"C" LANGUAGE EMULATION OF AN AUTOMAG PAINTBALL GUN
version 1.0
1 Introduction
This program calculates the behavior of an automag paintball gun using
a simple model of the gun. Please see the automag manual for an
explanation of the terminology. If the program is run as is, it will
emulate a 0.2 second wait while the gun charges up to about 450 psi, then
the trigger is pulled and a ball is ejected at about 290 fps. In order to
set up other scenarios, the timing loop in main() and the trigger
subroutine UpDate_t() must be modified by the user.
2 Explanation of the Program
At any point in time, the gun is considered to be completely described
by specifying a set of pressures, volumes, surface areas, positions and
velocities of certain key elements of the gun. In the computer program,
the subroutine tstep() takes the set of variables describing the state of
the gun, along with the time (t) and a time step (dt), and calculates the
new values for these variables at time t+dt. This "marching algorithm" is
accurate as long as the time step dt is small enough but if dt is too
small, it can take a long time for the program to calculate out to a
reasonable amount of time. A good value for dt seems to be about 1
millisecond. A check on whether a small enough time step is being used
is to cut the time step by a factor of 3 and see if the results are the
same. If the time step is increased by a factor of 3 and the results
are about the same, a larger time step can be used.
There are two types of subroutines used by tstep(); flow rate
subroutines starting with "Q" and moving part update subroutines starting
with "U".
2.1 Flow rate subroutines
There are 4 volumes or chambers in the gun: "s" signifies the source
(e.g. a CO2 bottle), "r" is the regulator chamber, "a" is the air
chamber, and "b" is the barrel. Thus, for example, PV.Pr is the regulator
chamber pressure and PV.Vr is the regulator chamber volume, and Qra() is
the subroutine that calculates the rate of flow from the regulator
chamber to the air chamber. The regulator valve connects the source and
the regulator chamber, the onoff valve connects the regulator chamber
and air chamber and the bolttopower tube oring is the "valve" that
connects the air chamber to the barrel. The flow rate subroutines
calculate the rate of flow of gas from one chamber to the other according
the the crude equation:
Q12 = d(P2*V2)/dt = d(P1*V1)/dt = (Cl + Z*x)*(P1P2) (1)
where P1 and V1 are the pressure and volume of the upstream chamber, P2
and V2 are the pressure and volume of the downstream chamber, Cl is a
leakage flow rate (in order that bad orings may be modelled), Z*x is the
normal flow rate where x is a measure of how open the valve is and Z is a
proportionality constant. (If x becomes negative it is assumed the flow
is stopped). Any pressure at time t+dt is calculated (assuming constant
volume V) by:
P(t+dt) = P(t) + (QinQout)*dt/V (2)
where Qin is the rate into the chamber and Qout is the rate out of the
chamber.
2.2 Moving Part Update Subroutines
The moving parts of the gun are moved by pressure differentials applied
to various surfaces, by springs, by friction, or by a finger moving the
trigger. Generally, the force (F) on a surface of area S due to a
pressure difference is given by:
F = S*(P1P2) (3)
where P1 and P2 are the pressures on either side of the surface. The
force on an element due to a spring is;
F = K*(xx0) (4)
where x is the length of the spring, x0 is its relaxed length, and K is
the spring constant. The force due to friction is taken to be:
F = R*V (4.5)
where V is the velocity of the part and R is a proportionality constant.
If a massive part is being moved, the position (x) and velocity (v) of
the part must be known in order to predict its motion. The acceleration
of the part is A=F/M where F is the force on the part and M is the mass
of the part. The position and velocity of the part at time t+dt is
calculated as:
x(t+dt) = x(t) + (v(t) + 0.5*A*dt)*dt (5a)
v(t+dt) = v(t) + A*dt (5b)
2.25 The Source Pressure
The pressure of the source (PV.Ps) is assumed to derive from the
equilibrium vapor pressure over liquid carbon dioxide at temperature PV.T.
The vapor pressure data was found in the CRC Handbook of Chemistry and
Physics, 74th edition, copyright 1993. The data is given between
temperatures of 140 and 300 degrees kelvin. The logarithm of the pressure
as a function of temperature was fitted with a third order polynomial and
the source pressure is calculated using the four coefficients of this
polynomial.
2.3 The Regulator
It is assumed that a gas source at pressure PV.Ps is applied to
the input. The regulator is diagrammed schematically below:

