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Simple question regarding 2 axis movement

  1. Feb 3, 2008 #1
    Hi there,

    I'm an electrical engineering student currently building a 2 axis solar tracker. I need to build the frame and I'm posting here because I have no mechanical knowledge at all ...

    The system is basically two motors, each for each axis which rotates a small solar panel (one rotates the panel up and down, the other rotates it left and right).
    Example of how it would operate : http://youtube.com/watch?v=ayjVNouuIqQ

    I need some ideas from you guys on how to have this kind of movement, ie. the arrangement of the motor, gear, pulley, etc. If you have any ideas or anything that I could Google to learn about, please leave a comment.

    I was thinking maybe have a round base with a pulley for left/right. Not so sure yet about the up/down movement.

    Thanks in advance.

  2. jcsd
  3. Feb 4, 2008 #2


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    Welcome to PF, Mel.
    What sort of linkage between the motors and the pivot points is irrelevant (pulleys, gears, cranks, whatever). All that you need to do is have one entire system mounted on top of the other. For instance, if you use a turntable for your lateral movement, mount the whole elevation mechanism on it. No matter which way it's facing, you can tilt it however you want to. As an example of that, think of the gun/turret assembly on a tank.
  4. Feb 4, 2008 #3
    Thanks for the reply, Danger.

    What I want to ask is how do I actually build the elevation movement system and lateral movement system ? As I've said I have no mechanical knowledge, so I'm hoping you guys can give me a guide or point me in the right direction.

  5. Feb 4, 2008 #4


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  6. Feb 4, 2008 #5


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    It's unfortunate that the sun doesn't follow the same path every day. If it did, you could just use a turntable with an elevation cam built into it. (Keep that in mind if you ever need something with a constant, uniform path.)
    I agree with Fred that lead screws (screwjacks--I've never heard of a 'ball screw', but it appears to be the same thing) are probably the easiest and most accurate way to go. They can be strong enough for things like industrial or automotive jacks, or accurate enough for use on optical benches (difference in scale; they can't be both). You could also use a worm gear driving a gear sector. You don't need 360 degree movement for either axis, since you'll only be using it in the daytime. Horizon-to-horizon is sufficient, and it can be manually reset to the starting point each time.
    As for the actual tracking aspect, you could input a trajectory each day. My preference, though, would be to include at least three sun-tracking sensors (photodetectors in the back of narrow tubes, for instance) with some sort of comparitor circuits to keep the system oriented so that the light impinging all three sensors is equal. I don't know any electronics, so I'll leave that up to others.
  7. Feb 4, 2008 #6

    So far, I've pretty much got the electronics part down (sensors,etc), only left the mechanical aspect of it.
    Yeah, it wont be 360 degree movement, and it'll move to the starting point each night. I'll probably have limit switches at the end.

    Can you elaborate a bit on 'worm gear driving a gear sector' ?
    And also how limit switches can be integrated into lead screws or worm gears ?

    Thanks for the info.
  8. Feb 4, 2008 #7

    I'm think thats not the type of movement I'm talking about ..? That is flat, I need a kinda rounded movement ? I hope you can get what I'm trying to say ...

    Another example:
    This one is a 1 axis. Can anyone explain how that is done ?
  9. Feb 4, 2008 #8


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    A worm gear and sector simply means that rather than a full-diameter driven gear, you only need part of one (think of a piece of pie rather than the whole pie). The worm itself is sort of like a screw thread, but the profile and pitch match the teeth of the sector. The gear ratio is determined by how many complete revolutions the worm has to make for a whole gear toothed like the sector to go around once, and can be very high. I can try to cobble up a rough sketch in 'Paint' if necessary; I don't have access to any real software since the wife spilled her coffee in my iBook. :grumpy:

    I can't see the drive mechanism in your video, but it would appear to be a simple spur-gear train that rotates the axle. The thing that the guy keeps covering with his hand is definitely a light sensor of some sort, but I don't know about the probe attached to the panel. It might be another sensor that's part of a comparitor system.
  10. Feb 5, 2008 #9
    In the picture below, is the drive mechanism a spur gear ? I attached a rough sketch below. Is this how it works ? If not, can you maybe sketch roughly how the drive works ?

    Is the main difference between spur and worm drive is that for spur the output shaft is parallel to the input, and for worm drive it is at a 90 degree angle from the input ?

    Sorry for the bad drawing !

    Attached Files:

  11. Feb 5, 2008 #10


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    No problem with the drawing; I just can't compare it to the photo because of the casing over the mechanism in the latter. I don't know how many gears in what configuration might be hidden there. Your picture is a perfectly valid interpretation.
    Oh, crap! I just realized that I have to get to work post haste. I'll log back on after I've done my start-up routine (an hour or more, depending upon the presence of my boss). I'll address the limit switch matter then, as well, since I forgot to last night.
  12. Feb 5, 2008 #11


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    Well... that was easy. Got to work, phoned the boss and told her to stay home, and here I am. I love my job.
    Anyhow, as to the limit switch situation... unlimited (pardon the expresion) options. Whatever type of switch you want to use, it has to be a Normally Closed type. You can use a push-button, lever, magnetic reed, whatever turns your crank. Just position it so that your travelling piece trips it at the end of its movement to shut off the drive motor. Also make sure that it's a momentary contact type, so that it will return to Closed after the piece lets it go.
  13. Feb 6, 2008 #12
    might be a bit late

