
\section{Background and motivation}

%The recent decades saw the design focus for jumping robots shifting from locomotion through hopping to reaching a better height and higher frequency, as well as the incorporation of other functions including gliding and running. 


The recent decades saw an increasing trend of robot designs taking ideas from natural organisms in the form of bioinspiration and biomimetics. This is due to the fact that the morphological versatility of living creatures indeed provide us with many examples of and insights into effective and efficient locomotion methods. Take jumping as an example, invertebrates such as locusts, fleas, crickets and leafhoppers use the catapult mechanism that has a lock-and-release system while jumping \cite{mo2020jumping}. Among the vertebrates, mountain goats jump with rapid pulling of forelegs and extension of hindlegs and spines, in which, they are able to store energy in tendons \cite{mcguigan2009dynamics}. The presence of elastic elements in musculoskeletal systems is the inspiration behind the series- and parallel- elastic actuators which have been utilised extensively in robotics. At the same time, linkage systems can be found in the knee joints of mammals, which alter the mechanical advantage at squad-down position for better leverage \cite{burgess2021review}. Many animals also possess the ability to manipulate aerodynamics. This usually takes the form of aerial abilities like flying and gliding, whereas some animals can also utilise the aerodynamic properties of their bodies without having full aerial capabilites. For example, Cheetahs' tails can generate more torque from aerodynamic drag than its inertia during turning \cite{patel2015conical}. It is also common for animals to be able to use several different locomotion methods and they can switch back and forth from one to another. There are even aquatic animals that constantly use jumping as their locomotion methods including Hydrophorus \cite{burrows2013jumping} and archerfish \cite{shih2017archer}. Therefore, bioinspiration and biomimetics can greatly aid the advancement of robot locomotion and especially in terms of providing insights into actuation by power modulation and aerodynamic utilisation. 

All aforementioned aspects of animal behaviour have been, in one way or another, incorporated into robotic locomotion thus giving inspirations to many past robot designs. Jumping and hopping robots in the past were usually designed to locomote through even terrain with little to none obstacles (examples in paper \cite{sheikh2013towards} and \cite{sayyad2007single}). This trend gradually shifted to finding mechanisms and methods that can enable the robot to jump higher (examples: TAUB \cite{zaitsev2015locust}, MultiMo-Bat \cite{woodward2011design}\cite{woodward2014multimo}, JumpRoACH \cite{jung2016integrated}, Frogbot \cite{burdick2003minimalist}, Glumper \cite{armour2007jumping}), and faster (examples: Jerboa \cite{brill2015tail}, MIT Cheetah \cite{park2015online}, StarlETH \cite{gehring2015optimization}, Minitaur \cite{kenneally2016design}). A new concept call jumping agility was introduced in the paper \cite{haldane2016robotic}. This value is the product of the jumping height and frequency and it describes the robot’s jumping ability comparing to the ballistic limit which is best jumping performance that a robot can achieve in natural earthly environment. This metric is relevant because some of the natural organisms (examples: Galago senegalensis \cite{aerts1998vertical}, Jumping Mouse \cite{harty2010role}, Bullfrog \cite{astley2013chasing}, Tree frog \cite{marsh1994jumping}) gets incredibly close to the ballistic limit even though being outclassed by the robot in terms of height or frequency alone. Few of the bionic robots managed to approach the ballistic limits when jumping compared to many natural organisms from whom they took inspirations. Usually, a single robot design is only able to succeed in either height or frequency, thus only one of the two metrics. Therefore, it is scientifically meaningful to design bionic robots whose jumping agility can catch up with their organic counterparts in terms of approaching the ballistic limit. Furthermore, there are robots that combine jumping and gliding abilities onto a single device \cite{zufferey2019consecutive}\cite{kovavc2011epfl}. They inevitably also took inspiration from animals like insects, birds and even mammals that possess both jumping ability and gliding. Nevertheless, both the bioinspired robots and their organic counterparts only use the gliding abilities as soft-landing solutions, which are functionally limiting. Thus, a promising strategy for improvement is for the jump-gliders to have longer, more stable, and more controllable glides.

