OpenModelica/C2/Developing-an-equation-based-model/English-timed

Time	Narration
00:01	Welcome to the spoken tutorial on Developing an equation based model.
00:06	In this tutorial, we are going to learn: how to create a textual model in OMEdit and simulate it,
00:14	how to declare variables and equations,
00:17	how to use Simulation Setup toolbox.
00:21	To record this tutorial, I am using OpenModelica 1.9.2 and Ubuntu Linux Operating System 14.04.
00:32	But, this process is identical in Windows, Mac OS X or FOSSEE OS on ARM.
00:40	To understand this tutorial, you need to know equation-based modeling of physical systems.
00:48	Let us simulate the motion of a ball of mass 'm' which is under free fall due to gravity.
00:54	The height of the ball from the surface of Earth is represented by variable h.
00:59	The velocity of the ball is represented by variable v.
01:04	Acceleration due to gravity is represented by g and is assumed to be constant.
01:10	Variables directed away from the surface of earth are considered positive.
01:16	The equations of motion for a freely falling body are as follows: dh/dt = v, dv/dt = g
01:27	The value of h at time t = 0 is 30 meters and the value of v at time t = 0 is 0.
01:37	Now, let me go to OMEdit. I have already launched it on my system.
01:43	To open OMEdit on Ubuntu Linux Operating System, click on Dash Home icon which appears at the top left, in launcher.
01:53	Type OMEdit in the search bar.
01:56	Click on the OMEdit icon.
01:59	On clicking OMEdit icon, you see a window such as this.
02:06	This window is known as "Welcome perspective".
02:09	OMEdit by default, opens in the "Welcome perspective".
02:14	At the bottom right-hand corner, you can locate buttons for ‘Welcome’, ‘Modeling’ and ‘Plotting’ perspectives.
02:23	click on ‘Modeling perspective’.
02:26	‘Modeling perspective’ has now opened.
02:29	I will be referring to the area between Libraries Browser on the left, Messages Browser at the bottom and Toolbar on the top, as the modeling area.
02:41	The toolbar has buttons related to file operations, graphical view and simulation.
02:51	We will learn more about these buttons, as we go along.
02:55	Now, we shall use ‘freeFall’ class file provided in the Code Files link on our Spoken Tutorial webpage.
03:02	Please download this file and save it on your system.
03:07	To open this class, go to the File menu in Menu bar.
03:13	Click on Open Model/Library File
03:17	Locate freeFall file that you downloaded and saved on your system and open it.
03:24	You may also use the tool named Open Model/Library File that my cursor is pointing to, for opening a file.
03:34	Note that freeFall icon appears in the Libraries Browser.
03:39	Libraries Browser shows all classes that are loaded in a session of OMEdit.
03:45	Right click on freeFall icon and select View Class.
03:52	The class has now opened in Diagram view.
03:56	Do not worry if the class does not open in Diagram view.
04:00	I shall show you how to switch between different views.
04:04	Go to the top of Modeling area.
04:07	Note that the second button stands for Diagram view.
04:10	Third button is Text View.
04:13	Click on it to switch to Text View.
04:17	The class has now opened in Text view.
04:20	Note that the first button stands for Icon View.
04:24	We will learn more about Icon view and Diagram view later on.
04:29	You may also create a new class named freeFall and type the required information.
04:36	To create a new class, go to File menu.
04:40	Select New Modelica Class.
04:43	A dialog box pops up, as shown.
04:46	In the Name field of this dialog box, type freeFall.
04:51	I am using a different name freeFall1 since freeFall class is already open in OMEdit.
04:58	Note that two classes cannot have the same name.
05:03	Click on Specialization drop-down menu. Select Class. Click on Ok.
05:10	A new class has been created.
05:13	You may also use the tool named New Modelica class to open a new class.
05:20	Let me delete annotation section.
05:23	Now, you may type the required information here and save this class.
05:29	To save this class, go to File menu in Menu bar and click on Save.
05:36	Choose an appropriate location for this file and save it.
05:41	Now, let us understand the syntax of Modelica using freeFall class.
05:47	So switch to freeFall class.
05:49	Go to top of Modelling area. Click on freeFall tab.
05:54	Programs in Modelica are arranged in the form of classes.
05:58	The first line of a class defines its name.
06:02	The name of this class is freeFall.
06:05	Every class must have an end statement to indicate where the class ends.
06:11	This class has variable declarations and equations.
06:15	Let me show you how to declare variables.
06:18	Real represents data-type.
06:21	h represents height of ball from surface of Earth.
06:25	start is an attribute of Real variable.
06:29	Every data-type has certain attributes, which specify useful information related to variables.
06:36	start attribute specifies the initial value of a variable.
06:41	The initial value of h is 30 units.
06:45	unit attribute specifies the unit of a variable.
06:49	The unit of h is metre.
06:52	Every variable declaration must end with a semi-colon.
06:57	v represents velocity of ball. It is of Real data-type.
07:02	The initial value of v is zero. Its unit is meter per second.
07:09	grepresents acceleration due to gravity. It is of Real data-type. And its unit is meter per second square.
07:18	parameter is a quantity that remains constant in a simulation run.
