We investigate utilizing inelastic bladder hydraulic artificial muscle actuators as muscle fibers. These muscle fibers are then grouped together to form a variable recruitment artificial muscle bundle. This muscle bundle configuration is biologically inspired, where in skeletal muscle, different numbers of motor units are recruited to match the load by increasing the number of motor neurons firing. This results in extremely efficient locomotion in nature. It is desired to use a similar methodology to increase the actuation efficiency of valve-controlled hydraulic systems. Such hydraulic control systems induce a pressure drop in the valves to throttle the flow to the cylinder actuators. Using the valves in this manner is simple but very inefficient. Hence, this paper presents selectively recruiting different numbers of the hydraulic artificial muscle fibers to match a required loading scenario similar to our bipedal robot. By using fewer of the muscle fibers to match a smaller load, less power is consumed from the hydraulic power unit because instead of inducing a pressure drop, the volume of fluid delivered is decreased. The potential efficiency improvements associated with this actuation scheme is compared to a traditional hydraulic system with differential cylinders.

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