Analyzing & Saving A Signal (Chapter 3)

• Building an Analysis VI

In the following exercises, you will build a VI that generates a signal, filters 

the signal, indicates if the signal exceeds a certain limit, and records the 

data. After you complete the exercises, the front panel of the VI will look simila

r to the front panel in Figure 3-1.

• Modifying a VI Created from a Template

Complete the following steps to create a VI that generates, analyzes, and displays a signal.

1. In the Getting Started window, click New to display the New dialog box.
2. From the Create New list, select VI»From Template»Tutorial (Getting Started)»Generate, Analyze, and Display. This template VI simulates a signal and analyzes it for its root mean square (RMS) value.
3. Click the OK button or double-click the name of the template to create a VI from the template.
4. If the Context Help window is not visible, press the <Ctrl-H> keys to display the window. (Mac OS X) Press the <Command-Shift-H> keys.
5. Display the block diagram by pressing the <Ctrl-E> keys.
6. Move the cursor over the Amplitude and Level Measurements Express VI, shown at left. The Context Help window displays information about the behavior of the Express VI. Keep the Context Help window open. It will provide useful information as you complete the rest of this exercise. 
7. Display the front panel and remove the RMS indicator, shown at left. You will not use the RMS functionality of the Amplitude and Level Measurements Express VI for this exercise. However, you can use the Generate, Analyze, and Display template VI with the RMS functionality in the future to reduce development time.
8. Display the block diagram and remove any broken wires that result from removing the RMS indicator. To remove all broken wires from the block diagram, you can press the <Ctrl-B> keys.
9. Then return to the front panel window and right-click the waveform graph indicator. Select Properties from the shortcut menu. The Graph Properties dialog box appears.
10. On the Appearance page, place a checkmark in the Visible checkbox in the Label section and enter Unfiltered Signal in the text box.
11. Click the OK button to save the configuration and close the Graph Properties dialog box.
13. Run the VI. The signal appears in the graph.
14. Click the STOP button to stop the VI.

• Adding a Signal

The Simulate Signal Express VI simulates a sine wave by default. You can customize the simulated signal by changing the options in the Configure Simulate Signal dialog box.

Complete the following steps to create an additional simulated signal that adds uniform white noise to the sine wave.

1. On the block diagram, use the Positioning tool to select the Simulate Signal Express VI. Hold down the <Ctrl> key and click and drag to create an additional Simulate Signal Express VI on the block diagram. (Mac OS X) Hold down the <Option> key and drag. (Linux) You also can hold down the middle mouse button and drag.
2. Release the mouse button to place the copied Simulate Signal Express VI below the original Simulate Signal Express VI. LabVIEW updates the name of the copied Simulate Signal Express VI to Simulate Signal2.
3. Double-click the Simulate Signal2 Express VI to display the Configure Simulate Signal dialog box.
4. Select Sine from the Signal type pull-down menu.
5. Enter 60 in the Frequency (Hz) text box.
6. Enter 0.1 in the Amplitude text box.
7. Place a checkmark in the Add noise checkbox to add noise to the sine signal.
8. Select Uniform White Noise from the Noise type pull-down menu.
9. Enter 0.1 in the Noise amplitude text box.
10. Enter -1 in the Seed number text box.
11. In the Timing section, select the Run as fast as possible option
12. In the Signal Name section, remove the checkmark from the Use signal type name checkbox.
13. Enter 60 Hz and Noise in the Signal name text box. When you change the signal name in the Configure Simulate Signal dialog box, LabVIEW changes the name of the signal output on the block diagram. Changing the signal name makes it easier for you to identify the signal type when you view the Express VI on the block diagram. The Result Preview section displays a random signal. The Configure Simulate Signal dialog box should appear similar to Figure 3-2.
14. Click the OK button to save the current configuration and close the Configure Simulate Signal dialog box.

• Adding Two Signals

To add two signals together to create one signal, you can use the Formula Express VI. Rather than merging two signals on one graph, the Formula Express VI adds both signals together to create a single signal on the graph. You can use this Express VI to add noise to a signal.

Complete the following steps to add the 60 Hz and Noise signal to the Sine signal.

1. In the block diagram window, triple-click the wire that connects the Sine output of the Simulate Signal Express VI to the Signals input of the Amplitude and Level Measurements Express VI and to the Unfiltered Signal indicator. Remove the wire.
2. On the Functions palette, click the Search button to search for the Formula Express VI, shown at left, and add it to the block diagram between the Simulate Signal Express VIs and the Amplitude and Level Measurements Express VI. The Configure Formula dialog box appears.
3. In the Label column, change the label for X1 to Sine and the label for X2 to 60 Hz and Noise. The Formula Express VI automatically enters the first input, Sine, in the Formula text box.
4. Click the + button and then the X2 button to add Sine and 60 Hz and Noise together in the Formula text box.
5. Click the OK button to save the current configuration and close the Configure Formula dialog box.
6. Use the Wiring tool to wire the Sine output of the Simulate Signal Express VI to the Sine input of the Formula Express VI.
7. Wire the 60 Hz and Noise output of the Simulate Signal2 Express VI to the 60 Hz and Noise input of the Formula Express VI.
8. Wire the Result output of the Formula Express VI to the Unfiltered Signal indicator and to the Signals input of the Amplitude and Level Measurements Express VI.
9. Display the front panel by pressing the <Ctrl-E> keys.
10. Run the VI. The signal with added noise appears in the graph.
11. Click the STOP button to stop the VI.
12. Select File»Save As and save the VI as Analysis.vi in an easily accessible location.