Potential applications for jumping robot with high agility usually involves travelling through tough terrains while doing inspections or sample collection. This nature opens up many opportunities for additional functions on the robots. The logical next step for a monopod jumper is naturally a bipedal runner. Jumping could be used as an obstacle-bypassing mechanism, but it cannot provide enough speed for fast locomotion. Thus, bipedal running should be a necessary function for any robot design to locomote in a practical situation. Densely forested areas are good examples of places that are not suitable for humans to work in and ideally need to be accessed by robots. Therefore, any aerial ability can be extremely helpful for robots to bypass obstacles that are too tall for jumping. Gliding ability, for example, gives the robot a method for fast locomotion as well as an aerial view of the terrain. Potential height booster could be a rocket-like combustion mechanism or power enough propellers capable of lifting the robot to mid-air. 

To summarise, there are several research gaps in the current state of bioinspired jumping robots. Firstly, even though the benefits of utilising series elastic actuation and power modulation in robotic jumping is known to the research community, insufficient studies have been done to incorporate the mechanism into functional jumping robots. Secondly, we see various examples of mechanical advantage manipulating component being widely present in both natural organisms and legged robots; however, few are combined with SEAs to explore the full benefits of both mechanisms. Thirdly, most past robot design that utilises aerodynamics are confined to having aerial abilities; but they are other forms of aerodynamic utilisation that can improve robotic locomotion such as balancing. Finally, robots that have full earthbound and aerial locomotion functions are still rear; being able to glide give raise to new requirements to the robot design including lightweightness. The detailed research gaps are illustrated along side literature review in \chapref{chap_lit}.



%There have been many designs that have achieved either impressive jumping frequencies or heights, and most of the novel designs have been bioinspired by natural organisms. For example, invertebrates such as locusts, fleas, crickets and leafhoppers use the catapult mechanism that has a lock-and-release system while jumping \cite{mo2020jumping}. Among the vertebrates, cats and dogs jump with rapid pulling of forelegs and extension of hindlegs and spines; while blackbucks and mountain goats are able to store energy in tendons of the hindlegs and spines. At the same time, linkage systems can be found in the knee joints of mammals, which alter the mechanical advantage at squad-down position for better leverage \cite{burgess2021review}. There are even aquatic animals that constantly use jumping as their locomotion methods including Hydrophorus and archerfish. However, few of the bionic robots managed to approach the ballistic limits when jumping compared to many natural organisms from whom they took inspirations. Usually, a single robot design is only able to succeed in either height or frequency, thus only one of the two metrics. Furthermore, there are robots that combine jumping and gliding abilities onto a single device. They inevitably also took inspiration from animals like insects, birds and even mammals that possess both jumping ability and gliding. Nevertheless, both the bioinspired robots and their organic counterparts only use the gliding abilities as soft-landing solutions, which are functionally limiting. Thus, a promising strategy for improvement is for the jump-gliders to have longer, more stable, and more controllable glides. Some animals can also utilise the aerodynamic properties of their bodies without having full aerial capabilites. For example, Cheetahs' tails can generate more torque from aerodynamic drag than its inertia during turning \cite{patel2015conical}. Therefore, bioinspiration and biomimetics can greatly aid the advancement of robot locomotion and especially in terms of providing insights into actuation by power modulation and aerodynamic utilisation. 


\section{Research questions}

The main research question of this thesis can be described as \textbf{``How can the aerodynamic drag-inducing tails and mechanical advantage-varying knee joints of mammals inspire improvement in the agility and precision of jumping robots?"} It can be further broken down into smaller points: 

\begin{enumerate}
    \item What are the best straight-line mechanisms for robotic jumping and how to use series-elastic actuation (SEA) to help them approach the ballistic limit?
    \item How can continuously variable transmission (CVT) mitigate the drawbacks of series-elastic actuators, and how can combining them improve robotic jumping? 
    \item Can aerodynamics aid robotic balancing; and can traditional reaction wheels be replaced by aerodynamic drag-based mechanisms for purpose of mass reduction?
    \item If an aerodynamic drag-based balancing mechanism is feasible, how to design it to be as effective and efficient as possible and what are the constraints?
    \item What are the best control strategies for jumping robots that locomote using CVT enhanced series-elastic actuators and balancing mechanisms based on aerodynamic effects and what advantages do they have compared to conventional robots?
\end{enumerate}


