As most of you may or may not know, I've been a big proponent of using PPM Sum (aka CPPM) with the Ardupilot since I first figured out how to make it work. The simplicity of just running a single wire from the Rx to the APM is very attractive. It eliminates 7 possible failure modes (losing the wire connecting channels 2-8) which could result in loss of control. There's only 1 wire to get right, but also one wire to get wrong. Fortunately, the APM can handle complete loss of signal well, and if you set your failsafe to handle complete signal loss, all is good.
I was using this on my HK450GT FBL helicopter, along with a FrSky D8R-SP Rx. Again, as anybody who knows me is aware, I'm a big fan of FrSky radio equipment. It is affordable, and high performance offering true diversity on the Rx, easy to use CPPM (just requires a jumper on two pins), the Rx is very tolerant of low voltage (it'll keep going LONG after the APM would have shut down, down below 3V). And rebooting and reacquiring the Tx signal takes less than 1 second.
All of these features are superior to the big name brands, Futaba, Spektrum, etc.
And the telemetry features offered by FrSky is just the icing on the cake. RSSI (signal strength), Rx voltage, external voltage (flight battery), and even the option for a telemetry hub with all kinds of goodies This is somewhat negated by using an APM, but still, you can put one of these on your Tx and have all the info right at your fingertips.
Anyway, back to my little 450 heli with a HUGE Rx mounted on it, the D8R-SP.
I discovered that FrSky offer a new Rx, the D4Fr which is a small, yet full-range Rx. And while it only has 4 channels out, it can be configured to do 8-channel CPPM with the simple addition of a jumper! Brilliant, and there's no reason why any modern compact receiver shouldn't be able to do this. This Rx does not have as many telemetry features as it's big brother. It can only report RSSI and Rx Voltage, it does not have connectors for the telemetry hub or external voltage. But that's fine, as the APM can be used to report voltage anyway. So I ordered one and it arrived Friday.
I started working on setting it up, when the wheels started turning in my brain. The "case" is just a flimsy plastic thing, almost useless anyway. And I'm only going to have a single cable going from the Rx to the APM, and it's so small. And the APM2 has those stand-off pins onto which the daughterboard is soldered. And those pins are kinda long... And I've got this piece of empty prototype board sitting around...
Pretty soon I had de-cased the Rx and stripped all the input pins off the APM2.
I soldered the proto-board to the daughter board standoffs, and then glued the Rx board onto the proto-board. I soldered jumpers on both the Rx and APM2 in order to set both into CPPM mode. And then I soldered a jumper wire for the CPPM signal, power and ground. Done.
The complete unit is compact. There's no wires to get in the way of whatever vibration-damping mount you are using. There are no wires to come disconnected. I suspect this setup is at least an order of magnitude more reliable than a standard 8-channel APM2 setup.
Even better, it's compact, and easy to move between vehicles.
Comments
Funny, I had two guys come up to me at the field, both with Spektrum systems, and told about the problems they were having. I've never had any.
@Mark, thanks, I've already bought this I could try:
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=...
But I'm not sure it will work with the telemetry system as it says it's for the V8? I'd like to get like a little extension, and then move my antenna off the module and attach it so it points upwards?
The model is all balsa, so I don't think it would have interference in the airplane? But I can give those longer ones a shot. I can stick one in the old 72MHz tube that is already in the model.
@Cliff, I'm not an expert, but I have seen statements that the FrSky is better, if not the best. Please understand, I'm pretty newbie when it comes to this high frequency stuff. I did all my flying with 72MHz, and I'm just learning here. So it's possible I've done something wrong. Like pointing the antenna at the airplane.
All of these features are superior to the big name brands, Futaba, Spektrum, etc.
I'm wondering: is the FrSky signal more robust than all those big brand names? I hear it uses a modified AFHSS--has it been tested as robust as the majors brands? Cause if the signal isn't robust, PPM-sum is sort of moot if the signal is unreliable. If you've been bitten by the DSM2 issues (e.g. signal drop in noisy areas) like I have, added features like PPM-sum add more risk (i.e. fail safe as one thing). And...
"The range was quite a bit shorter than I'd have liked..." sort of answers my question.
And yes, if you're Tx antenna is a typical dipole, pointing at the vehicle (as well as if the Rx is pointing too) is not the best direction. Still a great effort nonetheless, and getting the PPMsum for DSMX or S.bus sounds like a bullet proof signal and wiring solution.
@Robert Sinclair, upgrading a Turnigy 9x is pretty easy. I have some notes here:
http://eastbay-rc.blogspot.com/2012/03/frsky-installed-9x-module-re...
@R_Lefebvre, this is great! You're right about the Tx antenna, since the signal is basically a toroid being sent parallel to the axis of the antenna. Andreas and I both have this unit which might help.
http://www.hobbyking.com/hobbyking/store/__20108__2_4Ghz_7DB_Antenn...
There's two extended-length Rx antennas that might help, in case you're getting any interference on your plane:
http://www.hobbyking.com/hobbyking/store/__16667__FrSky_Receiver_an...
http://www.hobbyking.com/hobbyking/store/__16668__FrSky_Receiver_an...
I just came back from the flying field using the all-in-one. I put it in an old 80" Piper Cub with a Saito 4-stroke that I haven't flown in 8 years. It worked pretty well. The range was quite a bit shorter than I'd have liked, as it was warning me about the signal at about the distance that I would fly a regular RC airplane at.
It might have been because I was using a transmitter tray, and the result was the Tx antenna was basically pointing at the aircraft, which IIRC is bad?
FrSky also makes the Futaba FASST compatible TFR4B. This 4 channel full range rx is quite cheap, small and light and can output CPPM and RSI using the jumper method. It works like a charm with APM2 and my T8FG.
@Veikko: I don't know about the TFR6, but there is no mention of CPPM in the manual, so probably not.
@Veikko I read S-Bus is an inverted logic TTL serial connection, I think it would take a good bit of effort to support that vs PPM. Not sure if the hardware pin used for PPM sum is going to work without an hex inverter chip (thats how Mikrocopter does it if i'm not mistaken).
Ravi, it's the signal pins for 2 & 3 that need to be shorted together. Just like APM1. The wiki is wrong about APM2.
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