Multi-Pad-Launch-Controller Question

Discussion in 'Ground Support' started by Mike Haberer, Aug 15, 2019.

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  1. Aug 15, 2019 #1

    Mike Haberer

    Mike Haberer

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    I'm building a Multi-Pad-Launch-Controller originally designed by John Hruby and posted to the NAR.org site in May 2014 (parts and design being updated somewhat). I've attached my wiring diagrams for the launch controller and relay box. I'm still fiddling with the design components (I need low input voltage relays vs. standard 12v relays and a DB9 cable with a bigger gauge wire to minimize voltage drop to the relays) but have it working. The intent is to be able to launch up to four rockets concurrently. I'm using a 100 foot DB9 cable so I can comply with NAR safety code for HIJ engine flights (my launch pad converts from a four rod pad to a single rail pad using a jawstand).

    Here's my issue. The voltage drop over 100 feet is notable, but I can fix that with the two adjustments noted above. In testing, when I have one pad active, the voltage for relay one at the relay box is 10+ volts. All good. HOWEVER, when I activate pads 2, 3 and 4, the voltage across the active pads drops with each pad activated, roughly 7.5 volts with two pads, 6+ volts with three pads and 5+ volts with 4 pads. Voltage on these circuits at the controller end is nominal for all pads. Each circuit is independent, so why is the voltage dropping as I activate pads? I'm driving the controller box with a SLA 12 amp hour battery. I'm not an electrician, but I'm not a newby either and I'm stumped. Does anyone know why I am seeing this characteristic? Thanks in advance.
     

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  2. Aug 15, 2019 #2

    UhClem

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    Simple, all pads share a single common return path so that they are not independent.
     
  3. Aug 15, 2019 #3

    dwightr

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    Pin 9 on the DB9 cable is carrying the ground current from all 4 relays.
    Pins 5-7 are unused so just connect all three to pin 9 on both ends and let all four wires share the ground current load.
     
  4. Aug 15, 2019 #4

    Mike Haberer

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    Duh. Many thanks.
     
  5. Nov 6, 2019 #5

    scadaman29325

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    Nice. What do you mean by 'low input voltage relays vs standard 12v'? Also, what size wire to the pads.

    Thanks.
     
  6. Nov 6, 2019 #6

    Voyager1

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    5V input, perhaps?
     
  7. Nov 6, 2019 #7

    Mike Haberer

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    I used standard 12v automotive relays which require at least 5 or 6 volts to trip the relay. I was getting too much voltage drop over 100 feet on the DB9 cable I bought (the only commercially built one I could find) to trip them. Here is a link to a low voltage relay bank that I should have used (the 30 gauge, 100 ft. DB9 cable I bought would have worked with these; the voltage dropped to between 5 & 6 volts at 100 ft.).

    https://www.amazon.com/dp/B07BLKV64...olid=34100DE4CHPX5&psc=1&ref_=lv_ov_lig_dp_it

    I wound up building my own 100 ft. cable out of 16 gauge wire, a PITA, but I already had the relay box built and wired and replacing the relays would have been a complete rebuild.
     
  8. Nov 6, 2019 #8

    Steve Shannon

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    Going to a lower voltage relay might be exactly the wrong approach. Most relays will require just as much POWER to activate, meaning watts or volts times current. Volts are current times resistance. So, power is current squared times resistance.
    The killer is your voltage drop through your wires, which is the resistance of the wires times the current. If you switch to a higher activation voltage relay, say 24 volts or 48 volts, the current will be greatly reduced and you can go much further.
    Or you can try to find a relay that requires less current or wires that have less resistance.
     
  9. Nov 6, 2019 #9

    FredA

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    Use a 6V relay.
     
  10. Nov 6, 2019 #10

    Mike Haberer

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    Those, indeed, are the options. (Note: Auto relays only require 7-12 volts to activate, not 12v, so the contention above is incorrect. The whole point of relays in autos (or any other use) is to activate a high voltage operation (starting the car) using a low voltage input (from the key or on switch), thus not having to run heavy wire everywhere in the car, making the wiring harnesses smaller, lighter and less expensive). I chose to go with wires with less resistance (16 gauge vs. 30 gauge, I could have gotten by with 20-22 gauge in retrospect) instead of a lower voltage (5V) relay, since the relay box was already built.

