Modal testing
Encyclopedia
Modal testing is a 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.
, 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.
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.
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
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
Modal analysis
Modal analysis is the study of the dynamic properties of structures under vibrational excitation.Modal analysis is the field of measuring and analysing the dynamic response of structures and or fluids when excited by an input...
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
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
is supplied to the system with a known frequency content. Where structural resonances occur there will be an amplification
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
of the response, clearly seen in the response spectra. Using the response spectra and force spectra, a transfer function
Transfer function
A transfer function is a mathematical representation, in terms of spatial or temporal frequency, of the relation between the input and output of a linear time-invariant system. With optical imaging devices, for example, it is the Fourier transform of the point spread function i.e...
can be obtained. The transfer function (or frequency response function (FRF)) is often curve fitted
Curve fitting
Curve fitting is the process of constructing a curve, or mathematical function, that has the best fit to a series of data points, possibly subject to constraints. Curve fitting can involve either interpolation, where an exact fit to the data is required, or smoothing, in which a "smooth" function...
to estimate the modal parameters; however, there are many methods of modal parameter estimation
Estimation theory
Estimation theory is a branch of statistics and signal processing that deals with estimating the values of parameters based on measured/empirical data that has a random component. The parameters describe an underlying physical setting in such a way that their value affects the distribution of the...
and it is the topic of much research.
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 forceForce
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
, with a larger contact time causing a smaller range of bandwidth. A load cell
Load cell
A load cell is a transducer that is used to convert a force into electrical signal. This conversion is indirect and happens in two stages. Through a mechanical arrangement, the force being sensed deforms a strain gauge. The strain gauge measures the deformation as an electrical signal, because the...
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
Civil engineering
Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including works like roads, bridges, canals, dams, and buildings...
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
Window function
In signal processing, a window function is a mathematical function that is zero-valued outside of some chosen interval. For instance, a function that is constant inside the interval and zero elsewhere is called a rectangular window, which describes the shape of its graphical representation...
errors.
See also
- VibrationVibrationVibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
- CushioningCushioningPackage cushioning is used to help protect fragile items during shipment. It is common for a transport package to be dropped, kicked, and impacted: These events may produce potentially damaging shocks. Transportation vibration from conveyors, trucks, railroads, or aircraft can also damage some...
- Shock absorberShock absorberA shock absorber is a mechanical device designed to smooth out or damp shock impulse, and dissipate kinetic energy. It is a type of dashpot.-Nomenclature:...
- Shock (mechanics)Shock (mechanics)A mechanical or physical shock is a sudden acceleration or deceleration caused, for example, by impact, drop, kick, earthquake, or explosion. Shock is a transient physical excitation....
- Shock response spectrumShock response spectrumA Shock Response Spectrum is a graphical representation of an arbitrary transient acceleration input, such as shock in terms of how a Single Degree Of Freedom system responds to that input. Actually, it shows the peak acceleration response of an infinite number of SDOFs, each of which have...
- Shaker (testing device)Shaker (testing device)A shaker is a device used in vibration testing to excite the structure either for endurance testing or modal testing.see also :* Vibration...