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Modal and Operating Deflection Shape Testing

Modal and Operating Deflection Shape Testing

General, resonance surveys, bookkeeping, ODS

Impact Testing, Shaker Testing, Stepped Sine Testing

The structural dynamic response of a structure or system includes natural frequencies, damping and stiffness factors, and mode shapes. The study of these parameters is often referred to as modal testing or modal analysis.

Modal analysis can be done both experimentally through frequency response testing or mathematically using finite element analysis. Complex structural motion is reduced to individual vibration modes.

Experimental modal analysis is often used in the design cycle to verify and update FEA mathematical models. It can also be used for generating a new structural model for modification of a particular problem or verifying design structural performance.

However, the acquisition of modal data is done for a variety of other reasons and often the full analysis of the data is not needed. Simply knowing the resonances or understanding aspects of the resonance behavior often have advantages in many stages of engineering and troubleshooting. General resonance surveys and frequency response testing can assist to identify problem frequencies.

Operating deflection shape (ODS) analysis takes this a step further and shows the deformation of a structure at specific frequencies. ODS shows the response of a structure to both resonant and forced vibration and is often very representative of the real-world data.

Modal analysis is similar to ODS in that deformation of the structure can be viewed, but it is primarily concerned with resonance frequencies (or natural frequencies) of a structure. Modal analysis is typically performed by exciting the structure with a known force and measuring the response of the structure at many locations using accelerometers. There are operational modal analysis techniques for obtaining data without a force input. Frequency response functions are used to acquire modal data.

There are several challenges in the acquisition of modal data. These include dynamic range, phase matching, cabling, and bookkeeping.

Dynamic range and phase matching are key characteristics of a good dynamic signal analyzer that is used for modal analysis or structural testing. Fortunately, SignalCalc Dynamic Signal Analyzers offer excellent specifications for both dynamic range and phase matching in a single chassis or across thousands of channels.

Cabling and test setup for complex modal tests can be taxing to the most experienced test engineer or test technician. Many large scale models cover hundreds or even thousands of channels of simultaneous acquisition. Proper cable management techniques are important. Additionally, the use of distributed DSP architecture on front ends assists in this process. By allowing multiple hardware chassis to be distributed along great distances, cabling mess and error can be drastically reduced. Using uniform cable length from the sensor to the acquisition hardware helps to eliminate one variable of error from varying cable lengths. The The 900 Series Signal Analyzer system allows distributed data acquisition over multiple chassis to distances of 1000 m.

Cabling and bookkeeping go hand in hand in a test setup. Properly keeping track of the point and direction of each channel, while incrementing those if a roving transducer or input is used, can quickly become confusing if not done properly. SignalCalc Dynamic Signal Analyzers provide built-in bookkeeping and realtime exporting features to aid in this complex task.

Data Physics Modal Testing with Navcon JimBeam

Impact Testing

Impact testing is sometimes called bump testing, tap testing, or hammer resonance testing. It involves the use of an instrumented hammer to excite a structure in order to measure the response. The SignalCalc transfer function analysis suite provides users with every conceivable utility to ensure accurate frequency response function measurements. For instance, force and exponential windows can be user-defined, for the minimization of leakage.

SignalCalc analyzers provide a significantly improved implementation of the Force window designed to compensate for a known deficiency of the Response window. This “damping-compensated” force window is a truncated exponential shape, rather than being a simple rectangle. The exponential shape used has the same time-constant (response Width) as the exponential applied to the Response window and automatically corrects the amplitude of the force spectrum to track variations in response spectrum amplitude caused by the time-of-capture position within the exponential Response window.

As a result of the manual nature of impact testing, maintaining consistency of measurements due to variations in the location and amount of the applied impact is difficult. SignalCalc analyzers provide a Preview Average mode that allows the user to examine the time and frequency domain views of each capture to verify impact data before averaging the new capture into the measurement.

Shaker Testing

For larger or more complex structures it may be necessary to use an electrodynamic shaker to excite the structure with sufficient energy to achieve the required signal to noise levels. It is also useful to user shakers if extremely consistent and accurate results are required because shaker input eliminates the potential for human error that might be found with impact testing.

Shakers excite the structure through a narrow rod or “stinger.” The stinger is designed to transmit energy only in the lateral direction and aids in decoupling the dynamics of the shaker from the test article. Additionally shakers designed specifically for modal testing often use a spider flexure system or special bearing system to further decouple the shaker. For example, the SignalForce LMT-100 uses a special copper-beryllium spider flexure, and all other SignalForce modal shakers use a linear-rotary bearing for fully decoupling. The choice between a bearing or flexure system results in a tradeoff between shaker size and performance. For more information, explore SignalForce Modal Shakers.

SignalCalc Analyzers offer a large range of excitation signals to cater to a variety of measurement situations. Random, Burst Random, Chirp, and Swept Sine are among the fourteen different choices for signal generation for transfer function testing.

Stepped Sine Testing

A specific application, Stepped Sine Testing, allows the user to obtain test results by exciting a single frequency at a time. This application is closed-loop and allows the user to specify different measurement bands with different sweep speed and resolution. Stepped sine testing is particularly useful when testing systems that contain composites or closed-loop systems. Sine sweeps can be obtained for different input levels to fully evaluate the non-linearity of the structure or system.

The 900 Series Signal Analyzer make excellent analyzers for large and small scale structural dynamics testing.