Question re: Low battery RTH while flying Litchi waypoint mission

[BobWare]

Does a low battery RTH while flying a Litchi waypoint mission override the mission and execute immediately, or does the mission continue like it does in the case of lost RC signal?


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[Jayfdee]

I could be wrong, I often am, but I think If you have the smart return ticked it will RTH if it thinks it cannot complete the mission, which may be a bit conservative.
But good question, I would like a definitive answer, but my missions are always very safe with battery life.
One thing I have noticed though is that the actual mission time is usually only 75% of the predicted time, and so I push the boundary a little bit.

 

[RodPad]

@BobWare, Hey man, I just had this question when I woke this morning, I did a flight yesterday, one of the way points was accidentally / stupidly to high, and I thought about if it been windy above.

Some I'm looking at this search, at the moment.

Search Results for Query: litchi battery | DJI Phantom Drone Forum

Rod

 

[BobWare]

Thanks for the link to the other related thread, RodPad... Jayfdee, looks like you were right--the aircraft will ignore the rest of the mission and RTH when the low battery condition is triggered.


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[RodPad]

How many missions have you done, getting the elevations figured out.

Rod

 

[tevek]

Smart RTH kicks in during the mission if it thinks it's not going to make it. Happened to me last week and my fault trying to squeeze one more trip in on a half charged battery. First time in about 25 missions and a bit unnerving when it happened.

 

[RodPad]

I have always left Smart RTH off, Still making sure I fully understand the Dumb version.
Thanks...

Rod

 

[tevek]

I have always left Smart RTH off, Still making sure I fully understand the Dumb version.
Thanks...

Rod

Thought of doing the same, but I may have been lucky this time. A lot of people switch it off for distance flights, for obvious reasons, but I need all the help I can get.

 

[dwallersv]

Anyone have any "real" inside info on how the Smart RTH algorithm is implemented? I.e., does it calculate the needed juice based on a straight-line return to the home point from its current location (with some margin)? Include power for ascent? Include battery stats and model battery discharge/age?

I'm just trying to get an idea on how reliable it is -- can we just fly and forget (within some obvious limitations, of course)?

 

[smiller]

Anyone have any "real" inside info on how the Smart RTH algorithm is implemented? I.e., does it calculate the needed juice based on a straight-line return to the home point from its current location (with some margin)?

Pretty much. I don't think anyone knows all of the exact details but bottom line the algorithm seems to be pretty conservative (too conservative many might say) so it's usually pretty safe. One thing it cannot account for though is a significant headwind so if you are flying in high wind conditions keep that in mind.

 

[Meat]

Greetings All,

Just a couple of observations I made from a relatively recent waypoint mission of mine. The mission was impossibly long and SmartRTH was enabled. The vehicle returned safely to its start point with 13% power remaining.

First, the cruising speed for the mission was set to a rather leisurely 23mph. When SmartRTH kicked in, the quad maintained this speed throughout its 3.4 mile trek back to the home point. This might have been disastrous had the speed been even a bit lower.

Second, when SmartRTH engaged, the craft had been climbing sharply from 145ft to 300ft. The climb was aborted and the craft headed for home at 235ft AGL. It DID NOT climb to its set RTH altitude but maintained the altitude it happened to be at when SmartRTH took over. It wasn't a serious issue in this case but it would have been fatal for the quad had SmartRTH kicked in just a few seconds earlier at 145ft and the quad had taken the same route it went out on.

Just a couple of things to be aware of if you're relying on SmartRTH to keep your quad safe. It may not behave quite like you think it should.

 

[smiller]

Greetings All,

Just a couple of observations I made from a relatively recent waypoint mission of mine. The mission was impossibly long and SmartRTH was enabled. The vehicle returned safely to its start point with 13% power remaining.

First, the cruising speed for the mission was set to a rather leisurely 23mph. When SmartRTH kicked in, the quad maintained this speed throughout its 3.4 mile trek back to the home point. This might have been disastrous had the speed been even a bit lower.

Second, when SmartRTH engaged, the craft had been climbing sharply from 145ft to 300ft. The climb was aborted and the craft headed for home at 235ft AGL. It DID NOT climb to its set RTH altitude but maintained the altitude it happened to be at when SmartRTH took over. It wasn't a serious issue in this case but it would have been fatal for the quad had SmartRTH kicked in just a few seconds earlier at 145ft and the quad had taken the same route it went out on.

