Modal testing

Modal testing is the form of vibration testing of an object whereby the natural (modal) frequencies, modal masses, modal damping ratios and mode shapes of the object under test are determined.

A modal test consists of an acquisition phase and an analysis phase. The complete process is often referred to as a Modal Analysis or Experimental Modal Analysis.

There are several ways to do modal testing but impact hammer testing and shaker (vibration tester) testing are commonplace. In both cases energy is supplied to the system with a known frequency content. Where structural resonances occur there will be an amplification of the response, clearly seen in the response spectra. Using the response spectra and force spectra, a transfer function can be obtained. The transfer function (or frequency response function (FRF)) is often curve fitted to estimate the modal parameters; however, there are many methods of modal parameter estimation and it is the topic of much research.

Sample layout of a modal testing system

Impact Hammer Modal Testing

An ideal impact to a structure is a perfect impulse, which has an infinitely small duration, causing a constant amplitude in the frequency domain; this would result in all modes of vibration being excited with equal energy. The impact hammer test is designed to replicate this; however, in reality a hammer strike cannot last for an infinitely small duration, but has a known contact time. The duration of the contact time directly influences the frequency content of the force, with a larger contact time causing a smaller range of bandwidth. A load cell is attached to the end of the hammer to record the force. Impact hammer testing is ideal for small light weight structures; however as the size of the structure increases issues can occur due to a poor signal to noise ratio. This is common on large civil engineering structures.

Shaker Modal Testing

A shaker is a device that excites the object or structure according to its amplified input signal. Several input signals are available for modal testing, but the sine sweep and random frequency vibration profiles are by far the most commonly used signals.

Small objects or structures can be attached directly to the shaker table. With some types of shakers, an armature is often attached to the body to be tested by way of piano wire (pulling force) or stinger (Pushing force). When the signal is transmitted through the piano wire or the stinger, the object responds the same way as impact testing, by attenuating some and amplifying certain frequencies. These frequencies are measured as modal frequencies. Usually a load cell is placed between the shaker and the structure to obtain the excitation force.

For large civil engineering structures much larger shakers are used, which can weigh 100kg and above and apply a force of many hundreds of newtons. Several types of shakers are common: rotating mass shakers, electro-dynamic shakers, and electrohydraulic shakers. For rotating mass shakers the force can be calculated from knowing the mass and the speed of rotation; for the electro-dynamic shaker the force can be obtained through a load cell, or an accelerometer placed on the moving mass of the shaker. Shakers can have an advantage over the impact hammer as they can supply more energy to a structure over a longer period of time. However, problems can also be introduced; shakers can influence the dynamic properties of the structure and can also increase the complexity of analysis due to windowing errors.

Types of Modal Testing

Different types of modal testing can be performed.

Experimental Modal Analysis (EMA)

Experimental Modal Analysis tests can be performed both in the field and in more controlled lab environments. Testing in the lab has the advantage of a higher signal-to-noise ratio (SNR) and the ability to easily change the test setup. When performing EMA testing, objects are excited by artificial forces and both the inputs (excitations) signals and outputs (responses) signals are measured and used to estimate Modal Models.

Operating Deflection Shapes (ODS)

Operating Deflection Shapes is a simple way to do dynamic analysis and see how a machine or a structure moves by its operational conditions. ODS tests have no applied artificial forces and only response vibration signals are measured.

A Modal Model can not be estimated from ODS measurements but it provides structural deflection shapes which improves the structural analysis of operational DUTs.

ODS is used successfully for machine conditioning monitoring and in civil engineering applications e.g. on bridges, buildings, and other structures that are difficult to excite by applied artificial forces.

Operational Modal Analysis (OMA)

The test and measurement procedure for Operational Modal Analysis is similar to ODS, but their analysis part is different. Both ODS and OMA do not use external input forces but are based purely on response DOF measurements.

ODS provides basic information about amplitude and phase information of the DOFs on the measured operational DUT and enables geometry animation of the deflection shapes.

OMA estimates a Modal Model (like EMA) with natural frequencies, damping, and mode shapes of the measured operational DUT.  

OMA can be used to estimate a modal model in situations where it is difficult to do EMA. Such situations could e.g. be when monitoring running DUTs for health issues, when the size or location of the DUT makes it impractical to excite with external force, or when the operational structural conditions of the DUT must be analyzed.

See also

• Modal Analysis
• Vibration
• Cushioning
• Shock absorber
• Shock (mechanics)
• Shock response spectrum
• Shaker (testing device)