Modal tests are the basis for performing modal analysis and making conclusions on the structural dynamics of test objects. Modal Testing can be performed with artificial sources of excitation to vibrate the test object, or by running the test object in operating conditions.
Modal Tests and Modal Analysis are used to calculate and understand the dynamic behavior and structural properties of structures such as natural resonance frequencies, damping ratios and mode shapes. So that, structures can be tested, optimized and validated.
While performing the Modal Test, the structure can be excited in two ways by applying force (N):
1) Modal Hammer
2) Electrodynamic Shaker
Sensor is needed to see the response of the force applied to the structure. Usually accelerometers (g) are used to see the reaction.
Modal Testing and Modal Analysis generally includes:
• Modal shaker or impact hammer
• Power converters that receive input excitation signals
• Accelerometers that receive output signals
• DAQ device
• A computer with the Modal Test and Analysis software application
Modal analysis can be done in working conditions as Operational Modal Analysis (OMA) or it can be done by applying artificial excitation sources as Experimental Modal Analysis (EMA).
In the Experimental Modal Analysis, structures are excited by modal hammers or vibration shakers. The chosen shaker or modal hammer should be in the frequency range that it can excite the entire structure.
OMA estimates a modal model such as natural frequencies, damping, and mode shapes with measuring output (response) signals. Input force signal is not needed.
In some cases, it is either not possible or very difficult to make structures vibrate with impact hammers or shakers. In such cases, structures are loaded by natural loads (such as offshore structures, bridges, buildings etc..). Structures are not easily controlled or measured in this type of test.
EMA provides a modal model such as resonance frequencies, damping ratios and mode shapes of simple and complex structures. In EMA structures are excited by artificial forces and unlike OMA, both input (excitation) and output (response) signals are measured to get frequency response function. Structures can be excited with hammers or shakers.
EMA tests can be performed in field or controlled lab environments. Therefore, EMA is more preferred over OMA because of the controlled environment. In this way, structural excitations can be managed.
EMA can be classified according to the number of input and outputs:
-SISO (Single Input Single Output)
-SIMO (Single Input Multiple Outputs)
-MIMO (Multiple Inputs Multiple Outputs)
With the MIMO method, every part of the structure can easily excited. Thus, more accurate response can be obtained from all parts of the structure.
Operating Deflection Shapes (ODS) is a dynamic analysis to see how a structure moves within its operational conditions by measuring only output (response) vibration signals.
ODS is used for animations of structural deflection shapes, but it does not provide modal model like OMA and EMA.
ODS results consist of deflection shapes animated on geometry and determined response amplitude and phase information.
1. Choosing the support of a structure
The testing object should be able to vibrate freely in ways that best reveal the structural dynamics of the objects.
2. Type of excitation force
For Experimental Modal Analysis (EMA) different excitation (input) types can be selected. (Shaker or impact hammer).
• Impulse Hammers have some advantages like fast-set up time and being inexpensive. There is no need to fixture.
• Modal shakers are the best solution for larger complex structures where more in-depth analysis is required. Thanks to modal shakers angle and location of excitations are exact. Non linearities can be detected. Excitation of any frequency band is well.
3. Excitation places
For EMA testing, it is important to excite the object in positions that will affect it in a way that reveals most of its vibration characteristics.
4. Hardware and sensors used to measure forces and responses
For EMA testing, input excitations are usually measured with force transducers or impedance heads. An impedance head contains both a force sensor and an accelerometer. As an alternative to the impedance head, an accelerometer can be placed near the force sensor. Response signals are mostly measured with accelerometers.
To define it simply, an input signal is applied to the system, the output signal is measured, and the ratio of response to excitation gives the transfer function. FRF is calculated by measuring input and output of the system.
The system is assumed to be linear and time-invariant.
The transfer function is obtained to see the risky resonance values in mechanical structures. It is necessary to avoid frequency ranges where the stress value of the material is very high, that is, resonance frequency.
