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I designed and built this
custom launch control system in early 2000. It has been used at all
of the Tripoli
Idaho launches ever since then. The master control unit is designed to
handle a total of 32 launch pads organized as four banks of eight pads
each. The key design objectives were to make a very reliable system
that was light weight and relatively small, yet rugged and environmentally
sealed to handle the use and abuse out on the range. It is also
designed to use a simple 12 connector cable so that the size and weight of
all the launch wire is minimized. Four spools of wire allow pads to
be set up at 100 feet, 200 feet, 300 feet and 500 feet. These are
the standard safe distances for J, K, L and M motors. The bank
control units that sit out at the launch pads are simply daisy chained
together with the cable connections.
To enhance the safety of
this system, each bank unit has audible alarms that sound if the relays
have power flowing to them. So for example, if a power relay fails
by welding its contacts shut, the alarm will indicate it is unsafe to
connect igniters. Alarms also sound at each bank unit when it has
been enabled by the master control unit or when the launch button is being
pressed. The master control unit
also has alarms that indicate when the system is armed and when the launch
button is being pressed.
This system was also
designed to provide some key information back to the LCO. The master
control unit at the LCO station can read the battery voltage at the
control unit itself and also the battery voltage at each of the remote
bank units. It is also designed to read the igniter resistance at
each individual launch pad, and the current flowing through the igniter
when the launch button is depressed. Each remote bank unit is also
designed to allow a user to easily check for continuity at each launch pad
by simply pressing a button assigned to that pad.
12V batteries are needed at the master control unit and at each remote
bank unit. |
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This is the master control
unit with the top closed and locked. The control panel was built
into a watertight heavy duty plastic case available from
Pelican Cases.
The unit is roughly 10x8x4 inches and is extremely strong. It would
easily support the weight of someone standing on it. It has a handle on the front and
two circular connectors on the back for connecting the battery and launch
control cable. |
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This is the master control
unit with the lid opened. The red launch button is in the lower
right corner. Next to it is the safety key switch (key not shown).
In the lower left corner are the four bank select switches with LED
indicators that confirm which bank is selected. Across the middle of
the panel are eight pad select switches, also with LED indicators to
confirm which pads are selected. In the center top area is a meter
that is used for monitoring battery voltages, igniter resistances, and
launch currents. Four LEDs along the right side of the meter indicate
which measurement the meter is displaying. The power switch and fuse
are in the upper left corner of the control panel. |
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Closer view of the master
control panel. You can click on the photo to see a larger picture.
The LEDs used on this unit are extremely
bright so that they are easy to see even in direct sun light. In
fact they are so bright that it is almost painful to look at them if you
do so indoors! |
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This is the back side of
the master control unit with the top closed. There are two circular
connectors with captive covers on the back. The larger one is a 12
pin connector that mates with the launch control cable. The smaller
one is a 4 pin connector for battery power. The
Pelican Case has
also been modified so that the lid will open a full 180 degrees. |
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This is a remote bank
control unit. It is roughly 12x9x4 inches in size. It operates
8 individual launch pads. Each of the pads has a continuity test button
for it that is covered by a grey rubber boot to protect it from the
elements. In the top center of the unit is a meter that is mounted
inside a clear cover to protect it. The meter indicates the igniter
resistance when one of the continuity check buttons is pressed. |
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The remote bank unit has a
Sonalert Alarm mounted just below the meter face. This alarm will
sound if power is flowing to the relays inside the unit. There are
two circular connectors on the right side of the unit. These are for
the launch control cable. The cable from the master control unit can be
connected to either of these connectors. The other connector allows
another cable to extend on toward the next remote bank unit. The
banks are connected together in a simple daisy chain manner. |
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The leads from each launch
pad plug into one of the eight outlets around the front and back of the
bank control unit. Standard 120V outlets were used to help minimize
cost and to make setup and tear-down easy and fast. The leads to
each launch pad are just standard 20 foot extension cords with the female
end removed and a pair of alligator clips soldered to the leads. These
outlets are also protected by weather covers that open upward. |
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The battery connection to
the bank control unit is a standard 220V outlet like that used for the
dyer in your house. This provides plenty of current handling
capability for drag racing all eight pads at once. The battery cable
is just a standard 220V "pig tail" with battery clips connected on one
end. |
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Construction Details and Schematics
for the Master Control Unit |
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The master control unit has
8 switches and 8 LEDs that are used to select which pad should be
launched. The wiring for them is shown here. The switches are wired in a
type of 4x2 matrix arrangement that minimizes the number of wires needed
on the control cable. The eight switches produce 6 signals called
P1,P2,P3,P4 and G1 and G2. The 4x2 matrix works by putting power to
one or more of the P1-P2 lines as well as providing a ground path back
through one or more of the G1-G2 lines. The power and ground to this
switch matrix is controlled by a set of relay contacts off the master
control relay. The master control relay closes these contacts only
after a bank has been selected. |
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The bank select switches
are also wired in a scheme that reduces the number of control signals.
