Local annealing of shape memory alloys using laser scanning and computer vision

A complete set-up for local annealing of Shape Memory Alloys (SMA) is proposed. Such alloys, when plastically deformed at a given low temperature, have the ability to recover a previously memorised shape simply by heating up to a higher temperature. They find more and more applications in the fields of robotics and micro engineering. There is a tremendous advantage in using local annealing because this process can produce monolithic parts, which have different mechanical behavior at different location of the same body. Using this approach, it is possible to integrate all the functionality of a device within one piece of material. The set-up is based on a 2W-laser diode emitting at 805nm and a scanner head. The laser beam is coupled into an optical fiber of 60 mum in diameter. The fiber output is focused on the SMA work-piece using a relay lens system with a 1:1 magnification, resulting in a spot diameter of 60 mum. An imaging system is used to control the position of the laser spot on the sample. In order to displace the spot on the surface a tip/tilt laser scanner is used. The scanner is positioned in a pre-objective configuration and allows a scan field size of more than 10 x 10 mm(2). A graphical user interface of the scan field allows the user to quickly set up marks and alter their placement and power density. This is achieved by computer controlling X and Y positions of the scanner as well as the laser diode power. A SMA micro-gripper with a surface area less than 1 mm(2) and an opening of the jaws of 200 mum has been realised using this set-up. It is electrically actuated and a controlled force of 16mN can be applied to hold and release small objects such as graded index micro-lenses at a cycle time of typically 1s.