[Paparazzi-devel] Interference problems part 2

[Helmut Wabnig]

Some basic rules for model airplane wiring.


The pilot who called me for assistance experienced the following:
increasing or decreasing throttle would move the flaps and rudders
irregularly, and during full power flight the rudders made random
movements.

It's simply a grounding problem.

How to install proper ground on a model airplane?
We had immediate success by connecting the motor chassis, the motor
mounting bracket, the minus of the motor driver supply, the minus of
the servo battery, the minus (ground) of the receivers onto a common
ground wire which we laid through the whole plane body from  tip to
tail. 
Actually we use a braid from a shielded cable. Remove the insulation,
then push the wire mesh shield ends together which makes it easy
to slide off the cable, then pull to full lenght again. Finally the
mesh was about 5 mm wide flat litz wire. Alternatively one could use
thin copper tape which is available from electronics suppliers.
High frequency currents run on the outer surface of the conductors,
therefore it is important to have as much surface as possible on the 
ground wires. Connect each metal part to ground, e.g. motor mount.

metal      motor                servo
motor      battery              receiver    receiver
chassis    minus                minus       battery minus  
|---|      -----                 ---         -----
|---|      -----|                ---|        -----|
    |           |                   |             |
    |           |                   |             |    grounding
====================================================== litz
(use fixed pitch font)


Additionally we replaced the 3-pole servo flat-cable which
runs from the receiver to the motor driver from the rear
to the front over the full length of the plane with a twisted
cable. Twisting is almost as good as shielding.
Twisted cables normally do not have to be shielded in moderate
environments. How does twisting work?

TWISTED CABLES

----\    ------\   /-----
     \ /        \ /
      X          X
     / \        / \
----/   \------/   \-----

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ el. mag. field


The field induces interference currents in the wires.
By twisting the wires we change the polarity of the 
induced voltages every  twist, and so the
disturbances cancel each other.

It is absolute necessary to twist the power cables
from the motor battery to the driver board, because
there run high pulsed currents and what is true for
reception, is also true for transmitting, and the
electric noise transmitted from the power cable will
cancel itself when run over twisted cables.
Needless to say, make the wires as short as possible,
from the battery to the driver to the motor.
Those components must be placed as near as possible,
especially the driver and the motor.

Always use twisted servo cables and remove those
3 pole flat cables from your design if possible!


GROUNDING LOOPS

Grounding loops are not that much of concern as in audio
equipment or in microvolt sensors, but there is a general
simple rule to grounding loops:  avoid them.


motor                          servo
driver        cable            receiver
---  .......................> ----
---|                          ----|
   |        A R E A               |
   |                              |
=============================================== ground

The signal cable and the ground form a "loop" with a large
interior area. Induced noise is proportional to that area.



motor                          servo
driver                   receiver
---  .                          ----
---| .                        ^ ----|
   | .    twisted cable       .     |
   | ..........................     |
=============================================== ground

The grounding loop area is quite small now and we will receive
less noise. The same applies of course, the other way round,
to power cables with pulsed loads.


I do not recommend the use of ferrite beads, because if I need them,
my basic design is flawed. Sometimes ferrites help to overcome
problems, but I recommend to correctly design and install wirings and
shields the proper way. For example, I would lay out the interior of
a sensor compartment with thin copper foil or mesh or spray with
conductive paint to get a shielded chamber. Of course, I must take 
care to connect all shields to ground on several places.


Have a good flight & landing,

w.