Getting Started Using Universal Mechanism

Getting Started Using Universal Mechanism
Universal Mechanism 4.0
1
Getting Started
GETTING STARTED USING UNIVERSAL MECHANISM.............................................. 2
1.
MODEL OF A PENDULUM ..................................................................................... 3
1.1.
What we will learn.............................................................................................................3
1.2.
Model scheme.....................................................................................................................4
1.3.
Creating the model ............................................................................................................5
1.3.1.
Running UM Input and creating new model ...............................................................5
1.3.2.
Familiarizing yourself with Universal Mechanism .....................................................5
1.3.3.
Creating graphical objects ...........................................................................................7
1.3.3.1.
Scene image.........................................................................................................7
1.3.3.2.
Image of pendulum............................................................................................13
1.3.4.
Creating rigid bodies .................................................................................................14
1.3.5.
Creating joints ...........................................................................................................15
1.3.6.
Saving the model .......................................................................................................16
1.3.7.
Preparation for simulation .........................................................................................17
1.4.
Simulation of the motion.................................................................................................19
1.5.
Multibody pendulum.......................................................................................................25
2.
FREE AND FORCED OSCILLATIONS ................................................................. 26
2.1.
What we will learn...........................................................................................................26
2.2.
Model scheme...................................................................................................................26
2.3.
Creating the model ..........................................................................................................27
2.3.1.
Running UM Input and creating new model .............................................................27
2.3.2.
Creating graphical objects .........................................................................................28
2.3.3.
Creating rigid bodies .................................................................................................32
2.3.4.
Creating joints ...........................................................................................................34
2.3.5.
Creating force elements .............................................................................................37
2.3.6.
Visualization of spring and damper...........................................................................38
2.3.7.
Additional parameters................................................................................................40
2.3.8.
Preparation for simulation .........................................................................................42
2.4.
Simulation of the motion.................................................................................................43
2.4.1.
Free oscillations.........................................................................................................43
2.4.2.
Statistical analysis .....................................................................................................49
2.4.3.
Linear analysis...........................................................................................................50
2.4.4.
Forces oscillations .....................................................................................................52
3.
SUBSEQUENT STUDYING UNIVERSAL MECHANISM ...................................... 54
Universal Mechanism 4.0
2
Getting Started
Getting Started Using Universal Mechanism
This manual leads you through the basic possibilities of Universal Mechanism
software and shows you how to create and simulate models of several simple mechanical systems. It assumes that you go through the manual step by step sequentially.
Simulation of such mechanical systems as cars and railway vehicles has certainly its own features but basic concepts using UM still the same. These concepts
are shown in this manual.
Contact information
Universal Mechanism demo version you can download using the following
link: http://www.umlab.ru/um40demo.exe.
The latest UM version as well as up to date UM user’s manual available at
http://www.umlab.ru/download.htm.
Please, send you bug report, questions and suggestions to [email protected]
Address:
Russia, 241035, Bryansk,
bulv. 50-let Oktyabrya, 7
Bryansk State Technical University,
Laboratory of Computational Mechanics,
Prof. Dmitry Pogorelov
Phone, fax: +7 4832 568637.
Universal Mechanism 4.0
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Getting Started
1. Model of a pendulum
1.1. What we will learn
This lesson shows you how to create new model, add rigid bodies and joints,
generate and compile equations of motion, simulate dynamics of a model and obtain plots of various performances of the model. This lesson is devoted to general
overview of the UM possibilities and workflow.
At the end of the lesson we will have the model of the pendulum (you can find
the final model in the {um_root}\tutorial\eng\pendulum directory)1, which will
include one rigid body – pendulum itself, one rotational joint and graphical object
of the environment – support. After describing the model we will go through the
all stages of the working with the model: synthesis and compiling of equations of
motion, and then will come to the simulation of motion of the pendulum.
Support
Pendulum
Figure 1. Complete model
1
Pendulum model is also available at http://www.umlab.ru/download/40/eng/pendulum.zip
Universal Mechanism 4.0
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Getting Started
1.2. Model scheme
Before modeling the pendulum with the help of UM
we recommend to draw its sketch like you can see at the
left.
