OpenModelica/C2/Control-flow-and-Event-handling/English-timed

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 crosses 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 meter.
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 /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