\section{Aims and objectives}
The overall aims and objectives of this research can be summarized as follows: 
%\begin{itemize}
    %\item 1. Design and construct a jumping mechanism capable of high and continuous jumps with considerable frequencies enabled by a Continuously variable transmission-enhanced series elastic actuator. 
  %  \item 2. Design and construct a light-weight aerodynamic drag-enabled balancing mechanism capable of precise flight-phase orientation control.
    %\item 3. Incorporate the jumping and the balancing modules to create a integrated monopedal jumping robot; and develop a suitable control strategy.
   % \item 4. Evaluate the performance and quantify the improvements of such jumping robot compared to past studies. 
%\end{itemize}

\begin{enumerate}
    \item Design and construct a jumping mechanism capable of high and continuous jumps with considerable frequencies enabled by a SEA that is based on a suitable straight-line mechanism. 
    \item Incorporate the CVT into the jumping mechanism for the purpose of structural integrity and jump height improvement.   
    \item Design and construct a light-weight aerodynamic drag-enabled balancing mechanism capable of precise flight-phase orientation control.
    \item Conduct a thorough research on aerodynamic balancing and find the optimal drag-inducer (DI) design for the balancing mechanism.
    \item Incorporate the jumping and the balancing modules to create a integrated monopedal jumping robot; and develop a suitable control strategy.
\end{enumerate}



\section{Contributions}


As a result of this research, there are several contributions to the fields of legged, bioinspired, jumping robots. Some of these contributions are organised into papers, each one represents a definable stage of this research. I have four publications as the first author and several more as a co-author. This section will include a summary of all contributions made and work done for this PhD project, a list of published works as the first author with a brief introduction for each paper, and a list of papers that I contributed to as a co-author. 

\subsection{Summary of contributions and work done}

\begin{itemize}
    \item Extensive literature review is conducted on topics including bioinspired legged robots, jumpping robots, robotic balancing, and gliding robots; past studies regarding specific aspects such as straight-line mechanism, series-elastic actuation, continuously variable transmission, and aerodynamic drag are also reviewed. Through the literature review, crucial research gaps are found. They determine the objectives and focus of this research. 
    \item A robotic tail (AeroTail) device design capable of self-righting and balancing on a point-in-plane aerodynamically is developed. The contributions regarding this study consists of a simplistic mathematical model based on the inverted pendulum (IP) and control methods suitable for balancing the proposed AeroTail, as well as computational simulations and comparison studies to quantify and visualise the advantages of the AeroTail mechanism over traditional reaction-wheel-based (RW) tails under various circumstances. AeroTail is shown to achieve comparable balancing performance with a reduction of approximately 33.2\% in peak required balancing torque compared to a conventional reaction-wheel-based approach.
    \item A comprehensive study on optimising the DI design for the AeroTail to maximise its aerodynamic torque for robotic balancing is conducted. The contributions regarding this study include a specialised Computational Fluid Dynamics (CFD) simulations on a selection of shapes to explore how specific geometrical features influence aerodynamic drag, with detailed explanations of the observed effects, an optimisation framework to identify the optimal frame design that maximises aerodynamic drag while adhering to predefined constraints such as weight and dimensional limits, and finally, the implementation of practical experiments using the AeroTail test platform (equipped with the optimised DI designs) to assess their real-world performance on a rotational tail mechanism with variable wind speeds along its length. The last part involves a comparative analysis of the experimental findings against the CFD simulation results and the establishment of an analytical approach for quantifying the performance variation among different DI models. Experimental results demonstrate that the proposed optimised DI configuration can increase the achievable aerodynamic torque by up to approximately 20.8\% compared to baseline designs.
    \item A theoretical continuously variable transmission gearbox that can be incorporated into series-elastic actuators for robotic jumping is introduced. The contributions regarding this study consists of mathematical models for the hopping robot powertrain, including the CVT gearbox and a simplified model for an electric motor powered prismatic actuator suitable for iterative simulation, and a comparative simulation studies to examine the performances of both the C-SEA and regular SEA in various scenarios. The advantages and limitations of the mechanism are analysed.
    \item A jumping mechanism prototype is designed and constructed. This prototype transfers the theoretical C-SEA into a practical device. It utilises a rack-and-pinion based leg mechanism, a torsion spring enabled elastic element and a non-circular gear based CVT element. Experiments are conducted using the The jumper prototype; the results validate the findings from the simulations on the theoretical C-SEA and thus prove the advantage it has over conventional SEA.
    \item A simulation study of a monopedal jumping robot enabled by a C-SEA jumping mechanism and a AeroTail balancer is conducted. A SLIP based dynamics model is used to describe the jumping locomotion of such robot. An appropriate control strategy is implemented so the simulated robot can complete several locomotion tasks. The robot's performance is then compared to related works in the past. Results show that the integration of the AeroTail with the jumping robot can achieve stable jumping behaviour with an overall system energy consumption reduction of approximately 6.2\%, while preserving comparable jump height and stability performance. 
\end{itemize}

