Before we get started I need to
stress the dangers involved with working with high pressure pneumatics.
It is extremely dangerous if you do not know what you are doing
or if you are using components that are not rated for the correct
pressures the system could explode causing serious injury and
even death. Pneumatics should only be implemented in a robot by
an experienced builder or a pneumatics specialist. This section
is just designed to help give you an understanding of pneumatics
and how it works. That being said, we can get on with the lesson
:-)
Note: In this text the cylinder which contains all parts of the
pneumatic cylinder (body, piston, end caps, etc.) may be referred
to as cylinder or actuator. The movable part of the cylinder may
be referred to as the piston or ram.
What is pneumatics?
Pneumatic systems employ gas that
is compressed under extremely high pressure. The practical use
of pneumatics comes in putting that compressed gas to use, or
should I say the use of the rapid expansion of compressed gas.
At its most basic level a pneumatic system holds compressed gas
in a specially designed tank and then we release some of that
gas into an expandable chamber. The expandable part of the chamber
has a rod attached to it so that as it expands the rod moves outward.
Sounds pretty simple, right? Well, in theory it is, but it is
in application that things get complicated.
Parts of a pneumatics
system
What a gas!
The first part of a pneumatic system
may not sound like an actual part, the compressed gas itself.
There are three main gases used in the construction of a robot
pneumatic weapon system; Carbon Dioxide (CO2),
Nitrogen (N2), and High Pressure Air (HPA).
Nitrogen and HPA can be compressed in a tank to upwards of 5000
psi (pounds per square inch) but require larger tanks. CO2
on the other hand liquefies at around 850 psi which allows it
to take up less space and therefore need a smaller tank (The actual
pressure is dependant upon outside ambient temperature. The note
below will explain why but for the purpose of this help section
we'll use 850 psi). CO2 exhibits some really
unique properties under pressure and when it is vented. Without
going into the theories of thermodynamics let's suffice it to
say that as pressure drops so does the temperature and lower temperatures
create lower pressures to a point of equilibrium. In other words,
a drop in pressure results in a drop in temperature but that same
pressure will not come back until the ambient temperature rises
back to the same point.
When you let some of the gas out
of a CO2 tank (or any pressurized tank for
that matter [see below]) the pressure
inside the tank drops which makes the tank get cold (REAL cold).
Well, now that there isn't as much pressure in the tank the liquid
CO2 starts to boil. The boiling CO2
releases gas which increases the pressure until there is enough
pressure to keep the liquid from boiling any more, which happens
to be around 850 psi. Pretty cool, huh? So as long as you have
liquid CO2 in the tank, and it is kept at
the same temperature the pressure remains at a relative constant.
Note: In an environment like BattleBots where you have three
minutes per match there is not enough time for the tank to rise
back up to normal operating temperatures after each release of
gas. So the tank gets colder and colder which results in lower
and lower pressures inside the tank. So, your 850 psi tank at
the beginning of the match may only have 400 psi of pressure (or
lower) at the end of the match and be covered in frost.
But there is a drawback to CO2
systems. If some of the liquid CO2 gets
past your regulator (we'll get to regulators in a minute) then
it could boil up and increase pressure in the rest of your system.
What's so bad about that? Well, if all of your fittings are only
rated to 150 psi and all of a sudden there is over five times
that pressure in the system you could very easily blow fittings
and seals in the rest of your system. Once this happens you cannot
use it again until the seals and fitting are replaced with new
ones. Notice I said NEW ones? Never try to reuse blown seals or
fittings, you'll likely seriously injure yourself. You can help
prevent liquid CO2 from entering the rest
of your system by mounting the tank upright so that the regulator
is at the top of the tank or use an anti-siphon tank to help keep
it to a minimum. Also, having a burst valve rated for your regulated
pressure on the regulated side of your system is a very good idea
as regulators have been known to fail even under normal use.
So, to avoid problem like
that you may want to look into a Nitrogen system like those used
by many paintball shops, or High Pressure Air.
Because of the unique properties
of CO2 hobby uses of it are measured in
liquid ounces and pounds. Nitrogen and HPA are measured by volume.
