Order Magnitude and Phase (Sound and Vibration Measurement Suite)
December 2007
NI Part Number:
372416A-01
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You can compute the order magnitude and phase from an even-angle signal. You also can compute the magnitude and phase of a specific order from the order waveform. Unlike an order power spectrum, which provides information for all of the orders of a certain time block signal, the magnitude and phase provide information only for one particular order relative to time. Order magnitude and phase also can provide order information relative to speed when you perform a run-up or run-down test. Thus, order magnitude and phase can help you analyze sound or vibration signals by focusing on specific orders.
Use a spectral map or order power spectrum to identify the most significant orders in a sound or vibration signal. After identifying the most significant orders, you can extract an order waveform or its magnitude and phase to get detailed information for individual orders.
Many mechanical faults are associated with certain orders. Analyzing order magnitude and phase can help you detect mechanical faults directly. For example, a strong first order magnitude indicates imbalance of a machine in most cases. Analyzing the first order magnitude can help you identify the source of the imbalance. Moreover, the magnitude and phase of the first order can help you correct the imbalance when you add weights on the appropriate rotor positions.
Use the Order Tracking Express VI to calculate the order magnitude and phase from an even-angle signal. This Express VI can return the magnitude in RMS, peak, or peak-to-peak units. This Express VI also can return the phase in degrees or in radians.
Phase Definition
Phase describes the relative timing between two signals. Phase is the angle difference between a measured point and a reference point. You can use phase to locate the imbalance location on a rotor. Measuring the phase of vibration signals requires a reference signal or a reference trigger point. In machinery vibration analysis, tachometer pulses work as the reference trigger points.
The phase measurement in machinery vibration measurements uses the phase lag convention. Phase is the angle difference measured from the peak of a vibration signal backward in time to the reference trigger point.
The following illustration shows a phase of zero degrees and the relationship between a vibration signal and reference signal.
The shaft has a heavy spot and a keyway slot. When the keyway slot passes the proximity probe tachometer, the tachometer detects a trigger pulse. The heavy spot causes the shaft to vibrate as the shaft rotates. When the heavy spot passes the proximity probe, the vibration reaches a peak. The heavy spot passes the proximity probe and the keyway slot passes the tachometer simultaneously. Thus the peak of the vibration does not lag or lead the reference trigger point. At this point, the phase is zero degrees.
The following illustration shows a phase of 90 degrees and the relationship between a vibration signal and reference signal.
The peak of the vibration signal lags 90 degrees behind the trigger point. When the vibration signal reaches the peak, rotate the shaft backward or counter to the rotation direction until the keyway slot passes the tachometer. The number of degrees you rotate the shaft is the phase lag, or the phase value in machinery vibration measurement.