Difference between revisions of "OpenModelica/C2/Control-flow-and-Event-handling/English"

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Line 14: Line 14:
 
|| In this tutorial, we are going to learn:
 
|| In this tutorial, we are going to learn:
  
−
How to use '''if-else''' statement
+
* How to use '''if-else''' statement
  
−
How to handle '''time''' and '''state''' events
+
* How to handle '''time''' and '''state''' events
  
−
How to use '''when''' statement.
+
* How to use '''when''' statement.
  
 
|-
 
|-
Line 28: Line 28:
 
'''OpenModelica 1.9.2''' and
 
'''OpenModelica 1.9.2''' and
  
−
'''Ubuntu Operating system version 14.04'''
+
'''Ubuntu Operating''' system version 14.04.
 
   
 
   
−
But, this process is identical in Windows, '''Mac OS X''' or '''FOSSEE OS on ARM'''.
+
But, this process is identical in Windows, '''Mac OS X''' or '''FOSSEE OS''' on '''ARM'''.
  
 
|-
 
|-
Line 58: Line 58:
 
Free fall under gravity starts at time '''t = 0.5''' seconds
 
Free fall under gravity starts at time '''t = 0.5''' seconds
  
−
I have created a model named '''‘timeEventExample’''' which simulates this condition.  
+
I have created a model named '''timeEventExample''' which simulates this condition.  
  
 
|-
 
|-
−
||  
+
||Point to '''timeEventExample''' file.
−
||'''‘timeEventExample’''' file is available in '''Code Files''' link on spoken tutorial website.  
+
||'''timeEventExample''' file is available in '''Code Files''' link on spoken tutorial website.  
  
 
Please download and save all the files in Code Files link.  
 
Please download and save all the files in Code Files link.  
  
−
For your convenience, '''‘freeFall’''' class is also made available.
+
For your convenience, '''freeFall''' class is also made available.
−
 
+
  
 
|-
 
|-
Line 76: Line 75:
  
 
|-
 
|-
−
|| Click on Dash Home
+
|| Click on '''Dash Home'''
 
|| To open '''OMEdit''' on Ubuntu Operating System, click on '''Dash Home''' icon at the top left.
 
|| To open '''OMEdit''' on Ubuntu Operating System, click on '''Dash Home''' icon at the top left.
  
 
|-
 
|-
−
||  
+
|| Type '''OMEdit''' in the Search bar, click on '''OMEdit''' icon.
−
|| Type '''OMEdit''' in the Search bar and click on '''‘OMEdit’''' icon. Let me go back.
+
|| Type '''OMEdit''' in the Search bar and click on '''OMEdit''' icon. Let me go back.
  
 
|-
 
|-
Line 88: Line 87:
  
 
|-
 
|-
−
|| Click on '''‘Open Model/Library File’''' in tool bar.  
+
|| Click on '''Open Model/Library File''' in tool bar.  
−
|| To open '''‘timeEventExample’''' file, Click on the tool named '''‘Open Model/Library File’'''.
+
|| To open '''timeEventExample''' file, Click on the tool named '''Open Model/Library File'''.
  
 
|-
 
|-
−
|| Click on '''‘Open’'''
+
|| Click on '''Open'''.
−
|| Locate '''‘timeEventExample’''' file in your system and click on '''Open'''.
+
|| Locate '''timeEventExample''' file in your system and click on '''Open'''.
  
 
|-
 
|-
 
||  
 
||  
−
||You may alternatively use the '''file''' menu to open this file.  
+
||You may alternatively use the '''File''' menu to open this file.  
  
 
Note that '''timeEventExample''' has appeared in libraries browser.  
 
