RE: [Paparazzi-devel] radio interference (was: downlink only)

[gisela.noci]

Hi Martin.

These things are never easy to remedy! It is important to try ensure that
you do not have any ground loops though. For example, the power supply
ground (-ve) to the autopilot may go direct to autopilot, and then to the
RCS RX, and then to the servos from the RCS RX. This is not a good thing,
since each time the servo is activated, it causes a peak current flow
through the ground wire to the autopilot, and then to the power supply(
battery) Depending on the wire thickness and the quality of the connecters
in the path, there will inevitably be a voltage drop over these wires. This
volt drop in the ground lead can have the same effect as a pulse on the PWM
line to the servo, ie, the servo and/or autopilot may see a 'dip' in the
ground lead in the same way as it does a 'peak' on the signal lead. It all
boils down to what actually is the 'ground' reference. Routing of grounds is
most often the cause of ground loops and resulting glitches. It is always
best to try and achieve a single point grounding network, ie, have a single
ground point as the main ground connection, connect your battery ground to
this, and then lead of grounds to all the users in the system from that
point. At this ground point and close to the case of the modem and RCS RX,
wrap the ground wire 2 or 3 turns through a high permeability ferrite ring
core. It is also sound practice to follow the same technique with the
positive supply leads as well.

I implemented this method on all my planes, and do not have problems any
more. However, a further and more serious problem stems from the brushless
motor speed controller, although only relevant in the larger (higher
current) motor sizes. These controllers work by switching (pulse width
modulating) the DC to the motor, high frequencies and currents. The
resulting square wave switching generates very wide spread high intensity
radiated electromagnetic fields, which cover an incredibly wide range of
frequencies, including the all important 35MHz... This normally manifests
itself as follows.. You do a range check, a quick throttle blip, and all is
well. Prepare for takeoff, and go... Full throttle, aircraft lifts of, and
proceeds away from you, at which point the radiated emissions from the ESC
start to exceed the 35MHz transmitter signal, as the aircraft gets further
from the RCS TX, and suddenly you have no more control; failsafe and
crash....

I solved this as follows :

Choose a suitable place in the fuselage where you can stick a piece of
adhesive backed copper foil, at least 10cm X 15cm ( larger IS better..) use
the center of this as the single point ground ( solder all grounds to the
center, or fairly close to the center). Place a ferrite ring over each servo
lead, at the connector and near the servo. Place a ring at the receiver
connector inputs, on each lead into the receiver/out of the receiver. Open
the RX case and replace the antenna wire with a thin coax cable, length
equal to the original antenna wire, plus spare as you would require to route
in the aircraft. Solder the coax center to the antenna connection, and the
shield to the closest ground point in the RX module. ( Warrantee  , what
warrantee....). Wrap the receiver module completely, in the same type copper
foil. Using a section of the braid of some spare coax cable, solder a piece
of the braid to the copper shield around the RCS RX case, and solder the
other end of the braid to the main center ground connection.
Fit all the bits in the Aircraft. Route the antenna coax to and through the
required fuselage exit point. Leave intact about 300mm of coax protruding
the exit point , and from this point, strip the shield from the coax, so
that the center conducter is exposed, forming the main antenna, as before,
and trim to the same length as the original antenna. I had no more glitches
at all, with an AXI motor drawing 70amps on takeoff, 12amps at cruise.

Have fun.

Joe



-----Original Message-----
From: address@hidden
[mailto:address@hidden
] On Behalf Of Martin P
Sent: Tuesday, July 28, 2009 8:38 PM
To: address@hidden
Subject: [Paparazzi-devel] radio interference (was: downlink only)

Hello Joe!
 
Thank you for your long answer.
 
The cable I am talking about is the 3-wire cable with the PPM signal from
the receiver to the autopilot and 5V supply from the autopilot to the
receiver.
The receiver is a Yeti Rex, 35MHz, see
http://paparazzi.enac.fr/wiki/Other_Hardware#JETI_REX_5_plus_.28no_MPD.29_re
ceiver
The onboard modem is a 2G4 / 1mW Zigbee with a Nintendo WLAN antenna.
The autopilot is a Tiny.
You can see the setup here
http://picasaweb.google.com/lh/photo/wDgwmBGefqntD2ac6ElqQA?feat=directlink 
http://picasaweb.google.com/lh/photo/OWSuzgAoyvPwSZ5bp6QvNg?feat=directlink
The 35MHz antenna is taped to the left wing (not seen in the picture).  
 
