ISO 7626-5:2019
(Main)Mechanical vibration and shock — Experimental determination of mechanical mobility — Part 5: Measurements using impact excitation with an exciter which is not attached to the structure
Mechanical vibration and shock — Experimental determination of mechanical mobility — Part 5: Measurements using impact excitation with an exciter which is not attached to the structure
This document specifies procedures for measuring mechanical mobility and other frequency-response functions of structures excited by means of an impulsive force generated by an exciter which is not attached to the structure under test. It is applicable to the measurement of mobility, accelerance or dynamic compliance, either as a driving point measurement or as a transfer measurement, using impact excitation. Other excitation methods, such as step relaxation and transient random, lead to signal-processing requirements similar to those of impact data. However, such methods are outside the scope of this document because they involve the use of an exciter which is attached to the structure. The signal analysis methods covered are all based on the discrete Fourier transform (DFT), which is performed mostly by a fast Fourier transform (FFT) algorithm. This restriction in scope is based solely on the wide availability of equipment which implements these methods and on the large base of experience in using these methods. It is not intended to exclude the use of other methods currently under development. Impact excitation is also widely used to obtain uncalibrated frequency-response information. For example, a quick impact test which obtains approximate natural frequencies and mode shapes can be quite helpful in planning a random or sinusoidal test for accurate mobility measurements. These uses of impact excitation to obtain qualitative results can be a first stage for mobility measurements. This document is limited to the use of impact excitation techniques for making accurate mobility measurements.
Vibrations et chocs mécaniques — Détermination expérimentale de la mobilité mécanique — Partie 5: Mesurages à partir d'une excitation par choc appliquée par un excitateur non solidaire de la structure
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INTERNATIONAL ISO
STANDARD 7626-5
Second edition
2019-12
Mechanical vibration and shock —
Experimental determination of
mechanical mobility —
Part 5:
Measurements using impact excitation
with an exciter which is not attached
to the structure
Vibrations et chocs mécaniques — Détermination expérimentale de la
mobilité mécanique —
Partie 5: Mesurages à partir d'une excitation par choc appliquée par
un excitateur non solidaire de la structure
Reference number
ISO 7626-5:2019(E)
©
ISO 2019
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ISO 7626-5:2019(E)
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ISO 7626-5:2019(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General characteristics of impact measurements . 2
4.1 General description . 2
4.2 Advantages and limitations of impact excitation . 3
4.2.1 General. 3
4.2.2 Nonlinearity restrictions . 4
4.2.3 Signal-to-noise problems . 4
4.2.4 Limited frequency resolution . 4
4.2.5 Damping restrictions . 4
4.2.6 Dependence on operator skill . 5
5 Support of the structure under test . 5
5.1 General . 5
5.2 Ungrounded measurements . 5
5.3 Grounded measurements . 5
6 Application of the excitation . 5
6.1 Impactor design . 5
6.2 Force spectrum characteristics . 6
6.3 Control of the frequency range of excitation .10
6.4 Avoidance of impactor double hits .10
7 Transducer system .12
7.1 General .12
7.2 Impactor calibration.12
8 Processing of the transducer signals .13
8.1 Filtering .13
8.2 Transient capture characteristics .13
8.3 Sampling relationships .14
8.4 Avoidance of saturation (clipping) .15
8.5 Windowing techniques .15
8.5.1 Force signal .15
8.5.2 Windowing the response signals .19
8.6 Averaging techniques .23
9 Tests for validity of the measurements .24
9.1 Coherence function .24
9.2 Repeatability check .25
9.3 Reciprocity check .25
9.4 Linearity check .25
9.5 Comparison with measurements using an attached exciter .25
Annex A (informative) Correction of mobility measurements for the effects of exponential
windowing .26
Bibliography .28
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ISO 7626-5:2019(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 108, Mechanical vibration, shock and
condition monitoring.
This second edition cancels and replaces the first edition (ISO 7626-5:1994), which has been technically
revised.
