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nuckolls.bob(at)cox.net
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 Posted: Sun Oct 05, 2008 5:39 pm Post subject: EIS interference |
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Problem: The EIS engine monitors are annunciating random "phantom" alarm
messages, indicating a monitored parameter has exceeded a set limit. These
include high or low EGT&CHT alarms, voltage levels, pressure levels
etc. Of particular interest is that on the EFIS engine monitor page there
is a graphic representation of CHT/EGT readings on a time scale (similar to
a seismograph trace). This shows a trace history of these temps as
continuous lines advancing across the screen. On the trace lines of the
EGTs in particular there are obvious spikes (plus and minus). The duration
of each spike is virtually instantaneous and indicate an increase or
decrease in EGT of from 50-200 degrees, assuming they were accurate. The
spike frequency is random, sometimes occurring 10-15 seconds apart, often
much closer- frequency appears to be highest after engine start, and less
frequent as the battery recharges. These spikes appear to be systemic as
the alarms being generated are not limited to EGT/CHT (the only data shown
as traces) as noted above.
Since these are strong anomalies in perceived data, one should
not discount the probability that the noise is coming in through
the temperature sensor leads (thermocouples I presume) that
may or may not electrically float at the engine end. The idea
T/C installation does NOT make electrical contact with the surface
being measured so that signal ground occurs at the instrument and
is not duplicated at the crankcase.
At first is seemed to be caused by the Right magneto, as grounding it
eliminates the spikes, but many attempts at relocating the P-leads and even
disconnecting them entirely made no improvement. P-leads are 18 gauge
shielded, single conductor using the shields to complete the circuit to a
L/R/Start type keyswitch. Any electrical wiring or components near the mag
were relocated with no improvement. Suppressor wires and plugs were tried
with no change. Replacing the mag also did nothing.
Bad mags don't cause this . . . Try running the system with all
P-leads disconnected at the engine. Kill the engine with fuel cutoff.
Totally eliminate p-lead wiring as part of the propagation
pathway.
Suspecting the rather basic rectifier/regulator, I checked the Z-figures
and notes relating to the Jabiru and experimented with a 30,000uF 30VDC
electrolytic capacitor between the regulator output and ground. This
clipped the tops of the highest of the spikes shown on the EGT traces, but
had little other effect. Another experiment was to disconnect the
alternator output leads from the regulator and run strictly on the battery,
but the spikes persisted. (All connections to the regulator are
butt-spliced, soldered & shrink-wrapped) This leads me to believe it has
nothing to do with the charging system and is specific to the Right magneto.
Unlikely that the alternator/rectifier/regulator is a culprit here
too. There are no events that occur in normal or abnormal operation
of the power generation at 10-15 second intervals.
I've run out of ideas- all suggestions welcome!
I'd still like to experiment with a better rectifier/regulator, but unsure
where to hunt one down- perhaps others have found reliable substitutions?
(The B&C PRM1C-14 is limited to 14 amps continuous- 30-35 is required)
If you've run the system with the alternator OFF and the
problem persists, then it's not a problem with either the
alternator or the rectifier/regulator. Have you tried running
the EIS system from a dedicated set of dry cells? A power source
separate from the ship's bus.
Does the airplane have a single point firewall ground? What
is the airplane's structure?
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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Mark Phillips in TN
Joined: 10 Jan 2006
Posts: 431
Location: Columbia, TN
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| Posted: Sun Oct 05, 2008 6:15 pm Post subject: EIS interference |
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In a message dated 10/5/2008 8:40:14 P.M. Central Daylight Time, nuckolls.bob(at)cox.net writes:
Since these are strong anomalies in perceived data, one should
not discount the probability that the noise is coming in through
the temperature sensor leads (thermocouples I presume) that
may or may not electrically float at the engine end. The idea
T/C installation does NOT make electrical contact with the surface
being measured so that signal ground occurs at the instrument and
is not duplicated at the crankcase.
