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Scratchbuilt

Old dog, new tricks

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Hi everybody,

New to the RC-CAM forum, been away from the sport/obsession awhile. First got my feet wet with a Schluetter Heli-Boy back in 1979. Modded it as far as I could go and started over with a clean slate. Last flew my v.2.0 still-photo-purpose-built, industrial duty .60 glow-powered collective pitch heli in 1998, having built it in the winter of 82-83. Lately considering a full-on restoration lest it become a museum piece. "Image III" is still more modular and robust than any of the stacked frame jobs I see commercially available, if I do say so myself.

One of the first orders of business. I'm about to spring for a modern PCM radio, I'm thinking. Original avionics was a old (top of the line back in the day) Kraft Series 80 7-channel radio, updated to later narrow-band 72mHz FCC frequency allocation--but of course it's so old as to be highly suspect, and just junking the NiCDs and re-celling might would cost nearly as much as a cheap modern radio with mixing (oh but what a joy electronic mixing and servo reversing will be, instead of mechanical mixers and bellcranks!)

Also have a pair of 1W Freewave Technologies 900 MHz 115.2 kbps Frequency Hopping, Spread Spectrum serial modems to tinker with. While they don't have the bandwidth of WiFi or WiMax, they were fairly robust in a 27 mile point-to-point endlink (+10dB yagis on each end) which was my internet connection for a couple of years, since replaced by a satellite ISP (don't ask). Wondering if anyone here has attempted to integrate a Freewave modem pair for the control link? Have seen a number of the university UAV and autonomous heli competition projects that used Freewaves for telemetry links, what about for control? I'm sure I could get several miles LOS with dipoles, they're that good.

While I can fly without any bells and whistles, I'm figuring I'll want as many channels as I can get for a camera ship, and that a FAI 3D pro radio with maximum channels is what I have in my sights. Autonomous capability with something like the CARVEC system is rather intrigueing, would like to keep my options open and keep everything compatible.

I'm a few hundred miles N of Los Angeles, thinking of touring down to Ontario for the AMA Convention next week to get some hands on, solidify my thinking.

In particular, can anyone point me to a resource that might help me integrate the Freewaves in place of Tx/Rx modules with a modern helicopter radio--hopefully with a minimum of pain :) ? Or a source for another pre-configured FHSS heli radio that would integrate with an auto-pilot and 3-axis rate sensors (spendy or otherwise) ?

Thanks in advance,

Ivan

Edited by Scratchbuilt

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Or a source for another pre-configured FHSS heli radio that would integrate with an auto-pilot and 3-axis rate sensors (spendy or otherwise)

The new Spektrum DX6 radio system is a FHSS system. It has heli mixing, but is really being marketed for small park flyers due to its practical range limits: http://www.spektrumrc.com/. Dave Jones advertises a custom made FHSS system: http://www.auav.net/spread_spectrum_radio.htm

lvspark has a FHSS modem controlled SlowStick. It uses a laptop and PC joystick. I heard a claim of several miles range. He mentions his project here: http://www.rcuniverse.com/forum/fb.asp?m=3733237

There are other projects like what you propose out there, but few give out all the secretes. No doubt you will have to blaze a few of your own trails.

Edited by Mr.RC-Cam

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Many in the UAV industry have used modems like the Freewave to do r/c ontrol links. The main issue becomes system latency and bandwidth.

The Spektrum system is actually DSSS, and interestingly enough, it's an off the shelf wireless usb link. I bought one to play with and it is certainly a neat device for the money.

Michael

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Thanks for the links, guys, some interesting stuff.

I'd run across Dave Jones's site some time back and noted that his FHSS Silvertone looks to be about the same vintage as my Kraft Series 79 and 80 sets, thus providing the inspiration for possibly hacking in my Freewaves if I could. Hey, come to think of it, even back then these radios were called DIGITAL proportional... though not sure if by "digital" they meant thumbs :)

Truth be told, hacking Freewaves into a new heli radio myself without finding someone who's done a proof-of-concept on the circuitry ain't gonna happen, at least not for my control link (perhaps telemetry, but I'm thinking that 115.2kbps probably isn't enough bandwidth to fly a heli by an uncompressed video feed). I'm partial to the 900mHz band for it being less crowded hereabouts than 2.4gHz, and was really impressed with how robust these particular modems were as my internet end-link shot, even cheating by knife-edging over and around a mountain ridge 500 yds away. It's been a couple of years since we took them out of service, but from what I remember the latency only became a problem at the limits of reception, when whole (>1500 bit?) blocks of data would drop out and had to be resent. The block size is fully configurable in the modem setup menu, though more frequent error checksums would slow down throughput somewhat when the link was strong. Then again, how much bandwidth does even a 14ch R/C link requeire? (I'm thinking probably not very much, since most modeller-oriented radios are content using the 72mHz narrow-band regime).

