A force closure grasp is a term from dexterous manipulation that indicates a grasp that can resist any applied wrench, or force-torque. Force closure grasps are desirable because they can completely immobilize an object or impart an arbitrary wrench to an object. With fixed-base manipulators, determining the degree of force closure of a manipulation system is simplified to determining the degree of force closure of the gripper or end effector. In mobile manipulation, the manipulator base is not fixed to the ground so determining the set of wrenches that can be resisted is not strictly limited to the capabilities of the end effector. But due to the large differences in mass of the mobile base and end effector, it is generally safe to assume the degree of force closure is limited by the end effector and not by the ability of the mobile base to remain motionless. As aerial mobile manipulation has started to become an active area of research, the concept of force closure of the entire manipulation system needs to be considered. Conventional aerial platforms are not able to resist an arbitrary wrench so an end effector carried by such a vehicle will not be able to exhibit force closure. This is true because even current quad rotors lack both the number of degrees of freedom but also independence of the degrees of freedom due to the fact that the force vectors are all parallel. We have developed a hex-rotor system with six independent degrees of freedom providing force closure for a dexterous aerial vehicle for mobile manipulation tasks. The ability of the aerial mobile base to exert an arbitrary wrench coupled with a low degree of freedom manipulator will allow for an agile aerial mobile manipulator with true force closure.

In this paper, we present the development of Dexterous Hexrotor, a hexrotor UAV platform with canted thrusters, enabling dexterous interaction with structures. Aerial mobile manipulation is an emerging niche in the field of mobile manipulation. Although there has been a fair amount of study of free-flying satellites with graspers and yielded impressive results, it is hampered a lack of appropriate testbeds for aerial mobile manipulation. Typical helicopters or quadrotors cannot instantaneously resist or apply an arbitrary force in the plane perpendicular to the rotor axis. They lack of force closure (a term from the dexterous manipulation community), which makes them inadequate for complex mobile manipulation tasks. The Collaborative Mechatronics Lab is addressing this instrumentation gap with the development of Dexterous Hexrotor to eventually host a low-cost, lightweight Stewart-Gough platform that can be combined as a macro/micro mobile manipulation system. Based on the concept of force closure, the new type of 6 DoFs hexrotor UAV provides the unique capability of being able to resist any applied wrench, or generalized force-torque. In this paper, we describe how Dexterous Hexrotor provides this important capability. We also describe the flight test which Dexterous Hexrotor is exhibiting holonomic behavior.

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This paper presents a fully actuated UAV platform with a nonparallel design. Standard multirotor UAVs equipped with a number of parallel thrusters would result in underactuation. Fighting horizontal wind would require the robot to tilt its whole body toward the direction of the wind. We propose a hexrotor UAV with nonparallel thrusters which results in faster response to disturbances for precision position keeping. A case study is presented to show that hexrotor with a nonparallel design takes less time to resist wind gust than a standard design. We also give the results of a staged peg-in-hole task that measures the rising time of exerting forces using different actuation mechanisms.

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