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The Eyes


   The Eyes

  • Design Criteria for the Visual System
  • Physical Description of Cog's Eyes
  • Moving the Eyes

  • The Cog Shop
    MIT Artificial Intelligence Laboratory
    545 Technology Square, #920
    Cambridge, MA 02139

    write to the Cog Documentation Project: cdp@ai.mit.edu

    Design Criteria for the Visual System

    To approximate the complexities of the human visual system, we designed Cog's head and visual system around five goals:

    • Binocular
    • Active
    • Compact
    • Wide Field of View
    • High Resolution Area

    A binocular camera arrangement is necessary because disparity and approximate depth will be important factors for discriminating objects. We also require the system to be active, that is, the eyes should have a human-like speed and range of motion. The system should be compact enough to fit on the robot's head, and to allow the cameras to move with reasonable speed. As with human vision, the system should have both a wide field of view (for detection of motion and objects in the far fields) and very high resolution capability. (Humans and other animals have both a wide, low-resolution field of view and a very narrow, high resolution field of view called the fovea.)

    Physical Description of Cog's Eyes

    The camera system has four degrees of freedom (DOF) consisting of two active "eyes". To mimic human eye movements, each eye can rotate about a vertical axis (pan DOF) and a horizontal axis (tilt DOF). Human eyes actually have more than two degrees of freedom, but the pan and tilt DOFs are sufficient to scan the visual space. To approximate the range of motion of human eyes, mechanical stops were included on each eye to permit a 120 degree pan rotation and a 60 degree tilt rotation. Each eye consists of two black and white CCD cameras. Together, the camera ensemble approximates the wide peripheral view and high resolution fovea region as described below. Small remote head cameras were chosen so that each eye is compact and lightweight. Each camera is finger-sized, measuring approximately 17 mm in diameter and 53 mm in length (without connector), and weighs only 25 grams.

    The lower camera of each eye gives Cog a wide peripheral field of view 88.6 degrees (V) by 115.8 degrees (H). Although this is narrower than human peripheral vision, it is difficult to buy a lens with a wider field of view. The lens can focus from 10 mm to infinity. The upper camera of each eye gives Cog a high resolution fovea. The lens has a 15 mm focal length with a 18.4 degrees (V) by 24.4 degrees (H) field of view. This provides a fovea region significantly larger than that of the human eye, which is approximately 0.3 degrees. The lens focuses objects at a distance range of 90 mm to infinity. We could have simplified our design by using a single camera per eye. However, by using two cameras per eye we have a much higher resolution fovea than the single camera eye. We feel the increase in angular resolution significantly increases the functionality of the system at discerning fine features (faces and textures). This added functionality outweighs the additional mechanical complexity. The two images shown below were captured simultaneously from the wide-angle and foveal cameras in Cog's left eye:

    By minimizing the inertia of each eye, and using thin, flexible cables, the eyes can move quickly using small motors. Each fully assembled eye (cameras, connectors, and mounts) occupies a volume of approximately 42 mm(V) by 18 mm(H) by 88 mm(D) and weighs about 130 grams. Although significantly heavier and larger than their human counterpart, they are smaller and more lightweight than other active vision systems. To maintain an anthropomorphic appearance, the eyes were mounted in a head slightly larger than a human's.


    Moving the Eyes

    On average, the human eye performs 3 to 4 full range saccades per second. Given this goal, Cog's eye motor system is designed to perform three 120 degree pan saccades per second and three 60 degree tilt saccades per second (with 250 ms of stability in between saccades). To meet this requirement, Maxon 3.2 Watt motors with a 19.2:1 reduction were selected for the pan motors, and Maxon 2.5 Watt motors with 16.58:1 reduction were selected for the tilt motors.


    Representatives of the press who are interested in acquiring further information about the Cog project should contact Elizabeth Thomson, thomson@mit.edu, from the MIT News Office,  http://web.mit.edu/newsoffice/www/ .


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