• Filtering a Signal

Complete the following steps to configure the Filter Express VI to filter the 

signal using an infinite impulse response (IIR) filter.

1. Display the block diagram window and remove the wire that connects the Result output of the Formula Express VI to the Signals input of the Amplitude and Level Measurements Express VI.
2. Remove all broken wires that result from removing the wire.
3. Search for the Filter Express VI, shown at left, and add it to the block diagram between the Simulate Signal2 Express VI and the Amplitude and Level Measurements Express VI. The Configure Filter dialog box appears.
4. In the Filter Specifications section, change the Cut off Frequency (Hz) to 25.
5. Click the OK button to save the configuration and close the Configure Filter dialog box.
6. Display the front panel.
7. Click the Unfiltered Signal waveform graph indicator and press the <Ctrl> key while you drag with the Positioning tool to create an additional waveform graph indicator.
8. Triple-click the Unfiltered Signal 2 label above the new waveform graph indicator and enter Filtered Signal to change the label of the indicator. You also can change the label on the Appearance page of the Graph Properties dialog box.
9. On the block diagram, wire the Result output of the Formula Express VI to the Signal input of the Filter Express VI and to the Unfiltered Signal waveform graph indicator.
10. Wire the Filtered Signal output of the Filter Express VI to the Signals input of the Amplitude and Level Measurements Express VI and to the input of the Filtered Signal waveform graph indicator.
11. Select File»Save. The block diagram of the Analysis VI should appear similar to Figure 3-3.

• Modifying the Appearance of Graphs

Complete the following steps to change the format of the x-axis and y-axis of the Unfiltered Signal and Filtered Signal graphs.

1. In the front panel window, right-click the Unfiltered Signal graph indicator and select Properties from the shortcut menu. The Graph Properties dialog box appears.
2. On the Display Format page, select Time (X-Axis) from the top pull-down menu.
3. Select the Default editing mode option.
4. In the Type list, select Automatic formatting.
5. In the Digits field, enter 6 and select Significant digits from the Precision Type pull-down menu.
6. Place a checkmark in the Hide trailing zeros checkbox.
7. Select Amplitude (Y-Axis) from the top pull-down menu and repeat steps 3–6 so the y-axis configuration matches the x-axis configuration.
8. On the Scales page, select Amplitude (Y-Axis).
9. Remove the checkmark from the Autoscale checkbox
10. Enter -2.5 in the Minimum text box and 2.5 in the Maximum text box.
11. Click the OK button to save the configuration and close the Graph Properties dialog box.
12. Repeat steps 1–11 to configure the Filtered Signal graph indicator. The x-axis and the y-axis on the Unfiltered Signal and Filtered Signal graph indicators change to reflect the new configuration.

• Analyzing the Amplitude of a Signal

Complete the following steps to reconfigure the Express VI to measure the peak-to-peak amplitude values of the signal.

1. On the block diagram, double-click the Amplitude and Level Measurements Express VI to display the Configure Amplitude and Level Measurements dialog box
2. In the Amplitude Measurements section, remove the checkmark from the RMS checkbox.
3. Place a checkmark in the Peak to peak checkbox. Peak to peak appears in the Results section with the corresponding value of the measurement.
4. Click the OK button to save the current configuration and close the Configure Amplitude and Level Measurements dialog box. The RMS output of the Amplitude and Level Measurements Express VI changes to reflect the new Peak to Peak output, shown at left. You will use the Peak to Peak output in a later exercise.

• Controlling the Speed of Execution

To plot the points on the waveform graphs more slowly, you can add a time delay to the block diagram. A time delay slows the speed at which a VI runs.

Complete the following steps to control the speed at which the VI runs.

1. On the block diagram, search for the Time Delay Express VI.
2. Place the Time Delay Express VI inside the While Loop. The Configure Time Delay dialog box appears.
3. Enter 1.000 in the Time delay (seconds) text box and click the OK button
4. Display the front panel and run the VI. The VI runs more slowly. The loop iterates once every second.
5. Stop the VI.

• Adding a Warning Light

Complete the following steps to add a warning light to the VI.

1. Display the Controls palette by right-clicking any blank space in the front panel window.
2. On the Express palette, select the LEDs palette.
3. Select the round LED indicator and add it to the front panel to the left of the waveform graphs.
4. Double-click the Boolean label above the LED and enter Warning to change the label of the LED.
5. Select File»Save As to display the Save As dialog box.
6. Read the various dialog box options. Select the Copy and Substitute copy for original options to create a copy of the original VI and immediately edit the copy.
7. Click the Continue button and save the VI as Warning Light.vi in an easily accessible location.

