MAE 170 LabVIEW AssignmentsFall Quarter 2010

UC San Diego Department of MAE Compiled By Kevin Mandich

IntroductionThis document is meant to accompany the LabVIEW portion of the MAE 170 class at UC San Diego. It includes all of the assignments which will be completed during the quarter. In addition, it will provide some hints to make it easier to complete the assignments. This is not intended to be a complete manual for LabVIEW. If you don't come to class, you may have a very hard time understanding what is expected, and you will probably spend many frustrating hours at the computer trying to figure out how to make the programs work. In contrast, if you do attend lectures, you can gain tips and hints that will help to make the learning process a bit smoother.

Assignment 1CalculatorYour first assignment is to create a program which takes two user-inputted numbers, analyzes them, and provides an output upon execution of the program. The program will add, subtract, multiply and divide the two numbers; it will take the sine value of the first number; and it will display whether the first number is greater than, equal to or less than the second number. The Front Panel and Block Diagrams of an example of a successful program are shown below.

Helpful Concepts:Context HelpThis is a feature of LabVIEW that explains how LabVIEW functions operate. When you mouse over a function, the help for that function is displayed in the context help box. You can get context help by pressing Ctrl + H on the keyboard, or by clicking on the help menu, and selecting Show Context Help

Here the help for the Add function is displayed. The values for x and y are wired to the function, and the sum is wired out. By convention, inputs are on the left side of functions, and outputs are on the right.

Front Panel / Block DiagramAll LabVIEW programs have a front panel and a block diagram (also called a back panel). When you create a new program, both are created automatically, and when you save your program, both are saved. You can switch from one to the other by pressing Ctrl + E.

The front panel provides an interface which allows the user to interact with the program. Controls, such as knobs, slide bars and menu boxes allow the user to supply information to the program. Indicators, including graphs and dials, supply information to the user about the state of the program.

The block diagram is where all of the programming takes place. Here, functions are connected together by wires, in a manner similar to a circuit diagram. Whenever a control or indicator is placed on the front panel, an corresponding icon is automatically created on the block diagram. In the case of an indicator, data is wired to the icon on the block diagram and displayed on the front panel. For controls, data is wired from the control icon, and used elsewhere in the program.

Controls, Indicators and ConstantsData can be supplied to LabVIEW in two forms: Controls and constants. A control is something that can be changed while the program is running, and it has some type of icon on the front panel which the user manipulates to provide the input. A constant is provided during programming, but cannot be changed at run time. LabVIEW returns data to the user as indicators. These can be altered by the program while it runs, but cannot be changed by the user.

Controls

Constants

Indicators

Automatically Creating Inputs and OutputsThere are two ways to find the constants, controls and indicators that you need for your functions: the easy way and the hard way. The hard way is to search through all of the menus until you get what you are looking for. This can take a long time, and has been known to cause pulling of hair and gnashing of teeth. The easy way to get the input or output that you are looking for is to have LabVIEW create it for you. This is a sure way to speed up the programming process, and save yourself a lot of frustration. To have LabVIEW create an input or output, first put a function on the block diagram, then right click on the terminal you want to connect to, point to create, and choose constant, control or indicator.

Remember, inputs can only be constants or controls, and outputs can only be indicators.

Run ButtonThe small arrow in the upper left corner of the window is the run button. When this button is white, it means that there are no errors in your vi, and it is ready to run. If you click on the arrow when it is white, it will run your program. When the arrow is gray with and appears broken, it means that your program has errors in it. Rather than searching for the errors yourself, it is often easier to have LabVIEW find them for you. To do this, click on the broken run arrow. Labview will display a list of all the errors in your program. LabVIEW will even find the location of each error: double click an item on the list, and LabVIEW will highlight its location in your code.

Run Arrow No Errors

Run Arrow Broken, with errors

Show a list of errors by clicking on the broken run arrow

Assignment 2Analog to Digital ConversionThis assignment demonstrates the effect of discretizing an analog signal at different resolutions. Your program will create half of a sine wave by calculating 180 discrete points and connecting the dots, as seen in the graph below. It is left to the student to decipher why changing the number of bits changes the resolution of the shape. Only the graphical output of the VI is shown on the Front Panel. Your Front Panel must also contain a Vertical Fill Slide to control the number of bits, as well as a Numeric Indicator which displays the quantity 2^n, where n is the number of bits given by the Vertical Fill Slide.

