Recent Ph.D. Theses
| Name | Title | Year | Research Group |
|---|---|---|---|
| Mohammad Motaharifar | Control Structure Design and Implementation for Eye Surgery Training Dual Master Haptic System Abstract Surgery training have always been one of the most important challenges for the surgeons. Owing to the higher accuracy required in eye surgery than in other surgeries, any mistake made by the trainee might lead to undesired complications for the patient. Recently, dual-user haptic systems consisting of two haptic consoles, one for the trainer and one for the trainee, has been developed as an assistive tool to facilitate surgery training. In these systems, the trainer and the trainee collaboratively perform surgical tasks through their haptic consoles. In the proposed surgery training haptic system, the environment is directly manipulated by one surgeon and the other surgeon is indirectly involved in the procedure through the haptic system. This makes a special training framework which is applicable to not only virtual environment but also real environments such as physical phantoms, cadavers and patients with special focus on the vitrectomy surgery. Initially, the surgery is directly performed by the trainer and the position of the trainer is transformed to the trainee’s hands through the haptic system. Utilizing this step, the trainee can increase his/her expertise in different tasks by learning the true movements. After obtaining the required level of surgical skills, the trainee can perform the surgery directly on the environment with trainer’s supervision. To avoid undesired complications, the trainer is able to interfere into the procedure in the case of sudden mistakes happens by the trainee. Due to the interaction between the users in the dual user haptic systems, the decision of each user is affected by the other user. Hence, new application-based control structures are required to be further investigated. The majority of the previously proposed control methodologies for this system have not simultaneously considered special requirements of surgery training and stability analysis of the nonlinear closed-loop system which is the objective of this thesis. In order to describe the objectives and facilitate controller design for dual user haptic systems, this thesis introduces two training approaches including Expert Continual Action (ECA) and Expert Supervision & Intervention (ES&I). The ECA training approach is based on the trainer to perform all details of surgical operation, while the trainee freely experiences the task and receives the haptic guidance signals from the trainer. The control schemes based on ECA are appropriate for the most primary level of training in which the trainee does not have sufficient experience to perform the operation. On the other hand, the ES&I training approach is based on giving the trainee the chance to conduct the surgical operations and be supervised and corrected as needed, while the trainer is provided with the facilities to interfere in the operation and correct probable mistakes made by the trainee. In this approach, the trainer is not required to be involved at every stage of surgical procedures. Thus, the trainer is just given a supervisory role to interfere with the procedure only when a mistake is performed by the trainee. Notably, a unique feature of the proposed training structure is its application to remote training with real environment. In this case, one surgeon holds the surgical instrument and directly performs the surgical procedure, whereas the other surgeon is indirectly involved in the surgical operation through the haptic system. This requires a specially designed haptic system for each surgical task to provides the necessary movements required for that surgery. This issue is specially studied for the vitrectomy surgery and the designed haptic system is presented. In brief, the main contribution of this thesis is to introduce the above training approaches and develop some control schemes based on each training approach. Each control scheme is developed based on its respective training approach by utilizing the necessary mathematical tools of control theory. The stability of the closed-loop system for each control scheme is studied using input-to-state stability (ISS) analysis. Besides, some simulation and experimental results are presented to support the proposed methodologies. Finally, the specially designed haptic system for facilitating vitrectomy surgery training is presented. | 2019 | Surgical Robotics |
| Seyed Ahmad Khalilpour | Robust Control and Stability Evaluation of a Suspended Cable Robot considering the cable Dynamics Abstract Known for their lower costs and numerous applications, cable robots are attractive research fields in the robotic community. However, considering the fact that they require an accurate installation procedure and calibration routine, they have not yet found their true place in real-world applications. This thesis aims to propose a new control strategy that requires no meticulous calibration and installation procedures and can handle the uncertainties induced as a result of that. It should be noted that cable-driven robots generally cover a large-scale workspace. As such, the cable dynamics can not be ignored and phenomena such as cable flexibility may cause undesirable vibration. Therefore, in