As you can see, we drew a simple pendulum and chose two systems of coordinates (SC) - the base frame
OX0Y0Z0 (SC0) and the body-fixed frame (SC1). The
SC0 origin is placed in the center of the joint, the second
one (SC1) - at the mass center of the pendulum. The axes
of SC1 are directed along the pendulum principle axes of
inertia. The base frame exists in every object and, as a rule, is connected with the
Earth. There is only rotational joint connecting the pendulum and the base frame
(the wall which the pendulum is attached to).
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Getting Started
1.3. Creating the model
1.3.1.
Running UM Input and creating new model
Running UM Input program
1. Click Start/Programs/Universal Mechanism 4.0/UM Input.
Creating a new model
1. From the File menu point to New object MBS1.
The window of the constructor appears, see fig. 2.
1.3.2. Familiarizing yourself with Universal Mechanism
Take a few minutes to familiarizing yourself with the Universal Mechanism
constructor window, see Figure 2.
Tree of elements of a model in the left top corner of the constructor window is
used for getting access to elements of the model.
Animation window in the center shows the model or its elements. A frame is
shown in the center of animation window. There is the following identification for
axes: Red – X, Green – Y, Blue – Z (RGB). Point of view, zoom and other settings
can be changed via toolbar buttons. Using the context menu you can set perspective parameters, supporting grid, etc.
Inspector at the right-hand side of the constructor is the main tool for the description of elements. It shows parameters of an active element. It contains full information about current element of the model.
1
MBS means “multibody system”
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Getting Started
Tree of
elements
Animation
window
List of
parameters of
the model
Inspector
Figure 2. Constructor window
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1.3.3.
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Getting Started
Creating graphical objects
We recommend to start describing any mechanical system with creating a set of
graphical objects (GO) of the elements of the model.
1.3.3.1.
Scene image
Creating new graphical object - scene
Scene is a graphical object corresponding to fixed elements of the object. Describing the scene is optional. To create a scene you should make a usual graphical
object and assign it to the scene image. As for our example it is an image of the
fixed joint where the pendulum is attached to – support. In order to create the corresponding image you should do the following steps.
1. Point to Images element of Tree of elements.
2. Click
button in the top of the Inspector to create new graphical object,
see Figure 3.
Add new graphical
object
Figure 3. Adding a new element
Note: You can add new element of any type in the same way.
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Getting Started
Renaming the graphical object
As you create objects, UM automatically assigns names to them. Each name
consists of a string containing the element type and a unique integer ID for that
type. UM named the recently created graphical element GO1.
1. Point to the field with the name of the element and replace GO1 with Support, see Figure 4.
Name of graphical
object
Delete graphical
object
Copy graphical
object
Add graphical
element
Figure 4. Renaming the graphical object
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Getting Started
Creating graphical elements
Every graphical object (GO) can include any number of various graphical elements (GE). So you are able to create quite complicated images. Let's create three
elements - sphere, cone and box, which form the image of the support altogether.
Creating new graphical element: sphere
1. Click Add new graphical element button, see Figure 4.
New tab GE1 appears, see Figure 5.
Type of graphical
element
Figure 5. Type of the graphical element
2. Choose type for the new graphical element – Ellipsoid.
3. Point to the Parameters tab and set a = b = c = 0.05.
4. Point to the Color tab and set diffuse color to red.
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Getting Started
Creating new graphical element: cone
1.
Note:
Create new graphical element and set its type to Cone.
Do not add new graphical object instead new graphical element
within graphical object. In this example we create the only graph object – Support, which contains three graphical elements: sphere,
cone and box.
2. Point to Parameters tab and set R2 = 0.1; R1 = 0; h = 0.15.
3. Set diffuse color to red.
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Getting Started
Creating new graphical element: box
1. Create new graphical element and set its type to Box.
2. Point to Parameters tab and set A = 0.5; B = 0.5; C = 0.05.
3. Point to GE Position tab. Set Translation/Z to 0.15, see Figure 6.
Figure 6. Graphical element position
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Assigning Support as scene image
1. Point to Object item of Tree of elements.
2. Select Support in the field Scene image.
Getting Started
Universal Mechanism 4.0
1.3.3.2.
