Re: "hydraulics"
 

A full air tank and a couple solenoids and some tubing make a nasty dual spit wad shooter. It when through plastic and leather and lodged itself in the wood. I've be looking at those team with 7-8 solenoids on a single base and thinking cool. You could shoot like 8 spit wad a once and then shoot 8 more singles seperately or all together. Now I need to figure out how to reload. I bet you could lodge a spit wad in a speed controller. If it is lodged in another robot it is still on a robot.

Re: "hydraulics"
 

We made a tennis ball launcher out of the pneumatics - it was fun :D

We then continued to use it; we were launching soda bottles into the recycling bins...

Re: "hydraulics"
 

Yes, the most cited air/hydro differences are: compressibility (vs non), and a MUCH higher average working pressure in hydraulic systems.

But it is not true that a hydraulic system HAS to be limited by the pump's output. There are devices commonly known as "hydraulic accumulators" or "hydraulic surge suppressors". (You can web search on them.) These devices DO accumulate energy for hydraulic systems in the same way that the air tanks accumulate energy for pneumatic systems.

The main parts of a hydraulic accumulator are an expandable chamber for the fluid, working against a "spring force" of some kind. This is often implemented as a very high pressure vessel with a flexible diaphragm of some kind bisecting it and air on the other side to act as the spring, or as a hydraulic piston with a VERY heavy duty spring, air, or displaced vertical weight giving the back force.

For example: Smart multi-car hydraulic "elevator array" systems in buildings can use the unused elevator cars as "accumulators". They SWAP energy for "nearly free rides" with valves by sending one or more empty cars one way as the full one goes the other, or transferring the surplus "down" energy into a BIG standard accumulator to be reclaimed later for "up" use. Down can ALWAYS be had at any time by simply opening a valve. A Smart Array system (hydraulic OR electric based) can save the building owners a LOT in energy costs over the long haul.

Anyway... Standard compact hydraulic accumulators are NORMALLY used for one of these reasons (there may be others, but these are the most common):
A) To store energy for high frequency actuation you otherwise couldn't directly GET out of a pump;
B) Act as a "shock absorber" to help reduce "water hammer" effects, or
C) Allow a MUCH smaller "average capacity" pump to be used instead of a large "peak" one, for serious cost savings.

Just like with air, once an accumulator is pumped up, you can go crazy for a while with the actuators. However, pressures in a HYDRAULIC accumulator vessel can often exceed several THOUSAND PSI. The volume depends on your application.

The total energy stored is related to the total displacement volume times the maximum rated working pressure. IOW, a HECK of a lot of stored hydraulic energy can be had in just a TINY accumulator package, as much as a HUNDRED times higher than is available in OUR air based systems of the same size. "A couple thousand PSI" systems are fairly common. Anything over "a few thousand PSI" hydraulic systems DO exist, but are VERY dangerous, and take special plumbing parts and rules to avoid ruptures.

For safety, systems with hydraulic accumulators MUST include a way to CAREFULLY purge the energy stored in them. The easiest schematically is to add a simple bleeder/purge valve between the high pressure side and the oil reservoir.

BTW, hydraulic systems can be much more complex, too. Some components (like decent servo valves) may require microscopic oil filtering to eliminate even tiny contaminants for proper operation. I've once been in situation where even cracking a system for a SIMPLE change may lead to a DAY of lost time refiltering the entire oil load. But that's an extreme case...

I worked with accumulated systems when I helped a well known auto company develop their hydraulic active suspension system. Trust me, you quickly develop a HEALTHY respect for the kind of stored energy that can make a multi-ton car make significant corrective motions several hundred times a second, using only a hydraulic pump similar in size to our FIRST air pump, and a hydraulic accumulator not much bigger than a large grapefruit! Your health and safety hinges on proper procedures and rituals, and you think TWICE before taking ANY action in the lab.

During R&D tests, we even once had a full sized car jump over a foot straight up into the air on powerup due to a software error. (Whoops...) :D You might say, "there were puddles everywhere"... Trust me though, not ONE of them was the car's hydraulic fluid! ;)

...And THAT is why we don't include even NON-accumulated hydraulics in FIRST contests, and seriously limit working pressures! If you think an air piston can be dangerous, imagine a TINY hydraulic system which can EASILY produce THOUSANDS of pounds of crushing force in a gripper using with an itty bitty cylinder. In addition, hydraulic oil spray from a leaky high pressure connection intersecting a person can "pressure inject" oil directly under or even THROUGH your skin. NOT fun, and it may require some messy surgery to attempt to clean it out. (I've seen it happen... Luckily, it was an arm and not a FACE...)

But ACCUMULATED hydraulics are WORSE. They're that dangerous WELL AFTER the power is cut off. One mistake with a well sealed but unpurged multi-K PSI accumulator based system (even DAYS later) could easily be crippling, or fatal. On the suspension project, we had some SERIOUS safety classes before we could even get NEAR the project hardware.

