Analysis on stiffness and frictional effect of water hydraulic artificial muscle by pseudo-static isobaric experiments

As a novel actuator, water hydraulic artificial muscles (WHAMs) have the advantages of large force/weight ratio, good bionics, high positioning accuracy, and good compatibility with the ocean. It is more suitable as an actuator for underwater manipulators. To precisely control the movement and force of WHAMs, the static characteristics of WHAMs are analyzed by pseudo-static isobaric experiments. An accumulator was employed in the hydraulic circuit to stabilize the work pressure in WHAMs. Considering that the internal volume of the artificial muscle is constantly changing during the working process, the accumulator was specially selected after calculation on the volume change and pressure in the accumulator. Hysteresis phenomena in the force and length curves were observed at different working pressures. The main factor affecting the hysteresis phenomenon is the friction between the braided fiber strands, which is mainly related to the braid material of the muscles and the contact surface. The pseudo-static experimental results show that the different braid materials, initial angles, and wall thickness of the rubber tube have different effects on the experimental results. Combined with the mathematical model established by the empirical model that introduces the adjustment parameters and the friction between the braided fiber strands, the relative error between the model and the static experimental results is within ± 5 % in the middle 80 % part of the muscle contraction. This error is within ± 10% at the other 20% of the muscle contraction. Moreover, the WHAM has a nonlinear characteristic similar to the force-length character of human muscles. The stiffness of the WHAM increases linearly with the working pressure. The static physical model and stiffness analysis provide a theoretical basis for enhancing the high-precision position control of the WHAM.