# Observatory Drawbridge Shutter--linear motors

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

1. ### W. eWatsonGuest

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

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. ### ehsjrGuest

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. eWatsonGuest

....
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. ### ehsjrGuest

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

Ed

5. ### Bob EngelhardtGuest

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. eWatsonGuest

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. eWatsonGuest

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. ### ehsjrGuest

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