PhD Oral Exam - Xiaoming Zhang, Mechanical Engineering
When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
The Automated Fiber Placement (AFP) machines have brought significant improvement on composite manufacturing. However, the current AFP machines are designed for the manufacture of simple structures like shallow shells or tubes, and not capable of handling some applications with more complex shapes.
A cooperative AFP system is proposed to manufacture more complex composite components which pose high demand for trajectory planning than those by the current APF system. The system consists of a 6 degree-of-freedom (DOF) serial robot holding the fiber placement head, a 6-DOF revolute-spherical-spherical (RSS) parallel robot on which a 1-DOF mandrel holder is installed and an eye-to-hand photogrammetry sensor, i.e. C-track, to detect the poses of both end-effectors of parallel robot and serial robot.
Kinematic models of the parallel robot and the serial robot are built. The analysis of constraints and singularities is conducted for the cooperative AFP system. The definitions of the tool frames for the serial robot and the parallel robot are illustrated. Some kinematic parameters of the parallel robot are calibrated using the photogrammetry sensor.
Although, the cooperative AFP system increases the flexibility of composite manufacturing by adding more DOF, there might not be a feasible path for laying up the fiber in some cases due to the requirement of free from collisions and singularities. To meet the challenge, an innovative semi-offline trajectory synchronized algorithm is proposed to incorporate the on-line robot control in following the paths generated off-line especially when the generated paths are infeasible for the current multiple robots to realize. By adding correction to the path of the robots at the points where the collision and singularity occur, the fiber can be laid up continuously without interruption. The correction is calculated based on the pose tracking data of the parallel robot detected by the photogrammetry sensor on-line. Due to the flexibility of the 6-DOF parallel robot, the optimized offsets with varying movements are generated based on the different singularities and constraints. Experimental results demonstrate the successful avoidance of singularities and joint limits, and the designed cooperative AFP system can fulfill the movement needed for manufacturing a composite structure with Y-shape.