Note that '''timeEventExample''' has appeared in libraries browser.  
Line 115: Line 114:
  
 
'''model timeEventExample '''
 
'''model timeEventExample '''
−
|| The Name of this model is '''‘timeEventExample’'''.
+
|| The Name of this model is '''timeEventExample'''.
  
 
|-
 
|-
−
|| “'''Time event demonstration”'''
+
|| '''Time event demonstration'''
 
|| Comments such as this may be written on the first line of a model or class.
 
|| Comments such as this may be written on the first line of a model or class.
  
 
|-
 
|-
−
|| '''Real h(unit = “m”) “Height of ball”;'''
+
|| '''Real h(unit = “m”), "Height of ball";'''
  
 
'''Real v(unit = “m/s”) “Velocity of ball”;'''
 
'''Real v(unit = “m/s”) “Velocity of ball”;'''
  
 
'''parameter Real g(unit = “m/s2”) = -9.81 “Acceleration due to gravity”;'''  
 
'''parameter Real g(unit = “m/s2”) = -9.81 “Acceleration due to gravity”;'''  
−
|| '''‘h’, ‘v’''' and '''‘g’''' represent the same quantities as in ‘freeFall’ class.
+
|| '''h, v''' and '''g''' represent the same quantities as in '''freeFall''' class.
 
|-
 
|-
 
|| '''initial equation'''
 
|| '''initial equation'''
−
|| '''‘initial equation’''' section contains initial conditions.  
+
|| '''initial equation''' section contains initial conditions.  
  
 
|-
 
|-
 
|| '''h = 30;'''
 
|| '''h = 30;'''
−
|| Value of '''h''' at time '''t = 0''' is '''30''' m
+
|| Value of '''h''' at time '''t = 0''' is '''30m'''.
  
 
|-
 
|-
 
|| '''v = 0;'''
 
|| '''v = 0;'''
 
|| Value of '''v''' at time '''t = 0''' is '''0'''.
 
|| Value of '''v''' at time '''t = 0''' is '''0'''.
−
 
  
 
|-
 
|-
Line 151: Line 149:
 
|-
 
|-
 
||  
 
||  
−
|| Note that there is an '''‘if-else’''' block in equation section as shown.
+
|| Note that there is an '''if-else''' block in equation section as shown.
  
−
Let us try to understand each statement of this '''‘if-else’''' block.  
+
Let us try to understand each statement of this '''if-else''' block.  
  
 
|-
 
|-
 
|| '''if time >= 0.5 then'''
 
|| '''if time >= 0.5 then'''
−
|| '''if''' statement places a constraint on '''‘time’''' variable.
+
|| '''if''' statement places a constraint on '''time''' variable.
  
−
'''‘time’''' is a built-in variable in Modelica.  
+
'''time''' is a built-in variable in Modelica.  
  
 
It need not be explicitly declared in a class or model.
 
It need not be explicitly declared in a class or model.
Line 169: Line 167:
 
|| The ball experiences free fall after time '''0.5''' seconds.
 
|| The ball experiences free fall after time '''0.5''' seconds.
  
−
Hence, these two equations represent the same equations as those in '''‘freeFall’''' class.
+
Hence, these two equations represent the same equations as those in '''freeFall''' class.
  
 
|-
 
|-
 
|| '''else'''
 
|| '''else'''
−
|| '''‘else’''' branch contains equations which are executed when time is less than '''0.5''' seconds.
+
|| '''else''' branch contains equations which are executed when time is less than '''0.5''' seconds.
  
 
|-
 
|-
 
|| '''der(h) = v;'''
 
|| '''der(h) = v;'''
  
−
'''der(v) = 0; '''
+
'''der(v) = 0;'''
−
|| The ball is at rest when time is less than '''0.5''' seconds. Hence, these two equations hold good in that case.  
+
|| The ball is at rest when time is less than '''0.5''' seconds.
 +
 
 +
Hence, these two equations hold good in that case.  
  
 
|-
 
|-
 
|| '''end if;'''
 
|| '''end if;'''
−
|| '''‘end if’''' signifies the end of '''‘if-else’''' block
+
|| '''end if''' signifies the end of '''if-else''' block.
  
 
|-
 
|-
Line 195: Line 195:
 
|| '''Plotting perspective''' has now opened.
 
|| '''Plotting perspective''' has now opened.
  
