Shape-Memory-Alloy-Based Launch Lock
- Tuesday, 01 July 2014
This low-mass, low-power lock can be activated multiple times.
NASA’s Jet Propulsion Laboratory, Pasadena, California
Most NASA missions require the use of a launch lock for securing moving components during the launch or securing the payload before release. A launch lock is used to prevent unwanted motion and secure the controlled components. The current launch locks are based on pyrotechnic, electromechanical or NiTi-driven pin pullers that are one-time activation mechanisms. Generally, the use of piezoelectric activation provides high-precision nanometer accuracy, but they rely on friction to generate displacement. During launch, the generated vibrations can release the normal force between the actuator components, allowing the shaft’s free motion, which could result in damage to the actuated structures or instruments. This problem is common to other linear actuators that consist of a ball screw mechanism. There are many mechanisms that require the capability of being activated multiple times, and the disclosed concept addresses this need.
The proposed launch lock mechanism is driven by a shape memory alloy (SMA) that allows for controlled activation and repeatability of the launch. For this purpose, NiTi SMA rings are used as actuators. The device locks mechanisms with an actuator, prevents motion during launch, and releases it when receiving an activation command. The lock simplifies the current launch locks using a simple configuration with low mass and requiring low power. The mechanism is reusable for multiple activations, and does not introduce additional shocks or vibrations.
The lock consists of an SMA ring and a cylindrical target element. The target element can be mounted on one structure and the SMA element to the other structure through an adapter that allows a diameter change of the SMA ring. The target element has a large stiffness, while the adapter has a large axial stiffness but a low radial stiffness. The use of pretrained NiTi SMA material as an actuator allows generating 5% or more deformation by heating, using phase transition from martensite phase to austenite phase. Phase transition can be positive or negative. The phase transition temperature can be controlled by the composition of the selected alloy. The NiTi alloy has a martensite-to-austenite transition that starts at a temperature higher than 650 °C. A launch lock mechanism using SMA material is not expected to change the locking status by self-activation below this temperature.
This work was done by Mircea Badescu, Xiaoqi Bao, and Yoseph Bar-Cohen of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49294
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