Rocket Launching System - Design Compromises...

I've been looking around the Net, in an attempt to find RF transceivers that will allow me to set up my *potential* launching
system for wireless operation. I've come up with the following roadblocks...
Long-range (5000-10000 foot) radio modules are expensive. Since each group of ten pads would require one module, expansion adds up. Jerry - would *you* be willing to pay an additional $250 per ten-pad grouping for wireless...? ;-) There are lower-cost modules available, but their use would limit the effective range of the system as a whole. High-power pads would be limited to under 1000 feet from the LCO, so multi-M away cells would be out.
The transmitters I've found are available in analog-only or digital-only modes. In order to have a continuity tone sent back to the LCO console, I'd have to add a second transmitter in each 10-pad module, and a second receiver in the LCO system. Can personal 'analysis' of the continuity signal be eliminated...?
Thoughts...?
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Len Lekx wrote:

You want a two-way system for more reasons than just to get the continuity signal back to the range head. The primary reason is so that you will know if the remote cell is operating correctly. With a transmit only system, you simply transmit a launch command and hope that something happens. With a two-way system, you can get an acknowledgment of each command.
Plus you can get extra information back such as continuity status, battery condition, and other diagnostic stuff.
Range can be improved by replacing the typical rubber-duck antennas at the pads with a simple (and cheap to build) Yagi.
The system should be designed so that it can use both wired and wireless connections. Wired connections are fine for all of the closer pads (up to K motors or so) but the wireless system works best for those really long runs to the M pads and out.
And keep in mind that most of the modems available are Part 15 devices. That means that if some other RF source interferes with your system, tough. So being able to fall back to a wired mode is a good thing.
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On Sat, 01 Nov 2003 01:06:49 GMT, David Schultz

The plan *is* for it to be a two-way system. The LCO console sends commands out to the pad-group controllers, and they answer back. The difficulty comes in requiring a *third* channel for audio continuity transmitted to the LCO.

True enough. But I'd prefer to put the Yagi at the LCO table, and have each pad box use a rubber-duck antenna... easier to set up that way, since there's only one antenna to 'aim'.

That *is* an option I'm considering... but some of the earlier talk here centered around going totally wireless.
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Len Lekx wrote:

If you have a digital system (and I wouldn't trust an analog system), it is just different information in the data packets. No extra channel needed.

This would not work unless all of the pads were located along the same line of sight. I have not yet seen a range setup this way. Not the DARS, LDRS (Kansas), or Rocstock ranges.
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I believe Rocketfest is.
--
Jerry Irvine, Box 1242, Claremont, California 91711 USA
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On Sat, 01 Nov 2003 02:30:24 GMT, David Schultz

But - the continuity tones will be pseudo-analog. (555 timers at the 'pad' boxes rigged as voltage-controlled oscillators.)
How do I combine the digital 'command ack' signals with the continuity tones, and separate them at the other end...?
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Len Lekx wrote:

An ADC captures the continuity info. This is encoded into the data packet. The microcontroller on the other end pulls it out.
Very simple.
Unless you are trying to use a digital system that has a fixed format you can't control. Like a keyfob transmitter.
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On Sat, 01 Nov 2003 05:24:10 GMT, David Schultz

That's *kinda* what I'm planning. Communications are half-duplex, so the LCO 'Master' can't transmit while any of the 'slaves' are sending data. Since ADC info would be a continuous stream, that's ruled out.
Plus, I was intending to use the pad-relay boxes that John DeMar designed, which send an audio tone that reflects the resistance of the igniter. I'd have to do a frequency-to-voltage conversion, then an analog-to-digital one, to get it into transmittable form.
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Len Lekx wrote:

ADC info is NOT a continuous stream unless you make it that way.

