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Troubleshooting RX Brownout

#1
Hi everyone,

I've recently put together an FT Mini Guinea and had a great time for the first two flights. However since then, my transmitter briefly loses contact with the receiver a minute or two into the flight. The transmitter beeps to indicate that it's lost contact with the receiver, and I lose control and watch the plane crash. It's very repeatable and has gotten to the point where I need to rebuild the wing because its taken one too many hits.

I noticed as I approached the plane after the crash, the dropout beeping continues on the transmitter and I can hear the ESCs within the plane are continually playing their startup tone. This leads me to believe that the receiver is browning out and losing power momentarily. I've noticed that at least one of the aileron servos makes the whiney, "I'm straining under load" sound at neutral position, and the problem manifests much more quickly if I put my flaperons down. The ailerons themselves exert a bit of up pressure due to the glue reinforcement on the bevel hinge which the servos would be fighting against.

Would it be possible that both aileron servos being under constant load could draw enough current from the BEC to brown out the receiver? I'm using BLHeli 12A ESCs, and I've read that although its BEC is rated at 1A on paper, it's actually 500mAh in reality. I have two ESCs running in parallel, but only one of them is currently running power to the receiver.

Just wondering if that all sounds plausible, and if so, do any of these solutions sound ok:

a) Run both BEC power lines to the receiver to double the delivered current. From what I understand you can have both BECs running to the receiver as long as they both have linear regulators (which they do) and come from the same power source^ (which they do).
b) Upgrade aileron servos from 4g (recommended) up to 9g to handle the aileron load
c) Upgrade ESCs from 12A to 20A, which has a BEC rated at 2A/5V (I've got 2 spare lying around)
d) Get a standalone UBEC

Thanks in advance for any help/advice! I'd love to get the mini guinea back in the air because it's such a joy to fly!

Michael

^ https://www.rcpowers.com/community/threads/dual-motors-dual-becs-tutorial.10617/
 
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varg

Build cheap, crash cheap
#2
500mA is not adequate for 4 servos and an ESC control signal IMO. You mention the servos whining as if they're under load in the neutral position, when they're both down while using flaperons they are definitely under a decent load, if they're working against glue or foam in the neutral position it's certainly possible they're at their limits while flying. 2 stalled or even heavily loaded 4g servos could easily be drawing close to 500mA, add in the current requirements of a modern 2.4GHz telemetry enabled receiver, which is actually a transceiver, and you exceed 500mA quickly. If the 4g servos are at their limits it's possible a 9g servo upgrade wouldn't increase current draw at all, I'd go for it.

You can indeed share load between two linear voltage regulators, you can even do it with switching regulators if they're designed with parallel load sharing in mind, and it might be a good solution, but my preference would be to use a UBEC or larger ESCs if weight capacity allows.
 
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Hai-Lee

Old and Bold RC PILOT
#3
I have had a few instances of "Brownout" due to servo load. I test each design by slamming the sticks from one extreme to the other repeatedly so that each installed servo is moving and changing direction at the same time. (During pre-flight of course!)

I have even been able to "Brownout" a 3A BEC due in part to the servo current and the ESC operating temperature causing the BEC regulator to overheat and shutdown.

Avoidance measures: Lower rated servos, Less servos per BEC, Better cooling of the ESC, Less friction on pushrods, freer hinges, and adjust the throws of control surfaces mechanically and not by limiting travel in the transmitter settings.

When I design/scratch build I usually go for the smallest servos I can use for the initial flight trials my reasoning is that it is better to stall a low current servo or have it be a little sluggish than to have an inadvertent "brownout" followed EVERY time with a crash!

I found a test report online, (I forget where, sorry), that a generic 9Gram servo drew 370mA peak, (Stalled). so three 9 gram servos pushing hard can draw up to 1120mA which is over 1Amp.

Always test the cooling of the ESC prior to test flying! I test by restraining the aircraft and, (with the prop fitted), run the plane at full throttle for an entire battery, During the test also try to move the control surfaces as if you were actually flying the plane. Whilst it might seem a little silly it is far less embarrassing than the dreaded walk of shame!

Finally you can fit dedicated BEC for the Receiver alone and this way you will always have reception and hopefully retain enough control, (during a ESC shutdown), to minimise damage in an emergency landing.

I hope my experiences help!

Have fun!
 

Fluburtur

Cardboard Boy
#4
Spare BEC is always a good thing, I don't mind running power from the ESC for short range planes but I won't fly my mini talon without the BEC (or BECs sicne I got a 5v and 12v inside heh)

Hijacking the thread, I made a cardboard rc boat with a 1000kv motor and 4x4.5 quad prop on it (I run it like 10% throttle on 3S, too much power)
When I punch the throttle the power usage goes very high but still under 30amps but for some reason the rx browns out and doesn't reconnect, im using flysky RX and it did that with two different ones so I don't really know what happens.

Video of it happening, I made a better rudder so it shouldn't be servo power draw
 

Hai-Lee

Old and Bold RC PILOT
#5
Whilst I have not had an Airboat for over 30 years I do remember them having a sealed electronics compartment which was a little small.

You might not be getting the Rx to resume function because of something called "Thermal inertia". That is a term used to describe the time difference encountered in a rising or falling temperature in relation to the measured temperature.

As we use mostly silicon based electronics which are bonded to their mounting heatsink and then encapsulated then silicon electronics element is always at a different temperature to its heatsink except in the case of steady state operation. When the temperature rises rapidly internally the heatsink can still be relatively cold and the silicon circuit reach its over-temperature point and shut down, (sometimes the rise is so rapid that even the temperature circuit cannot respond and "Smoke" is born).

This heat can take a significant time to dissipate especially if the heatsink has attained a high temperature from external influences or environment.

Net result is that the temperature shutdown can occur over a very short period of overload and take a long time to recover. Additional considerations are that repeated and sudden rises in the silicon bonding to the heatsinking mount can be fatigued and reduce in contact area thereby reducing its ability to handle heat or dissipated power. Result is a temperature sensitive device that whilst still basically functional can be almost impossible to use.

The whole area of temperature dynamics in silicon based electronic components is a science in itself and this forum is not large enough to download information on every component.

In summary: If it browns out, COOL IT, (improve cooling airflow), If it still browns out then reduce the load, finally if it browns out with a light load and plenty of cooling, THROW IT OUT!

Have fun!
 
#7
Quick update! Running both power leads from the ESCs to the receiver looks to have solved the problem! As per Hai-Lee's advice I stress tested it on the bench for a battery with the flaperons down and pulling all sorts of wild motions with the control surfaces.

I guess it just needed a little bit more current in the end.

The other great news is that I got 15 minutes flight time with a 2200mAh battery, which I think is amazing for a twin engine.

Thanks again for your help.