ME 365 EXPERIMENT 3 INTRODUCTION TO LABVIEW

Transcription

1 ME 365 EXPERIMENT 3 INTRODUCTION TO LABVIEW Objectives: The goal of this exercise is to introduce the Laboratory Virtual Instrument Engineering Workbench, or LabVIEW software. LabVIEW is the primary software package that we will be using to acquire and analyze measurement data in ME365. This week's experiment will familiarize you with many of the LabVIEW fundamentals that we will rely on throughout the semester. After completing this lab, you should be able to: Construct a simple virtual instrument (VI). Navigate and use pre-made LabVIEW VIs. Know how to adjust sample rate and number of samples in order to capture a signal properly. Procedure: An Overview of LabVIEW: LabVIEW is a program development application or environment similar to the development systems you have used to write C or FORTRAN programs in some of your other courses. It is a general-purpose programming system which includes libraries of functions and development tools specifically tailored to data acquisition and instrument control. As you will see when you develop your own programs (called virtual instruments or VIs), LabVIEW is a graphic based programming language rather than text based like other languages you may have used. Another difference you will see as you develop VIs is that the program execution is data driven. That is, the program execution order is determined by the flow of data within the program. This is in contrast to other languages which are instruction driven. Log onto your computer and launch LabVIEW by going to Start Programs National Instrument LabVIEW When LabVIEW opens you will see the Getting Started window. In this dialog window, select Blank VI under the New heading. This will open two windows cascaded on top of each other. The top window is called the front panel of the VI. It acts as the user interface of the virtual instrument. It is called a front panel because it simulates the front panel of a physical instrument and includes such items as knobs, switches, buttons, gages, graphs, and other controls and indicators. The second window, partially obscured by the front panel, shows the wiring diagram for the VI. Inputs from the front panel are passed to control terminals located in the diagram window. Similarly, results from the VI are sent from indicator terminals in the diagram window to the corresponding front panel indicators. The diagram window contains a pictorial representation of the instrument's program, and describes how the instrument is to process data. 1

2 Front Panel Controls: If you haven't already done so, launch LabVIEW and select the front panel of a new VI. To create a new VI, click New and choose Blank VI and hit the OK button. A VI is either in edit mode, in which you can create or change a VI, or in run mode, in which you can execute a VI. You cannot edit a VI in run mode. Use the pull down menu titled Operate. At this time, make sure you are in edit mode. Across the top of the panel window are pull down menus labeled File, Edit, View, Operate, Project, Windows, and Help. Most of these pull down menus are very similar to those found in other applications. The arrow icon starts the execution of the VI even if the system is in edit mode. When you click on this icon while in edit mode, the VI is compiled, switched to run mode, run, and then switched back to edit mode when the VI is finished. The remaining editing icons are made available by pressing the shift key and the right mouse button simultaneously. This can also be called up by selecting the Tools Palette under the View dropdown menu Figure 1: The Tools Palette Figure 1 shows the Tools palette. The editing tools are used to perform specific tasks. The hand may be selected to operate front panel controls. This tool is also available in run mode. The top control, shaped like a wrench and screwdriver, enables or disables automatic tool selection. Manual tool selection can be done simply by clicking on the appropriate tool. The positioning tool looks like a normal mouse cursor, and may be used to select, move, and resize objects. The labeling tool has an upper case A on the icon, and is used to create and edit text labels. The icon shaped like a spool of wire is the wiring tool used to wire objects together on the block diagram; this tool also assigns controls and indicators on the front panel to terminals on the VI icon connector. The paint brush tool may be used to add color to objects and backgrounds. Adding Numeric Controls/Indicators to the Front Panel: Open the Numeric Controls and Indicators by clicking the right mouse button. A palette of controls and indicators should appear. Make a selection by using the mouse to 2

