Series and Parallel Elastic Actuation: Influence of Operating Positions on Design and Control

It is well-established that properly tuned elastic
elements can make robotic actuators more energy-efficient, especially
in cyclic tasks. Considering drive train topology, two
important subcategories of elastic actuators are Series Elastic
Actuation (SEA) or Parallel Elastic Actuation (PEA). There is still
no definite answer to the fundamental question which topology
consumes less energy in a given task. This paper approaches the
problem by studying oscillatory motions of a single degree-offreedom
link in a gravitational field. The imposed motion is a
sinusoid with non-zero offset requiring a static torque that needs
to be compensated by the actuation system. Simulations and
experiments show that SEA consumes less energy up to certain
offset angles. At high offsets, PEA becomes the more energyefficient
alternative, provided that its no-load angle is properly
tuned. Inverse dynamics simulations show how a threshold offset
angle can be determined for a given task.