Variable Disturbance Observer-Based Robust Force Control of Servo Riveting Actuators With Nonlinear Environmental Considerations

In the aircraft industry, servo riveting actuators (SRAs) face significant challenges in achieving precise force control due to the nonlinear riveting environments. In this article, a novel environment-coupled dynamic modeling method and a variable disturbance observer-based (VDO) two-degree-of-freedom (2DoF) control scheme (VDO-2DoFC) are proposed to address these issues. The SRA is modeled as a combination of a rack-and-pinion mechanism and a multilinear equivalent spring-damping system to capture its dynamics. In the proposed VDO-2DoFC, a semi-adaptive law is first developed to provide prior environmental knowledge for the control system. Then, based on the derived dynamic model and semi-adaptive law, the VDO and improved 2DoFC are respectively designed to enhance disturbance compensation and force tracking capabilities. Furthermore, the closed-loop stability of the proposed scheme is rigorously analyzed using the small gain theorem. Finally, in addition to a series of simulation verifying the superiority of the proposed method, experimental results from a self-designed dual-SRAs platform demonstrate that the proposed scheme not only achieves high bandwidth force control but also ensures optimal riveting quality consistency, achieving deviations of ±0.04 mm in driven head height and ±0.045 mm in driven head diameter, both of which are far superior to the acceptable quality range.