Googling around, it's apparent there are any number of folks using amateur R/C gear for business purposes. Used to be true that the FCC regs expressly prohibited commercial use of the amateur freqs. If that's still the case, this would seem to be a large incentive for development of commericial UAV radios using FHSS bands, and it's a bit surprising to me that there isn't much of a spread-spectrum module and receiver industry sprung up just yet. But it's looking hopeful, with stuff like Spektrum is developing.

What sort of range are you seeing with your DX6 set, Mluvara? While I'm reasonably sure those teeny servos won't cut it on my 12lb .60 size heli, it's closest to what I'm looking for so far.

Ivan

Edited by Scratchbuilt

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Haven't flown it yet (tomorrow), but just played with it around the shop. I've heard reports of 2000+ft. Needless to say, I've been curious about the robustness of the link and have been playing with other systems close to it. So far, so good.

BTW, the DX6 receiver probably won't handle high current loads required out of larger servos.

Michael

Edited by Mluvara

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The receiver does look kinda wimpy, and that's only one concern, another being how robustly built it is, i.e. how well soldered (because all helis vibrate at certain flight attitudes, even silky-smooth ones like mine).

Been following this 14 page Spektrum hacksie project this afternoon, maybe we can induce someone else to do the R&D recon on the servo amperage question:

http://www.rcgroups.com/forums/showthread.php?t=458314

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Quite honestly, this doesn't surprise me... That also includes the fact that some are hacking into and adding the modules to other transmitters. Well, technically speaking, it is not a module.

I bought it to put in some small electrics and that's all that it will go in.

Michael

Edited by Mluvara

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Apparently calling this Spread Spectrum is a bit of a stretch. Spread spectrum as I know it is frequency hopping, this is not, unfortunately. It's a pretty good implementation of a diversity system with a lock-out for in-use frequencies, but only upon startup... interference could easily swamp the signal and there's no hopping after the fact if it does...sigh.

Too, from what I know of the signal propagation issues, 2.4 GHz is fussier and offers less range than 900 MH (also a license-free band). This might be a good scheme for a park flier, or in a congested area if everybody starts using their technology, perhaps? Don't know particularly why they'd choose 2.4GHz other than the chips are essentially WiFi chips and therefore ubiquitous and cheap.

Anyhow, the Spektrum thing was an interesting diversion from my project for a time.

But back to the drawing board. I've got a couple/three of these RS232/DB9 serial modems that can be configured for any number of data protocols. What I'm thinking is that I ought to be able to pretty simply use a servo controller board on the flight pack side, something like this:

http://www.yostengineering.com/index.cgi?s...oCenterUSB.html

and on the Transmitter side, utilize the USB simulator port with an adapter to DB9, bypass the RF portion of the transmitter with my spread spectrum link. Need to figure out the protocol of the signal as it comes out of the transmitter.

I haven't got a modern radio yet-- do these use a standard USB cable or is there some black box involved as well?

Anybody done any simulator work to know what the signal looks like, and offer an opinion as to the feasibility of what I'm attempting to do?

-Ivan

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Spread spectrum as I know it is frequency hopping, this is not, unfortunately.

Spread Spectrum comes in two flavors: FHSS (freq hopping) and DSSS (direct Seq). The Spektrum Tx is a DSSS design based on a very low cost wireless USB controller IC. Both methods allow for more RF power than would be available if spread spectrum was not involved (license free consumer devices would have to be limited to under 1mW otherwise).

Don't know particularly why they'd choose 2.4GHz other than the chips are essentially WiFi chips and therefore ubiquitous and cheap.

It was a cost & availability issue. Silicon vendors are just not offering a wide variety of 900Mhz spread spectrum controller chips (900Mhz is essentially dead, mostly due to marketing bias).

I haven't got a modern radio yet-- do these use a standard USB cable or is there some black box involved as well?

The Tx's trainer jack, if present, provides a PPM formatted signal. The signal polarity, voltage levels, and channel order are not fully standardized among the various brands. Your o-scope is your friend in determining what to do.

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