  Legged robots constructed from soft materials are commonly claimed to
demonstrate safer, more robust environmental interactions than their rigid
counterparts. However, this motivating feature of soft robots requires more
rigorous development for comparison to rigid locomotion. This article presents
a soft legged robot platform, Horton, and a feedback control system with safety
guarantees on some aspects of its operation. The robot is constructed using a
series of soft limbs, actuated by thermal shape memory alloy (SMA) wire
muscles, with sensors for its position and its actuator temperatures. A
supervisory control scheme maintains safe actuator states during the operation
of a separate controller for the robot's pose. Experiments demonstrate that
Horton can lift its leg and maintain a balancing stance, a precursor to
locomotion. The supervisor is verified in hardware via a human interaction test
during balancing, keeping all SMA muscles below a temperature threshold. This
work represents the first demonstration of a safety-verified feedback system on
any soft legged robot.