Initially, I twisted the +5V, ground and PPM wires. For a picture, see
http://picasaweb.google.com/lh/photo/8m9HAOhXbbx1MPgfyTj5oQ?feat=directlink
 
I noticed radio interference when I did a range check on the field
(retracted antenna, 45m). I was able to control the servo (at this time,
only one servo was connected). OK, but I had spontanous servo movements too,
even when I was only 5m away.
 
Removed the modem, no change.
 
Next I strung the twisted PPM cable (including 5V supply) through a ferrite
core. This defineitely improved the situation.
 
Next I soldered capacitors to the 5V/gnd on the receiver (in case the
interference would be caused by the switching power supply). No improvement,
removed the capacitors.
 
Next was the shielding of the PPM signal. The shield is connected to ground
on the autopilot side only. The improvement was convincing, as I had found a
spot in the apartment where the servos moved wildly, and with the shielding
it was gone completely. Back on the field, I had the random servo movements
again (transmitter antenna retracted) and I were still able to control the
servos from across the field (40m).
 
Put in the ferrite core again. Seemed to help a little.
 
So what the hell, up in the air. It's an old, small, slow foamie (Multiplex
Minimag) and there was nobody around to get in danger, except myself. You
know that I know that I shouldn't do that. It was windy, too. I was so eager
to see it fly.
 
For the next steps:
A shielded lead from the fuselage to the actual antenna seems easy to do.
However, in the foamie, I am not really convinced that it will help. But
I'll try.
 
Ultrasonic height measurement is something I would like to do too, but in
future. Your report of a 4m plane automatically taking off and landing is
very impressive!
 
Thanks and Greetings, Martin
 
 

-------- Original-Nachricht --------
> Datum: Tue, 28 Jul 2009 08:24:51 +0200
> Von: "gisela.noci" <address@hidden>
> An: address@hidden
> Betreff: RE: [Paparazzi-devel] downlink only

