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Order Spectrum

An order spectrum is like a power spectrum in that it displays frequency content. However, an order spectrum provides additional insight for harmonically interrelated systems in rotating machinery.

An order refers to a frequency that is a certain multiple of a reference rotational speed. For example, a vibration signal with a frequency equal to twice the rotational frequency of a motor corresponds to an order of 2. Likewise, a vibration signal that has a frequency equal to 0.5 times the rotational frequency of the motor corresponds to an order of 0.5. For a rotating system, the primary shaft rotation drives the fundamental frequency. The order spectrum for this system quantifies the relative strength of the system harmonics with respect to shaft rotation. This approach has the following benefits:

  • Because the spectrum contains frequency ratios rather than absolute frequencies, the computation is insensitive to variations in shaft speed. This insensitivity decouples the change in the frequency location of a certain harmonic from the harmonic amplitude. In contrast, a power spectrum would be time-varying with shaft-speed variation.

  • Different harmonics point to different rotating components within the system. Features that detect changes in a single harmonic can therefore potentially isolate individual components or locations within the system.

Rotation Information

Specify the source of rotation rate.

  • Constant rotation speed — Use this option when you can represent the rotation speed of all members with a single scalar.

  • Time-varying rotation speed (RPM) — Use this option when you have a signal containing rpm information for every member.

  • Tacho signal — Use this option when you have a signal containing tachometer pulses.

Window Settings

  • Window type — For information on order spectrum window types, see 'Window' in the Name-Value Pair Arguments section of orderspectrum.

  • Overlap percent — A value of 0 for overlap percent means that adjoining segments do not overlap. A value of 100 means that adjoining segments shift by one sample. A larger overlap percentage produces a smoother result but increases the computation time.

Additional Information

The software stores the results of the computation in a new variable. The new variable name includes the source signal name with the suffix os.

For more information on order spectrum computation, see orderspectrum.