Dongkyoung Chwa

Antisway Tracking Control of Overhead Cranes With System Uncertainty and Actuator Nonlinearity Using an Adaptive Fuzzy Sliding-Mode Control

An adaptive fuzzy sliding-mode control (AFSMC) is presented for the robust antisway trajectory tracking of overhead cranes subject to both system uncertainty and actuator nonlinearity. First, a fuzzy sliding-mode control (FSMC) law is designed for the antisway trajectory tracking of the nominal plant. In association with a conventional trajectory tracking control law, this FSMC law guarantees asymptotic stability as well as improved transient response of the load sway dynamics while the trolley tracking error dynamics is rendered uniformly asymptotically stable. Second, a fuzzy uncertainty observer is designed to cope with system uncertainty as well as actuator nonlinearity present in an actual plant, and it is incorporated with the FSMC law for the development of the AFSMC law. In addition to stability analysis, the robust performance of the proposed AFSMC law is verified via numerical simulations and experiments.

Global Tracking Control of Underactuated Ships With Input and Velocity Constraints Using Dynamic Surface Control Method

This paper proposes a global tracking control method for underactuated ships with input and velocity constraints using the dynamic surface control (DSC) method, where the control structure is formed in a modular way that cascaded kinematic and dynamic linearizations can be achieved similarly as in the backstepping method. First, the first step linearization of the kinematics determines the pseudo (or auxiliary) surge velocity and yaw angle, which are used as the commands for the second-step linearization. Then, in the second-step linearization of dynamics, the actual torque inputs are designed to make the actual surge velocity and yaw angle follow these pseudo commands to achieve the position and yaw angle tracking. By employing the dynamic surface control method in the design of each kinematic and dynamic linearization law, we can obtain a control structure that is much simpler than the previous backstepping-based controllers such that it is beneficial from the practical application viewpoint. In addition, it is possible to track general reference trajectories, i.e., the reference yaw velocity need not be persistently exciting and there is no restriction on the initial yaw tracking error. In particular, global tracking control is achieved even in the presence of input and velocity constraints, unlike the DSC method which introduces the several filters in the backstepping design procedure to avoid the model differentiation and make it easier to be implemented and usually has semiglobal tracking performance. Finally, the stability analysis and numerical simulations are performed to confirm the effectiveness of the proposed scheme.

Swing-Up and Stabilization Control of Inverted-Pendulum Systems via Coupled Sliding-Mode Control Method

This paper presents a coupled sliding-mode control (SMC) of inverted-pendulum systems. An SMC law is designed to force a coupled sliding surface (which consists of sliding surfaces of both actuated and unactuated subsystems) to be reached in finite time, such that zero dynamics are generated in the form of a second-order damped and forced nonlinear differential equation. The stability analysis is provided to show that the resulting zero dynamics is guaranteed to be semiglobally asymptotically stable over the upper half-plane as well as over the whole plane except the horizon. This property is maintained even in the presence of the matched disturbance by virtue of the sliding-mode approach. Using the semiglobal nature of the stability of the zero dynamics, the aggressive swing-up (in one time, without swinging motion) and stabilization control can be achieved by a single coupled SMC law, without involving the switching (or hybrid) scheme in the previous works. The performance of the proposed method is demonstrated in both numerical simulations and experiments for the swing-up and stabilization control of inverted-pendulum systems such as cart-pendulum and Furuta-pendulum.

Tracking Control of Differential-Drive Wheeled Mobile Robots Using a Backstepping-Like Feedback Linearization

This paper proposes a tracking control method for differential-drive wheeled mobile robots with nonholonomic constraints by using a backstepping-like feedback linearization. Unlike previous backstepping controllers for wheeled mobile robots, a backstepping-like feedback control structure is proposed in the form of a cascaded kinematic and dynamic linearization to have a simpler and modular control structure. First, the pseudo commands for the forward linear velocity and the heading direction angle are designed based on kinematics. Then, the actual torque control inputs are designed to make the actual forward linear velocity and heading direction angle follow their corresponding pseudo commands. A stability analysis shows that the tracking errors of the posture (the position and heading direction angle) are globally ultimately bounded and its ultimate bound can be adjusted by the proper choice of control parameters. In addition, numerical simulations for various reference trajectories (e.g., a straight line, a circle, a sinusoidal curve, a spinning trajectory with no forward velocity and a nonzero rotational velocity, a cross-shaped trajectory changing the forward and backward directions, etc.) show the validity of the proposed scheme.

Nonlinear Tracking Control of 3-D Overhead Cranes Against the Initial Swing Angle and the Variation of Payload Weight

In this brief, we propose a nonlinear tracking control method of 3-D overhead crane systems which works well even in the presence of the initial swing angle and the variation of payload weight. Besides the practical importance of the overhead cranes, this study is also theoretically interesting because four variables (trolley and girder positions, two swing angles) should be controlled using two control inputs (trolley and girder forces). To control such an underactuated system as cranes, a simple proportional-derivative (PD) controller has been normally used. Unlike the conventional regulation control, the newly proposed nonlinear tracking control law further improves the performance and robustness, which is based on the feedback linearizing control by using the swing angular rate as well as the swing angle. The proposed nonlinear tracking control law eliminates the nonlinear characteristics of the system and achieves the satisfactory position control and swing suppression, even when the initial swing angle and the variation of payload weight exist. We present the stability analysis and simulation results to demonstrate the practical application of our scheme.