−
Select '''‘h’''' in the '''‘variables browser’'''.
+
Select '''h''' in the '''variables browser'''.
  
 
|-
 
|-
 
||  
 
||  
−
|| This generates '''‘h’''' vs '''‘time’''' plot.  
+
|| This generates '''h''' versus '''time''' plot.
 +
 
 +
Since the ball is at rest until time '''t=0.5''' seconds.
  
−
Since the ball is at rest until time t='''0.5''' seconds. Height remains the same.
+
Height remains the same.
  
 
|-
 
|-
Line 208: Line 210:
  
 
|-
 
|-
−
|| Click on '''Modeling perspective''' button
+
|| Click on '''Modeling perspective''' button.
 
|| To go back to '''Modeling perspective'''.
 
|| To go back to '''Modeling perspective'''.
  
Line 223: Line 225:
  
 
end if;
 
end if;
−
|| Now, let me delete the second equation in '''‘else’''' branch.  
+
|| Now, let me delete the second equation in '''else''' branch.  
  
 
Click on '''Save''' button in the toolbar to save this model.
 
Click on '''Save''' button in the toolbar to save this model.
  
 
|-
 
|-
−
|| Click on '''simulate''' button
+
|| Click on '''simulate''' button.
−
|| Click on '''‘Simulate’'''
+
|| Click on '''Simulate'''.
  
 
|-
 
|-
 
|| Hover the mouse over '''Messages browser''.
 
|| Hover the mouse over '''Messages browser''.
−
|| The simulation has failed. Go to the '''Messages browser''' and scroll up to see the error message.
+
|| The simulation has failed.
 +
 
 +
Go to the '''Messages browser''' and scroll up to see the error message.
  
 
The error message says that the number of equations in each branch should be the same.
 
The error message says that the number of equations in each branch should be the same.
  
−
Therefore, the number of equations in '''‘if’''' and '''‘else’''' branches should each be equal to the number of variables.
+
Therefore, the number of equations in '''if''' and '''else''' branches should each be equal to the number of variables.
  
 
|-
 
|-
Line 245: Line 249:
 
|-
 
|-
 
||  
 
||  
−
|| Note that '''der(h) = v''' is an equation which is commons to both '''‘if’''' and '''‘else’''' branches.
+
|| Note that '''der(h) = v''' is an equation which is commons to both '''if''' and '''else''' branches.  
−
 
+
−
Hence, the '''‘if-else’''' block can be replaced by two statements.
+
  
 +
Hence, the '''if-else''' block can be replaced by two statements.
  
 
|-
 
|-
 +
||
 
|| Let me show you how it is done.
 
|| Let me show you how it is done.
  
−
delete the '''‘if-else’''' block first.
+
Delete the '''if-else''' block first.
  
−
The equations to be typed here have been provided in a file named '''‘if else-statement.txt’'''.  
+
The equations to be typed here have been provided in a file named '''if else-statement.txt'''.  
 
   
 
   
 
|-
 
|-
 
||  
 
||  
−
|| These file is also available in '''‘Code Files’''' link.
+
|| These file is also available in '''Code Files''' link.
  
 
|-
 
|-
Line 267: Line 271:
 
|-
 
|-
 
|| /* gedit */
 
|| /* gedit */
−
|| Let me go to '''‘gedit’'''.
+
|| Let me go to '''gedit'''.
  
−
Windows users may open this file using '''‘Notepad’''' or any other text editor.
+
Windows users may open this file using '''Notepad''' or any other text editor.
  
 
|-
 
|-
Line 279: Line 283:
 
|-
 
|-
 
|| /* timeEventExample */
 
|| /* timeEventExample */
−
|| Go back to '''OMEdit'''. Do a right click and select '''Paste'''
+
|| Go back to '''OMEdit'''.
 +
 
 +
Do a right click and select '''Paste'''
  
 
|-
 
|-
−
||  
+
|| Click on '''Save''' button in the toolbar.
 
|| Click on '''Save''' button in the toolbar once again.
 
|| Click on '''Save''' button in the toolbar once again.
  