large cable-driven robots, the challenge of kinematic uncertainty is not the only problem to be solved and in addition to it, the cable dynamics should also be considered during the design procedure of the controller. The problem becomes more complicated when we consider the fact that in a deployable robot, accurate and expensive measurement tools are not generally available. In the field of robotics, most of the common control systems are based on a cascade structure in which the inner controller is assumed to be a fast high-bandwidth controller, and the outer controller (a user-specified control loop) is responsible for the robot’s main objective: trajectory tracking. The effectiveness of such an approach depends on the assumption that the inner loop controller is fast enough, and thus its dynamics may be ignored. In practice, such an assumption generally does not hold. Therefore, In chapter 2 of this thesis, we analyze the whole structure of the cascade controller and investigate the effect of selected structures and coefficients for the inner loop controller. In addition to this, an inner loop controller is designed to mitigate the adverse effects of dynamic uncertainties (of the actuators and power transmission systems, i.e. pulleys) on force tracking. These effects are more notable when the cables are directly driven by the motors. Chapter 3 extends the study presented in the previous chapter by deriving a model for a spatial cable-driven robot with flexible cables and employing the singularity perturbation theory for analyzing it. In this chapter, assuming a flexible dynamic model for cables, we propose a robust cascade control method and investigate the effects of inner-loop force controllers on the robot’s performance. In chapter 4, the cable model is extended and the effects of both cable mass and flexibility are investigated simultaneously. In this chapter, we employ the passivity theorem to propose a new control law which exploits an information fusion method. Finally, in order to illustrate the performance of the proposed controller, we present the results of an experiment on a deployable suspended cable-driven robot, which shows the effectiveness of the proposed controller in the presence of the uncertainties. | 2019 | Parallel and Cable Robotics |
| Alireza Norouzzadeh Ravari | Efficient representation of outdoor environment in mobile robotic simultaneous localization and mapping problem based on the information complexity Abstract Several algorithms are proposed for solving the Simultaneous Localization And Mapping (SLAM) problem for mobile robots. An efficient representation is required for large scale environment exploration and long term navigation, which is the main concern of this thesis. For this purpose, observations of sensors such as stereo camera or Microsoft Kinect are represented as a linear combination of atoms of a parametric dictionary by sparse modeling technique. The parametric dictionary is composed of Gaussian functions. In order to perform loop closure detection, the Normalized Compression Distance (NCD) is employed from information theory. The performance of this technique is analyzed in some indoor and outdoor environments. In addition, it has been proved that the developed environment representation is transformation invariant in the sense of Kolmogorov complexity. Furthermore, another environment is developed based on the Non Uniform Rational B-Spline (NURBS) for more accurate environment representation, simpler obstacle detection and smooth path planning. The NURBS-based environment representation is equipped with all of the sparse model benefits such as lower dimensionality, representation of information complexity, transformation invariance, parametric representation and uniqueness. Also, in contrast to the conventional environment representation methods, discrete sensor observation can be expressed in a continuous parametric space. This makes the obstacle detection simple and parametric representation of robot's path possible. The applicability of the proposed method is shown by several experiments on indoor and outdoor data-sets. | 2016 | Autonomous Robotics |
| Azadeh Zarif Loloei | Workspace Optimization of Spatial Cable-Driven Redundant Parallel Manipulators Abstract Since late 1980s, the study of cable driven parallel robots has received increasing attention. Using cables instead of rigid links in parallel robots makes them a suitable choice to remedy some of the traditional shortcomings of conventional robots. Replacing rigid links by the cables, however, introduces new challenges in the study of cable driven robots, of which design and workspace analysis are the most critical ones. One of the most important challenges propounded in cable mechanisms area is their workspace analysis. Researches ever done in this regard suffer from some deficiencies such as too much calculation in the numerical methods and complexity of the analytic methods and their weakness in analyzing the workspace of spatial robots with more than one degree of redundancy. Fundamental wrench introduced in this study is a new horizon in workspace analysis. Fundamental wrenches not only have presented an interesting insight in workspace analysis area, they have been also effective in calculating the optimal cable force in the presence of the external