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Grounds & noise

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Sam

Joined: 18 May 2008
Posts: 135

Posted: Tue Apr 28, 2009 4:38 am Post subject: Grounds & noise

This is a re-post from the Rotary Engines list, by Tracy Crook, owner of Real World Solutions, in response to a builder whose ECU seemed to reprogram itself.   It is very interesting to me because I am half way through trouble shooting a magneticly induced noise problem (see www.samhoskins.bolgspot.com).

 



[quote]My experience so far indicates that  electrical noise in various parts of

 the electrical system is the cause of the 'spontaneous' programming errors.

 This is a long subject that I am going to have to address.  It has to do

 with the physical arrangement of the aircraft electrical system, not an

 error in connections.  There is no  one simple fix for this kind of problem

 which is caused by builders having no guidlines on how to avoid this kind of

 thing.   Bad radio installation practices or poor antenna design or

 installation can also be a nightmare to explain or fix.



 I talked to a number of vendors of aircraft systems at SnF that are having a

 hell of a time addressing this.   Everything from auto pilots to Glass

 cockpit makers are having this problem.  Builders always conclude that we

 are passing the buck when we say that electrical noise is the problem.   And

 that's just a polite way of saying that "Your electrical system and layout

 sucks".    Even very knowledgable builders can fall into this problem.  Mike

 Wills made a technically perfect system with a Single Point ground scheme

 which is theoretically the right thing to do but the single point ground had

 a very high noise impedance due to the long distance between it and the main

 noise damper (the battery).



 Here is a VERY abreviated guideline.   If you do a single point ground

 system, make it at (or very near)  the negative terminal of the battery and

 ground the airframe close to it.   Have independant power feeds from the

 battery positive terminal for the noise producing things and the noise

 sensitive things.   Do these two things and noise problems will be rare.

[b]

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_________________
Sam Hoskins
www.samhoskins.blogspot.com

nuckolls.bob(at)aeroelect
Guest

Posted: Tue Apr 28, 2009 11:17 am Post subject: Grounds & noise

. . .  Even very knowledgeable builders can fall into this problem.  Mike

Wills made a technically perfect system with a Single Point ground scheme

which is theoretically the right thing to do but the single point ground had

a very high noise impedance due to the long distance between it and the main

noise damper (the battery).



Here is a VERY abbreviated guideline.   If you do a single point ground

system, make it at (or very near)  the negative terminal of the battery and

ground the airframe close to it.   Have independent power feeds from the

battery positive terminal for the noise producing things and the noise

sensitive things.   Do these two things and noise problems will be rare.

      I wish it were that simple. The battery's ability to shunt bus

      voltage perturbations to ground is very limited. I recall seeing

      a shunt impedance versus frequency plots of various batteries

      over the years and recall noting meager noise mitigation effects.

      If I can find some of those plots, I'll post them.



      I'd be willing to bet that the vast majority of antagonist/victim

      situations encountered by the OBAM aircraft community can be

      solved by an investigation of the DO-160/Mil-STD-704 characteristics

      of the major players.



      704 says . . .



      (1) The bus is loaded with certain kinds of crap.



      (2) Learn to control it at practical levels consistent

          with state of the art . . .



      (3) and live with the rest of it.



      160 says . . .



      (1) the electrical environment (conducted and radiated)

          is loaded with certain kinds of crap.

      (2) Control thy emissions to practical limits established

          by our state of the art . . .



      (2) Expect a certain amount of smelly stuff to be left

          over . . . learn to live with it.



      While there may be many an anecdotal report of success

      for the re-arrangement of wires, ground locations,

      filters, shields, etc. etc. they are seldom if ever

      joined by a qualitative and quantified description

      of the noise, propagation mode and the simple-ideas

      that led to an artful recipe for success. Most often,

      there's a sort of "swaptronics" approach to stirring

      the stew of ideas and parts until the problem goes away

      or at least becomes tolerable.



      I'm not suggesting that folks should not experiment

      to what ever amount of $time$ they're willing to

      spend. I will suggest that apparent successes that

      fall out of random experiments are at risk for not

      becoming recipes for success. I.e. repeatable

      experiments. I will also suggest that if your system

      is assembled per the recommendations of any of the

      Z-figures that the probability of having a "704

      issue" is small. This leaves newly discovered victims

      with their "160 shorts" down around their ankles.



      I'll see if I can find that data on batteries-as-filters.

        Bob . . .



        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

        ----------------------------------------

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