Modal Analysis- Frequently Asked Questions

Definitons

Experimental Modal Analysis

It is the process of characterizing the inherent dynamics of a physical system. The physical system can be an aircraft, automobile, gas turbine or MEMS gyroscope. The inherent dynamics can be defined using the natural frequencies, damping factors and mode shapes.

Accelerometer

The accelerometer is a sensor used to measure acceleration of a vibrating structure. There are many types of accelerometers. The most common type is the piezoelectric accelerometer and it is important to consider several parameters while selecting an accelerometer for experimental modal tests.

Impact Hammer

The impact hammer is typically a hammer with an in-built force sensor. The impact hammer is used to provide an impact force to excite the structure. Under the impact, when the structure vibrates, the accelerations of various points of the structure can be recorded using the accelerometers.

Shaker

The modal analysis of a physical system can be performed either by controlling the input forces or by using the excitation provided by operational loads. For example, a bridge vibrates under the ambient wind loads. The vibration response of the bridge can be recorded and this data can be analysed by operational modal analysis to derive the modal parameters

Damping

Almost all physical systems are subject to damping to some degree because energy is dissipated by friction and other resistances. Damping is very important in limiting the vibration levels at resonance.

Resonant Frequency

[Source: Wikipedia] In physics, resonance is the tendency of a system to oscillate at larger amplitude at some frequencies than at others. These are known as the system's resonant frequencies (or resonance frequencies). At these frequencies, even small periodic driving forces can produce large amplitude vibrations, because the system stores vibrational energy.

Interesting Fact: Resonance was discovered by Galileo Galilei with his investigations of pendulums and musical strings beginning in 1602.

Modal Parameters

Natural (resonant) frequency, damping and mode shape are collectively referred to as the modal parameters.

Curve Fitting

It refers to the procedure used to derive the modal parameters from the measured vibration response of a structure. It is a mathematical algorithm and there are many different types of algorithms. Some of them are listed below

• Circle Fitting
• Least Squares Complex Exponential
• Rational Fraction Polynomial
• Stochastic Subspace Identification
• Frequency Domain Decomposition