Line 294: Line 300:
 
|| /* Plotting perspective */
 
|| /* Plotting perspective */
  
−
Select '''‘h’''' in the '''‘variables browser’'''
+
Select '''h''' in the '''variables browser'''
  
−
||Select '''‘h’''' in the '''‘variables browser’''' once again.  
+
||Select '''h''' in the '''variables browser''' once again.  
  
−
Note that the '''‘h’''' vs '''‘time’''' plot is the same as in previous case which indicates that both models are the same.
+
Note that the '''h''' versus '''time''' plot is the same as in previous case which indicates that both models are the same.
  
 
To delete the result right click on '''timeEventExample''' and select '''Delete Result'''.
 
To delete the result right click on '''timeEventExample''' and select '''Delete Result'''.
Line 311: Line 317:
  
 
|-
 
|-
−
|| Slide:
+
||
 +
|| Now let me go back to the slides.
 +
|-
 +
||Slide:
  
−
|-
+
Caution
−
|| Now let me go back to the slides.
+
||Please note that the number of equations in '''if''' branch should be equal to the Number of equations in '''else''' branch.  
  
−
Please note that the number of equations in '''‘if’''' branch should be equal to the  Number of equations in '''‘else’''' branch and both of them should be equal to the Number of variables in the class or model.
+
And both of them should be equal to the Number of variables in the class or model.
  
 
|-
 
|-
Line 328: Line 337:
 
In a '''time event''', the time at which event occurs is precisely known.
 
In a '''time event''', the time at which event occurs is precisely known.
  
−
'''‘timeEventExample’''' handles a time event at '''t = 0.5''' seconds
+
'''timeEventExample''' handles a time event at '''t = 0.5''' seconds
  
 
'''state event''' occurs when a system variable cross a certain value.
 
'''state event''' occurs when a system variable cross a certain value.
−
 
  
 
|-
 
|-
Line 347: Line 355:
 
Magnitude of velocity changes if the collision is inelastic
 
Magnitude of velocity changes if the collision is inelastic
  
−
I have created a model named '''‘bouncingBall’''' to simulate this behavior.
+
I have created a model named '''bouncingBall''' to simulate this behavior.
  
 
This file is available on Spoken Tutorial Website.
 
This file is available on Spoken Tutorial Website.
Line 355: Line 363:
 
|| To view this model and simulate it, Let me go back to '''OMEdit'''.  
 
|| To view this model and simulate it, Let me go back to '''OMEdit'''.  
  
 +
|-
 +
|| Click on '''bouncingBall''' tab at the top of modeling area
 +
|| Click on '''OpenModel/LIbrary File''' tool.
  
 +
Locate '''bouncingBall''' file in your system and click on '''open'''.
  
 
|-
 
|-
−
|| Click on '''‘bouncingBall’''' tab at the top of modeling area
+
|| Right-click on '''bouncingBall'''
−
|| Click on '''‘OpenModel/LIbrary File’''' tool. Locate '''‘bouncingBall’''' file in your system and click on '''open'''.
+
|| Right-click on '''bouncingBall''' icon in libraries browser and select '''View Class'''.  
−
 
+
−
|-
+
−
|| Right-click on '''‘bouncingBall’'''
+
−
|| Right-click on '''‘bouncingBall’''' icon in libraries browser and select '''‘View Class’'''.  
+
  
−
If the model does not open in '''‘Text View’''' go to the top of modeling area and click on '''‘Text View’''' button.
+
If the model does not open in '''Text View''' go to the top of modeling area and click on '''Text View''' button.
  
 
|-
 
|-
Line 371: Line 379:
  
 
'''parameter Real e = 0.8 "Coefficient of restitution";'''  
 
'''parameter Real e = 0.8 "Coefficient of restitution";'''  
−
|| '''‘e’''' is coefficient of restitution for inelastic collision of ball with Earth.
+
|| '''e''' is coefficient of restitution for inelastic collision of ball with Earth.
  