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Getting Started
Image of pendulum
1. Return to the Images item in the Inspector.
2. Create new graphical object.
3. Rename new graphical object to Pendulum.
Note:
Do not forget to press Enter after any modification of the text data in
order to reflect this.
The pendulum image consists of two graphical elements: an ellipsoid and a
cone.
4. Add new graphical element Ellipsoid and set its parameters a = 0.05;
b = 0.2; c = 0.2. Set diffuse color to blue.
5. Add new graphical element Cone and set its parameters R2 = 0.03;
R1 = 0.03; h = 1. Set diffuse color to blue.
Now image of the pendulum is ready.
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Creating rigid bodies
The pendulum as a mechanical system consists of the only body.
1. Point to the Bodies item in the Inspector.
2. Create new body.
3. Rename body to Pendulum.
4. Select Pendulum from the drop-down list Image.
5. Set Mass = 1 (kg).
Getting Started
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1.3.5.
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Getting Started
Creating joints
The rotational joint connects the Pendulum and the Base0. To create new joint
do the following actions:
1. Point to Bodies/Pendulum.
2. Click the button Adjust joint
and select Rotational joint in the context
menu.
After that the rotational joint is created and named as jPendulum automatically.
Joint points and joint vectors describe the position of the rotation axis relative to
each of the bodies. Their coordinates must be given in the corresponding bodyfixed systems of coordinates.
3. In the fields Joint points/Pendulum set Z position to 1. So the pendulum
will swing around its upper point.
X component
Y component
Z component
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Getting Started
Saving the model
Now your model is described completely. And it is high time to save it. Let the
object name be Pendulum.
1. Select menu item File/Save as…
2. Set Path to {UM Path}\Pend, in the way how it is shown in the figure below.
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Getting Started
Preparation for simulation
Program package Universal Mechanism (UM) consists of two programs: UM
Input program - UMInput.exe and UM Simulation program - UMSimul.exe. The
UM Input is used for creating objects, generating their equations and compiling
them with the help of an external compiler. As a result you have got a dynamiclinked library UmTask.dll containing equations of motion of your object. The DLL
is always located in the object directory. When DLL of the object exists a model is
ready for simulation.
Now we should generate and compile equations of motion and start
UM Simulation for dynamical analysis of the system.
Paths to external compilers
Universal Mechanism supports using Borland Delphi, Borland C++ Builder,
Microsoft Visual C++ as external compilers.
1. Select Tools/Options menu item.
Your further actions depend on what external compiler you are going to use:
Delphi
2. Select Paths/Delphi tab.
3. Click Search Delphi button.
Borland C++ Builder, Microsoft Visual C++
2. Select Paths/C++ tab.
3. Click one of the following buttons Search Visual C or Search Borland
C++ Builder depending on which C compiler is installed on your PC.
If UM successfully detects external compiler all paths are set automatically.
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Getting Started
Generating and compiling equations of motion
1. Select Object/Generate equations.
If your description of the model is correct the corresponding dialog box appears. If your model description is not correct then tab Summary, which contains
all detected errors, appears.
2. Select Compile equations.
3. Click Generate button.
If generating and compiling equations of motions end successfully you’ll see
the message box: «Compiling successful. Object is ready for simulation.». The
model is ready to be loaded in the UM Simulation program.
Run UM Simulation program
1. Select Object/Simulation menu item.
UM Simulation program starts and opens the current model.
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Getting Started
1.4. Simulation of the motion
Now we are in the simulation program. We will open new animation window,
deflect the pendulum from vertical position to 1 radian and run simulation of dynamics of pendulum.
Creating new animation window
1. From the Tools menu, select Animation window. New animation window
appears. Familiarize yourself a bit with animation window.
Rotating
Point the mouse cursor to the animation window so that cursor looks like
the picture in the figure to the right. Press left mouse button and rotate the
model in the animation window.
Shifting
Point the mouse cursor to the animation window so that it has Rotating
shape, press Ctrl key and mouse cursor changes to Shifting mode. Press left
mouse button and shift model in the animation window.
