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Observatory Drawbridge Shutter--linear motors

Discussion in 'Electronic Basics' started by W. eWatson, Oct 26, 2012.

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  1. W. eWatson

    W. eWatson Guest

    I'm a bit further along with upgrading my observatory with electronics
    with a control system to rotate the dome, and raise and lower the
    shutter. I farmed out the PC board work to someone who has put relays,
    soft start controls, and other devices in the two NEMA enclosures. It'll
    probably take a month to get the boxes back. While waiting for that to
    happen, I've decided to take a shot a the lower shutter.

    As I probably mentioned here some time ago, there is a lower and upper
    shutter. The lower shutter is about 33"x32", and has a glass window in
    it. My guess it the upper shutter is 12'x3'. The shutters are on two
    tracks, and roll along with wheels. Latches between the two can couple
    them together, so they both move up and down together. If not latched,
    the lower shutter sits on the apron (the circular structure on which the
    dome sits) of the dome while the upper one moves up/down.

    The lower shutter was a mistake, when I first built the obs. It is quite
    unconventional. There may be 12 in the USA. As I quickly discovered,
    taking images through the window resulted in slightly distorted images.
    Not good. I had checked before buying the window shutter with five
    people who owned such a shutter. (It has something of a reasonable
    purpose for use in winter.) They all gave it thumbs up. I never asked
    what imaging through the glass produced good images. Nevertheless, the
    obs was still functional w/o the use of the window. The shutter weighs
    about 120-90 pounds.

    A typical lower shutter is like a drawbridge. That is it pivots on the
    apron, and swings in or out with, in some cases, a hand winch, pulleys
    and wires. It only needs to be positioned 10-15 degrees above the
    horizon. Looking below that the sky is avoided because of the
    atmospheric thickness. What I want is to take the current lower shutter
    and make it look like a drawbridge. I plan to hire someone to do this.
    However, my enclosure-box guy thinks I can operate with a linear motor.
    He suggests using it on the apron and centered in the shutter. He's
    short on details and working on another project now until he can get
    back to mine. So my question is how is a linear motor going to help in
    this situation. Why in the middle??? I haven't a clue.
     
  2. ehsjr

    ehsjr Guest

    See the following url for some desirable features of linear motors:
    http://www.baldor.com/products/linear_motors.asp

    Centered in the "drawbridge" avoids twisting torque that would
    result from the weight of the "drawbridge" if one corner is
    supported by the motor and the other is not.

    Ed
     
  3. W. eWatson

    W. eWatson Guest

    ....
    Probably eight months I bought a linear motor thinking it would do the
    job for pulling a chain down to detach the two shutters. It was
    purchased way before the drawbridge idea took hold. I purchased a
    Duff-Norton LT50-1". About $100. Months later I realized I didn't need
    it, and called them. They OKed the idea of an exchange at some point.

    In any case, looking at the link you gave indicates there are lots of
    linear motor types. That doesn't bother me now. What does is placing the
    motor in the center. (Yes, using one linear motor on a side would cause
    torque.)

    It would seem to me placing it in the center and adding a cable and
    maybe a pulley at the top of the lower shutter might somehow do the
    push-pull job. Still that makes no sense though. Where does one get the
    push-pull action? The shutter is at maybe a 85 degree angle, so how does
    the motor help push the shutter from the top, and how does it get the
    shutter back in its resting place by pulling?
     
  4. ehsjr

    ehsjr Guest

    If the shutter weighs ~120 pounds, why do you need anything
    mechanical to pull (or push) it down?

    Ed
     
  5. So, why don't you do that (have them latched & move together)?

    And ... 120# for a 33" x 32" shutter is really heavy. Does it really
    weigh that much?
     
  6. W. eWatson

    W. eWatson Guest

    That's pretty heavy in my book. In any case, it looks like the fellow
    who suggested the linear motor is going to come through with a sketch of
    how it works.
     
  7. W. eWatson

    W. eWatson Guest

    It's hard to tell. The glass window weighs 20 pounds. There's a lot of
    metal in the frame. The mfger doesn't even know how much it weighs. I
    may know in the next week or so, when we pull it out to rework it. I
    hope the weather holds out. We are getting into the rainy season.
     
  8. ehsjr

    ehsjr Guest


    Seemed heavy to me too, but I accepted the numbers you posted:
    "The shutter weighs about 120-90 pounds. "
    Great! I hope that will give you a clear picture of its
    operation. :)

    Ed
     
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