Finding turning angle from real IMU data

I collected some data while walking straight and then turning left 90 degrees using an IMU placed on the pelvis. I would like to find out the magnitude and direction of turning using Matlab. I have tried using the ‘ahrsfilter’ option to obtain the Euler angles.

• How can I align IMU axes with the reference frame axes of ahrsfilter to obtain the correct orientation?
• Will the Euler angles give the magnitude and direction of turning?
• I have tried to find out angles by integrating angular velocity obtained from the ‘ahrsfilter’. The results show a peak while turning 90 degrees but are unable to obtain the magnitude of the turn.

Please find the attached code, collected data and imu sensor axes.

*Note: The trial1_data.xlsx contains Frame No, Acc_x, Acc_y, Acc_z, Gyr_x, Gyr_y, Gyr_z, Mag_x, Mag_y, Mag_z data

Frame no from 387-567 is straight line walking. Frame no from 568-691 is 90 degrees left turning. Frame no 692-870 is again straight line walking.

clc;

clear all;

close all;

filename ='trial1_data.xlsx';

s=readmatrix(filename);

%Converting mm/s2 to m/s2

for i=2:4

for j=1:length(s)

accdata(j,i-1)=s(j,i)./1000;

end

end

% Converting deg/s to rad/s

for i=5:7

for j=1:length(s)

gyrdata(j,i-4)=s(j,i).*0.017453;

end

end

for i=8:10

for j=1:length(s)

magdata(j,i-7)=s(j,i);

end

end

% Estimating Orientation

fuse = ahrsfilter('SampleRate',300,'DecimationFactor',2);

[orientation,angularVelocity]= fuse(accdata, gyrdata, magdata);

% Plot Euler angles in degrees

y=rad2deg(unwrap(euler( orientation, 'YZX', 'frame')));

plot(y);

title('Orientation Estimate');

legend('Z-rotation', 'Y-rotation', 'X-rotation');

ylabel('Degrees');

% Creating a time vector.

xstart=0;

dx=0.01;

N=726;

x=xstart+(0:N-1)*dx;

x=x';

% Integrating Angular Velocity to obtain the angles

angles=cumtrapz(x,angularVelocity);

anglesd=angles./0.017453;

Answer 1

[Brian Fanous]

Can you clarify a bit more about what you mean by the "magnitude of the turn?"

What are you expecting to see in the plot? Should the 90 degree turn be on a different axis?

[Joselin George]

Thanks for the comment. Magnitude means the degrees of the turn.I would like to see 90 degrees on the plot.For the above data 0 degree for straight walking then 90degrees for the turn then 0 again for straight walking.