Hybrid Position-Force Feed Control of Heavy Load Forming Nonapod for Manufacturing Thin Wall and High Rib Parts

As load-bearing structures, thin wall and high rib parts (TWHRP) are widely used in aerospace equipment. A novel envelope forming process and the corresponding forming Nonapod with a parallel kinematic mechanism are developed for the manufacture of TWHRP. To realize high formability, tracking of tool motion and forming force simultaneously is necessary. However, the existing force-position control method utilizes an elastic contact model (spring-damper system), which cannot model the plastic forming force, making the existing force-position control method fail in the plastic force process. Therefore, in order to achieve force control along the feed direction and position control along other directions, a novel hybrid position-force feed control method (HPFCM) suitable for the envelope forming process is proposed. By taking into account both the change of link pose sourced from the feed motion and the deformation of the link caused by the forming load, rigid-flexible coupling dynamic model for high-precision motion tracking is established. By demonstrating that the normal forming force exhibits a near-linear relation with the contact area and assuming constant parameters for frictional forming force in consecutive control cycles, a real-time forming load prediction model based on the current force state and cone-disk plastic deformation hypothesis is developed for quick response force tracking. Through calculation of real-time forming load, forward of forming feed rate, and three-loop control of servo motors, an HPFCM is realized, tailored for the heavy load envelope forming process. Finally, aircraft bevel gears and aircraft window frames are formed with high precision. Compared to the position control method, the part error, forming load, and process time are highly reduced with HPFCM. This result shows high formability in the envelope forming of TWHRP and promising application prospects of the proposed HPFCM.