Helpful Concepts:For LoopsA for loop in LabVIEW is similar to for loops in other languages, in that is repeats a certain set of commands a given number of times. In LabVIEW, a for loop is actually represented by a square loop: any instructions inside of the loop are repeated.

Visible ItemsAll controls have certain items which can be made visible or invisible, including numeric indicators.

Waveform ChartsDisplay data that is wired into them in the form of vectors. A set number of points are stored, and data is not overwritten until that set number of points has been sent. The X and Y scales can be set to auto scale, or can be set to fixed lengths. To auto scale, right click on either axis, and select Auto scale X or Auto scale Y. To adjust the scales, double click on the lowest or highest value displayed, and type in the desired value.

Meaning of Wire Colors and SizesThe sizes and colors of the wires in LabVIEW are significant, and correspond to the type of data contained within the wire. The size of the wire relates to whether the data is a scalar, 1D vector or 2D vector. The color of the wire describes the type of variable contained in the wire. For instance, a variable is contained in a pink wire, an integer is contained in a blue wire, and a decimal number is contained in an orange wire. In order to choose the type of number you wish for a numeric control to produce, right click on the control and select Representation. The three values on the top line are Ext., Dbl, and SGL, and they stand for Extended precision, Double precision and Single precision decimal numbers. They range from the most to least values of stored accuracy. Since these are all decimal numbers, any of these will result in an orange wire. The next line down have I64 through I8, referring to 64 bit through 8 bit signed integer numbers. These numbers can take on positive and negative integer valued. Since they are integers, they will be represented by blue wires. The third line down displays U64 through U8, meaning 64 bit through 8 bit unsigned integers. These numbers can take on only positive integer values (and the 0 value). Since these numbers are integers, they will also be represented by blue wires.

Assignment 3Data Analysis using a Sub-VIIt is often very useful, when writing programs, to create functions which may be called by other routines. This helps to prevent redundancies, and can make large programs more organized and understandable. The LabVIEW analogy is called a Sub-VI. In general, a Sub-VI is very similar to some of the functions we have already used (i.e. sine, addition) in that the Sub-VI requires inputs, processes them, and then returns the results as outputs. In this assignment you are to create a program which reads two inputs, Number of Samples and Amplitude. You will then create a Sub-VI which reads these two inputs, analyzes them, and creates an output consisting of a 2-Dimensional array of the sine and cosine waves created from the input information. Your main program will read this output, unbundle the 2-D array into two 1-D arrays, read the RMS values of each wave, and display the superimposed images of the sine and cosine waves onto a Waveform Graph on your front panel. The Front Panel and Block Diagram of the main program are shown below:

The Sub-VI is shown as the white box with the words n/s, Wav, and Amp inside. As you can see, there are two inputs and one output. The Sub-VI will be constructed using the following pseudo-code: for i=1:n (n = number of samples) sine(i)=amplitude*sin(i/10) cosine(i)=amplitude*cos(i/10) end for loop 2D_array=(sine,cosine) (combing the two vectors into one unit) HINT: You can combine the 1-D arrays sine and cosine into a single 2-D array using the Build Array function in LabVIEW.

Helpful ConceptsUseful Functions RMS: Mathematics Probability and Statistics RMS Index Array Array Index Array

Creating a Sub-VI Make a vi from scratch as usual Once you have your inputs and outputs created go to the front panel, and right click on the icon in the top right hand corner of the screen o Click on Show Connectors o If there are not enough inputs and outputs, right click, then point to patterns, then choose the one you want Once you choose a pattern, connect each terminal to an input or output by first clicking on the terminal, then clicking on the input or output. Next go to the block diagram, right click on the icon in the upper right hand corner, and select Edit Icon. You should change the icon so that it will remind you how to connect wires later on. Save the vi on your computer Once you have created a sub-vi, you can place it in another vi from the back panel using the functions palette, by clicking on All Functions Select a vi. Simply browse for the vi, then place it as you would any other function.