 
It is a parameter and its value remains constant at 0.8 through out the simulation run.
 
It is a parameter and its value remains constant at 0.8 through out the simulation run.
  
 
|-
 
|-
−
|| '''parameter Real radius = 0.001 "Radius of the ball in “m'";'''
+
|| '''parameter Real radius = 0.001 "Radius of the ball in "m";'''
−
|| ‘radius’ represents the radius of ball in '''‘m’'''.
+
|| radius represents the radius of ball in '''m'''.
 
|-
 
|-
 
|| '''Real h(unit = “m”);'''
 
|| '''Real h(unit = “m”);'''
Line 384: Line 392:
  
 
'''parameter Real g(unit = “m/s2”) = -9.81;'''  
 
'''parameter Real g(unit = “m/s2”) = -9.81;'''  
−
|| '''‘h’, ‘v’''' and '''‘g’''' represent the same quantities as in '''timeEventExample'''
+
|| '''h, v''' and '''g''' represent the same quantities as in '''timeEventExample'''.
  
 
|-
 
|-
Line 390: Line 398:
  
 
'''der(v) = g; '''
 
'''der(v) = g; '''
−
|| These two equations of motions have already been discussed in '''‘freeFall’''' class.
+
|| These two equations of motions have already been discussed in '''freeFall''' class.
  
 
|-
 
|-
 
|| '''when h <= radius then'''
 
|| '''when h <= radius then'''
−
|| '''‘when’''' statement is used to signal an event.  
+
|| '''when''' statement is used to signal an event.  
  
 
It is used to specify the action to be taken when an event occurs.  
 
It is used to specify the action to be taken when an event occurs.  
Line 402: Line 410:
 
This statement is executed whenever the event occurs.
 
This statement is executed whenever the event occurs.
 
|-
 
|-
−
|| '''reinit(v, -e * pre(v)); '''
+
|| '''reinit(v, -e * pre(v));'''
−
|| '''‘reinit()’''' function is used to re-initialize a variable.  
+
|| '''reinit()''' function is used to re-initialize a variable.  
  
 
|-
 
|-
−
|| '''reinit(v, -e * pre(v)); '''
+
|| '''reinit(v, -e * pre(v));'''
 
|| Here, it re-initializes velocity of ball with the negative product of '''e''' and velocity of ball before touching the ground.
 
|| Here, it re-initializes velocity of ball with the negative product of '''e''' and velocity of ball before touching the ground.
  
Line 412: Line 420:
 
|| /* Simulation Options */
 
|| /* Simulation Options */
  
−
Click on '''‘Simulation Options’''' icon
+
Click on '''Simulation Options''' icon
 
|| To simulate this model
 
|| To simulate this model
  
−
Click on '''‘Simulation Setup’''' button in the toolbar.
+
Click on '''Simulation Setup''' button in the toolbar.
  
−
Under '''General''' tab, change '''‘Stop Time’''' field to 30 units and  
+
Under '''General''' tab, change '''Stop Time''' field to 30 units and  
  
−
Click on '''‘Simulate’'''.
+
Click on '''Simulate'''.
  
 
Close the pop up window.
 
Close the pop up window.
Line 425: Line 433:
 
|| /* Plotting perspective */
 
|| /* Plotting perspective */
  
−
Select'''‘h’''' in '''‘variables browser’'''  
+
Select '''h''' in '''variables browser'''  
  
−
|| Select'''‘h’''' in '''‘variables browser’'''  
+
|| Select '''h''' in '''variables browser'''.
  
−
note that '''‘h’''' drops below zero which is unacceptable.
+
Note that '''h''' drops below zero which is unacceptable.
  