Fuzzy Adaptive Tracking Control of Wheeled Mobile Robots With State-Dependent Kinematic and Dynamic Disturbances

Unlike most works based on pure nonholonomic constraint, this paper proposes a fuzzy adaptive tracking control method for wheeled mobile robots, where unknown slippage occurs and violates the nonholononomic constraint in the form of state-dependent kinematic and dynamic disturbances. These disturbances degrade tracking performance significantly and, therefore, should be compensated. To this end, the kinematics with state-dependent disturbances are rigorously derived based on the general form of slippage in the mobile robots, and fuzzy adaptive observers together with parameter adaptation laws are designed to estimate the state-dependent disturbances in both kinematics and dynamics. Because of the modular structure of the proposed method, it can be easily combined with the previous controllers based on the model with the pure nonholonomic constraint, such that the combination of the fuzzy adaptive observers with the previously proposed backstepping-like feedback linearization controller can guarantee the trajectory tracking errors to be globally ultimately bounded, even when the nonholonomic constraint is violated, and their ultimate bounds can be adjusted appropriately for various types of trajectories in the presence of large initial tracking errors and disturbances. Both the stability analysis and simulation results are provided to validate the proposed controller.

Hierarchical Formation Control Based on a Vector Field Method for Wheeled Mobile Robots

This paper proposes a hierarchical formation control using a target tracking control law based on the vector field method such that a decentralized and flexible formation control can be achieved without additional motion planning. Previously, many researchers have dealt with the control laws for the rigid formation, where the line of sight toward the leader is controlled for the leader-follower formation control. However, a width change or a collision of the formation can occur since a limited motion of the rigid formation can occur when the formation control maintains the line of sight. Therefore, the formation of multiple mobile robots is required to be flexible, keeping the width and curvature of the formation. To this end, a formation control law based on a vector field method is proposed, and a hierarchical formation structure is introduced in such a way that it consists of a line formation and a column formation based on the leader-follower formation strategy. First, a subgroup, which consists of several robots, is generated using the line formation, and then, the overall formation structure is constructed from several subgroups using the column formation. Finally, we show the stability of the whole formation. The stability analysis and simulation results of the proposed hierarchical formation control using this vector field method are included to demonstrate the practical applicability of the proposed method.

Variable Structure Control of the Active and Reactive Powers for a DFIG in Wind Turbines

The original direct power control (DPC) is known to give a fast response under transient conditions. However, active power, reactive power, and current pulsations occur in steady-state operation. The variable structure control (VSC) method for a doubly-fed-induction-generator-based wind turbine system is presented, using the principles of an active and reactive power controller known as modified DPC and where VSC and space-vector modulation are combined to ensure high-performance operation. The VSC scheme is designed following the modified DPC philosophy, which provides robust and fast power controls without frame transformation and the current controller that is used in the conventional field-oriented control drive. Simulation and experimental results demonstrate that the proposed methods preserve the effectiveness and robustness during variations of active and reactive power, rotor speed, and converter dc-link voltage.

Adaptive Bidirectional Platoon Control Using a Coupled Sliding Mode Control Method

This paper proposes an adaptive bidirectional platoon-control method for an interconnected vehicular system using a coupled sliding mode control (CSMC) to improve the performance and stability of the bidirectional platoon control and to guarantee string stability. The previous work in the field of platoon control is based on two strategies, i.e., the leader-predecessor and bidirectional strategies. In the case of the leader-predecessor strategy, all vehicles should use the information of all the leading and preceding vehicles. On the other hand, the bidirectional strategy uses the information of its neighboring preceding and following vehicles. Due to the drawbacks of the bidirectional strategy, most previous work has preferred to employ the leader-predecessor strategy, which can guarantee stability and improved performance. The bidirectional strategy is, however, advantageous in that its implementation of the actual system becomes much more feasible than that of the leader-predecessor strategy. Thus, to employ the platoon-control law to an actual system, we propose the platoon-control law using a CSMC method for an interconnected vehicular system based on the bidirectional strategy such that the problems arising from communication devices in the previous work can be overcome. In particular, unlike the previous work using the bidirectional strategy, the proposed adaptive platoon-control law can lead to improved control performance of the whole system and can guarantee string stability. The stability analysis and simulation results of the proposed method in the presence of uncertainties and disturbances are included to demonstrate the practical application of the proposed algorithm.

Obstacle Avoidance Method for Wheeled Mobile Robots Using Interval Type-2 Fuzzy Neural Network

This paper proposes an obstacle avoidance method in the position stabilization of the wheeled mobile robots using interval type-2 fuzzy neural network (IT2FNN). Previously, we have proposed the unified strategies of obstacle avoidance and shooting method of the robot soccer system using type-1 fuzzy neural network (T1FNN). Even though the previous T1FNN method can achieve the required tasks, the performance of the previous T1FNN method is not satisfactory in the following sense. The previous T1FNN cannot reduce the influence of uncertainties effectively because it uses the crisp set as the membership values. In addition, it can result in the large oscillation behavior during the obstacle avoidance. Accordingly, we should design the IT2FNN method to improve the performance with smoother behavior as well as improved obstacle avoidance. The proposed IT2FNN method has the fuzzy neural network structure different from the T1FNN. Since the IT2FNN uses the fuzzy set instead of the crisp set as the membership values and it is robust against uncertainties, the performance of the robot behavior can be significantly improved especially in the presence of obstacles. Both simulation and experimental results using the actual wheeled mobile robot with the vision information are provided to show the validity and the advantages of the proposed method.