For example, in BattleBots, a lightweight robot using Nitrogen
is restricted to 12 cubic feet of air which means that if the
tank were to be vented to room pressure the gas would fill up
12 cubic feet.
Let's get tanked
The
second part of a pneumatics system is the compressed gas storage,
otherwise known as the tank. Tanks range in size, weight, and
proofs (rated capacities) depending upon their use. Tanks should
be DOT-approved and can be made of steel or Carbon Filament wound
Aluminum (like wrapping it with carbon fiber) and will either
have the working pressure in psi stamped into the tank or will
have a certification sticker on it. Tanks may also be measured
in "bar" (not plural). 1 Bar is equivalent to about
14.5 psi which is equivalent to 100KPA which is equivalent to
1 Atmosphere. So a 69 bar tank is equivalent to 1000 psi. Many
if not most tanks are many of solid aluminum while there are some
that are made of steel. Carbon filament wound tanks can safely
handle pressures up to 5000 psi where as a comparable steel tank
would be too heavy to use in a robot.
Each tank should also have a burst
valve to keep the tank from excessive pressure. In general the
burst valve is rated for 120% of the tanks normal operating capacity.
If the pressure builds to above 120% of the rated pressure the
burst valve will pop open and vent the gas slowly to prevent the
tank from exploding. Tanks are generally hydro-tested to twice
the rated capacity to pass DOT-approval inspection. Common sources
of tanks are welding supply shops, diving shops, paintball stores,
and even fire extinguisher tanks!
Regulators
Regulators
are interesting pieces of hardware in that they can hold back
5000 psi of air and let only a enough air through to bring the
rest of the pneumatic system up to your designed operating pressure.
Regulators also generally have a purge valve to allow you to purge
all of the air out of a pressurized tank.
Regulators come in all shapes and
sizes. Some are rated for high pressure but have a low feed rate.
Other have a high feed rate but only work with low pressures.
To find one that has a high feed rate and can handle high pressure
is tough and usually expensive. The reason that we want a high
flow rate is because once the pneumatic piston fires you'll probably
want to reload as fast as possible to fire again. The reason that
we want high pressure is because our system was designed for a
specific pressure and any less than that only degrades performance.
Some regulators will already have gauges to show tank pressure
and another to show regulated pressure. Knowing both of these
is crucial and in some cases mandatory on your robot.
Note: You should ALWAYS have a way to bleed the system of pressurized
air on BOTH sides of the regulator, usually by means of a manually
operated purge valve. You never want to get parts of your body
near a fully pressurized system.
Buffer Tank
Buffer tanks are not necessarily
part of every pneumatic system. But, if you have the extra weight
allowance and space available they are very handy to have. A buffer
tank is just an extra tank in between your regulator and your
valve (which we'll get to in a minute) that stores extra gas.
So, what does that do for us? I'm glad you asked!
Let's say that you have a pneumatic
cylinder that has a 4 inch bore and a 6 inch stroke. That gives
us a total of about 75 cubic inches. Now, let's say that you are
using 1/2" pneumatic tubing between your regulator and your
valve and on to the cylinder and we are using 250 psi of CO2.
If you have 24" of total tubing you have almost 5 cubic inches
of compressed air in the feed lines. That will move the piston
about half an inch before the regulator has to start feeding more
air into chamber. The second the pressure in the feed lines drop
below the regulated pressure the regulator starts letting more
air through. But what if you have a regulator and is not a high
flow kind? Well then your high powered pneumatic flipper just
turned into a lifter.
Now, let's put a 75 CI buffer tank
in line before the valve. This time the regulator spends a little
longer initially filling the feed lines and the buffer tank. But,
when you fire the valves the buffer tank dumps its 75 cubic inches
of compressed gas along with the 5 that was already in the lines.
This time there is a lot more pressure immediately available to
the cylinder and we get the "pop" that we are looking
for in a flipper. The same amount of CO2
is used both times because we didn't increase the bore or the
throw of the pneumatic ram.
