Modeling and Design of Wearable Transducers for Assistive Technologies

Open Access
Author:
Safwat, Tahzib
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 22, 2019
Committee Members:
  • Christopher D. Rahn, Dissertation Advisor
  • Christopher D. Rahn, Committee Chair
  • Zoubeida Ounaies, Committee Member
  • Mary I Frecker, Committee Member
  • Mehdi Kiani, Outside Member
Keywords:
  • PVDF
  • copolymer
  • P(VDF-TrFE)
  • terpolymer
  • haptics
  • energy harvesting
  • self-powered
  • wearable
Abstract:
Wearable technology is a growing and highly interdisciplinary field, with research being conducted in many areas, including micro-electromechanical systems (MEMS), flexible sensors and actuators, energy harvesting, and human factors. In this dissertation, facets of wearable technology that are beneficial to healthcare and assistive technologies are explored. Polyvinylidene fluoride (PVDF) terpolymer actuators are applied to wearable haptics, electromagnetic (EM) harvesters are compared to PVDF films as strain energy harvesters, PVDF copolymer films are used for body-based energy harvesting, and a statistical analysis of an energy harvesting system model with storage is presented. People with prosthetic limbs can benefit from having haptic feedback that mimics a sense of touch. Electroactive polymer (EAP) actuators made of PVDF terpolymer, P(VDF-TrFE-CFE), have the potential to fill this need due to their compliance and ability to provide high strains. An EAP unimorph actuator is modeled for haptic feedback applications and shown to have flexibility, high frequency vibration capability (3mm end-to-end tip deflection at a resonance of 429Hz for a 10mm long beam), and high amplitude vibration at low frequency (tip deflection of up to 825μm for 10mm long beam) that provides a new way to send haptic signals by combining high frequency vibration with a slowly varying DC offset. A linear time-varying model is validated with experiments under static and slowly varying bias voltages. These actuators are demonstrated in a prosthetic feedback application and rotary motor prototype. The rotary motor is also used to calculate the maximum power density of the EAP active material, of at least 0.27W/g in the unimorph configuration, competitive with electric motors. Continuous monitoring of patients using low power sensors can help in tracking and treatment of chronic illnesses and other health conditions. Chest motion is investigated as a continuous source of strain energy from the body to power wearable health monitors. EM generator and piezoelectric PVDF film harvesters are modeled, designed, and experimentally tested. Static friction effects in the EM generators and their bulky profile make them poorly suited to this application. Piezoelectric PVDF films produce much lower power than EM, but their power scales with surface area and the films can be integrated into textiles. Under the force required by the EM harvester to break away from static friction, a PVDF film of 100cm^2 can produce up to 10µW. This, combined with their low weight and bending stiffness, make them ideal for wearables. To increase the power output from wearable piezoelectric energy harvesters, the PVDF copolymer, P(VDF-TrFE), connected in 33-mode via interdigitated electrodes (IDEs), is investigated. Copolymer is a highly compliant multifunctional material with superior electromechanical properties. Model-based analysis shows that its output power per force, a metric important for comfort, is greater than PVDF. A novel electrode design, interlaminar grid electrodes (IGEs), is used to align strain and poling directions similar to IDEs. However, electrodes of opposite electrical polarity are applied on different surfaces of the copolymer films, as in parallel plate electrode (PPE) configurations, helping to reduce fabrication defect-related shorting and open circuits. The ability to pattern electrodes with micron-scale features such as IGEs over large areas of copolymer thin films is a great asset to building wearable electromechanical devices. Additionally, the ability to pattern micron-sized electrode features at the interface between the layers of multilayer copolymer is required for IGEs and improves the performance of IDEs. A process to create multilayered copolymer with patterned electrodes using PVA and alumina capping layers is presented. Although alumina has closer dielectric properties to copolymer, very thin layers of PVA provide good chemical resistance without affecting the capacitance of the device. The self-power ability of a wearable device depends on how much energy is available for harvesting. Energy availability strongly depends on human behavior, including the activities that people do and the environments that they do them in. The intermittency of power generation limits the ability of an energy harvester to self-power a wearable device. Information from databases on human behavior and solar irradiance are used to estimate the average power that can be harvested from large populations. For three demographic groups (retired people, children, and office professionals), mechanical, thermoelectric, and photovoltaic harvesters worn on the wrist/forearm are simulated with supercapacitors that bridge power mismatches between the harvested power and power demand (assumed constant). The average solar power harvested, approximately 50-70µW, is an order of magnitude higher than mechanical or thermoelectric power, both of which harvest less than 10µW on average. Intermittency causes the solar (and mechanical) energy harvesters to achieve a 90% success rate of self-powering when the constant power consumption is a low 30-35% of their average harvested power. Thermoelectric generators, on the other hand, can provide 87% of their average harvested power with 90% success rate.