RTL based on remaining flight time estimation #18646

MaEtUgR · 2021-11-15T14:43:06Z

Describe problem solved by this pull request
The current range-based RTL from #16399 needs to be configured with a flight time per battery which would need to be adjusted to the environment, flight speed, payload, ... every time it is used and results in very coarse estimate. It's IMO also less transparent for user feedback because the rtl_limit_fraction is not an intuitive graspable number.

Describe your solution
The approach I implemented in summary uses the remaining battery time from the battery estimation #17828 and compares it against a precise estimate how long the RTL will take. It triggers the RTL a configurable margin before that such that there's enough time to return before the battery is empty. This results in a different return time depending on the average current consumption and distance the vehicle is away from where it wants to land again e.g. home.

Test data / coverage
This strategy was tested a lot before but not in the ported form of this pr.

- Compute RTL time also during RTL - Calculate correct altitude when finding destination

MaEtUgR · 2021-11-16T10:30:41Z

I rebased on top of master with the small conflict: https://github.com/PX4/PX4-Autopilot/pull/18645/files#diff-a58612b34e98d7c47e0b13294ccc0c5077dbab967450a7bc84acd596825a8151R86

RomanBapst · 2021-11-23T12:10:14Z

msg/rtl_flight_time.msg

-uint64 timestamp				# time since system start (microseconds)
-
-float32 rtl_time_s				# how long in seconds will the RTL take
-float32 rtl_limit_fraction			# what fraction of the allowable RTL time would be taken


@MaEtUgR Thanks for removing this, I always had trouble with this

sfuhrer

I think it's fine to bring it in without major changes, though we should think about the following for the next iteration:

I think it should be either the old battery state check triggering the warning/RTL/Land action, or this new range based RTL
VTOL average current calculation needs some revising, probably it makes most sense if it only is updated in FW flight, and for the MC part of the flight the user sets reasonable safety margins. Or we add separate estimate for MC flight as well
think about estimating power consumption or an unitless "state or charge per second" instead of current estimation (current increases towards end of flight)

sfuhrer · 2021-11-23T16:59:55Z

src/modules/commander/Commander.cpp

+		// Try to trigger RTL
+		if (main_state_transition(_status, commander_state_s::MAIN_STATE_AUTO_RTL, _status_flags,
+					  _internal_state) == TRANSITION_CHANGED) {
+			mavlink_log_emergency(&_mavlink_log_pub, "Flight time low, returning to land");


How about "Remaining" flight time low, return to land?

Suggested change

mavlink_log_emergency(&_mavlink_log_pub, "Flight time low, returning to land");

mavlink_log_emergency(&_mavlink_log_pub, "Remaining flight time low, returning to land");

I guess we should also add an event here and not only the legacy mavlink log?

sfuhrer · 2021-11-23T17:02:46Z

src/modules/commander/Commander.cpp

-		} else if (battery_at_home < _param_bat_low_thr.get()) {
-			battery_range_warning = battery_status_s::BATTERY_WARNING_LOW;
+		} else {
+			mavlink_log_emergency(&_mavlink_log_pub, "Flight time low, land now!");


Suggested change

mavlink_log_emergency(&_mavlink_log_pub, "Flight time low, land now!");

mavlink_log_emergency(&_mavlink_log_pub, "Remaining flight time low, land now!");

sfuhrer · 2021-11-23T17:15:46Z

src/modules/navigator/rtl_params.c

+ * @increment 1
+ * @group Return To Land
+ */
+PARAM_DEFINE_INT32(RTL_TIME_MARGIN, 110);


Not that it really matters, but it's an interesting default, why not making it 100s or 2min?

Taken over from some old real use case. Not sure how it ended up being this value. A round number makes more sense as default indeed.

sfuhrer · 2021-11-23T17:16:44Z

src/modules/navigator/rtl_params.c

+ *
+ * @unit s
+ * @min 0
+ * @max 300


300s sounds quite low as max, for vehicle flying multiple hours I would for sure want a bigger margin.
Would propose 3600 as max.

