Fail safe design for Fixed Wing plane using PixHawk


I am doing some due diligence before I use a PixHawk inside a fixed wing airplane for some custom long haul testing. I have specific questions and looking for people who have had past experience addressing the following questions. Thanks in advance for any help.

1. If the battery reaches a certain fail safe threshold and Pixhawk cuts the power off to the ESC(s), does it still provides any power to the TX receiver and the servos?

2. Is it possible through mission planner or through some custom code to make Pixhawk cut ESC power when the first threshold reaches but continue to provide power to Pixhawk and connected servos and the receiver?

3. Lastly, I want to trigger an output pin on pixhawk when the Lipo battery drop reaches a predefined threshold. Is that possible?

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  • @DavidJames - Thanks!

    I will test your findings in my lab as well. Let's see what I find.

    What do you think of designing a new power module for Pixhawk? :) may be we can market it. All such ideas of connecting two batteries in parallel, I find such concepts not very effective. Kills the whole idea of a true fail safe system what if one battery shorts or drains too fast, it will kill the other battery performance along with it.

  • 100KM

    I think that the 5v power supply on the standard 3DR power module will generate 5.3 volts down to a battery voltage of 6 volts or so.    I have only tested this myself accidentally by drawing the batteries down to 7 volts in the lab.  The 3DR power module uses this TI module

    The ESCs typically use linear 5 volt sources for their BECs.   You can get 5v switching BECs that would be independent from the ESC, more power, more efficient and work down closer to 5 volts.   

    The ESC Power cutoff to the motor is programmed independently from the Pixhawk.    You may be able to do this with the pixhawk, but the ESCs that I use all have some sort of motor power cutoff.

    Haven't tried that redundant supply hookup on the servo rail yet.    I plan to though.   Those $5 switching BECs from Hobby King are good down to 6 v.

    PTH08080W 2.25 A, 4.5-V to 18-V Input Wide Adjust Miniature Power Module |
    The PTH08080W is a highly integrated, low-cost switching regulator module that delivers up to 2.25 A of output current.
  • I think what I find annoying that the power module doesn't allow more than ONE battery connection.

    Ideally, the power module should have a built in switching mechanism where we can connect up to like 4 batteries. When the first battery drains, it switches to 2nd, and then 3rd and so on. This way Pixhawk provides battery condition via telemetry via one single connection to the Pixhawk like it does today.

    I might design a new power module for Pixhawk with built in multiple battery switching systems. I believe Pixhawk 2 has two battery connection but I still consider that a poor electronics design.

  • @DavidJames - Thanks, David. I know about two ways to power the PixHawk. That only addresses the powering of the unit.

    So you are saying when PixHawk goes into battery fail safe mode, it still continues to provide power to PixHawk till the Lipo battery voltage drops below what voltage?

    The voltage regulator that comes with PixHawk,         link doesn't provide any details on the minimum input voltage.

    I am not 100% sure what is the minimum voltage input required for it to continue to regulate it down to 5 or 6 V. The spec sheet doesn't provide any details.

    How do you program the ESC to cut power at a specific voltage drop? I thought Pixhawk takes care of that issue through battery fail safe feature.

    The last question remain is there any way to trigger a pin on Pixhawk to go high (+5V or +3.3V), if Lipo battery voltage drops to X value.

    Here is what's confusing. For example, if one is using a 4S battery.

    1. A 4S battery, max voltage is 16.8 V max
    2. To successfully recharge the battery again the voltage of the battery should not drop below 10V.
    3. In mission planner, if we set battery fail safe to let's say 11 V, that gives us one extra volt to keep the power on for the pixhawk.

    What I really wanted to accomplish is to trigger an external system when the battery voltage drops a certain voltage. not sure if it's possible without any custom programming.

    have you ever personally tried this two battery solution?


  • 100KM

    Hi,   The pixhawk typically gets 5v power from the power module on the voltage and current sense board and also from the 5 volts on the servo rail.   Here is a link to the section on powering the pixhawk.

    The 5 v power from the power module is independent of the ESC and I have found the 5 volts is maintained down to below 6 volts.    The ESCs battery eliminator(BEC) 5Vdc also is maintained down to at least 6v of battery voltage.

    I have found that the ESCs still maintain BEC 5v even as they are cutting back power to the motor.    The ESCs can typically be programmed to cut motor power partially or completely at some battery voltage.    Mine are set to cut off at 9.5 volts for 3S batteries.   In this configuration the motors cut out when battery voltage drops to 9.5 volts, but the pixhawk, the RC receiver and telemetry all remain powered.

    The ESC motor cut out can be overridden by cycling the throttle low.     You should be light on motor power use to avoid pulling the battery voltage below 6 volts.    The 5volt power modules vary so you will want to test them to determine when they cut out.    Some work right down to a 5 volt battery voltage.   Others might not even make it to 6v.

    Powering the Pixhawk — Plane documentation
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