    The challenge in going to higher voltage to go farther is you have to design the entire system from end to end to make the voltages work for the components you use (my rookie mistake when I picked my relays), which is dependent on the length and gauge of the wire you use, which limits you to that length or shorter. It's an inflexible design. Plus, finding longer cables (in my case I used a DB9 and if I put more enhancements into the system would have needed a DB25) is virtually impossible, prohibitively expensive and unwieldy to the nth degree.

    I built this for the relay to be 100 ft from the launch control to be compliant with H,I,J (non-complex rocket) launches, per the NAR HPR safety code. For anything farther than 100 feet I personally think that a wired solution is impractical and the relay system needs to be wireless. My next launch controller (if I decide to go down that path) will be wireless (more fun stuff to figure out...).
     
  11. Nov 6, 2019 #11

    Steve Shannon

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    The system I built in 2002 uses telephone extension wires, aka silver satin, small gauge (maybe 22 or 24 ga.). Each pad has two circuits: one that’s continuity, powered by the pad battery. It’s completely independent from the other circuit which includes the arming switch, selector switch, and push button launch switch. The system uses Tyco automotive relays at each pad box capable of supplying 40 amps. My box was used by our club until we upgraded to a Wilson FX system this past season. Even with those lightweight control wires the system worked reliably out to 375 feet. I was planning to change it to 48 volts control voltage (still 12 volt launch voltage) but we sprang for the wireless system instead.
     
  12. Nov 6, 2019 #12

    cwbullet

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    I love the WilsonFX system. I would love to upgrade our system to the newest. Maybe we will take up donations.
     
  13. Nov 6, 2019 #13

    Voyager1

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    In our original launch control system we have 28 AWG 9-core shielded cable running out to 100 metres to switch the relays in our pad boxes. However, the 12V/30A relays are driven by opto-coupled transistor drivers at the pad that only require about 20 mA to switch them, so never have any issues at that distance.

    We have upgraded to wireless, because all that cable is a pain.
     
  14. Nov 6, 2019 #14

    Mike Haberer

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    Good to know. This was my first effort and I modeled it off of a design I found. It was a learning exercise. I did consider upping the control voltage to 24v and tested it that way (the relays worked) but was not confident all of the control box components would all support a higher voltage in extended use, so I opted for a larger gauge wire to the relay.
     
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  15. Nov 6, 2019 #15

    Steve Shannon

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    Don’t forget the Tripoli Prefecture Improvement Program and NAR Section Grants.
     
  16. Nov 7, 2019 #16

    cwbullet

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    They do, we will have to apply.
     
  17. Nov 7, 2019 #17

    RocketRev

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    Hello Mike and everybody else too,

    Out of professional curiosity, I generally read thru every thread that I come across about somebody trying to build their own launch control system for high power rocketry. Folks inevitably get around to using relays at the pads. And that brings up the problem of trying to control voltage over long distances. But the critical issue with the controlled activation of relays over long distances is NOT JUST available voltage. It is available voltage + amperage and trying to harness them correctly over those same long distances. If your design is not dealing with controlling both the amperage and the voltage going into the relays at the pads, then you will inevitably end up with fused relays.

    And there's another kicker that's related to this that most folks seem to forget. When it comes to an analogue control system, you're going to need a different combinations of voltage/amperage for your control system design for each different distance. The voltage/amperage combo that will work at 100 feet between the controller and the pad will be different from the voltage/amperage combo that you're going to need for 200 feet, 400 feet, 500 feet, or at 1000 feet, etcetera.

    Until somebody figures this out there will inevitably be problems that will seem insurmountable. It might be possible that somebody will come up with the working solution for this problem, but I haven't seen it yet. I'll be honest and say that I agree with you Mike when you said, "It's an inflexible design. Plus, finding longer cables is virtually impossible, prohibitively expensive and unwieldy to the nth degree." And the more pads at the more distances you are trying to control makes things as you say virtually impossible, prohibitively expensive, and unwieldy to the nth degree.

