Contact-Force-Based Closed-Loop Control of Multi-Axis Additive Manufacturing With Continuous-Fiber-Reinforced Polymer

Abstract Continuous fiber-reinforced polymer (CFRP) has outstanding properties of lightweight and exceptional mechanical performance. Additive manufacturing (AM) with CFRP is an automated process for fabricating complex and customized structures. Multi-axis CFRP-AM introduces an increased degree of freedom (DOF) and enables the depositing of continuous fibers on complex curved surfaces with freeform spatial trajectories, which have wide applications such as aerostructures and the automotive industry. However, this enhancement in capability also brings challenges of high complexity and printing quality. Furthermore, due to the inherent tensile properties of continuous fibers, depositing on a complex surface increases the possibility of printing defects. Fiber bridges, slippages, and abrasion are much easier to occur particularly for complex surfaces with concave features. Existing research efforts mainly adopt pretreatments such as path and motion planning to achieve a more precise and efficient printing process, while lacking the focus on the improvement of the manufacturing process, particularly the capacity to deal with uncertainties and high dynamics when printing on complex surfaces. Therefore, in this paper, we proposed a contact-force-based closed-loop multi-axis CFRP-AM system to improve the fiber depositing accuracy on closed-shell structures with concave surfaces. A trajectory generation method based on conformal mapping is established, enabling the fiber path design on a 2D map and angle-preserved mapping onto the 3D surface. The 3D fiber path on the curved surface is transformed into an executable printing trajectory with robot motion and printer extrusion planning, thereby establishing an environment conducive to closed-loop control. A proportional-derivative (PD) controller is adopted, and the real-time vertical contact force is exploited as a feedback signal to dynamically adjust the layer height between the nozzle and the printing surface. A material extrusion multi-axis CFRP-AM platform combining a co-extrusion nozzle, a 6-axes industrial robot, a six-degree-of-freedom load cell, and a real-time closed-loop control system is built to verify the proposed control framework. To showcase the enhancements in print quality enabled by closed-loop control, a representative revolution part featuring a concave surface is designed. Dry basalt continuous fiber is deposited using the established closed-loop multi-axis CFRP-AM system. The results indicate that vertical contact force is effectively maintained around the desired value throughout the printing process, ensuring robust adhesion between the deposited fiber and the surface. Notably, the incidence of printing defects, including fiber bridges, is markedly reduced through the implementation of contact force feedback control.