  Soft robots have immense potential given their inherent safety and
adaptability, but challenges in soft actuator forces and design constraints
have limited scaling up soft robots to larger sizes. Electrothermal shape
memory alloy (SMA) artificial muscles have the potential to create these large
forces and high displacements, but consistently using these muscles under a
well-defined model, in-situ in a soft robot, remains an open challenge. This
article provides a system for maintaining the highest-possible consistent SMA
forces, over long lifetimes, by combining a fatigue testing protocol with a
supervisory control system for the muscles' internal temperature state. We
propose a design of a soft limb with swap-able SMA muscles, and deploy the limb
in a blocked-force test to quantify the relationship between the measured
maximum force at different temperatures over different lifetimes. Then, by
applying an invariance-based control system to maintain temperatures under our
long-life limit, we demonstrate consistent high forces in a practical task over
hundreds of cycles. The method we developed allows for practical implementation
of SMAs in soft robots through characterizing and controlling their behavior
in-situ, and provides a method to impose limits that maximize their consistent,
repeatable behavior.
