The effect of geometry and material properties on the performance of a small hydraulic McKibben muscle system

Fluidic McKibben artificial muscles are one of the most popular biomimetic actuators, showing similar static and dynamic performance to skeletal muscles. In particular, their pneumatic version offers high-generated force, high speed and high strain in comparison to other actuators. This paper investigates the development of a small-size, fully enclosed, hydraulic McKibben muscle powered by a low voltage pump. Hydraulic McKibben muscles with an outside diameter of 6 mm and a length ranging from 35 mm to 80 mm were investigated. These muscles are able to generate forces up to 26 N, strains up to 23%, power to mass of 30 W/kg and tension intensity of 1.78 N/mm2 at supply water pressure of 2.5 bar. The effects of injected pressure and inner tube stiffness on the actuation strain and force generation were studied and a simple model introduced to quantitatively estimate force and stroke generated for a given input pressure. This unique actuation system is lightweight and can be easily modified to be employed in small robotic systems where large movements in short time are required.