  This paper describes the design and control of a support and recovery system
for use with planar legged robots. The system operates in three modes. First,
it can be operated in a fully transparent mode where no forces are applied to
the robot. In this mode, the system follows the robot closely to be able to
quickly catch the robot if needed. Second, it can provide a vertical supportive
force to assist a robot during operation. Third, it can catch the robot and
pull it away from the ground after a failure to avoid falls and the associated
damages. In this mode, the system automatically resets the robot after a trial
allowing for multiple consecutive trials to be run without manual intervention.
The supportive forces are applied to the robot through an actuated cable and
pulley system that uses series elastic actuation with a unidirectional spring
to enable truly transparent operation. The nonlinear nature of this system
necessitates careful design of controllers to ensure predictable, safe
behaviors. In this paper we introduce the mechatronic design of the recovery
system, develop suitable controllers, and evaluate the system's performance on
the bipedal robot RAMone.
