tim77777
commented
7 years ago Whilst holding the device on the same plane at all times?
From: Kris Winer [mailto:notifications@github.com] Sent: Sunday, January 15, 2017 2:58 PM To: kriswiner/MPU-9250 Cc: tim77777; Comment Subject: Re: [kriswiner/MPU-9250] Doesn't seem to be working (#2)
The mag response surface is 3D. Move it any way you want as long as you sample all of the quasi-spherical shape of the 3D response surface for the best accuracy.
OK, dumb version:
Imaging the response surface is a flabby beachball and your mag is inside of it. Your job is to move it limited at the origin such that the imaginary vector perpendicular (up or down vector) to the top of the mag sensor sweeps out as much of the inside surface of the beach ball as you can in the time alloted for the calibration. Move the mag such that the imaginary vector sweeps all around, in all possible directions, don't leave anything out.
On Sat, Jan 14, 2017 at 7:47 PM, tim77777 notifications@github.com wrote:
Yes I read it, and I have read it many times before also. But I don’t get the figure 8. Do you move it in a figure 8 in all directions?
Regards
Tim
From: Kris Winer [mailto:notifications@github.com] Sent: Sunday, January 15, 2017 2:45 PM
To: kriswiner/MPU-9250 Cc: tim77777; Comment Subject: Re: [kriswiner/MPU-9250] Doesn't seem to be working (#2)
I see you did not read the material at the link.
You have to sample the entire 3D response surface in order to properly recenter it on the origin and respherize it.
On Sat, Jan 14, 2017 at 7:41 PM, tim77777 notifications@github.com wrote:
Ok, well thanks for all that. Bit beyond me I’m afraid.
But I do understand what you mean by “if you calibrate the magnetometer only in a plane it is not surprising to me that when you move out of the plane you get nonsense.”
But I cannot get it to work any other way than by spinning in a circle to calibrate it. How exactly is the figure eight supposed to go? I mean which direction or directions?
Regards
Tim
From: Kris Winer [mailto:notifications@github.com] Sent: Sunday, January 15, 2017 2:29 PM
To: kriswiner/MPU-9250 Cc: tim77777; Comment Subject: Re: [kriswiner/MPU-9250] Doesn't seem to be working (#2)
The axes are not aligned for the MPU9250. Use a NED scheme.
On Sat, Jan 14, 2017 at 7:20 PM, tim77777 notifications@github.com wrote:
Hi,
Would you be able to help me out with the formula for magnetic e-compass tilt compensation. Assuming that the magnetometer and accelerometer’s axes are aligned ( as with the LSM303D device), the formula is -
Compensated_x = mx cos(pitch) + mz sin(pitch);
Compensated_y = mx sin(roll) sin(pitch) + my cos(roll) - mz sin(roll) * cos(pitch);
However, being that the axes of the magnetometer in the MPU9250 are not aligned with the accelerometer, how would the formula change.
Regards
Tim
From: Kris Winer [mailto:notifications@github.com] Sent: Sunday, January 15, 2017 2:03 PM To: kriswiner/MPU-9250 Cc: tim77777; Comment Subject: Re: [kriswiner/MPU-9250] Doesn't seem to be working (#2)
I don't understand what you are doing here, but if you calibrate the magnetometer only in a plane it is not surprising to me that when you move out of the plane you get nonsense. There is nothing wrong with the datasheet.
I am using either Madgwick or Mahony open-source sensor fusion which with proper calibration can provide absolute 3D orientation estimation using MPU950 data with pitch and roll accurate to a degree or so and heading accurate to 4 degrees or so.
If what you are attempting can do the same or better it might be worth figuring out where it is going wrong. If not, I would switch to something that makes more sense.
I have no idea what you mean by tilt compensation. It looks like you are just depending on the accelerometer for pitch and roll. Why, I cannot understand.
