Work out: measuring and estimating the energy output of artificial muscle

Twisted and coiled polymer fiber actuators can provide a large work output of 2.1-2.6 kJ/kg at a constant external force. However, for realistic applications, variable external forces and periodic motion need to be considered. This paper measures the work loops of twisted and coiled polymer fiber actuators to obtain the work output per unit cycle. The paper finds for the first time that the softening property of the material due to temperature significantly deteriorates the work output. The upper limit of work output is determined by the crossover point of the work loop graph, reaching 0.39 kJ/kg in our initial studies. Deterioration of the work output is also caused by viscoelastic property. This paper further shows that a simple graphical method, that can be extended to other actuators, and can be used to estimate the crossover point and the maximum work from a stress-strain curve.