The four bank select switches generate three control lines BS1, BS2 and BSC.
BS1 and BS2 can be on or off and positive or negative with respect to BSC.
Each remote bank unit is wired to recognize its particular unique
combination of these signals to enable it. |
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The safety key provides
power to the launch button circuit. (The safety key can not be
removed once it has been turned to the armed position.) The LP
signal (Launch Power) is only active once a bank has been selected.
This will illuminate the ARMED LED when the safety key is in the ARMED
position. When the launch button is pressed the L (Launch) signal
will have power applied to it and this goes to the remote bank units via
the launch control cable. |
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The master control unit has
a metering system that has four different modes of operation. If no banks
are selected, then the meter reads the voltage for the battery at the
master control unit itself. Once a bank is selected, the meter will
show the voltage for the battery at the selected bank. When an individual
pad is selected, the meter shows the resistance of the igniter connected
at that pad. This allows the LCO to confirm good connections prior
to count down and launch. (It will also show when an igniter has been
burned yet the motor did not ignite.) While the launch button is
depressed the meter shows the amount of current actually flowing through
the leads going to the launch pad. This is useful for confirming the
bank is working properly and is applying power to the leads. The signals SH and SL on this schematic are on the launch cable and are driven by the
selected bank with the appropriate signals depending on the modes as
described above. (SH stands for Sense High and SL stands to Sense
Low.) |
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The master control unit has
an audible beeper built into it that has three different modes of
operation. When the safety key switch is first rotated to the ARMED
position, the beeper will produce a solid continuous tone for about one
second duration. It will then go silent, but after about 4 or 5
seconds will produce two short "reminder" beeps that the system is still
armed. The two short reminder beeps will continue to be produced
every 4-5 seconds as long as the system stays armed. Once the launch
button is pressed the beeper produces a continuous tone for as long as the
button is held down. |
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This schematic shows the 5V
power regulator circuit that supplies power to the digital logic that
operates the LCO warning beeper described above. It also shows the wiring
for the battery connector, the power switch, and the fuse. |
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This diagram shows the 12
signal connections to the launch cable connector on the back of the master
control unit. P1-P4 and G1-G2 are generated from the pad
select switches. BSC, BS1 and BS2 are generated by the bank select
switches. L is from the launch button and SH and SL are signals
returned from the remote bank unit that carry information to the metering
system.
The wire assignments inside
the 12 conductor cable are also shown by their color code. All
cables are built identically (except for length) and are completely
interchangeable with each other and are interchangeable end-for-end. |
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Construction Details and Schematics
for the Bank Control Units |
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This a wiring schematic for
the remote bank unit that is placed near the launch pads. The launch
battery is a 12V lead acid car battery and it connects into this unit
through a standard 220V outlet and plug. (J9 on the drawing.) The
leads from each pad connect into the controller through standard 120V
outlets and plugs. (J1-J8 on the drawing.) Each pad is selected via a set
of SPST relay contacts on relays U1-U8. These are automotive grade
40A relays. These relay contacts select which pad will receive
launch power, but the power is actually switched on and off with a high
capacity 100A industrial grade contractor switch. (U9 in the drawing. Part
number 576-3007 available at
Allied Electronics.)