• Setting a Warning Level Limit

To specify the value at which you want the LED to light, use the Comparison Express VI.

Complete the following steps to compare the peak-to-peak value to a limit you set.

1. On the block diagram, search for the Comparison Express VI and place it to the right of the Amplitude and Level Measurements Express VI. The Configure Comparison dialog box appears.
2. In the Compare Condition section, select the > Greater option
3. In the Comparison Inputs section, select Value and enter 2 in the Value numeric control to assign a constant value at which you want the LED to light.
4. Click the OK button to save the current configuration and close the Configure Comparison dialog box. The name of the Comparison Express VI changes to reflect the operation of the Express VI, shown at left. Greater indicates that the Express VI does a greater than comparison.
5. Wire the Peak to Peak output of the Amplitude and Level Measurements Express VI to the Operand 1 input of the Greater Express VI.
6. Move the cursor over the wire that connects the Peak to Peak output to the Operand 1 input.
7. When the Positioning tool appears, right-click the wire that connects the Peak to Peak output to the Operand 1 input and select Create» Numeric Indicator from the shortcut menu.

• Warning the User

After you specify the values at which you want the LED to light, you must wire the LED to the Greater Express VI.

Complete the following steps to provide a visual cue when the peak-to-peak value of the signal exceeds a specified limit.

1. In the block diagram window, move the Warning terminal to the right of the Greater Express VI. Make sure the Warning terminal is inside the While Loop, as shown in Figure 3-4.
2. Wire the Result output of the Greater Express VI to the Warning terminal.The block diagram should appear similar to Figure 3-4.
3. Display the front panel. A numeric indicator labeled Peak to Peak appears in the front panel window. This indicator displays the peak-to-peak value of the signal.
4. Run the VI. When the peak-to-peak value exceeds 2.0, the Warning indicator lights.
5. Click the STOP button to stop the VI.
6. Save the VI.

• Configuring a VI to Save Data to a File

To store information about the data a VI generates, use the Write To 

Measurement File Express VI.

Complete the following steps to build a VI that saves peak-to-peak values 

and other information to a LabVIEW data file.

1. Search for the Write To Measurement File Express VI and add it to the block diagram below and to the right of the Amplitude and Level Measurements Express VI.
2. In the Configure Write to Measurement File dialog box, locate the If a file already exists section and select the Append to file option to write all the data to the test.lvm file without erasing any existing data in the file.
3. In the Segment Headers section, select the One header only option to create only one header in the file to which LabVIEW writes the data. 
4. Enter the following text in the File Description text box: Sample of peak to peak values. LabVIEW appends the text you enter in this text box to the header of the file.
6. Click the OK button to save the current configuration and close the Configure Write To Measurement File dialog box.

• Saving Data to a File

Complete the following steps to generate the test.lvm file.

1. On the block diagram, wire the Peak to Peak output of the Amplitude and Level Measurements Express VI to the Signals input of the Write To Measurement File Express VI.
2. Select File»Save As and save the VI as Save Data.vi in an easily accessible location.
3. Display the front panel and run the VI.
4. Click the front panel STOP button.
5. To view the data you saved, open the test.lvm file in the LabVIEW Data directory with a spreadsheet or text-editing application. The file has a header that contains information about the Express VI. 
6. Close the file after you finish looking at it and return to the Save Data VI.

• Adding a Button That Stores Data When Clicked

Complete the following steps to add a button to the VI and configure how the button responds when a user clicks it.

1. Display the front panel and search the Controls palette for a rocker button. Select one of the rocker buttons and place it to the right of the waveform graphs.
2. Right-click the rocker button and select Properties from the shortcut menu to display the Boolean Properties dialog box.
3. Change the label of the button to Write to File.
4. On the Operation page of the Boolean Properties dialog box, select Latch when pressed from the Button behavior list.
6. Click the OK button to save the current configuration and close the Boolean Properties dialog box.
7. Save the VI.

• Saving Data When Prompted by a User

Complete the following steps to build a VI that logs data to a file when the

user clicks a button on the front panel.

1. In the block diagram window, double-click the Write To Measurement File Express VI to display the Configure Write To Measurement File dialog box.
2. Change the filename test.lvm in the Filename text box to Selected Samples.lvm to save the data to a different file.
3. Click the OK button to save the current configuration and close the Configure Write To Measurement File dialog box.
4. Right-click the Signals input of the Write To Measurement File Express VI and select Insert Input/Output from the shortcut menu to insert the Comment input.
5. Right-click the Comment input of the Write To Measurement File Express VI and select Select Input/Output»Enable from the shortcut menu to replace the Comment input with the Enable input.
6. Move the Write to File terminal to the left of the Write To Measurement File Express VI.
7. Wire the Write to File terminal to the Enable input of the Write To Measurement File Express VI.

• Viewing Saved Data

Complete the following steps to view the data that you save to the Selected Samples.lvm file.

1. Display the front panel and run the VI. Click the Write to File button several times.
2. Click the STOP button.
3. Open the Selected Samples. lvm file with a spreadsheet or text-editing application.
4. Close the file after you finish looking at it.
5. Save and close the VI.