In a Power Tower solar thermal power plant, alignment and control of the heliostats constitutes one of the largest costs of
both time and money. This is especially the case in systems where individual heliostats are small (~1m2). I describe a
closed-loop control system that generates the required feedback by inducing small mechanical vibrations in the heliostat
reflector surface using piezoelectric actuators. These vibrations induce time-dependent changes in the reflected wavefront
that can be detected by photosensors surrounding the thermal receiver target. Time and frequency encoding of the
vibrations allows identification of a misaligned heliostat from among the thousands in the system. Corrections can then be
applied to bring the reflected beam onto the receiver target. This technique can, in principle, control thousands of
heliostats simultaneously.Outdoor testing of a small-scale model of this system has confirmed that such a system is
effective and can achieve milliradian tracking accuracy. If such a system were implemented in a commercial plant, it
could relax the accuracy specification required of the heliostats as well as provide an automated alignment and calibration
system. This could significantly reduce the installed cost of the heliostat field.
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