Operating Deflection Shapes (ODS)

An Operating Deflection Shape (ODS) represents the actual deformation pattern of a structure at a specific frequency or time instant under real operating conditions. Unlike mode shapes, which are intrinsic properties of the structure, ODS includes contributions from all active modes and the applied forces.

ODS vs Mode Shapes

Property	Mode Shape	Operating Deflection Shape
Definition	Intrinsic structural property (eigenvalue solution)	Actual vibration pattern under operating forces
Force dependence	Independent of applied forces	Dependent on the specific loading condition
Frequency	Exists only at natural frequencies	Can be computed at any frequency
Composition	Single mode contribution	Superposition of all participating modes
Extraction method	Modal curve fitting (EMA/OMA)	Direct measurement (no curve fitting required)
Repeatability	Repeatable (structural property)	Changes with operating conditions

Key Insight: At or near a natural frequency, if one mode dominates the response, the ODS closely resembles that mode shape. Away from resonances, the ODS is a mixture of multiple modal contributions and does not correspond to any single mode.

When to Use ODS

Vibration troubleshooting: Identify which parts of the structure are moving the most at a problematic frequency (e.g., a customer complaint frequency).
Forced response visualization: Understand the actual deformation pattern under real operating loads, including multi-mode contributions.
Quick diagnostics: ODS requires no curve fitting — results are available immediately from measured data.
Source-path identification: Animate the structure at specific frequencies to trace vibration energy flow paths.
Before/after comparison: Compare ODS before and after a design change to verify that the modification reduced vibration at the target frequency.
Rotating machinery: Visualize deflection shapes at specific order frequencies (1x, 2x RPM, etc.).

Data Requirements

ODS analysis uses the same FRF or cross-spectral data imported in Step 2. The requirements are:

Requirement	Description
Channel-DOF mapping	All response channels must be mapped to geometry points and directions.
Phase reference	At least one channel must serve as the phase reference to maintain correct relative phase between DOFs.
Frequency resolution	Sufficient frequency resolution to distinguish closely spaced features (longer time records or finer FFT resolution).

Phase Reference

The phase reference channel establishes a common phase datum for all other channels. Without a consistent phase reference, the animated ODS will show incorrect relative motion between measurement points.

Selecting the Phase Reference

In the ODS settings panel, select the Phase Reference channel from the dropdown.
Choose a channel that has good signal-to-noise ratio across the frequency range of interest.
For EMA data with FRFs, the excitation (reference) channel is automatically used as the phase reference.
For output-only data, select a response channel near the primary excitation source.

Important: If multiple measurement runs were performed (roving sensor), ensure that a fixed reference sensor was present in all runs. Without a common reference, the relative phase between runs is lost and the ODS will be incorrect.

Frequency-Selective Animation

ODS animation shows the structural deformation at a single frequency selected by the user:

Selecting the Frequency

Method	How
Cursor on FRF plot	Click on the FRF Sum or any individual FRF plot. The ODS is computed at the clicked frequency.
Manual entry	Type the exact frequency in the ODS Frequency field and press `Enter`.
Sweep mode	Click Sweep to animate the ODS continuously from the lower to upper frequency, stepping through each frequency line.
Peak list	Use the auto-detected peak list to jump directly to resonance frequencies.

Animation Display

The ODS animation uses the same 3D viewport and controls as mode shape animation (see Validation). Additional ODS-specific features include:

Frequency readout: The current ODS frequency is displayed prominently at the top of the viewport.
Magnitude scaling: The deformation amplitude is proportional to the actual measured response magnitude at the selected frequency.
Phase display: Arrow vectors at each point show the relative phase. In-phase points move together; anti-phase points move in opposite directions.

ODS Parameters

Parameter	Default	Description
Frequency	—	The frequency at which to compute and animate the ODS.
Amplitude Scale	Auto	Scaling factor for the deformation display. Auto normalizes to the viewport size.
Phase Reference	Channel 1	Reference channel for phase normalization.
Interpolation	Linear	Interpolation method for surface deformation between measurement points (Linear / Cubic).
Display Mode	Magnitude + Phase	Choose Magnitude only, Phase only, or combined Magnitude + Phase display.

Interpreting ODS Results

At a Resonance Frequency

When the selected frequency coincides with a natural frequency and one mode dominates, the ODS closely resembles the corresponding mode shape. The phase distribution will be approximately 0 or 180 degrees (real normal mode behavior).

Away from Resonance

The ODS is a complex mixture of multiple modes. The phase distribution will show values between 0 and 360 degrees, and the deformation pattern does not correspond to any single mode. This is normal and expected.

Near Closely Spaced Modes

When two modes are close in frequency, the ODS between them transitions from one mode shape to the other. Sweeping through this frequency range reveals the transition clearly.

Tip: Use ODS sweep mode through a frequency range containing closely spaced modes to understand how the deformation pattern transitions between modes. This is often more informative than viewing individual mode shapes.

Use Cases

Vibration Troubleshooting Example

Customer reports excessive vibration at 125 Hz on a vehicle dashboard.
Measure acceleration at 30+ points on the dashboard structure with a fixed reference accelerometer.
Import the data and geometry into V-Listen.
Set the ODS frequency to 125 Hz.
The animated ODS reveals which panel region exhibits the largest displacement.
Engineering countermeasures (stiffening, damping treatment) can be targeted at the identified region.

Before/After Comparison

Measure ODS data before the design change.
Implement the structural modification.
Measure ODS data after the design change using the same geometry and DOF setup.
Use the 1x2 layout to animate both ODS side by side at the target frequency.
Quantify the reduction in displacement amplitude at the critical region.

ODS vs EMA Decision Guide

Question	If Yes, Use
Do I need the actual vibration pattern at a specific operating frequency?	ODS
Do I need intrinsic structural properties independent of loading?	EMA
Am I troubleshooting a vibration complaint at a known frequency?	ODS (fast, no curve fitting needed)
Do I need mass-normalized mode shapes for FE model updating?	EMA
Is the structure under operational loads that I cannot remove?	ODS (or OMA for modal parameters)

Tip: ODS and EMA are complementary techniques. A typical workflow is to first use ODS for quick diagnostics at problem frequencies, then perform a full EMA to understand the underlying modal behavior and guide design changes.

Export

ODS results can be exported using the same tools as mode shape validation:

AVI video of the animated ODS at a selected frequency.
PNG screenshot of the deformed geometry.
Excel table of displacement magnitudes and phases at each DOF.