The LDR sensors are placed in the four corners of the PV panel and are put in dark tubes with a small hole on the top to detect the illumination of the sun. The shading resulting from the dark tubes will create a light intensity differential on LDR sensors, i.e. sensors point different angles. The dark tubes are also considered a concentrator of radiation and are used to increase the solar tracker robustness.
The solar tracker above has been tested under a real environment (exposed to the sun) and it is able to instantly track the sun movement.