Pixhawk 6s - Sensing more then 250A safely

Friends please assist...

After parsing pages and pages of forum in search of an answer I finally feel I must ask for experienced opinion...

A common problem no doubt: Large 6s Hex; MN4014-9 motors; 16 inch props; Carrying a Flir thermal camera (Need I say I am hesitant to allow the 180A Attopilot to be a possible single point of failure!)

Would installing a current divider system to improve margin of safety be feasible? I propose one of the following:

1) In parallel over the Attopilot install an identical resistor arrangment as found on the Attopilot 180A - A 2watt resistor if I am not mistaken (availability of these??)

   > This would be a cheap solution

   > Would simply divide the current in half

   > The voltage readings should be unaffected;

   > When setting up MP one would specify a battery pack of half the real capacity.

2) If finding identical resistors to those in the Attopilot is going to be a problem, install a second 180A Attopilot in parallel

  > This second Attopilot will not be connected to the flight controller

  > Otherwise same as for the above.

  > A bit of a waste...

Alternatively could one not simply install low resistance shunts in parallel to a 6s power module?

3) The HKPilot32 10s 90A power module for example

  > If one where to know the resting resistance of this module a appropriate shunts could be used to allow just 1/4 of the current to pass via the power module and the remainder via the low resistance shunts.

> This appeals to me as I could save carrying one extra BEC, AttoPilot and cables.

Has anyone had any success with shunting current to improve the safety margin? / To allow the use of standard power modules on larger machines?

***Before somone mentions that we will never need more than 180A ---- If chasing a cable-car at near max throttle for over 1000m gain in altitude over 4min.... We will need exactly that amount of current!

Many thanks in advance.

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Replies

  • Some back of and envelope calculations while awaiting a more informed answer:

    It would appear that the HKPilot 10s 90A Power module is based on an AttoPilot 90A - using two 0.5 x 10^-3 Ohm resistors.

    If one where to make use of four 2W 0.5 x 10^-3 Ohm resistors in a parallel array then one would end up with a 8W 1.25 x 10^-4 Ohm shunt resistor (http://za.rs-online.com/web/p/surface-mount-fixed-resistors/7717799/)

    If the shunt were placed in parallel with the Hobbyking HKPilot 10s 90A power module the splittling ratio would (Module:Shunt) be 1:4 - Thus making my absolute max current of 250A a breeze for both the shunt and the power module:

    E.g. A 250A load would be split 50A through the power module and 200A into the shunt.

    Using P=I^2R the shunt would dissipate 5W and the power module 1.25W...

    Unfortunately I imagine a sensed current of 10A would indicate a real current of 50A.

    Thus, if my logic is sound, the standard power module should be able to serve large hex and octo machines and offer redundancy in the event of a melting power module.

    HKPower.jpg

  • I am well aware that placing a resistor of a calculated value over the 3.3v ADC port on the Pixhawk can allow for measurment of over 180A by a 180A Attopilot..... But this is not what I am trying to achieve.

    Aim is improved safety... Redundancy even! If the Attopilot were to melt only the readings would be lost... :) An expensive vehicle may be saved :)

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