Now, we need to remember that the
full 250 psi that is sitting in the buffer tank and the feed lines
does NOT fill the cylinder instantly. What actually happens is
the split second the valves open up is that the system equalizes
pressure until the regulator can catch up and bring it up to the
full 250 psi. That's the reason that I specifically chose a 75
CI buffer tank for the example. If we have 250 psi at 75 CI and
then we instantly double the volume of the system what happens
to the pressure? It gets cut in half. So, our 250 psi pneumatic
flipper is only using 125 psi at the instant the valve opens.
When the piston is fully retracted the pressure is at its highest.
As the piston moves forward under the pressure of air the volume
increases which decreases the overall pressure (again, until the
regulator catches up).
Granted the instant pressure drops the
regulator will start feeding more air and thus the cylinder has
more than 125 psi by the times it reaches full extension but I
just wanted to make a point about the usefulness of buffer tanks
:-)
Pneumatic hoses and fittings
Well, for all of the air to move
around we need to have a way to move it. That's where the hoses
and fittings come into play. To get the best air flow you need
to use the largest diameter hose that you can find that is rated
for the pressures that you will be using. You will also need to
find matching connectors and fittings throughout the system. It
does no good to have 1/2" hoses and fittings throughout your
whole system only to have a 1/8" port on your solenoid valves
(okay, so it's an extreme example but you get my drift). The 'push
to connect' low pressure fittings and hoses are the easiest to
work with for prototyping and low pressures.
You can get pneumatic hoses and
fittings that are rated for very high pressures. You can also
use hydraulic lines but they not really good for moving high volume
of air in a hurry but some will work. Hydraulic lines and fittings
are designed for extremely high pressure and are sometimes sheathed
with a steel mesh to help keep the hose from deforming and developing
bubbles.
There are some serious drawbacks
to using hydraulic rated equipment that should be addressed before
we go further. First, hydraulic equipment is rated for 'hydraulics'
(duh) and therefore are built to different standards. Hydraulic
components are usually metal to metal fittings as that is usually
enough to keep a liquid restrained but not necessarily a gas (meaning
it isn't 'bubble tight'). This is especially of note with regards
to hydraulic valves which, although are rated very high, still
use the metal to metal fittings which could allow air to seep
through to where it is not supposed to be (yet) and lead to a
possible dangerous situation.
It is a good idea to use PTFE tape
on all threaded connections as it helps seal any gaps that may
occur between the threads.
Other miscellaneous things that
you will need on your system is a shut off valve and a bleed valve.
Valves
The valve will probably be the
most critical (and consequently the most expensive) part of a
high power pneumatics system. It has to restrain the pressure
built up on one side and be able 'pop' completely open and not
restrict the air as it rushes through on its way to the cylinder.
There are many types of vales that can be used; Remotely Operated,
Manually Operated, and Solenoid Valves. For the context of this
help section we will just keep it to solenoid valves.
The reason that it is called a
'solenoid valve' is because there are really two parts; the valve
(and valve body) and then solenoid that activates the valve. The
solenoid opens a smaller valve that controls a small stream of
air that then pops open the large high flow valve.
There are several different
types of solenoid valves but we are just going to talk about the
three most common ones used in robots. There is a 3-port, a 4-port,
and a 5-port solenoid valve.
The 3-port solenoid valve is so
named because it has three ports; one from the tank, one going
to the cylinder, and one exhaust. Because there is only one going
to the cylinder we will be using a single acting cylinder (It
is possible to use a 3-port valve with a double acting cylinder
but that gets into advanced design and is therefore beyond the
scope of this help section). The valve opens, and pressurizes
the cylinder therefore extended the ram. Then the valve closes
which opens the exhaust port and the gas in the cylinder is allowed
to vent which equalizes it with the outside air and the ram retracts.
A 4-port valve is designed to be used
with a double acting cylinder. It has four ports; one from the
tank, one to the back of the cylinder, one to the front of the
cylinder, and one shared exhaust. In its normally closed position
it allows pressure to build up on either the front or the back
of the piston depending upon your design. When the valve activates
it redirects the compressed air to the opposite side of the piston
while simultaneously opening the exhaust port so that the air
that is currently in the cylinder can escape. If the air in the
cylinder were not allowed to escape then it would just build up
pressure when the ram piston tries to move and not allow the piston
to go anywhere.
