Optimal Synthesis of the Worm-Lever Mechanism for Humanoid Robots Shrug

Abstract

Emotions represent a significant means of nonverbal communication and their expression represents an important aspect of social robots functionality. There are two basic ways of expressing emotions. The first one is based on facial expressions that can be realized by moving a particular part of face or by displaying a picture on the screen that represents a face with characteristic features such as eyebrows, eyes, nose and mouth. Combining them is also possible. The second way of nonverbal communication is based on gestures, especially using arms. This paper presents an optimal synthesis of shrug mechanism for humanoid robots. Based on the set requirements the worm-lever mechanism is proposed. It has 1 DOF and enables simultaneous shrug of both shoulders. It consists of a worm which is meshed with two worm gears having different directions of rotation and two four-bar lever mechanisms whose input links are rigidly connected to the worm gears. Based on the kinematic-dynamic analysis the dynamic model is formed, the objective function is defined, the constraints are prescribed and the optimal synthesis is performed. The maximum torque on the input link of the lever mechanism, the driving torque of the complete worm-lever mechanism, the range of transmission angle and the rotation range of the worm gears are determined. The lever mechanism has high efficiency in all positions because the transmission angle has a high value during the whole movement. The worm mechanism enables a significant reduction of driving torque and has acceptable efficiency. The rotation range of the worm gear is small – the mechanism movement is very quick and therefore the shrug speed is large, which was the basic requirement for realization.