Build ArrayLabVIEW, like many other programming languages, can handle arrays of data. Almost any type of data can be made into an array, including numbers, strings and Booleans. Arrays can include any number of dimensions. Since LabVIEW doesn't have a separate data type for vectors, a one dimensional array is used. Once data is in an array, many operations can be performed on that array, including sorting, searching and transposing. Arrays can be constructed using a function called Build Array. This function can be found on the array palette, called Build Array.

Initially, the build array function has only one input

By dragging the bottom handle, it is possible to add as many inputs as necessary

It is possible to use either scalar values or arrays as inputs to the build array function. If scalar inputs are used, the output is a 1D array. If several 1D arrays are put into a Bundle Array function, the result will be a 2D array.

Scalar inputs create a 1D array

1D array inputs create a 2D array

It is also possible to append data onto an existing array. For instance, it is possible to add a number of scalars to the end of a 1D array, or to stack two 1D arrays one after the other, to create a longer 1D array. To do this, right click on the Build Array function, and choose Concatenate Arrays

Choose Concatenate Arrays to append data to an existing array

With Concatenate Inputs enabled, the dimension of the output is the same as the greatest input dimension

Assignment 4Event Structures & While LoopsEvent structures perform a pre-determined series of operations once a specific event is triggered (i.e. clicking the mouse or pressing ENTER). You can think of them as for loops than only perform one iteration when an event is triggered. While loops are likewise similar to for loops. However, instead of a predetermined number of iterations as defined in a for loop (the parameter N), While loops run continuously until a specified Boolean condition is met (i.e. a STOP button is pushed). Your program this week will read an input from a Vertical Slide Fill and display the results on a Waveform Chart, as shown below. Note that the Chart will only update when the value of the Slide Fill is changed. This is the triggering event.

The pseudo-code for the block diagram is shown below: while stop button is not pushed if vertical slide is changed chart value = vertical slide fill value plot the chart_value else if vertical_slide is not changed wait 10 ms (timeout event) end event end while loop HINT: To use the timeout feature (wait 10 ms in the code), simply wire a constant number, read in milliseconds. into the small blue hourglass at the top left corner of the event case. This sets the time before which the loop can enter another iteration. Once the time is up, and if an event has not happened, it will trigger the timeout case which is, in our case, blank. Thus, if you do not move the slide rule, nothing will happen.

Helpful ConceptsCreating an EventWhen you create an event loop, the only event you start with is the timeout event. To add an event, right-click on the text box at the top of the loop and choose Add Event Case. In this Edit Events window you will first choose an Event Source and then an Event. Note that you can also change any other events you have added to the loop with in this window. To scroll between different events in your loop, either press one of the horizontal arrows at the top of the loop or left-click the text box at the top, and choose which event you want to view.

Setting up a While LoopWhen you create a while loop, you can see a tiny red circle in a green box in the lower left corner. This is the Boolean condition for the loop. The default value is Stop if True. You can right-click this to change to the opposite, Continue if True, but we will use the default. If you connect a Boolean control to this (i.e. a STOP button on the front panel) you can then stop the loop from running at any time during the programs operation by pressing this button.

Assignment 5Shift Registers, Clusters, Build Array, and X-Y GraphsThe purpose of this assignment is to create a graph which will be updated when either of two knobs are rotated. One knob will provide the X position of a cursor, and the other, the Y position. The cursor should leave behind it a line showing all previous locations of the cursor, which will be present for the entire time that the VI is running. One possible Front Panel looks like this bootleg Etch-a-Sketch (please note you dont have to customize your VI at all; the default X-Y graph will suffice):

Pseudo-Code: while stop button is not pushed old_x_vector=new_x_vector (shift register) old_y_vector=new_y_vector (shift register) if x knob value is changed new_x_vector=(old_x_vector,current_x_knob_value) new_y_vector=(old_y_vector,current_y_knob_value) else if y knob value is changed new_x_vector=(old_x_vector,current_x_knob_value) new_y_vector=(old_y_vector,current_y_knob_value) else if nothing changes new_x_vector=old_x_vector new_y_vector=old_y_vector end if plot new x vector vx. Y vector on an X-Y graph store new_x_vector for next iteration (shift register) store new_y_vector for next iteration (shift register) end while Note: the notation (A,B) signifies that the variable B should be concatenated with the variable A, to create a vector. Here, the value B is added to the vector A, which contains all of the previous values.

