Design development and control of a shape memory alloy linear actuation based XYθ motion stage

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This article addresses the development and trajectory tracking analysis of the three degrees of freedom Shape memory alloy (SMA) actuation-based 4- P PR motion stage. The developed motion stage is of four legs of the PPR (Prismatic-Prismatic-Revolute) configuration, having the first prismatic joint as the active joint. SMA spring is incorporated as the linear actuator of the motion stage. The three degrees of freedom motion of the end-effector is achieved based on a new type of SMA-driven PPR joint configuration to provide two translations and one rotational motion. The advantages of the SMA as an actuator is such as lightweight, higher accuracy, and micron-level movement to establish linear motion and silent actuation. The kinematic and jacobian relations are derived for the developed motion stage. The actuation of the SMA (Nitinol) spring actuator is accomplished by the use of a programmable power supply. In addition, the close loop PID control scheme to the SMA linear actuator is investigated. The mechanism has some inherent errors, as geometric errors (kinematic error), friction between the elements of the mechanism, etc, are significant issues in tracking the trajectory of the motion stage. Thus, to address the above issues, this study incorporates the dual-loop control scheme for better trajectory tracking and error compensation of the proposed motion stage. The control method employs a redundant feedback approach, in which discrete joint space displacements and mobile platform coordinates, and task space coordinates are determined as feedback signal using suitable sensors. In this control scheme, the actual joint displacement errors are computed using the redundant feedback data and corrected the desired joint displacement trajectory to obtain the desired task space trajectory. To measure the effectiveness of the used control scheme, a real-time prototype of the proposed motion stage (4- P PR) is developed and investigated the performance of the control method using some simple geometric trajectories. It is observed from the experiments that the motion stage using SMA linear actuation technique has good tracking performance using the control scheme. The developed motion stage can be implemented in the micro-stereolithography process and micron-level positioning application.