forces. One of the assessments done on workspace is that of robot performance which is practicable through kinematic criteria. In this thesis, with due attention to the absence of a comprehensive criterion in cable mechanisms area and considering the attributes of the fundamental wrench, force sensitivity criterion has been proposed. Aside from expressing the ratio of cable force variations to the external forces, this index also analyses the robot singularity and controllability in the workspace. Controllability is one of the essential insights in cable robots workspace. The performance of Nasir spatial cable driven redundant parallel schemes has been further studied considering the proposed method in workspace analysis and force sensitivity criterion. Mani conceptual design has been proposed with the purpose of optimizing the previous designs. Then with the usage of multipurpose optimizing methods such as genetic algorithm, Pareto-optimal solution has been obtained in a way that the end functions have been optimized simultaneously. The results obtained from simulations confirm the practicability of the proposed methods in workspace analysis, cable kinematic criterion, conceptual designing and its optimization. | 2014 | Parallel and Cable Robotics |
| Mohammad A.Khosravi | Modeling and Robust Control of Redundant Parallel Cable Robots Considering Longitudinal Flexibility in Cables Abstract Since late 1980s, the study of cable driven parallel robots has received increasing attention. Using cables instead of rigid links in parallel robots, makes them a suitable choice to remedy some of the traditional shortcomings of conventional robots. Replacing rigid links by the cables, however, introduces new challenges in the study of cable driven robots, of which control and the dynamic behavior of the cables are the most critical ones. Cables are usually elastic elements and may encounter elongation and vibration. Therefore, elasticity in cables may cause position and orientation errors for the moving platform. This problem is a critical concern in applications where high bandwidth or high stiffness is a stringent requirement. It should be noted that modeling the dynamic effects of elastic cables is an extremely comprehensive task. Furthermore, it is also important to note that the obtained models must not only be sufficiently accurate, they must be usable for controller synthesis, as well. Therefore, in practice it is proposed to include only the dominant effects in the dynamic analysis. According to this approach, In this thesis axial spring is used to model dominant dynamics of the cable and a new and more precise model of the cable-driven robot with elastic cables is derived and being used in the control design and stability analysis. Using obtained model, three control algorithms are proposed to cope with vibrations due to inevitable elasticity of the cables. The first algorithm is formed in the cable length space and the stability of the closed-loop system is analysed through Lyapunov’s second method. Next, dynamics of the cable robot with elastic cables is rewritten to the standard form of singular perturbation theory. Using results of this theory, second and third control algorithms are proposed in the task space for this class of robots. Then, by separation of slow and fast variables stability analysis of the closed-loop system with proposed algorithms are performed. The third control algorithm uses popular PID control in its structure. Although it has simple structure which is a benefit in the implementation process, it can robustly stabilize the system. Simulation and experimental results verify the effectiveness of the proposed control algorithms and show that these algorithms can stabilize the system and efficiently cope with vibrations due to elasticity in the cables. | 2013 | Parallel and Cable Robotics |
| Ala Shariati Dehaghan | H∞ Control of Input delay systems with Derivative Feedback Abstract This thesis presents H∞ controller design for input delay systems with derivative feedback in presence of both constant and time-varying delay. By this control law, the resulting closed-loop system turns into a specific time-delay system of neutral type with both delayed-term coefficients depending on the control law parameters. proportional-derivative state feedback and output derivative feedback are two examples of this control law. In this thesis, these two examples are fully investigated. In some practical problems such as active vibration suppression systems the state-derivative signals are easier to access than the state variables. To this aim, an H ∞ -based state-derivative feedback control problem for input-delayed systems has been considered in this thesis, as an special case of proportional-derivative state feedback. Moreover, we have addressed an H ∞ PD controller for input-delayed systems, which leads to the aforementioned special closed-loop system of neutral type. It can be easily shown that designing a PD controller for an augmented plant model with an integrator, is equivalent to the design of a PI controller for the original plant model. Considering this fact and widespread application of PI controller in industrial plants, the significance of the developed theory will be better appreciated. Lyapunov-Krasovskii functional has been used for the design of both H ∞ proportional-derivative state feedback and H ∞ PD/PI controller for input delay systems. Consequently, new delay-dependent sufficient conditions for the existence of both H ∞ proportional-derivative state feedback and H ∞ state-derivative feedback in presence of uncertain delay are derived in terms of some matrix inequalities. Furthermore, descriptor model transformation is used to derive delay-dependent sufficient conditions for the existence of H ∞ PD/PI controller in terms of some matrix inequalities as well. The resulting H ∞ controllers stabilize the closed-loop neutral system and assure that the H ∞ -norm to be less than a prescribed level. Some application examples are presented to illustrate the effectiveness of the proposed methods. | 2012 | Dynamical Systems Analysis and Control |