    As you said, its an inflexible design. John's design is based upon several, decades older designs, none of which that I've ever seen, deal with the control of both available voltage and amperage in the control half of the design. And that's exactly why even in this short thread folks are already talking about going to 24 and even 48 volts in the control part of the system in order to overcome the various hurdles. All I can say to that is good luck, because there is no single analogue solution for for this problem because different relays have different voltage/amperage requirements for the control half of the equation. And controlling those same relays at the different distances required for high power rocketry adds another layer of difficulty into the design equation that in my experience is not being dealt with, let alone dealt with adequately.

    Dan Fox and I decided to find a way to work around the whole problem 25 years ago by eliminating trying to push control voltage/amperage out between the controller and the pad box. But as this is not a Wilson F/X thread, you can ask Chuck Haislip and Steve Shannon about that as they've already mentioned us.

    Brad
     
  18. Nov 7, 2019 #18

    RocketRev

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    Hello Chuck,

    Are you referring to getting one of our new controllers with all the bells and whistles for your club? Me? I love the RGB LEDs and the LCD remote voltage readout to say nothing of the Pelican case and the aluminum face plate....

    Your current controller will only handle the 8-banks of 8-pads each that you already have filled to capacity. You should probably think about an LCU-128x. Clearly we need to talk.

    Brad
     
  19. Nov 7, 2019 #19

    Reinhard

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    All the worries about wire gauge go away of one uses a driver for the relays. This way, only a very low current signal is required. The good old ULN2803, for example, is cheap and readily available. If one wants to really avoid semiconductors, reed relays are still around and can also be used to drive the main relays.

    One suggestion, if you're still open for changes at this stage:
    1) Change the pad arm switch to a relay or add one in addition to the local switch.
    2) Connect the beeper to the output of the arming relay, just like your LED already is.

    On the current design, if a relay gets stuck (single point of failure), the igniter will fire as soon as it is connected and the system is locally armed - both things that happen while you're close to the pad. With the arming relay, there is another layer of safety. Connecting the beeper directly to the relay will also provide an audible warning in case this relay gets stuck.

    Reinhard
     
  20. Nov 7, 2019 #20

    Mike Haberer

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    I agree completely and that is why I have both a technical design in the relay box that mitigates the fused relay issue as well as an arming procedure which, while obviously still prone to human error, attempts to address the shortfall in the design I followed to address the fused relay issue (I added this after the build when I realized the fused relay issue might still exist, albeit unlikely). For the technical bit, I have a 30A fuse on the relay's battery circuit to protect all non-relay components. In addition, the relays I used are also individually fused at 30A. It is highly doubtful that a relay will get fused under those circumstances. As for the procedural aspect, the relay procedure calls for arming the relay box (a manual SPST switch) and flicking the alligator clips together to see if they spark; if they do, the relay is fused and you go home. If there is no spark, you disarm the relay, attach the clips to the igniter and then step back and re-arm the relay box. My igniter leads to the pad for an HPR launch are 10 feet long, so you aren't RIGHT next to the pad when you arm the relay box. That said, the launch control I built will mostly be used for LPR and MPR launches, with the occasional HPR (H,I) launch. Anything bigger and I need to build a wireless relay system.
     
    Last edited: Nov 7, 2019
  21. Nov 7, 2019 #21

    Steve Shannon

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    Well, I disagree. A properly designed relay system certainly can be done using a single control voltage level without additional complexity. You just have to make sure you have enough current carrying ability for the longest distances. That’s why you go to a higher voltage, to keep the current low (if you want a simple and inexpensive relay system). Interposed relays or drivers like Reinhard mentioned is another way.

    Going digital is even better and wireless is much more convenient.
     
  22. Nov 8, 2019 #22

    UhClem

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    I am really mystified by this rant against analog systems and especially the claim that you need different voltages/currents at different distances to the pads. DARS has used an analog 12V system for more than 20 years. The design will work at distances from 10' to as much cable as you care to drag into the field. (Typically only 500'.) When I crunched the numbers many years ago I came up with a limit of ~17,000 feet. The actual limit is probably different but is certainly more than any practical cable length.
     
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