Patent ID: 7068028

Claim:
A method of detecting a position of a metallic object, the method comprising: (a) exciting a coil with an electrical signal to produce an electromagnetic field enveloping the metallic object; (b) allowing a frequency of the electrical signal to be at the resonant frequency of the coil as affected by the metallic object; (c) adjusting a control signal controlling a characteristic of the electrical signal so that the amplitude of oscillation substantially equals a predetermined standard level; (d) periodically coupling and decoupling a resistive element in parallel with the coil while performing (a) through (c); and (e) deriving a signal based upon the adjusted control signal during at least a first time period, wherein the derived signal is indicative of a difference between at least one first value of an intermediate signal functionally related to the adjusted control signal obtained when the resistive element is coupled in parallel with the coil and at least one second value of the intermediate signal obtained when the resistive element is decoupled, and wherein the derived signal is further indicative of the position of the metallic object.