On Sat, Jan 14, 2017 at 6:07 PM, tim77777 notifications@github.com wrote:
Hi,
For the magnetometer I was using mx = (float)magne_xmRes magCalibration[0]
- magBias[0]; I rotate the device in a circle. Figure 8 does not work, so I rotate it on a spinning seat. I can then read 0 – 360 degrees in a circle. I am including hard iron offsets, but not soft iron offsets. I am not factoring in magnetic declination because exact north is not relevant to this application, rather I simply need 0 – 360 degrees around a circle. To save time I am now using pre-calculated offsets. E.g. mx = (float)magne_x 1.28785 1.218 - 238;
For the pitch and roll angles I am not using quaternions or using a filter combining the gyro and accelerometer, but rather I am just using the raw accelerometer values
pitch = atan(-accele_x / sqrt(accele_y accele_y + accele_z accele_z));
roll = atan(accele_y / sqrt(accele_x accele_x + accele_z accele_z));
I am not doing any calibration for the accelerometer, but the readings are close to 0 – 2048 from flat to a right angle.
and that’s about it. The pitch and roll are in radians and seem to be about correct reading from 0 – 1.57 when the device is raised from flat to a right angle.
I am having to negate the X axis acceleration which seems wrong compared to the datasheet, but produces the correct positive values when rotating anti-clockwise around the accelerometer axes.
Why that is, is a mystery to me. I mean my read acceleration values are the opposite polarity when compared to the data sheet. Should be positive when rotating anti-clockwise, but my program shows it as negative. I wonder if the data sheet is wrong?
Anyway so the bottom line is that my compensated heading is way off and the values change significantly when tilted.
Have you implemented magnetic tilt compensation for the MPU9250?
Regards
Tim
From: Kris Winer [mailto:notifications@github.com] Sent: Sunday, January 15, 2017 3:43 AM
To: kriswiner/MPU-9250 Cc: tim77777; Comment Subject: Re: [kriswiner/MPU-9250] Doesn't seem to be working (#2)
How do you calibrate your sensors?
On Sat, Jan 14, 2017 at 3:16 AM, tim77777 notifications@github.com wrote:
Hi,
I am using a MPU9250 with a microchip controller and although I have most things working I have been unable to get magnetic e-compass tilt compensation to work. I have been looking at various code examples. However, I cannot find any code specifically for the MPU9250 that implements tilt compensation. And with the magnetometer and accelerometer of the 9250 having their X & Y axes exchanged, and the Z axis having its polarity exchanged, any code out there, such as for the HMC5883L, does not exactly match. Although I should be able to change the code to suit the axes and polarity, I have been unable to make it work.
Most examples use something like the following code for tilt compensation
Compensated_x = mxcos(pitch) + mysin(roll)sin(pitch) + mzcos(roll)*sin(pitch);
Compensated_ y = mycos(roll) - mzsin(roll);
compensated_heading = atan2( (double) Compensated_x, (double) Compensated_y ) * 57.2958;
if(compensated_heading < 0) { compensated_heading += 360;}
if(compensated_heading > 360){ compensated_heading -= 360;}
I thought I could simply swap the mx & my about, and negate mz, but it does not seem to be that simple.
I have derived mx, my & mz from your code
mx = (float)magne_xmResmagCalibration[0] - magBias[0];
my = (float)magne_ymResmagCalibration[1] - magBias[1];
mz = (float)magne_zmResmagCalibration[2] - magBias[2];
and when the device is lying flat the heading is correct. But when tilted it is way off. I have tried every which way of changing this code around to try and match the axes and polarity of the magnetometer versus the accelerometer, but without success. I am beginning to think that perhaps my pitch and roll are incorrect, or something else. And so I am wondering if the MPU9250 code of yours for the Arduino implements e-compass tilt compensation, so that I can compare code. I can’t see that it does, but I am wondering.
Regards
Tim
I have been looking at your code
From: Kris Winer [mailto:notifications@github.com] Sent: Saturday, January 14, 2017 10:54 AM To: kriswiner/MPU-9250 Cc: tim77777; Comment Subject: Re: [kriswiner/MPU-9250] Doesn't seem to be working (#2)
What's that?
On Fri, Jan 13, 2017 at 3:46 PM, tim77777 < notifications@github.com> wrote:
hi, do you have magnetic tilt compensation in your code?
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Hello Kris,
I have downloaded the source to the 9250 AHRS, but the system dones't seem to be functioning well at all.
The problems i'm experiencing are that the sensor fusion will settle on a value yaw value and whenever the IMU is rotated on the yaw axis, the yaw ill move slightly, and then return to the settled value it initialized to. The roll and the pitch axis are fine and working.
I'm not sure what I should be doing to help correct the problem.
Thanks