we are in the very early stage of testing a custom cable power supply (power tethering) for a 5 kg, 18” copter. We have a small backup battery and a switch that will always forward the source with the higher voltage to the copter. The battery has a lower voltage than the power tethering, so we will see if the power tethering fails, and we can set up a simple battery failsafe that lands automatically.
The battery is 10S, giving 39.5 V under load. The power tether is giving 48.4 Volts under load.
With the power tether, we mange to fly without issues with the motors running at an average PWM of 1466 - 1524 µs (we have not balanced the copter COG carefully enough on the first quick test). That should be close to 50% throttle.
But when we turn off the power supply, the copter can’t stay in the air anymore. That seems strange to me, because 48.4V is like 12S with 4V at each cell. 39.5 V is like 12 S with 3.3 V at each cell. I would expect that the copter does at least stay in the air even when totally underpowered. Is there some parameter that I am overlooking, that is maybe hard-limiting the max. throttle output?
Here is a log a this quick and dirty test:
At t = 14:29:05.7 , we are cutting the power tether (flying at 5cm altitude). But we have also done take-off test without the tether without success,
Your Motors don’t max out from parameter settings. But I can’t say if the limits are correct as they look like default values.
Also maybe your drone touch ground before the motors takes the load
What method have you used to make sure that all parameters have been correctly set for this 5Kg drone? This is not a toy, please tell me you have a process, a sequence of steps, to get the job done.
I have set these to 33 (min) and 42 (max). So number fors 10S (the battery). I will check again with numbers set to the actual expectable range (40 - 49 V). But will this have an effect at all? Does this limit anything or just scale gains as the documentation says?
I would think that mot_bat_volt_max and min might even be counter productive. They increase control loop parameters at low voltage, yielding faster I-term wind up during takeoff. I guess that when one motor maxes out due to this, take-off becomes impossible, correct?
Your actual params don’t show the values as you said
MOT_BAT_VOLT_MAX = 25,2V
MOT_BAT_VOLT_MIN = 19,2V
This is completely out of the range of your operating voltages
For me it seems that the ESC got a throttle up but reports RPM down, so it was not able to manage this rapid voltage change
Your process does not work.
AMC’s methodic process works on vehicles from 600g to 350Kg with many different propulsion systems and voltages. I bet one or more of the ATC_* parameters is incorrectly set like the ATC_MOT_MIX_MAX. Use AMC and it will tell you what to fix.
Once the parameters are correct the vehicle should be able to takeoff with the battery.
If the motors saturate and/or ArduPilot reports “motor thrust loss” then the voltage is just too low.
But to be 100% sure you first need to follow a method that guaranties that all parameters are correctly configured.
I have recorded another (better) log where I used a small 12 S emergency battery charged to 4.0V per cell (=48 V without load), and a power tether running at 48 V. The battery goes down to 45.5 V under load, the power tether stays at 48 V.
In this test, I turned off the tether to check the behavior on the emergency battery.
I can fly on the emergency battery until to a voltage of about 41.5 Volts (3.46 V per cell under load). Then, the copter becomes more or less uncontrollable. It feels like it doesn’t have enough power to stabilize the attitude anymore, and it would flip over if I didn’t land.
Usually, I would think that this happens because my motor KV is too low, and the motor PWM signals max out - the copter doesn’t have enough reserves to keep the attitude.
But when looking at the log, it seems that the motor PWMs stay around 50% when the copter becomes unflyable. I do not understand this. What parameter or what variable in the log should I look at to find the cause for this? I have never experienced something like this before.
I’ll have a look over the log, but be aware that a diode or similar in series with your onboard battery will be a problem for multirotor ESCs that give back some power during braking. Braking happens anytime there is a lower commanded throttle, and causes the supply voltage to rise potentially damaging electronics including the ESCs themselves, or causing ESCs to shutdown with an over-voltage alert.
Some ESCs allow you to turn off the braking energy recovery, and with very slightly reduce overall efficiency and ESCs may heat up more and their braking will be limited.
My suggestion would be to match the tether voltage to the typical battery voltage and remove the diode device. This may take some testing before you settle on a final configuration.
Thanks for your reply. We do not have diodes in series afaik. We use several TVS diodes in parallel to the power supplies, limiting the overvoltage generated by braking during tethered flight (usually mostly absorbed by a lipo) to about 65V. Do you think this is the issue? Shouldn’t I see a voltage drop or something in the log? We were also thinking that we have a design error somewhere resulting in too much resistance in the Lipo connection. But then I would also see a significant voltage drop when the current goes up. All I see is a normal voltage drop due to lipo resistance.
The log is large because I enabled raw gyro logging for something else:
Power supply selection happens with a dedicated IC (forgot the name, it is made for dual power supplies and always selects the source with the higher voltage by switching high power mosfets).
BATT_ARM_VOLT,44.30
BATT_CRT_VOLT,42.00
BATT_LOW_VOLT,43.20 // maybe as low as 43.0
BATT_FS_CRT_ACT,1
BATT_FS_LOW_ACT,2 // normally 2 but maybe use 1 while tethered
MOT_BAT_VOLT_MAX,50.40
MOT_BAT_VOLT_MIN,39.60
The battery is starting it’s nose dive to full discharge and wont be much use below about 43 volts.
It’s possible that your ESCs are not happy with that low voltage too, leading to the instability.
I can see Motor2 essentially stops and is commanded to maximum because it is not providing thrust - so that is your first one to go possibly because of the low voltage.
You might want to do some tests with MissionPlanner motor test to check if all the ESCs behave that way or if Motor2/ESC2 has a fault.
This gets frightening and rather dangerous, but the old way to do the compass/motor calibration was to move all the props around 1 position and flip them over. This way they are pushing down, allowing you to load up the motors without the copter flying away. Only use the MissionPlanner motor test - do not try to “fly” the copter in any normal modes in this state.
Keep everyone and everything a safe distance away from the copter.
Power selection is done with a LM5050 on a custom board. But a next step is to get rid of everything that we build, and just use the Lipo plugged directly into the copter without any extra stuff. I will report here.
The ESCs are AM32 70A 3-12S 4in1 ESC:
Thanks for your suggestions. All these battery failsafe behaviours is something that is planned to be implemented at a later stage (when we start tests at altitudes > 1 Meter). But you would agree that the behaviour is not exactly what you would expect?
The motor 2 going to max is very shortly before I touch the ground (when comparing with AccZ trace). The copter feels already very sluggish before that, I just wanted to keep it in the air as long as possible. But I had overlooked this before, good that you mention it.
Set the ESCs to ignore low voltage, let the flight controller look after that. Which also implies you have to have a good battery (and tether voltage source) and the battery thresholds must be correct.
