View source for OpenModelica/C2/Control-flow-and-Event-handling/English-timed

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||''' Time '''
||'''Narration'''

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||00:01
|| Welcome to the '''spoken tutorial''' on '''Control flow and Event handling'''.

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|| 00:07
|| 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.

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|| 00:19
|| To record this tutorial, I am using : '''OpenModelica 1.9.2''' and '''Ubuntu Operating system''' version '''14.04'''.
 
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|| 00:30
||But, this process is identical in '''Windows, Mac OS X''' or '''FOSSEE OS''' on '''ARM'''.

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|| 00:38
|| To understand and practice this tutorial, you need to know the following: equation-based '''modeling''' of physical systems ,  

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|| 00:47
||branching in any programming language , '''class''' definition in '''Modelica'''.

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|| 00:53
||Pre-requisite tutorials are mentioned on '''Spoken Tutorial''' website. Please go through them.

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|| 01:00
||'''freeFall''' class was already discussed in previous tutorials. 

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|| 01:05
||Now, let us add a constraint to it. 

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|| 01:08
||Ball is at rest until '''t = 0.5''' seconds.  Free fall under gravity starts at time '''t = 0.5''' seconds.

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|| 01:19
||I have created a '''model''' named '''timeEventExample''' which simulates this condition. 

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||01:25
||'''timeEventExample''' file is available in '''Code Files''' link on '''spoken tutorial''' website. 

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|| 01:32
||Please download and '''save''' all the files in '''Code Files''' link. 

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|| 01:38
||For your convenience, '''freeFall class''' is also made available.

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|| 01:43
|| Now let me go to '''OMEdit'''.

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|| 01:45
||I have already launched it on my system. 

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|| 01:49
|| To open '''OMEdit''' on '''Ubuntu Operating System''', click on '''Dash Home''' icon at the top left.

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|| 01:58
|| Type: '''OMEdit''' in the '''Search bar''' and click on '''OMEdit''' icon. Let me go back.

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|| 02:06
||Note: '''Windows''' users may similarly search for '''OMEdit''' in '''Start''' menu and open it.

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|| 02:13
|| To open '''timeEventExample''' file, click on the tool named '''Open Model/Library File'''.

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|| 02:21
|| Locate '''timeEventExample''' file in your system and click on '''Open'''.

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|| 02:28
||You may alternatively use the '''File''' menu to open this file. 

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|| 02:34
||Note that '''timeEventExample''' has appeared in '''Libraries browser'''. 

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||02:39
||Right-click on this icon and select '''View class'''. 

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|| 02:44
||The model has now opened in '''Diagram view'''. 

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||02:47
||Click on third button which is the '''Text View'''. 

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|| 02:51
||We have already discussed that '''model''' and '''class''' are used synonymously in '''Modelica'''. 

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|| 02:58
|| The name of this '''model''' is '''timeEventExample'''.

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|| 03:02
|| '''Comments''' such as this may be written on the first line of a '''model''' or '''class'''.

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|| 03:08
|| '''h, v''' and '''g''' represent the same quantities as in '''freeFall class'''.

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|| 03:16
|| '''initial equation''' section contains initial conditions. 

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|| 03:21
|| Value of '''h''' at time '''t = 0''' is '''30m'''. 

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|| 03:26
|| Value of '''v''' at time '''t = 0''' is '''0'''.

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|| 03:31
|| '''Equation''' signifies the beginning of equation section of this '''model'''.

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|| 03:36
|| Now, let us see how to insert the constraint explained in slides.

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|| 03:43
|| Note that there is an ''''if-else'block'''  in '''equation''' section as shown.

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|| 03:48
||Let us try to understand each statement of this ''''if-else' block'''. 

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|| 03:53
|| ''''if' statement''' places a constraint on '''time''' variable.

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|| 03:58
||'''time''' is a built-in variable in '''Modelica'''. 

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|| 04:01
||It need not be explicitly declared in a '''class''' or '''model'''.

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|| 04:06
|| The ball experiences free fall after time '''0.5''' seconds.

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|| 04:12
||Hence, these two equations represent the same equations as those in '''freeFall class'''.

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|| 04:19
|| ''''else' branch''' contains equations which are executed when time is less than '''0.5''' seconds.

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|| 04:27
|| The ball is at rest when time is less than '''0.5''' seconds.

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|| 04:32
||Hence, these two '''equations''' hold good in that case. 