>>>>>
Understand. CHT thermocouples used are TC wires crimped to ring terminals under spark plugs (supplied by GRT) and obviously grounded at head- EGTs are standard GRT probes inserted into exhaust pipes held in place by hose clamps. Experiment would be removing all CHT probes to eliminate grounding at head? This might pinpoint the antagonist, but without dedicated wells for bayonet-type probes, how to monitor CHTs?
Bad mags don't cause this . . . Try running the system with all
P-leads disconnected at the engine. Kill the engine with fuel cutoff.
Totally eliminate p-lead wiring as part of the propagation
pathway.
>>>>
Disconnecting Right mag had no effect- will try both...
Unlikely that the alternator/rectifier/regulator is a culprit here
too. There are no events that occur in normal or abnormal operation
of the power generation at 10-15 second intervals.
>>>>>
My tentative conclusion as well, but just checking!
If you've run the system with the alternator OFF and the
problem persists, then it's not a problem with either the
alternator or the rectifier/regulator. Have you tried running
the EIS system from a dedicated set of dry cells? A power source
separate from the ship's bus.
>>>>>
This was next on the experiment list, but if isolated power to instrument eliminates problem, doesn't that point back to a problem with power supplied to the EIS with a finger aimed at the alternator/regulator system? Any problem powering EIS from a separate PC-625 VLSA batt instead of the dry cells?
Does the airplane have a single point firewall ground? What
is the airplane's structure?
>>>>
They properly employ forest-of-tabs from B&C. Composite structure, stainless firewall over plywood bonded to structure.
Appreciate the reply Bob- will report on experiments...
Mark
New MapQuest Local shows what's happening at your destination. Dining, Movies, Events, News & more. Try it out!
[quote][b]
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nuckolls.bob(at)cox.net
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| Posted: Sun Oct 05, 2008 9:11 pm Post subject: EIS interference |
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At 10:13 PM 10/5/2008 -0400, you wrote:
| Quote: | In a message dated 10/5/2008 8:40:14 P.M. Central Daylight Time,
nuckolls.bob(at)cox.net writes:
Since these are strong anomalies in perceived data, one should
not discount the probability that the noise is coming in through
the temperature sensor leads (thermocouples I presume) that
may or may not electrically float at the engine end. The idea
T/C installation does NOT make electrical contact with the surface
being measured so that signal ground occurs at the instrument and
is not duplicated at the crankcase.
>>>>>
Understand. CHT thermocouples used are TC wires crimped to ring terminals
under spark plugs (supplied by GRT) and obviously grounded at head- EGTs
are standard GRT probes inserted into exhaust pipes held in place by hose
clamps. Experiment would be removing all CHT probes to eliminate
grounding at head? This might pinpoint the antagonist, but without
dedicated wells for bayonet-type probes, how to monitor CHTs?
|
Okay, I'm betting that the T/C signal for each
input goes to a AD595 or similar thermocouple
conditioning chip. These are high gain op amps
with limited common mode. When you ground the t/c
at the engine end, you MUST keep the signal ground
for the instrument tightly bonded to the crankcase
too. If you have too small, too shared, too long
of wires in ground system, common mode noises from
about anywhere in the system can get into those
microvolt-sensitive inputs to the TC conditioners.
| Quote: | >>>>>
This was next on the experiment list, but if isolated power to instrument
eliminates problem, doesn't that point back to a problem with power
supplied to the EIS with a finger aimed at the alternator/regulator
system? Any problem powering EIS from a separate PC-625 VLSA batt instead
of the dry cells?
|
any source of 12v will do. But only tells you what the
propagation path is . . . not the antagonist. Sub
the 12 supply (+) first. Don't mess with grounds.
If the noise is still there, then move the EIS ground
directly to the crankcase and see what happens.
| Quote: | Does the airplane have a single point firewall ground? What
is the airplane's structure?
>>>>
They properly employ forest-of-tabs from B&C. Composite structure,
stainless firewall over plywood bonded to structure.
|
Okay, make sure that the EIS instrument power ground
gets it's own dedicated wire to the forest of tabs.
Check that the bond strap from crankcase to forest
of tabs is suited to task. 4AWG wire or bigger.
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