| Hooman Sadjadian | Kinematic Modeling, Dynamic Analysis and Position Control of a Redundant Hydraulic Shoulder Manipulator with Proper Force Distribution Abstract | 2006 | Parallel and Cable Robotics |
| Sadjaad Ozgoli | Design and Implementation of Position Controller for Flexible Joint Robot Subject to Actuator Saturation Abstract The desire for higher performance from the structure and mechanical specifications of robot manipulators has spurred designers to come up with flexible joint robots (FJR). Several new applications such as space manipulators and articulated hands necessitate using FJRs. This necessity has emerged new control strategies required, since the traditional controllers implemented on FJRs have failed in performance. Since 1980's many attempts have been made to encounter this problem and now, several methods have been proposed including various linear, nonlinear, robust, adaptive and intelligent controllers. Among these, only a few researchers have considered practical limitations such as actuator saturation in the controller synthesis, although it is a real practical drawback to achieve good performance. On the other hand actuator saturation has been considered by the control community from early achievements of control engineering. During 50's and 60's at the beginning era of optimal control, researchers have been working on saturation, introducing bang-bang control methods. Over the last decade the control research community has shown a new interest in the study of the effects of saturation on the performance of systems. In fact it can be said that in the past, researchers were encountered actuator saturation as a drawback and they had developed methods to avoid it, while now researchers develop methods to achieve a desirable performance in the presence of actuator saturation encountered as a limitation. A common classical remedy for systems with bounded control is to reduce the bandwidth of the control system such that saturation seldom occurs. This is a trivial weak solution, since even for small reference commands and disturbances the possible performance of the system is significantly degraded. This idea (reduction in bandwidth by reduction in the closed loop gain) is practical and "easy", hence it motivates some researchers to propose an "adaptive" reduction in bandwidth consistent with the actuation levels. The "adaptation" process is done under supervision of a supervisory loop, and as proposed in a paper, it can be accomplished through complex computations, which seem not to be practically implementable. In order to come up with an online implementable controller in presence of saturation, we have proposed a fuzzy logic supervisory control in this work. In this topology, the fuzzy logic is set to be "out of the main loop", at a supervisory level, at the aim of preserving the essential properties of the main controller. This idea is then modified to use with composite controller for FJRs. It is observed in various simulations that by including this supervisory loop to the controller structure, the steady state performance of the system is preserved, and moreover, the stability of the overall system which may be affected by addition of a saturation block, will retain. In other words the supervisor can remove instability due to saturation. The stability analysis of the overall system, however, is essential for the closed loop structure for susceptible applications of the FJRs such as space robots. Robust stability proof for the "composite + supervisor" method has been also given in detail in this thesis. A second approach has been also proposed in this thesis. The robust methods proposed in this approach are simpler than the previous method (composite + supervisor) in structure, and moreover, they need only the feedback of the link position. In this approach using a frequency weighted penalty function of the control action is recommended in the mixed sensitivity minimization. Furthermore, in order to decrease the amplitude of the control action while keeping the desired bandwidth, a mixed minimization method has been also proposed. Numerical controller design has obtained using LMI method. Simulations verify the superior performance of the mixed method compared to that of the composite PID+PD controller and the mixed sensitivity controller. In this thesis, practical implementations of the proposed methods have also accomplished to verify the theoretical results. These implementations illustrate the effectiveness of the proposed methods in practice. | 2005 | Flexible Robotics |