  In this paper, we present an adjustable-equilibrium parallel elastic actuator
(AE-PEA). The actuator consists of a motor, an equilibrium adjusting mechanism,
and a spring arranged into a cylindrical geometry, similar to a motor-gearbox
assembly. The novel component of the actuator is the equilibrium adjusting
mechanism which (i) does not require external energy to maintain the
equilibrium position of the actuator even if the spring is deformed and (ii)
enables equilibrium position control with low energy cost by rotating the
spring while keeping it undeformed. Adjustable equilibrium parallel elastic
actuators resolve the main limitation of parallel elastic actuators (PEAs) by
enabling energy-efficient operation at different equilibrium positions, instead
of being limited to energy-efficient operation at a single equilibrium
position. We foresee the use of AE-PEAs in industrial robots, mobile robots,
exoskeletons, and prostheses, where efficient oscillatory motion and gravity
compensation at different positions are required.
