Bill Hawkins wrote: > Tom Van Baak wrote, > > "2) Instead of a fixed base, gnomon, and slowly moving shadow like > almost all sundials, you put a stepper or servo motor/encoder on the > base. Then place matched photodiodes on either side of the gnomon and > steer the whole sundial for constant *minimum* shadow. In real-time, a > > The scheme probably needs three photocells to be sure that the one > in the middle is darker than the others. Might be able to mask it > with a slit and use a fine wire gnomon, in a coarse/fine servo. > Could use a variable frequency motor and precision reduction, like > a phonograph turntable only much slower.
Bill, Back in the good old days before CCD arrays, people in the astronomy business used quadrant detectors for this sort of gizmo. A quadrant detector is a 2x2 silicon photodiode array. When the bright spot is in the middle, then the current through all four diodes is equal. When the object is off-center, the current is unbalanced. You can make a tracking servo using this detector that's entirely analog - no programming skills required! Of course, driving the alt-az mount requires derotating the detector array relative to the mount's alt-az axes. _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