> Hi there
>
> First time I am using this forum to comment on peoples comments, so hope I
> get it right.
>
> Cannot really comment on the main content of your problem Martin, but the
> last para bothers me. How did you 'shield' the 'receiver cable'? Which
> cable
> did you shield? Not the antenna cable , I trust. Or do you have an antenna
> (whip or trailing wire) external to the fuselage, and the use a piece of
> coax cable from the RX to the antenna? Are you using 2.4Ghz radio
> equipment
> ( for the radio control system, Spektrum or such like)or is it the 35MHz
> or
> similar stuff. If you are using Spektrum, what other equipment do you have
> on the aircraft, datalink, Video transmitters, etc, and are they in the
> 2.4GHz band as well? This is fraught with problems if so, since the very
> close proximity of these elements WILL cause desensing of the Radio
> control
> RX. There is nothing you can do about that; They ALL share the same 2.4GHz
> to 2.5GHz band. If the RX is in the 35MHz (or 70Mhz, etc) band, it would
> be
> better were is not for the fact the typical RX input selectivity is not
> really that good, and the receiver is also desensed. I have had bad
> experiences with JR 9 channel RX and the 9-Xtreme (900Mhz spread spectrum)
> datalinks. The very broad band signal from the datalink severely
> desensitized the RX, and range was reduced to less than 150meters!
> However,
> removing the RX crappy wire antenna, and and connecting a thin (2mm
> diameter) coax cable internally (shield to RX ground) and then routing the
> Coax to an exit point in the fuselage and then connecting the original
> antenna wire to the exiting coax center wire helped a lot. I easily get
> 600meter range now. 
>
> THE FIRST RULE IS : whatever you are testing, be SURE that you have a safe
> and tested means of taking control of the aircraft. Sure, a component
> failure can still foil you, but I cannot condone taking of, intentionally
> or
> otherwise(!), with your backup system ( manual RCS system) non-functional.
> FIRST fix that!
>
> I design Spread Spectrum systems, RF datalinks, Video link, etc, as well
> as
> antenna systems of all types, for the aviation industry, both military and
> commercial, so if there is any way I can help or advise, I will try. I am
> flying Paparazzi for about 7 months now, but have designed my own ground
> control station, and have extensively changed the autopilot - I fly V
> tails,
> with ONLY rudder/elevator, ie, no ailerons, as well as a 4 meter wingspan
> homebuilt A/C, fitted with ultrasonic height detection. This A/C does
> auto-takeoff and auto-land (beautifully!). But I cannot help with any of
> the
> issues related to the A/C  / GCS communication and control, since I have
> done my own....Found the Paparazzi GCS very emcopassing and complete, but
> far to complex, and impossible to certify , even in part, for use in
> restricted civil airspace.
>
> HAVE FUN, be safe...
>
> Joe
>
> -----Original Message-----
> From: address@hidden
>
[mailto:address@hidden
> ] On Behalf Of Martin P
> Sent: Monday, July 27, 2009 10:46 PM
> To: address@hidden
> Subject: Re: [Paparazzi-devel] downlink only
>
> Hi,
>
> I do not intend to do any settings via the transmitter.
>
> However, I do not seem to have 2-way-communication via the modems.
> Although
> I saw something with the scope, I am not able to use neither the settings
> tab in the ground station nor the settings dialog from the tools menu. The
> latter shows an empty window.
>
>
> On the field I used a modified flight plan which does not stop at the
> holding point. First of all, uplink seems not to work an secondly, most of
> the time I am alone on the field.
>
> Usually, when ground testing, the throttle reacted to the stick in AUTO1.
> Yesterday I switched to AUTO1 and the plane throtteled up and took off
> before I expected (I use a landing gear). Switched to MANUAL and climbed,
> then back to AUTO1. No throttle. Probably this is because I put the
> vmode="throttle" in the takeoff block to get executed anyway.
>
> I havent't seen any vmode statements other than specifying "throttle".
> What
> are the valid values for vmode?
> Could I automaitcally set the vmode to pitch in AUTO1?
>
> Managed to land manually, in spite severe radio interference. Radio
> interference in spite having changed the receiver cable to a shielded
> wire.
>
> So, I am still struggling and learning. Every help is greatly welcome.
>
> Greetings, Martin
>
> -------- Original-Nachricht --------
> > Datum: Fri, 24 Jul 2009 19:00:32 +0200
> > Von: Pascal Brisset <address@hidden>
> > An: address@hidden
> > Betreff: Re: [Paparazzi-devel] downlink only
>
> > Hi,
> >
> > Martin P wrote:
> > > I was wrong, I do have 2-way communication. Looked at pins 2 and 3
> with
> > the scope. (Could have done that earlier)
> > >
> > > Then I do not know why I was stuck in holding point block (before I
> > commented it out).
> > >
> > > With or without 2-way-communication, I am still interested in flying
> > without GCS interaction. Which means that, for the time being, I can
> only
> > follow a pre-determined track and then (attempt to) land.
> > >  
> >  The code to do a setting from RC channel is still there:
> >
> >
>
http://paparazzi.enac.fr/wiki/Telemetry#R.2FC_Transmitter_Data_Uplink_.28Obs
> olete.29
> >
> >  But.
> >  It has not been tested for months (years ?).
> >  We will remove it asap
> >
> > --Pascal
> >
> >
> >
> > > Greetz, Martin
> > > -------- Original-Nachricht --------
> > >  
> > >> Datum: Mon, 20 Jul 2009 23:11:58 +0200
> > >> Von: "Martin P" <address@hidden>
> > >> An: address@hidden
> > >> Betreff: [Paparazzi-devel] downlink only
> > >>    
> > >
> > >  
> > >> Hello,
> > >>
> > >> it seems that I have a common problem: GCS doesn't send to the plane.
> > >> Removing the datalink section didn't help.
> > >>
> > >> Anyway, I am not so much interested in the uplink. When I am alone I
> > >> cannot watch the plane and the computer screen at the same time. I
> know
> > that
> > >> Pascal Brisset strongly advises against removing the holding point
> > block from
> > >> the flight plan, but I might do just that.
> > >>
> > >> My starting procedure would be like this:
> > >> Switch on the transmitter and switch to MANUAL, throttle off.
> > >> Power the plane.
> > >> Wait for GPS and do GeoInit.
> > >> Instantly go to Takeoff (a beep or LED signal would be nice).
> > >> As I am still in MANUAL, the propeller doesn't spin.
> > >>
> > >> I don't know how to go on from this point. When I switched to AUTO2
> > >> (without propeller, for safety) I expected the motor to start, but it
> > didn't.
> > >> Does the autopilot wait for a "launch" command which it doesn't get?
> > Does the
> > >> autopilot wait until I have stared manually?
> > >>
> > >> Can you take off from the lawn (with a landing gear) or do you need
> to
> > >> throw the plane?
> > >>
> > >> Greetings, Martin
> > >>


_______________________________________________
Paparazzi-devel mailing list
address@hidden
http://lists.nongnu.org/mailman/listinfo/paparazzi-devel