−
We have seen similar behavior while simulating '''‘freeFall’''' class.
+
We have seen similar behavior while simulating '''freeFall''' class.
  
 
|-
 
|-
Line 451: Line 459:
  
 
'''Event-related operators'''
 
'''Event-related operators'''
−
|| '''‘when’''' statement is used to signal an event.
+
|| '''when''' statement is used to signal an event.
  
−
'''‘reinit(a,b)’''' assigns value of '''‘b’''' to variable '''‘a’''' and resumes simulation.
+
'''reinit(a,b)''' assigns value of '''b''' to variable '''a''' and resumes simulation.
  
−
'''pre(a)''' returns the value of variable '''‘a’''' before event.
+
'''pre(a)''' returns the value of variable '''a''' before event.
  
−
For example, '''reinit(a, 10)''' assigns a value of '''10''' to '''‘a’''' when  event occurs.
+
For example, '''reinit(a, 10)''' assigns a value of '''10''' to '''a''' when  event occurs.
  
−
The faulty behavior of '''bouncingBall''' model is corrected in '''‘bouncingBallWithHysteresis’'''.
+
The faulty behavior of '''bouncingBall''' model is corrected in '''bouncingBallWithHysteresis'''.
  
 
|-
 
|-
Line 466: Line 474:
 
'''Assignment'''
 
'''Assignment'''
  
−
||''‘bouncingBallWithHysteresis’''' model is available on spoken tutorial website.
+
||''bouncingBallWithHysteresis''' model is available on spoken tutorial website.
  
−
Simulate '''‘bouncingBallWithHysteresis‘''' and generate '''‘h’''' vs '''‘time’''' plot.  
+
Simulate '''bouncingBallWithHysteresis''' and generate '''h''' versus '''time''' plot.  
  
−
Notice the difference between '''‘bouncingBall‘''' and '''‘bouncingBallWithHysteresis‘'''
+
Notice the difference between '''bouncingBall''' and '''bouncingBallWithHysteresis'''
  
 
|-
 
|-

Revision as of 15:29, 1 March 2016

Visual Cue Narration
Slide Number 1

Title Slide

Welcome to the spoken tutorial on Control flow and Event handling.
Slide:

Learning Objectives

In this tutorial, we are going to learn:
  • How to use if-else statement
  • How to handle time and state events
  • How to use when statement.
Slide:

System Requirements

To record this tutorial, I am using

OpenModelica 1.9.2 and

Ubuntu Operating system version 14.04.

But, this process is identical in Windows, Mac OS X or FOSSEE OS on ARM.

Slide:

Pre-requisites

To understand and practice this tutorial, you need to know the following:
  • equation-based modeling of physical systems
  • branching in any programming language
  • class definition in Modelica

Pre-requisite tutorials are mentioned on Spoken Tutorial website. Please go through them.

Slide:

Problem Statement

freeFall class was already discussed in previous tutorials.

Now, let us add a constraint to it.

Ball is at rest until t = 0.5 seconds.

Free fall under gravity starts at time t = 0.5 seconds

I have created a model named timeEventExample which simulates this condition.

Point to timeEventExample file. timeEventExample file is available in Code Files link on spoken tutorial website.

Please download and save all the files in Code Files link.

For your convenience, freeFall class is also made available.

Now let me go to OMEdit.

I have already launched it on my system.

Click on Dash Home To open OMEdit on Ubuntu Operating System, click on Dash Home icon at the top left.
Type OMEdit in the Search bar, click on OMEdit icon. Type OMEdit in the Search bar and click on OMEdit icon. Let me go back.
Note: Windows users may similarly search for OMEdit in start menu and open it.
Click on Open Model/Library File in tool bar. To open timeEventExample file, Click on the tool named Open Model/Library File.
Click on Open. Locate timeEventExample file in your system and click on Open.
You may alternatively use the File menu to open this file.

Note that timeEventExample has appeared in libraries browser.

Right click on this icon and select view class.

The model has now opened in Diagram view.

Click on third button which is the Text View.

We have already discussed that model and class are used synonymously in Modelica.

