Design of a Self-Locking Component for McKibben Muscle Back-Stretchablility

McKibben muscles are widely utilised in biorobotics due to their high contraction ratio, remarkable power-to-weight ratio, and resemblance to skeletal muscles. However, the original design, constrained by the braided net, prevents elongation (back-stretchability) when inactive. This limitation means that McKibben muscles lack an essential characteristic of skeleton muscles: the ability to elongate when relaxed. This shortage results in the fact that when the McKibben muscles are used to mimic the antagonist muscle drives, they must be installed loosely (the McKibben muscle will sag while inactivated because the length of the McKibben muscle should be longer than the distance of two fixed points) to ensure the realization of antagonistic muscle movements. However, this compromise inevitably results in the waste of the contraction length of the McKibben muscle, leading to a reduction in the range of motion of the robot joints and the response speed. This paper proposed a Self-locking Component (SLC) enabling back-stretchability of McKibben muscle. When the McKibben muscle is inactivated, the component can contract with different stiffness springs to allow for elongation. Whenever the McKibben muscle with SLC is activated, the SLC will be locked with the McKibben muscle contracting. The elongation-load curve and max tensile force were tested. The results show a strong linear relationship with a max tensile force 50 N. Proposed self-locking component can achieve the self-locking and back-stretchable function.