Zoom in/zoom out
Point the mouse cursor to the animation window and press Shift key and
with the help of left mouse button zoom in/out the model.
After some practice you can get something like shown in the figure below.
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Start simulation
1.
From the Analysis menu, select Simulation.
Window of the Object simulation inspector appears.
Getting Started
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Getting Started
Initial conditions
You should deflect the pendulum a bit in order to obtain its motion. There exists a special tool for this purpose: a wizard of the initial conditions.
1. Select the Initial conditions tab.
You can see a complete list of the object coordinates. In our case there is only
one coordinate in jPendulum joint.
2. Set Coordinate to 1. Press Enter key.
Your pendulum has deflected.
Note:
Universal Mechanism uses System International (SI). Angular values
have dimensions of radian.
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Getting Started
Simulation
Now your model is ready for simulation. Simply start simulation process for the
10 seconds.
1. Click Integration button in the Object simulation inspector.
At the end of the simulation the Pause window appears. You can increase the
simulation time, change the numerical method etc.
2. Press the Interrupt button. Object simulation inspector appears.
Drawing plots
During the simulation you can see plots of various variables. Such as velocities,
accelerations, forces and so on. We will open new graphical window, create new
variable to plot – Y coordinate of the center of mass of the pendulum and draw its
plot.
Well, let us create new graphical window.
1. From the Tools menu, select Graphical window.
Open Wizard of variables.
2. From the Tools menu, select Wizard of variables.
The Wizard of variables is a special tool for creating variables, which can be
drawn in graphical windows or animated in animation windows (in cases of vectors or trajectories).
Let us draw a plot of Y coordinate of the mass center of the pendulum.
3. Select Linear var. tab (linear variables: coordinates, velocities, accelerations etc.).
4. Select Y in the Component group.
5. Then move the variable to the container with the help of the button
New variable r:y(Pendulum) appears in the container of variables.
.
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Getting Started
6. Select the variable in the Wizard of variables and drag it to the graphical
window.
7. Select the Object simulation inspector and click the Integration button.
You can see the plot of your variable in the graphical window.
Animation of vectors and trajectories
During the simulation you can animate various vector variables in an animation
window. Let us animate the vector of the mass center velocity. Firstly, we need to
create such variable in the Wizard of variables.
1.
Select the Wizard of variables and there select the Linear var. tab.
2.
Select v (velocity) in the Type group.
3.
Select V (vector) in the Component group.
4.
Add this variable to the container clicking the
5.
Drag new variable to the animation window.
button.
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Getting Started
A list of animated vectors is hidden by default. You can make it visible and
change its position with the help of the Position of list of vectors command of the
pop up menu of the animation window.
6.
Select animation window, click right mouse button and select Position of
the list of vectors/Left.
To draw a trajectory of the pendulum create a new variable with the help of the
master.
7.
Repeat all steps we made for the velocity, but the Type of the variable set
to r (radius-vector). Drag this variable to the animation window.
8.
Double click on the velocity item in the List of vectors and select red
color for the vector of velocity and than double click trajectory item and
select blue color for it.
9.
Click the Integration button in the Object simulation inspector.
Now you can see the vector of the velocity and trajectory of the center of the
mass of the pendulum. You should use the Scale of vectors command of a pop up
menu to specify its scale.
Double click on an element of the list of vectors or on a vector/trajectory image
to change the color of the vector and trajectory (in addition for the trajectory - to
change the number of points on the curve).
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Getting Started
1.5. Multibody pendulum
It is very easy to convert the object to a multibody system, which contains several bodies – a chain of pendulums.
1.
Close the UM Simulation program and come back to the UM Input program.
2.
Select Bodies and copy the pendulum two times.
Delete body
Copy body
Add new body
3.
Rename new bodies to Pendulum2 and Pendulum3.
4.
Select Joints and copy the joint two times too.
5.
Change the connecting bodies: Pendulum and Pendulum2 for the second
joint, Pendulum2 and Pendulum3 for the third one.
Note:
Use the
button in the top of the animation window to switch the
mode of window Full object / Single element.
6.
Generate and compile equations of motion.
7.
