nuckolls.bob(at)aeroelect
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 Posted: Sun Oct 11, 2009 8:26 am Post subject: Control Stick Switches – Best Practice? |
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At 09:18 AM 10/11/2009, you wrote:
<gordonrsmith921(at)yahoo.com>
Given: RV A/C with airframe tied directly to battery ground (rear Batteries).
I am planning the following switch functions on the control stick:
Trim (4 way), Com PTT, Autopilot Control, and perhaps Com channel
flip-flop and other functions.
All of these functions require a momentary pull to low (ground) to
activate/toggle a function. My original planning was to employ a
single ground conductor up the stick to the switches. This ground
conductor would be tied, locally, to A/C ground in the area of the stick.
After reviewing various Z figures it appears that a better practice
would be to bring the stick ground conductor to the Panel G3 (inside
firewall) or the Avx G5 (back side of panel). Which is preferred?
There's no "golden" recipe for success here. You
need to establish design goals (make a control stick
with lots of buttons control lots of things by pulling
a control line to ground).
Then do a failure mode effects analysis for the ways
that the control system might fail.
Is it proper to have a single ground conductor for all of the stick
switch functions? Should the PTT be given special consideration and
a separate ground path with perhaps both (to-from) paths shielded.
It's not a mater of "proper", it's a matter of criticality
for any single and then a combination of items in
the control system to be lost. If that single ground
breaks, what is the likely outcome of your flight?
Do you need to craft a specific plan-b for any of
those failures?
PS Engineering schematics show PTT wiring only from the mic jack (PTT
from the tip and low-ground from the barrel). Can the remote stick
PTT be wired directly from the Intercom, connecting to the respective
PTT mic pin and mic low connection at the Intercom? Should these
conductors be shielded in this case?
A twisted pair would be nice but shielding is
not necessary or beneficial.
Perhaps the bottom line might be:
1. 1 ground conductor for trim relay decks going to local ground.
2. 1 ground conductor for PTT going to Intercom or mic jack.
3. 1 ground conductor for the other toggle functions to Avx G5.
Overkill?
Can't answer that for you. If any failure deprived
you of any one or all of those features, how would
you deal with it? If subsequent arrival with the earth
is comfortable and sweat-free, then a single ground
would be fine. If not, then modify your planning
accordingly.
Bob . . .
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( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
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