07:24	The value of g remains constant throughout the simulation run, with a value of 9.81.
07:32	The negative sign is due to sign convention used.
07:36	The text in double quotes is a comment written with the declaration of g.
07:42	Comments provide useful information about the program. They are also useful for documentation.
07:49	Now, let me go back to the slides.
07:52	parameter is a quantity that remains constant in a simulation run.
07:57	Real, Integer, Boolean and String data-types are supported in Modelica.
08:03	start and unit attributes have already been defined.
08:07	min attribute specifies the minimum value of a variable.
08:10	Similarly, max attribute specifies the maximum value of a variable.
08:16	Let me go back to OMEdit.
08:19	‘equation’ signifies the beginning of equation section of a class.
08:25	This is an alternative way of inserting comments.
08:30	The two equations of motion for a freely falling body, as we have already discussed, have been included here.
08:38	der() is Modelica function for time derivative.
08:43	Hence, der(h) represents dh/dt.
08:48	And der(v) represents dv/dt.
08:52	Every equation must end with a semi-colon.
08:57	Let me show you how to simulate this class.
09:00	Click on simulate button in the toolbar.
09:04	Close the pop-up window.
09:07	This window is known as the Plotting perspective.
09:11	On successful simulation of a class, Plotting perspective opens automatically.
09:17	Variables browser displays information related to variables and parameters of a class.
09:24	Note that there are columns named Unit and Description.
09:29	Unit column specifies units of variables, as defined using Unit attribute.
09:37	Description column displays comments written in double quotes with variable declarations.
09:45	Let me show you how to generate a plot. Select h.
09:51	This generates the plot of h with respect to time -with h on y-axis and time on the x-axis
10:01	By default, the simulation is run from 0 to 1 unit of time.
10:07	The unit of time depends on the system of units used for other variables.
10:13	Let me de-select h.
10:17	It is always a good practice to delete the results, once the necessary plots have been generated.
10:25	To delete this result, right-click on freeFall and select Delete result.
10:33	The result has now been deleted.
10:36	Let me go back to Modeling perspective.
10:39	Click on Modeling button at bottom-right.
10:43	class is used synonymously with model, in Modelica.
10:48	One may use model instead of class here, to produce the same effect.
10:54	Now, let me show you how to change the time interval for simulation.
11:01	Click on Simulation Setup button present in the toolbar.
11:06	Under General tab, locate Stop time field. Change it to 5 units.
11:14	Click on Simulate. Close the pop-up window which appears.
11:21	Let me select h once again, in the Variables browser.
11:26	This generates h v/s time plot.
11:29	Notice that the time interval has increased to 5 units.
11:33	But, the value of h has gone below zero, which is unacceptable.
11:40	We will learn how to rectify this issue in the later tutorials.
11:45	Let me delete this result by right-clicking on freeFall and selecting Delete result.
11:53	Go back to Modeling perspective by clicking on Modeling perspective at bottom-right.
11:59	It is necessary to ensure that the number of equations is equal to the number of variables.
12:07	This class has two variables and two equations.
12:11	Now, let me delete the first equation and simulate this class, to see what happens.
12:18	I have deleted the first equation.
12:21	Notice that a star appears beside the name of the class in freeFall tab.
12:28	This indicates unsaved changes in the class.
12:31	Hence it is a good practice to save a class after a change has been made.
12:38	To save this class, go to the File menu and click on Save.
12:44	You may also use the Save button in toolbar, which my cursor points to, to save a file.
12:53	Now, let me simulate this class by clicking on Simulate button.
12:59	Note that an error message pops up in the Messages browser.
13:04	It says, there are too few equations and the model has 1 equation and 2 variables. Hence this cannot be simulated.
13:14	Let me insert the equation back in its place and click on Save button in the toolbar.
13:24	Click on Simulate button once again to simulate this class.
13:29	Note that the class simulates successfully, since the number of equations is equal to the number of variables.
13:37	Close the pop-up window.
13:40	Let me go back to the slides.
13:43	“der()” is Modelica function for time derivative.
13:48	There is no data flow direction for equations.
13:52	For example, der(h) = v may also be written as v = der(h).
14:00	Initial equations section is used to enter initial conditions.
14:05	We shall learn more about Initial equation later on.
14:10	As an assignment, write a model to simulate the differential equation dx/dt = -a into x , where a = 1, x belongs to R and the value of x at time t=0 is 5
14:28	This brings us to the end of this tutorial.
14:31	Watch the video at the following link. It summarizes the Spoken Tutorial project.
14:37	We conduct workshops using spoken tutorials; give certificates. Please contact us.
14:43	Spoken Tutorial project is funded by NMEICT, MHRD, Government of India.
14:49	We thank the development team of OpenModelica for their support.
14:53	Thank you.

Contributors and Content Editors

Jyotisolanki, PoojaMoolya, Sandhya.np14