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DumHed
Experienced Roboteer
Joined: 29 Jun 2004
Posts: 1219
Location: Sydney
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New DumBot on the way!
Well, inspired to play with pneumatics after driving El Bravo at Robowars I picked up some bits and pieces from Gary's favourite junk shop to take home and try out.
On new year's day I did some experimentation, including running the rams from LPG because my compressor is dead at the moment  Stinky!
I also put a pressure gauge on the LPG cylinder and found that the vapour pressure is a bit over 120psi on a reasonably warm day.
I tried out the idea of using car fuel injectors as valves for the gas, which worked pretty well, but they don't give much flow so the rams move slowly.
I dug out a few old fire extinguishers to use for pressure tanks, and made the world's most useless extinguisher by filling one with LPG
Today a friend came over, bringing an air compressor, so I could do some proper experiments, and a little bit safer 
We found that at about 125psi I can get over 50kg of push out of the smaller rams, and on the largest one it spun the dial around twice on a bathroom scale and then crushed it!
The scale is now mangled and reads a good 8kg lower than it used to
I modified a fire extinguisher for some more flow out of the nozzle, and pumped it up to around 100psi, then connected it to the medium sized ram I have (small diameter, but long stroke).
When the extinguisher button is pressed the ram extends faster than you can see - and the ram itself hasn't been modified at all yet for more speed. I think a decent solenoid to operate the extinguisher valve could work very well. This could drive a flipper or a hammer style weapon very nicely - and it seemed to do a fair few shots from the one tank pump up.
I've also designed a semi automatic control system that will basically fire the weapon, vent the pressure, reset it, and re arm the trigger from one input pulse, which would make it very easy to use, and wouldn't waste pressure by keeping the firing solenoid open for longer than needed.
I found another fire extinguisher that was a bit different, and wasn't designed in a way that makes it easy to attach filler tubes or output pipes. This is unfortunate because it seemed to have a larger valve setup than the other one, so it may have been even faster!
I decided to try out another idea with this one.
The hardest thing about getting air out of a tank is doing it quickly. Small valves are easier and more reliable to do, but they can't flow enough air for really fast acting rams, etc.
Large valves can flow more air, but they're obviously slower to open, so you end up with a slower pressure build up, which can waste some of the time you have to accelerate something like a hammer weapon.
I wanted a valve that can be controlled easily and quickly, but would dump a lot of air as fast as possible.
Turbo cars tend to have a blow off valve, which is normally held closed by a spring and turbo boost pressure, but opens when the driver releases the throttle, to vent off excess pressure - which reduces shock on the turbo, and gives better engine response when the throttle is pressed again.
These valves normally only handle pressures of under 1bar (under 15psi), but they flow a lot of air, and open very fast.
I had a spare factory Nissan blow off valve lying around, so I finally found a use for the aluminium welding / brazing rods I bought a while back, and attached its intake pipe directly to the end of the fire extinguisher bottle.
The blow off valve has a plunger valve in it, which would be pushed open by any pressure in the bottle, but it has an actuator with a diaphragm in it pushing on the other side, fed by a small pressure line.
As long as there's the same pressure in the actuator as there is in the bottle the valve will stay closed.
This proved to work quite well, with the BOV holding up to 100+psi of pressure, although it did leak very slightly. I think some grease on the valve surface may improve that.
With the tank pumped up to 100psi and the pressure feed to the BOV diaphragm suddenly disconnected the effect is a single bang! with all the air being released from the bottle instantly through the 1" outlet pipe.
As a quick test I put a steel pipe on the end and fired various things out the end
I think this thing would make an excellent spud gun!
I'm hoping to try this either on a modified pneumatic ram, or on a custom ram I'm putting together at the moment
One other thing I have only partially played with is building an internal combustion ram to drive a flipper. I figure if I ran it on a small butane canister there's very little chance of a fire, and it should produce quite good repeatable flips without the power dropping off or needing large air tanks.
So, the weapon experiments are well on their way, now I just need to sort out the drive system
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The Engine Whisperer
- fixer of things
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Tue Jan 04, 2005 12:17 am |
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DumHed
Experienced Roboteer
Joined: 29 Jun 2004
Posts: 1219
Location: Sydney
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Sure are!
I think I'll end up building the body out of fairly thin steel, because I have the tools to work with it and it's cheap.
I'd like to use alloy, but it does end up getting pretty beaten up, and I can't weld it with the gear I have.
So far I'm planning on a 3 wheeled setup, with a large single driving wheel at the back, and two small wheels at the front with steering.
The idea of 3 wheels is that they're all always on the ground, without having to use suspension - so I can keep good grip when getting bounced around, etc.
All the drive system weight will be as far back as possible to increase traction, and the front wheels will be right at the front, so they can take the force from the flipper.
I'll probably use one 300W scooter motor with chain drive to the rear wheel, as it's lighter than two 100W scooter motors and should be very reliable.
The drive train should be pretty efficient, so I won't need huge batteries, although I'd like to have an on board compressor for more flipper life.
For the pneumatics I think the easiest way will be to run two extinguisher bottles, with one's valve going to the flipper rams, and the other's to the hammer ram.
That way I can get high flow with easy control and not require any expensive solenoid valves.
I'll add an extra fitting into the bottom of each tank, and connect them together with a t-piece - to equalise pressure between the tanks and provide a spot for a fill valve.
I think I've worked out how to build the actual weapon section, which will also be quite simple and light - thanks to using the same pivot point for the flipper and the hammer, and the rams being positioned in an overalapping layout to save space and keep all the piping as short as possible.
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The Engine Whisperer
- fixer of things
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Tue Jan 04, 2005 2:38 pm |
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RoboWars Australia Forum Index
-> Builders Reports
be careful though, high pressure pnuematic bots would have to be about the most dangerous type there is.. blink, and you're dead if you're in the wrong place at the wrong time.