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|| 04:36
|| '''end if''' signifies the end of '''if-else block'''.

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|| 04:41
|| The '''model''' is now complete.

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|| 04:44
||Let us simulate it by clicking on '''Simulate''' button.

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||04:48
|| '''Plotting perspective''' has now opened.

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|| 04:51
||Select '''h''' in the '''Variables browser'''.

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|| 04:55
|| This generates '''h''' versus '''time''' plot. 

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|| 04:58
||Since the ball is at rest until time '''t=0.5''' seconds, height remains the same.

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|| 05:06
||To delete the result, right click on '''timeEventExample''' in '''Variables browser''' and select '''Delete Result'''.

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|| 05:15
|| To go back to '''Modeling perspective''', click on '''Modeling''' button at bottom right.

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|| 05:21
|| Now, let me delete the second '''equation''' in '''else''' branch. 

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|| 05:26
||Click on '''Save''' button in the toolbar, to save this '''model'''.

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|| 05:31
|| Click on '''Simulate'''.

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|| 05:34
|| The simulation has failed.

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||05:37
||Go to the '''Messages browser''' and scroll up to see the '''error message'''.

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|| 05:42
||The error message says that the number of '''equations''' in each '''branch''' should be the same.

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|| 05:49
||Therefore, the number of '''equations''' in '''if''' and '''else''' branches should each be equal to the number of variables.

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|| 05:58
||Let me insert the '''equation''' back in its place and click on '''Save''' button.

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|| 06:05
|| Note that '''der(h) = v''' is an equation which is common to both '''if''' and '''else branches'''. 

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|| 06:14
||Hence, the '''if-else''' block can be replaced by two '''statements'''.

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||06:20
|| Let me show you how it is done.

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|| 06:23
||Delete the '''if-else''' block first.

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||06:26
||The equations to be typed here have been provided in a file named '''if else-statement.txt'''. 
 
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|| 06:34
|| These file is also available in '''Code Files''' link.

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|| 06:39
|| Since it is a text file, I have used '''gedit''' to open it.

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|| 06:45
|| Let me go to '''gedit'''.

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|| 06:47
||'''Windows''' users may open this file using '''Notepad''' or any other text editor.

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||  06:54
|| '''Copy''' the two equations.

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||  06:57
|| Go back to '''OMEdit'''.

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|| 07:00
||Do a right-click and select '''Paste'''.

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|| 07:04
|| Click on '''Save''' button in the toolbar once again.

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||07:08
|| Click on '''Simulate'''.

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|| 07:11
||Close the pop up window.

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||07:14
||Select '''h''' in the '''Variables browser''' once again. 

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|| 07:18
||Note that the '''h''' versus '''time''' plot is the same as in previous case which indicates that both models are the same.

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||07:27
||To delete the result, right-click on '''timeEventExample''' and select '''Delete Result'''.

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|| 07:34
|| Click on '''Modeling''' button at bottom right.

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|| 07:38
|| This is an '''if-else''' statement which indicates that if '''time >= 0.5''' '''dv/dt = g''' otherwise '''dv/dt = 0'''.

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||07:52
|| Now let me go back to the '''slide'''s.

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||07:55
||Please note that the number of '''equations''' in '''if''' branch should be equal to the  number of equations in '''else''' branch. 

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|| 08:03
||And, both of them should be equal to the number of variables in the '''class''' or '''model'''.

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||08:10
|| '''Event''' is an abrupt change in behavior of a system.

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|| 08:15
||'''Events''' may be categorized into '''time event'''s and '''state event'''s.

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|| 08:20
||In a '''time event''', the time at which event occurs is precisely known.

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|| 08:25
||'''timeEventExample''' handles a time event at '''t = 0.5''' seconds

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|| 08:32
||'''state event''' occurs when a system variable crosses a certain value.

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||08:38
||Let us try to understand state events using an example.

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|| 08:43
||A ball under free fall faces a '''state event''' on touching the ground.

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|| 08:48
||The following behavior is observed when it hits the ground.

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|| 08:52
||Velocity of ball reverts in direction.

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|| 08:55
||Magnitude of velocity changes if the collision is inelastic.

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||09:00
||I have created a '''model''' named '''bouncingBall''' to simulate this behavior.

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|| 09:06
||This file is available on '''Spoken Tutorial''' Website.

