The human face is equipped with a complex physical structure and it has more than 100 muscles situated between skin surface and skull with very different shapes and functionality. They allow us to control even minimal muscular movements and to generate a myriad of different facial expressions.
The positions of the servo motors try to reflect the ones of the facial muscles to simulate expressions in a very realistic way. Servo motors actuates the skin along vectors corresponding to the natural muscle actions of the face, enabling animation control in an intuitive manner according to Ekman’s FACS (Facial Action Coding System). Paul Ekman developed the FACS as comprehensive language to code facial expressions in terms of atomic muscle movements, named facial action units (AUs). A single AU includes more than one muscle because the changes in appearance the muscles produced could not be distinguished. During the execution of an expression, the active AUs can be seen as estimates of the underlying muscle activations that caused the performed expression.
However there is an important difference in the way the human muscles and the robot servo motors actuate the face. In contrast to human muscles, the robot servos can produce only linear contractions. Human orbicular muscles, like the Orbicularis oculi and the Orbicularis oris, produce circular contractions. The movement of this kind of muscles is reproduced using more than one servo motor in the most realistic way as possible.