The main changes compared with the previous edition are as follows:
— updating of normative and informative references in the bibliography;
— redrawing of figures and graphs.
A list of all parts in the ISO 7626 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
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ISO 7626-5:2019(E)
Introduction
0.1 General introduction to the ISO 7626 series on mobility measurement
Dynamic characteristics of structures assumed to behave linearly can be determined as a function of
frequency from mobility measurements or measurements of the related frequency-response functions
(FRF), known as accelerance and dynamic compliance. Each of these frequency-response functions is
the phasor of the motion response at a point on a structure due to a unit force (or moment) excitation at
the same or any other point. The magnitude and the phase of these functions are frequency dependent.
Accelerance and dynamic compliance differ from mobility only in that the motion response is expressed
in terms of acceleration or displacement, respectively, instead of velocity. In order to simplify the
various parts of the ISO 7626 series, only the term “mobility” will be used. It is understood that all test
procedures and requirements described are also applicable to the determination of accelerance and
dynamic compliance.
Typical applications for mobility measurements are for:
a) predicting the dynamic response of structures to known or assumed input excitation;
b) determining the modal properties of a structure (natural frequencies, damping ratios and mode
shapes);
c) predicting the dynamic interaction of interconnected structures;
d) checking the validity and improving the accuracy of mathematical models of structures;
e) determining the frequency dependent dynamic properties (i.e. the complex modulus of elasticity)
of materials.
For some applications, a complete description of the dynamic characteristics can be required using
measurements of forces and linear velocities along three mutually perpendicular axes as well as
measurements of moments and rotational velocities about these three axes. This set of measurements
results in a 6 × 6 mobility matrix for each location of interest. For N locations on a structure, the system
thus has an overall mobility matrix of size 6N × 6N.
NOTE 1 In general, the measurement directions do not need to be perpendicular to each other, but only their
linear independence is needed.
For most practical applications, it is not necessary to know the entire 6N × 6N matrix. Often it is
sufficient to measure the driving-point mobility and a few transfer mobilities by exciting with a force
at a single point in a single direction and measuring the linear response motions at key points on the
structure. In other applications, only rotational mobilities can be of interest.
In order to simplify its use in the various mobility measurement tasks encountered in practice, ISO 7626
is published as a series comprising:
— ISO 7626-1, which covers basic definitions and transducers. The information in ISO 7626-1 is
common to most mobility measurement tasks.
— ISO 7626-2, which covers mobility measurements using single-point linear excitation with an
attached exciter.
— ISO 7626-5 (this document), which covers mobility measurements using impact excitation with an
exciter which is not attached to the structure.
Mechanical mobility is defined as the frequency-response function formed by the ratio of the phasor
of the linear or rotational velocity response to the phasor of the applied force or moment excitation. If
the response is measured with an accelerometer, conversion to velocity is used to obtain the mobility.
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ISO 7626-5:2019(E)
Alternatively, the ratio of acceleration to force, known as accelerance, can be used to characterize a
structure. In other cases, dynamic compliance, the ratio of displacement to force, can be used.
NOTE 2 Historically, frequency-response functions of structures have often been expressed in terms of the
reciprocal of one of the above-named dynamic characteristics. The arithmetic reciprocal of mechanical mobility
has often been called mechanical impedance. However, this is misleading because the arithmetic reciprocal of
mobility does not, in general, represent any of the elements of the impedance matrix of the structure. Mobility test
data cannot be used directly as part of an analytic impedance model of the structure. To achieve compatibility of
the data and the model, the impedance matrix of the model must be inverted to a mobility matrix, or vice versa.
This point is elaborated upon in ISO 7626-1:2011, Annex A.
0.2 Introduction to this document
Impact excitation has become a popular method for measuring the frequency response of structures
because of its inherent speed and relatively low cost to implement. However, the accuracy of mobility
measurements made by using impact excitation is highly dependent upon both the characteristics
of the test structure and the experimental techniques employed. With impact excitation, it can be
difficult or impossible in certain cases to obtain the accuracy which is attainable using steady state or
stationary excitation with an attached exciter, and the impact method carries an increased danger of
[6]
gross measurement errors . In spite of these limitations, impact testing can be an extremely useful
excitation technique when applied properly.
