Artificial muscles are characterized by a number of different performance measures, such as actuation strain, stress, strain rate, work and power output. A number of different testing methods are used to measure some of these parameters. The most commonly used test methods are the isotonic test that provides the actuation strain and the isometric test that gives a measure of the force or stress generated. Often the isotonic and isometric tests are performed at different levels of pre-strain. A simple mechanics based approach provides a theoretical framework that suggests that all these actuation parameters can be obtained by measuring the force-extension curves of the artificial muscle in the activated and non-activated states. A graphical method can provide estimates of the isotonic, isometric or any other test method that involves an external load.
This presentation provides an overview of the mechanics based simple theory. Experimental data is compared for three types of tensile artificial muscle: pneumatic braid; shape memory alloy spring and a twisted/coiled polymer fiber. Tests were performed under isotonic and isometric conditions and when operated against a spring. All three of these materials show non-idealities in their mechanical behaviour, including load-unload hysteresis and plastic deformation. When these non-idealities were taken into account, the mechanics based approach provided satisfactory estimates of the isotonic, isometric and spring actuation behaviour. The approach provides a simple and standardized method for characterizing the static actuation performance of any type of tensile actuator.