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Where to place IMU sensor on steel ship
#1
Hi,

I am wondering where I should place the IMU sensor on a steel boat.
Are the sensor all based on earth magnetic field, or like a mechanical gyro on gravity and really movement?
The urrent (autopilot, no nmea out) compass sensor is place on the wooden mast 1,5 meter above deck.
I can imagine this is not the best place for roll and pitch measurement.

If it's based on earth magnetic field it will not work if I place it inside the boat?

Thanks for the help
Sjoerd
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#2
I think it should work right inside after calibration.
Please report if you test this.
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#3
It would be very interesting to see the results on a steel boat. The newer openplotter version should autocalibrate which is probably needed because the steel hull itself can re-magnetize if it is in different orientations long enough.

I would not place the sensors too close to the steel itself. Ultimately you should experiment with different locations and report back!
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#4
I have just completed a trip, Tahiti to Fiji, using my plotter based on Openplotter. I built a fairly traditional plotter in an IP67 box with touch screen etc. and mounted it with sticky backed velcro strips on the steel coach roof, under the hard dodger. The IMU is currently mounted in the IP67 box which means that I have to be careful of the orientation of the plotter if I move it from inside to out. I'll be modifying it now to put the IMU in its own housing that I can more or less permanently mount somewhere remote from the plotter so that it is immune to the orientation/location of the actual plotter.

That's the background. In actual use the IMU worked, and calibrated, just fine wherever I put it. As it is in the top of the box it is never closer to the steel than about 100mm.

I had other issues with the IMU failing to run for more than about 8 hours continuously but I need to write a proper post on that separately.
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#5
(2017-09-02, 10:08 AM)Littlechay Wrote: I have just completed a trip, Tahiti to Fiji, using my plotter based on Openplotter. I built a fairly traditional plotter in an IP67 box with touch screen etc.  and mounted it with sticky backed velcro strips on the steel coach roof, under the hard dodger. The IMU is currently mounted in the IP67 box which means that I have to be careful of the orientation of the plotter if I move it from inside to out. I'll be modifying it now to put the IMU in its own housing that I can more or less permanently mount somewhere remote from the plotter so that it is immune to the orientation/location of the actual plotter.

That's the background. In actual use the IMU worked, and calibrated, just fine wherever I put it. As it is in the top of the box it is never closer to the steel than about 100mm.

I had other issues with the IMU failing to run for more than about 8 hours continuously but I need to write a proper post on that separately.

Thanks, always nice to hear of others experiences.

I'm surprised with the accuracy after playing with a MPU9250 down below on a steel boat, harder to find an accurate landmark to compare it to with a boat anchored.

Been running for days now non stop with no issues.
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#6
The imu can be aligned to any orientation from th compass calibration program

This  is where the 3d plot of the boat is rendered
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#7
(2017-09-02, 04:24 PM)seandepagnier Wrote: The imu can be aligned to any orientation from th compass calibration program

This  is where the 3d plot of the boat is rendered

Yes I know, I found that.  

But to be really useful I need to able to move my plotter.. tilt it up and down and rotate it to make it easier to reach from different seating positions or the way the light reflects on the screen etc.. so an external compass sensor makes more sense in my usage case. Very easy to do anyway. I'll report on progress.
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