Abstract: A measuring instrument and method for measuring paint and coating thickness on poorly conductive graphite substrates. The instrument uses a microwave amplifier, a measurement cavity, and an optional reference cavity with supporting electronics to measure the apparent changes in the length of the measurement cavity due to the changes in the paint and coating thickness. The measurement cavity resonates when the measurement cavity is set upon the test surface. The oscillating signal is sent through a) directly to a microwave frequency counter, or b) a reference cavity tuned to give the desired slope detection response to cover the paint and coating thickness range allowed. The filtered response of the reference cavity is converted to a dc level signal proportional to paint thickness. It is presumed that the expert user will have access to a PC allowing instant conversion—via a stepped calibration plate—to paint thickness in mils from electrical data.

Abstract: Thermoplastic welding is an emerging technology targeted at significantly reducing the manufacturing cost of aerospace structure by eliminating fasteners and the touch labor associated with fasteners to prepare, install, and inspect the assemblies. Quality welds are highly dependent upon achieving appropriate temperatures everywhere along the bond line. The present invention is a system that uses a secondary coil to detect misalignment between the induction coil and the susceptor which would result in an excessive bond line temperature (in portions of the weld) and to provide control signals for the coil or parasitic compensation to avoid the overheating.

Abstract: We form welded reinforced composites by heating a susceptor along the bond line with a time-varying magnetic field produced with a moving induction coil. To obtain more uniform heating across the susceptor when welding composite parts using induction heating, edge regions of lower absolute impedance or a lower longitudinal impedance than the transverse impedance counterbalance the higher current density and current that occurs near the edges. The susceptor has lower impedance at the edges by altering the aspect ratio (length/width) of openings in the susceptor, by folding the susceptor over onto itself, or both. Uniform heating is important to obtaining a uniform, consistent weld on which aerospace designers can rely.

Abstract: A method and apparatus for sensing the temperature of a susceptor in a bond line in an inductive welding process utilizing a conductive susceptor at the interface between the two plastic parts, and a magnetic coil that generates an alternating magnetic field and projects the field through the parts and around the susceptor. As the susceptor gets hot, its electrical resistance changes as a function of the thermal coefficient of resistance of the susceptor material, and that change of resistance is reflected back as a change in the drive coil impedance. An electrical circuit senses the varying resistance and that change in resistance is translated into a change of temperature on a temperature display, and may be used to adjust the power to the work coil or the speed of travel of the work coil along the bond line. The circuit includes a high power bridge with a sensitive null arm to sense changes in the susceptor impedance due to temperature changes.

Abstract: To obtain more uniform heating across the susceptor when welding composite parts using induction heating, we create edge regions of lower absolute impedance or a lower longitudinal impedance than the transverse impedance to counterbalance the higher current density and current that occurs near the edges. We achieve lower impedance at the edges by altering the aspect ratio (length/width) of openings in the susceptor, by folding the susceptor over onto itself, or both. Uniform heating is important to obtaining a uniform, consistent weld on which aerospace designers can rely.

Abstract: There is disclosed a high power induction heating coil for use in thermoplastic welding where a metallic susceptor is imbedded in a composite assembly and is heated by eddy currents generated by the coil. The coil is comprised of several windings of Litz wire composed of multiple individually insulated strands of copper wire. The coil is wound on a circular E core having a coolant conduit through the center of the core. Coolant is introduced through the central core and returned by circulating through the loosely wound coil to the coolant source.

Abstract: There is disclosed a high power induction heating coil for use in thermoplastic welding where a metallic susceptor is embedded in a composite assembly and is heated by eddy currents generated by the coil. The coil is comprised of several windings of Litz wire composed of multiple individually insulated strands of copper wire. The coil is wound on a circular E core having a coolant conduit through the center of the core. Coolant is introduced through the central core and returned by circulating through the loosely wound coil to the coolant source.