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

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|| 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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how to use '''when''' statement.
  
 
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|| To record this tutorial, I am using : '''OpenModelica 1.9.2''' and '''Ubuntu Operating''' system version 14.04.
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Revision as of 20:59, 17 August 2018

Time Narration
00:01 Welcome to the spoken tutorial on Control flow and Event handling.
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.

00:19 To record this tutorial, I am using : OpenModelica 1.9.2 and Ubuntu Operating system version 14.04.
00:30 But, this process is identical in Windows, Mac OS X or FOSSEE OS on ARM.
00:38 To understand and practice this tutorial, you need to know the following: equation-based modeling of physical systems ,
00:47 branching in any programming language , class definition in Modelica.
00:53 Pre-requisite tutorials are mentioned on Spoken Tutorial website. Please go through them.
01:00 freeFall class was already discussed in previous tutorials.
01:05 Now, let us add a constraint to it.
01:08 Ball is at rest until t = 0.5 seconds. Free fall under gravity starts at time t = 0.5 seconds.
01:19 I have created a model named timeEventExample which simulates this condition.
01:25 timeEventExample file is available in Code Files link on spoken tutorial website.
01:32 Please download and save all the files in Code Files link.
01:38 For your convenience, freeFall class is also made available.
01:43 Now let me go to OMEdit.
01:45 I have already launched it on my system.
01:49 To open OMEdit on Ubuntu Operating System, click on Dash Home icon at the top left.
01:58 Type: OMEdit in the Search bar and click on OMEdit icon. Let me go back.
02:06 Note: Windows users may similarly search for OMEdit in Start menu and open it.
02:13 To open timeEventExample file, click on the tool named Open Model/Library File.
02:21 Locate timeEventExample file in your system and click on Open.
02:28 You may alternatively use the File menu to open this file.
02:34 Note that timeEventExample has appeared in Libraries browser.
02:39 Right-click on this icon and select View class.
02:44 The model has now opened in Diagram view.
02:47 Click on third button which is the Text View.
02:51 We have already discussed that model and class are used synonymously in Modelica.
02:58 The name of this model is timeEventExample.
03:02 Comments such as this may be written on the first line of a model or class.
03:08 h, v and g represent the same quantities as in freeFall class.
03:16 initial equation section contains initial conditions.
03:21 Value of h at time t = 0 is 30m.
03:26 Value of v at time t = 0 is 0.
03:31 Equation signifies the beginning of equation section of this model.
03:36 Now, let us see how to insert the constraint explained in slides.
03:43 Note that there is an if-else block in equation section as shown.
03:48 Let us try to understand each statement of this if-else block.
03:53 if statement places a constraint on time variable.
03:58 time is a built-in variable in Modelica.
04:01 It need not be explicitly declared in a class or model.
04:06 The ball experiences free fall after time 0.5 seconds.
04:12 Hence, these two equations represent the same equations as those in freeFall class.
04:19 else branch contains equations which are executed when time is less than 0.5 seconds.
04:27 The ball is at rest when time is less than 0.5 seconds.
04:32 Hence, these two equations hold good in that case.
04:36 end if signifies the end of if-else block.
04:41 The model is now complete.
04:44 Let us simulate it by clicking on Simulate button.
04:48 Plotting perspective has now opened.
04:51 Select h in the Variables browser.
04:55 This generates h versus time plot.
04:58 Since the ball is at rest until time t=0.5 seconds, height remains the same.
05:06 To delete the result, right click on timeEventExample in Variables browser and select Delete Result.
05:15 To go back to Modeling perspective, click on Modeling button at bottom right.
05:21 Now, let me delete the second equation in else branch.
05:26 Click on Save button in the toolbar, to save this model.
05:31 Click on Simulate.
05:34 The simulation has failed.
05:37 Go to the Messages browser and scroll up to see the error message.
05:42 The error message says that the number of equations in each branch should be the same.
05:49 Therefore, the number of equations in if and else branches should each be equal to the number of variables.
05:58 Let me insert the equation back in its place and click on Save button.
06:05 Note that der(h) = v is an equation which is common to both if and else branches.
06:14 Hence, the if-else block can be replaced by two statements.
06:20 Let me show you how it is done.
06:23 Delete the if-else block first.
06:26 The equations to be typed here have been provided in a file named if else-statement.txt.
06:34 These file is also available in Code Files link.
06:39 Since it is a text file, I have used gedit to open it.
06:45 Let me go to gedit.
