EDDY CURRENT FLAW DETECTION SYSTEM WITH LEFT OFF COMPENSATION

• US 3,753,096 A
• Filed: 02/04/1971
• Issued: 08/14/1973
• Est. Priority Date: 02/04/1971
• Status: Expired due to Term

• Current Assignee: Staveley Instruments Incorporated
• Original Assignee: William Charles Wiers
• Inventors: Wiers, William Charles
• Primary Examiner(s): Corcoran, Robert J.
• Application Number:
  US05/112,710
• Time in Patent Office: 922 Days
• Field of Search: 324/37,40,41
• US Class Current: 324/233
• CPC Class Codes: G01N 27/9006 Details, e.g. in the structure or functioning of sensors
• CPC Class Codes: G01N 27/9046 by analysing electrical signals
• CPC Class Codes: G01N 27/9053 Compensating for probe to workpiece spacing

Abstract

A nondestructive testing system of the eddy current variety is disclosed herein for inspecting workpieces for defects. A search unit is provided for use with the system for inducing eddy currents on the surface of the workpiece and sensing the fields reradiated from such currents to produce a corresponding test signal. The search unit includes one or more magnetic cores and one or more windings which are constructed and arranged to improve the response characteristics of the search unit to discontinuities in the surface of the workpiece. Also, they are constructed and arranged to eliminate or at least drastically reduce variations in the search unit response characteristics resulting from variations in the lift-off distance, i.e., the space between the search unit and the surface of the workpiece.

Claims

1. An eddy current probe for inspecting a work-piece for defects, said probe including the combination of:

a first ferromagnetic core having an opening therein defining an inside diameter, a second ferromagnetic core having an outside diameter smaller than said inside diameter of the first core, said first core having a volume greater than that of said second core, said second core being at least partially disposed within said opening of the first core, a first winding on said first core, a second winding on second core, and electrical bridge means including the connection of said first and second windings as normally balanced windings of said bridge means, whereby electrical variation of both the first and second windings to lift-off of the probe are substantially the same to maintain the bridge means in a balanced condition whereas the second winding is substantially more sensitive to workpiece defects than the first winding so as to cause an unbalanced condition of said bridge means.

2. The combination set forth in claim 1, further defined by said inside diameter of the first core being substantially larger than said outside diameter of said second core such that the first and second cores and their associated first and second windings exhibit substantially independent magnetic fields.

3. A non-destructive eddy current testing system including:

a probe having inner and outer coaxially arranged annular cores of ferromagnetic material, each core having an associated coil wound thereon, and electrical bridge means for connecting said probe coils to electrical instrumentation, said bridge means including means for connecting said outer core coil and said inner core coil as normally balanced windings of said bridge means, whereby said bridge means maintains a substantially balanced electrical condition in response to lift-off of said probe relative to a workpiece and assumes an unbalanced electrical condition in reponse to a greater sensitivity of the inner core coil relative to the sensitivity of the outer core coil to workpiece defects.

4. An eddy current probe for inspecting a workpiece for defects, said probe including the combination of:

a first ferromagnetic core, said core being elongated and having an end portion with a face thereon for being disposed adjacent the workpiece, a first winding means on said core for creating eddy currents in the workpiece and creating a signal corresponding to the influence of such eddy currents on said first core and first winding means, a second ferromagnetic core, said second core at least partially surrounding said first core and spaced therefrom whereby magnetic fields from said first and second cores, are substantially separate from each other, the end portion of said first core protruding beyond a surrounding portion of said second core, second winding means disposed on said second core for creating eddy currents in the workpiece and creating a signal corresponding to the influence of such eddy currents on the second core and second winding means, and electrical bridge circuit including the connection of said first and second winding means as normally balanced windings of the bridge circuit such that said first and second cores and associated first and second windings exhibit substantially the same electrical response to the lift-off of the probe relatiVe to the workpiece so as to maintain the bridge circuit in electrical balance whereas said first core and first winding means exhibit a greater electromagnetic variation in response to workpiece defects than exhibited by the second core and second winding means so as to cause said bridge circuit to assume an unbalanced condition in response to the presence of a defect in the workpiece.

5. An eddy current search unit for inspecting a workpiece, said search unit including the combination:

an inner elongated ferromagnetic core having a face at one end for being positioned adjacent a surface of the workpiece, an outer annular ferromagnetic core disposed coaxially about said inner core, said outer core being substantially radially spaced apart from said inner core whereby the magnetic fields for said inner and outer cores are substantially independent from each other, an inner core winding means disposed on said inner core for inducing eddy currents in the workpiece, an outer core winding means disposed on said outer core for inducing eddy currents in the workpiece, electrical bridge means including the connection of said inner and outer winding means as normally balanced windings of said bridge means, and means responsive to the electrical condition of said bridge means to indicate defects within the workpiece, whereby said bridge means remains substantially balanced in response to lift-off of said inner and outer cores relative to a surface of the workpiece and assumes an unbalanced condition in response to a greater sensitivity of the inner core and inner core winding means to localized defects within the workpiece.

6. The search unit as defined in claim 5, said outer core having a substantially hollow cylindrical configuration with said inner core coaxially mounted therein, the face end of said inner core protruding beyond one axial end of said outer core, said axial end of said outer core having a beveled region, said beveled region being proportioned to balance out differences in the electro-magnetic variations of said winding means to the tilting of the axis of said cores relative to a surface of the workpiece.

7. An eddy current probe for inspecting a work-piece for defects, said probe including the combination of:

a first elongated ferromagnetic core having a first end thereon, a face on said end for being coupled to the workpiece, a second hollow ferromagnetic core, said second core having a volume of ferromagnetic material which is greater than the volume of ferromagnetic material in said first core, a face on said second core for being coupled to the workpiece, said second face being larger than said first face, an opening through said second face, said first core being disposed inside of the second core with said end thereof projecting through said opening whereby said first core is enclosed within said second core except for said end and said first face is positioned closer to said workpiece than said second face, first winding means on said first core for creating eddy currents in said workpiece in a small area corresponding to said first face and creating a signal in response to said eddy currents, second winding means on said second core for creating eddy currents in said workpiece over a large area corresponding to said second face and creating a signal in response to said eddy currents, means interconnecting said first and second winding means whereby electrical variations in the signal from the first and second windings means due to the lift-off between said faces and the workpiece are substantially cancelled whereby the first winding means is more sensitive to workpiece defects than the second winding as a result of an unbalanced condition of said signals resulting from defects in the workpiece.