**7. Inflatable structural elements**

Chapter 2.2 showed some examples of how inflatable structures can contribute to compliant robotics. This chapter shows how a modular design can help to integrate inflatable structures in robots independently from the drive concept. The main load cases of robotic structures are bending and torsion. In the shell of inflatable structures we have a state of plane stress. The shell cannot carry significant compressive force. However, when pressurized the shell is pretensioned. Compressive forces thus decrease this pretension in the shell. When the compressive forces overcome the pretension the whole structure deflects and yields the external loads.

The general fabrication process of structural elements is identical to the process described in chapter 3.2.1. The different load cases require different reinforcements in the shell. Two different layers are integrated in the shell in order to carry bending and torsion respectively. The fibers of the braided reinforcement follow the directions of the principal stresses for torsion on the surface of a cylinder as shown in figure 28(a). The reinforcement for pressure stability and bending are are shown in figure 28(b). This second reinforcement layer is a woven fabric tube with radial and axial fiber directions. The relationships between internal pressure and bending or torsional stiffness are presented in figure 28(c) and 28(d). These graphs show how the stiffness can be adjusted depending on the compliance requirements. Each front end of the structural element is equipped with a four screw flange, which allows for easy mounting and pressure sensor integration [36].
