Patent Publication Number: US-2007096728-A1

Title: Eddy current inspection apparatus and methods

Description:
BACKGROUND OF THE INVENTION  
      This invention relates generally to inspections apparatus and methods and more particularly, to eddy current probes and apparatus for performing inspections.  
      Eddy current probes for the inspection of accessible surfaces have been in use for many years and utilize various eddy current coil configurations. These coil configurations include single and multiple coil configurations. For example, multiple eddy current coils can be arranged in an array to scan an inspection surface, and data collected during the scan can converted to an image for review and evaluation.  
      Inspecting less accessible surfaces with an eddy current probe typically requires at least some disassembly of components adjacent the inspection surface. For example, to inspect an embossment of a first stage gas turbine wheel, at least some components adjacent the embossment typically are disassembled in order to gain access to the embossment along its length. Such disassembly provides access to the embossment and facilitates accurate scanning along an entire length of the embossment. Such disassembly also facilitates viewing the probe and surface during scanning.  
      Without disassembling such components, there may be noise signals generated due to variable lift-off of the probe from the surface and wobble of the probe relative to the surface. As a consequence, noise signals may prevent accurate inspection results. Furthermore, without such disassembly, there is no line-of-sight visibility to the embossment region. Disassembling components to perform an inspection, however, adds time and expense to such inspection.  
     BRIEF DESCRIPTION OF THE INVENTION  
      In one aspect, a method for inspecting an embossment of a wheel for a gas turbine using an inspection assembly is provided. The inspection assembly includes a fixture and an eddy current coil. The fixture includes a stability support and support bearings. The method includes positioning the assembly so that the eddy current coil is over the surface to be inspected and so that the support bearings are on the surface, and maintaining the stability support against a support surface.  
      In another aspect, an inspection assembly for inspecting a surface is provided. The inspection assembly includes a fixture having a handle, a center support, and a base. The center support extends from the handle to the base. The fixture further includes a stability support assembly, and a dual slide block arrangement coupled to a holder for engaging an eddy current coil. The assembly further includes an eddy current coil secured to the holder.  
      In yet another aspect, an inspection assembly for inspecting a surface is provided. The assembly includes a fixture having a handle, a center support, and a base. The center support extends from the handle to the base. The fixture further includes a stability support assembly, and a dual slide block arrangement coupled to a holder for engaging an eddy current coil. The dual slide block arrangement includes a first block providing axial adjustment of the holder relative to the surface to be inspected. The dual block arrangement further includes a second block providing radial adjustment of the holder relative to the surface to be inspected. The holder includes a pocket for engaging an eddy current coil, and the eddy current coil is secured in the pocket. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of an eddy current embossment inspection fixture and probe assembly.  
       FIG. 2  is a perspective view of a portion of the fixture shown in  FIG. 1 .  
       FIG. 3  is another view of a portion of the fixture shown in  FIG. 2 .  
       FIG. 4  is yet another view of the portion of the fixture shown in  FIG. 2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Set forth below is a description of an eddy current fixture and probe that can be utilized in connection with inspection of an embossment of a gas turbine first stage wheel. Such fixture and probe are not limited to practice with any particular first stage wheel embossment, and certain components of the fixture can be used separately and in connection with other fixture arrangements. Therefore, the following description is by way of example only, and many other fixture and probe configuration are possible depending on a particular surface to be inspected.  
      Generally, the fixture includes a set of spaced apart ball bearing supports to provide stable scanning of an embossment surface. The fixture also provides mechanical adjustment to position an eddy current probe accurately without a need for visually observing the inspection surface. Such fixture therefore facilitates inspection of remote embossment surfaces without rotor disassembly.  
      As shown in  FIG. 1 , an assembly  10  includes a probe scanning fixture  12 , an adjustable probe body head  14 , and a ferrite pot core eddy current (EC) transducer  16 , sometimes referred to herein as a sensor. Pot core eddy current transducers (two major components are the magnetic core material and the copper coil) are well known in the art and are commercially available from, for example, Ceramic Magnetics, Incorporated, Fairfield, N.J., for ceramic magnetic materials, and Xactex Corporation, Pasco, Wash., for coil windings. By using a pot core eddy current transducer, the inspection described herein can be performed with two circumferential scans. In addition, assembly  10  provides that an inspection of the embossment surface on a first stage wheel can be performed on a rotor that only has its buckets removed from the wheel being inspected. Therefore, when the buckets are removed for a dovetail inspection, an inspection of the embossment surface can be performed without further disassembly.  
