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Author Topic: Henkie's pit  (Read 436407 times)

Offline salaxi54

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Re: Henkie's pit
« Reply #1260 on: July 27, 2017, 02:40:01 AM »
@henkie: I've been entertaining the idea of using PHCC for a little while, but i'm hesitant for the moment. By the way, i can't help but notice that the PHCCusb2 pcb is still not available. After the DEDuino pcb experience, i'd much rather get ready pcbs. Getting too old for "extreme sports" lately. Maybe we should all start a talk about further PHCC development..
Anyway, your video shows the Caution panel is looking great! And the instruments too.
Great job my friend!  :cheerleader:

Offline henkie

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Re: Henkie's pit
« Reply #1261 on: August 12, 2017, 09:18:41 AM »
The mailman finally delivered a little package from China ... ITEAD delivered the V3 PCB for the altimeter control.
Yeah, version 3   ::)
Version 1 had a design error. The board was fine, but in my head I made a mistake   :DOH:
Version 2 works, but I still had to apply a few fixes, like cutting 3 traces at the back and inserting 3 resistors.
Version 3 has all fixes and design improvements that I learned from version 2 (I hope) and might solve the "jitter" issue I saw. I added 3 Sallen Key filters for the control voltage. I could have done an experimental setup using a breadboard, but that would take time. The design of a new PCB is just easier, especially considering that one PCB is less than 10 Euro, all-in. My time is much more valuable to me   :whistle:
So, soon I  will have to populate this PCB and hopefully see better results   :reading:

Offline Kukki

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Re: Henkie's pit
« Reply #1262 on: August 12, 2017, 12:08:54 PM »
TOTAL Cool Hank, im looking forward to see more of that  :thumbsup: :thumbsup: :thumbsup: :beer:
Kukki - (Skype: kukki_22)

Offline henkie

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Re: Henkie's pit
« Reply #1263 on: September 23, 2017, 06:07:39 AM »
Making little progress on the altimeter project. I am still trying to get it perfect using circuit board design #2, although I have board design #3 nearly completely soldered on my desk. But to solve my biggest issue I do not need version #3.
I am trying to obtain a "perfect" altitude indication when I specify some altitude. The hardware has an altitude resolution of approx 2 feet!
To see how well (or how bad...) the indication on the altimeter is, I entered data in an excel sheet.
Column A is the specified setpoint, that is, the altitude indication that I want on the altimeter. It is the straight monotonous climbing blue line in the plot.
Feeding this "raw" as-is to the hardware, I get the indicated altitude values entered in column B. That is the orange line "wobbling" along the blue line. Looks not too bad, but when you look at the data it is clearly far from perfect. To see how "bad" it is I have in column C the difference between the desired setpoint) indication and the actual shown indication on the altimeter. To have a good view, I multiplied the "error delta" values by 10. Surprisingly, this looks like a sine wave. That is visible in the plot as the grey line.
From this I get two observations.
There is a small "decay", that is, the slope of the sine wave tends to droop. I think I can explain this by the spring on the bellows inside the altimeter. The bellows are no longer functioning (I punctured them), but the springs are still there. As the altitude increases, the spring force increases and that might explain the small slope.
The other thing is the sine periodicity. The electrical periodicity of the altimeter is 10000 feet. That means that one full sine wave period represents 10000 feet. From the plot we see that the "error delta" periodicity is 6 sine waves. It is a bit difficult for me to explain, but I noticed that I also have 6 small "jumps" in the altitude indication, at positions where one of the 3 stator sine waves crosses zero. There are 6 zero-crossing, so that is no coincidence ...

In my simplistic reasoning, I hoped to get altimeter indication sort-of correct by applying the 6 sine wave periods as an added setpoint position to compensate the "error delta". That does work, up to some extend. The resulting indications are in column D and the yellow line in the plot. Again I also made a plot of the delta (difference between setpoint and actual indication, also multiplied by 10). That is the blue "wobble" on the horizontal line. It looks much better, but still not perfect, like I want to to be.
What I notice is that there is some repeating pattern, but if you try to see a sine wave in the plot you see that there is probably also some phase shift. If I would apply this correction the zero indication value will shift, which can be compensated, but I think this will become an endless looping effort to get a perfect indication.

