I used strong geared motors and four solar cells (12V, 100mA) for the control device. The system tracks according to the amount of current flowing over the single solar cells. If the sun moves the aluminium sheet casts shade on one solar cell. There are then different amounts of current flowing over the cells and the whole device rotates until the solar tracker is parallel to the sun.
This systems has to be improved because it’s not perfect, but I hope you get the idea.