That is more of a problem. I kind of suspected that there had to be a reason behind your messages. Now they make sense.
But now you don't need an ADC.
AC couple the signal to an op-amp. Amplify and clip the signal into something resembling a square wave. Feed this to an edge triggered interupt pin on the microcontroller. Now all you have to do is measure the time between interupts to decode the signal.
Now your controller sends a message that means: "pad X, tell me your continuity status": and it responds with: "it is N" where N is a number dependent on the frequency you decode.
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On Sat, 01 Nov 2003 15:20:02 GMT, David Schultz

No... but if you want to hear a continuous tone, it should be continuous enough to not make a difference. :-)

I'm still learning low-level programming, and I was hoping to avoid using interrupts for anything other than receiving the first "address" byte of the command sequence.
Shouldn't be *too* hard to implement, though...
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Len Lekx wrote:

You would have to sample at a high enough rate, of course. In the case of the ~1KHz continuity tone, you would have to have a >2KHz sample rate on the ADC and have enough bandwidth to send this over your digital link, plus overhead. Then put multiple pads on the same digital channel and you have a bandwidth problem. All of this would be useless because the continuity *information* changes at a relatively slow rate. If you had a status a few times a second from each pad, that would be more than enough info, and your datalink wouldn't get bogged down. Plus, it's simple enough to reproduce this tone on the other end by toggling a pin.

You could probably pole the pin to measure the period of the tone instead of using an interrupt, as long as you don't have anything else to do in the microcontroller that's critical.

Between David and me, you'll have enough ideas to keep you so busy you'll never finishing the project! ;)
-John
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On Sun, 02 Nov 2003 11:16:56 -0500, John DeMar

Well... since the LCO would only be able to 'select' one tone at a time, that removes the bandwidth problem. :-)

I got to thinking last night about that... what I came up with is this:
I use a tone-decoder set to the range of frequencies the pad-box can signal. This would produce a square-wave that gets sent to the interrupt pin. On interrupt, the MCU starts a timer that's programmed as a pulse-accumulator, which will count the pulses of the pre-decoder signal. It watches another pin, which is tied to the post-decoder signal, to determine when to stop counting. Since your pad boxes are set up to produce four pulsed-tones per second, I multiply the accumulated count by 4 to give me the frequency. That info gets sent back to the LCO, for reproduction at the console.
Sure beats my sleep-fogged idea of using a frequency-to-voltage converter, a sample-and-hold, and an ADC... :-)

Finished, heck! All the stuff I've got to figure out, I may never even get *STARTED*! ;-)
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wrote:

Probably not since I am a misfire alley guy (lanes not groups).
I have spent 20x that much on GSE over the fest history.

You are basing your assumptions on CENTRAL CONTROL, which I am a non-believer in.

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wrote:

Yeah. One LCO console to control all the pads, including away-cells.
What would you prefer...? One LCO for each pad used? :-)
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wrote:

YES. (You for yours).
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Len Lekx wrote:

All you have to do is put your PAD on a pad and let 'er rip!
JI's mottos: 'buyer beware' and 'flyer beware'. ;-)
-John (trying to ignore the leak in my JI filter)
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Two way data rf modems are expensive but very little info has to be transmitted. High baud rate is not needed. All data can be put into a packet with a pad address and commands. Add the tell me three times protocol for reliability. A pic can handle it. The problem is finding cheap transmitters and receivers with range greater than a couple hundred ft. with license exempt. Would a preamp boost the range of something like the LINX receivers work? I was going to try the kh series but never got around to it . Gary Deaver

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The RF-900CLP analog transceiever is available for $19ea. If you prefer, the RF-2400 is a "data" modem at twice the price.
You can easily send digital data over an "analog" transceiver. The data modems typically just add a data slicer on the RX side. You can build this yourself if you like.
Range is stated as 1,800' line of site but with this can be increased by using an antenna with a bit of gain.
Deaver wrote:

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On Sat, 01 Nov 2003 05:23:10 GMT, David Schultz

True. The big question is doing analog and digital simultaneously. Separating the 56kbps digital data from the ~1khz 'analog' isn't something I'm good at. :-)
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Len Lekx wrote:

Actually, with that kind of frequency relationship, it should be easy enough to combine and split them with simple RC networks...
-dave w
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