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.

Fluidic soft actuators have drawn more and more research interest in recent years, but it is not clear on their behaviors of dynamical energy consumption, which is critical to power system design. To solve this problem, this work develops an analytical model of required energy including elastic potential energy and dissipated energy for fiber-reinforced fluidic soft actuators based on the theory of viscoelasticity. Since the parameters of soft material are difficult to obtain, the key parameters in the model are identified by experimental data and genetic algorithm. To verify the general performance of the developed model, a series of fiber-reinforced fluidic soft actuators with different dimensions are fabricated and tested. The average errors of the experimental value and the predicted value are equal to 0.045 J, 0.003 J, 0.001 J, and 0.011 J, which are less than 15% of the nominal values. The experimental results show that the model can capture the energy characteristics of the soft actuators under various operating conditions with expected accuracy. Finally, the energy consumption of a robotic arm composed of three soft actuators is also evaluated. This work can promote the understanding on the energy requirement of soft robots and provide further reference to the optimization design of power system.

Soft robots have attracted increasing attention due to inherent environmental adaptability and reliable human-machine interaction. However, there is relatively few research about their energy efficiency which acts as an important indicator. This paper firstly derives the time-domain model of energy consumption for a fluidic soft robotic arm. With the introduction of forward kinematics, the trajectory of the soft robotic arm is optimized for energy saving under motion constraints and solved using interior point method. A series of experiments are implemented to evaluate the performance of the proposed model and the optimized trajectory. The results show that the time-based model can capture the dynamical energy behaviors of the fluidic soft actuators under various motions. It is also found that the energy consumption of the soft robotic arm is effectively reduced when the trajectory optimization is applied. This work can provide further reference to the energy-based optimization of the soft robots.