Capacitive Eye Tracker made of Carbon-Nanotube Paper Composite for Human-Machine Interface and Neuroscience Applications

Abstract

The uniqueness of eyes, facial geometry, and gaze direction make eye tracking a very challenging technological pursuit. The gold standard for an eye-tracking device has been the scleral search coil system. Despite dominant camera-based eye-tracking systems, their bulky equipment's obtrusiveness and high-power consumption are considered challenging for wearable applications. Piezoelectric sensors, electrooculography (EOG), and capacitive sensors have been attempted for eye tracking but failed due to low sensitivity. The capacitive sensors are unique in that eye movement can be monitored in a noncontact manner, but the sensitivity dampens as the number of sensors increases due to enlarged parasitic capacitance. In this dissertation, the capacitive sensors are reviewed with respect to their history, working principles, sensing mechanism, fabrication methods, recent nanoscale and microscale regimes, and current applications. An analytical and numerical study is presented to understand in-plane and fringing capacitances. Single- and differential sensing configurations are analyzed in terms of sensitivity and linearity by analytical equations and numerical simulations. Micro and nanostructured materials to construct capacitive sensors will be assessed in the contexts of target parameters, including pressure, strain, force, liquid level, humidity, temperature, displacement, and acceleration. The applications of capacitive sensors are presented in the emerging fields of wearable sensors, human-machine interface (HMI), biomedical implementation, human health monitoring, robotics, and industrial monitoring. Based on the knowledge, the capacitive interaction between a novel sensor and eye movement for wearable eye-tracking is studied. The capacitive sensors are made of a pair of asymmetric electrodes; one comprising carbon nanotube-paper composite fibers (CPC) and the other being a rectangular metal electrode. The interaction between the asymmetric sensor and a spherical object mimicking an eyeball is analyzed numerically. Using a face simulator, both single- and differential capacitive measurements are characterized with respect to proximity, geometry, and human body charge. Using a prototype eye tracker, multiple sensor locations are studied to determine the optimal configurations. The capacitive responses to vertical and horizontal gaze directions are analyzed in comparison to those of a commercial eye-tracking system. The performance is demonstrated for sensitive eye-movement tracking, closed-eye monitoring, and human-machine interface. Eye tracker performance is validated in comparison to the scleral search coil, the current gold standard in eye tracking. The capacitive interaction between the CPC electrode and a spherical eyeball is analyzed by a numerical study to understand how the shape and scale of the eyeball affect capacitive interaction. A non-human primate (NHP), implanted with a scleral search coil, is utilized to conduct an eyeball interaction study. The CPC eye tracker’s response to smooth pursuit and saccadic movements is investigated and validated against the scleral search coil. This research has important implications for the development of capacitive, wearable eye trackers, which can facilitate fields of human-machine interface, cognitive monitoring, neuroscience research, and rehabilitation.