Using force-displacement relations to obtain actuation parameters from artificial muscles

Many different test methods are currently used to characterise the output of artificial muscle materials but few studies report the full range of possible force and displacements that can be generated by a given material when activated with a given input stimulus but when operated against different external loads. The measurement of the loading and unloading force extension curves in tension in both the un-activated and activated states is investigated as a means for efficiently characterising the full range of outputs for three different types of artificial muscles: pneumatically operated braided muscle and thermally operated shape memory alloy spring and twisted / coiled polymer fiber. A graphical method of analysis was applied whereby the force-extension curves obtained before and after actuator activation were plotted on the same axes. By overlaying the external loading conditions, the graphical method provided the equilibrium starting and finishing forces and displacements and successfully predicted the isotonic strokes, isometric forces and combined force and displacement generated when the actuator was operated against an external spring. Complications in the interpretation of the force-stroke curves were encountered as all three artificial muscles displayed a degree of loading-unloading hysteresis and non-ideal mechanical behavior.