3 highlight the desired control, then clicking the left mouse button. Position the control to the desired location on the front panel and click the left mouse button. Practice placing a knob on the front panel. Move the knob around by using the positioning tool and dragging it with the left mouse button. Notice that when you move the positioning tool cursor near the corners of the knob a "frame corner" appears. This is a resizing handle. Clicking and dragging on a handle allows you to resize an object. Use the Labeling tool to change the first and last values of the numbers around the outside of the knob. In LabVIEW parlance, to pop-up means to right-click on an object. Pop-up on the knob you have placed on the front panel and look though the types of settings and properties you can set for this object. You can change the precision, number base, etc. of the knob. You can also change the knob from a control to an indicator or hide/reveal a label that is bound to the knob. Bound labels are always connected to an object and move along with the object if it is relocated. Bound labels also identify component terminals in the wiring diagram window. Adding Boolean Controls/Indicators to the Front Panel: Boolean (true/false) controls and indicators are available from the Boolean palette of the Controls palette. Boolean controls simulate push-buttons or toggle/slide switches and come in a variety of shapes. Boolean indicators include different shapes of LEDs or light bulbs. Popping up a menu on a Boolean object allows you to customize their behavior to some degree. Practice adding some Boolean controls or indicators to your front panel. It is also a good idea to begin getting in the habit of affixing bound labels to your front panel elements. Adding Graphs to the Front Panel: LabVIEW supports several types of graph and chart indicators. A graph indicator is a two-dimensional display of one or more plots. A graph indicator acquires and plots data all at once, as opposed to a chart which also displays one or more plots, but acquires the data and updates the display point by point. Graphs and charts are available from the Graph palette of the Controls palette. Graph or chart pop-up menus allow you to customize such items as X and Y axis scaling, linear or logarithmic spacing, and legends. Try adding a graph or chart to your front panel before proceeding. The Wiring Diagram Panel: Now select Show Diagram from the Windows menu. This will place you in the wiring diagram window of the VI. It is in this window that you will connect your front panel controls and indicators with arithmetic operators, functions, and other structures. Notice that any front panel elements that you may have added appear here along with their bound labels. Pop-up anywhere in the diagram window to examine the Functions palette. Many of the function palettes used in the lab can be found under the palette labeled Programming. For example, to find the Structures palette, right-click in any blank section of the block diagram window, go to the Programming palette and then go to the first palette at the top labeled Structures. This sub palette contains common programming 3

4 structures. Numeric constants are similarly found in the palette labeled Numeric also under the All Functions palette. Look through the other sub palettes in the Functions palette to get an idea of what building blocks are available to you. Programming Structures: LabVIEW supports several program flow structures that are common to other languages that you may have used. FOR and WHILE loops are supported to control repetitive operations, as are CASE structures which can be used to make decisions. The icon for each structure is a resizable box with a distinctive border. Any objects placed inside the structure will be controlled by the structure. Place a For loop and a While loop somewhere in the diagram window now. Be sure to identify the parts of these structures as they are discussed here. A For loop executes its contents a number of times specified by the count terminal, N. The count terminal may be set explicitly by wiring a value from outside the loop to the upper left-hand corner of the count terminal. The other edges of the count terminal are exposed to the inside of the loop so that you can access the count internally. The iteration terminal, i, contains the current number of completed iterations. For loops can also iterate over the elements of an array. In this case, simply wiring an array into the for loop will create an auto-index tunnel and not require an explicit connection to the count terminal, N. The for loop will iterate over the entire array. If multiple arrays are tunneled into the for loop, the loop will iterate element by element until the smallest array is completed. A While loop executes its contents until a Boolean value you wire to the conditional terminal is FALSE. The conditional terminal looks like a rotational arrow. LabVIEW checks the conditional value at the end of each iteration, and continues if the value is TRUE, thus the loop always executes at least once. The default value of the conditional terminal is FALSE, so if it is left unwired, the loop iterates only once. The iteration terminal in a While loop behaves exactly like its For loop counterpart. LabVIEW creates terminals, or "tunnels," for passing data into or out of a structure automatically where wires connecting outside nodes and inside nodes cross the structure boundary. Tunnels always have one edge exposed to the inside of the structure and one edge exposed to the outside. A tunnel always resides on the border of the structure, but you can move it anywhere along that border by dragging it. Tunnels into for loops can behave as an indexer with the array data type. This causes the for loop to lookup the element specified by the iteration terminal i and to loop until the array has been fully parsed. If you wish to pass the entire array into a for loop, you can right click the tunnel and select Disable indexing. Building Your First VI - An Electronic Thermometer Begin by closing the current VI and opening a fresh one, or by deleting all objects in the wiring diagram and front panel windows. 4