Now, I don't mean to scare anyone off from the field. Accumulated Hydraulic systems are VERY USEFUL, and some things simply can't be easily done WITHOUT them. But they're VERY serious systems, not something to "play with" as a first time user. Air is MUCH better for learning on, and teaches you the same things with fewer safety issues. NEXT comes Basic Hydraulics, and THEN Accumulated and "High Pressure" Hydraulics.

I'm SURE we'll be sticking with low pressure, "limited storage" air for THIS contest. I just want people to know that the field of "Accumulated Hydraulics" EXISTS, and can be quite exciting!

(I hope this has been informative...)

- Keith

Re: "hydraulics"
 

we might have done something similar with a single tank, 100 psi and launching a drywall screw through a piece of 1/4" plywood....also rigged it to a air horn for some ear drumming good fun, we almost defeaned, well half the team at 100 psi....hearing protection for the people running the horn a must, after the first try my ears were ringing for the rest of the day...

Re: "hydraulics"
 

try clamping a fire extinguisher to a skateboard and then knocking the head off of the thing with a hammer.

Re: "hydraulics" - HOLD ON!
 

This post contains a great deal of very positive and beneficial information. It is also starting to contain some ideas and actions that are extremely dangerous and could cause serious injuries, even death, if something goes wrong. You are proposing messing with some powerful energy sources.

Please refrain from this type of discussion in this forum. Focus on robotics and topics related to them and lets keep everyone healthy.

Re: "hydraulics"
 

I think I'm going to have to stick with hydraulics because that is what powers the Jaws-of-Life. Yes they move slow, but when you hear the hinge of a car door pop, It's well worth the wait.

Re: "hydraulics"
 

For some reason the question "Can I sit on it?" has been asked often.

Edit: Even when its not moving

Re: "hydraulics"
 

its fun to try to ride on them.

Re: "hydraulics"
 

yeah, freshmen are comfy... :rolleyes:

Re: "hydraulics"
 

Anyone who doubts that accumulated hydraulic systems are fast hasn't ridden any of the new "rocket coasters" such as Xcelerator, Storm Runner, Top Thrill Dragster, or (soon) Kingda Ka. Accelerating a train carrying 16 people from 0 to 120mps in 3 seconds isn't slow.

Re: "hydraulics"
 

Sorry Shaun, but no hydraulics are allowed in this contest... Pneumatics only, at the above mentioned specs. :(

But I'll admit, a couple of years back I saw a 12HP hydraulic driven ZTR lawnmower for sale and was sorely tempted to buy it. I could just imagine hacking it into my own personal Transformer style "FIRST Robot of Doom" for an outdoor "yard version" of a standard FIRST contest bot... ;)

Gee... Can you imagine playing Stack Attack in a gravel pit "arena", using people riding in the (secured) cages of hacked Bobcat forklifts, trying to collect and stack large wooden crates over a large center hillock? :)

- Keith

Re: "hydraulics"
 

Ummm, isn't air a fluid? I mean, to all intents and purposes doesn't fluid dynamics deal with "liquids" and "gasses?"

Re: "hydraulics"
 

Yes it is a "fluid", but please don't confuse the behavior of "liquid" vs "gas" forms.

It's mostly a question of semantics. You'll mostly find the term "Hydraulics" is traditionally used to refer to "liquid only" power transfer mediums, and "Pneumatics" to refer to "gaseous only" power transfer mediums. As long as you're not compressing the "working fluid" enough to phase change the medium, this distinction is fine.

The major difference is "compressibility". Gasses are compressible, whereas fluids are not. IOW, pneumatic systems provide its own "spring function" between the energy source and the load, while hydraulic systems act more like a hard mechanical linkage. Otherwise, they have similar characteristics. Both will give you controllable Mechanical Advantage, very flexible energy transfer without complex mechanical linkages (hoses are simpler than gears and shafts!), and (unlike electric motors) "infinite stallability without harm".

"Fluid Power" is the superset term, that covers BOTH pneumatics AND hydraulics. It is often used by university departments when teaching the subject. You should also definitely use it when referring to any power transfer medium that may undergo a phase change between liquid and gas depending on condition. (But honestly, most designs normally try hard to AVOID those situations... You either want compressibility, or you don't!) :)

<edit>
The confusion comes in because the COMMON usage of the term "fluid" in English IMPLIES a liquid. In scientific terms though, the "working fluid" can be any phase of material.

Oh yes, before someone chimes in with a joke, solid working fluids ARE possible, as long as they are particle systems. Things like sand, or a bunch of tiny ball bearings CAN be used as a "working fluid" (though I'm not sure it would be a good IDEA for use in OUR situation!) . Particle systems are a rather "weird" working fluid, but you CAN transfer force through them! :)
</edit>

- Keith