Voltage / Current Sensor Ground Issues
« on: June 22, 2022, 12:40:18 am »
I have calibrated and tested my voltage / current sensor on the bench.  When I run the bench power supply variable voltage up and down, telemetry agrees with the voltage.  When I fly it (a 40 amp max total draw quad) the voltage is invalid.  I shows about 8 volts from a 12 volt pack.  I know this is wrong because my lipo alarm acts normally and does not come on till later in the flight when it should.

This is a cheap 90A sensor.  The power wires are 16 AWG which is too small.  That's one issue, but probably not the cause.

The sensor board has a BEC but I am not using it, so I presume I am getting a difference in ground between the FC where the ADC is (powered by ESCs BEC at the end of the power wires) and sensor board that is on the power wires closer to the battery.

Is this a standard problem?

I suppose I could use the sensor's BEC and disconnect both ground and +5V from the ESCs.

I am thinking of coding a current to voltage factor into the sensor flight code.  I think that would fix the issue too.
« Last Edit: June 22, 2022, 02:19:16 pm by TheOtherCliff »


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Re: Voltage / Current Sensor Ground Issues
« Reply #1 on: June 22, 2022, 03:48:29 am »
I usually have problems with inaccurate Amp readings but the Voltage normally okay.

Re: Voltage / Current Sensor Ground Issues
« Reply #2 on: June 22, 2022, 07:25:28 am »
My voltage readings are OK at low amps.

I am not able to get consistent low current readings either, even with trimming the zero current.  It changes with the day or the battery.

Do you use the BEC built into the current/voltage sensor to power the FC?

How much max current in your vehicle and what gauge power wires?



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Re: Voltage / Current Sensor Ground Issues
« Reply #3 on: June 22, 2022, 10:35:30 am »
Probably a ground loop issue here?

How many ground wires go to the flight controller? It should be only one ground wire! ESCs / servos should have only their signal wires connected to the FC.  Then there shouldn't be invalid voltage readings, regardless of current being drawn.

BTW, connecting together the +5V coming from BECs built in the ESCs is a key to disaster :)  Voltage sources shouldn't be wired in parallel. There are always minor differences in BEC voltages that may float with temperature and once the difference is big enough this will stress too much one of the BECs, causing more and more current to be drawn from one BEC to another in attempt to equilise the voltages, this will heat the BEC even further, causing bigger drift in its output voltage, etc... Finally the BEC temperature rises so much that it causes the ESC to enter thermal protection mode and the quad, presumably, to crash...

Shunt type current sensors we commonly use are not precise for small currents, I prefer calibrating them for typical in-flight currents (8-15A, for example). The easy and precise way is using a current/Ah meter, compare its readings to what the GCS reports for mAh used (via Oplink telemetry) and correct the calibration coefficient respetively.

Zero current trim should not be used with this type of sensors, it makes sense and is intended for hall effect current sensors (like ACS758LCB-100B).

To have best calibration results, telemetry should be via Oplink, and no USB cable connected.


Re: Voltage / Current Sensor Ground Issues
« Reply #4 on: June 22, 2022, 02:41:28 pm »
Some ESC's BECs have two LM7805 5v regulators in parallel.  It has been my assumption that paralleling ESC's BECs would be similar to what is already going on inside a single ESC.  I know that this will put more load on one than another, due to manufacturing tolerances.  I leave my 4 ESC BECs connected in parallel and I can tell that one gets warmer than the others because it is the one actually being used (highest voltage) leaving the others basically switched off.

I have heard though, that it is not good to parallel switching BECs...

About: Current Sensor Ground Issues
Setting up a proper single point ground is one of the solutions I thought about and the correct way to do it.  Problem is that cheap sensors are leaving the BEC unpopulated now days, and that basically means a single ground is difficult to do (or requires adding a BEC at the current sensor for sake of the FC ADC).

If it was a simple matter of power wires with varying ground potential along the length; with sensor in one place and ADC ground in another, then a fix of having a current to voltage tweak seemed a simple fix.  I coded it up and it seems to work.  I will flight test it next flying session.