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An order power spectrum provides a quantitative description of the amplitude or power of the orders in a signal. An order power spectrum provides a view of all order components of a signal and can help you find significant orders and compare the level of different order components.

When analyzing machinery sound and vibration, you typically compute an order power spectrum after displaying the spectral map. After you identify a certain block of a signal as the signal of interest on the spectral map, you can compute an order power spectrum on this signal block to get more detailed order information. You can identify the characteristic order components, form a quantitative spectrum measurement of the orders, and compare the amplitudes of different orders. The following front panel shows a typical order power spectrum for a gearbox.

You can identify the significant orders, like the 48^th and 72^nd orders, measure the amplitude of the orders, and compare different orders in this plot.

Use the Order Spectrum Express VI to compute the order power spectrum of a signal.

Spectrum Averaging

Averaging successive measurements can improve measurement accuracy. You typically average spectra instead of directly averaging a time signal. The Order Spectrum Express VI supports spectrum averaging. You can choose from the following averaging modes to perform spectrum averaging.

• RMS averaging
• Vector averaging
• Peak hold

RMS Averaging

RMS averaging averages the power of a signal. The averaged RMS spectrum does not contain phase information. Thus, performing RMS averaging on spectra can reduce signal fluctuation but not the noise floor. Use RMS averaging when you apply an order power spectrum to an unknown even-angle signal. RMS averaging works with signals with different order components and generates spectrum results. The Order Spectrum Express VI computes RMS averaging for an order power spectrum according to the following equation:

where X is the complex Fast Fourier Transform (FFT) of the even-angle signal X, and X* is the complex conjugate of X.

Vector Averaging

Vector averaging, also called coherent averaging, or time or angle synchronous averaging, can reduce the noise floor in an even-angle signal. Vector averaging computes the complex quantity and averages the real and imaginary parts of the even-angle signal separately. Use vector averaging when you already have some knowledge of the signal and want to reduce the noise floor in the signal. The Order Spectrum Express VI computes vector averaging for order power spectra according to the following equation:

where X is the complex FFT of the even-angle signal X, and X* is the complex conjugate of X.

When performing vector averaging, use a triggered even-angle signal. Otherwise, you might eliminate strong order components in the averaged spectrum. The Order Spectrum Express VI automatically generates a triggered even-angle signal for vector averaging. You also can use the OAT Output Triggered Even Angle Signal VI to generate a triggered signal for vector averaging. You must specify a trigger period when you use the OAT Output Triggered Even Angle Signal VI. The trigger period determines the frequency for triggering the signal output. The trigger period must be equal to the period of significant order components in the signal. For example, when the significant order component is the 0.5^th order, set the trigger period to two revolutions. The OAT Output Triggered Even Angle Signal VI uses a trigger period that is the smallest integer multiple of the trigger period you specified and greater than the block size. Suppose the sampling rate is 160 samples per revolution and the block size is 512. The OAT Output Triggered Even Angle Signal VI uses four revolutions as the trigger period, as shown in the following illustration:

Peak Hold

Peak hold averaging is performed at each individual order and retains the RMS peak levels of the averaged quantities from one FFT spectrum to the next. Peak hold averaging is most useful when configuring a measurement system or when applying a limit to an order spectrum. The Order Spectrum Express VI computes peak hold averaging for order power spectra according to the following equation:

where X is the complex FFT of the even-angle signal X, and X* is the complex conjugate of X.

Weighting Mode

You can choose from the following weighting modes when performing RMS or vector spectrum averaging: linear or exponential.

Linear weighting weights each individual spectrum by the same amount in the averaged spectrum. Use linear weighting for analysis purposes.

Exponential weighting weights the most recent spectrum more than the previous spectra, which makes the averaged spectrum more responsive to changes in the input signal. This responsiveness makes exponential weighting ideal for the configuration phase of a measurement. Exponential weighting also is useful for monitoring applications because the averaged spectrum responds to a singular event. A linearly averaged spectrum might not respond noticeably to a singular event, especially with a large number of averages.

Extended Measurement

You can use the Extended Measurements VIs to perform extended measurements on an order power spectrum, such as unit conversion, spectrum peak search, power in band, and limit testing.

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