Turning the idea sideways - does anyone use pneumatics to spin up a spinner? They seem a great way of getting a large amount of power easily compared to the electrical methods, allowing a near instantaneous spin up, with just a little motor to keep it spinning?
I imagine you'd exhaust your CO2 supply after only a few spin-ups, even quicker if you had to spend long periods of time without hitting the other robot. I'd expect the energy density of a lipo is more than that in the compressed gas at the temperatures and pressures in a featherweight. I think pneumatics are useful for getting things accelerated and moving over very short bursts, like flippers and axes. There are a few pneumatic grabbers too. Maybe if you had a motor to keep it going, but if you're talking something like Carbide's bar I think you might as well have just the motor. What's more you're now relying on two separate systems to activate one weapon. If one goes down, the weapon does too.
Last edited by R9000; 18th August 2016 at 10:51.
I was thinking purely for accelerating up to speed, with a tiny motor to keep it spinning to overcome air/bearing drag. I probably need to do some calcs to see how much air would be needed to get a decent spinner spinning at speed and compare that to an appropriately sized motor.
The latent heat of vaporisation of CO2 is 574kJ/kg the energy density of a typical LiPo is about 500kJ/kg - so not much to call it.
Unfortunately you need a container for the CO2, which for the relatively small amounts involved drives the density right down.
Not impossible - but probably not an advantage.
Si
How about pneumatics to cock the spring? Have plans for something similar...Originally Posted by Will
Hi Will,
I am in Rochdale and currently building myself so it would be good to put heads together, especially if you need help.
Your pneumatics have to develop more force than the spring to be able to compress it - so there must be an argument for just using the pneumatics and not bothering with the spring!
Si
Thats right, agreed. The main argument for the spring rather than pneumatics is the simpicity of the air system required to close the spring when compared to the air system required as a flipper. For an effective flipper using LP pneumatics you would require a buffer tank, large bore fittings and qev etc, basically you need a system which is optimised for flow and that takes up space.
On the other hand, if you use a spring, you get instantaneous power without the related problems of flow etc on pneumatic systems. The pneumatics to close the spring will not be effected by port sizes and flow etc because it does not matter if it takes a couple of seconds to cock the spring.....I think a set up with springs takes up less space and can possibly be more powerful...that said, it is not straight forward to build as it's not something I've seen built, so there's not much help around or build projects from where to take ideas.
I have a CAD design of one, which I'd like to build at some point...but I need to find time for it!
Let me share an alternative with you. This arrangement is known by several names, including a 'cannon valve' as they are often used in Potato Cannons!
http://www.fightingrobots.co.uk/images/attach/jpg.gif
It's made from two concentric tubes. The outer forms the reservoir and the inner a barrel.
To charge, you push air through the hole shown. The pressure moves a sliding piston to the right. The piston has a thin diaphragm (thick rubber works well) bolted to the right hand face. This forms a seal with the end of the barrel but can flex under pressure allowing air into the reservoir.
http://www.fightingrobots.co.uk/images/attach/jpg.gif
Once it's charged, you release the pressure in the space to the left of the sliding piston (3 port pneumatic valve with exhaust). The pressure on the reservoir side of the piston is higher than on the charge side - so the piston is shot to the left.
http://www.fightingrobots.co.uk/images/attach/jpg.gif
Once the piston reaches the end of it's travel, the diaphragm seals - and the only place the air can go is into the barrel pushing the plunger (or potato!) out.
http://www.fightingrobots.co.uk/images/attach/jpg.gif
This gives you a valve with the same port size as the bore of the barrel which opens incredibly fast.
I've used this to fire a potato over 500m at 8 Bar pressure - but I've also used it in the configuration above to move a load attached to the piston rod very fast (known as a 'Jerk Rig').
They are very easy to make, even out of drain pipe (so long as the pressure is not too high). You need to calculate the volume of the reservoir to be about 25% greater than the volume of the barrel for the best power / size ratio.
Also, the sliding piston only needs to move 15mm or so - I drew it bigger to make the diagrams clearer.
Si
Let me share an alternative with you. This arrangement is known by several names, including a 'cannon valve' as they are often used in Potato Cannons!
It's made from two concentric tubes. The outer forms the reservoir and the inner a barrel.
To charge, you push air through the hole shown. The pressure moves a sliding piston to the right. The piston has a thin diaphragm (thick rubber works well) bolted to the right hand face. This forms a seal with the end of the barrel but can flex under pressure allowing air into the reservoir.
Once it's charged, you release the pressure in the space to the left of the sliding piston (3 port pneumatic valve with exhaust). The pressure on the reservoir side of the piston is higher than on the charge side - so the piston is shot to the left.
Once the piston reaches the end of it's travel, the diaphragm seals - and the only place the air can go is into the barrel pushing the plunger (or potato!) out.
This gives you a valve with the same port size as the bore of the barrel which opens incredibly fast.
I've used this to fire a potato over 500m at 8 Bar pressure - but I've also used it in the configuration above to move a load attached to the piston rod very fast (known as a 'Jerk Rig').
They are very easy to make, even out of drain pipe (so long as the pressure is not too high). You need to calculate the volume of the reservoir to be about 25% greater than the volume of the barrel for the best power / size ratio.
Also, the sliding piston only needs to move 15mm or so - I drew it bigger to make the diagrams clearer.
At school (40 years ago!) I made one out of a Bic Biro as the barrel and a piece of plastic tube as the reservoir. I used a syringe and two non return valves to pump it up. It fired bits of tissue paper with enough speed to really hurt over the width of a classroom! It nearly ended my time at school prematurely!!! - don't try this at home (school) kids!
Si
Last edited by bigsi; 7th September 2016 at 14:33.
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