Noncontact Human-Machine Interface With Planar Probing Coils in a Differential Sensing Architecture

This paper presents a noncontact human-machine interface which is capable of sensing and indicating the position of a human hand that is approaching a probing coil. High sensitivity is achieved by utilizing a structure including two oscillators and a mixer to compare and extract the electromagnetic shift between two probing coils. The probing coils chosen in this system have a planar structure and small size, which can be easily built on a printed circuit board or integrated into a portable device for hand detection. The hand-coil interface is analyzed quantitatively for both the inductive and the capacitive links. Based on the analysis, a circuit model for the hand-coil interaction is presented. The proposed sensing circuit was built on an evaluation board with two commercial planar coils. A small phantom was used to imitate the hand electrical characteristic for a reliable and repeatable experimental setup. The measured results showed a hand-detection sensitivity of 10 kHz/mm at the output of the mixer stage and 30 mV/mm at the final output of the proposed interface.