Recent Ph.D. Theses
Efficient representation of outdoor environment in mobile robotic simultaneous localization and mapping problem based on the information complexity
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.
|Azadeh Zarif Loloei|
Workspace Optimization of Spatial Cable-Driven Redundant Parallel Manipulators
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|
H∞ Control of Input delay systems with Derivative Feedback
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|
|Mohammad Azam khosravi|
Modeling and Robust Control of Redundant Parallel Cable Robots Considering Longitudinal Flexibility in Cables
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|
Kinematic Modeling, Dynamic Analysis and Position Control of a Redundant Hydraulic Shoulder Manipulator with Proper Force Distribution
|2006||Parallel and Cable Robotics|
Design and Implementation of Position Controller for Flexible Joint Robot Subject to Actuator Saturation
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.