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A magnitude plot can help you analyze a sound or vibration signal by focusing on particular orders.

Use the Order Tracking Express VI to compute the magnitude of designated orders. Recall that the Colormap plot showed that the seventh order of the computer fan run-up data has a strong amplitude at 2,600 RPM. You can use a magnitude plot to observe this order in more detail.

Complete the following steps to compute and display the magnitude of the seventh order of the computer fan run-up data.

1. Press the <Ctrl-E> keys to switch to the block diagram of the Getting Started Exercise VI.
2. Place the Order Tracking Express VI on the block diagram. A configuration dialog box appears.
   
    
   
   
3. On the Configuration page, set the Orders to track to 7.
4. Set the X-axis selection to Speed.
5. Click the OK button to save the current configuration and close the configuration dialog box.
6. Wire the waveform signal output of the oax_Load Data (Analog Tacho, single channel) VI to the input signal input of the Order Tracking Express VI.
7. Wire the speed profile output of the Analog Tacho Processing Express VI to the speed profile input of the Order Tracking Express VI.
8. Wire the magnitudes output of the Order Tracking Express VI to the Magnitude indicator.
   
   The block diagram should appear similar to the following block diagram.

9. Run the Getting Started Exercise VI to calculate and display the magnitude of the seventh order of the computer fan run-up data.
10. Save this VI.

On the front panel, click the Order Tracking tab to display the Magnitude plot for the seventh order of the computer fan run-up data. You can see that this order has greater amplitude around 2,600 RPM, with a dip between 2,450 RPM and 2,650 RPM, as shown in the following figure. This dip is difficult to observe with the colormap alone.

You also can use the Order Tracking Express VI to compare the magnitudes of different orders. For example, if you add a second element to the Orders to track input and set the element to 8, the Magnitude plot displays the seventh and eighth orders together, as shown in the following figure.

As you observed with both the Colormap plot and the Order power spectrum plot, the amplitude of the eighth order increases as the amplitude of the seventh order decreases between 2,800 RPM and 3,000 RPM. The Magnitude plot provides a detailed view of exactly how the two orders change in magnitude with changing speed.

In the configuration dialog box of the Order Tracking Express VI, you can set the X-axis selection of the Magnitude plot to display the order magnitude against time, speed, or number of revolutions. You also can use the Magnitude View options to specify the type of quantitative measurement you want to use to calculate and display the magnitude.

Whereas an order power spectrum shows the magnitude or power values of all orders at a specific period in time or at a specific speed, the Magnitude plot provides detailed information about particular orders that you specify.

Previous: 4. Analyzing the Order Power Spectrum

Next: 6. Extracting Order Waveforms

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