This document provides a guide to the use of impact excitation for mobility measurements.
Accurate mobility measurements always require careful attention to equipment selection and to
the measurement techniques employed; these factors are especially important when using impact
excitation. Furthermore, the characteristics of the test structure, especially its degree of nonlinearity,
limit the accuracy which can be achieved. Subclause 4.2 describes these limitations on the use of impact
excitation.
Because the exciter is not attached to the structure, this method makes it practical to measure a series
of transfer mobilities of a structure by moving the excitation successively to each desired point on the
structure, while the response motion transducer remains at a single fixed location and direction. Due
to the principle of dynamic reciprocity, such measurements should be equal, assuming linearity, to the
results obtained using an attached exciter at the same fixed location and direction with the response
transducer relocated to each desired point on the structure. However, it can be difficult to impact the
structure in all desired directions at certain locations, and in such cases, it can be more practical to use
impact excitation at the fixed location and direction and relocate a multi-axis response transducer to
the desired response locations.
NOTE 1 When a multi-axis transducer is used at a fixed location for a modal test and if the impact is applied
in one direction of the transducer at each point, then only the mode shape components in that direction are
obtained.
NOTE 2 The mass of the multiaxial transducer can change the mass properties of the structure leading
to an inconsistent set of measured transfer functions. This can cause serious problems in using the FRFs for
experimental modal analysis.
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INTERNATIONAL STANDARD ISO 7626-5:2019(E)
Mechanical vibration and shock — Experimental
determination of mechanical mobility —
Part 5:
Measurements using impact excitation with an exciter
which is not attached to the structure
1 Scope
This document specifies procedures for measuring mechanical mobility and other frequency-response
functions of structures excited by means of an impulsive force generated by an exciter which is not
attached to the structure under test.
It is applicable to the measurement of mobility, accelerance or dynamic compliance, either as a driving
point measurement or as a transfer measurement, using impact excitation. Other excitation methods,
such as step relaxation and transient random, lead to signal-processing requirements similar to those
of impact data. However, such methods are outside the scope of this document because they involve the
use of an exciter which is attached to the structure.
The signal analysis methods covered are all based on the discrete Fourier transform (DFT), which
is performed mostly by a fast Fourier transform (FFT) algorithm. This restriction in scope is based
solely on the wide availability of equipment which implements these methods and on the large base
of experience in using these methods. It is not intended to exclude the use of other methods currently
under development.
Impact excitation is also widely used to obtain uncalibrated frequency-response information. For
example, a quick impact test which obtains approximate natural frequencies and mode shapes can be
quite helpful in planning a random or sinusoidal test for accurate mobility measurements. These uses of
impact excitation to obtain qualitative results can be a first stage for mobility measurements.
This document is limited to the use of impact excitation techniques for making accurate mobility
measurements.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 2041, Mechanical vibration, shock and condition monitoring — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 2041 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
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ISO 7626-5:2019(E)
3.1
frequency-response function
FRF
frequency-dependent ratio of Fourier transform of the motion-response of a linear system to the one of
the excitation force
Note 1 to entry: Frequency-response functions are properties of linear dynamic systems which do not depend
on the type of excitation function. Excitation may be harmonic, random or transient functions of time. The test
results obtained with one type of excitation may thus be used for predicting the response of the linear system to
any other type of excitation.
Note 2 to entry: Linearity of the system is a condition which, in practice, may be met only approximately,
depending on the type of system and on the magnitude of the input. Care has to be taken to avoid nonlinear
effects, particularly when applying impulse excitation. Structures which are known to be nonlinear (for example,
certain riveted structures) should not be tested with impulse excitation and great care is required when using
random excitation for testing such structures.
Note 3 to entry: Motion may be expressed in terms of either displacement, velocity or acceleration; the
corresponding frequency-response function designations are dynamic compliance, mobility and accelerance or
dynamic stiffness, impedance, and effective mass, respectively.