Run UM Simulation.
8.
Create a new animation window.
9.
Select the Analysis/Simulation menu item.
10. Set a proper initial position of the chain with the help of the Initial conditions tab.
11. Click Integration to run the simulation.
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Getting Started
2. Free and forced oscillations
2.1. What we will learn
In this lesson we will learn how to add forces, preset movement of a body as a
time function and use parameterization of a model. We will use Linear analysis
for obtaining the equilibrium position of a system, natural frequencies and forms.
As well as we will analyze the spectrum of output data using the Statistics tool.
2.2. Model scheme
The example of simulation of free and forced damped oscillations is considered.
In this lesson we will create the model shown in the Fig. 2.1. Model consists of
two rigid bodies Top and Brick, two translational joints, a linear spring and a
damper. We will set vertical coordinate of the upper body as a sinusoid function.
You can find the final model in the {um_root}\tutorial\oscillator directory or
download it using the following link:
http://www.umlab.ru/download/40/oscillator.zip
Asin(ωt)
Top
µ
c
Brick
m
Figure 2.1. Model scheme
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Getting Started
2.3. Creating the model
2.3.1.
Running UM Input and creating new model
Running UM Input program
1. Click Start/Programs/Universal Mechanism 4.0/UM Input.
Creating a new model
1. From the File menu point to New object. The window of the constructor appears.
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Getting Started
Creating graphical objects
Top
We will create a thin rectangular plate as a graphical object for the Top body.
1.
2.
3.
4.
5.
Create new graphical object.
Set its name to Top.
Add new graphical element – Box.
Set parameters and GE position for the Box as it is shown below.
Select the Color tab and choose blue for diffuse and specular colors.
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Brick
1.
2.
3.
4.
5.
Let us describe the brick as a cube of 0.2 m side length.
Create new graphical object.
Set its name to Brick.
Add new graphical element Box into this graphical object.
Set its parameters and GE position.
Select the Color tab and set red for diffuse and specular colors.
Getting Started
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Spring
1.
2.
3.
4.
Now we will create the graphical object for the spring.
Create new graphical object.
Set its name to Spring.
Add new graphical element – Spring.
Set Spring parameters as it is shown below.
Getting Started
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Getting Started
Damper
1.
2.
3.
4.
5.
6.
Now we come to the last graphical object in this model – damper.
Create new graphical object.
Set its name to Damper.
Add new graphical element for the Damper – Cone.
Set parameters as follows:
R2 = 0.02;
R1 = 0.02;
h = 1.
Select the Color tab and choose blue for diffuse and specular colors.
Add one more Cone with the following parameters: R2 = 0.04; R1 = 0.04;
h = 0.5. In the GE Position tab set Translation/Z to 0.25. Select red diffuse
and specular colors.
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Creating rigid bodies
Top
Create new rigid body – Top.
1.
2.
3.
4.
Add new rigid body.
Set its name to Top.
In the Image list select Top.
Leave Mass and Inertia tensor empty.
Getting Started
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Getting Started
Brick
Now we will create one more rigid body – Brick. Its mass we will express via
parameter (identifier) m. Such a parameterization gives us a possibility to change
its mass easily and quickly obtain results for various values of the mass of the
brick without regeneration equations of motion. Otherwise we would have to generate equations every time we want to change its mass.
1.
2.
3.
4.
5.
6.
Add new rigid body.
Rename it to Brick.
In the Image list select Brick.
Set Mass to m and press Enter. New Initialization of values window appears.
Set Value to 10. Press Enter.
This new parameter appears in the parameter list in the bottom left corner of the
constructor window, see the figure below.
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Getting Started
Creating joints
Joint for the top
The Top body moves along the vertical direction according A·sin(ω·t) function.
Now we will describe the translational joint between the base and the top and set
the coordinate in this joint as a time function.
1. Select the Top body in the tree of elements.
2. Click the button. Select Create joint and in the drop-down list select Translational, see the Fig. 2.2, left. The new joint of this type is created. Now you
can see parameters of the joint.
3. Select the Geometry tab and set joint parameters as it is shown in the Fig. 2.2,
right.
