Method and apparatus for detecting weld junctions in steel sheets using an inductive sensor to detect thickness variations in the sheet

A method for detecting weld junctures in a sheet constructed of magnetic material, said weld juncture being defined by areas of differing sheet thickness, which method comprises:

• transporting said sheet material at a given velocity while supporting a first side of said sheet;
• obtaining a varying electrical signal whose variations are a function of variations in the thickness of said sheet by sensing magnetically a second side of said sheet;
• developing a rate of change signal by differentiating said varying electrical signal as a function of time;
• calculating an allowable rate of change signal using said given velocity and an acceptable variation in sheet thickness in a weld juncture area;
• comparing said allowable rate of change signal with said rate of change signal from said developing step;
• setting a latch if any difference between the calculated rate of change signal and the allowable rate of change signal is unacceptable:
• using said set latch to activate a reject mechanism;
• and
• resetting said latch subsequent to said activating of said reject mechanism.

Apparatus for detecting a weld juncture in a sheet material that is being transported at a given velocity, said weld juncture being defined by an area of differing thickness, which apparatus comprises:

• means for producing a rate of change signal that is a function of changes in the thickness of said sheet material and said given velocity including sensor means for obtaining a varying signal that is a function of said changes in said thickness of said sheet material and differentiator means operatively connected to said sensor means for differentiating said varying signal with respect to said given velocity;
• and
• a comparator operatively connected to said producing means for comparing said rate of change signal with a calculated allowable rate of change signal,
• said differentiating means comprising
• a first operational amplifier for amplifying said varying signal;
• a first resistor operatively connecting an output of said first operational amplifier to an input thereof;
• and
• a first capacitor operatively connecting said output of said first operational amplifier to said input thereof.

Apparatus for detecting a weld juncture in a sheet material that is being transported at a high-speed velocity of approximately 600 feet per minute, said weld juncture being defined by an area of differing sheet thickness, which apparatus comprises:

• an inductive sensor for producing a varying signal that is a function of changes in the thickness of said sheet material and its velocity of approximately 600 feet per minute;
• a support surface disposed a fixed distance from said sensor over which said sheet is transported;
• a differentiator operatively connected to said sensor for differentiating said varying signal as a function of time;
• and
• a comparator operatively connected to said differentiator for comparing said differentiated signal with an allowable value.