Design and Control of a Parallel Elastic Actuator With Adjustable Equilibrium Position

Y. Chen; E. Chatziandreou; C. W. Mathews; B. P. Johnson; D. J. Braun

doi:10.1109/LRA.2026.3681100

Publications

Design and Control of a Parallel Elastic Actuator With Adjustable Equilibrium Position

Y. Chen, E. Chatziandreou, C. W. Mathews, B. P. Johnson and D. J. Braun

IEEE Robotics and Automation Letters, vol. 11, no. 6, pp. 6512–6519, 2026

DOI

@article{Chen2026,
  author  = {Y. Chen and E. Chatziandreou and C. W. Mathews and B. P. Johnson and D. J. Braun},
  title   = {Design and Control of a Parallel Elastic Actuator With Adjustable Equilibrium Position},
  journal = {IEEE Robotics and Automation Letters},
  volume  = {11},
  number  = {6},
  pages   = {6512--6519},
  year    = {2026},
  doi     = {10.1109/LRA.2026.3681100}
}

This paper presents a parallel elastic actuator with an adjustable equilibrium position (AE‑PEA). A large direct-drive motor works in parallel with a 3D-printed torsional spring, while a small motor continuously adjusts the equilibrium position of the spring. The actuator combines three features usually found in different actuators: high torque-control bandwidth, like direct-drive actuators; energy storage and shock mitigation, like parallel elastic actuators; and an adjustable equilibrium position, like series elastic actuators.

Why it matters: A spring in parallel with a motor can effectively carry a load or store energy, but is limited to working about a fixed equilibrium position. Adjusting the equilibrium position lets the same spring help across a range of positions and tasks, while the direct-drive motor keeps fast, precise torque control. This makes the actuator suited to robot joints that both hold static loads and perform fast oscillating motions.