Lets say there are two signals x(t) and y(t) which are input and output signals respectively. X(f) and Y(f) are the complex spectrums coming from x(t) and y(t). The division of response (output) to excitation (input) in the frequency domain basically gives the transfer function:
H(f) = Y(f) (output signal) / X(f) (input signal)
In mechanical structures:
FRF is obtained by exciting the structure with a modal hammer or shaker (measuring the force), and by measuring the response of the structure with accelerometers (measuring the acceleration).
In electrical circuits:
FRF is obtained by applying voltage to the circuit on the input and measuring the voltage on the output.
Coherence data provides important information for modal test validation. It shows how input and output values are related to each other to identify resonance and anti-resonance points.
Coherence value is between 0 and 1. Coherence value of 1 indicates that the measured response is caused totally by the measured input. If it is less than 1, it indicates that the measured response is not only caused by the measured input.
Coherence value must be based on averaged measurements.
When the FRF is very high, for example at resonance point, the coherence value is approximately 1. However, if the coherence value approaches to zero at a point where the FRF is high, it might not necessarily be a real resonance.
Different types of vibration shakers can be used as modal shakers:
Each type of shaker has different advantages. Shaker selection depends on the scenario of the test performed. Maximum level of excitation force, frequency range, and most importantly, the way how the device under test is placed are some of the important parameters while selecting a shaker.
Permanent Magnet Shakers mainly used for environmental testing and sensor calibration.
DUT is placed on the top of the shaker armature. The vibrating surface area can be expanded by using a head expander according to sample sizes.
• Vibration testing of micro parts and modal testing of assemblies and electronics
• Shock Test
• Sensor Calibration
• Resonance Test
• Educational and various researches
Modal vibration shakers are used to excite large and complex structures and to obtain high-quality modal data. Compared to modal hammers, modal shakers have ability to excite the structure over a wider frequency range and with various signal types. Modal shakers are ideal to obtain optimal and accurate test results. For complex structures, using multiple shakers provide more realistic force excitations and better inspection of mod shapes.
Modal shakers are mounted to the structure with a stinger and force is transferred from shaker to the structure via this stinger. The structure under test and/or the modal shaker can be hung with components such as an elastic rope or spring during the test to simulate free-free boundary conditions.
Modal Shakers are designed with an open-hole armature for the stinger, such that, the stinger can be adjusted to the length required for the DUT without moving the shaker, which simplifies installation.
• Electronic boards and sub-components
• Machinery, vehicles, aircraft and constructions
Inertial shakers are used for structures that require excitation in lower frequency bands. They are compact, lightweight and used as hand held.
Inertial shakers and modal shakers are similar in usage area. They are used also in tests for examining vibrational behavior of structures such as modal shakers. However, compared to the modal shakers, the connection styles are different. The inertial shaker’s own body vibrates. For this reason, inertial shakers are fixed directly to the structure.
Modal Hammer is used for hammer impact tests. Impact Hammer is generally preferred to test a simple structure or used as a quick preview before more complex modal shaker testing. Impact hammer is used for exciting the structure with a short impulse. Thus, they are giving a broad frequency excitation. With one or more acceleration sensors the response is measured.
The hammer has a force sensor integrated and an interchangeable hammer tip that can have different stiffness. Regarding the frequency range to be included in the modal test, it is important to choose the correct hammer tip. Impact hammers come in different sizes and features depending on the type of structure.
Impact Hammers have some advantages like fast-set up time and being inexpensive. There is no need to fixture.
First, most appropriate lenght of the modal hammer should selected. Other features to consider are below:
• Regarding the frequency response and frequency range, choosing the most suitable pulse tip
• Hammer size and weight
• Measuring range – sufficient level of impact force
• Sensitivity – providing a balance between very weak and overloaded levels
• Burst Random
• Pseudo Random
• Periodic Random
• Sinusoidal Chirp
• Sine Scan and Graded Sine
Sensors should be selected based on the acceleration range, temperature range, frequency range etc… The number of sensors must also be decided.
Controller selection should be determined by test conditions. Common vibration test types are:
RSTD (Resonance Search and Tracking Dwell)
Sine on Random
Sine on Sine
Random on Random
Sine and Random on Random
Shock Response Spectrum