A 5-port valve is also designed
to be used with a double acting cylinder but has an added exhaust
port. This increases the efficiency of air flow leaving the cylinder
which allows it to extend or retract faster. The diagram to the
right in this section shows how a 5-port solenoid valve works.
The Actuator
The actuator is the business end
of a pneumatics system. All of the parts listed above are to make
the actuator (cylinder) move, and move with authority. There are
three main flavors of actuators, each with their own advantages
and disadvantages; Single Acting, Single Acting Spring Return,
and Double Acting. Inside the cylinder is a disc that is sealed
against the walls of the cylinder. Then there is a rod attached
to the disc which extends out one end of the cylinder. The rod
is where we will attach things to to make things move, usually
via a clevis. There are end caps on each end of the cylinder to
keep the piston from shooting out of the cylinder when the piston
slams into it at high speed. Actuators are typically made out
of high grade aluminum or steel (usually the stainless variety).
There are also a variety of mounting styles.
Front Block mount
Front Nose mount
Rear Pivot mount
Rear Trunion mount
A single acting cylinder has only
one inlet port and therefore only one power stroke. This is usually
at the back of the cylinder so that the power stroke is the 'push'
stroke. These require some other means of retracting the piston
to its starting position, like gravity. Because of this standard
single acting cylinders have a slow reload time. On the plus side
it only has one inlet and therefore you get more shots per tank
full. Single acting cylinders are more commonly found on flipper
bots.
Single
acting spring return cylinders are just like the standard single
acting cylinders with the exception that they have a spring inside
of them. At the completion of the power stroke the spring helps
to push the piston back to its starting position. Like the standard
single acting cylinder this one allows you get more shots per
tank full than a double acting cylinder and it has the added bonus
of a spring return to help speed up reload times. But, alas, it's
not all roses. Because there is a spring inside the cylinder it
will take pressure to compress it which takes away from power
that you could potentially be putting into flipping the opponent.
This is usually a minor issue but the bigger issue is the fact
that single acting spring return cylinders tend to be longer to
accomodate the spring and therefore it makes it a little tougher
to fit inside a bot.
The last one is the double acting
cylinder. It is called double acting because it has a power stroke
on the push AND pull. The picture to the left is of a double acting
cylinder and you will notice an inlet port on each end cap. This
type of cylinder is used primarily for pneumatic spike bots and
hammer bots. The reason that you would want this on a spike bot
is because if you get the spike through the opponent you probably
want to get it back out, right? Hammer bots are pretty obvious.
Below is an example of how a double
acting cylinder is used in conjunction with a 5-port valve. Press
the Fire Piston button to watch it in action. Keep in mind that
this is a very slow motion example. Robots like Toro can go from
rest to full extension in 1/100th of a second. To put that into
perspective, in the time that it takes this demonstraion to actuate
the piston once Toro could have done it over 200 times!
Uses of Pneumatics in a Robot
There are many different
uses of pneumatics in a robot. First let's look at flipper
robots. In the Pneumatic flipper #1 diagram you can see how
the gurus at Inertia
Labs set up T-Minus and The Matador. If you want to see
cutting edge combat pneumatics then look no further than Alexander
Rose and Reason Bradley. This design maximizes the leverage
of a flipping arm by putting the lifting force as close to
the opponent as possible. If it were positioned further back
up the arm and closer to the pivot point then the arm could
theoretically lift higher but it would require more force.
For example, if the ram were connected to the arm halfway
between where it currently is and the pivot point of the arm
then it would take twice the force to lift the same amount
of weight.
The guys over at
Team
WhoopAss have their own version of a high powered pneumatic
flipper bot in Hexidecimator and Hexy Jr. Take a look at
Pneumatic flipper #2 to the left. This variation connects
the pneumatic ram to the flipper by a pivot joint behind
the flipping mechanism.
You'll notice in this instance
that the ram is connected to the flipper arm halfway between
the pivot point and the contact surface. This may sound
like they are losing out on force but they are using twice
the bore with half the throw so the overall effect is the
same. So, you can see, it is all in how you implement your
design.
