  Exoskeletons which amplify the strength of their operators can enable
heavy-duty manipulation of unknown objects. However, this type of behavior is
difficult to accomplish; it requires the exoskeleton to sense and amplify the
operator's interaction forces while remaining stable. But, the goals of
amplification and robust stability when connected to the operator fundamentally
conflict. As a solution, we introduce a design with a spring in series with the
force sensitive cuff. This allows us to design an exoskeleton compliance
behavior which is nominally passive, even with high amplification ratios. In
practice, time delay and discrete time filters prevent our strategy from
actually achieving passivity, but the designed compliance still makes the
exoskeleton more robust to spring-like human behaviors. Our exoskeleton is
actuated by a series elastic actuator (SEA), which introduces another spring
into the system. We show that shaping the cuff compliance for the exoskeleton
can be made into approximately the same problem as shaping the spring
compliance of an SEA. We therefore introduce a feedback controller and gain
tuning method which takes advantage of an existing compliance shaping technique
for SEAs. We call our strategy the "double compliance shaping" method. With
large amplification ratios, this controller tends to amplify nonlinear
transmission friction effects, so we additionally propose a "transmission
disturbance observer" to mitigate this drawback. Our methods are validated on a
single-degree-of-freedom elbow exoskeleton.
