Patent ID: 7756576

Claim:
An apparatus for position sensing, comprising: a probe, comprising at least one probe electrode, which is adapted to be inserted into a body of a living subject, said body having a body surface; a plurality of electroconductive body surface patches, which are adapted to be fixed to said body surface at respective surface locations, wherein said body surface patches each comprise a first electrode and a second electrode, said second electrode being electrically isolated from said first electrode; electrical circuitry having at least one switch for arranging said first electrode in a series circuit with said second electrode in a calibration mode of operation and for arranging said first electrode in a parallel circuit with said second electrode in a sensing mode of operation; and a controller, operative to control said electrical circuitry, wherein said controller is adapted to be coupled to said probe and to said body surface patches so as to pass first electrical signals through said first electrode and said second electrode thereof in said calibration mode of operation, and to pass respective second electrical signals through the body between said probe electrode and said body surface patches, wherein said controller is operative in said calibration mode of operation to determine a local electrical impedance of said body surface and of an interface thereof with a respective one of said body surface patches, and to determine position coordinates of said probe in said sensing mode of operation by measuring respective impedance characteristics of said second electrical signals, and wherein said impedance characteristics of said second electrical signals are adjusted by said controller according to respective instances of said local electrical impedance of said body surface.