Sunday, February 18, 2018

Another EFIS screen

Made a new screen with bar graphs. The idea was to have a bit less clutter, and everything should be green if all is OK. See the oil pressure for an example. Somewhat better I think, but I'm not totally satisfied. I will keep them both, and improve stuff when I get the plane in the air.

Small EFIS fixes and thoughts about power supply

Added time variables to the EFIS. UTC, local time, flight time and stop watch. This also includes a count down timer that pops up, like on the default screen 1,2 etc.

But it's getting a bit cluttered. Perhaps bar graphs (big fat ones) and numbers would be better than these round dials and numbers. I have to try, I still have a couple of pages left in the EFIS. Programming it, once understanding the principles, is really easy though, rather fun actually.

The optically isolated DC-DC solution (Aviogard) may take some time to mature it seems. The Aviogard from MGL is not a viable solution as of today. There is one other guy doing something similar (I think), but not very much of details other than a battery charger as of yet. Its a really interesting site though. Experimental Avionics. If I have learned anything from this, it is that the backup battery MUST be on the isolated side, so it will supply power to the avionics also if the DC-DC converter should fail (a very real scenario as I have experienced). This also means a real battery charger solution must be in place. And while doing that, why not install a modern type of battery, NiMH or LiFePO4. (having said that, I have already ordered a Mascot DC-DC charger, and will rebuild the Aviogard with a new UQQ, configured in accordance to the specs, and try it on the kitchen table at least).

Anyway, a plan B is obviously needed, or building will stop. The obvious plan B is a traditional system, but with a backup battery for avionics. Plan B basically consist of removing the Aviogard, replacing it with a simple switch, essentially what is called an "avionics (master)/switch" in most certified aircraft. I will lose the galvanic protection completely, but:

  • I will be able to isolate the avionics when starting the engine AND have power to everything through the backup battery.
  • The backup battery is charged by the main system, and can be turned on and off.
  • I can switch on and off the backup battery switch and avionics switch to see the voltage from each directly on the EFIS, monitoring each system without an extra gadget (IEFIS Extender)
  • I can isolate the avionics in flight running only from the backup battery if needed.

The plan B will look something like this:

The reversed biased schottky diode is mainly to prevent accidentally starting the engine with the backup battery, which will completely overload the backup battery and wires. No power should ever run from the secondary system to the primary system. This will also simplify the backup mode.

  • Starting:
    • PM ON
    • SM OFF
    • SB ON
  • Normal operation
    • Everything ON
    • If everything in the primary system goes down, the diode will assure all power from the backup is going to the avionics. The engine runs fine on mags alone.
  • Testing
    • SM ON/OFF - reading V from backup battery and main 

Tuesday, February 13, 2018

Aviogard and Verhees Delta

I received an answer from MGL (Rainier Lamers) about the Aviogard. I mailed them before it went toast though. He said I could wire a 20W light bulb in series on the positive terminal to the backup battery. Then in series with the bulb, I could wire a reversed biased schottky diode. Yes, I guess I could, if I had a working Aviogard :-)

Sometimes I wish I had more electrical engineering skills, because that solution looks to be exactly what is needed (I have never heard of a schottky diode before, even less a reversed biased one). I would have replaced the bulb with something more durable though. This will prevent the battery from draining too many amps when charging, an the diode will allow full power from the backup battery.

I have to sort out the broken Aviogard first in any case.

I also received plans for the Verhees Delta. ATM this looks like a much more fun project than the RV-4 when the Onex soon is finished.The Delta looks to be a perfect match for the D-motor also. I have lots of time studying the drawings before deciding anything, a year or two from now. I have only had a short glimpse at the drawings, but they look excellent, and the design is straightforward, it's only one wing. A very good builders guide is also included.


Sunday, February 11, 2018

EFIS screens

Consider myself finished with the EFIS screens. The default 1-5 screens are standard, but I have taken away the stuff I don't have, and changed a bit on the engine module. Screen 6, the insanely ugly "VFR" screen, I have removed entirely, and replaced with an info sort of screen. Screen 7 is entirely of my own design. The idea was to have a kind of "aerobatic" screen, or "sporty VFR".

