DEVICE AND PROCESS FOR DETERMINING FILM THICKNESS AND PATTERN REGISTER IN CELLS PLATED ON INDUCTIVE DEBIT CARDS

A device for determining the thickness of conductive metal patterns plated on insulating surfaces, such as credit cells in prepaid telephone debit cards, comprising a plurality of sensing modules equal in number to the credit cells in said cards, each module consisting of an oscillator (30) wherein the inductance is provided by a pair of collinear sensing coils (24a, 24b), aligned with the true position of its relative credit cell (25), the output (26) of said sensing assembly being connected to the input of an A/D converter (40) whose output is connected to computing means (11, 21) provided with memory means. Said oscillators are enabled by turns, the output voltage being converted to a numeric value. The converter parameters are adjusted for each sensing module according to coefficients stored in said memory, to compensate for component variations, sensing coil position and so on. The resultant numeric value is processed by the computing means according to a transfer curve (53) which is specific for each sensing module, so as to furnish the film thickness. Said coefficients and transfer curves are previously determined for each one of the positions in the cell array by means of individual calibration, using standard cards of known metal film thickness.