Note that I use a formula to "convert" any altitude value to the data to control the altimeter. I guess that a lookup table (10000 entries!) will always be a solution, but building the table would be a lot of work   ::)

If you want to look at the data, I also included the excel file.
Any helpful insights are appreciated!   :notes:
Henk

Offline henkie

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Re: Henkie's pit
« Reply #1264 on: September 23, 2017, 10:00:22 AM »
I got a small goodie from Martin ("pegasus") in the mail this week.
It is a "simple" bracket to mount the right side map light onto the glareshield.
Bending metal beyond 90 degrees is out of my capabilities, so I was very happy that Martin had this available.
I had to drill the holes a bit larger, but that was not a problem.
Needless to say that it is a perfect fit, as all of Martin's products are!  :thumbsup:
To mount the map light onto the bracket, adding a soldering lip for the "ground" connection, required a bit
longer screw. Of course, the threading was not "standard European", but with a nut I made a solid mounting.
Getting the bracket with map light in the pit was a bit difficult, because I did not want to remove the DED,
nor the warning lights. Using some adhesive tape to keep the nuts in place I managed to get the screws in   :whistle:

Offline Focaldesign

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Re: Henkie's pit
« Reply #1265 on: September 23, 2017, 11:18:40 AM »
Looking very nice!

Offline Kukki

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Re: Henkie's pit
« Reply #1266 on: September 24, 2017, 07:08:23 AM »
TOTAL NICE  :thumbsup: :thumbsup: :thumbsup: :thumbsup:
Kukki - (Skype: kukki_22)

Offline henkie

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Re: Henkie's pit
« Reply #1267 on: October 10, 2017, 01:21:22 PM »
Here are my findings of the experiments with an altimeter.
To get an altimeter working you have two options.
Probably the easiest option is to remove the inner mechanics and install a stepper motor with
a "zero-position" sensor. The local pressure setting knob can be connected to a potentiometer
so that you can read its position using an analog input. Possibly a rotary encoder could also
be used here but that would only be for "zero altitude" adjustment.

The second option is to get the altimeter working *without* modifications to the instrument.
That proved to be too difficult. The altimeter indication is either generated based on the
atmospheric pressure ("stand-by mode") or the indication is generated based on information
from the air data computer which drives a synchro inside the altimeter. The first problem is
that when the altimeter is driven by the synchro, the atmospheric pressure bellows are still
"in the measurement loop". In reality that is probably a check for air data computer failure.
If there is a failure (air data computer or power supply) the altimeter automatically reverts
back to stand-by mode of operation. Again, in reality, if you are at 20000 feet the atmospheric
pressure is substantially lower than at sea level. Thus, the pressure bellows excert a different
pressure on the mechanics of the altimeter. That force must be counteracted by the synchro.
At a certain altitude the force generated by the synchro "looses" from the force generated by
the bellows. As a result, the altimeter reverts back to stand-by mode operation.
That is of course not what we can use in our pit.

So, getting the altimeter working *without* modification is only possible if the atmospheric
pressure sensed by the instrument matches within a certain degree the pressure at the indicated
altitude. Might be possible to realize, but way too complicated.
Therefore, I *removed* the bellows. Only puncturing them and keep them inside the instrument
is no solution, because there is still a sort of spring load between the bellows and the
altimeter mechanics. After the bellows are removed (loosening one screw per bellow, not too
difficult to do) the altimeter indication only depends on the synchro. However, the bellows
have (had) a second function. They dampen the indication pointer, so that it makes a smooth
movement. Small jittering of the indication is dampened, and with the bellows gone, you can
see the indication pointer (and the number drums!) make a small jitter. That jitter is not
always visible, but when it is, it is aweful to see. Maybe a small piece of cloth like felten
against the indication shaft is sufficient to dampen the jitter.

As said, the altitude indication now only depends on the control of the synchro. It turns
out that the altimeter is an extremely sensitive instrument. One 360 degrees rotation of
the synchro axis represents 10000 feet altitude (that is 10 times a full rotation). More
technically said, one electrical period represents 10000 feet. One degree rotation is
already a bit more than 2 feet!
It was clear to me that the ADI synchro design ("SDI") would not be able to produce a
sufficiently accurate indication; it would be too coarse for a smooth movement. The new
design does not use PWM to DC control voltage conversion using a low-pass filter, but
uses three 10-bit DACs that give 4096 discrete positions resulting in 2.2 feet resolution.
However, the VCA (Voltage Controlled Amplifier) does not have a linear response. The gain
or attenuation of the VCA is controlled by the DC voltage from the DAC output, but it is
logarithmic instead of linear. The result is that in some altitude ranges the resolution
(number of "steps" per 10 feet difference) is better than in some other rang.

