Difference between revisions of "OpenModelica/C2/Developing-an-equation-based-model/English-timed"
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Latest revision as of 15:22, 16 August 2019
| 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 | g represents 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. |
