Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Biomedical Engineering

Supervisor

Trejos, Ana Luisa

Abstract

Throughout your lifetime, it is nearly impossible to avoid at least one musculoskeletal condition that requires rehabilitation, and for a world containing billions of people, the costs associated with these conditions have become a huge economic burden. Nevertheless, the scientific community has become increasingly aware of the growing potential for wearable mechatronic systems to assist with upper-limb musculoskeletal rehabilitation; however, one limiting factor to their clinical acceptance lies in the development of actuation solutions. Conventional actuators are rigid and cylindrical in nature, requiring additional features to provide the proper forces, torques, and motion direction, required for human interaction. Another type of actuator, called twisted coiled actuators (TCAs), made from low-cost nylon thread, has recently been shown to provide many of the necessary performance specifications in a soft and flexible form factor, with additional properties akin to biological muscles. The aim of this thesis is to provide solutions to some of the setbacks surrounding this new actuator and analyze its feasibility for use in portable and wearable devices in the context of rehabilitation.

TCAs already show biomimetic properties such as variable stiffness, linear contraction, and flexibility, and like our own muscles, require an opposing force to extend following a contraction. Antagonistic arrangements are found throughout the body to provide the ability to extend and contract muscles allowing simultaneous control of position and stiffness around a joint. Therefore, the first part of this thesis focused on the biomimetic control of TCAs by characterizing the stiffness and position of two opposing TCAs in terms of the activation intensity. An empirical model was then used to implement a controller that could regulate the stiffness with an accuracy of 98.95%, while simultaneously controlling the position, with 92.7% accuracy.

TCAs are thermomechanical actuators, meaning that they contract when heated, and rely on external cooling to reverse their motion. This poses a problem when considering the requirements for human interaction, since the slow rate of natural cooling reduces the operating bandwidth to ineffective levels. This thesis introduces multiple studies around the implementation and evaluation of an active cooling system in order to provide faster cooling rates when needed.

First, a study compared the cooling performance of TCAs within an enclosed tube for two cooling mediums: pressurized air and water. The cooling rate was evaluated for three tube diameters and three activation levels of each mediums in order to evaluate the effects of these design parameters on the thermal characteristics and overall performance. The results of this study highlight the feasibility of both methods and the importance of geometry on the final outcome and determined that pressurized air cooling would be used moving forward.

An analytical model was then developed to represent the performance of a pressurized active cooling design as a function of important system parameters, such as the material properties of the enclosure, the mechanical dimensions, and the magnitude of the inlet air pressure. Experimental validation showed accurate model predication of the temperature with 92.7% accuracy during heating, and 94.5% during cooling. The implications of this study are that this model can provide an accurate and predictable method for tuning and evaluating the thermal characteristics of actively cooled TCAs.

Finally, the feasibility of actively cooled TCAs was evaluated for a simplified application involving wrist extension. To do this, a TCA-driven wearable wrist extension brace was conceptualized based on anthropomorphic data, and then modelled as a single degree-of-freedom joint in order to approximate the kinematic and dynamic system of equations. A numerical and physical representation of the system was developed and used to evaluate the performance of an inverse dynamics control law. The results showed position accuracy up to 98.8% for slower input commands, which declined to 66% for higher bandwidth inputs.

Summary for Lay Audience

Health of the bones, muscles, and nerves is important in allowing people to perform everyday tasks, making it a critical factor when it comes to living an active and independent life. Therefore, it is in our best interest to continue to improve the physical therapy process by implementing modern solutions, reducing costs, decreasing recovery time, and enhancing patient outcome. The aim of this thesis is to grow the field of smart wearable devices by exploring the unlocked potential of a new type of artificial muscle known as twisted coiled actuators (TCAs), which are made from nylon thread making them soft, light, and flexible. When TCAs are heated, they can contract to produce forces over 100 times greater than biological muscles of the same weight and with the same amount of displacement. These properties make them very desirable for use in wearable braces, especially in the field of physical therapy.

In order to make TCAs behave like muscles, this work presents a biomimetic (to mimic nature) approach to allow a pair of opposing TCAs to control the stiffness and position of a joint, much like our muscles when adapting to external forces. The success of this study signifies the ability of TCAs to assist biological muscles without obstructing natural motion when implemented into wearable devices.

Another issue with TCAs is that, in order to contract, they need to be heated to temperatures over 100 °C, which not only makes them unsafe to the human touch, but also very slow without additional cooling. The work presented in this thesis focuses on an active cooling system that encloses the TCA inside a tube, thus allowing it to cool at a controlled rate, while improving the safety to the wearer. Throughout this thesis, this new cooling design was tested based water or pressurized air cooling, along with various tube diameters. A mathematical model was also formulated in order to allow future researchers to make predictions about the performance based on variations in the system design. Finally, the feasibility of TCAs for use in rehabilitation devices was tested based on a wrist extension brace, where the performance was measured using both physical and simulated, replicas of the wrist brace system.

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