Helpful Concepts:Shift RegistersShift registers provide a way to store a value during one iteration of a loop, and retrieve the value during the next iteration of the loop. Shift registers can be used with for loops as well as while loops. Shift registers can store almost all data types, including strings, numbers and Boolean. They can also store arrays of these data types. The easiest way to create a shift register is to create a for loop or a while loop, then right click on the side of the loop, and choose Add Shift Register. Wiring a variable or constant into the left terminal, from outside the loop, provides an initial value. \

ClustersClusters are a way of combining several pieces of data into one package. It is possible to have different types of data in one cluster, or a cluster can contain multiple instances of the same data type. The XY graph, described below, only has one input terminal, and requires a cluster of two arrays: an array of x coordinates and an array of y coordinates.

The easiest way to build a cluster is by using the bundle function. This function can be found on the functions palette, under Cluster, Class and Variant. The bundle function only has two terminals initially, but you can add more terminals by dragging the bottom of the icon, as shown:

The bundle function only has two terminals initially

It is possible to add more terminals by dragging the bottom of the icon

XY GraphAn XY graph is just what it sounds like, a figure which displays a number of points that are supplied as X,Y pairs. The XY graph can be found on the front panel, on the controls palette, under the graph menu. The input format for an XY graph is an array of X coordinates bundled together with an array of Y coordinates, to make a cluster of two arrays.

An array of X coordinates is bundled with an array of Y coordinates, and the cluster of two arrays is plotted on the XY Graph.

Assignment 6Tunnels & IndexingFor this assignment you will be creating what is known as a Lissajous Figure.

Pseudo-Code: for i = 0 to 3 f_ratio = 4-i for j = 0 to 89 phase = 4*j k = 0 angle_1 = 0 while angle1 < 720 angle2 = f_ratio*angle_1 + phase sinewave1 = cos(angle_1*pi/180) sinewave2 = cos(angle_2*pi/180) chart sine waves angle_1 = 2*k k = k + 1 end graph sinewave1 vs sinewave2 wait 100 milliseconds end end

Helpful Concepts:Tunnels and Indexing:Any time that a signal is wired into or out of a structure (i.e. a for loop, while loop, case structure, etc.), a small square, called a tunnel, is created. When using for loops and while loops, it is possible to choose either a vector or scalar output by altering the output tunnel. If a signal is wired out of a for loop or while loop, the value for that signal will be calculated each time that the loop iterates. Thus it is possible to have many values calculated for a single output. It is up to the programmer to decide whether to use all calculated values, or to use only the last value calculated. This selection is made by choosing either an indexed or non-indexed output tunnel. An indexed tunnel provides all calculated values, while an non-indexed tunnel gives only the last calculated value.

Non-indexed Tunnel

Rich-click, choose Enable Indexing

Indexed Tunnel

Assignment 7Local Variables and Sequence StructuresIn this program you will pre-assign a username and password. If, upon execution of the program, the username and password typed into the text input prompts are correct, a message will pop up saying Message Granted. Otherwise, the user will receive an Access Denied, Try Again message.

Pseudo Code: username = ______ password = ______ while stopsign = false if OK button value changes if Username == username and Password == password Popup Dialog: Access Granted stopsign = True else if OK button values changes popup dialogue: Access Denied, Try Again stopsign = false end if else if CANCEL button value changes stopsign = True else stopsign = False (the Timeout event) end if end while

Helpful Concepts:Sequence StructureA sequence structure carries out a sequence of events. There are two types of sequence structures: flat and stacked. In a flat structure, as shown in the block diagram, there are multiple frames which execute in a left-to-right sequential order. Code within the first frame will execute completely. After it is finished, the program moves onto the next frame, and so on. A stacked structure works in the same way, except you only see one frame at a time, much like the event structure. There is a text box at the top of the loop that will tell you which frame you are on. To add or delete a frame in either structure, simply right-click the border of the loop and choose the corresponding option. You can add a sequence either before or after the frame you are currently on.

Local VariablesA local variable allows you to read from or write to a variable, whether it is a control or an indicator. Essentially, with a local variable, you can access a Front Panel object as both an input and an output. To create a local variable, simply right-click a string control in the Front Panel, choose CREATE Local Variable, and place the local variable box onto your Block Diagram by left-clicking where you want it to be placed.