Bode Plot and Polar Plot (Sound and Vibration Measurement Suite)
December 2007
NI Part Number:
372416A-01
»View Product Info
Use a Bode plot or polar plot to analyze order magnitudes and phases in different representations. You usually combine polar plots and Bode plots to describe the locus of a rotational speed vector signal during speed changes.
Bode Plot
A Bode plot displays order magnitudes and phases as a function of rotational speed or frequency. You typically use Bode plots for transient analysis in both run-up and run-down tests. Transient analysis is the analysis of nonstationary signals. A Bode plot can help you identify the resonance speed of a rotor or examine the rotor dynamics on an order basis. The x-axis in a Bode plot is speed or frequency, which enables you to see the changes in magnitude and phase over speed or frequency. The following front panel shows the Bode plot for a rotor in a run-up test.
Notice that the magnitude reaches the peak when the rotor passes the resonance speed area at approximately 3,500 RPM. The phase shifts 180 degrees after the speed passes the resonance range, which is approximately from 1,300 RPM to 4,700 RPM.
Use the OAT Order Magnitude and Phase VI to display a Bode plot.
Polar Plot
A polar plot displays data in polar coordinates, which enables you to see phase changes in the range of zero to 360 degrees. The polar plot zero degree point always is located at the angular position of a transducer. You can compare data from orthogonally-mounted proximity probe pairs with a polar plot.
Use the OAT Polar Plot VI to display a polar plot. The OAT Polar Plot VI defines the phase of a vector signal as a phase lag value. Thus the phase increases in the direction counter to the shaft rotation in a polar plot that the OAT Polar Plot VI displays. Specify the probe angle value and shaft rotation direction in the channel settings control of the OAT Polar Plot VI to set the angular position of the transducer. This VI rotates the plot accordingly.
Similar to the Bode plot, the polar plot displays the changes in magnitude and phase over speed for the rotor in the run-up test, as shown in the following illustration.
In the previous illustration, you can see that the phase angle shifts 180 degrees after the speed passes the resonance range. In the CCW rotation polar plot, the proximity probe is at zero degrees and the shaft rotates in a counter clockwise (CCW) direction. In the CW rotation polar plot, the proximity probe is at 90 degrees and the shaft rotates in a clockwise (CW) direction. You can see how the zero degree position changes according to the angular position of the proximity probe and how the phase changes against the shaft rotational direction.