calibra_anatomical

Reorient the obtained data so that they coincide approximately with the defined anatomical standar orientation.

Taking as input a static signal -in which the only acceleration is that of gravity-, it calculates the rotation matrix that realigns the reference axes so that the first acceleration correspond to the vertical axes in the anatomical standar orientation.

Syntax

Mrot=calibra_anatomical(Acc_static)

Mrot=calibra_anatomical(Acc_static, R_approx)

Description

Mrot=calibra_anatomical(Acc_static) the input parameter Acc_static is a matrix with triaxial accelerations recorded in a static position (used for calibration). It must be an nx3 matrix.

A rotation matrix is computed which can be used to change the sensor frame orientation to one where the first column is pointing against gravity. We call that the anatomical standar orientation.

If sensor data stores as columns of a matrix (nx3), the way to obtain the data referred to this anatomical axes will be to make the multiplication:

dat_anat=(data_sensor*Mrot')

dat_anat=(Mrot*sensor_data')'

it is assumed that the sensor has been placed closely oriented to a standar anatomical orientation, defined as:

column 1 = vertical axis, superior direction (against gravity).
column 2 = medio lateral axis, with changed sign acceleration
column 3 = anteroposterior axis acceleration, anterior direction.

--------------------------------------------------------------

Mrot=calibra_antomical(Acc_static, R_approx) when the sensor is not oriented close to the standar anatomical orientation, an estimation of the initial orientation must be provided, parameter R_approx.

It can be used to provide this initial orientation in two different ways.

As a vector, with three numbers, indicating in which column are placed the antero-posterior, medio-lateral and vertical accelerations, respectively (with a negative sign if the axis is inverted). For example, a first-gen Xsens wired sensor as in the figure, its relation to the standar orientation corresponds to R_approx = [ 3 , 1 , 2 ]
As a rotation matrix, to align the axes in the antero-posterior, medio-lateral and vertical order.

Examples

% Cargamos los datos de un experimento de calibracion ya conocido:
filename = '../simur_data/test.log';
data=load(filename);
Error using load
Unable to find file or directory '../simur_data/test.log'.
acc=data(:,2:4);
%Dibujamos los 500 primeros datos de aceleración
plot(acc(1:1000,:));
%Los primeros 250 datos los usamos para calibración. Son estáticos.
%La primera columna se corresponde aproximadamente con la aceleración
%vertical
%La segunda columna se corresponde aproximadamente con la mediolateral
%cambiada de signo
%La tercera columna se corresponde aproximadamente con la anteroposterior
 
%Definimos la rotación previa como :
R=[ 3 , -2, 1]; %columna ap, columna ml, columna vert
 
% Matriz de calibracion:
Mrot=calibra_anatomical(acc(1:250,:), R)
% Los datos calibrados son:
plot (acc(1:1000,:)*Mrot');
%Igualmente se calibran los giros o magnéticos
giro=data(:,5:7);
plot(giro(1500:2500,:)*Mrot');

Argumentos de entrada

%    AccStatic          - vector con la velocidad de giro en el eje mediolateral
%    Rapprox            - velocidad mínima a alcanzar para detectar eventos. Aconsejado 150

Argumentos de salida

% Mrot - Rotation matrix