blount2000
blount2000
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April 20th, 2015 at 4:22:51 PM permalink
I'm trying to figure out the best configuration to open two doors. I have two small electric servos which will each be winding up a string on a spool to open two separate doors.

The doors (solid green lines) are hinged at the top.
The strings (dotted blue lines) can be as long as necessary and are connected to the bottom of each door.
The doors need to open upward to nearly 90 degrees (dotted green lines).
The doors will be opening from a resting closed vertical position which is gently held shut magnetically.

Long story short, I'm trying to decide where to mount the servos in relation to the location of the doors, as well as deciding where (or if?) the strings need to be "anchored/redirected" with an eye hook or pulley or something versus being attached directly to the servos.

For example, my first thought would be to mount the first servo at point "2", and then connect the first string directly to the servo at point "B" (which are both right above the bottom edge of the door when it is open).

But then, since the servos aren't real strong, I wonder if more "angled force" may be needed to initially get the door open since it is held closed with a small magnet. Which makes me think maybe mounting the first servo at point "2" with the first string connected at point "C" (using an eye hook) might be better.


The whole thing may ultimately end up being an exercise in trial & error, but are there any initial opinions on which combination of points might be the best choices for each of the servos/strings?



You serious, Clark?
OnceDear
OnceDear
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April 20th, 2015 at 4:55:49 PM permalink
Hi Blount

If I understand your drawing correctly, you are pulling those stings generally upwards and you want to know a good relative position?
They seem small doors, presumably lightweight.

Lets just think of the leftmost door. I take it that these are lift up doors?

lets say the string is pulling upwards at a point nearly above the hinge. Point 1

As you possibly realise, having the servo there will give it a hard time. It will really struggle as the door starts to open, because you are not pulling the magnet away so much as trying to scrunch up the door.. If the door ever did get moving , your string would still be doing most pulling just to scrunch the door with sod all lifting going on. That's bad.

Now, lets say the servo and pully are a long way away from the hinge. Say 5 door heights away.Point 9
Now, the string has it's best shot at breaking the attraction of the magnet. It has its best shot at moving the door, for a while.
BUT the trade off is that as the door progresses, it gets to a point where the string is mostly wasting its time in trying to stretch the door. That's bad.

So somewhere in between. I'd guestimate if your drawing is to scale, somewhere between 3 and 4

BUT, what would really make a difference is if you could make the door 'weight neutral' with some counter weights or counter springs. Arrange something that makes it that you can lift the door with a finger or two. That's how many garage doors work. It means your servo and strings can be much more light duty, maybe not much more heavy duty than would be needed to break the magnet hold.
Next time you look at an elevator with people in, remember, the engine at the top of the lift shaft is barely strong enough to lift the elevator cart, let alone a full one. It would depend on having a blooming heavy counterweight going down the side of the shaft as the elevator goes up. That's the sort of principle to adopt. that sort of thing would also mean that the speed of the door opening would tend to be more similar to when the dor is being lowered. As you have it, reversing the servo to lower the door might just drop it with a thud. but lift it with grinding agony :o)

I hope that helped.

OnceDear
B.Sc Physics with Electronics.
Psalm 25:16 Turn to me and be gracious to me, for I am lonely and afflicted. Proverbs 18:2 A fool finds no satisfaction in trying to understand, for he would rather express his own opinion.
blount2000
blount2000
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April 20th, 2015 at 6:14:45 PM permalink
Thanks OnceDear, that was indeed very helpful and you spot on described/explained my concerns (much more clearly than I was able to). My only previous experience with servos is being a participant in the crashing & complete destruction of an R/C model airplane about 20 years ago. It was a spectacular (and brief) 30-second virgin flight.

You are correct that the doors are small and lightweight. They are 8" x 8" pieces of 0.1" thick plexi-glass.

I have to admit (somewhat sheepishly) that this project is not an earth shattering endeavor.

My daughter has three cats, and only one of them is allowed to go outside. Whenever we go out of town, the one "indoor-outdoor" cat gets mighty miserable since we are not there to let it out. So I am trying to build an "air-lock" type thing to work with my existing pet door which will give me the capability to allow the one cat to go outside (without risking the possibility of the other two indoor cats getting out). I have a USB servo controller which I plan to hook up to my computer here and then operate remotely when we are out of town.

We already monitor the cats with webcams when we are gone, so this is intended to work in conjunction with that. I plan to open the first door, let the one cat in, close the first door so the other two cats can't get in there, and then open the second door so the one cat can go outside. Both plexi-glass doors open freely coming in from the outside, so the outdoor cat can just come back inside when it pleases (which is the current situation with the single existing pet door).

Thanks again for taking the time to answer my question so thoroughly. I'll try the servo placement you suggested, and hopefully I can make it work without needing a counter weight.

My daughter (and her cats) send their appreciation as well. :)

You serious, Clark?
beachbumbabs
beachbumbabs
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April 20th, 2015 at 7:00:39 PM permalink
I would do it like this x2 doors.



