Doctor of Philosophy

dissertationThe optimization of novel stretchable fingernail sensors for detecting fingertip touch force direction is introduced. The fingernail sensor uses optical reflectance photoplethysmography to measure the change in blood perfusion in the fingernail bed when the finger pad touches a surface with various forces. This "fingernail sensing" technique involves mounting an array of LEDs (Light Emitting Diodes) and photodetectors on the fingernail surface to detect changes in the reflection intensity as a function of applied force. The intensity changes correspond to changes in blood volume underneath the fingernail and allow for fingertip force detection without haptic obstruction, which has several applications in the area of human-machine interaction. This dissertation experimentally determines the optimal optical parameters for the transmittance of light through the human fingernail bed. Specifically, the effect of varying the wavelength and optical path length on light transmittance through the nail bed are thoroughly investigated. Light transmittance through the human fingernail is optimized when using green light (525nm) and when placing optoelectronic pairs as close together as possible. The optimal locations of the optoelectronic devices are predicted by introducing an optical model that describes light transmittance between an LED and a photodiode in the fingernail area based on optical experimentation. A reduced configuration is derived from the optimal optoelectronic locations in order to facilitate iv the fabrication of the optimized fingernail sensor without significantly compromising the recognition accuracy. This results in an overall force direction recognition accuracy of 95%. Using novel fabrication techniques, we successfully build a stretchable fingernail sensor prototype, which fully conforms to the two-dimensional fingernail surface and is independent of its geometry. Namely, we overcome the challenges of patterning conductive lines on a stretchable substrate, and embedding rigid optical components in a stretchable platform while maintaining electrical conductivity. A finite element analysis is conducted to optimize the electrical contact resistance between the optoelectronic components and underlying stretchable conductors, as a function of the bending curvature and substrate thickness. The functionality of the stretchable sensor is tested in relation to the design parameters. Finally, applications and potential impacts of this work are discussed

Design and analysis of fingernail sensors for measurement of fingertip touch fouce and finger posture

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2002.Includes bibliographical references (leaves 142-148).A new type of wearable sensor for detecting fingertip touch force and finger posture is presented. Unlike traditional electronic gloves, in which sensors are embedded along the finger and on the fingerpads, this new device does not constrict finger motion and allows the fingers to directly contact the environment without obstructing the human's natural haptic senses. The fingertip touch force and finger posture are detected by measuring changes in the coloration of the fingernail; hence, the sensor is mounted on the fingernail and does not interfere with bending or touching actions. Specifically, the fingernail is instrumented with miniature light emitting diodes (LEDs) and photodetectors in order to measure changes in the reflection intensity when the fingertip is pressed against a surface or when the finger is bent. The changes in intensity are then used to determine changes in the blood volume under the fingernail, a technique termed "reflectance photoplethysmography." By arranging the LEDs and photodetectors in a spatial array, the two-dimensional pattern of blood volume can be measured and used to predict the touch force and posture. This thesis first underscores the role of the fingernail sensor as a means of indirectly detecting fingertip touch force and finger posture by measuring the internal state of the finger. Desired functionality and principles of photoplethysmography are used to create a set of design goals and guidelines for such a sensor.(cont.) A working miniaturized prototype nail sensor is designed, built, tested, and analyzed. Based on fingertip anatomy and photographic evidence, mechanical and hemodynamic models are created in order to understand the mechanism of the blood volume change at multiple locations within the fingernail bed. These models are verified through experiment and simulation. Next, data-driven, mathematical models or filters are designed to comprehensively predict normal touching forces, shear touching forces, and finger bending based on readings from the sensor. A method to experimentally calibrate the filters is designed, implemented, and validated. Using these filters, the sensors are capable of predicting forces to within 0.5 N RMS error and posture angle to within 10 degrees RMS error. Performances of the filters are analyzed, compared, and used to suggest design guidelines for the next generation of sensors. Finally, applications to human-machine interface are discussed and tested, and potential impacts of this work on the fields of virtual reality and robotics are proposed.by Stephen A. Mascaro.Ph.D