/* timeEventExample */

model timeEventExample

The Name of this model is timeEventExample.
Time event demonstration Comments such as this may be written on the first line of a model or class.
Real h(unit = “m”), "Height of ball";

Real v(unit = “m/s”) “Velocity of ball”;

parameter Real g(unit = “m/s2”) = -9.81 “Acceleration due to gravity”;

h, v and g represent the same quantities as in freeFall class.
initial equation initial equation section contains initial conditions.
h = 30; Value of h at time t = 0 is 30m.
v = 0; Value of v at time t = 0 is 0.
Equation signifies the beginning of equation section of this model.
Now, let us see how to insert the constraint explained in slides.
Note that there is an if-else block in equation section as shown.

Let us try to understand each statement of this if-else block.

if time >= 0.5 then if statement places a constraint on time variable.

time is a built-in variable in Modelica.

It need not be explicitly declared in a class or model.

der(h) = v;

der(v) = g;

The ball experiences free fall after time 0.5 seconds.

Hence, these two equations represent the same equations as those in freeFall class.

else else branch contains equations which are executed when time is less than 0.5 seconds.
der(h) = v;

der(v) = 0;

The ball is at rest when time is less than 0.5 seconds.

Hence, these two equations hold good in that case.

end if; end if signifies the end of if-else block.
Click on simulate button The model is now complete.

let us simulate it by clicking on simulate button.

/* Plotting perspective */ Plotting perspective has now opened.

Select h in the variables browser.

This generates h versus time plot.

Since the ball is at rest until time t=0.5 seconds.

Height remains the same.

To delete the result right click on timeEventExample in Variables browser and select Delete Result.
Click on Modeling perspective button. To go back to Modeling perspective.

Click on Modeling button at bottom right.

/* timeEventExample */

else

 der(h) = v;

 Delete // der(v) = 0;

end if;

Now, let me delete the second equation in else branch.

Click on Save button in the toolbar to save this model.

Click on simulate button. Click on Simulate.
Hover the mouse over 'Messages browser. The simulation has failed.

Go to the Messages browser and scroll up to see the error message.

The error message says that the number of equations in each branch should be the same.

Therefore, the number of equations in if and else branches should each be equal to the number of variables.

Let me insert the equation back in its place and click on save button.
Note that der(h) = v is an equation which is commons to both if and else branches.

Hence, the if-else block can be replaced by two statements.

Let me show you how it is done.

Delete the if-else block first.

The equations to be typed here have been provided in a file named if else-statement.txt.

These file is also available in Code Files link.
Since it is a text file I have used gedit to open it.
/* gedit */ Let me go to gedit.

Windows users may open this file using Notepad or any other text editor.

der(h) = v;

der(v) = if time >= 0.5 then g else 0;

Copy the two equations.
/* timeEventExample */ Go back to OMEdit.

Do a right click and select Paste

Click on Save button in the toolbar. Click on Save button in the toolbar once again.
Click on simulate button Click on Simulate.

Close the pop up window.

/* Plotting perspective */

Select h in the variables browser

Select h in the variables browser once again.

Note that the h versus time plot is the same as in previous case which indicates that both models are the same.

To delete the result right click on timeEventExample and select Delete Result.

/* Switch to Modeling perspective */ Click on Modeling button at bottom right.
der(v) = if time >= 0.5 then g else 0; This is an if-else statement which indicates that if time >= 0.5 dv/dt = g otherwise dv/dt = 0.
Now let me go back to the slides.
Slide:

Caution

Please note that the number of equations in if branch should be equal to the Number of equations in else branch.

And both of them should be equal to the Number of variables in the class or model.

Slide:

Event

Event is an abrupt change in behavior of a system.

Events may be categorized into time events and state events.

In a time event, the time at which event occurs is precisely known.

timeEventExample handles a time event at t = 0.5 seconds

state event occurs when a system variable cross a certain value.

Slide:

Problem Statement

Let us try to understand state events using an example.

A ball under free fall faces a state event on touching the ground.

