In this article, an adaptive sliding-mode disturbance observer (ASMDO)-based finite-time control scheme with prescribed performance is proposed for an unmanned aerial manipulator (UAM) under uncertainties and external disturbances. First, to take into account the dynamic characteristics of the UAM, a dynamic model of the UAM with state-dependent uncertainties and external disturbances is introduced. Then, note that a priori bounded uncertainty may impose a priori constraint on the system state before obtaining closed-loop stability. To remove this assumption, an ASMDO with a nested adaptive structure is introduced to effectively estimate and compensate the external disturbances and state-dependent uncertainties in finite time without the information of the upper bound of the uncertainties and disturbances and their derivatives. Furthermore, based on the proposed ASMDO, the finite-time control scheme with the prescribed performance is presented to ensure finite-time convergence and implement the specified transient and steady-state performance. The Lyapunov tools are utilized to analyze the stability of the proposed controller. Finally, the correctness and performance of the proposed controller are illustrated through numerical simulation comparisons and outdoor experimental comparisons.

This article presents the problem of autonomous control of an unmanned aerial manipulator (UAM) developed for operation with unknown disturbances, wherein the disturbances from the coupling effect between the UAM and the external environment need to be considered. Regarding the coupling force as a disturbance to the entire UAM system, an adaptive prescribed performance control (APPC) scheme utilizing the knowledge of prescribed performance is proposed to guarantee the transient and steady-state performance responses. Also, an adaptive law is designed to estimate the upper boundary parameters of the UAM system uncertainties and disturbances, wherein the restrictive constant boundary assumptions and the prior information of the upper bound are not required in the controller design. Furthermore, to enable safe manipulation in a realistic situation, an end-effector trajectory generation method is presented satisfying the joint angle limitation. For the validation of the proposed method, the simulation results of numerical simulation comparisons are shown. Moreover, experimental scenarios including stable flight and simulated co-work with humans in complex environments are designed to verify the proposed method. Note to Practitioners—This article is motivated by the problem of aerial manipulation under unknown disturbances, which may be caused by the wide movement of the manipulator and the sudden loading or unloading of an object. Existing approaches for aerial manipulation often require the assumption of a constant or slowly varying external disturbance. However, a priori bounded disturbance might impose a priori bound on the system state before obtaining closed-loop stability. In this article, the proposed controller with an adaptive law is designed to estimate the upper boundary parameters of the overall disturbances and ensure the predefined performance, so that the prior information of the upper bound of disturbances is not required. The performance of the proposed control strategy is demonstrated via numerical simulation comparisons and experiments, including stale flight and simulated co-work with humans in a complex environment.

In this article, an adaptive force tracking impedance control strategy is investigated for an aerial manipulator in physical interaction with uncertain contact environments. Based on the modified target impedance model, an adaptive impedance control method is proposed to accomplish aerial interaction in uncertain environments while maintaining a stable contact force, wherein the environment parameters of location and stiffness are estimated online to generate a reference position trajectory. Then, in order to ensure the tracking performance of the aerial manipulator, a robust pose tracking controller is designed, including a barrier function-based position controller and an adaptive attitude controller. Both proposed position and attitude controllers can ensure finite-time convergence of the state variable without the priori boundary information of disturbances. In particular, the position state variable can converge to a predefined neighborhood of zero from any initial state, and the control gain is not overestimated. The stability of the proposed strategy is analyzed via Lyapunov tools. Simulations and real-world experiments are conducted to illustrate the feasibility and performance of the proposed control strategy. Note to Practitioners —The motivation of this article is to investigate an adaptive force tracking impedance control strategy for aerial physical interaction with uncertain contact environments. In the existing impedance control schemes for aerial manipulators, the environment parameter of location or stiffness is often required to be utilized in controller design. However, in practical cases, the environmental parameters are not known precisely. Thus, this article presents an adaptive impedance method to automatically generate the reference position trajectory and achieve a stable contact force. Additionally, the tracking performance of the aerial manipulator is inevitably subject to uncertainties and disturbances. To ensure tracking convergence, traditional robust controllers generally involve high control gains than the known upper bounds of the disturbances. The main disadvantage of those controllers is that the control gain is often overestimated when the disturbance decreases. To address this issue, a barrier function-based position controller is proposed for the aerial manipulator, where the priori boundary information of disturbances is not needed and the control gain is adaptively adjusted according to the amplitude of disturbances. The stability and convergence of the proposed strategy are analyzed mathematically, and the experiments using an aerial manipulator provide promising results.

