Pixhawk installed in Vulture 2 spaceplane

We've wrapped the installation of the Pixhawk in our aircraft, and have now just got to get it talking to our onboard Raspberry Pi.

For your viewing pleasure, here's a schematic of the Pixhawk and R/C rig. There are full details on the install here.

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Moderator
Comment by Gary Mortimer on March 10, 2014 at 7:05am

Will 4 AA's be enough?? Good against the cold I guess.

Comment by Lester Haines on March 10, 2014 at 7:09am

Yup, we've done the calculation, and four AAs have enough juice. The Ultimate Lithiums are the only ones we'll use (good to -40 degs C or thereabouts, according to the manufacturer), and we've sent them aloft on various occasions without problems. 


Developer
Comment by Andrew Tridgell on March 10, 2014 at 11:48am

Hi Lester,

Good stuff!

A couple of comments. I haven't tried that sort of battery before - it would be interesting to see how it performs. If you have some time, can you install the latest plane firmware and get a tlog with it all powered up, while you are rapidly moving the servos. The most recent firmwares will log a bunch of data on power status, including servo rail and brick input. What I'll be looking for are transients in voltage that may cause an issue. Note that with the digital servos the transients could be positive, and it may be that you need a zener clamping diode on the servo rail.

For the RPi hookup, have you seen this page. It is something that Randy put together to help people get their RPis hooked up to a Pixhawk properly.

On the radio links, is the 3DR Radio just for short range telemetry, or do you want it to hold link at long range? It is designed as a short range (few km) link. For a much longer range radio look at the RFD900 or RFD900u at rfdesign.com.au. With the right config that would give you full MAVLink control the whole flight.

Cheers, Tridge


Developer
Comment by Andrew Tridgell on March 10, 2014 at 11:57am

A couple more thoughts.

Are rubber bands good to the temperatures you will reach, or will they get fragile?

Is the Pixhawk mounting foam good to low temperatures? As I understand it you won't have a motor, so vibration will be minimal. I'd probably just hard mount the Pixhawk and not bother with the foam.

I'm also always nervous about radio transmitters that close to a GPS. I think it will be OK as we usually don't see interference between a SiK radio and a GPS, but more distance couldn't hurt :-)

Cheers, Tridge


Developer
Comment by Andrew Tridgell on March 10, 2014 at 11:59am

next thought ... I don't think the safety switch will be of a lot of benefit for your setup. You may like to avoid it with the latest firmware by setting BRD_SAFETYENABLE=0. The main benefit will be that if you reset in flight for some reason you won't need to send a rescue shuttle up to push the button :-)

Comment by Quadzimodo on March 10, 2014 at 12:45pm

Tridge - Those Energiser Lithiums are just incredible in my big SB-900 Speedlight, massively outlasting anything else out there.  They are valued in pro photo not only for their endurance, but also because of much faster recharging between flash discharges.  They should be perfect for this application.

Comment by Lester Haines on March 10, 2014 at 12:59pm

Hi Andrew -

Just to take your brainstorm points in order ;-)

Batteries - I refer you to Quadzimodo's comment. The Utlimate Lithiums are the mutt's nuts. We have no doubt they can do the job - tested up to 35k metres, no problem.

Firmware - Yup, I'll get that sorted as soon as I've done painting the Vulture (my current task), and report back.

RPi hook-up - I'm waiting for the Pi breakout board before proceeding. Thanks for the link. I need it!

3DR radio - It's just for short range. We don't need long-range capability, but coincidentally are looking at such a transmitter for live video feed from the launch platform. I'd pay ten bucks to see that footage.

Rubber bands and foam mounts - Actually, there's possibly an issue with too much heat internally at altitude, with the electronics unable to radiate heat away. Dave's had this problem before. He's confident the interior will remain well above external ambient.

GPS/transmitter - Yes. Unfortunately, it's a very tight squeeze in there. Took some working out how to get it all in. We'll see if it's an issue in testing.

Safety switch - That had crossed my mind. Since it's only for arming on the ground just before when the plane's assembled (that's essential, since it's a real hassle to disassemble), I thought we could programme the switch to disconnect after launch, as part of the custom flight parameters

Comment by Quadzimodo on March 10, 2014 at 1:19pm

Lester - Isn't "mutt's nuts" a development on the term "dog's bollocks", meaning nonsense or crazy? To quote an iconic scene from Australian Action Film Heritage, and to be specific, the Energiser Lithiums are "the d-d-d-d-ducks guts".

Comment by Lester Haines on March 10, 2014 at 1:26pm

Quadzimodo - Ha! Mutt's nuts is indeed an evolution of "dog's bollocks", but in Blighty simply means "the best", without connotations of "mad". Great clip, and "duck's guts" is a classic. I'm a big fan of Downunder English, and have been in Oz a couple of times.


Developer
Comment by Andrew Tridgell on March 10, 2014 at 1:28pm

Hi Lester,

I just had a look at the datasheet for those batteries, and while it is impressive, it does show that it won't handle -40 degrees C with significant current draw.

What is your current draw when everything is turned on? I'd guess around 600mA? Have a look at the "temperature effects on capacity" graph for those batteries. At 1A the capacity drops to zero at -40C.

If the fuselage does stay warm then you'll be fine. If it gets really cold I think you'll lose power.

Regarding the RPi breakout board, why do you need that? You can connect directly to that USB TTL serial adapter or plug hookup wires straight into the ttyAMA0 serial port.

Have you worked out the expected CoG of the plane?

Regarding live video transmitters, if you are referring to an analog one those are notorious for causing GPS interference (as well as interference with pretty much any radio receiver you can imagine). Or are you using the Pi cam and digital transmission?

It may be worth getting a a RFD900u at least. It should give you enough range to get full control all the way through the mission (with a good ground antenna). That also means you'll have telemetry logs even if the mission fails and you lose the plane with its on-board logs. You'll just need to check with your teams HAMs if the freq range of the RFD900u is something they can transmit on in the UK (at 100mW).

Note that you can also do comms relaying between the Pixhawk and the RPi, in either direction or both directions. On my planes I have two comms links, one on the embedded Linux box and one on the Pixhawk. Both computers send their own telemetry over their own links while also relaying over the other link. That means I have full control over both the embedded Linux box and the Pixhawk as long as at least one radio link is up. I also relay images (slowly, low res!) over the RFD900 link from the camera connected to the Linux box.

Cheers, Tridge

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