Cocatre-Zilgien J.H., Delcomyn F., and Hart J.M. (1996). Performance of a muscle-like "leaky" pneumatic actuator powered by modulated air pulses. J. Robot. Syst. 13(6): 379-390.

Abstract. There is current interest in the development of legged robotic devices inspired by biological walking systems. We have developed an actuator for this type of device that closely emulates the performance of muscle. The actuator is a "leaky" single-effect air cylinder controlled through a 3-way valve that either permits charging the cylinder with compressed air or opens it to the atmosphere. To emulate muscular activation with trains of identical nerve impulses that vary in frequency, we controlled the pressure in the cylinder through pulse frequency modulation. To allow a comparison between our cylinder and muscle, we evaluated the open loop performance of the cylinder and valve system in isometric (no motion) and linear isotonic (constant load) conditions. We demonstrate that this mode of activation of the cylinder makes its behavior quite close to that of muscle. Furthermore, our cylinder/valve system generated twice the tension developed by muscle of the same cross-sectional area, with half its power-to-weight ratio.

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