Date of Award

Spring 1-1-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Christoph Keplinger

Second Advisor

SeHee Lee

Third Advisor

Jana Milford

Fourth Advisor

Mark Rentschler

Fifth Advisor

Gregory Whiting

Abstract

As technology becomes more integrated into our daily lives, there is an ever-increasing demand for versatile and cost-effective electromechanical transducers which convert mechanical energy into electrical energy or vice versa and are used as [i] actuators to produce movement, [ii] sensors that react to mechanical stimuli, and [iii] generators to produce electricity. Traditional electromechanical transducers are comprised of metals, magnets, and electrical conductors which are highly capable in select applications with controlled and predictable environments, but also ill-suited for applications such as interactions with humans and operation at low frequencies. However, biological systems, through millions of years of evolution, can easily function in these applications and countless others, due to the use of soft and compliant materials and ionic systems, enabling wide adaptability and versatility. This work takes inspiration from nature to develop new electromechanical transducers which incorporate soft and flexible materials with ionic conductors for applications in energy generation and soft robotics.

The first part of this work introduces a variable electric double layer (EDL) generator, an energy generation system that harnessed mechanical motion to influence the capacitance of a charged EDL and increase the electrical energy. Traditional electromechanical transducers for energy generation rely on electromagnetic principles that are ill-suited to capture low frequency mechanical motion. Conversely the variable EDL generator introduced in this work provides a model system in which all experimental parameters are easy to access in order to gain a detailed understanding of the energy flows of this conversion process rooted in electrostatic principles. The system uses titanium electrodes in NaCl aqueous electrolyte and operates as a charge pump. Analysis of the voltage-charge work-conjugate plane confirmed net positive electrical energy generation and also enabled exploration of possible avenues to improve the performance of the variable EDL generator through materials and electrical circuit optimization. An outlook for the variable EDL generator is discussed in which external mechanical input such as oscillating ocean waves could drive the generator.

The second part of this work continues to take inspiration from nature by again using ionic conductors and combining them with soft and flexible materials to create robotic actuators capable of producing life-like motion. This work combines the benefits and avoids the pitfalls of two prior muscle-mimetic soft actuators through the marriage of electrostatic and hydraulic principles to introduce a new material system termed a hydraulically amplified self-healing electrostatic (HASEL) actuator. A key characteristic of HASEL actuators is the ability to electrically self-healing from dielectric breakdown due to the use of a liquid dielectric. Additional work shows that the capacitive structure of HASEL actuators enables the ability to simultaneously actuate and sense capacitance (and thus position). Another key characteristic of an actuator is the efficiency, and thus the efficiency of HASEL actuators is experimentally measured both for the electromechanical transduction process as well as the full system efficiency of a HASEL actuator driven by a portable driving electronics which will be especially valuable when exploring portable applications. Lastly an outlook for HASEL actuators is discussed with possible enhancements in material selection, self-healing, self-sensing, and efficiency.

The third part of this work aims to introduce a new multifunctional material – a transparent, self‐healing, highly stretchable ionic conductor – that is used in dielectric elastomer actuators (DEA), another soft robotic actuator. This work demonstrates the ability of a DEA made with the self-healing material to mechanically self-heal from severe mechanical damage and continue to function, which was shown in contrast to a DEA made with another type of ionic conductor that does not have the ability to self-heal. We also performed experimental analysis of mechanical and electrical properties of the self-healing material. Possible follow on work is suggested to enhance the self-healing abilities of both dielectric elastomer actuators as well as HASEL actuators.

In closing, this work explores the transition of HASEL actuators from the academic lab into commercial applications. Key considerations toward this goal are discussed, including the technology readiness level of HASEL actuators, remaining technical challenges, and immediate next steps to study fabrication and lifetime.

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Available for download on Wednesday, July 07, 2021

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