OPTIMAL GRASPING STRATEGY OF SPACE TUMBLING TARGET BASED ON MANIPULABILITY
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摘要: 由于目标的翻滚运动, 空间双臂机器人对动态目标的抓捕相比于静态目标更具有挑战性. 对抓捕策略进行优化可以提高空间双臂机器人对翻滚目标的操作能力以保证任务的成功. 本文提出了一种基于能力评估的抓捕策略优选方法. 空间双臂机器人捕获目标时, 双臂末端执行器与目标同时接触形成闭链系统, 闭链约束的引入使操作能力的评估更加复杂. 在对双臂空间机器人协调操作翻滚目标的运动学与动力学分析基础上, 建立了考虑闭链约束的协调工作空间, 并分析了基于任务兼容度的消旋能力评估指标. 建立的协调工作空间同时包含位置和姿态信息, 可以用于灵巧度的计算. 接着, 基于协调工作空间的全局灵巧度指标确定机械臂末端执行器对目标的最优抓捕点, 以及考虑相机视角约束和末端执行器对目标速度跟踪约束下的力任务兼容度指标确定空间双臂机器人捕获翻滚目标时的最优抓捕构型. 利用能力评估确定抓捕策略可以充分利用双臂的协调性以增加对动态目标的操作能力, 通过仿真验证了所提抓捕策略的可行性和有效性.Abstract: Due to the tumbling motion of the target, the dual-arm space robot to grasp a dynamic target is more challenging compared to a static target. Besides, optimization of the grasping strategy can improve the manipulability of the space robot to operate on the tumbling targets to ensure the success of the grasping mission. In this paper, a method for grasping strategy optimization is proposed based on the manipulability evaluation. When the dual-arm space robot cooperatively grasping a target, the dual-arm end-effectors contact the target simultaneously and a closed kinematics chain is formed. As a result, the formation of the closed-chain constraint complicates the evaluation of the manipulability of the space robot. First, the kinematics and dynamics of a dual-arm space robot manipulating a target are analyzed in this paper. Following this, a cooperative workspace considering the closed-chain constraint is established, and a task compatibility based detumbling manipulability metric is analyzed. The established cooperative workspace contains both the position and the attitude information of the manipulated target in the task space, which can be used for the calculation of dexterity. Then, the optimal grasping points of the end-effectors to grasping a target are determined based on the global dexterity metric, as well as the optimal grasping configuration of the space robot to grasp a tumbling target is found based on the force task compatibility metric considering the field-of-view constraint of the camera and the velocity tracking constraint of the end-effectors to the tumbling motion of the target. Using the manipulability metrics to determine the grasping strategy can make full use of the coordination of both arms to increase the manipulability of the space robot to manipulate dynamic targets, and simulations are conducted using a 7 degree of freedom dual-arm space robot to verify the feasibility and effectiveness of the proposed grasping strategy.
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Key words:
- dual-arm space robot /
- tumbling target /
- workspace /
- task compatibility /
- grasping strategy
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表 1 可行抓捕点相对于目标坐标系位姿
Table 1. Feasible grasping poses relative to target frame
GP Position/m Attitude P1 [−1.22 0 0.58] [0 0 1; 1 0 0; 0 1 0] P2 [−1.22 0.35 0.18] [0 0 1; 0 1 0; −1 0 0] P3 [−1.22 0 −0.22] [0 0 1; −1 0 0; 0 −1 0] P4 [−1.22 −0.35 0.18] [0 0 1; 0 −1 0; 1 0 0] P5 [−1.22 0 1.08] [0 0 1; 1 0 0; 0 1 0] P6 [−1.22 0.75 0.18] [0 0 1; 0 1 0; −1 0 0] P7 [−1.22 0 −0.72] [0 0 1; −1 0 0; 0 −1 0] P8 [−1.22 −0.75 0.18] [0 0 1; 0 −1 0; 1 0 0] 表 2 双臂末端执行器协调操作的可行抓捕点对
Table 2. Feasible grasping point pairs for dual-arm end-effectors cooperative manipulation
GP P1 P2 P3 P4 P5 P6 P7 P8 P1 × √ √ − √ √ √ − P2 − × √ √ √ √ √ √ P3 − − × − √ √ √ − P4 − − − × − − − − P5 − − − − × √ √ − P6 − − − − − × √ √ P7 − − − − − − × − P8 − − − − − − − × 表 3 空间机器人系统的运动学和动力学参数
Table 3. Kinematic and dynamic parameters of system
Joint $ a/{\rm{m}}$ $ \alpha/(^ \circ )$ $ b/{\rm{m}}$ $ q/(^ \circ )$ $ {{m}}/{\rm{kg}}$ $ I_{{xx}}/({\rm{kg}}\cdot {\rm{m}}^2)$ $ I_{{yy}}/ ({\rm{kg}}\cdot {\rm{m}}^2)$ $ I_{{zz}}/ ({\rm{kg}}\cdot {\rm{m}}^2)$ 0 2.52 0 ±0.446 0 400 128 340 340 1 0 $ \mp $90 0 $ q_1^j$ 3 0.0041 0.0041 0.0096 2 0 −90 0.168 $ q_2^j$ 8 1.3824 0.0256 1.3824 3 0 90 1.450 $ q_3^j$ 2 0.0047 0.0064 0.0047 4 0 90 0.168 $ q_4^j$ 6 0.8712 0.0192 0.8712 5 0 90 1.290 $ q_5^j$ 2 0.0047 0.0064 0.0047 6 0 −90 0.168 $ q_6^j$ 2 0.0047 0.0047 0.0064 7 0 90 0.44 $ q_7^j$ 4 0.0645 0.0645 0.0128 target − − − − 100 50 50 50 -
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