Reconsidering the McKibben muscle: Energetics, operating fluid, and bladder material

Abstract

In spite of extensive modeling and characterization efforts, little is known about the energetics of McKibben muscle actuators. This article experimentally investigates the effectiveness of traditional McKibben muscles at converting fluid energy delivered to the actuator to mechanical output work over full actuation cycles. Once these efficiency metrics are established, a comparison of the efficiencies of traditional pneumatic fluidic artificial muscles and hydraulic fluidic artificial muscles is presented. Two new hydraulic oil compatible bladder materials are tested-an elastomeric Viton bladder and an inelastic low-density polyethylene bladder. The performance of these muscle variants is compared by measuring blocked force and free contraction as a function of pressure, hysteresis, and energy efficiencies. The measurement of fluid volume delivered to the fluidic artificial muscles over their actuation ranges is shown to be useful for evaluating the accuracy of existing cylindrical volume models. Models of the energy conversion efficiency are developed and compared to the experimental data. The results show that using an inelastic bladder significantly improves the efficiency, force capacity, and contraction range of McKibben muscles; however, it also increases the actuator's hysteretic behavior. Powering the muscles hydraulically and operating at higher pressures improves the efficiency as well.

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

“…Such an assumption is generally verified in the case of pneumatic artificial muscles working in a typical [1bar-5bars]-range if a sufficiently thin inner tube made of a soft rubber was chosen. Meller et al [21] have recently demonstrated the effects of bladder stiffness on HAM performance with increasingly stiff bladders significantly limiting the maximum strain achieved and slightly reducing the blocked force. A semi-empirical approach was introduced to account for bladder stiffness in the ideal model by introducing fitting parameters  F and   for force and strain, respectively [21]:…”

“…As noted by Tiwari et al [20] and Meller et al [21], the use of bulky compressors can be avoided in hydraulic artificial muscles (HAMs), thus making compact design possible. Meller et al [21] have also clearly demonstrated that the HAMs have approximately doubled the energy conversion efficiency of PAMs.…”

“…As noted by Tiwari et al [20] and Meller et al [21], the use of bulky compressors can be avoided in hydraulic artificial muscles (HAMs), thus making compact design possible. Meller et al [21] have also clearly demonstrated that the HAMs have approximately doubled the energy conversion efficiency of PAMs. Moreover, it has been shown that it was possible to design relatively 'large' pneumatic McKibben muscles to actuate human limb-size robots [22], or even very powerful ones thanks to the use of a particular strong external braided sleeve [23].…”

Thermally activated paraffin-filled McKibben muscles

“…To consider the impact of bladder stiffness on the muscle performance, theoretical (Sangian et al, 2015) and semi-empirical (Meller et al, 2014) modifications have been added to the model.…”

“…The muscle usually operates with pressurized gas/water, and the system requires a compressor/pump as well as a gas/water storage container (Meller et al, 2014;Tiwari et al, 2012;Mori et al, 2010;Moon et al, 2006). The pressurized fluids are used to increase the volume of the inner bladder and subsequently deform the braided sleeve that make up the McKibben muscle.…”