Flexible Touchpads Based on Inductive Sensors Using Conductive Composite Polymer and Flexible Metal PCB

In this thesis, the design, fabrication, testing and characterization of two tactile sensor array technologies are presented. The two sensor systems both use inductance as a transduction principle and are designed to be implemented in flexible wearable systems. The tactile sensor arrays feature flexible PCB substrates and/or flexible conductive composite polymer (CCP) structures, resulting in highly flexible tactile 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. In the first method, where the system operates based on the principle of eddy current generation, movement of the conductive target changes the inductance of a sensing coil. In the second method, the contact between the target and the conductive membrane triggers a touch detect in the detection mechanism. An electronic circuit that accurately measures inductances is developed to detect the change in the inductance of each sensor’s coil when the user presses the target element of the sensor. Different sizes and geometries of coils in both flexible PCB metal and CCP are investigated to determine which couple best with the CCP that forms the target for the inductive coils. Using COMSOL, a COMSOL simulation of the coil geometries is also conducted in order to study the generated magnetic field and distribution of the magnetic flux density at the center of the coils. Techniques for patterning two-layer inductive coils on flexible PCBs are described. A low cost microfabrication technique to create inductive flexible coils using embedded CCP in polydimethylsiloxane as an alternative to PCB metal coils is also presented. The inductance for a sensor composed of PCB metal coils and CCP target are measured to be approximately 33.1 μH and 42.9 μH for circular and square geometries, respectively, before being pressed. When pressed, a 40% change in the inductance is observed, a change which is easily detected. In the case of coils made of CCP, although the measured inductance values are shifted because of the internal resistance of the coils, a 35% change in the inductance was observed when the conductive target was pressed.