Closed loop control of heliostats

Tracking control in current heliostats is performed with an open loop, without any verification that the radiation is actually arriving at the desired target. Errors due to open-loop tracking control are often around 1-2 mrad and can accumulate during operation. A significant reduction of tracking error by closing the control loop is presented. The method includes a dynamic measurement of the actual radiation incident around the receiver's aperture (spillage), detection of aiming errors, and feedback of a correction signal to the tracking algorithm. The measurement does not interfere with the receiver operation. The detection method can distinguish among different heliostats in the field, producing individual corrections to each heliostat. The closed loop control system was developed and successfully operated at the Weizmann Institute heliostat field. Both large errors and gradual drift errors were detected and corrected automatically. Resolution of the closed loop detection algorithm can reach 0.1 mrad, which is insignificant in the overall heliostat beam quality.