Two sensors are installed at the service robot, one on each side of the guiding wire. When the robot is set so that the guide wires are directly between the two sensors, the measured magnetic field is the same for both coils. If the robot moves to one side or another, or if it changes direc‐ tion of the leading wire, the intensity of the magnetic fields of both sensors will become uneven. This difference of magnetic fields is used to control the motor to guide the robot, which performs certain changes of direction until the signals from the two sensors are re‐aligned. When using coloured tapes on the floor to define paths, then service robots use a system of optical sensors capable of tracking colour. The tapes can be glued or painted, or a spray can be used to indicate the path.