Tips and tricks
===============
.. container:: python
.. highlight:: python
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
retrieving them in the user function.
..
REMARK:
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
.. figure:: figure/UserModel_snapshot_Step.png
:alt: Steps to add user model in MBsysPad
User model gui illustration
..
WARNING:
Press enter when modifying a parameter value to ensure that the
change is correctly taken into account.
..
REMARK:
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.
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. However, you may need to re-generate the
project with CMake to take into account the newly generated user model
files.
..
REMARK:
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.
.. container:: python
- 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:
- K, C and L0 are stored in the dictionary ``MbsData.user_model``
with the name and values defined in MBsysPad.
.. code:: python
def user_JointForces(mbs_data, tsim):
#...
# set the joint force in joint 2
id_J = 2
K = mbs_data.user_model['Spring']['K']
C = mbs_data.user_model['Spring']['C']
L0 = mbs_data.user_model['Spring']['L0']
mbs_data.Qq[id_J] = - ( K*(mbs_data.q[id]-L0) + C*mbs_data.qd[id_J] )
#...
Get the ID of a specific joint / body / link / “F sensor” / “S sensor”
----------------------------------------------------------------------
Instead of hardcoding the joint index in user functions, it is possible
to get the joint (or another element) index on basis of its name using
MBsysLab functions.
..
REMARK:
Joint numbering in MBsysPad may be modified when modifying the tree
structure of the system. Since the user has no full control on the
joint numbering, using function to retrieve the ID make the code more
robust with respect to topology modifications.
Only the function to retrieve a joint ID is illustrated, see the end
of the section for the equivalent function.
.. _back-to-the-spring-pendulum-example-1:
Back to the spring-pendulum example
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The model defined in the *Bodies and joints* section is used to
illustrate this feature. The goal is to Get the slider joint id via a
function in the *user_JointForce* function.
.. figure:: figure/PendulumSpringExample.png
:alt: The initial example model consists of a pendulum with a mass sliding along
Pendulum spring illustration
..
REMARK:
Step 2, 4 and 5 are not impacted by theses function. See the *Bodies
and joints* part for more information.
.. _step-1-draw-your-multibody-system-1:
Step 1: Draw your multibody system
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Open the existing \*.mbs file project with MBsysPad.
- Select the T3 joint (q2)
- Give an explicit name to the joint (in this example: “Spring_Slider”)
.. figure:: figure/tipsAndTrick_GiveJointName.png
:alt: The initial example model consists of a pendulum with a mass sliding along
Joint name illustration
Save your project.
.. _step-3-write-your-user-function-1:
Step 3: Write your user function
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. container:: python
- Modify the spring-damper law:
- Edit the *user_JointForces* function (open the file from the
userfctR subfolder of your project)
- Look at the fourth line (“id_J =”)
.. code:: python
def user_JointForces(mbs_data, tsim):
#...
# set the joint force in joint named 'Spring_Slider'
id_J = mbs_data.joint_id['Spring_Slider']
K = mbs_data.user_model['Spring']['K']
C = mbs_data.user_model['Spring']['C']
L0 = mbs_data.user_model['Spring']['L0']
mbs_data.Qq[id_J] = - ( K*(mbs_data.q[id_J]-L0) + C*mbs_data.qd[id_J] )
#...
Equivalent function for other element
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. container:: python
.. code:: python
#...
# The *_Name refers to the name of the element defined in MBsysPad
Joint_ID = mbs_data.joint_id['Joint_Name']
Link_ID = mbs_data.link_id['Link_Name']
F_Sensor_ID = mbs_data.extforce_id['F_Sensor_Name']
S_Sensor_ID = mbs_data.sensor_id['S_Sensor_Name']
Body_ID = mbs_data.body_id['Body_name']
Point_ID = mbs_data.points_id['Body_name']['Point_name']
#...
Initial conditions
------------------
The initial condition `previously defined in
MBsysPad <./bodiesJoint.html>`__ can also be defined in the code after
project loading.
..
REMARK:
Step 1, 2, 3 and 5 are not impacted by theses modifications.
