Variable-Actuated Parallel Robot

A parallel robot possesses the multi-branched closed-loop structure, which consists of two or more kinematic branches connecting the base and the end effector. Parallel robots possess the distinctive features of high stiffness, high load capacity, and high accuracy with closed kinematic chains. However, the multi-branched closed-loop structure also brings some unavoidable shortcomings to parallel robots, such as the limited workspace and complex singularities. Singularity further reduces the available workspace.

Since the number of joints is much greater than the degrees-of-freedom number in parallel robots, only part of the joints is actuated while others are not. Therefore, various actuation modes, i.e., different choosing schemes of active joints, are available for parallel robots. Different actuation modes will inevitably change the kinematic performance of the parallel robot. Actuation conversion is a potential way to improve the kinematic and dynamic performance of the parallel robot.

Motivated by this idea, we investigated the variable-actuated kinematic chains, variable-actuated parallel robots and the method for improving the performance via actuation mode conversion. Several variable-actuated kinematic branch chains with nine driving modes and real-time driving mode conversion ability were designed. Based on the proposed variable-actuated kinematic chains, the variable-actuated 5R, 3-RRR and 3-RPaS parallel robots were designed. The kinematics performance of the variable-actuated parallel robots was analyzed by using the motion/force transmission index. The change rule of the kinematic performance of the variable-actuated parallel robot under different actuation modes were clarified. A new approach to improve the kinematic performance of parallel robots by transforming actuation modes was established.

Improve robot's performance through actuation mode conversion

通过图9的图谱可以发现，在工作空间内的某一点，当机构采用不同的驱动模式工作时，其性能存在明显差异。各类驱动模式的运动学性能较优区域在工作空间的位置各不相同。在工作空间内任一点，为得到最优的运动学性能，机构应采用在该点性能最优的驱动模式进行工作。将在该点的OLTI值取得最大值的驱动模式称为该点的最优驱动模式。由此可得参数为R = 500mm，r = 100mm，l1 = l2 = 350mm，动平台转角为15°时3-RRR机构在工作空间内的最优驱动模式变换图谱如图11所示。

得到机构动平台角度从-90°和90°变化时，机构在驱动模式I和最优驱动模式工作时OLTI指标分布对比如图12所示。对整个立体工作空间中各离散点的OLTI指标值进行统计，得OLTI指标数值在各区间的比例分布如图13所示。采用模式I驱动时，机构在整体工作空间中OLTI指标值小于0.3区域占比为49.81%，大于0.7区域占比为12.21%。而当机构采用最优模式驱动时，上述两个值分别为12.86%和30.6%，采用变驱动的方式明显提高了机构性能。由图12进一步可知，当机构采用最优驱动模式工作时，可达工作空间内部不再出现OLTI=0的区域，即可达工作空间内不再存在奇异点。

同样，变驱动5R机构和变驱动3-RPaS机构也可以通过工作空间内的驱动变换提升空间内的运动学性能，并消除内部奇异，使得工作空间的利用率最大化。

Related Publications:

Liping Wang, Zhaokun Zhang, Zhufeng Shao, Xiaoqiang Tang. Analysis and Optimization of a Novel Planar 5R Parallel Mechanism with Variable Actuation Modes. Robotics and Computer-Integrated Manufacturing, 2019, 56: 178–190. DOI: 10.1016/j.rcim.2018.09.010.

Liping Wang, Zhaokun Zhang, Zhufeng Shao. Kinematic Performance Analysis and Promotion of a Spatial 3-RPaS Parallel Manipulator with Multiple Actuation Modes. Journal of Mechanical Science and Technology, 2019, 33(2): 889-902. DOI: 10.1007/s12206-019-0146-z.

Zhaokun Zhang, Liping Wang, Zhufeng Shao. Improving the Kinematic Performance of a Planar 3-RRR Parallel Manipulator through Actuation Mode Conversion. Mechanism and Machine Theory, 2018, 130: 86-108. DOI: 10.1016/j.mechmachtheory.2018.08.011.