Just a couple of things to be aware of if you're relying on SmartRTH to keep your quad safe. It may not behave quite like you think it should.

Interesting. I would think that SmartRTH would be under the control of the aircraft low-level programming, not the control app. A couple of questions:

1. You mentioned your aircraft returned at 235 feet. What value was your preset RTH set to?

2. Was the return home speed exactly 23 mph (same as your Litchi cruise speed) ? I ask because because the 'smartest' RTH speed for the RTH algorithm to choose would be the max endurance speed, which is not the same as maximum possible speed. The max endurance speed might be fairly close to your Litchi mission speed, making the two easy to confuse.

 

[RodPad]

@Meat, Good Stuff!

Rod

 

[Meat]

Interesting. I would think that SmartRTH would be under the control of the aircraft low-level programming, not the control app.

Yeah...but keep in mind that, once the waypoint mission is uploaded to the craft, the app no longer has any input, particularly when the quad is out of range of the controller.

1. You mentioned your aircraft returned at 235 feet. What value was your preset RTH set to?

395'

2. Was the return home speed exactly 23 mph (same as your Litchi cruise speed) ?

Yes, exactly. It was traveling at 23mph when it reestablished contact with the controller. Viewing the video from the SDcard, it didn't appear to alter its speed when contact was reestablished.

 

[smiller]

Thanks. It sounds like the three most important things to remember with respect to a Litchi mission are:

1. Planning
2. Planning
3. Planning

 

[Meat]

For sure. Actually, I hadn't given the SmartRTH speed much thought at all. As for the altitude, I guess I just assumed that it would be the same as the regular RTH.

It's been mentioned on here many times that SmartRTH is NOT the same as normal RTH and that's very clear to me now

 

[smiller]

My understanding is that in all RTH modes the unit should ascend to the preset minimum RTH altitude (or if already above the RTH setting, at the current altitude), although the DJI manual is typically ambiguous about this. The behavior I would expect with any low-battery RTH procedure would be to ascend to RTH altitude, but it's possible that an automatic RTH if in autonomous mode (no connection to the controller) can only occur at current altitude/speed settings. This is kind of difficult/scary to test since you must have the unit both out-of-range and in a low battery condition, not a situation many people aim for, although I guess you could test it safely by simply setting RTH battery level to something high, like 50%.

Bottom line, while Litchi does note that gimbal tracking and speed cannot be controlled while under LOS conditions, if it is also true that even an emergency low-battery RTH during a Litchi mission will be performed only at the current altitude and speed and the RTH altitude setting will be ignored if the unit is in LOS then this is potentially very dangerous and should be clearly outlined in the documentation.

 

[Meat]

Absolutely.

Of course, one can always test this by switching off the controller to simulate a loss-of-signal situation. A controlled and predictable SmartRTH test. Perhaps I'll try this at some point. Set up another impossibly long waypoint mission but keep the waypoints close to home, then sit and wait...

 

[DanMan32]

That would test fail-safe RTH and not smartRTH. However both should be applying the same RTH rules.
Fail-safe won't trigger during a mission.
You can't set battery level for RTH, that's calculated automatically as smart RTH. You can only change critical low battery which causes it to land where it is.
Low Battery warning setting only notifies you that you have reached that level. No other action is taken.
RTH I would think should autonomously go to set height but during a mission that could be a different story.

 

[Meat]

@DanMan32 - I was addressing smiller's 2nd paragraph. I believe he was wondering if there was possibly a difference in the behavior of SmartRTH when the controller was NOT connected, as some other functions are affected (and documented) such as gimbal pitch and speed.

What I would like to test is the behavior of SmartRTH with no signal but I don't particularly wish to intentionally send my quad several miles out with a low battery just to do this. By switching off the controller and simulating a loss-of-signal situation, I could let the quad run a long mission with the controller disconnected but keep the unit within line of sight until the battery runs down and SmartRTH occurs. I can then observe whether the quad ascends to the set RTH altitude or simply remains at the altitude it was at. Also, whether the quad accelerates during a SmartRTH or whether it continues at the cruising speed set in the mission.

I believe I already know the answers to these questions but I'd like to confirm them under more controlled conditions.