5 Thermometer Control Panel: One of the first steps in creating any VI is to build a front panel that contains the necessary controls and indicators. Use the following procedure to create your front panel. 1. Position the cursor near the center of the front panel window. 2. Pop-up (by pressing the right mouse button) and select the thermometer icon. It is located under the Express Menu, then the Numeric Indicators Menu, and finally Thermometer. 3. The label appears above the thermometer's digital display as a gray rectangle with Thermometer written in it. Change it to read "Temperature". The label border expands to accommodate the size of the text as you type. To remind you to end an editing operation, an Enter button appears on the far left of the tools palette. If you do not enter text into the label before performing the next keyboard or mouse action, the label disappears. If this happens, pop-up on the thermometer and select Label from the Show option in the pop-up menu. 4. The default scale on the temperature indicator ranges from 0.0 to You can change the scale by highlighting the high and low scale markers with the operating or labeling tool and typing new values. Highlight 0.00 by double-clicking on it, type 70.0, and press the <ENTER> key on the numeric keypad. Similarly, change to LabVIEW automatically scales the intermediate increments. Thermometer Wiring Diagram: Next, we'll begin to wire the VI to perform a desired function. Select Show Block Diagram from the Windows menu. The untitled diagram is now the active window. It should already contain the terminal for the temperature indicator and the label you created. 1. Select the Positioning tool from the Edit Palette (Shift and right mouse button). Use the positioning tool to move the temperature indicator terminal to the right portion of the wiring diagram. Dragging the terminal also moves the label. Dragging the label moves only the label. 2. Find the Demo Voltage Read Function and place it to the left on the wiring diagram. To find the Demo Voltage Read function, right click on the wiring diagram to get the Functions pop-up menu. Then go to Select a VI and select the DemoVoltageRead VI from the ME365_support.llb library. 3. Pop-up between the Demo Voltage Read function and the Temperature terminal and choose the Multiply function. This is found in the Mathematics Menu followed by the Numeric Menu. 5

6 4. Pop-up below the Demo Voltage Read node and choose a Numeric Constant. This is found in the Programming Menu followed by the Numeric Menu. Edit the constant to have a value of With the Positioning tool, arrange the objects on the block diagram as shown in figure 2. Figure 2: Block Diagram 6. Select the Wiring tool. (When working in the block diagram, you can toggle between the Wiring tool and the Positioning tool by pressing the space bar.) 7. Position the Wiring tool over the Demo Voltage Read node titled Measured Voltage. The node flashes, indicating that you can wire to it. 8. Click and release the left mouse button, and move the mouse to the right. As you move the tool away from the node, the wire spools off horizontally. 9. Click the left mouse button. This tacks the wire so you can change direction. Move the Wiring tool over the top-left input terminal on the Multiply icon. It will flash, signaling that you can make a connection. Click the mouse button to complete the operation. A colored line indicates a good connection between the Demo Voltage Read and the Multiply node. A bad wire appears as a black dashed line, indicating a faulty connection. If you create a bad wire, select Remove Bad Wires from the Edit Menu and rewire the objects. Continue to wire the diagram as shown in figure 3. The order or direction in which you wire objects is unimportant. If, during the process of wiring the VI, you notice that the arrow on the Run button appears "broken," then there is an error in the diagram. Select Remove Bad Wires from the Edit menu to remove any small wire fragments that you may have left in the diagram. The Run button arrow should not appear broken when all bad wires are removed and the necessary connections are made. 6

