This article presents a robust predictor-based sliding-mode control (RPSMC) for multirotor unmanned aerial vehicles (UAVs) to ensure desired tracking control under time delays, which appear in practice by communications, complex computation, and actuator delays. Many UAV applications have difficulty in control and operation due to various types of time delays, resulting in repeated commands, severe control instability, and then mission failure. However, existing controllers have limitations in solving time delay problems. In this article, the RPSMC with the prediction of future disturbance and reference trajectory is developed for the multirotor UAV to minimize the effects of time delay, robustly deal with external disturbances, and further achieve desired tracking control. The performance of RPSMC for the multirotor UAV is verified under various time delays and disturbances in numerical simulations. The results show the robustness and fast control convergence compared with proportional-integral-differential and conventional predictor-based controllers. Experimental results with step response and tracking of circular trajectory demonstrate the feasibility and performance of the RPSMC for UAVs in the presence of various time delays and disturbances.

Dear Editor, This letter proposes a robust control strategy for the autonomous landing of a quadrotor on a moving target. Specifically, a force command that consists of a cascade dynamics estimator and an optimal guaranteed cost control law is exploited for the position-loop tracking. Then, an orientation constraint torque command is employed for the attitude-loop tracking such that the quadrotor refrains from flipping during the landing operation. Stability analysis indicates that the overall closed-loop system is asymptotically stable. Finally, flight experiments validate and access the theoretical results.

SUMMARY: In this paper, we investigate dynamic walking as a convergence to the system's own limit cycles, not to artificially generated trajectories, which is one of the lessons in the concept of passive dynamic walking. For flexible walking, gait transitions can be performed by moving from one limit cycle to another one, and thus, the flexibility depends on the range in which limit cycles exist. To design a bipedal walker based on this approach, we explore period-1 passive limit cycles formed by natural dynamics and analyze them. We use a biped model with knees and point feet to perform numerical simulations by changing the center of mass locations of the legs. As a result, we obtain mass distributions for the maximum flexibility, which can be attained from very limited location sets. We discuss the effect of parameter variations on passive dynamic walking and how to improve robot design by analyzing walking performance. Finally, we present a practical application to a real bipedal walker, designed to exhibit more flexible walking based on this study.

This paper presents to develop a robust control system of spherical motion platform (SMP) capable of controlling six degree-of-freedom motion, particularly unlimited rotational motion for better virtual reality. The SMP is the new design of motion manipulator/simulator driven by the frictional force between a set of spherical wheels and cockpit sphere. However, it has intrinsic difficulties in control due to numerous uncertainties, including slipping motion between spherical wheels and cockpit sphere. Inaccurate motion control may degrade human perception as well as tracking performance. In this paper, sliding mode control based on disturbance observer is developed for the SMP to achieve precise motion tracking performance and provide a more realistic environment regardless of kinematic and dynamic uncertainties. Numerical simulation and experimental results with proportional-integral-differential (PID) and the robust controllers are compared for step and sinusoidal inputs to demonstrate the feasibility of motion control and tracking performance in spite of uncertainties. Finally, the SMP is operated using two representative flight maneuvers, as a flight simulator. The results show the SMP with the control system can be utilized to provide motion cue for a human on board for virtual reality.

This paper presents a new control strategy for a spherical motion platform capable of three DOFs translational and three DOFs unlimited rotational motion undergoing time delay problems. The control is based on a state prediction and preview method to make a state correctly track a time-varying desired trajectory despite input time delay and disturbance. The spherical motion platform can suffer from a delay effect since it needs a wireless data link to control a cockpit sphere without any mechanical linkage. The delay decreases operation efficiency and further causes control instability. The proposed prediction-based preview control (PPC) offers that a state vector under input time delay is predicted using a disturbance predictor attenuating the disturbance effect. In addition, a predictor-based previewed reference with the state prediction is applied to the controller to achieve desired tracking control. In numerical simulations, it is shown that the proposed controller stabilizes the state to the desired trajectory under delay and disturbance, whereas most existing controllers only show delayed tracking. Finally, the proposed controller is applied to the motion platform, and the result shows excellent performance minimizing the effect of time delay and disturbance.

In this paper, a six degree-of-freedom (DOF) motion platform based on spherical wheels is presented to improve design and motion control from the previous. The motion platform developed here is capable three DOFs unlimited rotation and three DOFs translation which has large workspace of linear motion with better stability and high rigidity. In particular, the new design offers better motion control, uniformity, safety in motion range by additional actuators providing better motion capability for virtual reality. Kinematic analysis of the platform is investigated to provide a basis of motion controllability. In particular, the rotational motion can be independently controlled from the translation by active driving control. The design and motion analysis are demonstrated from both numerical simulation and experiment using a prototype in an open-loop control. Slipping motion and friction as a main driving torque are experimentally evaluated for non-slip condition. The comparison results prove the design feasibility and motion control capability.