The following behavior is observed when it hits the ground.

Velocity of ball reverts in direction.

Magnitude of velocity changes if the collision is inelastic

I have created a model named bouncingBall to simulate this behavior.

This file is available on Spoken Tutorial Website.

/* Switch to OMEdit */ To view this model and simulate it, Let me go back to OMEdit.
Click on bouncingBall tab at the top of modeling area Click on OpenModel/LIbrary File tool.

Locate bouncingBall file in your system and click on open.

Right-click on bouncingBall Right-click on bouncingBall icon in libraries browser and select View Class.

If the model does not open in Text View go to the top of modeling area and click on Text View button.

/* bouncingBall */

parameter Real e = 0.8 "Coefficient of restitution";

e is coefficient of restitution for inelastic collision of ball with Earth.

It is a parameter and its value remains constant at 0.8 through out the simulation run.

parameter Real radius = 0.001 "Radius of the ball in "m"; radius represents the radius of ball in m.
Real h(unit = “m”);

Real v(unit = “m”);

parameter Real g(unit = “m/s2”) = -9.81;

h, v and g represent the same quantities as in timeEventExample.
der(h) = v;

der(v) = g;

These two equations of motions have already been discussed in freeFall class.
when h <= radius then when statement is used to signal an event.

It is used to specify the action to be taken when an event occurs.

h <= radius holds true when the ball is in contact with ground.

This statement is executed whenever the event occurs.

reinit(v, -e * pre(v)); reinit() function is used to re-initialize a variable.
reinit(v, -e * pre(v)); Here, it re-initializes velocity of ball with the negative product of e and velocity of ball before touching the ground.
/* Simulation Options */

Click on Simulation Options icon

To simulate this model

Click on Simulation Setup button in the toolbar.

Under General tab, change Stop Time field to 30 units and

Click on Simulate.

Close the pop up window.

/* Plotting perspective */

Select h in variables browser

Select h in variables browser.

Note that h drops below zero which is unacceptable.

We have seen similar behavior while simulating freeFall class.

This faulty behavior is due to numerical errors.

A discussion of numerical errors is beyond the scope of this tutorial.

Please refer to Zeno behavior of bouncing ball for a better understanding.

Let me deselect h

Let us go back to the slides
Slide:

Event-related operators

when statement is used to signal an event.

reinit(a,b) assigns value of b to variable a and resumes simulation.

pre(a) returns the value of variable a before event.

For example, reinit(a, 10) assigns a value of 10 to a when event occurs.

The faulty behavior of bouncingBall model is corrected in bouncingBallWithHysteresis.

Slide:

Assignment

bouncingBallWithHysteresis' model is available on spoken tutorial website.

Simulate bouncingBallWithHysteresis and generate h versus time plot.

Notice the difference between bouncingBall and bouncingBallWithHysteresis

This brings us to the end of this tutorial.
Slide:

About the Spoken Tutorial Project

Watch the video available at following link.

http://spoken-tutorial.org/ http://spoken-tutorial.org] /What\_is\_a\_Spoken\_Tutorial

It summarises the Spoken Tutorial project

Slide:

Spoken Tutorial Workshops

We conduct workshops using spoken tutorials

Give certificates

Please contact us.

Slide:

Forum

If you have any questions related to this spoken tutorial, please visit the following website.
Slide:

Textbook Companion Project

Under Textbook Companion Project we coordinate coding of solved examples of popular books.

We give honorarium and certificates to those who do this.

For more details please visit the following website.

Slide:

Lab Migration Project

Under Lab Migration Project we help migrate commercial simulator labs to OpenModelica.

For more information please visit the website.

Slide:

Acknowledgements

Spoken Tutorial Project is supported by NMEICT, MHRD, Government of India.
Slide:

Thanks

We thank the development team of OpenModelica for their support.
Thank you for joining me in this tutorial. Goodbye.

Contributors and Content Editors

Kaushik Datta, Nancyvarkey

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