Tips and tricks

User models

Instead of assigning parameter in the code, parameters values can be introduced via “User Model” in MBsysPad.

Back to the spring-pendulum example

The goal is to store the parameters of the model in the “User Model” and retriving them in the user function.


Step 4 and 5 are not impacted by theses function. See the Bodies and joints part for more information

Step 1: Draw your multibody system

  • In MBsysPad, click on “User Model” (under the joint button)
  • Click the upper “Add” button to add a new user model
  • Give a name to the user model
  • Add 3 parameters for the spring (K, C, L0)
    • Click the lower “Add” button to add a parameter
    • In the right part of the window, give a name and a value to the parameter
Steps to add user model in MBsysPad

User model gui illustration


Press enter when modifying a parameter value to ensure that the change is correctly taken into account.


User model only serves for storing data. If nothing else is done, they do not modify the behaviour of the system. They must be accessed in user functions to be taken into account.

Generate the user models C files

The user models are not loaded with the *.mbs file. They need to be generated before compiling your model. The generation is done in the MBSysPad interface, open your project and click on Tools->Generate C-specific user files. You have to save your project before the generation, otherwise your current modifications will not be included.


For this step you must have compiled the User file generation application. Please refer to the installation instructions

Generation of the user models

Generation of the user models

This action should create in your userfctR/ folder of your project the following files:

  1. user_model.h
  2. user_model.c
  3. user_all_id.h

Step 2: Generate your multibody equations

It is not necessary to regenerate the multibody equations after a modification or a creation of user models since the symbolic files are not affected by user models.


When modifying a user model parameter in MBsysPad, you must save the model in MBsysPad and reload the model in your code so that the modification take effect.

Step 3: Write your user function

The parameters defined in MBsysPad can then be accessed via the mbs data structure in the code.

C section:

  • Edit the user_JointForces function (open the file from the userfctR subfolder of your project);
  • Modify the spring-damper law to get the parameter values from the user model:
    • The file user_model.h is now included.
    • The attribution of K, C and L0 use mbs_data->user_model->Spring.PARAM to retrieve the value of the parameter defined in MBsysPad.
#include "user_model.h"

double* user_JointForces(MbsData *mbs_data, double tsim)
    // set the joint force in joint 2
    int id = 2;
    UserModel *um = mbs_data->user_model;
    double K = um->Spring.K;
    double C = um->Spring.C;
    double L0 = um->Spring.L0;

    mbs_data->Qq[id] = - ( K*(mbs_data->q[id]-L0) + C*mbs_data->qd[id] );

Initial conditions

The initial condition previously defined in MBsysPad can also be defined in the code after project loading.


Step 1, 2, 3 and 5 are not impacted by theses modifications.

Back to the spring-pendulum example

The goal is to modify in the main script the initial rotation and rotationnal speed of the pendulum.

Step 4: Run your simulation

C section:

  • Edit the main function and add the following command
    • After the project loading (load() function)
    • Before the coordinate paritioning (mbs_new_part function) and time integration (mbs_new_dirdyn function)

// Define the initial condition of the joint 1 (rotation of the pendulum)
int Joint_ID = 1;

mbs_data->q[Joint_ID] = 0.2;
// The initial rotation of the pendulum is set to 0.2 rad

mbs_data->qd[Joint_ID] = 0.01;
// The initial angular velocity of the pendulum is set to 0.01 rad/s



The initial conditions are stored in the mbs_data->q0, ->qd0 and ->qdd0 vectors at the loading of the .mbs file. If you need to access to q0 (or qd0 or qdd0) during the computation you need to update these values aswell.


If you change the value of an independent joint involved in a kinematic loop, you can fail to close the loop (max Newton-Rapson iteration reached).

If you change the value of a dependent joint, you only change the initial configuration of the Newton-Rapson iteration.

MBsysPad 2D diagram manipulation

Moving several elements at once

Press the SHIFT key to move a component and all its descendence. Moving the base while pressing SHIFT will move all the drawing.

Press the SHIFT key to move a component and all its descendence

Moving a component and all its descendence

Modifying the attach point of a body, joint, anchor point, sensor…

Select the element (body, joint, anchor point, sensor…) you want to modify. A small grey square appears next to the attach point of your element. Click on it (and keep the button pressed) and move the the mouse to the desired attach point.

Use the small grey square to modify the attach point

Modifying the attach point of a body, joint, anchor point, sensor


The attach point is colored in red while moving the mouse. Check that you have reached the desired location.

Saving a custom quantity

It is often useful to get the time history of a custom variable which is not contained by default in the result structure returned by the direct dynamics module. The variables contained by default are:

  • The position, velocity and acceleration of each joint at each time step
  • The state vectors at each time step (see User derivatives if you don’t know what are state vectors)

The way to save custom quantity highly differs between the codes (Matlab, python, C).

In C, this can be done using the set_output function.

Back to the spring-pendulum example

The goal is to store the spring force and the damper force of the link.


Step 1, 2, 4 and 5 are not impacted by theses modifications.

Step 3: Write your user function

Python section:

  • Edit the user_LinkForces function (open the file from the userfctR subfolder of your project) and:
    • Include set_output.h
    • Add the call to the set_output function
#include "set_output.h"
double user_LinkForces(double Z, double Zd, MbsData *mbs_data, double tsim, int ilnk)
    double Flink = 0;

        // get the link id
        case MyLink_id :
            // retrieve the user model
            UserModel *um = mbs_data->user_model;
            double K  = um->mylink.K;
            double C  = um->mylink.C;
            double Z0 = um->mylink.L0;

            Flink = K*(Z-Z0)+C*Zd;

            // Save the spring and damper forces
            set_output(K*(Z-Z0), "Fspring");
            set_output(C*Zd, "Fdamper");


    return  Flink;

The results will be saved in the resultR/ folder.