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  1. Thread: uNav AHRS

    by brtaylor
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    Just a quick update on my end. I'll be mostly a...

    Just a quick update on my end. I'll be mostly a ghost until the second week in March. One of my colleagues modified our original 15 state EKF to use floats wherever possible, which is basically...
  2. Thread: uNav AHRS

    by brtaylor
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    Not yet. I haven't looked yet at the IMU timing...

    Not yet. I haven't looked yet at the IMU timing compared to a "true" source like GPS time to see how bad the IMU timing drifts. For the INS, we're more concerned about the delay between the IMU and...
  3. Thread: uNav AHRS

    by brtaylor
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    Not really looking for mag data from the GPS....

    Not really looking for mag data from the GPS. Could be nice, since it could be further away from some electronics and provide a better source of heading data, but just another sensor to write drivers...
  4. Thread: uNav AHRS

    by brtaylor
    Replies
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    I'm finally starting to read back through the...

    I'm finally starting to read back through the thread, but might not fully get back to developing until the second week of March, once all proposals are submitted (can't wait!).

    I would like to...
  5. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    Thanks for the longer explanation! Makes sense...

    Thanks for the longer explanation! Makes sense now.
  6. Thread: uNav AHRS

    by brtaylor
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    25,460

    I must be dense this morning. What order of...

    I must be dense this morning. What order of function calls reproduces the issue and what's the issue that you see?

    Yes, grant writing for the next month...
  7. Thread: uNav AHRS

    by brtaylor
    Replies
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    25,460

    The loose coupling like you describe could be...

    The loose coupling like you describe could be really interesting. With respect to the accelerometer integration not working well, are you subtracting gravity after transforming accel from body to...
  8. Thread: uNav AHRS

    by brtaylor
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    I don't see where that came from, it's not in my...

    I don't see where that came from, it's not in my code.
  9. Thread: uNav AHRS

    by brtaylor
    Replies
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    Still here! And keeping up with the work you and...

    Still here! And keeping up with the work you and Don are doing; although, I haven't had a chance yet to download the DCM, Madgwick, Mahony, and CF algorithms yet.

    1. It shouldn't be aligned with...
  10. Thread: uNav AHRS

    by brtaylor
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    We could put together a library of different AHRS...

    We could put together a library of different AHRS classes and use a common set of methods for all of them so that it's easy to switch between.
  11. Thread: uNav AHRS

    by brtaylor
    Replies
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    That's a big can of worms. It's easy enough to do...

    That's a big can of worms. It's easy enough to do simple logging to the SD card, but my understanding is that to do so in a low latency way takes a bit of work and you'll need an onboard buffer so...
  12. Thread: uNav AHRS

    by brtaylor
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    One last option I hadn't considered earlier is...

    One last option I hadn't considered earlier is using DCM matrices. Based on what I've read they lie somewhere between quaternion and euler angle based approaches in terms of accuracy vs performance...
  13. Thread: uNav AHRS

    by brtaylor
    Replies
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    25,460

    How do you mean gyro drift correction? We...

    How do you mean gyro drift correction?

    We know that in practice the gyro bias will drift, but assuming the drift is slow enough, I would think you could treat the bias as a constant and let the...
  14. Thread: uNav AHRS

    by brtaylor
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    572
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    25,460

    Definitely agree with that!

    Definitely agree with that!
  15. Thread: uNav AHRS

    by brtaylor
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    I was thinking of actually continuing with Euler...

    I was thinking of actually continuing with Euler based attitude in order to have a smoother transition in degraded states (i.e. extended GPS outage). I *think* the Euler singularity can be countered...
  16. Thread: uNav AHRS

    by brtaylor
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    I like that we can easily separate the pitch and...

    I like that we can easily separate the pitch and roll attitude from the heading using Euler angles. I think this lends itself well to approaches for sensing and rejecting magnetic disturbances, while...
  17. Thread: uNav AHRS

    by brtaylor
    Replies
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    25,460

    Shoot, I know what it is. I was just checking the...