 > to onoff valve
 
 D
  E    
 A 88888 B C 
     
 D 
 
 

v
to source
A Regulator nut
B Regulator piston
C Regulator valve
D Regulator seal
E Regulator spring pack ("888" represents the spring)
It is assumed that the left end of the regulator valve is always in
contact with the regulator piston. (actually the regulator valve spring
contacts the right end of the regulator valve and keeps it pushed against
the regulator piston) As gas enters from the source, it flows past the
regulator valve into the part of the regulator chamber holding the
regulator piston. The pressure forces the piston to the left, and the
regulator valve moves with it. This process continues until the regulator
valve head comes in contact with the regulator seal and stops the flow.
Equation (1) is used to calculate the rate of flow of gas past the
regulator valve, with:
Qsr = (Zr.Cl + Zr.Z*Zr.x)*(PV.PsPV.Pr) (6)
Zr.x distance from the bottom of the regulator valve
head to the top of the regulator seal.
PV.Ps source pressure
PV.Pr regulator chamber pressure
Zr.Z flow rate/length for the regulator valve
Zr.Cl leakage rate for the regulator valve
The position of the regulator valve is determined by the position of
the regulator piston. The regulator piston is pushed to the left by the
regulator spring and pushed to the right by the higher pressure in the
regulator chamber. The regulator piston can only move so far to the right
before it runs into a stop. The force of the regulator spring can be
increased by moving the regulator nut to the right, compressing the
spring. Lets say Zr.x0 is the value of Zr.x when the regulator piston is
against the stop and Zr.T/Zr.H is the distance the regulator nut
compresses the spring beyond its relaxed length when the regulator piston
is against the stop. Zr.T is the number of turns of the regulator nut
(clockwise) and Zr.H is the number of turns per inch of the regulator
nut. The position of the regulator piston is found by assuming that the
spring force and the pressure force are always equal:
Zr.K*(L0  L) = Zr.S*(PV.PrPV.Px) (7)
where L0 is the relaxed length of the regulator spring and L is its
compressed length. When the spring is compressed, it is compressed from
the left Zr.T/Zr.H inches by the regulator nut and Zr.x0Zr.x by the
pressure force. That means
L0 = L + Zr.T/Zr.H + Zr.x0  Zr.x (8)
The above two equations can be solved for Zr.x
Zr.x = Zr.x0 + Zr.T/Zr.H  (PV.PrPV.Px)*Zr.S/Zr.K; (8.5)
which is the equation used inUpDate_r();
2.4 The OnOff valve
A schematic diagram of the onoff valve is given below:

to regulator < 
o o
  > to air chamber


AB
Sear
Equation (1) is used to calculate the flow through the onoff valve with:
Qra = (Zo.Cl + Zo.Z*Zo.x)*(PV.PrPV.Pa)
Zo.x distance that the top of the regulator pin is below
the onoff oring
PV.Pr regulator pressure
PV.Pa air chamber pressure
Zr.Z flow rate/length for the onoff valve
Zr.Cl leakage rate for the onoff valve
The position of the onoff pin is controlled by the sear which is in turn
controlled by the trigger. It is assumed that the bottom of the onoff
pin is connected to the sear but actually the regulator pressure pushes
it against the sear. If the trigger is released, the sear is tilted at
zero degrees. As the trigger is pulled, The onoff valve position (Zo.x)
decreases according to:
Zo.x = Zt.xs*sin(Zo.BZo.Bo)
where Zt.xs is the length of the sear arm connecting its pivot point
("B" in the diagram) to the point of contact with the onoff pin ("A"
in the diagram). Zo.Bo is the angle at which the onoff pin contacts
the teflon onoff oring. This equation is implemented in UpDate_o()
except it is assumed that the angle Zo.BZo.Bo is small enough so that
sin(Zo.BZo.Bo) can be replaced with Zo.BZo.Bo itself.
2.5 The Power Tube
A schematic diagram of the power tube is given below:

O 888888888

Air Chamber  bolt  Barrel

O 


 

o
Sear
Equation (1) is used to calculate the flow through the power tube valve:
Qab = (Zp.Cl + Zp.Z*Zp.x)*(PV.PaPV.Pb) (9)
Zp.x distance that the bolt is to the right of the power tube
oring ("O" on the diagram)
PV.Pa air chamber pressure
PV.Pb barrel pressure
Zr.Z flow rate/length past the power tube oring
Zr.Cl leakage rate past the power tube oring
The position of the bolt is controlled by the pressure difference between
the air chamber and the barrel, the power tube spring ("888" in the
diagram), and the sear. If the bolt moves Zp.xc inches to the left of the
power tube oring, it is latched by the sear, as long as the sear is at
an angle of Zt.Bl or greater. If the bolt at the latch point, it is
stopped from moving left. If the bolt moves to the right by Zp.xm inches,
it strikes a stop in the main body and is stopped from moving right.
anywhere between these two points, equations (3) and (4) give
the force on the bolt except that if the bolt is in contact with the
power tube oring, it is assumed that there is an additional frictional
force on the bold according to equation (4.5) and equations 5a and 5b
then determine the motion. This is implemented in subroutine UpDate_p().
2.6 The Ball and Barrel
The ball is pushed down the barrel by the difference in pressure between
the barrel and the atmosphere. A complication here is that the barrel
volume (the volume of the barrel between the bolt and the ball) is
changing as the ball moves down the barrel. The barrel volume (Vb) is
given by:
Vb = 0.25*pi*Zb.D*Zb.D*(Zb.x+0.1) (10)
where Vb(t) is calculated before Zb.x is updated and Vb(t+dt) is
calculated after Zb.x is updated. The rate equation is then:
P(t+dt)Vb(t+dt) = PV.Pb*Vb(t) + (QabQbx)*dt (11)
which can be solved for PV.Pb(t+dt). Equations 3, 5a, and 5b are then
used to update the position of the ball. If the ball moves beyond the end
of the barrel, the barrel pressure is atmospheric.
I tried worrying about whether the ball was in contact with the foamie
and being pushed by the bolt or whether it was being pushed by barrel
pressure, but it seemed like it was always being pushed by barrel
pressure, so I didn't bother with including this in the model, even
though it needs to be included. Also, it shouldn't be too hard to model
a vented barrel to determine how much more gas efficient an undrilled
barrel is.
2.7 Estimation of Constants
Most of the geometric values were obtained by direct measurement of the
gun itself, or were obtained from the automag manual.
2.7.1 The Power tube spring constant
The spring constant of the power tube spring was estimated by placing
a 5pound weight on the spring and noting the deflection.
2.7.2 The Regulator Spring Pack Spring Constant
The manual says that the gun is nominally charged to 450 psi. Not
knowing how much the regulator nut compresses the spring pack under
these conditions, I assumed it didn't compress it at all. That means the
spring constant for the spring pack is found by solving equation (8.5)
with Zr.x = 0 and Zr.T=0:
Zr.K = Zr.S*(450PV.x)/Zr.x0 = 1367.5 (12)
This value for Zr.K can probably be improved if I can find out how much
the regulator spring pack is compressed when the gun is charged to 450
psi.
2.7.3 Air chamber volume
The manual says that a 7 oz tank of CO2 will provide 400 good shots for
the gun. There are probably about 6 usable ounces in the tank so that
70 shots per ounce of CO2 is a reasonable estimate of gas consumption
per shot. Assuming that the air chamber fill pressure is is 450 psi, and
the temperature is 70 degrees F, we can solve the ideal gas equation for