Figure 2.2. Creating translational joint
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Getting Started
4. Point to Description tab.
5. Turn on the Prescribed function of time check box.
6. Set the Type of description to Expression, and then input a*sin(omega*t), see
the Fig. 2.3, and press Enter.
Figure 2.3. Prescribed function of time
7. In the Initialization of values window set a = 0.05 (m) and omega = 10 (rad/s).
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Getting Started
Joint for the brick
1.
2.
3.
4.
5.
Select the Brick bod y in the tree of elements.
Click the button.
In the drop-down list select Prismatic again.
Select the Top as the first body instead Base0, see the figure below.
Set the rest parameters of the joint as it is shown below.
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Getting Started
Creating force elements
Now we will describe elastic and damping force elements between the top and
the brick. Let us use c parameter for the stiffness coefficient of the spring and mu
parameters for the damping coefficient of the damper. Length of the unloaded
spring let us denote as l0.
1.
2.
3.
4.
Select the jBrick joint.
Select the Joint force tab.
In the Joint force list select the Linear.
In the c field input с (stiffness coefficient), in the x0 field input l0 and set d to
mu, see Fig. 2.4. Press Enter. Set values of parameters as follows: c = 250,
l0 = 0.4, mu=5.
Figure 2.4. Elastic and damping joint forces
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Getting Started
Visualization of spring and damper
After all we have completely described object from the mechanical point of
view. We described all elements we need: rigid bodies, joints and force elements.
However our model now looks not so good – spring and damper introduced as
joint forces that cannot be visualized, see the Fig. 2.5, left. In order to visualize
spring and damper we will create two bipolar forces in the model. Their values we
set to zero. That is why these bipolar forces will not influence on the dynamics of
the model, but give us a possibility to show the spring and the damper, see the
Fig. 2.5, right.
Figure 2.5. Visualization of forces
Note.
There are several possible ways to describe elastic and damping forces
in our model. We used the way to describe them as joint forces, but it is
not the only right way. We could introduce them as bipolar as well. And
in this latter case we would visualize them and introduce forces at once
without intricate describing additional fake bipolar forces.
But such a way leads to a following problem. Our ideal case that we
consider here allows to model the situation when the length of the spring
and damper equal to zero. Imagine that the brick has so large amplitude
that the attachment points of the spring and damper will be on the same
level. In such a case we have degeneration of bipolar forces that act
along the line between the attachment points. When we have zero length
we could not find the direction of the bipolar forces. Joint forces have no
such a degeneration, because they direction always coincide with the
axis of the joint. That is why we used very joint forces here.
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Getting Started
1. So, select the Bipolar force in the tree of elements.
2. Add two bipolar force elements. Set their parameters as it is shown in the
Fig. 2.6.
Figure 2.6. Fictitious bipolar forces
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Getting Started
Additional parameters
Using UM you can express one parameter via others. Here we will add two new
parameters in the model – accurate analytical values of the natural frequency and
the critical damping coefficient. Our model is very simple that is why we can obtain analytical solutions easily.
Natural frequency can be obtained according to the following formula:
k=
c
, where
m
k – natural frequency, rad/s;
с – stiffness coefficient, N/m;
m – mass of the body, kg.
Critical damping coefficient can be found as:
µ* = 2 cm , where
µ * – critical damping coefficient, Ns/m.
1. Well, add new identifiers (parameters) to our model. Click the
button in the
list parameters or select the New identifier menu command from the context
menu.
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Getting Started
2. Fill out the Add identifier form as it is shown in the figure below.
3. Add one more identifier: mu_star = 2*sqrt(c*m). It is a critical damping coefficient.
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Getting Started
Preparation for simulation
1. Save the model as Oscillator (use menu command File/Save as).
2. From the Object menu select the Generate equations item. New dialog box
appears. Turn on the Compile equations check box.
3. Click the Generate button.
In the case of successful generation and compiling equations of motion you will
see the following message: «Compiling successful. Object is ready for simulation». It is true. The model is really ready. Let us start its simulation.
4. Click the Close button.
Now we will come to the simulation program.
5. From the Object menu select Simulation, or simply press F9 key.
The simulation programs starts and opens the current model.
