Opto Isolated ESCs + Pixhawk

A

Ancistrus01

Member
Hi Guys,

Today I've finished my X8 heavy-lifter build. Currently I am charging batteries and hopefully everything will work fine! (I am nervous as hell! :-D)
I am using Pixhawk, U7s 420kv, FLAMEs 80A...
I found very strange and I think pretty dangerous thing.

Opto Isolated ESCs (FLAMEs are optoisolated however even KDE ESCs are optoisolated) needs 5V on their servoplug.
Unfortunately Pixhawk doesn't provide 5V on 'ServoRail'... You can find more information here: http://copter.ardupilot.com/wiki/connect-escs-and-motors/ (Scroll down...)

What does it mean?
You have to use external BEC for powering the Pixhawks servorail! Without powering the servorail from external BEC, ESCs can't start and they keep beeping in 1 sec. interval.

I can't imagine what would happened if the external BEC stopped working?!
It is horrible imagination, that whole rig is holding on one small BEC and one battery!..

What do you think would happen when the BEC would stop supply voltage?
Personaly I think that motors would stop, because they couldn't 'read' signal from FC...
Any suggestions how to prevent this?

Thank you very much,
Have a nice day.
Adam
 


dazzab

dazzab

Member
Yes, you need a BEC. Make sure it's a good quality one like Castle Creations. I use Pixhawk on multirotors and planes. On both I have a 5V BEC on the output rail because even with a normal ESC that supplies 5V to the rail, with planes it may not be enough to run all the servos. With a multi rotor and normal ESCs you don't need the BEC but if the power to the Pixhawk from the power module fails it will automatically switch to the power from the output rail being provided by the BEC.

As for your entire rig relying on a single BEC, there are so many single points of failure on a multi rotor it's not even funny. Check your motors that are holding the props. One little cheap C clip and the magnets of the motor are lifting all that weight. Multirotors are just an accident waiting to happen. All you can do is to mitigate the risk as best as possible.
 

Hoki

Hoki

Member
what are the odds somethings going to fail. I think the best thing you can do is get good parts and solder well and make everything waterproof.
 

A

Ancistrus01

Member
Dazzab: You are 100% right! Multirotors are really dangerous and have to be operated with HUGE responsibility! I am trying to eliminate risks as much as I can! It sometimes drives me crazy because I can't make thing redundant as I would personally want...

I am thinking of using Triple redundant power source... Using two BECs (one Castle Creation and one Turnigy (because you know, it is better to use BECs from different manufactures) and scheme shown below on this page: http://copter.ardupilot.com/wiki/common-powering-the-pixhawk/ (Advanced Power Supply Configuration).

BTW. Are you using Zener diode on ServoRail?

One last thing I want to order for my heavylifter is safety parachute. Local company called Galaxy GRS is manufacturing ballistic safety parachutes for multirotors. http://www.galaxysky.cz/multicopters-s71-en
 

dazzab

dazzab

Member
BTW. Are you using Zener diode on ServoRail?
Click to expand...
I know I should but I'm using a good BEC so I doubt the voltage will go high enough to cause any issues. For those interested, here's the info about using a Zener on the output rail.
One last thing I want to order for my heavylifter is safety parachute. Local company called Galaxy GRS is manufacturing ballistic safety parachutes for multirotors. [URL]http://www.galaxysky.cz/multicopters-s71-en[/URL]
Click to expand...
I've been the whole route of considering kill switches, parachutes etc. It just adds complexity and typically I'm far too low for a parachute to be useful. The extra weight just isn't worth it in my case.
 


P

Paul-H

Member
If you only have ESC's connected to the Pixhawk you don't need to add a BEC to the servo rail.

I have 4 ESC's connected to mine and they are all OPTO's and all work fine, and no other power is connected to the servo rail.

Its my understanding you only need to add power to the servo rail if you are needing to power servos or want to add power redundancy.

OPTO ESC's should work fine with just signal and ground connected.

Unless I read what the op is trying to do incorrectly.

Paul
 

KDE Direct

KDE Direct

KDE Direct, LLC.
True OPTO-Isolated ESCs do require power to the servo rail, or the ESCs will fail to arm properly (minimal voltage to power the internal circuitry). The KDE Direct XF ESCs are indeed truly OPTO-Isolated to the control-servo lead, so power is required via a UBEC to the servo rail on the Pixhawk, to properly arm the electronics.

Many of the low-cost ESCs in the market that claim "OPTO-Isolation" are not actually using the correct circuitry, but simply stating this as a marketing tactic. Many brands have simply taken a traditional low-cost ESC, disabled the internal BEC, and then call it "OPTO", even though there is no optical-isolation circuitry at all.

You can test this via quick easily by measuring (with an ohmmeter or multimeter) for conductivity between the negative-rail of the control servo lead and the main ground lead, or simply - if they arm properly on the Pixhawk with no connected BEC, then it's a tell-tale sign the OPTO-circuitry is truly not there.
 

A

Ancistrus01

Member
Thank you for explanation! I am really using true Opto-Isolated BECs.

What would happened if UBEC accidentally stopped supply 5V into power rail?

Thank you!
 

KDE Direct

KDE Direct

KDE Direct, LLC.
If the UBEC fails to supply the 5V power line (the OPTO-Circuitry is rated from 5V up to 35V maximum), then the OPTO-isolation will be unable to properly transmit the control signal. In this regard, the ESCs would loose connection to the flight-controller, and go into fail-safe mode after two (2) seconds of consecutive missing communications. In this regard, choosing a high-quality UBEC is important, as the durability and trust-worthiness of the unit is important for safe operation. The UBEC we offer, the KDEXF-UBEC22, is a popular-choice for this as a robust solution to prevent concerns and electrical-failures: https://www.kdedirect.com/collections/xf-multi-rotor-electronics/products/kdexf-ubec22.
 

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