In this paper, we present the design of a novel 3-RUS/RRR redundantly actuated parallel mechanism for ankle rehabilitation based on the principle from the conceptual design. The proposed mechanism can actualize the rotational movements of the ankle in three directions while at the same time the mechanism center of rotations can match the ankle axes of rotations compared with other multi-degree-of-freedom devices, owing to the structural characteristics of the special constraint limb and platform. A new actuator redundancy scheme is used, which not only still maintains all inherent advantages from actuator redundancy but also possesses the kinematic partially decouple feature that improves the flexibility of the robotic system. Kinematic performances, such as dexterity, singularity and stiffness, are analyzed based on the computed Jacobian. Then simulation is performed. All the results show that the redundant robot has no singularity, better dexterity and stiffness within the prescribed workspace in comparison with the corresponding 3-RUS/RRR nonredundant robot, and is suitable for rehabilitation exercise.

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In the airborne passive radar system, space-time adaptive processing (STAP) employs the reference signal for clutter covariance matrix estimation. However, the clutter caused by the multipath signals involved in the reference channel (MP clutter) can corrupt the estimated covariance matrix. When the target under detection and the MP clutter have the same Doppler frequencies and spatial frequencies, the STAP would mistake the target as clutter and target detection performance would degrade. To cope with this problem, a novel cascaded clutter suppression method is proposed. In the proposed algorithm, based on the sparsity of the MP clutter in the range-Doppler domain, the cost function with a sparse constraint is first introduced. Then, an l 1-based recursive least squares algorithm is used to solve the resulting optimization problem, and MP clutter can be eliminated by the derived weight vector. Finally, the residual clutter is suppressed by the existing STAP algorithm. A range of simulations demonstrates that the proposed algorithm has superior performance in the presence of a contaminated reference signal. INDEX TERMS Space-time adaptive processing, cascaded method, clutter suppression, airborne passive radar.

Link to this article: http://journals.cambridge.org/abstract_S0263574714000241How to cite this article: Congzhe Wang, Yuefa Fang, Sheng Guo and Changchun Zhou (2015). Design and kinematic analysis of redundantly actuated parallel mechanisms for ankle rehabilitation. Robotica, 33, pp 366-384 SUMMARYIn this paper, we present the design of two serial spherical mechanisms to substitute for a single spherical joint that is usually used to connect the platform with the base in three degrees of freedom parallel mechanisms. According to the principle derived from the conceptual design, through using the two serial spherical mechanisms as the constraint limb, several redundantly actuated parallel mechanisms are proposed for ankle rehabilitation. The proposed parallel mechanisms all can perform the rotational movements of the ankle in three directions while at the same time the mechanism center of rotations can match the ankle axes of rotations compared with other multi-degree-of-freedom devices, due to the structural characteristics of the special constraint limb and platform. Two special parallel mechanisms are selected to analyze their kinematical performances, such as workspace, dexterity, singularity, and stiffness, based on the computed Jacobian. The results show that the proposed scheme of actuator redundancy can guarantee that the redundantly actuated parallel mechanisms have no singularity, better dexterity, and stiffness within the prescribed workspace in comparison with the corresponding non-redundant parallel mechanisms. In addition, the proposed mechanisms possess certain reconfigurable capacity based on control strategies or rehabilitation modes to obtain sound performance for completing ankle rehabilitation exercise.

A systematic method is developed for the type synthesis of 4-DOF nonoverconstrained parallel mechanisms with three translations and one rotation inspired by the design of H-4. First, the motion requirements of primary platform and secondary platform of the 4-DOF nonoverconstrained parallel mechanisms are analyzed, and the conflict between the number of actuators and the constraint system for nonoverconstrained parallel mechanism is solved. Then, the research topic of type synthesis of 4-DOF nonoverconstrained parallel mechanisms is transformed into the type synthesis of the secondary platform with three translational DOF linked by two chains. On the basis of the screw and reciprocal theory, all possible secondary limbs with 3-DOF, 4-DOF, and 5-DOF are synthesized, respectively. Finally, the configurations and spatial assembly conditions of all possible secondary limbs are provided and some typical mechanisms are sketched as examples.