Checking Magnetic Compass Placement

If the GPS/compass unit is placed too close to the electronics they may interfere with operation. This can result in loss of control so it is important to ensure there is no interference before attempting to tune or fly the craft.

  1. To begin, secure your aircraft to your bench - no freedom of motion is required (or desired) for this procedure. For safety, it is ideal if the craft is located away from the ground control operator or other people but where it can still be observed.
  2. Attach all props so the system can be tested under full load.
  3. Load QGroundControl, connect to the craft, and select the Pro–>Plot feature
  4. Scroll down the list and select HIGHRES_IMU.xmag, HIGHRES_IMU.ymag, HIGHRES_IMU.zmag
  5. Change the time axis to 2 minutes or so to slow it down
  6. You should see three horizontal lines. Some noise will be present.
  7. Arm the aircraft and slowly apply throttle while watching the compass outputs on the plot. If any of the outputs rise with increasing throttle (as illustrated in the image below) then the compass unit should be moved to a new location. Also try to keep the two battery connection wires in the power harness close together (avoid current loops).
  8. Repeat until a location with little coupling (less than the noise floor) is found.

Example setup

Below is how I set up my octorotor for mag coupling testing. The compass/GPS is shown in the new location midway out on one of the arms to minimize interference. I had to build a custom longer cable to reach.

Response

Here is a photo of my magnetic coupling with the original compass placement (top of PCB, center of craft). The initial spikes are from me lifting it to press the arm button. The slow ramp is the compass response to me increasing the throttle. This is bad and resulted in some pretty scary flying and two broken landing gear. One of the broken landing gear found new life as a compass mount (see above) after cutting off the leg portion.

FIME Add photo of good response

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