This paper presents the design of a low cost system that can be used as a spherical humanoid robot head to display expressive animations for social robotics. The system offers a versatile canvas for Human Robot Interaction (HRI), especially for face to face communication. To maximize flexibility, both in the style and apparent motion of the robot’s head, we exploit the relatively recent availability of low-cost portable projectors in a retro-projected animated face (RAF). The optical mechanical system is comprised of a projector whose light is reflected off a hemispherical mirror and onto a 360 degree section of the spherical head with sufficient resolution and illumination. We derive the forward and inverse mapping relation between the pixel coordinates on the projection plane of the projector, and the outer spherical surface to offer fast graphic generation. Calibration of the system is achieved by controlling three parameters of image translation and scaling, resulting in a specifically devised light cone whose edges are tangential to the hemispherical mirror. Several facial expressions are tested in illuminated indoor environments to show its potential as a modular low cost robot head for HRI.

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