The contractor is only activated when the master control unit has selected
the bank and the launch button is pressed. Push buttons S1-S8 on
this drawing are for performing the continuity test on pads 1-8
respectively. |
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This next schematic shows
how the pad selection relay coil connections are wired. Once the
bank is selected, relay U10 will close the contacts connected to G1, G2
and L. This allows power on lines P1-P4 to flow through the relay
coils and into either G1 or G2. This activates the selected relays
U1-U8 as per the switch settings on the master control unit. When
the launch button is pressed, the L control line applies power to the
contractor coil shown as U9 in the drawing. This power from L flows back
through either G1 or G2 depending on which one is connected to ground at
the time. |
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The remote bank unit is
designed to send three different signals back to the master control unit
via sense lines SH and SL. When the bank is selected, but no pads
are selected, the bank sends back a signal indicating battery voltage at
the bank unit. Once a pad is selected, the unit sends back a signal
that represents the igniter resistance for that pad. And when the
launch button is pressed, it sends back a signal indicating how much
current is flowing to the pads. When the bank unit is not selected,
then the unit completely disconnects from the SH and SL lines so
that some other bank can drive them as needed. These four different
modes are achieved with relays U13, U14 and U15 on this schematic. When
the bank is selected, relay U13 closes. When a pad is selected,
relay U15 closes. And when the launch button is pressed, relay U14 closes. |
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The master launch
controller can operate four different banks. This schematic shows
how a remote bank is hardwired to respond when BS1 is positive with
respect to BSC. This is the state of the lines when the master
controller has enabled "Bank A". Other combinations are
also valid. Bank B is selected when BS2 is positive with respect to
BSC. Bank C is selected when BS1 is negative with respect to
BSC and Bank D is selected when BS2 is negative with respect to BSC.
Each bank just needs to have the diode D22 wired appropriately. |
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The electronics in the bank
unit are powered by a 5.6V regulated power supply that is based on an
LM317 three terminal adjustable voltage regulator. It takes the +12V
input and provides a stable +5.6V output. 5.6V was selected because
it provided just enough voltage headroom for the LT1078 op-amp in the
resistance measurement circuit yet also was low enough for the LTS 15-NP
current sensor. |
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The current sensor
circuit sends a signal back to the master control unit that allows the LCO
to monitor how much current is flowing in the igniter while the launch
button is pressed. This can be very useful when checking for shorts or
opens and is also a nice way to verify the launch system is operating
properly and that the battery is delivering sufficient current.
The current sensor circuit
is based on an LTS 15-NP Hall effect sensor. That sensor provides a
calibrated output of 0.625V per 15A.
A
data sheet for it is here. It is available at
Digi-Key for about
$19.20 and is part number 398-1001-ND. The output of the
sensor is amplified by an LT1078 op-amp circuit with a nominal gain of
about 4.8X. This provides a scale factor where 1V corresponds to 5A
which is what the metering system on the master controller is expecting.
The real beauty of this
current sensor is that it does not introduce any additional voltage drops
in series with the launch current. The launch current simply passes
through the sensor body via a heavy gauge wire and a Hall effect element
inside the sensor senses the magnetic field produced around the wire by
the current flowing in the wire. The output of the Hall effect
element is a voltage that is proportional to the strength of the magnetic
field and therefore proportional to the current in the wire. Very
neat! |
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The resistance measurement
circuit sends a signal back to the master control unit that allows the LCO
to read the actual resistance of the igniter at the selected pad. A
reading close to zero is a short and a reading more than a few ohms
usually indicates an open. Most igniters are in the 1-2 ohm range.
The igniter resistance is also displayed on the meter mounted on the face
of the bank unit itself. This allows a flyer to check the connection
by reading the actual resistance. The circuit also provides an
audible beep that indicates continuity through the igniter regardless of
the actual resistance.