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Getting Started
2.4. Simulation of the motion
Let us consider some particular cases of oscillations: free damped oscillations
and forced oscillations without damping.
2.4.1.
Free oscillations
Free damped oscillations
1. Open new animation window (Tools/Animation window).
Open new graphical window, where we will plot time history of the vertical position of the brick.
2. Open new graphical window (Tools/Graphical window).
3. Open Wizard of variables (Tools/Wizard of variables).
4. Select the Linear var. (linear variables) tab, select Brick in the list of bodies,
button to create
set Type to r (coordinate), set Component to Z. Click the
new variable. The variable appears in the container of variables. Drag the variable to the graphical window. Close the Wizard of variables.
5. From menu Analysis select Simulation. The Object simulation inspector appears.
6. Arrange windows on the desktop as you prefer, for example, as it is shown in
the Fig. 2.7.
Figure 2.7. Desktop of the simulation program
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7. Select the Object simulation inspector and click the Identifiers tab.
8. Set a to 0 and press Enter. So we set zero amplitude of the oscillations of the
Top body, in other words we fix the Top in order to analyze free oscillations.
Figure 2.8. Parameters of the model
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9. Select the Initial conditions tab. In the Coordinate/1.1 input 0.1. We need to
shift the brick a bit because its position at zero coordinate is quite near to its
equilibrium position that gives us small amplitude of oscillations if we do not
shift the body.
10. Select the Solver tab. Set Simulation time to 25 (seconds).
11. Run simulation clicking the Integration button.
Process of the numerical simulation starts for 25 seconds period. You can see
oscillations of the Brick in the animation window and time history of the vertical
position of the brick.
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12. Click the 100% button in the drop-down tool panel in the top or click the Show
all menu command in the context menu, see the Fig. 2.9. Plot now fits the window.
Figure 2.9. Graphical windows after the first experiment
Free oscillations without damping
Now we will turn off damping and compare plots for damped and free oscillations. Using zero damping coefficient gives us free oscillations.
1. Select the graphical window. Point to the r:z(Brick) variable in the list of variables. Open context menu. Select the Copy as static variable menu command.
The second variable appears.
2. Select the Pause inspector and click the Interrupt button. Object simulation
inspector appears.
Note.
The r:z(Brick) variable, which we dragged from the Wizard of variables, will be recalculated for every numerical experiment. It is
so-called dynamic variable. In order to compare plots for different experiments we need to copy dynamic variables as static ones. Static variables are not changed from one experiment to another.
3. Select the Object simulation inspector and point to the Identifiers tab.
4. Set mu = 0 and press Enter. So we have just turned off damping.
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5. Click the Integration button.
It will take you some seconds to finish the simulation. In the Fig. 2.10 you can
see the graphical window after two numerical experiments.
Figure 2.10. Graphical windows after the first experiment
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Free oscillation: critical damping
As we showed above critical damping coefficient is mu = 100 Ns/m. Let us
analyze the motion of the Brick in such a case.
1. Point to the graphical windows. Select the first variable r:z(Brick) and copy it
as a static one again (use the Copy as static variable item from the context
menu).
2. Select the Pause inspector and click the Interrupt button. Object simulation
inspector appears.
3. Select the Identifiers tab and set mu = 100.
4. Click Integration.
Now you can see that the motion of the brick is non-periodic, see the Fig. 2.11.
Figure 2.11. Graphical window after three numerical experiments
5. Make numerical experiments for other values of the damping coefficient. Do
not forget to copy variables as static ones.
6. If you changed the value of the damping coefficient, set it again to
mu = 100 Ns/m.
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Statistical analysis
Now we will come through some additional tools for analysis of results of the
simulation.
1. From the Tools menu select Statistics. New Statistics window appears.
2. Drag the variable, which corresponds to free oscillations, from the graphical
window to the Statistics window.
3. Select the Statistics window and point to Power spectral density.
The characteristic shape of the power spectral density shows the process has the
only frequency, which corresponds to natural frequency. We have the accurate analytical solution – 5 rad/s. Not let us obtain this frequency numerically from the plot
of the power spectral density, see the Fig. 2.12. It is approximately 0.78 Hz, see
abscissa in the left bottom corner, 0.78 Hz gives us 0.78·2π = 4.9 rad/s. You can
see that numerically obtained values are quite close to analytical one.