06:47 Windows users may open this file using Notepad or any other text editor.
06:54 Copy the two equations.
06:57 Go back to OMEdit.
07:00 Do a right-click and select Paste
07:04 Click on Save button in the toolbar once again.
07:08 Click on Simulate.
07:11 Close the pop up window.
07:14 Select h in the Variables browser once again.
07:18 Note that the h versus time plot is the same as in previous case which indicates that both models are the same.
07:27 To delete the result, right-click on timeEventExample and select Delete Result.
07:34 Click on Modeling button at bottom right.
07:38 This is an if-else statement which indicates that if time >= 0.5 dv/dt = g otherwise dv/dt = 0.
07:52 Now let me go back to the slides.
07:55 Please note that the number of equations in if branch should be equal to the number of equations in else branch.
08:03 And both of them should be equal to the number of variables in the class or model.
08:10 Event is an abrupt change in behavior of a system.
08:15 Events may be categorized into time events and state events.
08:20 In a time event, the time at which event occurs is precisely known.
08:25 timeEventExample handles a time event at t = 0.5 seconds
08:32 state event occurs when a system variable cross a certain value.
08:38 Let us try to understand state events using an example.
08:43 A ball under free fall, faces a state event on touching the ground.
08:48 The following behavior is observed when it hits the ground.
08:52 Velocity of ball reverts in direction.
08:55 Magnitude of velocity changes if the collision is inelastic.
09:00 I have created a model named bouncingBall to simulate this behavior.
09:06 This file is available on Spoken Tutorial Website.
09:10 To view this model and simulate it, let me go back to OMEdit.
09:15 Click on OpenModel/LIbrary File tool.
09:19 Locate bouncingBall file in your system and click on Open.
09:25 Right-click on bouncingBall icon in Libraries browser and select View Class.
09:33 If the model does not open in Text View, go to the top of modeling area and click on Text View button.
09:41 e is coefficient of restitution for inelastic collision of ball with Earth.
09:48 It is a parameter and its value remains constant at 0.8 throughout the simulation run.
09:56 radius represents the radius of ball in m.
10:00 h, v and g represent the same quantities as in timeEventExample.
10:07 These two equations of motions have already been discussed in freeFall class.
10:13 when statement is used to signal an event.
10:17 It is used to specify the action to be taken when an event occurs.
10:22 h <= radius holds true when the ball is in contact with ground.
10:29 This statement is executed whenever the event occurs.
10:34 reinit() function is used to re-initialize a variable.
10:39 Here, it re-initializes velocity of ball with the negative product of e and velocity of ball before touching the ground.
10:49 To simulate this model Click on Simulation Setup button in the toolbar.
10:55 Under General tab, change Stop Time field to 30 units and Click on Simulate.
11:04 Close the pop-up window.
11:06 Select h in Variables browser.
11:10 Note that h drops below zero, which is unacceptable.
11:16 We have seen similar behavior while simulating freeFall class.
11:21 This faulty behavior is due to numerical errors.
11:25 A discussion of numerical errors is beyond the scope of this tutorial.
11:30 Please refer to Zeno behavior of bouncing ball for a better understanding.
11:36 Let me de-select h.
11:39 Let us go back to the slides.
11:42 when statement is used to signal an event.
11:45 reinit(a,b) assigns value of b to variable a and resumes simulation.
11:54 pre(a) returns the value of variable a before event.
11:59 For example, reinit(a, 10) assigns a value of 10 to a when event occurs.
12:08 The faulty behavior of bouncingBall model is corrected in bouncingBallWithHysteresis.
12:15 bouncingBallWithHysteresis model is available on spoken tutorial website.
12:21 Simulate bouncingBallWithHysteresis and generate h versus time plot.
12:27 Notice the difference between bouncingBall and bouncingBallWithHysteresis
12:33 This brings us to the end of this tutorial.
12:36 Watch the video available at following link.http://spoken-tutorial.org/ http://spoken-tutorial.org] /What\_is\_a\_Spoken\_Tutorial
12:40 It summarises the Spoken Tutorial project
12:42 We conduct workshops using spoken tutorials Give certificates Please contact us.
12:48 If you have any questions related to this spoken tutorial, please visit the following website.
12:55 Under Textbook Companion Project we coordinate coding of solved examples of popular books.
13:03 We give honorarium and certificates to those who do this. For more details please visit the following website.
13:10 Under Lab Migration Project we help migrate commercial simulator labs to OpenModelica.
13:17 For more information please visit the website.
13:21 Spoken Tutorial Project is supported by NMEICT, MHRD, Government of India.
13:28 We thank the development team of OpenModelica for their support.
13:33 Thank you for joining me in this tutorial. Goodbye.

Contributors and Content Editors

Jyotisolanki, Sandhya.np14

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