Design and implementation of a 300% strain soft artificial muscle

We present the inverse pneumatic artificial muscle (IPAM), a new soft actuator that is powered by pneumatics in a manner inverse to traditional pneumatic muscles: low pressure, rather than high, contracts the muscle. The IPAM improves on the 50-year-old standard in soft pneumatic actuators, the McKibben muscle, but retains many of the advantages that have drawn roboticists to this artificial muscle over the years. The McKibben muscle produces up to 40% strain, and has nonlinear control with friction and hysteresis, whereas the IPAM attains strains of over 300% and has a nearly linear mapping between input pressure and force/length output and no sliding friction. Crucially, the IPAM retains the soft structure, low weight, compliance, and robustness that the McKibben muscle boasts. We present a simple model to describe the behavior of the muscle, as well as force, displacement, pressure, and speed tests validating the model and characterizing the IPAM's performance. Further, we present two practical implementations using the IPAM: an active brace and a robotic finger.