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Compass heading incorrect


(Michael) #1

We’ve finally got our BlueROV2 completed, connected, and operational. Everything seems to be working correctly and we are excited to test drive it in the pool. We did the compass calibration according to the instructions, and it seemed to complete successfully.

Unfortunately, the indicated compass heading in QGC is incorrect. When I line up the BlueROV2 on a heading of exactly 0 degrees using a separate and reliable compass, QGC indicates a heading of 319 degrees - 41 degrees off. Is there a way introduce some kind of offset to true it up?

Thanks.

Mike


(TCIII) #2

@mike3,
If your ROV is near any kind of iron (steel) objects when you are seeing this offset, then that is the cause.
I have found that the rebar in a swimming pool can cause the compass in my OROV 2.8 ROV to have a 45 degree offset compared to being out of the pool.
Regards,
TCIII AVD


(Michael) #3

It’s just sitting on a piece of plywood on top of plastic water barrels. No metal or batteries or power nearby. Good thought, though.


(Tim Pierce) #4

any chance you used a nonstandard bolt that might be more magnetic than expected? or a magnet on off switch?


#5

When you say “a separate and reliable compass” what type of compass - is it GPS or an old-school magnetic needle compass?

The reason I ask is the MEMS sensors inside the magnetometer are much more likely to concur with a magnetic needle than a GPS. A GPS doesn’t use magnetism and will tell you where true north is in relation to the Earth’s axis of rotation. Magnetic north is totally different and (last time I checked) is somewhere in Canada.

41 degrees of error is quite a lot but if you live in the northern latitudes this could explain the anomaly. My system uses a Bosch IMU and it disagrees with my iPhone compass by about 21 degrees (both compasses are MEMS), I’m putting this down to some kind of adjustment applied by Apple so that their maps orient correctly to true north.

Due to this, we have an offset control in our software which will realign the the Bosch reading with GPS true north and display the corrected heading on the topside software.


(undersearobotics.com) #6

Just so there isn’t any confusion, the “heading” displayed by a GPS is based on the GPS’s historical data. In other words (and unless it has a separate heading sensor), the GPS must be moving in order for it to calculate the heading traveled, There are two types of compasses, a magnetic compass (which relies on the earth’s magnetic field to display “magnetic north”) or a gyro compass (which must be preset to a calculated “geographic north” and then checked / maintained regularly to verify it is pointing in the correct direction). The only compasses I’ve ever seen on an ROV have been magnetic compasses or sensors using a magnetic sensor.


#7

@Paul, absolutely true, maybe I should have that more clear.

In our application we need to work to platform charts which are GPS aligned, hence the need for the offset.


(undersearobotics.com) #8

@Sarawak_ROV - By “GPS aligned” I’m guessing you mean a chart that is using “geographic North” versus “magnetic North”. The difference between these two references is easily calculated by adding / subtracting the “magnetic declination” for that area. Most GPS’s have a setting to select either magnetic or true north. If not using a GPS you can find the declination by looking at a nautical chart or one of many declination calculator.


(Rusty) #9

@mike3 - I would guess that this is a magnetic declination issue. Where are you located, roughly? The declination varies by location and as @Sarawak_ROV mentioned, it can be fairly dramatic at very northern or southern locations.

The two parameters that should be changed to adjust declination are the following:

You should disable auto declination and then enter the declination for your location. Let me know if that helps.

Note that I’m assuming that your “accurate and reliable” compass is something like a phone or boat navigation system that knows the proper declination already. If it’s a physical compass with a needle, then it effectively has 0 degree declination already.


(Todd Sparkes) #10

Paul,
As you stated an ROV compass is used as a reference only and is not survey quality. All ROV systems usually come with a simple compass. If a job where accuracy is required most companies rent a TOGS or Octans gyro. These are very expensive units. For normal ROV purposes a compass is a means of finding a general direction.

Regards
Todd


#11

You’re absolutely right. The idea is that we have a primary means of navigation but once the ROV gets down amongst the platform risers and pipework, that will amount to very little - the pipelines contain a lot of residual magnetism due to previous MFL inspections.

Once we lose confidence in the magnetometer reading, the platform and subsea layout maps will give us the references we need.


(Michael) #12

Thanks all, for your replies and suggestions. I’ve been attending sidescan sonar training and this is my first day back in the office. I will check the parameters that Rusty indicates above and report back.

Thanks again!

Mike