PV.Va:
PV.Pa*PV.Va = n*R*PV.T
where n = 1/70 oz = 9.2904e2 mole and R=8.314 J/mole/deg K. This gives
an air chamber volume of 0.463 in^3
2.7.4 The Valve Flow Rates/Length
The manual says the gun charges up to 450 psi in about a third of a
second. The flow rate constant Zr.Z was found by hitormiss until the
air chamber filled up to 95 percent of its steady state value (i.e. 430
psi) in about 0.33 seconds. The same value of flow rate/length was used
for both the regulator valve and the onoff valve.
2.7.5 Power Tube Flow Rates/Length
When the air chamber is filled up to 450 psi, it is assumed that the
ball will be ejected from an 11 inch barrel at 300 fps. The Power Tube
Flow Rate/Length was adjusted until this was the case.
2.7.6 Power Tube Friction Coefficient
If the friction coefficient is set to zero, in the "lawn sprinkler"
scenario the bolt motion is dominated by the resonant frequency of
the bolt and its spring. If the coefficient is much greater than 3.0
all evidence of this natural frequency is lost. I chose a value of
1.0 only because its interesting. The bolt motion has about equal
components of 5 and 10 cycles per second. Some thinking has to be
done to get the value of this constant correct.
3. Scenarios
A print statement is in the main() program which outputs a number of
variables. A few different scenarios are given by the following
modifications to the program along with a description of the expected
printed output
3.1 Simple Shot  run program as is. The regulator chamber should
charge to 449.54 psi and the air chamber should charge to 449.02 psi at
0.005 seconds, then the trigger should go on and a ball should be ejected
at 296.69 fps.
3.2 fast stroke: The regulator and air chambers are precharged to
about 450 psi and the trigger subroutine turns the trigger on to
begin with. The first ball comes out at 296.67 fps. The trigger
subroutine waits until the air chamber is emptied, then turns the trigger
off (0.0176 sec). 0.2 seconds later at 0.2176 sec, the second ball comes
out at 263.92 fps, becuase the air chamber could not fully charge in
0.2 seconds.
Zt.B = Zt.Bb; /* Trigger pulled */
PV.Pr = 449.; /* regulator chamber charged */
PV.Pa = 449.; /* regulator chamber charged */
dt = .0002;
n = 1150;
 Insert into UpDate_t() 
static double delt;
double cycle;
cycle = 0.2; /* time trigger is off (sec) */
if(Zt.B >= Zt.Bb)
{
if((Zb.x >= Zb.L) && (PV.Pa <= 1.005*PV.Px))
{
Zt.B = 0.0; /* trigger off */
Zp.x = 0.1; /* bolt latched */
Zp.v = 0.0;
Zb.x = 0.0; /* ball loaded */
Zb.v = 0.0; /* ball motionless */
delt = t; /* reset timer */
}
}
else
{
if(tdelt >= cycle)Zt.B = Zt.Bb; /* Trigger on */
}
ret: return(0);

3.3 Lawn sprinkler. A leakage in the onoff valve is put in, and no
ball is loaded (This is done by putting the position of the ball
Zb.x outside the barrel, i.e. Zb.x >= Zb.L) The trigger is always
on, and the regulator and air chamber pressures are set roughly
to a steady state value. (They needn't be).
Zb.x = Zb.L; /* no ball */
Zt.B = Zt.Bb; /* trigger down */
Zp.X = 0; /* bolt at zero */
Zo.C = 0.01 /* onoff leak */
PV.Pr = 450; /* steady state */
PV.Pa = 34.7; /* steady state */
n = 3000;
dt = .0002;
 Insert into UpDate_t() 
{
Zt.B=Zt.Bb
return(0);
}

3.4 Cold Gun  Low temperatures lower the vapor pressure of the CO2 which
results in slow charging of the gun and possibly incomplete charging.
Set the temperature PV.T to 30 degrees F. and run the short stroke
scenario. The second ball will come out at 239.00 fps, which is even less
than before, this being due to the lower source gas pressure.