4S vs 5S

Discussion in 'Cinestar 8' started by William Johnston, Oct 25, 2012.

  1. William Johnston

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    There seems to be some debate as to whether switching to a 5s battery would actually cause you draw less current or not. In theory, you should draw less current the higher the voltage to maintain the same power. So I thought I would test it out. What I want to test was is to maintain the propeller at the same speed and alter the voltage. Maintaining the same propeller speed is a bit difficult because of the large steps in the motor test values. Nevertheless, I was able to keep the propeller speed between tests close.

    Here is what I found:

    16.8 volts at a motor test value of 55 spins the propellers at 2730 rpm and consumes 2.31 amps.
    21 volts at a motor test value of 40 spins the propeller at 2760 rpm and consumes 2.12 amps.

    So it seems to hold with theory. It takes less current at the higher voltage to maintain the same speed.

    What's weird is that it takes way more power to maintain the same speed at the higher voltage; 38.8 watts at 16.8 vs 44.5 watts at 21 volts. I would have expected those two number to be closer.
     
  2. Pavlos Antoniou

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    On the qd site you can find a video that compare 4s to 5s..!

     
  3. William Johnston

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    I have seen that. And it sort of shows that the 5S pulls less current. The trouble with that test though is that they don't control for the load on the motors. That is, the two flights are not flown exactly the same. It may be, for example, that the copter was flown more aggressively on the 4S flight. So you can't really draw any conclusions from the test.

    My test was done on the bench with power from a power supply.
     
  4. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    That's a good point, William. I wonder whether we could devise a "standard" flight -- perhaps just take off and hover on AH at a preset altitude for some number of minutes? It wouldn't necessarily show the peak current/voltage sag, but might be probative of such questions as ESC temps?

    Andy.
     
  5. William Johnston

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    Even hovering outside presents a problem as you can't control the wind. This Friday I'll fly it indoors at the Universities RFID lab. It's a huge warehouse. I'm trying to convince them that RFID stands for Robot Flying InDoors. :)

    You could come up with a standard flight using way-point navigation. You just need a program that generates the way-point coordinates from a starting coordinate. Then everyone could fly the same pattern.
     
  6. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    I've arranged to do some test flights in a corner of a TV studio (20 feet before you get into the lighting). I'm hoping that studio (which is 4,200 square feet and square shaped will allow me to hover about six feet above ground to do some discharge curves.

    The concerns are:
    1. Will the studio be large enough to avoid the C8 having to fly in its own downwash -- which makes for very turbulent air. I'll be "flying" it all the time.

    2. Without a GPS lock (I'm pretty sure the studio is shielded), will I still get GPX files? I believe Josh said on another post that logging happens without GPS lock, but I'll find out for sure this afternoon.

    I'm going to try just a very simple flight profile: take-off, go to hover six feet above ground and stay pretty much in one place and see how long the batteries last until 14.4 volts -- I might keep going below that just to see what the decay curve looks like, but they have a hard concrete floor in the studio. :(

    I'll have a 3-Axis gimbal and Sony CX760 as payload for these flight this afternoon. I'll come back and try a 5D Mk III later. I'll be testing QC6200s, QC8000, and Maxamps 10900. I'll test single and dual battery combinations.

    William: If you would be kind enough to provide a copy of your analysis tool in its current form (ours is still in design), I'll be happy to run the GPX files through it and post the results.

    I was also thinking of trying wrangle Excel into plotting discharge curves by extracting CSV files of the GPX data.

    Andy.
     
  7. William Johnston

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    Yes. In my mind it goes something like this... I gun the engines to take off, but I have given them too much gas and it shoots up. Before I can lower the gas, it hits the ceiling. The propellers shatter and it comes down fast smashing to bits on the hard floor. I hope it doesn't go like that.

    I dropped the latest program here:
    http://workspace.cast.uark.edu/wgj/outgoing/AnalyzeGPX.jar
     
  8. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Thanks for the jar. I owe you a jar (English term for a pint of beer).
    I'll be posting my results shortly.

    The flying was fine. 20 foot ceiling and 70 feet x 50 feet flying space. The biggest problem was I got quite a few spectators and I didn't want to be too close to them. I put mover's blankets over the studio cameras under the operating rule "Don't break anything you cannot buy" and those cameras/lens/teleprompters probably come close to $850K.

    Thanks
    Andy.
     
  9. Gary Haynes

    Gary Haynes Administrator
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  10. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Well, my tests were somewhat inconclusive, I'm afraid. I had hoped to analyze the GPX files for flight duration and battery discharge curves, but, contrary to what I thought, the FC board does NOT log data if it cannot get a GPX fix. At least mine didn't today. Didn't even open up a folder with today's date.

    I suspect it may be slightly more complex than that -- as in, it may log flight data if you have appropriate settings via MK Tool.

    Just anecdotally, the flight times seemed in proportion to battery capacity, with the additional weight of the battery factored in --- but that's just based on what the flight times seemed like. I should probably have used a stop-watch as well, but, there you go.

    Anyone got any idea how to get flight logging without a GPS lock?
    I'll retry the experiments when next I can next can get studio time....

    Andy.
     
  11. Tabb Firchau

    Tabb Firchau Administrator
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    Hey guys, I think you could realize the benefits of 5S if you were to at the same time switch to a lower kv motor. Right now you are holding motor KV constant and increasing voltage. Thus the ESC's are running at a lower part throttle setting and heating up more.

    If you went to a lower KV you would have lower AVG current draw, equiv throttle percentage, and likely lower temps on the ESC and batteries. The only unknown is how efficient the MK ESC is as it approaches it's voltage rating limit.

    Tabb
     
  12. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    To answer my own question regarding getting a GPX file, I found the source code from the Navi Control board that's causing the problem. It's from file "logging.c" -- for any of you who are programmers and understand the C programming language, you can figure it why no log file gets created:

    Lines 314-320:
    case LOGFILE_CLOSED:
    if((GPSData.Status != INVALID) && (GPSData.Flags & FLAG_GPSFIXOK) && (GPSData.SatFix == SATFIX_3D) && (FC.StatusFlags & FC_STATUS_FLY))
    {
    logfilestate = LOGFILE_START;
    }
    part = 0;
    break;

    For those of you who are not programmers, here's the translation:

    In the program that controls GPX logging, there is logic that tests whether or not to start a GPX log file. The log file will be started if the following conditions are met (the AND requires that ALL of the conditions must be true):

    1. The GPS Data is not invalid (which really means it is valid!)
    AND
    2. The GPS board has obtained a fix.
    AND
    3. The GPS fix is good enough to give a three dimensional fix.
    AND
    4. The Flight Control Board is indicating that the aircraft is flying.

    So, bottom line: If you don't have a GPS fix, the Navigation Board will not create a log file. :(

    So flying indoors in a shielded room: no log file.
    I should have checked the source code before I bothered to do those flight tests, shouldn't I?

    Has anyone on this forum attempted to edit the source code, recompile and upgrade the firmware? I think the Navi Control board waypoint distance limitations are tucked away in the boot loader and are therefore inaccessible to changing, so there's no risk of damaging those settings.

    Thanks
    Andy.
     
  13. William Johnston

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    I was afraid of that. I read that somewhere and it kind of makes sense. The GPX file is the GPS exchange format. So if there is no GPS data, then there is no GPX file. All the interesting data is in the <extensions> section.

    It's a design flaw, if you ask me. There is no reason not to write out all the other data. It could be written into a seperate XML file.
     
  14. Josh Lambeth

    Josh Lambeth Well-Known Member

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    My apologies on steering you guys in the wrong direction in regards to it recording without a GPS fix... Apparently I must have had one when I was inside the building I was flying since I was able to see the GPX file after... Odd

    Josh
     
  15. William Johnston

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    That would mean that the temperature is related to power rather than current.


    edit:

    Which is what the theory says:
    "The equilibrium temperature of a transistor is proportional to power"
     
  16. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    No sweat, Josh. I've downloaded the 0.88o NC source code. If I can get time, I might hack it to remove that restriction so it will record GPX flight logs regardless of GPS lock.

    Andy.
     
  17. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    I agree with you, William. They could just as easily (based on the code), just put in zeros for the GPS coordinates and other values that depend on the data from the uBlox GPS. I've asked one of my in-house programmers to look at setting up the development environment so we can compile the source and see if we can regenerate the existing binaries. If we can do that, then we'll get serious about correcting the design flaw. I see no reason not to emit a GPX file -- all the logic is there to do it. Some of the values will be non-sensical, but the <extensions> will be mostly valid.

    Andy.
     
  18. William Johnston

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    So in conclusion. Switching from a 4S to a 5S will lower your current draw which will increase your flight time. However, your ESC temperature will go up, so if you are close to the thermal limit with a 4S battery than switching to a 5S will push your ESCs into their current limiting state and end your flight.
     
  19. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Should that be ..."and end your flight unexpectedly while your copter might be at significant altitude, thus ending your camera gear too?"

    Andy.
     
  20. William Johnston

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    It would also be nice to see the data from when the engines start rather then from after takeoff.
     

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