How to add a new failsafe for battery level?

smilie · April 4, 2019, 8:57am

Hello Guys,

i think about to have two or more stages for battery fail-save.

Battery half: return to land.
Battery empty: land

is this possible and how it is possible?

Many thanks

JulianOes · April 4, 2019, 2:50pm

Isn’t this already what is implemented, except that you need to adjust the limits?

@MaEtUgR can you confirm?

smilie · April 4, 2019, 3:06pm

I see only one option in Safety.

Empty Battery

Can you explain me, how this works with two levels?

JulianOes · April 4, 2019, 3:09pm

The battery simulation for SITL is here:

github.com

PX4/Firmware/blob/46c4a68b6218b5df9c1df31819585095513c1c47/src/modules/simulator/simulator_mavlink.cpp#L385-L413


	// battery simulation (limit update to 100Hz)
	if (hrt_elapsed_time(&_battery_status.timestamp) >= 10_ms) {


		const float discharge_interval_us = _param_sim_bat_drain.get() * 1000 * 1000;


		bool armed = (_vehicle_status.arming_state == vehicle_status_s::ARMING_STATE_ARMED);


		if (!armed || batt_sim_start == 0 || batt_sim_start > now_us) {
			batt_sim_start = now_us;
		}


		float ibatt = -1.0f; // no current sensor in simulation
		const float minimum_percentage = 0.499f; // change this value if you want to simulate low battery reaction


		/* Simulate the voltage of a linearly draining battery but stop at the minimum percentage */
		float battery_percentage = 1.0f - (now_us - batt_sim_start) / discharge_interval_us;


		battery_percentage = math::max(battery_percentage, minimum_percentage);
		float vbatt = math::gradual(battery_percentage, 0.f, 1.f, _battery.empty_cell_voltage(), _battery.full_cell_voltage());
		vbatt *= _battery.cell_count();

This file has been truncated. show original

So you could adapt this simulation to go all the way down to 0 and play with it to find out what happens

smilie · April 4, 2019, 3:27pm

?

i see there no info.
You are shure to understand my questiion?

I seach a parameter, which defined the action by half of battery.

Thanks for help

JulianOes · April 4, 2019, 4:54pm

the params are: https://dev.px4.io/en/advanced/parameter_reference.html#BAT_CRIT_THR

https://dev.px4.io/en/advanced/parameter_reference.html#BAT_EMERGEN_THR

https://dev.px4.io/en/advanced/parameter_reference.html#BAT_LOW_THR

smilie · April 5, 2019, 7:29am

Ok, now i understand.
Critical - Failsave (return)
Emergency - Land
Low - Warning

Failsave means “return mode at critical low level” ?
Emergency means “reaching dangerously low levels” ?

So i have to increase the “Critical level” for return.

thanks

MaEtUgR · April 22, 2019, 8:24am

Yes, by default the thresholds are:
BAT_LOW_THR 15%, BAT_CRIT_THR 7%, BAT_EMERGEN_THR 5%
which results in:

15% - “OK”
7-15% - Low - Default: Warning for user, configurable by COM_LOW_BAT_ACT
5-7% - Critical - Return to launch
< 5% - Emergency - Land on the spot

See PX4-Autopilot/src/lib/battery/battery_params.c at c50c44cc582956aefd32e39a6c7a5210ba4e1585 · PX4/PX4-Autopilot · GitHub

You can set these parameters on your vehicle to set different thresholds. As @JulianOes mentioned you can test the reactions in SITL simulation on your computer by setting the minimal simulated percentage (default is 50%) to the value 0.0f here: PX4-Autopilot/src/modules/simulator/simulator_mavlink.cpp at c50c44cc582956aefd32e39a6c7a5210ba4e1585 · PX4/PX4-Autopilot · GitHub

I strongly recommend to you to follow these instructions such that you get meaningful battery percentage values out of your power module setup and configuration: Redirecting to latest version of document (master)