7 Figure 3: Completed Wiring Diagram A VI can be run from either the diagram window or the front panel, but for this example select Show Panel from the Windows menu, and run the VI several times by clicking on the Run button. To run the VI repeatedly click on the Continuous Run button, which looks like two arrows going around in a circle. To stop the VI, click on the stop sign button. At this point, it would be a good idea to save your work. To save a VI that has been newly written: 1. Select Save from the File menu. You are presented with a save dialog box, similar to the ones you've used in Excel or Word. 2. Change the Save directory to your own computer account or flash drive. 3. Type therm.vi in the dialog box that appears and click OK. During the course of the semester, you will be called upon to modify existing VIs. In order to store your modifications, you should use the Save As command. This will allow you to make a copy of the modified VI. Thermometer Icon: It is possible to call one VI within a wiring diagram of another. This is similar to the notion of creating subroutines or subprograms in other computer languages. Since LabVIEW is graphically based, you must create an icon and connector for any VI you wish to make use of elsewhere. The icon is the graphical symbol for the VI, and the connector assigns controls and indicators to input and output VI terminals. 1. Switch to the Panel window if you are not already there and get in edit mode (found under the operate fly-out menu). 2. Pop-up on the blank icon in the top right corner of the panel window and select Edit Icon. 7

8 3. Clear the icon area by selecting Layers tab of the Icon Editor, highlighting the User Layer VI Icon and clicking the red X button. Figure 4 shows the Layers tab. Figure 4: Edit Icon Window The tools on the right side of this window are used to create the icon in the pixel editing area, which is the large square box. The normal-size image of the icon appears in lower left corner of the large icon editing region. 3. Use the fill tool, Glyphs, Icon Text tab to make an icon like the one shown in figure 5. When you are finished, click on the OK button of the Icon Editor window. Your icon replaces the blank icon. Figure 5: Completed Icon 8

9 Data is passed to and from a sub VI through the terminals of its connector pane. You define connections by choosing the number of terminals you want for the VI and by assigning a front panel control or indicator to each of those terminals. Only the controls (inputs) and indicators (outputs) you will use during programming need terminals on the connector pane. 4. Pop-up on the icon (with the right mouse button) you just created and select Show Connector. 5. Pop-up (with the right mouse button) on the connector pane and choose Patterns. Each of the rectangles on the connector represents a terminal area for input to or output from the VI. You can select other patterns if needed by selecting them here. Choose a pattern with one input and one output. 6. Now you must assign the terminal on the connector to the temperature indicator. Select the Wiring tool. 7. Click on the terminal in the connector with the Wiring tool. It turns black. Then click on the temperature indicator with the same wiring tool. LabVIEW highlights it. The assignment is now complete. 8. Pop-up on the connector and select Show Icon. 9. Close the VI by clicking on the close box of the panel window. When the dialog box asks you to save changes, click on the Yes button. Congratulations! You have just built your first VI! Now we will see how to use it as a "sub VI" in the next VI we build. Building the Second VI - A Temperature Monitoring Instrument: We will now build a new virtual instrument that will monitor the temperature using the Thermometer VI we just finished and will plot the data on a strip chart recorder. Control Panel: The VI needs a switch to start and stop the acquisition and a strip chart to display the temperature. Open a new VI by clicking on the New VI button in the dialog box. An empty panel and diagram window will come up on the screen of your computer. 1. Choose a toggle switch from the Boolean Palette of the Controls Menu. Label the switch "Power." 2. Choose the waveform chart from the Graph Palette of the Controls Menu. Label it "Temperature History." Change the range to accommodate temperature values. With the operating tool, double click on the lower limit and type Similarly change the upper limit of the strip chart to

