Design and modeling of a hydraulic soft actuator with three degrees of freedom

This paper focuses on the design and modeling of a hydraulic soft actuator which can perform space motion with multiple degrees of freedom (DOF). The cylinder soft actuator made of elastomer has three fiber-reinforced hydraulic chambers distributed symmetrically. By controlling the pressure vector in the chambers, the motion of the soft actuator can be expressed by three independent variables including axial elongation, bending angle, and azimuth angle. This paper firstly presents the actuation principle, structural design, and fabrication method of the soft actuator. Secondly, the relationship between the input pressure and the axial length of the chamber is derived by taking the nonlinear deformation behavior of the elastomer into account, which is seldom considered in the state of the art. Thirdly, the kinematics of the soft actuator is modeled to predict the coordinates of its tip position with the piecewise constant-curvature (PCC) assumption. Finally, an experimental platform is set up and a series of experiments are implemented to identify some parameters which are difficult to be quantified analytically as well as to validate the developed model. The results show that the measured data are in good accordance with the predicted data. In addition, a model-based open-loop control experiment is carried out to realize the path following.