INDEX
Formulae pertaining to
the pneumatic systems performance
|
Energy = Force x Distance Distance = Volume / Area Energy = Pressure x Volume ( psig x cu-in => in-lbs ) |
Power = Energy / Time Power = Pressure x Volume / Time ( Units = in-lbs/sec) |
Flow rate = Volume / Time Power = Pressure x Flow rate ( Psig x cu-in/sec => in-lbs/sec ) |
Pipe
thread sizes are odd ...... the thread size indicates the MAX BORE of
the fitting ....... NOT the measurement of the thread itself !
The picture on the left attempts to show the relationship between pipe
threads .....their size and designation ... together with the fitting
and the tube that could be used.......and have been drawn thus to show the
disparity between the two.
The tubes shown in this picture DO NOT match the fittings they are paired
with ......at least as far as flow goes.
For max flow the bore of the tube should be the same as or bigger than the
pipe thread designation.
E.G... The 3/8" O.D. tube has a bore of 0.28" and should be used with the
1/4" fitting if you require the maximum flow potential..
Optimising your Flippers pneumatic SYSTEM.
The
following has been picked as an example .. but the same principles apply to
bigger pneumatic systems.
Here's
a fairly standard co2 powered pneumatic setup for a small robot consisting
of a 20oz paintball cylinder / regulator combination plus a 3/8 inch 5 port valve
and a double acting ram with a 2 inch bore 8 inch stroke and 1/4 inch
ports.
OK .. Let's try and identify the problems this setup has for a Fighting
Robot
Let's assume we need the activation to be as quick
as possible with max power.
So what we need is to supply the max pressure that the ram can take and fill
it as quickly as possible.
The regulator should easily be able to supply sufficient pressure so this is
rarely a problem ...but ...
supplying enough gas as possible ... it's called FLOW ...and the lack of
this is a major design flaw in many Robots.
All the elements in a pneumatic circuit have a flow rating called
Cv or Kv
and a number associated with it.
The aim is to get as big a Cv or Kv number as possible and to be able to
supply sufficient gas for that flow rating
The following ... as far as flow ratings ( Cv's
and Kv's ) are concerned... is somewhat simplified.. subject to my
Ummm ...educated guesstimates of values...and only a guide.
I prefer Imperial units ........ so ........I'm going to use them .........
Cv is the imperial unit.
The actual Cv you get from all your components
is the square root of the inverse of the sum of the squares of the inverses
of all your individual Cv's ...
Huh!
Well we only really need to MAXIMISE our flow so let's
ignore that formula...J.
Starting
at the beginning.- The 20oz Paintball Cylinder
I cannot find any meaningful data on paintball regulator flow ...so the next
bit is my evaluation
The valve on the cylinder is a restriction it has a small outlet orifice and
CANNOT by itself supply an acceptable volume of gas in a short time
I.E. 10th's of a second ... the paintball regulator has much
the same problem,
LOW FLOW!
Next on
the list, tubing, fittings, valve and ram.
The braided hose from the regulator probably has a Cv around 0.2 this is
plumbed into an 3/8" port 5/2 solenoid valve with a Cv of 2.3 out of
that into 1/4" O.D. tube 0.180 inch bore
with a Cv of 1.1 ( based on the possible flow thru a one foot length of
straight tube) then into the 1/4" ports of the ram via two fittings ...1/8"
fitting and 1/4" reducer.
The ram however has a Cv of 2.5 -- I've roughly
based
this
on
the possible flow thru a 1 foot section of 1/4"
bore
straight tube.
The best possible Cv of this system ..as is
..would be the smallest of all of these ...0.2... which
would fill the ram to a pressure of 10 bar or 150 psi in around 1 seconds ..
(NOT to be confused with the stroke time which you get on the bench with an
unloaded ram).
Whoops! ...The above statement is untrue ... It's a smaller Cv and a longer
fill time ...Why? ...Well any bend or change in gas direction has a
detrimental effect on the FLOW.