    Shoot, I know what it is. I was just checking the timing output, which is correct, but I forgot to divide by 1000000 to put dt into seconds.

    Not at a computer right now to fix it.
  18. Thread: uNav AHRS

    by brtaylor
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    Which signal are you pulling from the uBlox for...

    Which signal are you pulling from the uBlox for the timing?

    One thing the lab at the University of Minnesota has been noticing is that it appears that the uBlox GPS is taking a couple of frames to...
  19. Thread: uNav AHRS

    by brtaylor
    Replies
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    25,460

    That's a part of the code where I'm just...

    That's a part of the code where I'm just gathering sensor statistics for one minute. I switched from micros to elapsedMicros on line 209 for the dt calculation (_t is defined as a type elapsedMicros...
  20. Thread: uNav AHRS

    by brtaylor
    Replies
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    25,460

    Doh! I think I figured it out - I'm not handling...

    Doh! I think I figured it out - I'm not handling the micros() rollover correctly, used for computing dt. I should modify the code to use elapsedMicros(), which is rollover safe and better for...
  21. Thread: uNav AHRS

    by brtaylor
    Replies
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    Strange, I just ran a super long test with my...

    Strange, I just ran a super long test with my MPU-9250 on I2C and a 100 Hz IMU and filter update rate. For the first hour, everything seemed perfect. Was away for another 90 minutes or so to find the...
  22. Thread: uNav AHRS

    by brtaylor
    Replies
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    If you look at the top of the update function,...

    If you look at the top of the update function, youíll find a good recursive routine Iím using to compute the IMU variance. Should be able to apply that to the innovations.

    EDIT: youíll want to...
  23. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    Yeah, oops! In my haste yesterday, I accidentally...

    Yeah, oops! In my haste yesterday, I accidentally left some of my test code in. I was using this to see how well the EKF was tracking the raw yaw measurements. The commented out portion of code...
  24. Thread: uNav AHRS

    by brtaylor
    Replies
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    25,460

    Don't have much time today, but I noticed an...

    Don't have much time today, but I noticed an issue with my yaw measurement update. The attached code fixes it. The issue had to do with handling the -180 to +180 switchover. Now I unwrap the measured...
  25. Thread: uNav AHRS

    by brtaylor
    Replies
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    25,460

    For my own edification, I created an Euler based...

    For my own edification, I created an Euler based EKF AHRS. It's a two stage filter, the first stage filter works on the pitch and roll angles (2 state EKF) and the second stage filter works on the...
  26. Thread: uNav AHRS

    by brtaylor
    Replies
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    25,460

    I use the UBX-NAV-PVT packet in my uBlox library:...

    I use the UBX-NAV-PVT packet in my uBlox library:
    https://github.com/bolderflight/UBLOX

    The baud is selectable in the library.
  27. Thread: uNav AHRS

    by brtaylor
    Replies
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    25,460

    The one thing to watch out for is that the...

    The one thing to watch out for is that the version of the 7 state EKF on github only updates the magetometer in combination with an accelerometer update.
  28. Thread: uNav AHRS

    by brtaylor
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    25,460

    In his thesis, he is keeping the magnetometer...

    In his thesis, he is keeping the magnetometer from affecting the pitch and roll channels. The issue is that the accelerometer is affecting the yaw channel.
  29. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    Some interesting observations so far: 1. I can...

    Some interesting observations so far:

    1. I can reproduce the drift that Don and Mike are seeing if I "fly" the IMU by hand. I'm using a book clamped to a work table to set the initial alignment...
  30. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    Thanks for the thoughts on tuning! I'll have to...

    Thanks for the thoughts on tuning! I'll have to investigate that more. I'll also be interested to hear what you and Mike find as you play with the filter. When you're checking margins, is it similar...
  31. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    That makes sense to me. I'm modeling the process...

