This study deals with the problem of an aerial manipulator pushing a movable structure. Contrary to physical interaction with a static structure, suitable consideration on the interacting force during the motion of the structure is required to stably perform this movable structure interaction. To accomplish the task of pushing a structure while ensuring the stability of the aerial manipulator, we present a nonlinear DOB based robust control approach by regarding the interaction force as a disturbance to the system. Furthermore, to utilize the proposed controller for pushing a movable structure, we propose an algorithm to generate an end-effector position reference that enables safe operation in a realistic situation. We validate the proposed control framework with successful demonstrations on pushing two types of movable structure, a heavy rolling cart (42 [kg]), and a real-like hinged door.

This paper presents an acceleration control of a hexarotor unmanned aerial vehicle (UAV) in the earth-fixed frame with a disturbance observer (DOB). Unlike conventional cascade control structures where the outer-loop position controller generates the desired attitude command, the position controller in this paper generates the desired acceleration command in X, Y, Z axis of the earth-fixed frame. With acceleration control combined with DOB, the UAV could manage the lateral disturbance force that makes the trajectory tracking very challenging. This is a new concept compared with existing DOB-based UAV control approaches which aim to cancel the moment disturbance for precise attitude control. The small-gain theorem is used for stability analysis and Q-filter bandwidth design. Both simulation and actual experiment are shown to validate the performance of the proposed design.

In this article, we introduce a translational force control method with disturbance observer (DOB)-based force disturbance cancellation for precise 3-D acceleration control of a multi-rotor unmanned aerial vehicle (UAV). The acceleration control of the multi-rotor requires conversion of the desired acceleration signal to the desired roll, pitch, and total thrust. However, because the attitude dynamics and the thrust dynamics are different, simple kinematic signal conversion without consideration of those difference can cause serious performance degradation in acceleration tracking. Unlike most existing translational force control techniques that are based on such simple inversion, our new method allows controlling the acceleration of the multi-rotor more precisely by considering the dynamics of the multi-rotor during the kinematic inversion. By combining the DOB with the translational force system that includes the improved conversion technique, we achieve robustness with respect to the external force disturbances that hinder the accurate acceleration control. μ-analysis is performed to ensure the robust stability of the overall closed-loop system, considering the combined effect of various possible model uncertainties. Both simulation and experiment are conducted to validate the proposed technique, which confirms the satisfactory performance to track the desired acceleration of the multi-rotor.

This paper presents a practical swing-angle estimation method for slung load operations of the multi-rotor unmanned aerial vehicle (UAV), which is essential to maintain the safety during the operation. In order not to rely on extra sensors for monitoring the swing angle, the proposed method in this paper offers an autonomous swing-angle estimation using only an inertial measurement unit (IMU) and a single load cell attached to the slung load. The disturbance observer (DOB) derived external force estimation is performed to estimate the swing angle. The unique structure of the proposed DOB-based disturbance force estimation technique utilizes the IMU data only. Both simulation and actual experiment are performed to validate the feasibility of the proposed algorithm.

This paper introduces a fully-actuated multirotor flight system utilizing the T3 (Tilting Thruster Type)-Multirotor, a new type of multirotor platform that enables six controllable degrees-of-freedom flight with minimal structural heterogeneity compared to conventional multirotor designs. This new multirotor platform consists of upper and lower parts (or thruster and fuselage parts), with a unique kinematic structure and dedicated servomechanism that controls the relative attitude between the two parts. With the new mechanism, the fuselage of the T3-Multirotor can control the translational and rotational motions independently of each other, allowing 6-degrees of freedom motion that was not possible with conventional multirotors. A dedicated robust control algorithm is developed based on a thorough analysis of system dynamics to derive accurate 6-degrees of freedom motion of the platform. The flight control performance of the platform is validated through simulations and actual experiments. Several flight tasks are also performed to demonstrate the potential of the T3-Multirotor in overcoming the limitations of conventional multirotors.

In this paper, we introduce a new quadrotor fail-safe flight solution that can perform the same four controllable degrees-of-freedom flight as a standard multirotor even when a single thruster fails. The new solution employs a novel multirotor platform known as the T3-Multirotor and utilizes a distinctive strategy of actively controlling the center of gravity position to restore the controllable degrees of freedom. A dedicated control structure is introduced, along with a detailed analysis of the dynamic characteristics of the platform that change during emergency flights. Experimental results are provided to validate the feasibility of the proposed fail-safe flight strategy.