【正文】 Taking an NIDAQmx Measurement in LabVIEW. Note The following exercises require that you have installed NIDAQmx and an NIDAQmxsupported device. Refer to the National Instruments Web site at NIDAQmx installed or an NIDAQmxsupported device, refer to the LabVIEW Measurements Manual for information about using Traditional NIDAQ for data acquisition. Complete the following exercises to create an NIDAQmx task that continuously takes a voltage reading and plots the data on a waveform graph. You can plete the exercises in approximately 30 minutes. Creating an NIDAQmx Task In NIDAQmx, a task is a collection of one or more channels, timing, triggering, and other properties that apply to the task itself. Conceptually, a task represents ameasurement or generation you want to perform. For example, you can create a task to measure temperature from one or more channels on a DAQ device. Complete the following steps to create and configure a task that reads a voltage level from a DAQ device. 1. Open a new VI. 2. Select the DAQ Assistant Express VI, shown at left, on the Input palette and place it on 10 the block diagram. The DAQ Assistant launches and a Create New dialog box appears. 3. Click the Analog Input button to display the Analog Input options. 4. Select Voltage to create a new voltage analog input dialog box displays a list of channels on each DAQ device installed. The number of channels listed depends on the number of channels you have on the DAQ device. 5. In the My Physical Channels listbox, select the physical channel to which the signal is connected, such as ai0, and then click the Finish button. The DAQ Assistant opens a new window, shown in Figure 41, which displays options for configuring the channel you selected to plete a task. Figure 41. Configuring a Task Using the DAQ Assistant 6. In the Input Range section of the Settings tab, enter 10 for the Max value and enter 10 for the Min value. 7. In the Task Timing tab, select the Acquire N Samples option. 8. Enter a value of 1000 in the Samples To Read input box. 11 Testing the Task You can test the task to verify that you correctly configured the channel. Complete the following steps to confirm that you are acquiring data. 1. Click the Test button, show。Control to create the Enable switch. 4. Rightclick the wire linking the Sine output in the Simulate Signal Express VI to the Signals input in the Signal Compression Express VI and select Create187。 HowTo Help to open the LabVIEW Help. 2. Click the Search tab and type sample pression in the Type in the word(s) to search for text box. Notice that this word choice reflects what you want this Express VI to do— press, or reduce, the number of samples in a signal. 3. Select the Sample Compression Express VI topic to display the topic that describes the Sample Compression Express VI. 4. After you read the description of the Express VI, click the Place on the block diagram button, shown at left, to select the Express VI. 5. Move the cursor to the block diagram. Notice how LabVIEW attaches the Sample Compression Express VI to the cursor. 6. Place the Sample Compression Express VI on the block diagram to the right of the Simulate Signal Express VI. 7. Configure the Sample Compression Express VI to reduce the signal by a factor of 25 using the mean of these values. 8. Close the Configure Sample Compression dialog box. 9. Using the Wiring tool, wire the Sine output in the Simulate Signal Express VI to the Signals input in the Sample Compression Express VI. Customizing the Front Panel In the previous exercises, you added controls and indicators to the front panel using the Controls palette. You also can add controls and indicators from the block diagram. Complete 19 the following steps to create controls and indicators. 1. Rightclick the Mean output in the Sample Compression Express VI and select Create187。New VI from the front panel or block diagram menu bar. 2. If the Functions palette is not visible, rightclick any blank space on the block diagram to bring up the temporary version of the Functions palette. Click the thumbtack, shown at left, in the upper left corner of the Functions palette to place the palette on the screen. Note You can rightclick on a blank space on the block diagram or the front panel to bring up the Functions or Controls palettes. Adding an Express VI that Simulates a Signal Complete the following steps to find the Express VI you want to use and then add it to the block diagram. 1. If the Context Help window is not visible, press the CtrlH keys to open the Context Help window. You also can press the Show Context Help Window button, shown at left, to open the Context Help window. 2. Select the Input palette on the Functions palette and move the cursor over the Express VIs on the Input that the Context Help window displays information about the function of each Express VI. 3. From the information provided in the Context Help window, find theExpress VI that can output a sine wave signal. 4. Select the Express VI and place it on the block diagram. The Configure Simulate Signal dialog box appears. 5. Idle the cursor over the various options in the Configure Simulate Signal dialog box, such as Frequency (Hz), Amplitude, and Samples per second (Hz). Read the information that appears in the Context Help window. 6. Configure the Simulate Signal Express VI to generate a sine wave with a frequency of and amplitude of 2. 7. Notice how the signal displayed in the Result Preview window changes to reflect the configured sine wave. 8. Close the Configure Simulate Signal dialog box. 18 9. Move the cursor over