    That makes sense to me. I'm modeling the process noise as only gyro covariance. In reality, there are some unmodeled dynamics and other effects (possibly even gyro rate saturation if you really move...
  32. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    Interesting. I'm not seeing drift, but I also...

    Interesting. I'm not seeing drift, but I also haven't tried rapid movement yet. Any suggestions for tuning to help constrain it?
  33. Thread: uNav AHRS

    by brtaylor
    Replies
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    25,460

    I havenít, but i could do more testing (different...

    I havenít, but i could do more testing (different IMUs, microcontrollers, etc).
  34. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    I just pushed a function to GitHub that should...

    I just pushed a function to GitHub that should work.
  35. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    FYI, I've cleaned up the 7 state library and...

    FYI, I've cleaned up the 7 state library and pushed it to GitHub:
    https://github.com/bolderflight/uNavAHRS
  36. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    Yes, that's correct.

    Yes, that's correct.
  37. Thread: uNav AHRS

    by brtaylor
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    25,460

    Funny, I combined it into a single step in the 7...

    Funny, I combined it into a single step in the 7 state version of the EKF. I still need to work through whether one approach would be more technically correct than the other.
  38. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    Thanks! I co-taught a course on navigation...

    Thanks! I co-taught a course on navigation filters for a few years and am planning on doing something with all this code. I'm thinking derivations alongside examples that can be run to demonstrate...
  39. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    Attached is the 7 state EKF that I derived. Per...

    Attached is the 7 state EKF that I derived. Per the discussion earlier, this one is using doubles instead of floats as a test. The performance isn't bad, for me running a Teensy 3.6 at a CPU of 240...
  40. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    The attached is updated. I *think* the equations...

    The attached is updated. I *think* the equations are correct now, but it's still a work in progress.
  41. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    Just processing speed; however, the literature...

    Just processing speed; however, the literature recommends using as high of precision as possible for Kalman filters. Trying doubles might be worth a shot, I haven't checked to see if we're hitting...
  42. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    While deriving a 7 state EKF, I found some errors...

    While deriving a 7 state EKF, I found some errors with my 4 state EKF attached above. Standby for a fix.
  43. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    That's interesting, I'll have to look into...

    That's interesting, I'll have to look into whether I'm over-running the frame or not.
  44. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    Attached is one more version of the 4 state EKF....

    Attached is one more version of the 4 state EKF. This is the same as above, except using measured heading for a magnetometer measurement update instead of Matthew Watson's approach. I also added the...
  45. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    Attached is a preliminary 4 state EKF. I...

    Attached is a preliminary 4 state EKF. I developed the time update and accel measurement update. The magnetometer measurement update is from Matthew Watson's thesis. Right now, I take two minutes...
  46. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    Hi Mike, Can you email me the thesis that this...

    Hi Mike,

    Can you email me the thesis that this code is from as well? brian.taylor(at)bolderflight.com

    I re-derived the 4 state EKF equations and implemented them from scratch. I take a couple...
  47. Thread: uNav AHRS

    by brtaylor
    Replies
    572
    Views
    25,460

    For this filter, the absolute heading accuracy is...

    For this filter, the absolute heading accuracy is only as good as the heading given by the magnetometer, which is only as good as the magnetometer calibration. The filter is initializing with a...
  48. Thread: uNav AHRS

    by brtaylor
    Replies
    572
    Views
    25,460

    In the MPU-9250 library I transform the data so...

    In the MPU-9250 library I transform the data so the gyro, accel, and mag are all on a NED system.
  49. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    Hi Fernando, This library doesn't output...

    Hi Fernando,

    This library doesn't output position. The 15 state EKF (https://forum.pjrc.com/threads/48856-uNav-INS) does, but it's not meant to go for more than a few seconds without a GPS update....
  50. Thread: uNav AHRS

    by brtaylor
    Replies
    572
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    25,460

    Yes, I plan to continue working on and debugging...

    Yes, I plan to continue working on and debugging this library. I recently did a little tuning, the process noise seemed to be set by default way too low for the potential dynamics between time...
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