accelcal

Calibration parameters for accelerometer

Since R2023b

Syntax

[A,b] = accelcal(D)

[A,b] = accelcal(XUP,XDOWN,YUP,YDOWN,ZUP,ZDOWN)

[A,b] = accelcal(___,Gravity=g)

Description

[A,b] = accelcal(D) returns matrix A and vector b used to correct uncalibrated accelerometer measurements based on the calibration data D.

After obtaining A and b, obtain the calibrated data C from uncalibrated data U by using C = U*A + b, where U is a M-by-3 matrix and each row of U is an uncalibrated accelerometer measurement.

[A,b] = accelcal(XUP,XDOWN,YUP,YDOWN,ZUP,ZDOWN) specifies the measurement data at six calibration orientations as shown in Accelerometer Calibration Orientations.

[A,b] = accelcal(___,Gravity=g) specifies the value of the Earth gravity constant g.

Examples

Calibrate Accelerometer Measurements

Create an imuSensor object for generating measurement data. By default, the reference frame of the object is the north-east-down (NED) frame. Specify the accelerometer parameters so that the measurements contain a constant bias and some random noise.

accelParams =  accelparams(ConstantBias=[.1 .2 -.1],...
    NoiseDensity=1e-2*ones(1,3));
imu = imuSensor(Accelerometer=accelParams);
disp(imu.ReferenceFrame)

NED

Define the Euler angles for six calibration orientations and convert them to quaternions.

orients = [...
    0 90 0 % xUp
    0 0 -90 % yUp
    0 180 0 % zUp
    0 -90 0 % xDown 
    0 0 90 % yDown    
    0 0 0]; % zDown
quats = quaternion(orients,"eulerd","ZYX","frame");
N = numel(quats);

Assume the external accelerations and angular velocities are both 0. Simulate the IMU sensor to get the calibration measurements.

exAcc = zeros(N,3);
angVel = zeros(N,3);
D = imu(exAcc,angVel,quats)

D = 6×3

   -9.6490    0.2225   -0.1925
    0.0146   -9.5593    0.0153
   -0.0210    0.1928   -9.9271
    9.9363    0.1840   -0.0210
    0.0008    9.9094   -0.0655
    0.2828    0.1528    9.7232

Use the accelcal function to obtain the calibration parameters.

[A,b] = accelcal(D)

A = 3×3

    1.0018    0.0019   -0.0087
    0.0006    1.0078    0.0041
   -0.0154    0.0020    0.9986

b = 1×3

   -0.0956   -0.1852    0.0779

Verify the calibration parameters by using the calibration measurements.

DCali = D*A+b

DCali = 6×3

   -9.7587    0.0199   -0.0298
   -0.0872   -9.8187    0.0534
    0.0365   -0.0111   -9.8339
    9.8589    0.0196   -0.0284
   -0.0875    9.8012    0.0535
    0.0380   -0.0109    9.7852

Create some random orientations, obtain the accelerometer measurements, and calibrate the measurements.

M = 10;
randomQuats = randrot(M,1);
U = imu(zeros(M,3),zeros(M,3),randomQuats);
C = U*A+b

C = 10×3

    9.1400    3.5374   -1.3008
   -7.1185   -6.8126   -0.7088
    7.1372   -6.8065   -0.5943
   -7.9362    1.5605    5.6203
    7.4397   -2.3143   -6.2363
    7.0758   -4.0794   -5.7321
    3.1250   -5.4306    7.2655
    0.8261   -9.7286    1.3509
    0.8486    2.8606   -9.2705
   -2.4922    8.5000   -3.9532

Input Arguments

D — Measurements at six calibration orientations

N-by-3 matrix

Measurements at the six calibration orientations of the accelerometer, specified as an N-by-3 matrix. N is the total number of measurements. Each row of the matrix is a measurement at one of the six calibration orientations of the accelerometer, as shown in Accelerometer Calibration Orientations. For best results, the matrix should contain an equal number of measurements for all the six orientations.