A simple use of pneumatics
in a hammer bot is a bot like Jerry Clarkin's Hammertime from
Team
Hammertime. Jerry actually sets his up in a pull stroke
beneath the pivot point of the hammer for the downward swing
but the principle is the same. This bot uses a double acting
pneumatic cylinder to actuate the hammer.
Here is a little
more advanced use of pneumatics in a hammer bot. One of
the most powerful, if not THE most powerful, hammerbots
is Jascha Little's The
Judge. The Judge uses a massive double sprocket mounted
to the base of the hammer arm with chains running around
the sprockets to opposite sides of a movable bed. When the
bed is pulled back it rotates the sprockets which in turn
rotates the hammer a full 180 degrees and beyond. Then to
get the hammer reloaded the bed moves in the opposite direction.
Jascha actually custom built
a microcontroller to control the solenoid valve to achieve
maximum hammer velocity with a minimum of gas. And to say
that The Judge swings fast and reloads quickly is quite
an understatement.
Getting Bored
With all of these single and double
acting cylinders what is the difference between them? Two things:
Bore and Throw. The bore is how large, in diameter the piston
is. The throw is how far the piston moves over the course of its
stroke. As you will see in the next section the bore has a major
role in determining how much force your cylinder will generate.
May the Force be with you
Force is the end goal for everyone
using pneumatics. It is what does all the work for us. Force is
calculated by the Pressure times the Area. The Area is calculated
by using basic Algebra with regards to the Bore. To get the Area
we take half the bore and square it then multiply it by Pi (hmmmm...
that sounds an awful lot like the area of a circle "Pi(r)^2").
Once we have the Area we multiply it by the pressure to get the
force. Here they are again in easy format:
Force = Area x Pressure
Area = (1/2 Bore)^2 x Pi
So, how do we increase the force? Two ways; increase pressure
or increase the bore of the cylinder.
Note: If you have been paying attention to all of the diagrams
you will probably realized that the bore on the front side of
the piston is less than the bore on the back side because of the
rod that is attached to it. Believe it or not it does have an
effect on the final numbers so be sure to take them into consideration
when doing your calculations.
How many shots can I get?
One of the questions that I have
heard many times is, "How many shots will I get with 'a'
amount of 'gas' in a 'b' sized tank with 'c' psi regulated pressure
and a cylinder with 'd' bore and 'e' throw?" Well, to determine
that we will need to start with this formula to determine how
much gas we have available from the tank:
P1 x V1 = P2
x V2
Note: This formula is known as Boyle's Law and basically states
"The volume of a given mass of gas is inversely proportional
to the absolute pressure if the temperature remains constant."
We will assume for the moment that the temperature does remain
constant for the duration of our example.
P1 is your input pressure and V1
is your input volume. P2 is your output
pressure and V2 is your output volume. So
let's say that we have a 88 cubic inch HPA tank with a 2500 psi
tank pressure, 250 psi regulated pressure, and a double acting
cylinder with a 4" bore and a 6" throw with a 1"
rod to actuate our hammer bot. Lets find out just how much compressed
air that we really have in the tank:
2500 x 88 = 250 x V2
220000 = 250 x V2
220000 / 250 = V2
V2 = 880 cubic inches at 250 psi
Now, let's figure out how much volume we have in the cylinder
using the formula Volume = Area (bore) x Length (throw)
Push Stroke
V = ((1/2 bore)^2 x Pi) x throw
V = ((1/2 4)^2 x Pi) x 6
V = (2^2 x Pi) x 6
V =(4 x Pi) x 6
V = 12.56 x 6
V = 75.36 cubic inches
Pull Stroke
Vrod = ((.5)^2 x Pi) x 6
Vrod = 4.71 cubic inches
75.36 - 4.71 = 70.65 cubic inches
Now that we have total volume for the push stroke (75.36) and
the pull stroke (70.65) we can add them together to get 146.01
cubic inches. Now, we divide that number into the available volume
that the tank has:
Remember our formula for determining Force? Let's apply it to
determine how much force we are generating on each stroke.