Screens 7 (it's actually very crisp and clear on the EFIS, but my camera can't really sort out the light properly):

Screen 6:

Screen 1-5

Aviogard take 3

Googling around for DC-DC chargers, it seems that charging lead-acid batteries is a rather standard procedure. Mascot is a large producer of chargers, power supplies etc (they also have a DC-DC charger). They describe the charging process in 3 stages:
  1. Boost charge
  2. Top-up charge
  3. Float charge
The following explanation is found at their site:

At Battery University the same graph is shown:

The Aviogard will only work in the float charge stage, thus a real charger must be used to get the battery to that stage. The reason is mainly the boost stage where the current must be limited by the charger. The only way to do this is by lowering the voltage. The UQQ cannot do this without entering a "hiccup mode", because it tries to keep the voltage constant at all times. When a battery with "low" charge is connected (actually anything less than max charge), it will try to supply the current to get up to 13.8V. The current needed to do that will easily be 12A or more for my 6 AH backup battery. At the boost charge stage a real charger automatically restricts the current. According to Battery University it should be max 30% of the total capacity of the battery, but could be lower. For my 6 AH backup battery, this means max 1.8A.

The only way for the Aviogard to work with a backup battery in a "plug'n play" configuration, is to have a separate circuit for the battery that limits the current to the battery by limiting the voltage. The Aviogard doesn't have a separate line for the battery, the UQQ is 1 in, 1 out.

Mascot do indeed have a DC-DC charger. It weighs 250 grams and costs NOK 750 (€75). This opens up some possibilities.

What to do next? I have several possibilities. The plug'n play solution that I thought the Aviogard would serve, obviously does not work, which is simply incredibly irritating to say the least. The default backup battery solution for the Aviogard is not a viable solution in an aircraft at all. Even though by trying real hard it can be made doable in some sense, it will cause more problems and grief than it solves in an everyday setting.
  1. Send the Aviogard back, and forget about it
  2. Use the Aviogard with no backup battery
  3. Use the Aviogard with the Mascot charger on the output side
  4. Send the Aviogard back and use a manual backup battery solution.
Just for the record. The Aviogard is a nice device IMO. It uses a UQQ optically insulated DC-DC converter, which is a high quality, excellently documented device from what I can tell. MGL has packaged it nicely with ferrites and capacitors and a nice and practical IO block. Why it is not used according to it's specs is a mystery though, and why they are marketing it as a good battery backup solution is equally strange.

Well. My plan for the whole electrical system was based on the Aviogard with a backup battery and the ECB and using all electric instruments. The main purpose was to get a very simple and nice solution with battery backup and optically isolated power, with emphasis on simple. It didn't really work out. Modifying the Aviogard so it adheres to the UQQ specs and installing a Mascot charger, seems to be one solution ATM. This means adjusting the Aviogard to 13.2V, and connecting the sense pins correctly. It also means fixing the PCB lid. On the other hand, optically isolated DC-DC converters are used en mass in the automotive industry, so maybe I will just get one of those?

The backup battery needs more considerations no matter which solution I chose. None of these DC-DC converters are designed to charge batteries, because it will defeat the very purpose of supplying constant and clean voltage to delicate instruments, at least when there is no separate battery line output. I could install a 3 way switch instead of the two way. This switch will have charge-off-on, switching between the Mascot charger and the Aviogard. My modified electrical diagram would look something like this:

At start up the switch will be in "off". Then turned to "charge" when the engine is running, and left there. When the Mascot shows a green light, it will be turned "on". That way I will be sure the UQQ does not enter a "hiccup mode", and the battery will be kept charged with 13.2V continuously. I guess this will work, but there are several other possibilities also. One is to use two "Aviogards" in parallell, where the second has it's own battery that can be switched out from the primary circuit. This will prevent the "Aviogards" from having to charge batteries. Instead of the switch, I could just have a Mascot charger, but by then there will be an awful lot of gadgets. I have to think about this a little, and also ask Reinier at MGL about this Aviogard that is broken.

Aviogard take 2

Tested a bit more. The backup battery needs to be fully charged; newly maxed out from the charger, for the Aviogard to "accept" it, and not go into some kind of indefinit "break down mode" or whatever by making ticking noises and heating up. Even when fully charged, it would still enter in this indefinite loop at every other try. I switched batteries. Took the half way charged smaller battery as the input, and the larger one as backup and turned on the power. The idea was to see what happens when the "main" battery slowly discharges, a typical situation when the alternator stop working.

That seemed to work. The Aviogard switches off and the backup takes over. It did not switch cleanly off though, it happened with lots av ticking and heating up, the same way as if a not fully charged backup battery was installed. I could see on the EFIS the voltage varied also.

After that the Aviogard was dead. It does not turn on again. It smells burned also.

Either I have a faulty device (it obviously is now), or the Aviogard simply does not work properly with a backup battery. I'm just glad I was curios enough to test it on the table before mounting it in the plane.

Because of the smell, I had to take off the cover and look inside. Not much in there actually. It didn't smell good, but looked OK (to me). It looks like this:

And there are a few capacitors and resistors mounted on inside of the cover. The main unit is just an off the shelves isolated DC-DC converter. It's a Murata Power Solution UQQ series unit. More precisely is is a UQQ-12/8-Q12NB-C.  It's sold by RS for instance, and I bet other component warehouses. This is useful, because full documentation is readily available.

The schematics of the UQQ looks like this:

The pinout and some specs:

The interesting part is the note that say Important! Always connect the sense pins; see Application Notes. With emphasis on always. These sense pins are not connected on my device! I don't know what that means in detail though, but the Aviogard is certainly not made according to the specs of the UQQ.

Further down the following can be read about the sense pins: Note: The sense and VOUT lines are internally connected through low-value resistors. Nevertheless, if sense is not used for remote regulation, the user must [emphasis in specs] connect + sense to + VOUT and -sense to -VOUT at the converter pins. Sense is intended to correct small output accuracy errors caused by the resistive ohmic drop in output wiring as output current increases. This output drop (the difference between Sense and VOUT when measured at the converter) should not be allowed to exceed 0.5V. Consider using heavier wire if this drop is excessive. Sense is connected at the load and corrects for resistive errors only. Be careful where it is connected. Any long, distributed wiring and/or signifi cant inductance introduced into the Sense control loop can adversely affect overall system stability. If in doubt, test the application, and observe the DC-DC’s output transient response during step loads. There should be no appreciable ringing or oscillation. You may also adjust the output trim slightly to compensate for voltage loss in any external fi lter elements. Do not exceed maximum power ratings.

Then the trim. The UQQ is nominally 12V out, but can be trimmed up to +10% and down to -20%. This means MAX 13.2V and minimum 9.6V. The trimming is done as explained below in the specs:

Now, the trimming on my unit was done between the "trim" and Vout+, but should be done between "trim" and "sense+". Of course, if sense+ and Vout+ were connected (as they should, but isn't), this wouldn't matter.

I think this is the root to all problems.The Aviogard is trimmed to 13.8V. This is +15% and thus above the recommended +10% by an amount of 50%. The sense+ and Vout+ was not connected. According to the red square in the specs above: Trim adjustments greater than the specified +10%/–20% can have an adverse affect on the converter’s performance and are not recommended. Excessive voltage differences between VOUT and Sense, in conjunction with trim adjustment of the output voltage, can cause the overvoltage protection circuitry to activate (see Performance Specifi cations for overvoltage limits).

This "hiccup" mode looks very much like what happened to my unit.

This all seems strange to me. It's not obvious how the unit is expected to maintain 13.8V while charging a battery that has dropped to 11-12V without entering into a never ending "hiccup mode", also due another reason:

The UQQ is used in the Aviogard without adhering to the specs of the UQQ. The UQQ was obviously never ment to charge batteries, as it will enters a "hiccup mode" that it won't get out of without removing the load, ie. removing the backup battery.

A closer look at the inside of the cover, and I could see the fault. It was the output line on the PCB that had burned off. Probably due to hiccup mode?