The long diagonal string would be taut when the door is closed, so when the servo starts winding, you have an angle on pulling the lip away from the magnet. As the pulley is retracted, the attachment line gets closer and closer to the vertical. When the winding reaches the stirrup, the door is horizontal. If it's a one-way winder motor, releasing it should allow the door + pulley weight to close it firmly. Or a 2 way winder can let it down gently until the magnet captures it.

The pulley could be just a slick metal or plastic bar for the string to slide over, but that would lend itself more to twisting during retraction/extension. A channeled pulley would be best.
If the House lost every hand, they wouldn't deal the game.
Dalex64
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April 20th, 2015 at 7:18:12 PM permalink
Howabout a rotating cylinder?

It could have an opening at each end, off center, such that when one opening is down, the other is up. You could have fixed panels at either end with openings that line up with the holes at the end of the cylinder only when the hole has rotated into the down position.

Then you only need an on/off motor that spins one way.

Make sure the cylinder is longer than the cat. Don't want to pinch a tail. Unfortunately that will leave room for more than one cat.


In any design you'll have the problem of first training the cats to enter a space with no open exit, then keeping two cats from squeezing in at the same time. Believe me, they can.


When I first saw this question by the title I thought it might involve me picking a door to win a prize.
Dieter
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Dieter
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April 20th, 2015 at 7:37:24 PM permalink
You said servos, not motors. They should be able to apply force in both directions, if I understand correctly.

My instinct says that the servo should drive a drum, the cable (string) should wrap a few times around the drum, one end should go to the door, and one end should go to a counterweight.

The servo then doesn't have to provide all the force, the force to move the door.

This is sort of how elevators work.
May the cards fall in your favor.
blount2000
blount2000
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April 21st, 2015 at 6:53:45 AM permalink
Thanks for the additional replies/ideas!

I had thought of doing something similar to the concept of the rotating drum with offset access points where only one exit would be available at any given time. My thought was to have the plexi-glass doors be situated in "guide tracks" and always remain totally vertical. They would then be raised/lowered straight up and down sort of like a floodgate (versus swinging outward on a hinge). But then I couldn't figure out exactly how the cat was going to get back in the house by himself whenever he was ready to come inside.

Anyway, the project is coming along slowly but surely. I decided to make the upper half of the "corridor" out of pvc pipe so we can see the cat go through on the webcam and make sure he isn't stuck in there. Plus, hopefully the openness of the top half will make the cat more comfortable with the idea of entering a big ol' box!


You serious, Clark?
Dalex64
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April 21st, 2015 at 7:59:28 AM permalink
I'm not sure how much the cat would enjoy being rotated in my drum idea, either.

with what you have, you could put the "ends" of the cylinder over the openings, with a long axle running along the top of your box.

you could then cut down the cylindar ends to look more like triangular hand-fans, or pie-slice-shapes. have them offset from eachother at 90 degrees. just rotate the axle 90 degrees left and right to expose one opening at a time.

with an automatic cat re-entry, possibly through detection via a pressure switch or electronic eye, you aren't worried about the cat bringing presents into the house for you and the other cats?
OnceDear
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April 21st, 2015 at 10:11:14 AM permalink
I liked BBB's design a lot.

Meanwhile, I know rthe build has progressed but if it was intention that one door opens as the other one closes, might there be a case for connecting the doors together such that one counterweights the other? Guilotine doors might do for that?
Psalm 25:16 Turn to me and be gracious to me, for I am lonely and afflicted. Proverbs 18:2 A fool finds no satisfaction in trying to understand, for he would rather express his own opinion.
blount2000
blount2000
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April 21st, 2015 at 2:14:28 PM permalink
Quote: Dalex64

I'm not sure how much the cat would enjoy being rotated in my drum idea, either.

with what you have, you could put the "ends" of the cylinder over the openings, with a long axle running along the top of your box.

you could then cut down the cylindar ends to look more like triangular hand-fans, or pie-slice-shapes. have them offset from eachother at 90 degrees. just rotate the axle 90 degrees left and right to expose one opening at a time.

with an automatic cat re-entry, possibly through detection via a pressure switch or electronic eye, you aren't worried about the cat bringing presents into the house for you and the other cats?


We've been lucky so far, and the only thing the cat has brought in through the existing pet window was a live bird which he released into the living room. He's fairly lazy, so I'm hoping he doesn't change his ways and start catching & bringing a bunch of stuff inside.


Quote: OnceDear

I liked BBB's design a lot.

Meanwhile, I know the build has progressed but if it was intention that one door opens as the other one closes, might there be a case for connecting the doors together such that one counterweights the other? Guilotine doors might do for that?


I'm hopeful that the two separate doors with individual servos will give me enough control to make certain that both doors are closed/secure once the cat is inside the corridor before opening the one door to the outside. With my luck (and building skills), I'm afraid I'd somehow get both doors stuck in a halfway open position if they were connected!
You serious, Clark?
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