[Paparazzi-devel] Re: Paparazzi UAV Project

[antoine . drouin]

Selon Hugo Vincent <address@hidden>:

> Hello again Antoine,

> Like i said, i am new to RC, so: what is the advantage of PCM? just 
> better range and signal quality? or something more subtle...

It is for robustness to radio jamming. With the PPM type (analog) you cannot
tell for sure is the signal you are receiving is correct and originating from
your transmitter. As this signal is used to switch the autopilot on and off, it
is a little risky.

For now, we do a sanity check on pulses width and filter the mode toggling
channel. It has improved things but I really would prefer a PCM encoding type.

Problem is that PCM is dependant on radio gear manufacturer and they won't
disclose the specifications of their encoding.
A great job has been done by Trammel and  Coert S. Langkemper in reverse
engineering the Futaba PCM encoding.

see  http://www.flightsense.nl/pcm1024/docs/pcm1024_signal_analyzed.pdf

and http://autopilot.sourceforge.net/pcm.html

> ok, how much of the 1300g is batteries?

450 - 500 g for 8x2400nicd or 3300 nimh

300g for 2P3S 2000mAh lipo


> I did a project at work with Lithium Ion cells on a handheld computer 
> design we were working on -- a charger and power supply unit. At least 
> with the cells we were using, they prefer constant, lower current draw, 
> i.e. not 30A, and not on-and-off, but if they had to, they can supply 
> fairly the high currents. I am not 100% sure about the differences 
> between LiIon and LiPo, but have heard LiPo have a shorter life cycle 
> (# of charges) but have a better energy density. I have heard that 
> after around 1000 charges for LiIon or 500 charges for LiPo, the total 
> capacity is about 1/2 of what it was new. I would guess that high 
> discharge rates would decrease them somewhat.
> 
> For comparison, the RCtoys Predator 
> (http://www.rctoys.com/predator.php) apparently runs 1 hr 30 min on 
> LiPo, and 16-20 min with the NiCad. There battery pack is 7.4V, 
> 4200mAh, for US$139. A quick look at www.jelu.se which sells (among 
> other things), surplus Nokia LiIon cells of 3.6V and 1200mAh capacity 
> for about US$13. So 6 of them for about US$80 would give 10.8V and 
> 4400mAh!!! Maybe too heavy though.

The battery I have bought are 3.6V kokam 2400mAH. I intend to use 6 of them
(10.8V and 4000mAh). They are rated for 6c to 8c current -> 24-30 A

I expect 30 minutes with the stock speeds400 and maybe up to one hour with
brushless motors and good props.

> The charging is a lot more complicated for Lithium, than for Nickel 
> cells. But you can get nice single chip solutions to do it (we used the 
> MAX1649 + an ATtiny micro).
> 

 What I find cool with nicd (and nimh to a certain extend) is the fast charge.
You can charge in half an hour before going to fly. Lipo don't stand such 
current.

Regards

Antoine