  %  \item We also introduced a novel gliding mechanism that can be coupled with jumping robots. The profile and the aspect ratio of the basic wing was designed so that the glider is able to have long and efficient flights. The aeroelastic hinged winglets provide a natural stabilizing factor to the glider as well as being able to released from spring tension to alleviates load on the wing when the protection provided by the elasticity is not sufficient.

\subsection{Publications as the first author}

% \textit{``AeroTail: a bio-inspired aerodynamic tail mechanism for robotic balancing"} and \textit{``Optimising aerodynamic drag for enhanced robotic balancing"} are both on the balancing mechanism. The former one details the theoretical concept of a aerodynamically balanced tail mechanism whereas the latter includes a study on its component known as drag-inducer and experimental validations of this concept. Paper \textit{Advancing Robotic Jumping with CVT Enhanced SEA} is the theoretical and simulation-wise study of the jumping mechanism. The work around prototyping the jumping mechanism and its experimental validation has not been arranged into the form of papers. Finally, the conceptual design of a glider capable of carrying jumping robots for integrated locomotion is introduced in \textit{Gliding mechanism with hinged aeroelastic wings} as a foundation to future works.

The publications as the first author are listed here. Some of the contributions mentioned in the previous section, namely, the mechanical design of the jumper prototype and the robot locomotion simulation study, are still in the process of being organised into paper. The publication of those papers will be a part of the future works.

\begin{enumerate}
    \item \textbf{Sun, Jingcheng, and Chengxu Zhou. ``Robust control for AeroTail and C-SEA enabled agile robotic jumping." (working title, paper in the writing process)} is the paper that we are currently working on regarding the control strategies of the integrated planar jumping robot. This work will include the optimal control methods for a jumping robot utilising a C-SEA leg mechanism and an AeroTail balancer. The groundworks for this paper have been laid in the form of simulation studies and the related contents can be found in \chapref{chap_robot}.
 
    \item \textbf{Sun, Jingcheng, and Chengxu Zhou. ``Advancing Robotic Jumping with CVT Enhanced SEA." In Annual Conference Towards Autonomous Robotic Systems, pp. 73-84. Cham: Springer Nature Switzerland, 2024.} introduced a theoretical continuously variable transmission gearbox that can be incorporated into series-elastic actuators for robotic jumping \cite{sun2024advancing}. The work in this paper serves as the theoretical introduction and the simulation study of the jumping mechanism. \chapref{chap_jump} corresponds to the content of this paper.
    
    \item \textbf{Sun, Jingcheng, and Chengxu Zhou. ``Optimising aerodynamic drag for enhanced robotic balancing" Engineering Applications of Computational Fluid Mechanics 18, no. 1 (2024): 2430659.} presents a comprehensive study on optimising the DI design for the AeroTail to maximise its aerodynamic torque for robotic balancing \cite{sun2024optimising}. The work in this paper finalises the design of the most crucial component, the DI, on an AeroTail. The previous theoretical findings regarding the AeroTail are also validated as a result. More details can be found in \chapref{chap_di}
    
    \item \textbf{Sun, Jingcheng, and Chengxu Zhou. ``AeroTail: a bio-inspired aerodynamic tail mechanism for robotic balancing" In 2023 28th International Conference on Automation and Computing (ICAC), pp. 1-6. IEEE, 2023.} presents the development of a robotic tail (AeroTail) device design capable of self-righting and balancing on a point-in-plane aerodynamically \cite{sun2023aerotail}. The work included in this paper details the theoretical concept of an aerodynamically balanced tail mechanism, which lays the foundation to further researches on aerodynamic balancing. \chapref{chap_balance} corresponds to the content of this paper.
    