When interpreting the data in D, the function by default assumes that the Earth gravity constant g is 9.81m/s² or 1, whichever is closer to the mean of the norm of the data.

Data Types: single | double

XUP — Measurements at x-up orientation

N₁-by-3 matrix

Measurements at the x-up orientation of the accelerometer, specified as an N₁-by-3 matrix. N₁ is the total number of x-up measurements. Each row of the matrix is a measurement of the accelerometer. See the Accelerometer Calibration Orientations section for the definition of the x-up orientation.

Data Types: single | double

XDOWN — Measurements at x-down orientation

N₂-by-3 matrix

Measurements at the x-down orientation of the accelerometer, specified as an N₂-by-3 matrix. N₂ is the total number of x-down measurements. Each row of the matrix is a measurement of the accelerometer. See the Accelerometer Calibration Orientations section for the definition of the x-down orientation.

Data Types: single | double

YUP — Measurements at y-up orientation

N₃-by-3 matrix

Measurements at the y-up orientation of the accelerometer, specified as an N₃-by-3 matrix. N₃ is the total number of y-up measurements. Each row of the matrix is a measurement of the accelerometer. See the Accelerometer Calibration Orientations section for the definition of the y-up orientation.

Data Types: single | double

YDOWN — Measurements at y-down orientation

N₄-by-3 matrix

Measurements at the y-down orientation of the accelerometer, specified as an N₄-by-3 matrix. N₄ is the total number of y-down measurements. Each row of the matrix is a measurement of the accelerometer. See the Accelerometer Calibration Orientations section for the definition of the y-down orientation.

Data Types: single | double

ZUP — Measurements at z-up orientation

N₅-by-3 matrix

Measurements at the z-up orientation of the accelerometer, specified as an N₅-by-3 matrix. N₅ is the total number of z-up measurements. Each row of the matrix is a measurement of the accelerometer. See the Accelerometer Calibration Orientations section for the definition of the z-up orientation.

Data Types: single | double

ZDOWN — Measurements at z-down orientation

N₆-by-3 matrix

Measurements at the z-down orientation of the accelerometer, specified as an N₆-by-3 matrix. N₆ is the total number of z-down measurements. Each row of the matrix is a measurement of the accelerometer. See the Accelerometer Calibration Orientations section for the definition of the z-down orientation.

Data Types: single | double

g — Earth gravity constant

positive scalar

Earth gravity constant, specified as a positive scalar. If you do not specify this argument, the function assumes the Earth gravity constant g is 9.81m/s² or 1, whichever is closer to the mean of the norm of the data in D.

Example: 9.807

Data Types: single | double

Output Arguments

A — Calibration matrix

3-by-3 matrix

Calibration matrix, returned as a 3-by-3 matrix.

Data Types: single | double

b — Calibration offset vector

1-by-3 vector

Calibration offset vector, returned as a 1-by-3 vector.

Data Types: single | double

More About

Accelerometer Calibration Orientations

To obtain the measurement data for calibrating the accelerator, align the orientation of the accelerometer measurement frame (X-Y-Z) with one of the six orientations shown in this diagram. For example, to obtain an x-up measurement, align the positive-X direction of the accelerator measurement frame in the opposite direction of the gravity force as shown in the first figure.

If the accelerometer does not have any calibration errors and its measurement frame is perfectly aligned as shown in the figure, you can obtain ideal measurements in this table.

Orientation	aX	aY	aZ
X-Up	-g	0	0
X-Down	+g	0	0
Y-Up	0	-g	0
Y-Down	0	+g	0
Z-Up	0	0	-g
Z-Down	0	0	+g

In the table, a_X, a_Y, and a_Z are the X-, Y-, and Z-acceleration components, respectively. g is the local Earth gravity constant.

References

[1] AN4508 Application Note: Parameters and Calibration of a Low-G 3-Axis Accelerometer.

Version History

Introduced in R2023b