RomanBapst · 2021-11-24T06:08:18Z

@sfuhrer

VTOL average current calculation needs some revising, probably it makes most sense if it only is updated in FW flight, and for the MC part of the flight the user sets reasonable safety margins. Or we add separate estimate for MC flight as well

What issues do you currently see?

think about estimating power consumption or an unitless "state or charge per second" instead of current estimation (current increases towards end of flight)

I think this could make sense. If we can get a reasonable "state of energy" estimate in [Wh] then the accuracy might be improved since it would be independent from current increase towards the end.

sfuhrer · 2021-11-24T12:46:16Z

What issues do you currently see?

Let's say the VTOL uses about 10x as much energy for hover as for cruise. If you update the consumption average estimation independently of the flight phase, then the remaining flight times drops by a factor of 10 when you transition to hover, likely triggering a RTL if the vehicle is at some distance to home.

But as I said, let's tackle that in a follow up.

MaEtUgR · 2021-11-24T13:00:40Z

Thanks for the review! I'm quickly fixing the smaller issues @sfuhrer pointed out and then we can follow up with further improvements. I generally agree with all the points and am happy to discuss the details.

MaEtUgR · 2021-11-24T13:01:59Z

Or may I merge this and create an immediate follow-up for the fixes? That would help the work flow 😇

sfuhrer · 2021-11-24T13:05:24Z

Or may I merge this and create an immediate follow-up for the fixes? That would help the work flow innocent

Whatever you prefer

MaEtUgR · 2021-11-24T13:22:09Z

I added the follow up here: #18711 Good input!

hamishwillee · 2022-07-07T04:10:27Z

@MaEtUgR What is the docs implication of this?

I see RTL_TIME_FACTOR is new. I don't see any information about this in https://docs.px4.io/main/en/config/safety.html or http://localhost:8080/px4_user_guide/en/flight_modes/return.html

Specifically, what should a user set or not set to use this? It appears to be triggered of a low battery - what battery level? What if there are multiple batteries? How is it enabled/disabled?

MaEtUgR added the Admin: Enhancement (improvement) 💡 label Nov 15, 2021

MaEtUgR requested a review from RomanBapst November 15, 2021 14:43

MaEtUgR self-assigned this Nov 15, 2021

MaEtUgR mentioned this pull request Nov 15, 2021

Estimate remaining flight time #17828

Merged

MaEtUgR added 3 commits November 16, 2021 11:16

Commander: call battery_status_check on update instead of early return

4d48b60

Compute RTL time and react if lower than flight time

bbd1e5c

- Compute RTL time also during RTL - Calculate correct altitude when finding destination

navigator: Remove deprecated RangeRTL unit tests

6664ffb

MaEtUgR force-pushed the rtl-remaining-flight-time branch from ef8ce01 to 6664ffb Compare November 16, 2021 10:24

MaEtUgR requested a review from sfuhrer November 22, 2021 09:07

RomanBapst reviewed Nov 23, 2021

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sfuhrer approved these changes Nov 23, 2021

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RomanBapst approved these changes Nov 24, 2021

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MaEtUgR merged commit ea3b99e into master Nov 24, 2021

MaEtUgR deleted the rtl-remaining-flight-time branch November 24, 2021 13:10

MaEtUgR mentioned this pull request Nov 24, 2021

Follow up to RTL based on remaining flight time #18711

Merged

hamishwillee mentioned this pull request Jul 13, 2022

PX4 v1.13 - Common changes require docs PX4/PX4-user_guide#1940

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RTL based on remaining flight time estimation #18646

RTL based on remaining flight time estimation #18646

MaEtUgR commented Nov 15, 2021

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hamishwillee commented Jul 7, 2022

	mavlink_log_emergency(&_mavlink_log_pub, "Flight time low, returning to land");
	mavlink_log_emergency(&_mavlink_log_pub, "Remaining flight time low, returning to land");

RTL based on remaining flight time estimation #18646

RTL based on remaining flight time estimation #18646

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RomanBapst commented Nov 24, 2021

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hamishwillee commented Jul 7, 2022