  Springs can provide force at zero net energy cost by recycling negative
mechanical work to benefit motor-driven robots or spring-augmented humans.
However, humans have limited force and range of motion, and motors have a
limited ability to produce force. These limits constrain how much energy a
conventional spring can store and, consequently, how much assistance a spring
can provide. In this paper, we introduce an approach to accumulating negative
work in assistive springs over several motion cycles. We show that, by
utilizing a novel floating spring mechanism, the weight of a human or robot can
be used to iteratively increase spring compression, irrespective of the
potential energy stored by the spring. Decoupling the force required to
compress a spring from the energy stored by a spring advances prior works, and
could enable spring-driven robots and humans to perform physically demanding
tasks without the use of large actuators.
