Composite index-based stiffness optimization and deformation error compensation for a weak rigidity industrial robot

In order to solve the issue of relatively noticeable end deformation due to the weak rigidity structures in a highly dexterous 3T2R industrial robot, a composite index-based method of quasi-static robot stiffness optimization and deformation error compensation is proposed. The proposed method aims to enhance the quasi-static stiffness performance and reduce the quasi-static end deformation for this weak rigidity industrial robot. First, structural characteristics of the weak rigidity industrial robot are analyzed, and a quasi-static robot stiffness model is established. Second, a quasi-static robot stiffness identification approach is presented, considering weakly rigid joint transmission chains and flexible rod of the studied robot. The effects of the weakly rigid joint transmission chains and the flexible rod on the robot end deformation are investigated. Quasi-static robot stiffness identification experiment is conducted to obtain the quasi-static stiffness of each joint of the studied robot. Third, a composite stiffness index is constructed by using a compliance ellipsoid, incorporating both quasi-static normal stiffness evaluation index and quasi-static plane stiffness evaluation index. And a composite index-based method of quasi-static robot stiffness optimization and deformation error compensation is proposed to minimize the quasi-static robot end deformation of the studied weak rigidity industrial robot. Finally, the effectiveness of the presented quasi-static robot stiffness identification approach, and the effectiveness of the proposed composite index-based method of quasi-static robot stiffness optimization and deformation error compensation, are validated experimentally. Experimental results indicate that the error rates between the measured comprehensive robot end deformations and the theoretical counterparts at 4 kg, 8 kg, and 16 kg quasi-static loads are 10.61%, 11.82%, and 8.89%, respectively. The comprehensive position deformation is reduced by 23.43% before and after the quasi-static robot stiffness optimization. After performing quasi-static robot end deformation error compensation for parallelogram, triangle, and rectilinear segment, the reduced comprehensive position deformations range from 28.61% to 81.61%.