The last test concerns the interception of a hole on the ground of bout 20 cm deep, in front of the right foot, condition that is very similar to a step down. In Fig. 15 we show that only the right infrared sensor intercept the obstacle and advise the user of the presence of the step. The user can understand that the obstacle is surmountable because the vibrating signal increase and decrease very quickly and never arrive to the maximum value of vibration