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Hi Steve,
Always impress with your digging into these
anomalies!
So what are the relevant factors looking at the black
box.
One thing we know is that the interval between
pulses becomes greater as the rpm is lowered. While the peak voltage
probably remains fairly constant - the average voltage will decrease due to the
lower pulse duty cycle. Also based on your analysis, a 1K ohm load on the
input circuit reduces the pulse peak voltage level from approx +13.5 volts to
around +6.00 volts peak - which given the same pulse duty cycle would mean
even a lower average voltage - after which the engine runs OK at
lower rpm.
I suspect the EC has a limiting circuit of some type to
reduce the +13.5 volt peaks to that suitable for input to a microcontroller chip
(generally +5 volts TTY). I use a combination of a zener diode and a
dropping resistor in my EFISM such that any voltage above 5 volts causes the
diode to conduct and clip/limit the peak to around 5 volts. There are, of
course, many other voltage limiting circuit methods with different
characteristics that might be affected differently with a lower peak or average
voltage or the 1K loading.
Analog SWAG:
Interaction of pulse train and a capacitive element
of the EC
IF for example the circuit was such that the pulse train
kept some capacitive element repeatedly trigger so as to be discharged
between pulses with the rpm above a specific rpm, then as the duration
between pulses became longer (with the lower rpm) it might reach a critical
point where the capacitor never becomes completely discharged -I.e
the discharging pulse does not occur frequently enough to bring/keep the
capacitor to zero volt level between pulses - as an example. Adding
the resistor may result in a quicker discharge circuit than
without the resistor- thereby again permitting capacitor to completely discharge
between pulses and the circuit to work at lower rpms . This quicker
discharge and the lower peak voltage may combine to permit the
capacitor to completely discharge at your lower rpm. The lower
voltage seen may only be a side effect and not the reason for the circuit
now working. Just one thought.
Digital SWAG:
I don't see how the 1K resistor would play in this
scenario, but I'm throwing it in for consideration.
One problem I ran into early on with my EFISM
was that the Pulse circuit in the microchip had only a word (16
bits) wide register to store pulse timing data, meaning that no more that
65535 CPU clock tics could be stored. In my case with an 8Mhz clock tic it
mean that any Low rpm with an interval of greater than 122 msec (approx 500 rpm
or lower) would overflow the word memory size - causing errors. If the CPU
clock tic is at a higher rate the minimum rpm is raised and if lower cock tic
rate the min rpm level is lower.
I had to add a flag to signal when the pulse
duration memory register overflowed and then use that flag (when set) to
add 65535 to the pulse interval clock tic count to calculate the correct
rpm, otherwise the rpm calculated was in error.
So again, this is probably not a factor - as I don't see
where the 1K resistor or lower peak voltage would play a role, but thought I
would throw it out for consideration.
All I can think of for the moment
Ed
Sent: Tuesday, November 06, 2012 7:50 PM
Subject: [FlyRotary] Re: Renesis CAS & EC-2
upgrade?
A follow-up on the attempt to
use Renesis CAS with
an EC2 controller set up for '86 stock Mazda
CAS.
There appeared to be a correlation between loosing the rotor 2 controller
signals with crossing the staging threshold on startup. This was tested by
starting the engine with the throttle open sufficiently that the manifold
pressure should not drop to less than the staging threshold. The result
was that the engine still ran only on rotor 1 until RPM reached at least 1400
RPM. It just didn't take as long to reach this RPM as before.
This behavior and examination of controller signals recorded during the startup
disproved the appearance of a correlation between loosing rotor
2 controller signals and staging.
1K ohm resistors were then added between
each Renesis CAS signal and ground.
With these resistors installed, the engine started easily and ran normally at
all throttle settings. Since the engine was operated with this setup for
only a few minutes, it is unknown if this is a reliable solution. If it
isn't, it is certainly a step in the right direction. Plots of
the CAS signals with and without the 1K
ohm resistors are attached. The voltage scales on the two plots are the
same, but voltages read from the plots must be multiplied by a factor
of 3 to give absolute values in volts.
Higher resolution plots of the CAS signals
are available on request. Any feedback on why adding the resistors is
effective would be welcome.
RV6A, 1986 13B NA,
RD1A, EC2
From: Steven W. Boese
Sent: Monday, November 05,
2012 1:50 PM
To: Rotary motors in aircraft
Subject: Renesis
CAS & EC-2 upgrade?
I have the
2011 Renesis engine running on my test stand...sort of. On
startup, when the manifold pressure drops below the staging threshold, both
ignition signals and the primary injector signal from the EC2
are missing on rotor 2 on the next cycle after the dropping of the secondary
injector signals on rotors 1 and 2. This occurs at about 870
RPM. The engine then runs on just rotor 1 until the RPM reaches the
range from 1420 to 1750 at which time the rotor 2 coil signals and primary
injector signal return. The engine runs normally up to 5350 RPM
which is as fast as the prop load will allow. If the engine is idled into
the rpm range less than around 1700, it goes back to running only on rotor 1
with the rotor 2 primary injector and coil signals missing.
The CAS
for this engine is set up as 2 Renesis CAS units, one
triggered from a eccentric shaft mounted 12 tooth wheel and the
second CAS triggered from a single pin on the side of the 12 tooth
wheel. The 12 teeth are the same size and shape as those on the
stock Renesis wheel. It was hoped that this would allow use of
the same EC2 controllers that were used on the '86 engine with the
stock CAS with the 24 and 2 tooth pickups.
Searching the archives
indicates that trying a 1K ohm resistor from
each Renesis CAS to ground and/or limiting
the CAS voltage with the circuit suggested by Bobby Hughes might be
worth trying. Traces of the Renesis CAS and the
stock '86 CAS are attached. The recording uses a factor of 3
divider so the voltage scale for each trace must be multiplied by 3 to give
actual volts. The white trace is of the 1 tooth Renesis or the
2 tooth "86 CAS while the colored trace is of the 12
tooth Renesis or 24 tooth "86 CAS. The amber vertical
and horizontal lines are software generated cursors, not EC2
signals. One thing of note may be that the Renesis 1 tooth
signal crosses zero between triggers whereas the 2 tooth '86 signal remains
above zero between triggers. There is no discernable change in
the Renesis CAS signals when the transition to missing
the Rotor 2 signals takes place. There is also no discernable noise on
the CAS signals under the measurement conditions of 3000 samples per
second.
Any suggestions for
investigating this issue would be appreciated.
Steve
Boese
RV6A,
1986 13B NA, RD1A,
EC2
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