Hello. I did the compass motor calibration various times and checked the results.
I have APM 2.6, external compass + gps module UBlox Neo-6M, turnigy plush 12Amp ESC's, and a 2.7Ah battery. The apm is mounted on an anti-vibration stand on the top of the 250mm sized quadcopter. The compass is elevated by 13cms above the APM. So it's very distant from all electronics. The APM lies ~3 cm away from the power distribution board and the battery power cable. There is no way to move it further than that. I power the flight controller through the apm power module. On the output rail i plug all 4 esc's signal cables plus all the BEC power cables. I put the props upside down and made sure the vehicle wasn't moving. This is the reading i get when i run the compass motor calibration test:
What do you think of this level of interference? It's a lot right?
Next i educated myself more about the magnetic fields on the craft and i did some mods on the craft. I have twisted all wires especially the power wires. I resoldered the ESC's to the power distribution board in a proper way (they were a bit loose before). Also, on the output rail of the apm i now connect only the ESC signal cables and did some other minor fine adjustments. Now here's the result :
Only a tiny improvement. I expected it to have improved significantly.
Also the current readings are a bit off (it's using the power module as a current sensor to measure them right?). I diligently measured the current draw from the quadcopter with my trusty wattmeter and it was ~7Amp less than what the second picture says (and @ 100% throttle).
Considering that i have the compass module so far away and everything i have said above i don't think that these interference values are acceptable. But i may be wrong. So what do you think? What is going on, what can i improve, what do you think of my set-up? I'm new to flying drones. Any input and advice will be very appreciated. Thanks in advance!
Replies
Agreed, you're probably ready to fly.
Your readings look great. You did not notice much of a change because you are reading nearly Zero interference, and the change within the margin of error of the sensor. Graph will prob change slightly every time.
Regarding the current, the current sensor is not extremely accurate, but if you use a multi-meter to calibrate the current measurement, you will get a more accurate reading of your current use. Same holds true for the Volt measurement. Multi-Meter can measure the voltage to at least 1 or 2 decimal places, making your calibration quicker and more accurate. In the end, it will take time and many tests for you to get the power module measuring right on the mark. If you use different battery, the ready accuracy will not be as accurate anymore as the 'internal resistance' of the battery will be different.
Other thing to notice is that some batteries, like AGA Power, discharge the majority of the capacity between 3.9 and 3.6 volts, and I was setting my failsafe to trigger at 3.7 v, losing 40% of potential flight time.
Even if the true accuracy of your power module voltage is not great, you will be able to find out what your critical voltage is and set your failsafe accordingly. Doing a boring hover flight will give you this information.
If your fail-safes are all set(especially RC signal) then you can just figure this stuff out as you start testing your drone. Just be careful until everything proves its integrity.
Most consumer grade DVMs are fused for a maximum current of 10 Amps. This is just barely enough to get the current calibration started, and if you overshoot the fuse blows and you're done.
Besides, in order to calibrate both voltage and current under a load you must use two meters, or use one meter and do two separate calibration runs. A better approach is to use and in-line Watt Meter. I have two power modules. One is on an APM and the other is on a Pixhawk. Both are calibrated to within +- 0.02 VDC, and +- 200mAh.
Watch the video:
http://copter.ardupilot.com/wiki/common-3dr-power-module/
You are misreading the graphs.
The Legend is at the top left corner of the graph.
The GREEN line is CURRENT.
The RED line at the bottom is magnetic interference.
You're fine. Go fly..
wow. You must think i'm stupid. That was so silly of me. That is so embarrassing. *facepalm*. It was late at night actually and when i woke up this morning i figured it out by myself. I wanted to delete this discussion just so others won't take notice, but unfortunately i can't.
Not at all.