The first PCB design had two errors. One I could live with, but the other was a real issue,
because it caused an inversion of the signals. So, PCB #2 was made, and I used that one for
a lot of testing and patching to improve the design. That lead to PCB #3. I think this is it.
My first tests showed that the added filters do indeed remove the "jitter" on the indication.
When you update the altitude position, the movement is no longer an abrupt jumpt to the new
setpoint, but it is a lot more a smooth movement. That implies that you do not need to send
an altitude update at a high rate. But nothing comes for free. The smooth movement also causes
some damping, thus the actual altitude is not attained perfectly. The used filters have a time
constant of approximately 1 sec (!). By changing 6 capacitors that time constant can be lowered.
The damping of jitter and smooth movement will be a bit lower, but I expect that the actual
altitude will be indicated better. If that is needed? The "altitude error" on the given
setpoint is less than 10 feet (0.5 of an indication mark on the scale).

The non-linear behavior is solved. Curve fitting and trying to find a working algorithm was
not a solution. It takes a bit of time, but I created a look-up table (trial/error/correction).
The table has 200 entries, one for every 50 feet increment. So, at every 50 feet increment
position of the setpoint altitude, the altimeter gives the exact indication (which, of course,
depending on your efforts to create an accurate table). The positions between two 50 feet
increments are linearly interpolated. That seems to work very nicely, so I am happy with this
end result. Using a different altimeter may show different results, but using the lookup table
approach you can solve that.

The only thing I have no solution for at the moment is the "jump" that occurs at 6 specific
altitude settings per 10000 feet. Although the setting just changes one bit, the altimeter
makes a jump. The jump is not as bad as it was, but it is still a "jump" in altitude of some
40 feet, 2 scale divisions, and that is visible. Maybe I can come up with a solution (or
somebody else), but for now I want to forget how many hours/days/weeks and months I have
spent on this "simple" thing!  Now I only have to update the documentation.

Offline Nigel

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Re: Henkie's pit
« Reply #1268 on: October 10, 2017, 03:37:34 PM »
Great news, Henk! Congrats. Your hard work is always worth the effort and much appreciated, believe me. Thanks. Now if I can find a real altimeter, I'll replace my Malwin.

Offline Sandman

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Re: Henkie's pit
« Reply #1269 on: October 11, 2017, 12:24:55 AM »
Wow Henk!

I always find it interesting to read about your various tests (some work, some don't) to come up with workable solutions... I can only imagine the hours you have invested in getting to this point.  I stand in awe.

Reminds me of something Albert Einstein once said...


"It’s not that I’m so smart...
it’s just that I stay with problems longer.”

Offline henkie

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Re: Henkie's pit
« Reply #1270 on: October 11, 2017, 07:05:17 AM »
Thanks guys  :bounce2:
Although the 6 "jumps" are still there (per 10000 feet), they are a lot smaller, but still visible.
However, I can live with this imperfection. After almost one year, I just wanted to get my desk cleaned up.
Moving on to the next instrument ... The Fuel Flow Indicator  :whistle:

I have attached the documentation. If there are errors/omissions, an update is easy  ;D

Offline raptor

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Re: Henkie's pit
« Reply #1271 on: October 11, 2017, 08:12:55 AM »
Just amazing... Love every post m8...

Offline Kukki

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Re: Henkie's pit
« Reply #1272 on: October 11, 2017, 10:28:57 AM »
Its FRICKING AWESOME with you Henk.  A BIG Kudus to you my friend, you are truly an inspiration for all of us here.

Professor Henkie  :vprules: :thumbsup: :beer:

Keep up the GREAT work my friend  :thumbsup:
Kukki - (Skype: kukki_22)

Offline Leech87

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Re: Henkie's pit
« Reply #1273 on: October 11, 2017, 02:26:14 PM »

Moving on to the next instrument ... The Fuel Flow Indicator  :whistle:


Oh that's sweet. Finally I will be able to get my real FFI working by using the brain cells of someone else.
Hope the Fuel quantity indicator is also on you list.