טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentCohen Malka Yoav
SubjectA fast and powerful release mechanism based on pulse
heating of shape memory wires
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Doron Shilo
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Release and deployment devices, which are in common use in many defense and civilian applications, are usually based on an actuator that moves a safety pin against a resisting force. The actuator is required to have small volume and to consume small amount of energy, but at the same time to provide large work output during a short time. Existing actuators, such as solenoids, are restricted by their small work per volume capability. Pyrotechnic actuators provide large work per volume, but cannot be tested prior to their application.

Actuators based on shape memory alloys (SMA) provide the highest work per volume amongst all other actuators, except for pyrotechnics.  However, existing SMA actuators suffer from two major limitations: (1) slow actuation time and (2) small energy efficiency. This research suggests a novel solution that is aimed for overcoming these limitations by means of a novel mode of SMA actuation and a novel mechanical release mechanism.

The slow response time of existing SMA actuators is restricted by the rate of heat transfer rather than the rate of the phase transformation. Recently, the group of Prof. Shilo demonstrated a new mode of SMA actuation, in which NiTi wires are heated by a short (ms-scale) electric pulse. In the proposed research, the new SMA actuation mode will be implemented in the development of a novel fast and powerful actuator that is suitable for release mechanisms. The results presented in this research demonstrate several advantages of this actuator, including a release time of about 1 ms and a threefold improvement of the energy efficiency in comparison with conventional NiTi actuators.

Another novel aspect of the suggested research is the mechanical release mechanism that exploits the fast and powerful response of the new NiTi actuator and the fact that the mass of the safety pin is much smaller than the mass that connect to the load. The release mechanism is designed to form conditions in which the safety pin moves faster than the larger mass that presses on it. As a result, the contact between the masses is disconnected and the friction force drops to almost zero. In accordance, the work that is required for moving the safety pin is much smaller than the product of the friction force (under slow motion conditions) by the displacement of the pin. The results demonstrated this concept and showed a reduction of the required energy by almost an order of magnitude with respect to the energy consumption of a conventional NiTi actuator.

The research presents an SMA actuator, which provides a unique combination of large work per volume and short response time. A dedicated experimental setup, which enabled measuring the force applied by the actuator and the displacement of the safety pin, was used for studying the dynamic response of the actuator and release mechanism. Analysis of experimental results provided information on various dynamic variables of the system, including variables that were not measured directly, such as the contact force and the friction forces as a function of time.