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|| 09:10
|| To view this '''model''' and simulate it, let me go back to '''OMEdit'''. 

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|| 09:15
|| Click on '''OpenModel/LIbrary File''' tool.

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|| 09:19
||Locate '''bouncingBall''' file in your system and click on '''Open'''.

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|| 09:25
|| Right-click on '''bouncingBall''' icon in '''Libraries browser''' and select '''View Class'''. 

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|| 09:33
||If the '''model''' does not open in '''Text View,''' go to the top of '''modeling area''' and click on '''Text View''' button.

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|| 09:41
|| '''e''' is '''coefficient of restitution''' for inelastic collision of ball with Earth.

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|| 09:48
||It is a parameter and its value remains constant at 0.8 throughout the '''simulation run'''.

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|| 09:56
|| '''radius''' represents the radius of ball in meter.

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||10:00
|| '''h, v''' and '''g''' represent the same quantities as in '''timeEventExample'''.

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|| 10:07
|| These two equations of motions have already been discussed in '''freeFall class'''.

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|| 10:13
|| '''when''' statement is used to signal an '''event'''. 

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|| 10:17
||It is used to specify the action to be taken when an event occurs. 

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|| 10:22
||'''h <= radius''' holds '''true''' when the ball is in contact with ground.

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|| 10:29
||This '''statement''' is executed whenever the event occurs.

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|| 10:34
|| '''reinit()''' function is used to re-initialize a variable. 

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|| 10:39
|| Here, it re-initializes velocity of ball with the negative product of '''e''' and velocity of ball before touching the ground.

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|| 10:49
|| To simulate this model, click on '''Simulation Setup''' button in the toolbar.

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|| 10:55
||Under '''General''' tab, change '''Stop Time''' field to '''30 unit'''s and  click on '''Simulate'''.

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|| 11:04
||Close the pop-up window.

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|| 11:06
|| Select '''h''' in '''Variables browser'''. 

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|| 11:10
||Note that '''h''' drops below zero which is unacceptable.

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|| 11:16
||We have seen similar behavior while simulating '''freeFall class'''.

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|| 11:21
|| This faulty behavior is due to numerical errors.

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|| 11:25
||A discussion of numerical errors is beyond the scope of this tutorial. 

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|| 11:30
||Please refer to '''Zeno''' behavior of bouncing ball for a better understanding.

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||11:36
||Let me de-select '''h'''.

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|| 11:39
|| Let us go back to the slides.

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|| 11:42
|| '''when''' statement is used to signal an event.

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|| 11:45
||'''reinit(a,b)''' assigns value of '''b''' to variable '''a''' and resumes simulation.

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|| 11:54
||'''pre(a)''' returns the value of variable '''a''' before event.

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|| 11:59
||For example, '''reinit(a, 10)''' assigns a value of '''10''' to '''a''' when  event occurs.

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|| 12:08
||The faulty behavior of '''bouncingBall''' model is corrected in '''bouncingBallWithHysteresis'''.

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||12:15
||'''bouncingBallWithHysteresis''' model is available on '''spoken tutorial''' website.

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|| 12:21
||Simulate '''bouncingBallWithHysteresis''' and generate '''h''' versus '''time''' plot. 

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|| 12:27
||Notice the difference between '''bouncingBall''' and '''bouncingBallWithHysteresis'''

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|| 12:33
|| This brings us to the end of this tutorial.

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|| 12:36
|| Watch the video available at following link.
'''http://spoken-tutorial.org/What_is_a_Spoken_Tutorial'''

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|| 12:40
||It summarises the '''Spoken Tutorial''' project.

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|| 12:42
|| We conduct workshops using spoken tutorials, give certificates. Please contact us.

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|| 12:48
|| If you have any questions related to this spoken tutorial, please visit the following website.

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|| 12:55
|| Under '''Textbook Companion Project''' we coordinate coding of solved examples of popular books.

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|| 13:03
||We give honorarium and certificates to those who do this. For more details, please visit the following website.

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|| 13:10
|| Under '''Lab Migration Project''', we help migrate commercial simulator labs to OpenModelica. 

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|| 13:17
||For more information please visit the website.

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|| 13:21
|| '''Spoken Tutorial Project''' is supported by '''NMEICT, MHRD''', Government of India.

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|| 13:28
|| We thank the development team of '''OpenModelica''' for their support.

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|| 13:33
|| Thank you for joining me in this tutorial. Goodbye.
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