Author Topic: Reflection drift in late afternoon/evening  (Read 987 times)

alobo

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Reflection drift in late afternoon/evening
« on: June 05, 2015, 04:58:41 AM »
Hey Gabriel,

I've put together a heliostat using 12V 28BYJ-48 steppers. It's not entirely accurate as things go, but acceptable - a full degree of looseness in the azimuth, for example. But this is a small project.

Anyway, it seems to track the sun fairly well; in heliostat mode, I have it configured to reflect at a target directly due south, on the horizon (zero degrees in both axes). It seems to track fine in the mornings, but some time after noon and progressing into the evening the reflection drifts slowly upwards as the sun goes down. The gear ratios should be identical for both axes, and there doesn't seem to be a similar issue in the azimuth - it's loose, but the reflection stays more or less in the same location.

Any ideas? It could be some friction in the design of the altitude motor, preventing it from moving accurately, I guess, but if there are any other causes it would be good to rule them out as well.

Peace!


Gabriel

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Re: Reflection drift in late afternoon/evening
« Reply #1 on: June 05, 2015, 05:32:54 AM »
Hi alobo,

It's just a thought, but you might try commenting out this line of code located in the void loop.
SunsAltitude = SunsAltitude + (1.02/tan((SunsAltitude + 10.3/(SunsAltitude + 5.11)) * pi/180.0))/60.0;//Refraction Compensation: Meeus Pg. 105
Although, if that is the problem, it seems like it would give you the same problem in the morning. It's worth a shot though. It occurs to me that I can't really test that code in reality since my place is shaded either early in the morning or late in the evening.

One other thing you can try is to reset the machine after it has drifted some to see if the reflection goes back to where it is supposed to be after it resets. If it does, you know that the stepper motors are probably losing steps.

One final idea is to experiment with the manual control through the serial monitor in sun tracking mode. That way you can manually type in different angles and measure to see if the machine is actually moving to them. It's a bit difficult to do this accurately, but it might do the trick.

Thanks!
Gabriel


alobo

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Re: Reflection drift in late afternoon/evening
« Reply #2 on: June 11, 2015, 02:39:54 AM »
Hey Gabriel, thanks for the suggestions!

Being in India, the monsoon season is just about to begin and sunlight has been (and will be) sporadic at best. I'll have to wait to try this out.

Gabriel

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Re: Reflection drift in late afternoon/evening
« Reply #3 on: June 12, 2015, 06:10:43 PM »
Ha ha, yeah I feel your pain. Let me know how it goes. Thanks!

alobo

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Re: Reflection drift in late afternoon/evening
« Reply #4 on: July 17, 2015, 09:20:52 PM »
So we finally had a few days of somewhat reliable sun. It seems the reflection drifting was because of a combination of
1) incorrect north-south alignment, and
2) the altitude motor missing steps.

If the sun comes out some more I'll try it with a fresh motor and double-checked wiring.

None of the nearby buildings are in line with any cardinal directions. I used a nearby landmark that looked to be in the right place on Google Maps but it seems it was nearly 5 degrees off. Using the direction-sensor on my smartphone only gives me accuracy to within one degree. That was the issue I faced, anyway, I'm sure others have their own workarounds/solutions. I finally used what I should have done at first, and measured the shortest shadow of the day on a pole, and lined up with that. (luckily the sun was out long enough to measure the shadow).