Social Humanoid Robot SARA: Development and Realization of the Shrug Mechanism

One of the basic requirements of socially interactive robots is that they are able to effectively communicate, verbally and non-verbally. In addition to communication via speech, non-verbal communication of the robot is of utmost importance enabling different informations to be communicated in a simple and intuitive manner as well as expressing emotions. The paper presents the development and realization of the shrug mechanism as an addition to the assortment of non-verbal communication of socially interactive robots. The realized shrug mechanism has 1 DOF and enables lifting/lowering both shoulders together. It consists of a ballscrew shaft/nut mechanism that is coupled with a six-link planar lever mechanism. Within the kinematic-dynamic analysis, the basic parameters of the lever mechanism are defined - driving force on the input link, the vertical stroke of the end point of the shoulder and the dynamic efficiency of the lever mechanism. By optimal synthesis the driving force on the input link is reduced and, at the same time, the stroke of the input link is the smallest, while the stroke of the output links is the largest. That is why the movement of shoulders shrug is fast and short-termed, which was the basic requirement for realization of a movement that largely resembles to human-like movement. In addition, the mechanism is efficient in all positions during motion because the values of the transmission angle are within the prescribed values for the lever mechanisms. Ballscrew shaft/nut mechanism, besides enabling additional torque reduction, is also low backlash which is significant for motion control. The realized shrug mechanism enables high speed of shoulder shrug for small driving torque, has low backlash and high efficiency, compact design and small mass and dimensions.