Lab (2) - Visualization and Plotting in MATLAB

Objectives

• To learn to generate data in MATLAB
• To learn to visualize the data as plots
Preparation
We will use MATLAB in this lab.  To start MATLAB, double click on MATLAB icon on your PC.

In this lab manual every time that we show >> we are referring to the MATLAB command prompt where you can type a MATLAB command.  Then you have to press Enter to execute that command. MathWorks provides an excellent online tutorial for plotting, click here to bring that page up.

This lab is designed to help you learn some of the most powerful tools in MATLAB that allows you to create different visual representations of your data.   In general  MATLAB Graphics provides tools for:
• Basic Plotting – describes how to plot vector and matrix data in 2-D representations.
• Formatting Graphs – describes how to customize the format of a plot to help explain your data. This section includes information about adding labels, titles, and annotations to a plot.
• Creating Specialized Plots – describes how to create bar graphs, histograms, and contour plots.
• Basic Printing and Exporting – describes frequently used ways to print or export a MATLAB figure, and explains the basic printing and exporting interfaces.
• Handle Graphics Objects – introduces MATLAB’s graphics objects and properties.
• Figure Properties – discusses the features that are implemented with figure properties.
• Axes Properties – discusses the features that are implemented with axes properties.
Preparing Data
As it is the case for any visualization case we need data to generate plots or visualization.  We can generate data using formulas or mathematical relationships or we can use the data from an experiment to generate visual representation of that data which helps us better understand the data.

In our first example we will generate data using the projectile motion from physics.  The projectile motion is a two-dimensional motion where the projectile moves in x (distance) and in y (height) directions.  In this motion the projectile is usually sent of from its initial location with an initial speed (v0) at an initial angle (angle0).

Suppose our projectile motion starts with an initial speed of v0 = 40 m/s at angle0 = 35 degree from ground (initial height of 0).

We want to create 4 different plots to:
• Visualize the horizontal distance of the projectile from its initial location at different times.
• Visualize the height of the projectile from its initial location at different times.
• Visualize the horizontal speed at different times.
• Visualize the vertical speed at different times.
So we need to have time-dependent formulas for  distanceheight,  horizontalSpeed, and verticalSpeed.  We know that the initial horizontal and vertical speeds are defined as:
• InitialVerticalSpeed = v0*sin(angle0)
• InitialHorizontalSpeed = v0*cos(angle0)
Here are the formulas we need to compute the above four parameters (g = 9.8 m/sec^2):

distance =  InitialHorizontalSpeed*time
height =  -(1/2)*g*time^2 + InitialVerticalSpeed * time + initialHeight
horizontalSpeed =  InitialHorizontalSpeed
verticalSpeed = -g*time + IntialVerticalSpeed

Three of these are time-dependent, thus their values change with time.  We need to compute these at some time instances.  We will use the total time that the projectile is traveling before it comes to stop and will create equally spaced time instances between 0 to TMAX.  TMAX is computed as:

TMAX = 2*v0*sin(angle0)/g

Lab Activity (1)
Now we will create time intervals between 0 to TMAX using a simple MATLAB command.  But first we need to enter the initial values for speed, angle, and height.  After each command press enter to execute that command.  The values shown in font green are those that MATLAB displays.  Pay attention that I have converted the angle to Radian by multiplying it by (pi/180).

>> v0 = 40
v0 =
40

>> angle0 = 35 * (pi/180)
angle0 =
0.6109

>> g = 9.8
g =
9.8000

>> initialHeight = 0
initialHeight =
0

>> InitialVerticalSpeed = v0*sin(angle0)
IntialVerticalSpeed =
22.9431

>> InitialHorizontalSpeed = v0*cos(angle0)
InitialHorizontalSpeed =
32.7661

>> TMAX =  2*v0*sin(angle0)/g
TMAX =
4.6823

In this example our projectile motion lasts 4.6823 seconds. Now let's create the time intervals to monitor the motion from the time that the projectile is sent of to the time that it will come to stop.  The following command automatically generates different times and will store all of them in a variable that we will also time (actually time is what we refer to as an array of times).