Force = Area x Pressure and we know that we have an area equal
to 12.56 square inches on the face of the piston in the 'push'
stroke and (12.56 - .785) 11.775 square inches on the face of
the piston in the 'pull stroke.
Push Force = 12.56 x 250
Pull Force = 11.775 x 250
Putting all of this information together we get 880 (tank volume)/
146.01 (total cylinder volume)= 6.02 total shots and reloads (12
total actuations) with 3140 pounds of force on the push stroke
and 2944 pounds of force on the return stroke.
So, we get only 6 shots with our uber hammer bot. You can see
that you would have a pretty powerful hammer bot but you would
only get a limited number of shots. You could decrease the regulated
pressure and get more shots with less power or you can go to a
bigger tank and get more shots that way if you have the available
weight and space.
Just for grins, if we vented the entire contents of the tank
into the air how much would that be?
2500 x 88 = 14.5 x V2 (we use 14.5 because
that is the pressure at one atmosphere which is normal air pressure)
220000 = 14.5 x V2
220000 / 14.5 = V2
V2 = 15172.4 cubic inches
15172.4 / 1728 = 8.78 cubic feet (1728 equals one cubic foot of
volume)
Note: The above calculations are under ideal circumstances and
anyone who has built a robot before will tell you that you NEVER
have ideal situations. So, whatever number you get you'll probably
get a little bit less.
And lastly, there is no substitution for real world testing!
As I have always said, "Numbers look good on paper but you
never really know until you build it." And remember
to use your head and be safe!
Hey, chill out!
Okay, ready to get into some more
math? Remember earlier when I stated that Boyle's Law says, "The
volume of a given mass of gas is inversely proportional to the
absolute pressure if the temperature remains constant?"
So, what happens if we change the temperature? Well, there's a
new law that comes into play called the General Gas Law which
states the relation between Pressure and Temperature
P1 / P2 = T1
/ T2.
P1 = Initial Pressure
P2 = Final Pressure
T1 = Initial Temperature (Absolute)
T2 = Final Temperature (Absolute)
Note: Temperature means "Absolute Temperature". Since
the temperature at which molecules stop moving is -460 degrees
Fahrenheit, also known as, Absolute Zero, we have to add 460 to
whatever temperature above 0 degrees Fahrenheit that we want to
work with. This is known as the Rankine Scale. So, 50 degrees
Fahrenheit (50 + 460) equals 510 Rankine. With me so far? Okay,
let's get back to the math.
Let's put an electric heat wrap on the tank but not turn it on
yet. Now, let's say that the ambient temperature in the BattleBox
is 85 degrees (trust me it feels like 100 in there :-p) and we
have 2500 psi in our 88 ci tank at this temperature. Before the
match starts we flip the switch that turns on the heat wrap and
(for argument's sake) it gets the tank up to 170 degrees Farenheit.
Sounds like we just doubled the temperature so the pressure should
be double, right? Well, not quite, remember we are working with
absolute temperatures here. So, the absolute temperature at the
beginning of the match is really 545. At 170 degrees Fahrenheit
the absolute temperature is only 630. Not even close to double
the temperature. So, if we apply the General Gas Law 2500 (P1)
/ x (P2) = 545 (T1)
/ 630 (T2) we get x = 2890 psi.
Well, now that we have more pressure in the same amount of space
I would bet that it would have an effect upon how many shots we
can get out of our system. Replacing 2890 for 2500 in the equation
above we get 1017.28 ci at 250 psi available to us instead of
only 880. If we finish the equation we get a total of 7 (well
6.97 but who's counting?) shots. That gives us 7 foward swings
and 7 reloads. That gives us one whole extra chance to smack the
snot out of the opponent. Had the answer been 6.5 we could have
gotten 7 swings but only 6 reloads so we'd be dragging a limp
hammer around the box until the match was over.
Now because we are dealing with BattleBots rules here the Technical
Regulations say that a bot can carry no more than 2500 psi of
N2 or HPA on board at any time (8.2.2.a of Tech Reg 2.2). This
is why it is stated in the Technical Regulations section 8.9.5
Pneumatic Heaters NOT Allowed.
