This paper presents development of a theoretical model for predicting the torque generated by a pneumatic soft-and-rigid hybrid actuator at a given actuation pressure and bending angle. The soft actuator with a novel architecture is comprised of half bellow-shaped flexible hollow structure (soft section) inbetween block-shaped semi-rigid sections made of silicone rubber materials. This actuator provides forward and backward bending motion when air pressure and vacuum are applied into the soft section, respectively. A simplified steady-state model was developed based on the quasi-static assumption when the system dynamics is slow enough that all of the interacting forces comes to an equilibrium at any given bending angle during the motion. The elastomeric material was modeled using the Yeoh 3rd order model to capture its nonlinear behavior. The model was studied for the soft actuators with different geometrical features in two cases of the free end motion (without external force) and the constrained end motion (with external force). Experimental testing and finite element simulations were carried out corresponding to the case-studies to validate the model. Comparison of the results obtained by these two approaches show a good agreement with the theoretical model's prediction.
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