Context: Elastic actuators can store energy and release it for explosive, efficient motions. The Bi‑Stiffness Actuator (BSA) concept addresses the key hurdle—precise energy‑release timing—by decoupling spring preload from release via brake/clutch sequencing. 

Remaining challenge: Can we implement the BSA concept in a compact, robot-integrable joint without compromising timing precision, sensing fidelity, or control authority?

Our answer: iBSA: a fully integrated BSA joint—compact mechanics (cage‑spring with integrated torque sensing) plus embedded high‑rate control (with DOB)—that preloads and releases on schedule within a robot‑scale envelope.

Results: Under matched limits, iBSA outperforms our earlier modular BSA; at full speed with a disturbance observer it goes further—while remaining small and system‑ready for multi‑DoF robots.

Three takeaways

- BSA solves timing; integration makes it practical.

- Timing is preserved in a compact, robot‑ready joint.

- Embedded high‑rate control + DOB improve tracking and boost repeatable peak performance.