    %\item \textbf{Sun, Jingcheng, Junfeng Yang, Robert Richardson, and Chengxu Zhou. ``Gliding mechanism with hinged aeroelastic wings" In UKRAS22 Conference “Robotics for Unconstrained Environments” Proceedings, pp. 80-81. EPSRC UK-Robotics and Autonomous Systems (UK-RAS) Network, 2022.} introduced a novel gliding mechanism that can be coupled with jumping robots \cite{sun2022gliding}. The work regarding the conceptual design of a glider capable of carrying jumping robots for integrated locomotion in included in this paper. More details can be found in \textbf{Appendix~\ref{app_glide}} .
\end{enumerate}


\begin{comment}
    Firstly, several versions of a continuous jumping robot are capable of power modulation and jumps of impressive heights. The different versions may use similar or various mechanisms to maximize certain metrics including height, frequency and jumping agility. Secondly, a gliding mechanism will be created to be integrated into the jumping robot; it can also be a separable glider that combines with the jumper to provide it with gliding ability. The gliding mechanism is expected to perform long, steady and controllable glides with novel foldable wing designs for maneuvering or compactness. Furthermore, a bipedal running version of the jumper will be designed to improve its compatibility in realistic scenarios. Finally, an evaluation system will be developed to examine how well the robots perform in experimental environments, simulated or realistic scenarios. Data will be collected in order to compare the results of this research with past examples of similar robots and to put together a thorough analysis of any advantages and limitations of the designs. 
\end{comment}

\subsection{Publications as a Co-author}

I have also contributed in the research and writing of several other works during my PhD project. Although these works do not necessarily fit into the narrative of this thesis, they are all within the realm of legged, bioinspired robotics. They are listed here. 

\begin{enumerate}
    \item ``Performance and usability evaluation scheme for mobile manipulator teleoperation" Wan, Yuhui, \textbf{Jingcheng Sun}, Christopher Peers, Joseph Humphreys, Dimitrios Kanoulas, and Chengxu Zhou. IEEE Transactions on Human-Machine Systems 53, no. 5 (2023): 844-854. \cite{wan2023performance}
    \item ``Teleoperating a legged manipulator through whole body control" \\ Humphreys, Joseph, Christopher Peers, Jun Li, Yuhui Wan, \textbf{Jingcheng Sun}, Robert Richardson, and Chengxu Zhou. In Annual Conference Towards Autonomous Robotic Systems, pp. 63-77. Cham: Springer International Publishing, 2022. \cite{humphreys2022teleoperating}
    \item ``Vision-based Gesture Tracking for Teleoperating Mobile Manipulators" Wang, Tianyu, Yuhui Wan, Christopher Peers, \textbf{Jingcheng Sun}, and Chengxu Zhou. In UKRAS22 Conference “Robotics for Unconstrained Environments” Proceedings, pp. 52-53. EPSRC UK-Robotics and Autonomous Systems (UK-RAS) Network, 2022. \cite{wang2022vision}
    \item ``Trigger-Assisted Ambidextrous Control Framework for Teleoperation of Two Legged Manipulators" Peers, Christopher, Joseph Humphreys, Yuhui Wan, Jun Li, \textbf{Jingcheng Sun}, Robert Richardson, and Chengxu Zhou. In Annual Conference Towards Autonomous Robotic Systems, pp. 50-62. Cham: Springer International Publishing, 2022. \cite{peers2022trigger}
\end{enumerate}

\begin{comment}
    \textbf{``Performance and usability evaluation scheme for mobile manipulator teleoperation" Wan, Yuhui, Jingcheng Sun, Christopher Peers, Joseph Humphreys, Dimitrios Kanoulas, and Chengxu Zhou. IEEE Transactions on Human-Machine Systems 53, no. 5 (2023): 844-854.} This paper presents a standardized human–robot teleoperation interface (HRTI) evaluation scheme for mobile manipulators. It comprises a set of robot motion tests,objective measures,subjective measures, and a prediction model to provide a comprehensive evaluation. Based on an extended Fitts’ law, the prediction model employs the time and mission difficulty index to forecast system performance in future missions \cite{wan2023performance}.

\textbf{``Teleoperating a legged manipulator through whole body control" \\ Humphreys, Joseph, Christopher Peers, Jun Li, Yuhui Wan, Jingcheng Sun, Robert Richardson, and Chengxu Zhou. In Annual Conference Towards Autonomous Robotic Systems, pp. 63-77. Cham: Springer International Publishing, 2022.} In this paper, we present a highly functional teleoperation system, that integrates a full-body inertia-based motion capture suit and three intuitive teleoperation strategies with a Whole-Body Control (WBC) framework, for quadrupedal legged manipulators. The overall system has been tested and validated through a physics-based simulation and a hardware test, demonstrating all functionality of the system, which in turn has been used to evaluate its effectiveness \cite{humphreys2022teleoperating}.