      Fixture  12  includes a handle  18  and a main body  20 . Main body  20  includes legs  22  and a center brace  24  that extend to a base  26 . A stability support assembly  28  is provided at top of center brace  24 . Fixture also includes a dual slide block arrangement that permits offsetting eddy current probe axially relative to fixture so that an adjacent track, or surface, can be scanned.  
      As shown in  FIG. 2 , adjustable head  14  includes a dual slide block arrangement that provides for both axial motion and radial motion of transducer  16  relative to fixture  12 . Such dual slide block arrangements are well known in the art and are commercially available from, for example, CBM Fabrications, Ballston Lake, N.Y. More particularly, head  14  includes a first block  30  providing axial adjustment of transducer  16 , and a second block  32  providing radial adjustment of transducer  16 , relative to the surface to be inspected. An opening  34  is provided in a cross brace  36  to permit adjustment of probe block  30 , as described below in more detail.  
      As shown in  FIG. 3 , stability support assembly  28  has a T cross-sectional shape support  38  secured to center brace  24  by screws  40 . Ball bearing  42  is engaged to support  38  so that ball bearing  42  rolls along a support surface. For example, bearing  42  is positioned to ride on a surface region just below a bottom of a dovetail slot during an inspection operation, and provides stability for an inspection operation. In addition, a reference coil, instrumentation connector, and probe coil connections may be secured to fixture  12  at a location generally indicated by  44 .  
      As also shown in  FIG. 3 , adjustable head  14  includes a holder  46  that has a pocket  48  for housing a pot core eddy current coil. Also, a lip  50  is provided as a mechanical stop to limit axial adjustment of head  14 .  
      As shown in  FIG. 4 , a mechanical guide  52  extends from cross brace  36  to first block  30  so that radial adjustment of the location of the coil can be made by adjusting a screw  54 . In addition, guide ball bearings  56  are provided to run against a lip, and support ball bearings  58  are provided to adjust probe coil clearance relative to the inspection surface.  
      To perform an inspection, for example, of an embossment of a wheel for a gas turbine using inspection assembly  10 , and prior to positioning assembly  10  to perform the inspection, assembly  10  is adjusted so that transducer  16  is at a pre-selected orientation relative to fixture  12 . Particularly, the dual slide block arrangement having axial adjustable block  30  and radial adjustment block  32  is adjusted to orient transducer in a desired position. Once adjusted, assembly  10  is positioned so that eddy current coil, i.e., transducer  16 , is over the surface to be inspected and so that support bearings  58  are on the surface while maintaining stability support  28  against a support surface. Assembly  10  is then moved along the surface to be inspected so that support bearings  58  roll along the surface being inspected and stability support  28  is maintained in contact with the support surface.  
      The combination of support  28 , and bearings  56  and  58 , along with adjustment of head  14 , facilitate keeping scanning induced noise to a practical minimum. For example, in inspecting the embossment in a first stage wheel, there are at least two sources of motion-induced noise. A first noise source results from not running smoothly enough on the inspection surface. A second noise source results from wobbling motion transverse to the scan direction. Assembly  10  facilitates minimizing noise from these noise sources  
      Further, with respect to assembly  10 , support ball bearings  58  are spaced from transducer  16 . Transducer  16  is approximately centered between supports  22 . In this way, if one of supports  22  is raised up due to a local surface irregularity, the displacement at transducer  16  is one-half the displacement at support  22 . Furthermore, the angular tilt of transducer  16  from local normality to the inspection surface may also be reduced. Such an arrangement facilitates reducing mechanical motion of sensor  16  relative to the inspection surface, which in turn reduces this source of electrical noise. With respect to wobble motion, use of ball-bearing support  28  near handle  18  provides the support by running on surface just below the dovetail opening on the wheel face.  
      While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.