  Although there has been recent progress in control of multi-joint prosthetic
legs for rhythmic tasks such as walking, control of these systems for
non-rhythmic motions and general real-world maneuvers is still an open problem.
In this article, we develop a new controller that is capable of both rhythmic
(constant-speed) walking, transitions between speeds and/or tasks, and some
common volitional leg motions. We introduce a new piecewise holonomic phase
variable, which, through a finite state machine, forms the basis of our
controller. The phase variable is constructed by measuring the thigh angle, and
the transitions in the finite state machine are formulated through sensing foot
contact along with attributes of a nominal reference gait trajectory. The
controller was implemented on a powered knee-ankle prosthesis and tested with a
transfemoral amputee subject, who successfully performed a wide range of
rhythmic and non-rhythmic tasks, including slow and fast walking, quick start
and stop, backward walking, walking over obstacles, and kicking a soccer ball.
Use of the powered leg resulted in clinically significant reductions in amputee
compensations for rhythmic tasks (including vaulting and hip circumduction)
when compared to use of the take-home passive leg. In addition, considerable
improvements were also observed in the performance for non-rhythmic tasks. The
proposed approach is expected to provide a better understanding of rhythmic and
non-rhythmic motions in a unified framework, which in turn can lead to more
reliable control of multi-joint prostheses for a wider range of real-world
tasks.
