# Research on the Attitude Control Strategy of TBM Digging with V-Shaped Propulsion System
## 1 Introduction  
As global urbanization progresses, the demand for subway transportation and infrastructure, including underground comprehensive pipeline corridors, has surged, significantly driving the adoption and development of full-section rock Tunnel Boring Machine (TBM) technology [1, 2]. The propulsion system is integral to the TBM boring process, with traditional designs typically employing a parallel arrangement of partitioned hydraulic cylinders to achieve posture adjustment, deflection correction, and steering by controlling the displacement of cylinders in each partition [3]. While the parallel-arranged hydraulic cylinder propulsion system offers high boring efficiency, it results in a longer shield structure and larger turning radius. These limitations hinder traditional TBMs in addressing complex tunnel layouts and specialized scenarios such as mine tunnels with variable geological conditions [4, 5]. To address this challenge, a V-shaped TBM propulsion system has been developed, leveraging six-degree-of-freedom parallel robot technology [6]. Figure 1 illustrates the structure of the V-shaped TBM propulsion system, where the hydraulic cylinders are uniformly distributed between the front and support shields in a \"V\"configuration. A counter-torque is generated by modulating the thrust differential among the cylinders, enabling the TBM to achieve tunneling with a minimal turning radius.  
The performance of the TBM propulsion system significantly influences the equipment's boring attitude. Inconsistent attitude control can lead to issues such as the tunnel step phenomenon, over-excavation, or under-excavation, severely compromising tunnel molding quality and the stability of subsequent construction [7; 8]. Consequently, the study of tunneling attitude control for TBM propulsion systems has become a critical focus in related research domains. Regarding the digging attitude control in traditional TBM construction, both domestic and international scholars have conducted extensive research. For example, [PERSON] et al. [9] proposed an automatic control method utilizing a double closed-loop feedback mechanism, which regulates inclination and speed through primary and local feedback to precisely control the shield's digging trajectory. Addressing the lagging issues in attitude control during shield construction, [PERSON] et al. [10] developed an automatic shield attitude control system based on an optimal deviation correction curve model and fuzzy control theory, enabling precise control of the digging attitude under specific operating conditions. [PERSON] et al. [11] employed reinforcement learning to optimize the attitude control strategy for serpentine motion during TBM excavation, introducing the CACP algorithm to minimize cumulative deviations between the digging axis and the design axis. [PERSON] et al. [12] utilized a deep learning-based online optimization framework to predict and estimate digging trajectory deviations, thereby supporting TBM attitude and position control.  
Despite its emergence, research on the V-shaped TBM propulsion system remains relatively limited. The enhanced coupling characteristics of the system significantly increase the control complexity of the V-type propulsion system. In practical applications, operators primarily depend on personal experience for manual regulation and control, which poses significant limitations to the system's accuracy and stability. This study introduces a digging attitude control strategy tailored to the V-TBM propulsion system, utilizing a nonlinear control approach in the attitude adjustment process to enhance control accuracy and stability.