Launch Controller

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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.

Master control unit with cover closed.

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.

Master control unit with cover opened.

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.

Closer view of master control panel.

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!

Back side of the master control unit with cover closed.

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.

Bank "A" control unit.

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.

Another view of Bank "A" control unit.

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.

Pad connections use a standard 120V outlet and plug.

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.

The battery connection uses a standard 220V output and plug.

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.

 

Construction Details and Schematics

for the Master Control Unit

Click here to see the Pad Selection circuit schematic

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.

Click here to see the Bank Selection circuit schematic

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.

Click here to see the Launch Button circuit schematic

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.

Click here to see the Metering Circuit schematic

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.)

Click here to see the LCO Warning Beeper circuit schematic

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.

Click here to see the Power Regulator circuit schematic

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.

Click here to see the Cable and Connector pin assignments

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.

 

Construction Details and Schematics

for the Bank Control Units

Click here to see the Pad Selection Relay circuit schematic

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.

Click here to see the connections to the Relay Coils

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.

Click here to see the Sense Selection Relay circuit schematic

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.

Click here to see the Bank Address Selection circuit schematic

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.

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.

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!

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.

Click here to see the Audible Continuity circuit schematic

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.

Click here to see the Cable and Connector pin assignments

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. 

The Italian Version!

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

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.

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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 All photos not otherwise credited were taken by Vern Knowles

Vern Knowles © 2002, 2003, 2004, 2005, 2006, 2007 All Rights Reserved