos(φ_f)sin(φ_e)[sin(φ_e)cos(φ_f)−cos(φ_e)sin(φ_f)cos(θ_f−θ_e)]

[0106] +cos(φ_e)sin(φ_f)[sin(φ_f)cos(φ_e)−cos(φ_f)sin(φ_e)cos(θ_f−θ_e)]

[0107] +sin²(φ_e)sin²(φ_f)sin²(θ_f−θ_e)θ_p=sin⁻¹{[cos(β)sin(φ_f)cos(θ_f)−cos(β_f)sin(φ_e)cos(θ_e)

[0108] −(SIN(φ_f)cos(θ_f)cos(θ_e)−sin(φ_e)cos(θ_e)cos(φ_f)cos(φ_p)]/(sin(φ_p)sin(φ_f)sin(φ_e)sin(θ_e−θ_f))}

[0109] Thus, new values of θ_pand φ_pare calculated for the i^thiteration of the invented method in step 48 of FIG. 2 according to substeps 50 through 88 shown in FIG. 3.

[0110] After θ_pand φ_pare calculated according to step 48, it is necessary to determine, as shown in step 90 of FIG. 3, which quadrant θ_pis in to determine the actual value for θ_pfor the i_thiteration of the invented method. The final value of θ_pis determined in step 90 according to the following rules:

[0111] If sin(θ_p)>0 and cos(θ_p)>0, then θ_p=θ_p

[0112] If sin(θ_p)>0 and cos(θ_p)<0, then θ_p=180−θ_p

[0113] If sin(θ_p)<0 and cos(θ_p)<0, then θ_p=180−θ_p

[0114] If sin(θ_p)<0 and cos(θ_p)>0, then θ_p=360+θ_p

[0115] After φ_pand θ_pare calculated for the i^thiteration of the invented method, they are checked to determine if the point that they define on the surface 11 of the spherical work piece 10 is a solution in that the refracted portion of the transmitted pulse actually propagates to the selected focal point 16. This task is shown in step 92 of FIG. 2. The point defined by φ_pand θ_pis a solution, and thus defines the location of the preferred refraction point 12 for the particular phase element being considered, if it satisfies the following equation:

[0116] BB=U₁Tan(θ_i)+U₂Tan(θ₁)  (6)

[0117] where, θ_t=sin⁻¹[(c₂/c₁)sin(θ_i)] and c1 and c2 are the speed of sound in the coupling material and the speed of sound in the spherically bounded material, respectively. The other parameters in equation (10) are defined as follows:

[0118] BB=X₁+X₂,

[0119] X₁=[(x_p−x₁)²+(y_p−y₁)²+(z_p−z₁)²]^½

[0120] X₂=[(x_p−x₂)²+(y_p−y₂)²+(z_p−z₂)²]^½

[0121] U₁=[(x_ei−x₁)²+(y_ei−y₁)²+(z_ei−z₁)₂]^½

[0122] U₂=[(x_f−x₂)²+(y_f−y₂)²+(z_f−z₂)²]^½

[0123] and the components are given by,

[0124] x₁=(cos²(θ_p)+cos ²(φ_p)sin²(θ_p))x_e−sin²(φ_p)sin (θ_p)cos(θ_p)y_e−sin (φ_p)cos(φ_p)sin(θ_p)z_e

[0125] +sin²(φ_p)sin²(θ_p)x+sin²(φ_p)sin(θ_p)cos(θ_p)y_p+sin (φ_p)cos(φ_p)sin(θ_p)z_p

[0126] y₁=−sin²(φ_p)sin(θ_p)cos(θ_p)x_e+(cos²(θ_p)cos²(θ_p)+sin²(θ_p))y_e−sin(φ_p)cos(φ_p)sin(θ_p)z_e

[0127] +sin²(φ_p)sin(θ_p)cos(θ_p)x_p+sin²(φ_p)cos²(θ_p)y_p+sin(φ_p)cos(φ_p)cos(θ_p)z_p

[0128] z₁=sin(φ_p)sin(θ_p)cos(φ_p)(x_p−x_e)+sin(φ_p)cos(φ_p)cos(θ_p)(y_p−y_e)+cos²(φ_p)(z_p−z_e)+z_e

[0129] x₂=(cos²(θ_p)+cos²(φ_p)sin²(θ_p))x_f−sin²(φ_p)sin(θ_p)cos(θ_p)y_f−sin (φ_p)cos(φ_p)sin(θ_p)z_f+

[0130] sin²(φ_p)sin²(θ_p)x_p+sin²(φ_p)sin(θ_p)y_p+sin(φ_p)cos (φ_p)sin(θ_p)z_p

[0131] y₂=−sin²(φ_p)sin(θ_p)cos(θ_p)x_f+(cos²(φ_p)cos²(θ_p)+sin²(θ_p))y_f−sin(φ_p)cos(φ_p)sin(θ_p)z_f+

[0132] sin(φ_p)sin(θ_p)cos(θ_p)x_p+sin²(φ_p)cos²(θ_p)y_p+sin(φ_p)cos(φ_p)cos(θ_p)z_p

[0133] z₂=sin(φ_p)sin(θ_p)cos(φ_p)(x_p−x_f)+sin(φ_p)cos(φ_p)cos(θ_p)(y_p−y_f)+cos²(φ_p)(z_p−z_f)+z_f

[0134] The values of θ_pand φ_pare considered to satisfy equation (6) if equation (6) has converged sufficiently with respect to previous values of φ_pand θ_p. That is, φ_pand θ_pare considered to define the location of the preferred focal point 16 if

[0135] test_error>U₁Tan(θ_i)+U₂Tan(θ_t)−BB,

[0136] where test_error is a pre-selected allowable margin of error.

