Flexible touchpads based on inductive sensors using embedded conductive composite polymer

Abstract

We present the design, fabrication, and preliminary testing of a flexible array of sensor switches intended for applications in wearable electronics and sensor systems. The touch pad sensor arrays feature flexible printed circuit board (flexible PCB) substrates and/or flexible conductive composite polymer (CCP) structures, resulting in highly flexible switch arrays. Each switch consists of 4 elements: fascia, target, spacer and a sensor coil. The user presses the fascia, bringing the target in contact with the sensor coil. Any change in the position of the target changes the coil inductance due to the generation of eddy currents, which are detected by an electronic circuit and custom software. Contact between the target and coil also measurably changes the inductance of the coils. Different sizes and geometries (square, circular, hexagonal and octagonal) of coils in both flexible PCB metal (copper) and CCP were investigated to determine which couple best with the CCP that forms the target for the inductive coils. We describe techniques for patterning two-layer inductive coils on flexible PCBs. Using this process, we demonstrate coil trace thicknesses of 200 micrometers. We also present a new low cost microfabrication technique to create inductive flexible coils using embedded CCP in polydimethylsiloxane (PDMS) as an alternative to flexible PCB metal coils. We further describe an electronic circuit that accurately measures inductances as low as 500 nH that is used to detect the change in the inductance of a sensor’s coil when the user presses the target element of the sensor. The inductance for a sensor composed of CCP square coils and CCP target was measured to be approximately 35 μH before being pressed. When pressed, the inductance dropped to 3.8 μH, a change which was easily detected.