Nowadays the technological community is concerned about the human-machine interface to increase the real device capabilities to achieve or overcome the user expectancy, as well as to increment the accurate interpretation of humans command by the machine. This study aimed at developing more understandable, efficient and reliable ways to communicate with devices. In this work, PC-based touch-screen acts as the principal Human-Machine Interface to interact with an assistive robot. This work is presented using a cognitive approach to improving understanding related to the way of thinking of the final user and give a tool to the designer to interpret this information.