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Extract Signal Envelopes

When you import high-bandwidth signal data into Diagnostic Feature Designer, you can extract low-bandwidth signal envelopes from the signal data to isolate low-frequency vibration modulation near machine resonances. This low-frequency signal can be useful for detecting faults in rotating machinery.

Parameter Settings and General Workflow

The Signal Envelope tab, which you access from the Filtering & Averaging menu, provides the parameters for extracting the envelope of a selected signal. You can extract the envelope using the full bandwidth of the signal. However, results are often better if you first apply a bandpass filter to the signal to isolate the signal components near the resonances.

The following paragraphs describe the available parameters and controls.

Envelope Settings

  • Envelope — Choose Upper or Lower.

  • Method and method parameter

    • Hilbert — No parameter

    • FIRFilter Order

    • RMSWindow Length in samples or time units

    • PeakMaximum Separation in samples or time units

Bandpass Filtering

  • Use Bandpass Filter — Select to specify bandpass filtering

  • Center Frequency — Center frequency of the bandpass filter in selectable units

  • Bandwidth — Bandwidth of the bandpass filter in selectable units

You can iterate on your settings and preview the results without creating a new variable by clicking Preview. The app shows the envelope corresponding to the current settings alongside the original signal so that you can visually compare the two plots.

When you are satisfied with shape of the envelope, click Apply to create the new envelope variable. You can process and extract features from the signal envelope as you would with other time-domain signals. If you have selected Plot Results, then the app generates an envelope plot.

Extract a Signal Envelope

As an example, consider the following single signal which has been imported into the app.

Signal trace in the app. The variable Sound is selected in the Variables pane on the left. The signal is plotted on the right.

Select Signal Envelope Option

From the Filtering & Averaging menu, select the Signal Envelope option.

Filtering & Averaging menu. Signal Envelope is the second item from the bottom.

Specify and Preview Full Bandwidth Signal

Specify a signal envelope using the full bandwidth of the signal and click Preview.

Signal Envelope tab. The parameters are on the left. The Preview button is the leftmost button of the buttons on the right.

The previewed envelope captures the general shape of the original signal.

The settings and controls are in the tab at the top. The Variables pane is on the left. The preview plot is on the right. The top plot shows the envelope. The bottom plot shows the signal.

Apply Envelope Settings

Click Apply to create and plot the new envelope variable.

The new envelope variable is the bottom item in the Variables pane on the left. The envelope is plotted on the right.

Use Bandpass Filter

Applying a bandpass filter can improve results. Specify and preview another envelope that includes a bandpass filter of 1200 Hz, with a center frequency of 200 Hz.

The settings are along the top. Use Bandpass Filter is selected. The preview plot is on the right.

The envelope computed using a band-limited filter captures the shape of the signal more accurately than the unfiltered envelope.

See Also

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