10 3. Position the components as shown in figure 6. Figure 6: Front Panel with Labels Wiring Diagram: 1. Select Show Block Diagram from the Window Menu. 2. Move the two terminals to the top of the window. If necessary, pop-up on the terminals and select Show Labels. (You can select more than one object at a time by clicking on the first object to select it, then holding the shift key while clicking on all additional objects.) 3. Pop-up in a free area of the diagram and choose Select a VI from the functions menu. Locate and open the VI library you created and select the Thermometer VI program you just saved in the earlier portion of the laboratory. Move the Temp icon up to the top of the diagram and out of the way. 4. Add a While loop by popping-up in an open area of the diagram, the While Loop is under Execution Control. Resize the While loop placing it around all three objects. 10

11 5. Wire the diagram as shown in figure 7. Please check the Stop/Continue if True setting is the same as in figure 7 (in the right bottom corner). Figure 7: Initial Block Diagram of Temperature Monitor 6. Select the Wait Until Next ms Multiple Function. It is found by right clicking on an empty portion of the block diagram. Select the View All (the double arrow down), followed by the Programming Menu, followed by the Timing Menu, and finally the Wait Until Next ms Multiple Function (it looks like a metronome). 7. Select a Numeric Constant from the Numeric Palette. It appears highlighted on the block diagram. Type in a value of 1000 and wire the constant to the Wait Until Next ms Multiple Function. This block ensures that no iteration occurs before the specified number of milliseconds has elapsed. In this case, the loop will execute once per second. Your wiring diagram should look like the one shown in figure 8. 11

12 Figure 8: Final Wiring Diagram of Temperature Monitor with Timing Run your VI using the run button, not the continuous run button. The VI has been wired so that turning off the power switch on the front panel will halt the program. Save your new VI. Printing VIs: Print your VI to turn in to your laboratory instructor at the end of the period. To do this, choose Print under the File menu. The printout of your VI will include the front panel, the wiring diagram, and a hierarchy of the sub VIs called by the top-level VI. This printout gives you a record of the source code of the instrument you have constructed. A General Waveform Measurement VI: Please close all VIs and load the Multichannel Scan.vi located in the ME365.llb library located in the ME365 folder on the Desktop. Look at the front panel of the instrument, as shown in figure 9. Along the left side of the front panel are several controllers used to set the channels that are scanned, the number of sample points in each channel, the sampling rate in samples per second, the input range of each channel, and whether the data will be saved to disk. Next to the controls is a waveform indicator. A title is displayed above the indicator. The default value is Multi Channel Scan, however it may be changed to a more descriptive title. To the right side of the plot area of the indicator is a legend in which 12

13 displays the channels selected. If the data is to be saved to a file, then the title is stored along with the date and time from the system clock. Legend labels are used to label the data columns in the spreadsheet file. Take a few minutes to examine the wiring diagram for this VI. Can you identify some of the components and structures we used earlier? Measurements with the VI: Figure 9: Front Panel of Multichannel Scan.vi In this section you will be asked to measure a 5 volt RMS 500 Hz sine wave with the VI and record the signal in an EXCEL spreadsheet file. You are to collect exactly 10 cycles of the waveform with 20 points in each cycle. Use these values to calculate the Sample Rate and Samples/channel that you will use in Multichannel Scan.vi. 1. Set the function generator to produce the required signal (REMEMBER High Z Setting). 2. Connect the signal to the channel 0 input on the data acquisition board interface box. 3. Set the VI to collect the signal and save it to a file. Since only one input channel is being used, the Samples per Channel is the total number of data points that you wish to collect. Options in the Channel Groups section allow the user to specify input ranges for individual channels. The default values are fine for this application. To save the data, click Save Data button and toggle the switch to its down position. After the data is collected you will be prompted for a file name. Save the data on your 13

14 account drive or external memory device as an EXCEL file with a.xls extension. 4. Switch to Windows and launch EXCEL. Load the file you just saved into EXCEL and view the data. Make a plot of the signal using Chart Wizard to be turned into your laboratory instructor. 14