Once again the IR sensors are useful to perform another important task. In this case they are used in an opposite way: all it's fine when the sensors reveal an object very close (the floor). If one or more sensors don't see anything it means that this is a non safe path and the robot travels backward.
There are 4 sensors along the circumference to be sure that they can discover a possible fall down even if encountered on a side while turning.
As already seen for other components of the robot, the sensors are custom tailored to perfectly fit in the base slots.
A transmitter LED sends an infrared beam. The beam bounces to the floor and go back to the receiver beside separated by a screen to avoid direct interference. The signals are elaborated by the controller to decide how to drive the motors for the safest path.
Another safety system is obtained by checking if all the wheels are in contact with the floor...
... With a mechanical switch on the traction wheels...
... And with an optical switch on the swivel castor.
update on 30-12-2015

Safety

How to avoid damages to robot and humans