# Optimization Method of the Clamping Force for Large Cabin Parts
## 1 Introduction  
The large cabin part belongs to the category of the large axial part, which is the basic part and the critical component of common equipment, special equipment or large heavy equipment. It occupies an irreplaceable position and has been widely used in some key industries such as transportation, construction machinery, marine ships, aerospace, national defense and scientific research. In the exploration and utilization of space, civil aerospace, as a strategic emerging power, has been playing an increasingly important role in the field of aeronautics and astronautics. At the same time, the maturity of aerospace technology also requires the automation and intelligence of civil aerospace to reach a higher level. For civil aerospace enterprises, how to quickly realize pose adjustment and docking of a large cabin part or pose detection and adjustment of a solid rocket motor has become a key problem. If the above functional requirements and other processing requirements of large cabin parts are realized smoothly, the accurate and stable clamping of the part should be achieved. A large cabin part has the characteristics of large size, heavy load, complex clamping, difficult pose adjustment and so on. At present, civil aerospace enterprisesstill use the traditional fixed clamping device. The contact area between the clamping device and the large cabin part is usually designed with fixed curvature and rigid contact. Due to the manufacturing process of large cabin parts, their shape and diameter have some dimensional errors. The actual operation of the clamping device requires frequent manual adjustment by changing the thickness of the filling layer between the clamping device and the parts to contain the dimensional errors. It not only involves complicated disassembly but also seriously affects the civil aerospace enterprises' production efficiency. Furthermore, the clamping contact surface of the clamping mechanism with the existing constant curvature cannot effectively guarantee the stable clamping of large cabin parts. The instability of large cabin parts will cause serious safety problems. It can be concluded that the traditional fixed clamping device is time-consuming and has high cost and low clamping efficiency. In addition, the conventional fixed clamping device can cause stability problems and damage to large cabin parts. The ring pneumatic fixture can solve the above problems well and is shown in Figure 1.  
The ring pneumatic fixture is a kind of annular centripetal fixture that can be connected with a bracket to the ground firmly and be used to achieve grinding, painting or other processing steps of the workpiece. It can also be connected to a vehicle platform to realize the workpiece's transportation, docking and other assembly requirements, as shown in Figure 2. The ring pneumatic fixture contains several clamping mechanisms. The clamping mechanism contains self-adaptive joints consisting of a linear spring, torsional spring or other underactuated elements that can make the ring pneumatic fixture able to realize shape- and self-adaptive clamping. Each clamping mechanism contacts the surface of the workpiece and exerts a certain clamping force on the workpiece, finally realizing the shape-adaptive and stable clamping of the workpiece. The design inspiration for the ring pneumatic fixture comes from the underactuated mechanism. It can adapt to the different shapes of the clamped objects and achieve envelope clamping of the objects, which has strong clamping flexibility. The main purpose of the ring pneumatic fixture is to realize the assembly of large cabin parts. As a multi-functional fixture system device, the ring pneumatic fixture is gradually replacing the traditional single  
Figure 1: 3D model of the ring pneumatic fixture: 1. Body of the ring pneumatic fixture, 2. Felt at the end of the clamping mechanism, 3. Clamping mechanism. function fixture. The feature of shape-adaptive clamping is first proposed in the study of the underactuated mechanism.