.. _back-to-the-spring-pendulum-example-2:
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
^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. container:: python
- Open the main.py file and add the following command
- After the project loading (*load()* function)
- Before the coordinate partitioning and time integration
(*run()* function for both but they are called on different
python class)
.. code:: python
#...
# Define the initial condition of the joint 1 (rotation of the pendulum)
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
#...
..
REMARK:
The initial conditions are stored in the 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.
The function ``MbsData.reset()`` reset the coordinates values to
the initial one. The considered initial value are the one stored
in q0, qd0 and qdd0.
..
REMARK:
If you change the value of an independent joint involved in a
kinematic loop, you can fail to close the loop (max Newton-Raphson
iteration reached).
If you change the value of a dependent joint, you only change the
initial configuration of the Newton-Raphson 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.
.. figure:: figure/tipsTrick_snapshot_moveSubTree.png
:alt: 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.
.. figure:: figure/tipsTrick_snapshot_modifyTree.png
:alt: Use the small grey square to modify the attach point
Modifying the attach point of a body, joint, anchor point, sensor
..
REMARK:
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 <./userDerivatives.html>`__ if you don’t know what are
state vectors)
The way to save custom quantity highly differs between the codes
(Matlab, python, C).
.. container:: python
In Python and MbsysC, in the opposite of Matlab version, also save
the following value by default:
- All values of links forces (z, zd, force value)
- All joints force/torque value
- All force/torque required in the driven joints
Python offers to save several type of value (with ``mbs_data`` being
a ``MBsysPy.MbsData`` instance):
- A single value: ``mbs_data.set_output(value, name)``
- A vector of values:
- In ``user_dirdyn_init`` function, declare the vector size and
name with the function:
``mbs_data.define_output_vector(name, size)``
- Fill the vector in any of your user function or in the
``user_dirdyn_loop`` function with:
- A specific vector:
``mbs_data.set_output_vector(vector, name)`` (the size of
the defined and provided vector must match).
- Multiple values composing the vector:
``mbs_data.set_output_value(value, index, name)`` (the index
start at 1 and its maximal value is the defined size)
By default we allow the user to save up to 12 single value, if you
need more output value update the option ``max_save_user`` with the
function ``MbsDirdyn.set_options()``.
.. _back-to-the-spring-pendulum-example-3:
Back to the spring-pendulum example
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The goal is to store the spring force and the damper force of the link.
..
REMARK:
Step 1, 2, 4 and 5 are not impacted by theses modifications.
.. _step-3-write-your-user-function-2:
Step 3: Write your user function
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. container:: python
- Edit the link_forces.py file from the userfctR subfolder of your
project
.. code:: python
def user_LinkForces(Z, Zd, mbs_data, tsim, identity):
#...
Flink = K*(Z-Z0)+C*Zd
# Save the spring and damper forces
mbs_data.set_output(K*(Z-Z0), 'Fspring')
mbs_data.set_output(C*Zd, 'Fdamper')
#...
The saved value will be accessible in the results
(``MbsDirdyn.results``) instance in the dictionary ``outputs``.
Integrator
----------
Integrator type
~~~~~~~~~~~~~~~
.. container:: python
With the Python implementation of Robotran, you can use different
integrator by setting the field ``integrator`` of the class
``MbsDirdyn``. The available integrator are listed in the
documentation of ``MbsDirdyn.set_options()`` function
(``import MBsysPy`` and write
``print(MBsysPy.MbsDirdyn.set_options.__doc__)``). Please refer to
the `MbsysC
documentation `__
for full details about the integrators.
Force the integrator to pass at specific time value
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. container:: python
With the Python implementation of Robotran, you can force the
integrator to return the results at fixed time step. do to so,
activate the ``flag_waypoint`` options of ``MbsDirdyn`` and set the
step size in the option ``delta_t_wp``. More details in the
documentation of the ``MbsDirdyn.set_options()`` function Please
refer to the `MbsysPy
documentation `__
or write ``import MBsysPy`` and write
``print(MBsysPy.MbsDirdyn.set_options.__doc__)``).