This article investigates the active physical interaction problem of an aerial manipulator in terms of capability, reliability, and costs. Then, an active physical interaction control architecture is presented for the aerial manipulator to achieve both stable motion and interaction behaviors with external wrench estimation. Specifically, an external wrench estimator in absence of the acceleration and wrench measurements is designed to regulate the interaction point of the aerial manipulator with a minimal sensor condition. Next, a force tracking impedance control strategy with variable stiffness is presented to guarantee the contact stability and force tracking of the aerial manipulator with uncertain contact targets. Further, utilizing the knowledge of prescribed performance and terminal sliding mode surface, a pose controller is proposed to implement the dynamic response speed and accuracy control of the aerial manipulator, which provides a prerequisite for the realization of reliable physical interaction tasks. The stability of the proposed control architecture is analyzed through Lyapunov tools. Moreover, the feasibility and performance of the proposed control architecture are validated via simulations and real-world contact experiments.

This article investigates a robust variable impedance control methodology for aerial manipulators to realize compliant and safe interaction tasks. Considering that the stability characteristics are generally overlooked in existing variable impedance controllers of the aerial manipulator, state-independent stability conditions are applied for time-varying impedance profiles to ensure the exponential stability of the desired variable impedance dynamics (DVID) as well as the boundedness of the state variables in the DVID. A command trajectory variable is introduced for converting the impedance control issue to a particular tracking issue, and then, a robust variable impedance controller based on the wrench estimator is designed to guarantee the exponential convergence of the translational states and impedance error of the aerial manipulator. The designed impedance controller is structurally simple and results in low implementation costs. Next, an improved attitude control approach with the command filter is developed for global flight attitude stability without any singularities or ambiguities, where the filter is introduced to avoid computing the derivative signals of the generalized force input. Finally, the effectiveness of the proposed control method is illustrated via numerical simulations and interaction experiments with different targets in real scenarios.

This article aims to investigate the research achievements related to aerial robots with physical interaction. Various morphologies of aerial physical interaction (APhI) robot prototypes with fixed wing, flapping wing, single main rotor, conventional underactuated multirotor, fully actuated multirotor, even deformed multirotor, and multiple platforms are reviewed for different APhI tasks associated with momentary, loose, and strong interaction coupling. This review also covers APhI robot rigid dynamics and robot-environment coupled interaction dynamics modeling methods, interaction wrench measurement/estimation, decoupled and coupled control, active aerial interaction control, and task-constrained planning approaches. Finally, future development directions and prospects are initially anticipated for aerial robots with physical interaction. Note to Practitioners —Aerial physical interaction (APhI) has been a hot topic in the field of aerial robots in recent years, which is a reflection of the advanced capabilities of aerial robots. However, APhI robots face challenges such as difficulty in flight stability and weak adaptability to dynamic environments while exerting active influence on environments. Under this background, this review aims to offer a reference for researchers and practitioners engaged in the related field from the aspects of system design, modeling, control, and task-constrained planning, which hopes to help them apply APhI robots to polar scientific expeditions, complex environment sampling, infrastructure inspection and maintenance, and other application areas. Further, this review also highlights the design idea of rigid-soft integrated APhI robots from the perspective of design-mechanism-performance to enhance interaction stability and safety.