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
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.
Ravi, it's the signal pins for 2 & 3 that need to be shorted together. Just like APM1. The wiki is wrong about APM2.
can someone suggest which jumper on APM 2 has to be shorted to supply PPM to IN1.
@All: I did a quick test with this and it seems to work. The only issue is that it seem that having the telemetry transmitter so close to the GPS puck causes a bit of trouble. It takes about 5-10 minutes to get a lock, but then after that it's fine. It doesn't seem to affect the performance. If that is a problem for you and you don't want telemetry, maybe you could use this Rx?
http://www.hobbyking.com/hobbyking/store/__22692__FrSky_TFR4B_2_4Gh...
This isn't a problem for me as I use a seperate battery for the APM2 typically, so I just plug it in first and by the time I've set everything up, it's good to go.
I would LOVE to set up the APM to output telemetry to the D4FR-ii Rx. That would be GREAT. It's a bit beyond me but maybe we can work on it together. Ideally I'd like to use the same protocol as the sensor hub, so that the remote display works the same way. This appears to be the protocol that the Sensor Hub uses.
http://www.frsky-rc.com/uploadfile/201107/20110727110301692.pdf
I love that FrSky provides this info!
@Chris Huitema, yeah, I have a few older FrSky 8Ch Rx, and I had to flash them for CPPM as well. It was a little more involved because the older one required an extra electronic widget and had to be decased. But I got it done.
@Veikko, you're on your own for that one. ;) I have no SBus equipement, and not a lot of interest in delving into it. It's expensive and closed source.
@Daniel, yes, the APM1 can also take CPPM input if you have the updated firmware. If you bought recently, it should have it already. It the unit is old, it does not, and you'll need to update the PPM firmware. You'll have to figure out the details for mounting, as it's not as obvious, but should be possible.
very nice. well done!
Have not done any extensive research but, should also be easy to use the Flytron Open LRS (already in open circuit board form) Just jumper two pins for serial output!
The missing link, well done that is a top idea. Especially as I already have some FRSKY gear heading my way to try!
Genius !Good Post! What I like is you guys have dealt with this stuff so much you just have to look at it and you know if it will work or not and are not afraid to cut into a circuit board for the betterment of the product for your own use I have been pricing some of the stuff for my own set up it is not cheap.I have been following along slowly gathering knowledge I learn something new every day when I turn on the computer
@Jack, I've got all the open source I can handle already. I need at least one thing to "just work". ;)
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