The circuit works by
providing a 1ma "test current" that flows through the igniter and
generates a 1-2mV voltage drop. This is not enough to fire the
igniter, but is just enough to reliably detect. The resistance of
the igniter is proportional to the voltage drop it produces with the 1ma
of test current.
The LT1078 op-amp amplifies
the voltage by about 1000X. This produces an output that is
calibrated to 1V = 1 ohm which is also what the metering system in the
master control unit is expecting. There is also a meter mounted on
the bank unit itself that will show the same reading. That meter is
1ma full scale so the 2.2K resistor and 2K potentiometer set the final
meter calibration.
The two 2N3904 transistors
create a continuity detection circuit that activates the piezo-beeper if
the igniter has continuity (regardless of actual resistance.) The
circuit also activates the meter via the 2N7000 N-chan FET. That way
the meter is off (not left "pegged" against the full scale stop) whenever
there is no igniter connected. The 5.6V power supply allows just
enough voltage headroom so that the LT1078 op-amp will saturate at about
4.1V. This provides a good upper limit for driving the metering
system to avoid over driving and damaging the meter mechanism.
The resistance measurement
circuit requires a low-offset, low drift, high gain op-amp like the
LT1078. The LT1078 is available as a dual so one of the two
amplifiers can be used in the resistance measurement circuit and the other
one can be used in the current sensor circuit. It is also available
at Digi-Key as part
number LT1078CN8-ND. |
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Since the 5.6V power
regulator, current sensor circuit and the resistance measurement circuit
are relatively complex, it is also possible to just leave them out and use
this simpler continuity check circuit instead. (Use one or the
other, don't use both at the same time!)
This circuit
allows a simple continuity check to be performed. It works by
providing a very small (0.5ma) current out through the relay contacts of
the selected pad. If the current flows out the pad lines then
transistor Q1 turns off and beeper U17 will sound. The continuity is
also indicated on the meter at the remote pad unit (U16) and a signal is
sent back to the master control unit via the Rsen+ and Rsen- lines.
This allows the master unit to read igniter continuity (or lack of it) on
the meter system at the LCO station. The meter will simply go to
mid-scale if there is continuity. To get the meter to actually read
accurately, the more complex resistance measurement circuit is required. |
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This diagram shows the 12
signal connections to the launch cable connectors on the side of the
remote bank unit. There are two connectors there but each is wired
identically. One allows signals from either the master controller or a
previous bank to enter this bank unit. The other connector allows
the signals to pass through this unit and travel to the next bank further
out at the next set of launch pads.
P1-P4 and G1-G2 are generated from the pad
select switches on the master controller. BSC, BS1 and BS2 are generated by the bank select
switches on the master controller. L is from the launch button and SH and SL are signals
returned from the remote bank unit that carry information to the metering
system on the master control panel.
The wire assignments inside
the 12 conductor cable are also shown by their color code. |
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The Italian
Version! |
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This photo shows the
Italian version of the master control unit.
In 2005 I was contacted by
Tripoli Italy prefect Stefano Figini. His club was interested in
building a launch controller based on my design presented above. A
few design changes had to be made in order to use components more readily
available in Europe. A few changes were also made to suit the needs
of the club. However, after about two months of construction and after
trading a number of emails to work out a few technical difficulties they
had a working launch system! They have also done a great
job presenting the design on their web site.
Click here to see all the Italian design details |
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This is one of the four
bank control units that were built for the Italian version. Each
bank can launch four pads giving a total of 16 pads for the complete
system. |
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ACME is
an Italian rocketry group founded in 1999 with the purpose of putting
non-professional rocketry activity under control of a governing body and
to fill the void of information on the hobby. The name ACME (pron.
ACK-MEH) is an Italian word meaning "the highest point" or "apogee."
The logo shows the Looney Tunes© cartoon to express the happy attitude of
the group! ACME is Tripoli Rocketry Association prefecture #079 (Tripoli
Italy) |
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