Control circuit for resonant sensing device

Claim 1

A SONIC TESTING DEVICE FOR MEASURING THE HARDNESS OF AN OBJECT COMPRISING IN COMBINATION: A TRANSDUCER ASSEMBLY WHICH INCLUDES A MAGNETOSTRICTIVE ROD ADAPTED TO BE SONICALLY VIBRATED AND HAVING ONE END WHICH IS ADAPTED TO ENGAGE BY FORCED CONTACT THE SURFACE OF THE OBJECT; AN ELECTROMAGNETIC MEANS DISPOSED TO SONICALLY VIBRATE, WHEN ENERGIZED, SAID ROD; A PIEZOELECTRIC MEANS COUPLED TO SAID ROD FOR PRODUCING AN ELECTRICAL FEEDBACK SIGNAL RESPONSIVE TO THE VIBRATIONS OF SAID ROD; A FEEDBACK ALTERNATING CURRENT AMPLIFYING MEANS COUPLED BETWEEN SAID ELECTROMAGNETIC MEANS AND SAID PIEZOELECTRIC MEANS TO PROVIDE AN ALTERNATING CURRENT SIGNAL TO SAID ELECTROMAGNETIC MEANS IN RESPONSE TO THE FEEDBACK SIGNAL FROM SAID PIEZOELECTRIC MEANS; THE SERIES CONNECTION OF A SHAPING CURCUIT, A MULTIVIBRATOR, A LIMITER CIRCUIT AND A FREQUENCY-TO-CURRENT CONVERTER COUPLED TO SAID AMPLIFYING MEANS FOR PROVIDING AN OUTPUT SIGNAL WHOSE MAGNITUDE IS RESPONSIVE TO THE FREQUENCY OF VIBRATIONS OF SAID ROD, SAID CONVERTER INCLUDING A CAPACITIVE REACTANCE AND AN INDUCTIVE REACTANCE CIRCUIT BRANCH, AND A METER CONNECTED TO INDICATE THE DIFFERENCE IN CURRENT THROUGH SAID BRANCHES AS A FUNCTION OF THE FREQUENCY OF SAID ALTERNATING CURRENT SIGNAL, SUCH CURRENT BEING A MEASURE OF THE HARDNESS OF THE OBJECT AT THE AREA OF ENGAGEMENT OF SAID ROD.