Sensors are used to compute the kinematics of a point of the system. They can compute the following information for a given point:

  • The absolute position vector
  • The absolute rotation matrix of the body on which the point is attached
  • The absolute velocity vector
  • The absolute angular velocity vector of the body
  • The absolute acceleration
  • The absolute angular acceleration vector of the body
  • The Jacobian matrix of the sensor, i.e. the relation between generalized velocities and the sensor velocity and angular velocity

There are two kinds of sensors:

  • S sensor: sensor attached to a given point;
  • F sensor: sensor used to compute external force (see the external force);

Note that there also exist Gen sensor: it is equivalent to S sensors attached on each joint of the system.

Back to the pendulum-spring example

The goal is to capture the vertical acceleration of the point located at the middle of the blue rod.

A sensor has to be placed at the middle of the blue rod

Position of the sensor

Step 1: Draw your multibody system

Open the existing *.mbs file project with MBsysPad:

  • Set back the pendulum joint to independent and the crank joint to dependent;
  • Add an anchor point on the rod and enter its coordinates;
  • Add a sensor to the anchor point:
    • Click on the Sensor button;
    • Click on the anchor point at the end of the rod;
    • A “S” is added next to the anchor point;
    • Click on the “S” to edit its properties and give it a name;

Step 2: Generate your multibody equations

Regenerate the multibody equations with the same options as previously.

Step 3: Write your user function

Edit the user_DrivenJoints function to remove the imposed trajectory of the crank. You can delete or comment the part which imposes the trajectory of the crank.


Imposing the trajectory of an independent or dependant joint will lead to inconsistent results.

Matlab section:

In matlab there are 2 options in order to store the result of a sensor:

  1. During the simulation in the user_DirDyn_io function, this is similar to saving a custom quantity;
  2. After the simulation, during the post-process.

For both option the acceleration is accessible via the MBS_user.resdirdyn structure:

  • MBS_user.resdirdyn.tsim: the time
  • MBS_user.resdirdyn.rodAccZ: the value of the acceleration at the correspondig time

Option 1: on-line sensor calculation

  • Open the user_dirDyn_io function:
    • In the ‘init’ case (“process starting”), initialize the result vectors.
    • In the ‘else’ part (“process running”), calculate the sensor kinematics using the mbs_comp_S_sensor function and save the desired value.
function [] = user_DirDyn_io(mbs_data,tsim,step,flag)

    if (nargin > 3)         %   process starting
        switch flag
            case 'init'     % initialization of the vectors
                MBS_user.resdirdyn.tsim = zeros(step,1);
                MBS_user.resdirdyn.rodAccZ = zeros(step,1);
            otherwise       %   unused

    else                    %   process running
        MBS_user.resdirdyn.tsim(step) = tsim;
        id = mbs_get_S_sensor_id(MBS_info, 'RodSensor');
        rodSens = mbs_comp_S_sensor(mbs_data,tsim,id);
        MBS_user.resdirdyn.rodAccZ(step) = rodSens.A(3);



The calculation of sensor kinematics can be done in any user function if it is useful for defining the constitutive law.

Option 2: post-process calculation

In the main script, after the direct dynamics, add a for loop for calculating the sensor kinematics for each time step of the solution:


%% 2. Direct dynamics [mbs_exe_dirdyn]
[mbs_dirdyn,mbs_data] = mbs_exe_dirdyn(mbs_data,opt.dirdyn);

%% Sensor calculation
MBS_user.resdirdyn.rodAccZ = zeros(length(mbs_dirdyn.tsim),1);
sensId = mbs_get_S_sensor_id(MBS_info, 'RodSensor');

for i=1:length(mbs_dirdyn.tsim)
    % Restoring the state at time i
    t = mbs_dirdyn.tsim(i);
    mbs_data.q = mbs_dirdyn.q(i,:);
    mbs_data.qd = mbs_dirdyn.qd(i,:);
    mbs_data.qdd = mbs_dirdyn.qdd(i,:);

    % Calculate the sensor and save the acceleration at time i
    [rodSens] = mbs_comp_S_sensor(mbs_data,t,sensId);
    MBS_user.resdirdyn.rodAccZ(i) = rodSens.A(3);


Check the results

Plot the graph of the sensor acceleration (results ares avilaible in resultsR/ folder) and check your results with the following graph. The evolution of the system is the same as for the Cuts part of the tutorial

Plot the time history of the snesor acceleration of the pendulum spring example

Time history of the joint position