Note.
To pick the frequency more precisely use changing scale of the window
as it is shown in the Fig. 2.12.
Figure 2.12. Power spectral density of the free oscillations
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Linear analysis
Let us consider an example of using the Linear analysis. With the help of this
tool we will find the equilibrium position of the system, its natural frequencies and
forms, define how much the actual damping ration relative to the critical one.
Well, at first you need to close Pause and Object simulation inspector windows.
1. Select the Pause windows and click Interrupt.
2. Select the Object simulation inspector and click Close.
Open Linear analysis window
3. From the Analysis menu select Linear analysis. The Linear analysis window
appears.
Equilibrium position
4. In the Linear analysis window select the Equilibrium tab. Click the Compute
button. You can see the message “Equilibrium position is successfully computed!”. The brick in the animation window is now in its equilibrium position.
Note.
Obtained coordinates, which correspond to equilibrium position, you
can save to a file. To do it use the button in the Initial conditions tab.
This file with initial conditions you can loaded using Object simulation
inspector in order to start simulation form the equilibrium position if
necessary.
Natural frequencies and forms
5. Select the Frequencies tab. In the left list you can see the natural frequencies of
the system. As you can see our system has only one frequency and this frequency is 0.795775 Hz, what corresponds to 5.0000 rad/s.
6. Click the Show button to start animation of the natural forms. Adjust appropriate Amplitude and Rate. Click the Stop button to finish animation.
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Stability
Let us find the roots of the linearized system. It gives us the information about
stability of the model.
7. Set the Compute to Eigenvalues. You can see that the real parts of roots are
negative, therefore system is stable.
Damping ratio
8. Let us describe the damping ratio of the system. Click the right mouse button
on the list of eigenvalues and from the context menu select the Frequency + damping ratio menu command. We have Beta = 100 %, that corresponds to critical damping.
Note.
Damping ration shows us if all forms are damped properly and thus
change damping coefficients or geometry of attachment point of dampers if necessary.
9. You can change the value of the mu identifier in the Identifiers tab and see
what will happen with damping ratio.
10. Close the Linear analysis window.
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Forces oscillations
Let us consider simulation of forced oscillations without damping.
1. Delete all variables from the graphical window except the first (dynamic) one.
2. From the Analysis menu select Simulation.
3. Select the Identifiers tab. Set the following values: a = 0.05, omega = 8,
mu = 0.
4. Run integration. Now you can see that the body Top also moves. The time history of the vertical position of the Brick is given in the Fig. 2.13.
Figure 2.13. Forced oscillation (omega = 8 rad/s)
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Resonance
In conclusion we consider the resonance case, when the excitation frequency is
equal to the natural frequency of the system.
1. In the Pause window click the Interrupt button.
2. In the Object simulation inspector set omega = 5.
3. Run integration. As we expected in the resonance case the amplitude of the oscillations increases in the long run, see the Fig. 2.14.
Figure 2.14. Forced oscillations: resonance case
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3. Subsequent studying Universal Mechanism
You have come through two examples of dynamical systems (pendulum and
sprung body) and have seen the basic tools and features of the UM Base version.
The Getting Started series includes other manuals that devoted to the rest modules of the Universal Mechanism. Here they are:
• Getting Started: simulation of road vehicles;
• Getting Started: railway vehicle dynamics;
• Getting Started: Matlab/Simulink interface;
• Getting Started: optimization module;
• Getting Started: elastic bodies with UM FEM.
Library of simple models: how to…
The Part 7 of the UM User’s Manual is devoted to consideration of simple
models that show you how to create/model various graphical elements, joints and
force elements. Studying these examples helps you familiarize yourself with basics
of Universal Mechanism and approaches for simulation of objects of different
kind. The library of models is in the {um_root}\library directory.
The part7.pdf you can find in the {um_root}\manual directory or download
using the following link: http://www.umlab.ru/download/docs/eng/part7.pdf.
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