NiTi Torsion Spring based Lock-Release Mechanism using an Aperture Diaphragm

Compact, lock-release mechanisms are needed in applications ranging from small robotics and UAV payload release to medical devices and other space-constrained systems. This work presents the design, development, and performance testing of a compact, self-resetting lock-release mechanism actuated by a NiTi (Nitinol) shape-memory-alloy torsion spring. Upon Joule heating, the SMA torsion spring provides direct rotary actuation of an aperture diaphragm, enlarging the aperture and releasing a captive shaft. The prototype demonstrates reliable, repeatable actuation over multiple cycles.

Traditional SMA-based release systems commonly rely on additional conversion or transmission mechanisms to transform SMA motion into usable rotary output; such intermediates increase part count, footprint, machining complexity, and assembly effort. By providing direct torsional output, the proposed design eliminates those intermediates, reduces spatial requirements, and simplifies assembly. Compared with axial (tension/compression) spring layouts, torsion springs store twist in a single plane and produce rotation directly. Hence, the mechanism requires less out-of-plane space and can deliver higher torque for a given amount of spring material — a practical advantage for compact designs. A simple torsion-spring bias returns the diaphragm to the locked position upon cooling, enabling automatic relocking without the need for external reset systems.