These motors run at a nearly constant rate. ALl you have to do to know position is to keep track of the time they are on. It remains calibated because at the end of every pass (it ends at zero elevation) so you konw you are at 0 (include say a 10% overshoot after the pass to be sure it is at 0).
Same for Azimuth. Each time you find yourself within 45 degrees of the mechanical stop at south (or north), after the pass, send the rotator to the stop with again, 10% overshoot, this re-establishes that reference.
So, no feedback needed (for the beamwidths of VHF and UHF anyway). Oh, and you dont need elevation at all really for all LEO satellites. See http://aprs.org/rotator1.html
Bob, WB4APR
On Fri, Aug 9, 2013 at 6:00 AM, Robert C. Campbell kb3pmr@gmail.com wrote:
I am using reed switches for both my AZ and EL visual feed back on my newest build. I have had to slow the motors down with a PWM so that the reeds don't skip a count. Both AZ and EL are still within the speed of keeping up with ISS. You can determine the ratio of motor turns to one revolution of the AZ full 360 or 90/180 for the EL and use that ratio to determine the position in degrees. Repeat-ability seems very good. If you are going visual/manual positioning you can check on utube or instructables.com and find a easy build for a modified $ 2.00 Dollar General calculator that then becomes a pulse counter by simply connecting the enter button to your reed and entering the ratio and pressing the plus sign. Then each rotation the calculator will tell you the degrees the antenna is looking at. Yes, it does need reset after each pass, and yes, if you run by the bird and go back the calculator will give you some extra counts since it is counting reed switch contacts not true degrees of indication. Purchase 5 of them, one for practice, 2 for EL and 2 for AL then if you start from the same standard position each time one can add and one can subtract. This should work fine assuming that the driver maintains mental focus. It is a small fun project and cheaper then happy hour at the local pub. The Pulse Width Modulator works good with this because you can adjust the speed of the motors to keep up with the bird. This of course assumes you are working with direct current motors.
Bob _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb