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Attopilot 180 A, non linear voltage&current reading

Posted by Dean Hitrec on June 21, 2015 at 2:01pm

Hello people!

I'm having problems with my Attopilot 180A voltage and current sensor.

I know people had similar problems but can't find any solution.

Here is my story:

I have 1.000 mm octocopter (6S 16.000 mAh), around 50 A at hover, Pixhawk.

I've installed Attopilot 180A voltage and current sensor in order to have voltage, current, but most importantly for me, mAh reading to be as safe as possible when flying. I also have Futaba 14SG with telemetry so I'm also tracking my battery status (voltage) on the transmitter.

The problem is that Attopilot doesn't seem to have linear voltage (nor current) output under load. I've ruled out everything else because I did the following:

1. I've calibrated the voltage reading with handheld voltmeter and fully charged 6S. The voltage info on the OSD/MP is absolutely correct without load.

2. When I start to draw about 10-20 A, voltage on the OSD/MP drops too much, 0.4-0.5 V under actual value (measured with inline watt meter and handheld voltmeter)

3. Interestingly enough, when I connect 4S pack, without any load the reading is perfect. So, conclusion is that when current starts to flow shit start to happen.

4. Now, to rule out the Pixhawk, I've measured the output on the Attopilot alone and compare that to the voltmeter. Here is what I get: 24,87 V (no load) -> 1,579 V; 24,67 V (around 7A) -> 1,559 V; 24,49 V (around 10 A) -> 1,546 V; 24,41 V (around 20 A) -> 1,537 V

If you start doing the math you will see that Attopilots voltage drops too much when under load. So, if I calibrate the voltage reading with no load, then the reading will be inaccurate when under load. If I calibrate it under 10 A load, then it will be inaccurate without the load and under different load (50 A).

Since these Attopilot board are very simple devices I don't know why is this happening. Should I try to buy another Attopilot 180A board (or any alternative if something like that exists) or I can fix this somehow

Thanks a lot!!

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Tags: 180a, attopilot, current, inaccurate, nonlinear, voltage
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Replies

  • vyoufinder November 29, 2019 at 8:23am

    I've decided on the Mauch power module, because it claims to be more accurate and also use less power to get wasted into a resistor.  I don't know how to know which 200A module to get though.  There is  28v for 2s-4s packs, and a 60v model for 4s-14s packs.  I don't know how to know what I need.  How can I figure this out?  Should I still use my zener diode on the servo side of the Pixhawk when using the Mauch modules?

  • Moderator
    Nathaniel Caner July 21, 2015 at 11:40am

    I also think BATT_AMP_OFFSET plays a role as well. I've never been sure how to implement it though :) Like Peddals2Paddles, I too calibrate the sensor at 50-60% throttle (the practical range where I spend most of the flight). As in this video here:

    https://www.youtube.com/watch?v=tEA0Or-1n18

    Regards,

    Nathaniel ~KD2DEY

    • Hampus Lundin > Nathaniel Caner November 16, 2015 at 10:25am

      Has anyone found a solution to this? I have the exact problem with Attopilot and also Hobbyking Power Module. It is unacceptable that it doesn't work reliably, since my wattmeter can measure the voltage at any load!

  • Pedals2Paddles July 21, 2015 at 8:32am

    In my experience, the 3DR power module does the same thing and so does the Hobby King power module.  Since there is nothing I nor you can do about it, you can only take the lesser of two evils.  Instead of calibrating it to be accurate on the ground, calibrate it to be accurate in the air.

    I calibrate the voltage and current sensor using Mission Planner in the Pixhawk at 50-60% throttle.  Most of my flying is 25-35 amps, so I bring it up to that level, measure the current and voltage on my meter, and enter those readings into Mission Planner.  Now I know I have accurate readings in flight and the subsequent slightly inaccurate readings are all while sitting on the ground.

    I do this by turning the propellers upside down, just like you do for the Compass Motor Calibration.  That way I can spool the motors up to 30 amps on the bench and meter it.

  • Peter Seddon July 19, 2015 at 9:30am

    You are measuring the battery current and voltage going into 8-ESC which due to the switching going on in the ESC's will be far from smooth current/voltage. As you vary the speed of the ESC's so the way the current gets chopped up will vary. The output from the Atto sensor will be smoothed to try and better reflect average dc current but that smoothing could easily be different to the smoothing on your inline wattmeter. So every device that measures that chopped up dc current from the battery is likely to give a different reading. Both the current and to a lesser extent the voltage readings will be affected. And then you have other issues with how the measured current is digitised. 

    What can you do? Not a lot other than trying to get the same level and type of smoothing or filtering on each measuring device, which I would suggest is not easy.Accept the limitations of pulsed dc current and voltage measurement and be aware of the inaccuracies. 

    Peter

        

    • Dean Hitrec > Peter Seddon July 19, 2015 at 12:44pm

      I don't think this can cause that big of a difference. Because I've measured current and voltage with different devices, including one in Futaba R7008SB receiver. All of them are in 0.1V the same. So, I don't really get how the Futabas R7008SB receiver which is connected basically at the same place as the attopilot can measure the voltage very precisely (same as my Hobbyking wattmeter and high quality handheld multimeter) and the attopilot board is way way off under load.

    • Adolfo Rodrigues > Peter Seddon July 19, 2015 at 11:49am
      I would agree with you if my own voltage divider behaved the same way and it does not. It gives the correct reading without and with load. The only diference I can think of is the point were it measures the battery.
      The attopilot voltage divider reads the voltage before the shunt resistor and mine after. Both have the same groung reference atm. Confused :S

      Granted my resistors and filter capacitor got different values but still...

      The only thing I have to try is connect my voltage divider to pixhawk and see if it then gives correct voltage.
      • Dean Hitrec > Adolfo Rodrigues July 19, 2015 at 12:50pm

        This is all very interesting because:

        1.) My Futaba R7008SB telemetry reads the voltage perfectly and it's connected basically to the same place.

        2.) Voltage from the attopilot is not linear so Pixhawk can't do anything about that. I would say the problem is Attopilot. But...

        3.) Both brand new attopilot boards I've tried behave the same. They can't all be faulty so I think that maybe this is not Attopilot or they just can't do what they are marketed for.

        Really I have no clue what to do.

        • Adolfo Rodrigues > Dean Hitrec July 19, 2015 at 2:31pm
          Tomorrow I have some testing to do. I'm going to connect my voltage divider to pixhawk and a frsky lipo sensor to the battery and see if they match while in flight. If they do I'll just continue using my voltage divider and stop using attopilot for voltage. I to cannot make it measure correctly. However I will try to make a change on the attopilot itself if I have time.

          I'll post my findings
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