[0137] If the current values of θ_pand φ_pdo not constitute a solution, then the disclosed method is iterated according to a modified version of Newton's well-known root finding technique. A new θ_i+1is calculated, as set forth in step 94 according to the following equations:

[0138] ftest=U₁Tan(θ_i)+U₂Tan(θ_t)−BB

[0139] fftest=(U₁/cos(θ_i)²)+(U₂/cos(θ_t)²)((c2/c1)cos(θ_i))/[1−((c2/c1)sin(θ_i))²]^½

[0140] θ_i+1=θ_i−delta(ftest/fftest)

[0141] where delta is a pre-selected value. The value of delta may be selected by the user of the method or by a computer algorithm.

[0142] As stated, the entire iteration process stops when equation (6) converges. When equation (6) converges, θ_pand φ_pdefine the preferred refraction point 12 and the current angles θ_pand φ_pare considered to be θ_pand φ_p. The iteration process may also be designed to cease after a maximum number of iterations if a solution is not found before.

[0143] Once the preferred refraction point 12 is determined, the associated pulse firing times are calculated as shown in step 96. The element delays, do(k), for each element k of a phased array 14 located above the spherical work piece 10 is calculated as follows:

[0144] do(k)=(ro₁−ro₁(k))/c₁+(ro₂−ro₂(k))/c₂

[0145] where, ro₁=[(x_c−x_p0)²+(y_c−y_p0)²+(z_c−z_p0)²]^½,(x_c, y_c, z_c) are the coordinates of the center of the phased array, (x_p0, y_p0, z_p0) is the point on the surface 11 of the spherical work piece 10 where a wave propagated from the center of the phased array 14 would refract toward the focal point 16, ro₁(k)=[(x_pk−x_ek)²+(y_pk−y_ek)²+(z_pk−z_ek)²]^½, (x_pk, y_pk,z_pk) is the point on the surface 11 where the wave from the k^thelement refracts towards the focal point 16, ro₂=[(x_f−x_p0)²+(y_f−y_p0)₂+(z_f−z_p0)²]^½, (x_f,y_f,z_f) is the focal point, and ro₂(k)=[(x_f−x_pk)²+(y_f−y_pk)²+(z_f−z_pk)²]^½. After the time delay, do(k), has been calculated for each of the k phase elements, the minimum time delay, domin, is identified from the set of all time delays, do(k). This task is shown in step 98 of FIG. 2. Finally, as set forth in step 100, the focal law sequence, FLO(k), for all of the k phase elements is calculated by subtracting the identified minimum time delay, domin, from all of the element time delays, do(k). That is, FLO(k)=do(k)−domin. Thus, for each phase element, k, FLO(k) provides the preferred pulse firing times.

[0146] As already noted, a second embodiment of this invention is shown in FIG. 4 where phased array 14 is located on the other side of the spherical work piece 10. In this second embodiment, the focal point 16 is located inside of the spherical work piece 10 as before. Otherwise, the components and configuration of the environment where the disclosed invention is used is the same as the first embodiment. The same methodology as described above and shown in the flowcharts in FIGS. 2 and 3 is used to determine each preferred refraction point 12 corresponding to the various phase elements. That is, the methodology depends upon the following relationship:

[0147] β_e=β₀−β

[0148] The only difference in the second embodiment shown in FIG. 2 relating to the calculation of the preferred refraction point 12 is that the mathematical definition of β_eis as follows:

[0149] β_e=θ_i−sin⁻¹[(R₀/R_f)sin(θ_i)].

[0150] In contrast, in the first embodiment shown in FIG. 1, β is defined by this formula.

[0151] A second difference between the first and second embodiments relates to the method of determining the timing sequence for the k phase elements. As in the first embodiment, the preferred time delay for each phase element is calculated (step 96) after the various preferred refraction points 12 are determined, as in steps 48 through 94. When the disclosed invention is used in an environment where the phased array 14 is located on the other side of the spherical work piece 10, the preferred delay associated with the k^thphase element is defined as follows:

[0152] di(k)=(ri₁−ri₁(k))/c₁+(ri₂−ri₂(k))/c₂,

[0153] where, c₁is the speed of sound in the material comprising the spherical work piece 10 and c₂is the speed of sound in the coupling material 8. The other parameters are, ri₁=[(x_f−x_po)²+(y_f−y_po)²+(z_f−z_po)²]^½, (x_po, y_po, z_po) is the point on the inside surface 11 where a wave propagated from the focal point 16 would refract toward the center of the array, ri₁(k)=[(x_pk−x_f)²+(y_pk−y_f)²+(z_pk−z_f)²]^½, (x_pk,y_pk,z_pk) is the point on the inside surface where the wave from the focal point refracts towards the k^thelement, ri₂=[(x_ek−x_pk)²+(y_ek−y_pk)₂+(z_ek−z_pk)²]^½, (x_ek,y_ek,z_ek) is the k^thelement, and ri₂(k)=[(x_ek−x_pk)²+(y_ek−y_pk)²+(z_ek−z_pk)²]^½. Except for the two identified differences, the methodology for determining preferred refraction points and delay times is the same whether the phased array is located above or on the other side of the spherical work piece 10.

[0154] While preferred embodiments of the present invention have been described herein, it is apparent that the basic construction can be altered to provide other embodiments which utilize the processes and compositions of this invention. Therefore, it will be appreciated that the scope of this invention is to be defined by the claims appended hereto rather than by the specific embodiments which have been presented hereinbefore by way of example.