Device for accurately detecting the position of a ferromagnetic material inside biological tissue

A device for accurately detecting at a specific limited distance the position of an object having ferromagnetic properties inside biological tissue, while being relatively insensitive to changes in external temperature and the like, comprising:

• a probe having a primary winding and at least two secondary windings,
• a means for producing an alternating output to activate said primary winding, said primary winding, when activated, generating a limited field which induces a.c. voltages in said secondary windings, said voltages in said secondary windings being approximately equal and cancelling each other so that said probe has an approximately zero output voltage when the object to be detected is outside a specific limited distance from one of said secondary windings, but creating unequal voltages in said secondary windings which do not cancel each other so that said probe has a net alternating voltage when the object is within a specific limited distance from one of said secondary windings,
• a detection means connected to said secondary windings for detecting any net alternating voltage from said probe, said detection means comprising a means for amplifying any such net alternating voltage from said probe, and a means for converting any such net alternating voltage as amplified by said means for amplifying to a d.c. voltage, the magnitude of which reflects the proximity of the object to one of said secondary windings,
• a conversion means for converting the d.c. voltage from said means for converting, said conversion means comprising a voltage to frequency converter having a threshold voltage means which sets a level that must be exceeded by the d.c. voltage from said means for converting in order to produce an output signal from said conversion means thereby limiting the detection distance and providing insensitivity to external temperature,
• said voltage to frequency converter including an error amplifier connected to an oscillator circuit having a pair of outputs, said outputs of said oscillator circuit being complementary and respectively connected to a pair of analog switches which form a servo loop with an input of said error amplifier so that said error amplifier compares each output of said oscillator circuit with the input received by said error amplifier from said means for converting,
• one of said oscillator circuit outputs providing the output signal from said conversion means,
• said means for converting further comprising a synchronous converter, said synchronous converter having first and second analog switches, each said switch being alternately triggered by a different, complementary output from said means for producing, said synchronous converter also having a first capacitor, said first capacitor being charged with the low voltage from any amplified net alternating output from said means for amplifying when said first analog switch is activated by a first output from said means for producing,
• an alarm means which is activated by the output signal from said conversion means and a power source, said power source connected to said means for producing, said detection means and said conversion means.