\textbf{``Vision-based Gesture Tracking for Teleoperating Mobile Manipulators" Wang, Tianyu, Yuhui Wan, Christopher Peers, Jingcheng Sun, and Chengxu Zhou. In UKRAS22 Conference “Robotics for Unconstrained Environments” Proceedings, pp. 52-53. EPSRC UK-Robotics and Autonomous Systems (UK-RAS) Network, 2022.} This work provides a vision-based method with a camera to reduce costs and make teleoperation accessible for consumers. Teleoperation strategies are composed of coordinates and hand gestures. We conduct a simulation study on the Husky robotic platform with a manipulator in PyBullet and demonstrate the control framework through various gestures \cite{wang2022vision}.

\textbf{``Trigger-Assisted Ambidextrous Control Framework for Teleoperation of Two Legged Manipulators" Peers, Christopher, Joseph Humphreys, Yuhui Wan, Jun Li, Jingcheng Sun, Robert Richardson, and Chengxu Zhou. In Annual Conference Towards Autonomous Robotic Systems, pp. 50-62. Cham: Springer International Publishing, 2022.} presents a motion-capture based control framework for the purpose of effectively teleoperating two legged manipulators without significant delays caused by the switching of controllers. The functionality and ease of use of the control framework is demonstrated through a real life experiment where the operator controls two quadrupedal manipulator robots to open a spray can. The experiment was successfully accomplished by the proposed teleoperation framework \cite{peers2022trigger}.
\end{comment}


\section{Scope of research}

Even though the incorporation of a gliding mechanism onto a fully functional jumping robot is a major motivation of this research, the work regarding the gliding function will only be lightly touched upon in this thesis as an introduction and foundation to future works. Instead, the main focus of the thesis is the development of a light-weight jumping robot that can potentially be integrated with a gliding mechanism when there is such demand. The idea of using a aerodynamic drag enabled balancing mechanism lays the foundation to a potential gliding-jumping robot because weight reduction and increasing the utilisation of aerodynamics are critical necessities for a conventionally earthbound robot to gain aerial capability. Therefore, the minimalistic nature of the results on the gliding function in no way undermines the importance of a balancing mechanism enabled by the utilisation of aerodynamics. 

%This research will face several foreseeable challenges when working towards the objectives. Firstly, the weight margin of the robot is very confined in order to reach superior heights using existing accessible motors. To solve this challenge, the materials used on the robot should be carefully considered; the jumping mechanism and transmission should also be carefully selected and designed to minimize material use and thus weight while functionality is barely sacrificed. Secondly, the design philosophy of the gliding system is not intuitively clear: whether a separable glider that can be attached to the jumper or a well-integrated jump-glider is the optimal option is still up to debate. The fact that many components and devices on the jumper can be re-purposed onto the glider without major changes to overall structure also adds further room for improvements. Finally, even though there has been progress in most aspects of the research, the design for a height booster to initiate the gliding process is yet to be settled. Some initial work has been put into using combustion mechanism to rocket the device, as well as simply lifting the robot with a quadcopter drone.  


\section{Thesis outline}

In the rest of this thesis, the following contents will be addressed. In \chapref{chap_lit} is the literature review, which will cover and analyse notable past examples of robots that either inspired or influenced this research. \chapref{chap_balance} is the summary of work done around the development of AeroTail, a robotic balancing mechanism that generates torque using aerodynamic drag. In \chapref{chap_di}, an optimisation study is conducted for the drag-inducing components on the AeroTail. \chapref{chap_jump} includes the research on a series-elastically actuated jumping mechanism that utilises continuously variable transmission. Following the previous chapters, \chapref{chap_robot} introduces a simulation study of the integrated jumping robot combining the jumping and balancing modules. Finally, \chapref{chap_conclusion} is the conclusion of this thesis, in which the plans and initial progress for future works are also included. Other supporting contents are included in \textbf{Appendix \ref{app_supp}}. Summary of notations and acronyms can be found after the conclusion chapter.