    This may be a bit too late to help now but i might have a way to do this quite easily. why not try using stepper motors? the circuitry is simple for them, and they are pretty strong too. i wasnt able to see how big a solar panel you are talking about moving, but if its not huge you should be able to just attach the panel, or whatever is holding the panel, straight to the shaft of the motor. this would also work for the side to side rotation as well. the motors would need driver chips though. i could be wrong about these things, i have down a lot of programming and building with stepper motors using quick basic, a printer port, and an old laptop. you wouldnt need a stop sensor if you did things this way because you would already have a huge amount of control over the motors. of course programming the tracks that it follows would probably be a bit tedious using qbasic alone.

    *edit* just re-read your post and noticed that you said you already have all the electronics down just need the mechanics. the stepper motors would probably require you to modify your circuit setup i am unsure since i dont know what your setup is right now. if you want to pursue this idea i can try to help you out. i know enough about the circuitry for stepper motors and the programming for them to rotate a light bulb for an x axis and rotate a pen holder for a y. basically i can draw on lightbulbs with a good amount of precision. if i dont know enough i have a contact who could probably help as well.
    Last edited: Feb 6, 2008
  14. Feb 6, 2008 #13


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    Of course a really clever solution would use a polar mount and an elliptical cam on the declination axis with a mechanism that advanced it very slightly with each days rotation so it followed the changing path of the sun.
    Then you would only need one motor - and only need to reset the declination once/twice a year.
  15. Feb 6, 2008 #14
    i get the feeling that if i knew what all those things were i would be concurring.
  16. Feb 6, 2008 #15


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    It's basically the same as a telescope mount, there are two obvious way of doing it.

    The modern way is an Alt-Az, as you have drawn, one motor tracks in azimuth and another in altitude. But to accurately follow the curve of the sun (or star) path accross the sky you have to move both motors at continually changing speeds. Up to the 70s there wasn't the computing power to do this!

    Earlier telescopes used a polar mount where the base axis is tilted toward the pole and the other axis tips in a fork mounted on the first. The advantage of this is that you set the updown tilt (called declination) to point at the sun and then drive the base at 1rev/24hours. This was much easier to do with clockwork or falling weights before the invention of electricity.

    Every day the declination (the height in the sky) of the sun changes slightly.
    There were some amazing gear based analog computer type mechanisms built to automatically adjust telescopes to track the sun each day by slowing advancing one axis each day.
  17. Feb 11, 2008 #16

    Hey Danger, sorry for the late reply. I didn't have Internet access these past few days and couldn't reply. I did read your reply on Wednesday though, using my cellphone ..

    OK, so I will probably use that basic spur gear system, since it seems like the simplest.
    What about for the azimuth/horizontal axis ? What type of arrangement would be the most efficient ?

    Thanks for the info on the limit switches, now I roughly know what to do. I'll think about the specifics once I get the frame/gears up.

    Hi Josh, no don't worry, you're not too late to give your suggestions :)
    Actually, I wanted to use steppers in the beginning. However, because higher current is required and the motor and its driver is a bit more expensive, I decided to go with DC motors instead.
    Thanks for the offer though.

    Hi mgb_phys. Is this type of mount simpler than the Alt-Az method ? Can you elaborate on this and maybe post a diagram that I can take a look at ?

    guys, what is the smallest size (meaning the voltage/power ratings) of DC motor do you think I can use to rotate an approx 0.8kg panel, taking into account the spur gears ?

    Thank you all for your help.
  18. Feb 11, 2008 #17


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    An equitorial mount is simpler to build and much simpler to drive - but isn't as compact as an alt-az. If you only need to roughly track the sun then the electronics and software for an alt-az are simple, but you do need some sort of micro to calculate the rates.
    An equitorail only needs one motor running at constant rate to track the sun.

    There is an animation here http://science.howstuffworks.com/telescope5.htm

    Given the very low speeds I would use a worm gear and a very low power motor. You will probably have to experiment with the power, it depends on the balance of the mount and the friction of the bearings.
    The bearings on a large telescopes are so good it is possible to push a 250t telescope by hand!
  19. Feb 11, 2008 #18
    hey man, glad the suggestion wasnt too late. i see what you mean about the cost and the current though. good luck with your project man.
  20. Feb 11, 2008 #19


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    Well, this is a pretty slow day at work, so I decided to kill some time sketching up the kind of thing that I was talking about. I absolutely detest 'Paint', but it's all that I have available now. Notice that I didn't put a lot of effort into making it neat or even accurate. For clarity (and out of laziness), I've eliminated all support brackets, bearings, sensors, and most wiring from the picture. It should give you a rough idea of what I meant, though. The only problem is, I can't resize it larger because my Photoshop is also in the dead laptop. This is as big as 'Paint' will let me go. You'll need a magnifying glass. :grumpy:
    Last edited: Feb 11, 2008
  21. Feb 11, 2008 #20


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    This is clever - it uses the suns heat to evaporate a liquid on one side of the mount which then condenses on the other side and the weight swings the panel around to the follow the sun, all without any power/computers/drives.

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