Okay, now that we know that the heaters are not allowed, and
we know the relation of Pressure to Temperature, what would the
temperature of the gas be after one shot if the ambient temperature
of the gas starts out at 85 degrees Fahrenheit? Well, this one
is gonna take a little more math because the pressures are different
on both sides of the regulator and we need to know how much gas
gets used after one shot.
First, lets determine how many units of Atmosphere we have available
in the tank by multiplying the pressure by the volume:
2500 x 88 = 220000
Now let's figure out how much of that gets used up when we fire
our weapon. We know that the volume on the push stroke is 75.36
ci and we are running it at 250 psi. Now we multiply those together:
250 x 75.36 = 18840
So now we now have (220000 - 18840) 201160 units left that are
stuffed into an 88 ci tank
201160 / 88 = 2285.91
We now have 2285.91 psi of HPA left in the tank after one shot.
That means that we just dropped in pressure so, by the General
Gas Law, there must be a corresponding drop in temperature of
the gas. (Remember to add 460 to the temperatures!)
2500 (P1) / 2285.91 (P2)
= 545 (T1) / (460 + x (T2))
1.094 = 545 / 460 + x
x + 460 = (545 / 1.094)
x + 460 = 498.17
x = 498.17 - 460
x = 38.17 degrees Fahrenheit
So, the temperature of the gas dropped almost 47 degrees after
just one shot. Now the tank itself won't be that cold because
of thermodynamics but that is some serioulsy nasty math that we
don't want to get into at this point. Now remember, this is just
after one actuation of our cylinder. We still need to reload.
So, if we apply the same math to the reload function (I'll let
you do it on your own to see if you get the same thing) we get
a gas temperature of -5.57 degrees Fahrenheit.
Theoretically, you could work out all twelve actuations and get
down pretty close to absolute zero but in reality it never comes
close.
Wrapping up
Well, there you have it. A somewhat in depth look at what it
takes to build a pneumatics system in a robot. It's not something
to be taken lightly but if you have the time, energy, and patience
you to can build one of the most exciting bots to watch in the
arena. I hope you have enjoyed reading this and hopefully you
have learned a thing or two along the way. And, I have said it
before and I'll say it again, have fun but please use your head
and be safe when building a robot of any kind. Cheers! -Buzz
The
second part of a pneumatics system is the compressed gas storage,
otherwise known as the tank. Tanks range in size, weight, and
proofs (rated capacities) depending upon their use. Tanks should
be DOT-approved and can be made of steel or Carbon Filament wound
Aluminum (like wrapping it with carbon fiber) and will either
have the working pressure in psi stamped into the tank or will
have a certification sticker on it. Tanks may also be measured
in "bar" (not plural). 1 Bar is equivalent to about
14.5 psi which is equivalent to 100KPA which is equivalent to
1 Atmosphere. So a 69 bar tank is equivalent to 1000 psi. Many
if not most tanks are many of solid aluminum while there are some
that are made of steel. Carbon filament wound tanks can safely
handle pressures up to 5000 psi where as a comparable steel tank
would be too heavy to use in a robot.
Regulators
are interesting pieces of hardware in that they can hold back
5000 psi of air and let only a enough air through to bring the
rest of the pneumatic system up to your designed operating pressure.
Regulators also generally have a purge valve to allow you to purge
all of the air out of a pressurized tank.
Single
acting spring return cylinders are just like the standard single
acting cylinders with the exception that they have a spring inside
of them. At the completion of the power stroke the spring helps
to push the piston back to its starting position. Like the standard
single acting cylinder this one allows you get more shots per
tank full than a double acting cylinder and it has the added bonus
of a spring return to help speed up reload times. But, alas, it's
not all roses. Because there is a spring inside the cylinder it
will take pressure to compress it which takes away from power
that you could potentially be putting into flipping the opponent.
This is usually a minor issue but the bigger issue is the fact
that single acting spring return cylinders tend to be longer to
accomodate the spring and therefore it makes it a little tougher
to fit inside a bot.
