Underwater multiple-degree-of-freedom robots possess a broad range of application potentials in diverse fields such as marine resource exploration, scientific investigation, and engineering construction and maintenance. However, during the execution of large-scale, long-distance underwater tasks, the conventional rigid serial and parallel manipulators frequently face the challenge of inadequate working range. Cable-driven parallel robots offer advantages such as large workspace, small inertia, and strong load capacity. However, prevalent cable-driven parallel robots for underwater applications are typically passively tensioned by gravity or buoyancy with their drive units (winches) mounted on a static platform, which constraints their motion ability and reconfigurability. Hence, a winch-integrated underwater cable-driven parallel robot based on a variable thrust mechanism is presented. The proposed robot adopts a hybrid drive form of six cables and a propeller. The thrust generated by the propeller, equivalent to cable tension, is adjustable in terms of magnitude and direction.

First, the overall mechanical structure of the robot is examined, and its kinematic and static models are established. On the basis of analyzing the judgment criterion of the wrench-feasible workspace (WFW), the wrench-feasibility testing problem under variable thrust is transformed into a constrained quadratic programming problem through the linear approximation method, and a new WFW calculation method is obtained. Then, a set of structural and force parameters of the robot are provided to evaluate and compare the WFWs of the robot with varied moving platform orientations and external forces under constant- and variable-direction thrusts. In addition, a large-span spiral trajectory is selected, a two-norm force index is implemented to optimize the thrust and cable tensions, and then the changes of all driving forces during the quasi-static motion of the robot on the trajectory are assessed under the two different thrust strategies.

Calculation and analysis reveal that under constant-direction thrust, the WFW of the robot appears columnar. Although the moving platform can extend over 10 meters in the Z direction, its motion ranges in the X and Y directions are small, and the WFW is influenced by the orientation of the moving platform and the external forces, which suggest that the robot is susceptible to out-of-control phenomenon. By contrast, under variable-direction thrust, the WFW becomes a cone-shaped space; compared with the condition of constant-direction thrust, the X and Y direction motion ranges of the moving platform increase, and the volume of the robot workspace remarkably improves. The simulation for spiral trajectory motion also reveals that under constant-direction thrust, the cable tensions vary substantially, which facilitates exceeding the limit and causes problems such as slack. The change of thrust direction can considerably alleviate the variability of the tensions, and guarantee that they remain within feasible limits, hence expanding the robot's range of motion.

Results reveal the remarkable improvement outcome of the variable thrust mechanism on the WFW of the robot, which solves the problem of inadequate working range of the existing underwater multiple-degree-of-freedom robots. This paper can provide a reference for further studies on the design and analysis of underwater cable-driven parallel robots.

The driving wheel is a challenge during the reconstruction of Su Song's astronomical clock-tower. Two approaches, the "flip-scoop" and "fixed-scoop" methods, have been identified based on whether the scoop can turn independently. Recent research reports that both options are inevitably inconsistent with the original text, indicating that conformity with the original text is no longer the only criterion for evaluating the merits of these approaches. Therefore, future models should not solely follow these options. New schemes can be designed by learning from their advantages while addressing their shortcomings and components that do not match the original text. Additionally, for museum exhibitions, long-term model stability is crucial. The force data of key components exactly determine the overall stability of the whole model. Therefore, in this study, the force data of some key components in the two schemes are obtained by constructing models in the software and calculations, which will provide valuable references for the reconstruction process.

This study establishes two models in the software based on the "flip-scoop" and "fixed-scoop" methods according to the original size in Song Dynasty. The force data between the forward upper lock and the driving wheel are calculated using the laws of a rigid body in rotational motion. Subsequently, the pressure between the forward upper lock and the driving wheel is analyzed.

The results revealed that the force between the forward upper lock and the driving wheel was nearly three times higher in the "flip-scoop" model compared with the "fixed-scoop" model. The reason for this disparity was the different structural characteristics between the two schemes. The "fixed-scoop" model incorporated buffering components that reduced the rotation speed of the driving wheel and thus weaken the force between the forward upper lock and the driving wheel. However, the "flip-scoop" model lacked components to help achieve similar effects. More critically, while the driving wheel periodically struck the forward upper lock, it was also subjected to the reaction force of the forward upper lock. In the "fixed-scoop" model, the forward upper lock struck the bottom of the scoop, while in the "flip-scoop" model, the forward upper lock struck the edge of the spoke on the side of the driving wheel. This results in a much smaller spoke impacted contact area in the "flip-scoop" model than in the "fixed-scoop" model, leading to higher pressure on the driving wheel in the "flip-scoop" model than that in the "fixed-scoop" model. Furthermore, this increased pressure exacerbated the potential risk of deformation and damage to the spokes. Additionally, once the pivot wheel was damaged, the influence on the stability of the entire model would be irreversible.

In summary, the differences in the buffering components and the contact areas between the driving wheel and forward upper lock make the driving wheel and forward upper lock in the "fixed-scoop" model suffer less impact and render its operational stability. Future reconstruction models can be designed based on this advantage.