Control of shape memory alloy actuated gripper using Sliding Mode Control

One major challenge is to develop high performance light weight end effectors for various applications in fields like medical, space, aerospace and deep water explorations. For this reason today new robots able to guarantee more dexterity and workspace are under study. Smart actuators like shape memory alloys (SMA) can fill this gap due to their very high energy density (power to weight ratio). They share many common aspects with traditional robotic assemblies such as positioning of manipulators, velocity, jerk and force control, tactile feedback, task planning, collision avoidance, grasping, part orientation, etc. End-effectors like grippers utilizing SMAs as direct drive actuating elements can find vast applications in industrial automation, assembling parts, semiconductor industries, non-invasive medical surgeries and processing of biological objects. This article presents a gripper actuated by SMA wire. The mechanical structure of the gripper includes a pair of jaws-one fixed and other movable, a SMA wire for actuation and a biasing torsion spring for the strain recovery in SMA wire. An experimental setup of the SMA gripper is established. The SMA actuator is heated by the electric current. A noncontact laser displacement sensor is engaged to measure the position of the movable jaw. In order to control the position of the devised gripper, a model of the gripper is estimated from open loop experimental data using system identification technique. Finally, Sliding Mode Control (SMC) technique is designed and compiled to track various stationary and variable (dynamic) trajectories to suit applications that can manipulate objects of varied dimensions. Experimental results show that the gripper is able to track the position rapidly and precisely.