Accel Bias

[PierreGogendeau]

Hi,

I was looking at your file "MPU9250_MS5637_AHRS_t3" and there is something that I don't understand. at the line 373 when you used this function :

accelgyrocalMPU9250(gyroBias, accelBias); // Calibrate gyro and accelerometers, load biases in bias registers

Here you store in the internal registers of the MPU9250 biais from the Gyro and Accel. Then line 478 in the main loop you substract it again :

// Now we'll calculate the accleration value into actual g's ax = (float)MPU9250Data[0]*aRes - accelBias[0]; // get actual g value, this depends on scale being set ay = (float)MPU9250Data[1]*aRes - accelBias[1]; az = (float)MPU9250Data[2]*aRes - accelBias[2];

I thought that when you store the offset value in the MPU9250, it do it by itself to substract the offset. Maybe there is something that I miss understood ?

Thanks in advance for your help

Cheers

Pierre Gogendeau

[kriswiner]

I don't like the way the MPU9250 handles the accel bias when written to the register so I donlt usually write the value there, rather I subtract it manually in the main code.

On Wed, Jun 28, 2017 at 10:50 PM, PierreGogendeau notifications@github.com wrote:

Hi,

I was looking at your file "MPU9250_MS5637_AHRS_t3" and there is something that I don't understand. at the line 373 when you used this function :

accelgyrocalMPU9250(gyroBias, accelBias); // Calibrate gyro and accelerometers, load biases in bias registers

Here you store in the internal registers of the MPU9250 biais from the Gyro and Accel. Then line 478 in the main loop you substract it again :

// Now we'll calculate the accleration value into actual g's ax = (float)MPU9250Data[0]aRes - accelBias[0]; // get actual g value, this depends on scale being set ay = (float)MPU9250Data[1]aRes - accelBias[1]; az = (float)MPU9250Data[2]*aRes - accelBias[2];

I thought that when you store the offset value in the MPU9250, it do it by itself to substract the offset. Maybe there is something that I miss understood ?

Thanks in advance for your help

Cheers

Pierre Gogendeau

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[PierreGogendeau]

Ok thank you for your very very fast answer. Have a good day.

[lindig]

I believe the bias is corrected by the chip and again in the main loop - and this is wrong. Not doing it in the main loop I can observe that the values are much closer to zero when the chip is not moving. So I suspect a bug here.

[kriswiner]

The only way the "chip" can correct offset bias is if the user writes the offset into the bias registers. So yes, double correcting the biases would produce incorrect results...

On Tue, Feb 5, 2019 at 1:34 PM Christian Lindig notifications@github.com wrote:

I believe the bias is corrected by the chip and again in the main loop - and this is wrong. Not doing it in the main loop I can observe that the values are much closer to zero when the chip is not moving. So I suspect a bug here.

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[lindig]

In MPU6050BasicExample.ino mpu.calibrateMPU6050() is called, which does write the data to the chip, and it subtracts the bias explicitly in the main loop.

[kriswiner]

Then, of course, this is wrong. These sketches are examples, nbot perfect black boxes. Buyer beware...

On Tue, Feb 5, 2019 at 1:39 PM Christian Lindig notifications@github.com wrote:

In MPU6050BasicExample.ino mpu.calibrateMPU6050() is called, which does write the data to the chip, and it subtracts the bias explicitly in the main loop.

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