For example a one foot section of 3/8" bore
straight tube would have its flow rating reduced from a
Cv
of 6.7 down to 4.2 with only one right angle bend and down to a Cv of 3.3
with two bends.
This
accounts for the low Cv of the 3/8" port 5/2 valve because there are
two bends or 90 degree changes in gas flow direction within the valve ... as
can be seen in the sketch.
Hmmm ... O.K .. Lets call the total system Cv
0.14..that will give us a fill time of 1.5 seconds...so NOT a flipper just a
lifter.
Solutions:-
The easiest way to overcome the low supply flow is to insert a separate
reservoir with an outlet sized to suit your system called a BUFFER TANK.
A Buffer Tank
is a reservoir (of the correct pressure rating) sized to contain a volume of
gas of at least twice the rams volume.
The
buffer tank takes the gas from the paintball bottle / regulator and stores
it ready for your immediate use, it can supply gas at a MUCH higher rate
than the paintball bottle / regulator combination and it will only take a
second or two to refill the buffer after each activation/firing of the ram.
We now have the
potential to supply sufficient gas.
Just adding a buffer removes the major flow restriction and increases the
system Cv to around 1 and brings the fill time down to 0.2 second!
Refining the system further by replacing the 1/4" O.D.
0.180" bore tube with 3/8" O.D. Tube that has a
0.28" bore and using the correct fittings
entering the ram would give an extra improvement with a fill time of around
0.1 second.
But!...Tsk!? Isn't there
always one of those....You are dumping gas into the ram from a buffer tank
and therefore sharing/dividing this volume of gas between it and the
ram...therefore the pressure at the start of your stroke will be 10 bar..but
your final end of stroke pressure will be lower.
So..for a minimum fill time the elements in
the pneumatic circuit should have their flow matched as closely as possible
to the flow potential of the port or ports of the ram and an adequate supply
of gas to match this flow should be available.
Let's
review the above
.
All Co2 bottle /regulator combinations ..that I'm aware of ..are unable to
supply sufficient flow for a really effective weapon.
The tube attached to any 5/2 spool type solenoid valve has the potential of
flowing 2 or 3 times more gas than the valve itself.. So a 5/2 valve with
the same port size as the ram is not by itself a good choice.
Solutions:- Add a buffer tank -( this is in my
opinion a MUST )-Add an extra 2/2 valve ..Enlarge the port on the ram ..Add
extra inlet ports to the ram..or any combination that will allow you to achieve your required
FLOW.
BUT WAIT! ..We've forgotten that
although getting the gas IN is the major issue.. exhausting the gas OUT of
the retract side of the ram can also be a problem ..it can cause
significant back pressure slowing down the rams
extend stroke.
Luckily the solution to this is fairly straight forward and cheap.
Add a QEV ..Quick Exhaust Valve.. to the rams retract port..that's the
silver fitting in the following picture ... it has a higher Cv rating than
the 5 port valve and exhausts the gas directly to atmosphere rather than
having to follow a convoluted route thru the pipe work and out the exhaust
port of the 5/2 valve.
This
mod by itself will reduce the back pressure.
.......Next.....
We can go one step further ...as shown in the picture? add in a
self relieving pressure
regulator... this mod allows you to have a lower pressure in the retract
side...so less gas to exhaust and a further reduction in back
pressure....plus the added bonus of using less gas!
Finally
there is a maximum extension speed that your cylinder can reach for a given
leverage ratio and mass to be thrown..... based on Acceleration = Force /
Mass and Velocity = Sqrt (2 * Acceleration * Distance travelled ).
We can then calculate the flow needed from the maximum extension speed.
Use the embedded version
below or download
Excel Spreadsheet Download
| Flipper Calc for Throw and Flow. |
You can find a spreadsheet that takes into
account the changes in leverage and pressure here... Flipper
calculator
Odds and Sods
Some pictures of my strange pneumatic arrangement ....Using a QEV as an inlet!
PIC 1 PIC2 PIC3
Setup that Mario de Jongh made PIC
30 bar version PIC1 PIC 2 PIC 3 PIC4
Explanation of the use of a QEV as an inlet valve HERE.