Apparatus and method for detecting structural cracks using a movable detector

Method for detecting a crack in a workpiece, the method comprising the following steps:

• a. detecting a change in an eddy current in the workpiece due to the presence of the crack when a detector is moved near the crack;
• b. generating a measurement signal which is a function of the change in the eddy current due to the presence of the crack such that the measurement signal has a first component when the detector moves in a first direction relative to the workpiece and a second component when the detector moves in a second direction relative to the workpiece;
• c. removing the second component from the measurement signal by
• (i) generating a first signal, represented by a vector having an endpoint x1,y1 of a vector coordinate system, resulting from movement of the detector relative to the workpiece,
• (ii) generating a second signal, represented by a vector having an endpoint x2,y2 of the vector coordinate system, resulting from movement of the detector relative to the workpiece, such that the second signal differs from the first signal due to movement of the detector in the second direction relative to the workpiece,
• (iii) calculating a compensation vector extending between endpoint x1,y1 and endpoint x2,y2,
• (iv) rotating the vector coordinate system so that an axis of the vector coordinate system is parallel to the compensation vector,
• (iv) referencing the measurement signal to the rotated vector coordinate system, and
• (v) removing the second component of the measurement signal by ignoring any components of the measurement signal in the rotated vector coordinate system which are parallel to the compensation vector;
• and
• d. generating a visual display of the crack as a function of the first component of the measurement signal.

Apparatus for detecting a crack in a workpiece, the apparatus comprising:

• a. means, which are movable with respect to the workpiece, for detecting a change in an eddy current in the workpiece due to the presence of the crack when the detecting means is moved near the crack during an x clock count and a y clock count, the detecting means including a first row of detector elements for detecting the presence of the crack and for generating first signals at the x clock count and first signals at the y clock count, and a second row of detector elements for detecting the presence of the crack and for generating second signals at the clock count, the first and second row of detector elements being located in the detecting means such that the first row of detecting elements are moved near the crack before the second row of detecting elements are moved near the crack;
• b. means for generating a signal which is a function of the change in the eddy current due to the presence of the crack, the signal generating means including means for storing the first and second signals, the storing means including a first location for storing the first signals obtained at the x clock count and for storing the second signals obtained at the y clock count, and a second location for storing the first signals obtained at the y clock count such that the signals stored in the first location are representative of a first portion of the crack and the signals stored in the second location are representative of a second portion of the crack;
• and
• c. means for generating a visual display of the workpiece such that the signals from the first and second columns generate a visual display of the first and second portions of the crack.