In conclusion of this section, we show in Fig. 7 a sample of the data recorded (upper panel) and the noise of the ultrasonic sensor(lower panel). As for the infrared sensor, we placed the ultrasonic sensor on a tripod with a distance of 1 m from the wall, using a sample frequency of 50 ms. The distribution is symmetric with respect to the zero and is bimodal. The noise does not influence the system because it falls completely in the resolution range of the sensor.