Dual-Motor Position Control Based on a Synchronous State Observer

High-end vertical five-axis machining centers commonly adopt dual-motor direct-drive configurations for their cradle-type A-axis to improve dynamic performance; however, this approach introduces control challenges in balancing counteracting torque and synchronization accuracy due to high-rigidity coupling. To address this issue, this study presents a novel error compensation control strategy based on a synchronous state observer. First, a system dynamic model incorporating dual-axis coupling effects is developed to systematically investigate the coupling mechanism between synchronization error and counteracting torque. Based on this model, a synchronous state observer is designed, which achieves real-time reconstruction and feedforward compensation of synchronization disturbances induced by factors such as transmission parameter mismatches and inter-axis torque imbalance, thereby enabling coordinated control of high-precision position synchronization and torque balance. The effectiveness of the proposed method is verified through simulation and experiments conducted on a VMC630 vertical five-axis machining center. Results show that under various speed and acceleration conditions, the maximum position synchronization error remained below $6.3\mathrm{e}-4$${}^{\circ }$, with comparable convergence performance; the current deviation between the dual motors was constrained to within $\pm 0.25$$\mathrm{A}$, demonstrating effective mitigation of counteracting torque. In machining tests of S-shaped specimens, all measured contour deviations fell within the $\pm 0.060$$\mathrm{m}$$\mathrm{m}$ tolerance range, and the specimens exhibited excellent contour consistency and surface quality. These results validate the proposed strategy’s status as an engineering-viable solution for precision motion control in high-rigidity coupled dual-motor systems.