Modeling and Simulation Optimization of Vehicle Door Sag Based on Nonlinear Control Theory

Based on nonlinear control theory, the door is modeled and simulation - optimized to reduce its sag and residual displacement, and enhance vertical stiffness. Taking the front and rear doors of a certain model as the research object, the performance under 20° and full opening ($72^{\circ} / 70^{\circ}$) conditions was analyzed. The results show that the sag of the front door is reduced by $13.53 \%$ and $35.93 \%$, and the residual displacement is reduced by $84.91 \%$ and $82.76 \%$ when the door is opened at 20° and 72°, respectively; the sag of the rear door is reduced by $62.63 \%$ and $67.81 \%$, and the residual displacement is reduced by $99.60 \%$ and $99.47 \%$ when the door is opened at 20° and 70°, respectively. It shows that the optimized design not only reduces the door sag, but also significantly improves the door recovery ability and reduces the risk of deformation in long-term use. The computer simulation means can provide a scientific method for the door design, which helps to improve the overall design quality of the automobile.