>> time = 0: TMAX/10:TMAX

This simple call has three parts:
The first part indicates the time you want to start the time interval from  (we start from 0 in our case)
The second part indicates the time steps (our time step is 1/10th of the TMAX)
The last part indicates the maximum time (maximum time in our case is the TMAX)

If you type the command:
>> time = 0: TMAX/10 : TMAX
you will produce:
time =
Columns 1 through 6
0    0.4682    0.9365    1.4047    1.8729    2.3411
Columns 7 through 11
2.8094    3.2776    3.7458    4.2140    4.6823

Now we can use this time intervals to compute distanceheight,  horizontalSpeed, and verticalSpeed at these time intervals.  Type the following commands:

>> distance =  InitialHorizontalSpeed*time
Columns 1 through 6
0   15.3419   30.6838   46.0258   61.3677   76.7096
Columns 7 through 11
92.0515  107.3934  122.7354  138.0773  153.4192

>> height =  -(1/2)*g*time .^2  +  InitialVerticalSpeed * time + initialHeight
Columns 1 through 6
0    9.6683   17.1880   22.5593   25.7821   26.8563
Columns 7 through 11
25.7821   22.5593   17.1880    9.6683    0.0000

horizontalSpeed
=  InitialHorizontalSpeed
32.7661

verticalSpeed = -g*time + InitialVerticalSpeed
verticalSpeed =
Columns 1 through 6
22.9431   18.3544   13.7658    9.1772    4.5886         0
Columns 7 through 11
-4.5886   -9.1772  -13.7658  -18.3544  -22.9431

Now we have generated data for distanceheight,  horizontalSpeed, and verticalSpeed at these time intervals.  Let's plot them to see how they behave with time.  That is where we can benefit from the power of MATLAB for doing this kind of work.

A simple plot of  distance vs. time can be obtained using this simple MATLAB command:
>> plot( time, distance)

Which results in this plot.

So, this plot shows that distance changes linearly with time.  In fact, we can observe that distance has increased by time.  This is a good graph but we wish to add something to it to help viewers understand it better.  Things like a tittle, caption, etc...

By executing the following three lines, I can add the tittle, and x and y labels.

>> title('Plot of Projectile Distance Vs. Time')
>> xlabel('time (sec)')
>> ylabel('distance (m)')

Below is the outcome.

Here is the list of all commands so far.  You can save it in a .m file and execute it.

v0 = 40
angle0 = 35 * (pi/180)
g = 9.8
initialHeight = 0
InitialVerticalSpeed = v0*sin(angle0)
InitialHorizontalSpeed = v0*cos(angle0)
TMAX = 2*v0*sin(angle0)/g
time = 0: TMAX/10 : TMAX
distance = InitialHorizontalSpeed*time
height = -(1/2)*g*time .^2 + InitialVerticalSpeed*time + initialHeight
verticalSpeed = -g*time + InitialVerticalSpeed
plot( time, distance)
title('Plot of Projectile Distance Vs. Time')
xlabel('time (sec)')
ylabel('distance (m)')

How to create a Script?
Step 0: Click on File, then New, then Script -  This will open a blank page
Step 1: Cut and paste the above code (everything in font red)
Step 2: Save the file using File, then Save As in your working Directory
Step 3: To execute this program at the MATLAB prompt type the name of the file you just saved (without .m at the end)

From now on you can modify this file as you wish and execute it to generate the graph.  You can even add other things in this file.

Now, let me show you something even nicer.  You can actually save this graph in several different formats for use in your class presentations.  For example you can save this plot as a jpg image or as a bmp image and then insert it into your PowerPoint documents.  Here are the steps to do this:

Step 1: On the plot, left click on the File at the top left, then left click on Save As...
Step 2: Choose the directory in which you want to save the file - at the top Save in: Use the arrow to browse to Desktop
Step3: Choose a file name for this image - for example File name: DistanceVSTime.jpg
Step4: Choose a file format - Save a type:  JPEG image *.jpg
Step5: Click on Save

Once you created a graph you can use the MATLAB Graphical User Interface to make modifications or add things to it.  Chapter 4 of the MATLAB Plotting Manual describes this in detail.

Lab Activity (2)

Let's follow the same procedure and create the graph for height of the projectile vs. time.  This time we will create a plot but I want you to use the GUI to add the labels, legends, titles, etc.  Note that we have already created the values for height and have stored them in variable height, so what we need to do now is to plot that.

>> plot( time, height)

You will get:

Now use the GUI to add things to this graph.    Below you will see the MATLAB screen for the graph in which you will view your graph.   You can get to the Edit Mode by clicking on the white arrow at the top of the screen, then by double clicking on the object you wish to modify.