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Post by kendra on Apr 9, 2017 10:24:58 GMT -7
We did some testing of the tracking system this weekend and a few times the dish pointed exactly opposite the payloads when we launch the tracking.
We are thinking that there was an issue with the calibration of the IMU. We were able to get "3"s for all of the sensors before tracking. However, we didn't have a standard format for doing the calibration. We laid it flat, put it on end, put it on its side, but did not do the figure 8 movement for the magnetic field. So, we are thinking that it didn't actually read magnetic north.
So, my question is, what is the correct way to calibrate the IMU? Should you do certain axes first? Do the figure 8 first?
Thanks,
Kendra
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Post by leviw on Apr 9, 2017 15:57:26 GMT -7
To my understanding, the IMU begins calibrating as soon as you hit update settings. It has a 4 sections, system, gyro, magnetometer, and accelerometer that are all calibrated (even though it only uses the magnetometer.) You do not need to calibrate them in any particular order, or rather, they are all calibrating simultaneously.
The magnetometer calibrates by waving the IMU in roughly a figure 8 shape. This one is usually quite easy.
The accelerometer calibrates best for us by holding the IMU on edge (perpendicular to the ground) and turning it roughly 45 degrees, about once per second. We change from one axis to another and this usually takes us about 30 seconds total, but sometimes longer.
System and the gyro calibrate by holding still. We try to keep the IMU moving throughout the process so that these are the last to calibrate. That way when we place the IMU back on the ground station, it is aligned with the ground station when these finish calibrating.
That's important because as far as I can tell, the tracking software only reads from the magnetometer once, moments after calibration. So if you're still holding the magnetometer or the tracker is not centered when it takes that reading, your system will point in the wrong direction. Also, be aware that there are several magnetic pieces on the tracker including the back of the ubiquity dish, and the servos. If the IMU is near either of these it will not return the correct heading.
If someone has a different understanding of the system, please correct me.
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Post by David MSGC on Apr 10, 2017 16:21:37 GMT -7
It does not matter what order things are calibrated but the magnetometer will calibrate quicker if the accelerometer is calibrated. Not saying your antenna is assembled incorrectly but almost all previous cases which the antenna pointed 180 degrees off the pan servo and related mechanical parts were assemble incorrectly. Previous teams tried to fix the problem without disassembly and reassembly but so far that is how they 'fixed' the problem was to reassemble the antenna per the documentation.
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Post by bachri on Apr 11, 2017 23:07:58 GMT -7
We had the exact same problem but I am happy to report has been fixed! After two weeks of troubleshooting and re-zeroing we realized the IMU was mounted backward. IMU should point away, in the direction away from the back of the dish. Our dish always pointed opposite because the IMU had been mounted such that it pointed along the the direction the dish was pointing at.
David mentioned mechanical assembly, I think this would be the case if the dish had always consistently pointed along the wrong direction, but if it had tested well at least one time, then this would suggest the problem is not mechanical. I don't know how important is the 5 seconds window to mount IMU after calibration, but have someone looking the calibration numbers and prompt you so that you mount the IMU very quickly and before the dish makes a move.
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