I need to hoist a flag about 7 meters high for a sailing club (automated start sequence).
There is a flag pole on site similar to this one:
Possible solutions:
DC motor timing belt with guide rails and slider blocks.
DC motor with spool (winch).
Threaded rod
Gears and chain
Rope and pulleys
Pneumatic linear actuator
Requirements:
Easy to assemble / maintain.
Weatherproof (extreme sun / rain).
Able to be stored away when not in use.
No fixed components out of reach high up on flag pole.
Reliable.
Cheap.
12V.
A 10 meter timing belt costs $135 USD and 14 meters of belt may stretch over time and cause significant strain on DC motor axle bearings. Timing belts are used typically for lengths up to 1 meter or so. Chains rust. Pulley and rope is unreliable. I like the idea of pushing a piston in a cylinder (or pipe) using air and a very small compressor/solenoid, but it would be impossible to attach a flag to the piston inside the cylinder. DC motor requires limit switches and its own power supply (or noise filtering), stepper motor is slower but does not require limit switches.
On the bright side, this application does not require precision. Slider blocks could simply be stainless steel wire with a wooden / PVC pipe sliding along. I prefer a continuous belt concept over a treaded rod or winch/spool type of concept as it is (arguably) more reliable.
To lower a flag the most basic way would be to attach a weight to the bottom of the flag and have gravity pull it down.
I can't think of any real life situation where one would encounter a device that does fast linear motion over 10 meters or so similar to hoisting a flag...
ISAF flag start sequence:
5 minutes to GO - flag 1 hoisted (fleet flag)
4 minutes to GO - flag 2 hoisted (prepatory flag)
3 minutes to GO - No action
2 minutes to GO - No action
1 minute to GO - flag 2 lowered (prepatory flag)
GO - flag 1 lowered (fleet flag)
7 meter galvanised steel RHS (rectangular half section) square pipe.
mount a (larger diameter) pulley for a timing belt in the top.
run timing belt along the outside of the pipe.
clamp DC motor / stepper motor to base of pipe and use a second pulley to drive timing belt.
(- hang the whole contraption with a hook off the flag pole for easy removal).
Thinking of funfair rides I would think air, piston and gravity would make for a better concept than any belt though...
Or perhaps have the motor climb itself up along the square pipe using rollerblade wheels...it would look a bit like how an elevator cage hangs in a frame.
...or perhaps instead of hoisting/lowering a flag 7 meters, use a container ("chute") to hide the flag when lowered. This would reduce the hoisting/lowering distance to 1 to 2 meters or so(?)
Inner pipe: 4 meter long round galvanized steel post.
Outer pipe: 4 meter long round PVC pipe as outer sleeve.
A cylinder like this fully extended could cover 7 meters.
For hoisting a flag, if using a continuous air supply, the seals do not have to be airtight as the air just needs to push the pipe up and keep it there for 5 minutes max.
Drawbacks:
The red end stops should be located 1 meter from the top; without a significant overlap the PVC sleeve would come apart from the inner pipe.
A flag could never be lowered to the ground using a piston/cylinder due to the cylinder height (4 meters).
Solution:
Hide the cylinder and flag in the lowered state behind a plank or board (or inside a container or 'chute' as mentioned earlier), similar to a hand puppet show:
This way, the travel distance only needs to cover the height of the flag! You can mount this entire contraption in a high and visible spot on a flag pole if need be.
I suspect the truth of the matter is that flagpoles use cord and pulleys because these are the simplest and thus most reliable system. Yes, to the the extent that they are unreliable, they are nevertheless less unreliable than long sliders, belts which only work as long as they run correctly on the pulleys and worst of all - lightweight pneumatic systems with sliding seals.
There is a need to "furl" the flag and perhaps, protect it. Mounting such a thing partway up the mast is going to be absurdly ugly, particularly more than one.
I would suggest a horizontal housing at the bottom for the flags with a (smooth) roller at the top of its opening so that the cord - and flag - rolls under this and into the (capacious) housing. The winch is at the far end of the housing but the cord is a continuous loop, only passing round the drum a few times, tensioned by a weight on a pulley. Three flags, three winches, three loops, one housing.
You do not need limit switches as such - neither car windows (AFAIK) nor garage doors use them. They sense the motor current, and since you propose to use an intelligent (Arduino) controller, you use algorithms to automatically "unstick" things by reversing and trying a few times if operations do not go to plan.
No, I specified (a little more complex) that the wire/ cord rolls over a roller and horizontally into the housing which affords some shelter, and that the tension weight pull on another pulley (at the bottom, also need a pulley or two to take it out of the housing; think of a "T" on its side for the housing).
The entry to the housing can be made a little narrow so that the flags are quite inaccessible when stowed.
I omitted to mention having a "stop" (either a cable buckle or even just a complex knot) either side of the flag, one to jam at the top pulley and the other to jam at the guide just before the winch, to stall the motor and trip the current sense. But as I said, the electronics should "know" how long - maximum and minimum - it takes to hoist or furl and respond accordingly to fault conditions.
I would just go with a rope looped around a pulley at the base of the flagpole. It's all simple and well-tried technology and there's no reason why it wouldn't be reliable. If there's a problem, the rope is right there for you to operate by hand. This is something that ought to be easy to test in prototype form without any electronic development, just using a DC motor, gearbox and pulley connected manually to a battery so you can see the thing going up and down. Subsequently I'd envisage a full solution being loosely attached to the base of the flagpole and designed so that it could be suspended on the rope to maintain a constant tension, with limit switches triggered by something attached to the rope. This self-tensioning design also means that you can take the tension off the rope and pull it through manually in an emergency without having to dismantle anything.
I like the idea of pushing a piston in a cylinder (or pipe) using air and a very small compressor/solenoid, but it would be impossible to attach a flag to the piston inside the cylinder.
Attach a steel rope to the top and bottom of your piston, forming a loop. Have the bottom of the rope pass through the bottom of the cylinder with a greased packing gland to create (mostly) airtight chamber. Guide the rope loop outside the cylinder with pulleys and remove all slack.
Now, apply pressure to the lower cylinder chamber, piston goes up, flag attached to rope loop goes down. Release pressure, gravity pulls piston down, flag goes up. If you want the flag to raise faster, apply a vacuum to the lower chamber.
If you want the flag to raise quickly without vacuum, turn the whole thing upside down (though you'll need to run the air pressure hose to the top if your pole).
Club has decided they want to stick with lights (which is an approximation/interpretation of the rules) rather than flags. Video of prototype can be made available if of interest...it is all a bit depending on whether or not club wants to go ahead with my proof of concept(s) before spending $$$.
Thanks to all for your suggestions; much appreciated!
Yes, you basically understood my idea except that I was thinking of having the rope pass round the drum (a couple of times) and back out of the housing, over another pulley and around a pulley attached to the tension weight.
The rope could be pulled manually if necessary, and the weight would not need to move up and down over any great range.
I was envisioning the driven pulley and the bottom pulley being the same thing, with the drive mechanism actually suspended off the rope to provide the tension, hence avoiding the need for any separate tensioning mechanism.
A driven pulley is the way to go as it is simple compared with the other ideas and so will be more reliable.
Don't they use a cannon to start races ?
I saw an old film with Kevin Bacon in it recently "Tremors", not as bad as expected. In it fun is poked at a couple of survivalists. At one point a survivalist is improvising an explosive device. He is asked what he is using as a fuse and replies that it is cannon fuse. When asked what he had the cannon fuse for he looks rather puzzled and then replies "Er, my cannon of course".