IMU orientation?

[Guttagrynna]

I am considering using an IMU directly attached to the tinypilot computer. Now I still don't understand to what extent I need to orient the IMU so that it is pointing its x-axis forward and is level. This is important for me because it restricts the ways I can mount the box containing the entire system in the boat. Could someone clarify? I have tried a bit of search for "orientation" here in the forum but haven't managed to find anything.

[seandepagnier]

You can mount the sensors in any orientation.

Two of the angles of rotation are resolved when you press the level button, or level the sensors in the calibration. The third angle is somewhat optional to align but set with heading offset manually.

[ironman]

Some discussion on the orientation can be found in this thread https://forum.openmarine.net/showthread....2#pid17922

[Guttagrynna]

Some discussion on the orientation can be found in this thread https://forum.openmarine.net/showthread....2#pid17922

Thanks, that was useful. I intend to run two sensors, one with Openplotter and Pypilot in compass only mode, where the heading is shared with the NMEA2000 network. Maybe that is a tall order, given what Sean writes about the accuracy of the sensors. I am not getting very good results yet. I have been working with placement away from sources of disturbance, and with the quality and robustness of my wiring, but still not robust and good results.

[seandepagnier]

well, you may need to calibrate the accelerometers:
https://pypilot.org/wiki/doku.php?id=calibration

[fishy]

Some discussion on the orientation can be found in this thread https://forum.openmarine.net/showthread....2#pid17922

Hi Ironman,

Is your IMU tilt sensitive / corrected? I have a cheap MPU-9250/6500 that goes waaay of course in magnetic reading when I tilt the thing. With a 30 degree tilt I et a 90 degree variation in magnetic reading it is simply not useful sailing.

Yes, it is calibrated in PyPilot. 

Can you post a link to where you bought your sensor? There is so very little information about the differences between sensors (on the sites where I can find them) and I would really like a compass to be part of my setup...

Anyone else who has an accurate compass and is willing to share where it was bought (preferably in Europe) I'd love to hear about it!

[Guttagrynna]

Thanks for replies. Regardimg calibration is it possibly, and even ever nevessary to compöetely erase the calibration and start fresh
? For example if I change sensor.

[ironman]

Is your IMU tilt sensitive / corrected? I have a cheap MPU-9250/6500 that goes waaay of course in magnetic reading when I tilt the thing. With a 30 degree tilt I et a 90 degree variation in magnetic reading it is simply not useful sailing.

I presume so, because it steers all right for me, and I normally sail under heel. But I have no way to tell for sure. Any tests done on your workbench are without value: the only real test is on the boat. I've tried to heel the boat in the harbour, but even with the purchase of the whole mast length, I can only tilt it like five degrees.

Can you post a link to where you bought your sensor? There is so very little information about the differences between sensors (on the sites where I can find them) and I would really like a compass to be part of my setup...

I bought mine here https://www.dennisdeal.com/search?q=DD1216625 a while ago, but I would not rely on two shipments being from the same manufacturer or batch.

Having said all that, I also struggle getting consistent compass headings. For maintaining a heading the pypilot imu often works all right, but sometimes the boat slowly changes heading, and then I switch to wind mode, and then it corrects itself. Also, I changed my setup so it resets pypilot.conf at restart to a safe configuration, which includes the compass calibration data. A reliable HDM reading is another thing, which complcates gps/track following mode, and in that mode I typically need to change the compass offset for different headings, which is exactly what I would not expect to be required. For me, I blame my own setup, having the IMU strapped to the tiller pilot. I have all the parts to make a boatimu version only for HDM purposes, but so far this has not seen the light yet.

[fishy]

Hi Ironman,

Thanks for the reply. I agree heeling the boat to test errors in magnetic readout would be, well, silly. Behind a desk, with the sensor in-hand it is quite a bit easier
I find it surprising (and really rather confusing) how all these cards look exactly the same but seem to have a wide spread in quality / reliability ( and price! I've seen them range from €3 to €60 with no visible differences, and barely any specs to make an informed choice). I'll certainly look at Dennisdeal though.

[seandepagnier]

I order sometimes 60 mpu9255 and all are good. Then order 20 more and all are bad and I must discard them all. I have not had much better luck with icm20948 either.