I. Introduction
In the last years, robots are becoming a significant part in our society, providing a valuable assistance in a variety of fields. Some examples are their use for taking care of elder people ([1], [26]), assisting in surgery ([12], [24]), education ([17], [20]), entertainment applications ([21], [9]), etc. However, all these are only sporadically found in practical applications due to the difficulties and constraints that entail the inclusion of robots in the daily life.