EMA Stability Diagram

The Stability Diagram tab is the third step of Modal Analysis in EDM, after EMA Band Selection. The Stability Diagram tab features a configurable plot with parameter control over the modal order, estimation method, Mode Indicator Function type, and pole selection.

EDM curve-fits the FRFs in the selected frequency band many times, using a higher model order each time. Poles that stay at the same frequency and damping as the model order increases are real modes of the structure. You pick these poles, and EDM calculates the mode shapes from them. It also displays Pole information for modal order frequency and damping ratio. Selected poles are used in mode calculation.

The tab has two main areas:

Settings panel- holds the calculation settings, the information about the selected pole, the list of selected poles (modes), and the options for saving the mode shapes.

Chart- shows the stability diagram. The horizontal axis is frequency and the vertical axis is the model order. A MIF curve from the same band is drawn behind the poles to help you see where the modes are.

The settings in the Stabilization group control the calculation.

Start frequency / End frequency- The frequency band from the Band Selection tab. These values cannot be changed here. To change them, go back to Band Selection.

Estimation method- The curve-fitting method used to find the poles:

• Time Domain- The standard method. Always available.
• Poly-X- A frequency-domain method. Available only if your license includes it.
• SSI- Stochastic Subspace Identification. Available only for Operational Modal Analysis (OMA) tests, and only if your license includes it.

Max modal order- The modal order is a user defined number which is used to determine the highest order of the polynomial curve of best fit. The diagram has one row for each model order up to this value. As the hint in the tab says, the modal order is typically four times the number of modes you are curve-fitting. If the value is too high for the data, EDM lowers it automatically and shows a message, for example “Max Modes will reset to 24.”

Click Calculate stabilization to calculate the diagram. A progress window opens. You can cancel the calculation from this window.

Each symbol on the diagram is a pole found at one model order. EDM compares each pole with the poles at the model order below it and marks it with one of these symbols:

Real modes usually show as a vertical column of green Stable Pole symbols that continues over many model orders, often at a peak or valley of the MIF curve. Poles that appear at random places, or only at a few model orders, are usually caused by noise or by the math of the curve fit, and should not be selected.

When you move the mouse over a pole, the Poles and Modes group shows its information:

Frequency- The natural frequency of the pole, in Hz.

Damping- The damping ratio of the pole, in %.

Modal order- The model order (row) where the pole was found.

Type- The pole type: Stable Pole, Stable Frequency, or New Pole.

The Tolerances group sets how close a pole must be to a pole at the lower model order to be called stable.

Frequency- The largest allowed difference in frequency, in %. The default is 1%.

Damping- The largest allowed difference in damping, in %. The default is 5%.

Click Refresh in the Tolerances group to classify the poles again with the new tolerances. This does not run the calculation again.

NOTE: Clicking Refresh in the Tolerances group removes all the poles that you have selected. Set the tolerances before you select poles.

The MIF/Imaginary Sum group sets which curve is drawn behind the poles: Multivariate MIF, Complex MIF, Real MIF, or Imaginary Sum. See the Band Selection page for the differences between them. Choose a type, then click Refresh in this group to redraw the curve. Your selected poles are kept.

Each pole that you select becomes one mode in the results.

To select a pole:

1. Move the mouse over a Stable Pole symbol in a stable column. Check its information in the Poles and Modes group.
2. Right-click the pole and click Add Cursor.

A cursor is added at the pole, and the pole is added to the list of modes, for example Mode 1: 125.43 Hz, 1.25 %. The modes are sorted by frequency.

Screenshot: The right-click menu open on a pole with Add Cursor highlighted, and the list of selected modes in the Poles and Modes group.

NOTE: Some rules apply when you select poles:

• You can select only one pole at each frequency. If you try to add a second pole at the same frequency, EDM shows a message.
• The number of selected poles can be at most half of the Max modal order. To select more modes, increase the Max modal order and calculate again.
• If Auto select pole in stability pole is selected in the Band Selection tab, EDM selects the poles that stay stable over many model orders automatically after the calculation. Check the automatic selection, and add or remove poles as needed.

To remove poles:

Delete pole- Select one or more modes in the list and click Delete pole. Hold Ctrl or Shift to select more than one mode.

Remove Cursor- Right-click the chart, point to Remove Cursor, and click the pole you want to remove.

Remove All Cursor- Right-click the chart and click Remove All Cursor to remove all the selected poles.

Right-click on the chart to open the menu.

Screenshot: The right-click menu open on the Stability Diagram chart, showing the Remove Cursor submenu.

Zoom Back- Returns to the previous zoom level.

Un-zoom All- Returns to the full view of the chart.

Add Cursor- Selects the pole under the mouse. See Selecting Poles.

Remove Cursor- Removes one selected pole.

Remove All Cursor- Removes all the selected poles.

X Axis Format- Shows the frequency axis on a Linear or Log scale.

Y Axis Format- Shows the MIF curve on a Linear or Log scale. This option is available only when a Multivariate, Complex, or Real MIF is shown.

TIP: Zoom in on a mode to make it easier to pick the right pole. When you are zoomed in, use the scroll bars or hold the middle mouse button and drag to move around the chart.

When you have selected the poles, choose where to save the results:

Save to new shape- Saves the modes to a new shape. Type a name in Shape name. The name can contain only letters, numbers, and underscores (_). It must be different from the names of the existing shapes, and it cannot contain Default_.

Save to existing shape- Adds the modes to a shape that you choose from Select shape. Use this to combine modes from several frequency bands into one shape. The existing shape must use the same DOFs, and its frequency band must not overlap the band that you are fitting now.

Then click Calculate modes. EDM calculates the mode shapes, saves them to the shape, and opens the Animation tab. The message “The modal shapes are calculated successfully.” is shown.

Screenshot: Close-up of the shape-saving options (Save to new shape, Save to existing shape, Shape name, Select shape) and the Calculate modes button.

NOTE: If EDM shows “Name isn't unique, or FRF is different, or the frequency band overlaps”, then either the shape name is already used, or the existing shape you chose has different DOFs or an overlapping frequency band. Use a different name, or choose a different shape.

Why does the diagram show only red New Pole symbols?- The data may be noisy, the band may not contain a clear mode, or the tolerances may be too tight. Check the FRFs and the band in Band Selection, try a higher Max modal order, or increase the tolerances and click Refresh.

Why can't I add a cursor?- Make sure that the mouse is over a pole symbol when you right-click. You also cannot add a pole at a frequency that is already selected, or select more than half of the Max modal order in poles.

Why did my selected poles disappear?- Clicking Refresh in the Tolerances group removes the selected poles. Changing the signal selection in Modal Data Selection also clears the Stability Diagram.

Why is Poly-X or SSI not in the Estimation method list?- These methods need a license option. SSI is also available only for OMA tests. Contact support@go-ci.com for more information about your license.

How many modes should I select?- Select one pole for each column of stable poles that lines up with a peak or valley of the MIF curve. Each pole you select becomes one mode in the results.