SMA-actuated bionic eye for humanoid expression

Inspired by the mechanism of human facial expression, this paper designs and implements a bionic eye system driven by shape memory alloy (SMA) springs. To meet the requirements of multidirectional eye movement, a spherical joint actuation model based on antagonistic SMA spring arrangement is proposed. Four SMA springs are evenly spaced at 90° intervals around the spherical joint. By differentially contracting the opposing springs, precise upward, downward, leftward, and rightward eye rotations are achieved, and real-time force sensor feedback is used to construct a closed-loop PID controller. For eyelid opening and closing, an SMA-PVC bias actuation mechanism is designed. When electrically heated, the SMA spring contracts to open the eyelid; upon cooling, the elastic recovery of the PVC sheet closes the eyelid. An angle sensor is employed to precisely control the opening angle. Experimental results show that the maximum rotation angle of the eyeball is ±20°, corresponding to an SMA differential force of 0.4 N, with a rotation angle error not exceeding 1°. The tracking error of eyelid opening and closing in step response does not exceed 3.7%, and the step tracking error for multi-directional eye rotation does not exceed 4.7%. The proposed system is capable of producing a rich variety of human-like micro-expressions, significantly enhancing the emotional expression capability of humanoid robots.