Robotic manipulators are highly nonlinear and coupled systems that are subject to different types of disturbances such as joint frictions, unknown payloads, varying contact points, and unmodeled dynamics. These disturbances, when unaccounted for, adversely affect the performance of the manipulator. Employing a disturbance observer is a common method to reject such disturbances. In addition to disturbance rejection, disturbance observers can be used in force control applications. Recently, research has been done regarding the design of nonlinear disturbance observers (NLDOs) for robotic manipulators. In spite of good results in terms of disturbance tracking, the previously designed nonlinear disturbance observers can merely be used for planar serial manipulators with revolute joints (Chen, W.H., Ballance, D.J., Gawthorp, P.J., O'Reilly, J., 2000. A nonlinear disturbance observer for robotic manipulators. IEEE Trans. Ind. Electron. 47, 932-938), (Nikoobin, A., Haghighi, R., 2009. Lyapunov-based nonlinear disturbance observer for serial n-link manipulators. J. Intell. Robot. Syst. 55,[135][136][137][138][139][140][141][142][143][144][145][146][147][148][149][150][151][152][153]. In this paper, a general systematic approach is proposed to solve the disturbance observer design problem for robotic manipulators without restrictions on the number of degrees-of-freedom (DOFs), the types of joints, or the manipulator configuration. Moreover, this design method does not need the exact dynamic model of the serial robotic manipulator. This method also * Corresponding author, Tel.: +1 (647) unifies the previously proposed linear and nonlinear disturbance observers in a general framework. Simulations are presented for a 4-DOF SCARA manipulator to show the effectiveness of the proposed disturbance observer design method. Experimental results using a PHANToM Omni haptic device further illustrate the effectiveness of the design method.

Abstract-This paper investigates the problem of maneuvering control for planar snake robots. The control objective is to make the center of mass of the snake robot converge to a desired path and traverse the path with a desired velocity. The proposed feedback control strategy enforces virtual constraints encoding a lateral undulatory gait, parametrized by states of dynamic compensators used to regulate the orientation and forward speed of the snake robot.Index Terms-Virtual holonomic constraints, biologically inspired robot, path following, snake robot, hierarchical control, reduction theorems, stability of closed sets.

Teleoperation systems are subject to different types of disturbances. Such disturbances, when unaccounted for, may cause poor performance and even instability of the teleoperation system. This paper presents a novel nonlinear bilateral control scheme using the concept of disturbance observer based control for nonlinear teleoperation systems. Lumping the effects of dynamic uncertainties and external disturbances into a single disturbance term enables us to design a disturbance observer in order to suppress these disturbances and alleviate their adverse effects on the teleoperation system. A disturbance observer based control law is proposed for nonlinear teleoperation systems which will guarantee global asymptotic force tracking and global exponential position and disturbance tracking when the bilateral teleoperation system is experiencing slow-varying disturbances. In the case of fastvarying disturbances, the tracking errors are shown to be globally uniformly ultimately bounded, with an ultimate bound that can be made as small as desired using the design parameters. Simulations are presented to show the effectiveness of the proposed approach.

Developing methods for autonomous landing of an unmanned aerial vehicle (UAV) on a mobile platform has been an active area of research over the past decade, as it offers an attractive solution for cases where rapid deployment and recovery of a fleet of UAVs, continuous flight tasks, extended operational ranges, and mobile recharging stations are desired. In this work, we present a new autonomous landing method that can be implemented on micro UAVs that require high-bandwidth feedback control loops for safe landing under various uncertainties and wind disturbances. We present our system architecture, including dynamic modeling of the UAV with a gimbaled camera, implementation of a Kalman filter for optimal localization of the mobile platform, and development of model predictive control (MPC), for guidance of UAVs. We demonstrate autonomous landing with an error of less than 37 cm from the center of a mobile platform traveling at a speed of up to 12 m/s under the condition of noisy measurements and wind disturbances.

This article investigates the problem of enforcing a virtual holonomic constraint (VHC) on a mechanical system with degree of underactuation one while simultaneously stabilizing a closed orbit on the constraint manifold. This problem, which to date is open, arises when designing controllers to induce complex repetitive motions in robots. In this paper, we propose a solution which relies on the parameterization of the VHC by the output of a double integrator. While the original control inputs are used to enforce the VHC, the control input of the double-integrator is designed to asymptotically stabilize the closed orbit and make the state of the double-integrator converge to zero. The proposed design is applied to the problem of making a PVTOL aircraft follow a circle on the vertical plane with a desired speed profile, while guaranteeing that the aircraft does not roll over for suitable initial conditions. Virtual holonomic constraints (VHCs) have been recognized to be key to solving complex motion control problems in robotics. There is an increasing body of evidence from bipedal robotics [12,13,34], snake robot locomotion [27], and repetitive motion planning [1,31] that VHCs constitute a new motion control paradigm, an alternative to the traditional reference tracking framework. The key difference with the standard motion control paradigm of robotics is that, in the VHC framework, the desired motion is parameterized by the states of the mechanical system, rather than by time.Geometrically, a VHC is a subset 1 of the configuration manifold of the mechanical system. Enforcing a VHC means stabilizing the subset of the state space of the mechanical system where the generalized coordinates of the mechanical system satisfy the VHC, while the generalized velocity is tangent to the VHC. This subset is called the constraint manifold.Grizzle and collaborators (see, e.g., [34]) have shown that the enforcement of certain VHCs on a biped robot leads, under certain conditions, to the orbital stabilization of a hybrid closed orbit corresponding to a repetitive walking gait. The orbit in question lies on the constraint manifold, and the mechanism stabilizing it is the dissipation of energy that occurs when a foot impacts the ground. In a mechanical system without impacts, this stabilization mechanism disappears, and the enforcement of the VHC alone is insufficient to achieve the ultimate objective of stabilizing a repetitive motion. Some researchers [30,32] have addressed this problem by using the VHC exclusively ⋆ This paper was not presented at any IFAC meeting. Corresponding author M. Maggiore. Tel. (Fax