Note 4 to entry: In practice, the discrete Fourier transform (DFT) by the fast Fourier transform (FFT) is used as
an approximation of the continuous Fourier transform. The errors of this approximation can be reduced to levels
below those of other measurement errors. Hence, the use of the DFT does not necessarily limit the accuracy of the
measurement.
3.2
frequency range of interest
span, in hertz, from the lowest frequency to the highest frequency at which mobility data are to be
obtained in a given test series
3.3
power spectral density
square of absolute value of the FFT of a signal multiplied by 2/T where T is the length of the time signal,
meaning mean-square value of a time signal per unit bandwidth
3.4
energy spectral density
power spectral density (3.3) multiplied by the length of the record in seconds, which is used in the
spectral calculation of a transient signal
Note 1 to entry: This definition assumes that the transient signal is entirely contained within the record.
4 General characteristics of impact measurements
4.1 General description
The instrumentation required for mobility measurements using impact excitation consists of an impact
hammer with built-in force transducer, one or more motion-response transducers with their associated
signal conditioners and a digital Fourier transform analysis system or analyser having at least two
simultaneous input channels. The instrumentation system is shown schematically in Figure 1. This
document provides information on the selection and use of these components.
The force and response signals from each impact are anti-aliasing filtered and then digitally sampled
using the pre-triggering or transient capture mode of the analyser. Each of the resulting digital records
should represent a single impact event. The discrete Fourier transform of each record is computed
by the analyser. Frequency domain averaging of several frequency-response functions obtained from
impacts at a given point may be performed to improve the estimate.
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ISO 7626-5:2019(E)
Key
1 structure under test 6 output device (printer/plotter)
2 motion-response transducer(s) 7 amplifiers and analogue anti-aliasing filter
3 signal conditioners 8 analogue/digital converter
4 storage oscilloscope 9 DFT/FFT and FRF computation
5 display 10 signal analyser
________ basic feature
-------- optional feature
DFT discrete Fourier transform
FFT fast Fourier transform
FRF frequency response function
Figure 1 — Instrumentation block diagram for impact excitation
4.2 Advantages and limitations of impact excitation
4.2.1 General
Impact excitation offers the following intrinsic advantages compared with the use of an attached
exciter:
a) measurement speed;
b) ease of installation;
c) ease of relocating the excitation point;
d) no change of structure, which can be caused by the exciter attachment method (see ISO 7626-2).
On the other hand, the following limitations of impact excitation shall be taken into account:
a) nonlinearity restrictions;
b) signal-to-noise problems;
c) limited frequency resolution;
d) damping restrictions;
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ISO 7626-5:2019(E)
e) dependence on operator skill.
These limitations are discussed in 4.2.2 to 4.2.6.
4.2.2 Nonlinearity restrictions
Mobility measurements on structures which exhibit a significant degree of nonlinearity always demand
special precautions. In such cases, the use of sinusoidal or random excitation with an attached exciter is
preferred, if practical, instead of the impact-excitation technique.
With the impact-excitation technique, the energy needed to drive the response signal to a certain
magnitude is put into the structure during a limited part of the time period used for analysis. Compared
with sinusoidal or random excitation, the force of the impact pulse therefore can be much larger and the
effects of nonlinearity are thus likely to be increased.
For measurements on systems with a significant degree of nonlinearity, it is very important to keep a
level of the force used for the excitation or a level of the system response. In this aspect, the sinusoidal
excitation techniques are preferable. If a hand-held hammer is used to generate the impacts, the
individual force amplitudes can vary significantly. The repeatability of such a measurement can be
poor for nonlinear systems.
4.2.3 Signal-to-noise problems
Because the average signal levels are low compared with the peak levels, impact measurements require
a very low noise testing environment and the maximum possible dynamic range in the measurement
system. This requirement can rule out the use of current analogue tape-recording techniques.
A significant noise problem can occur because the force signal duration is short compared with the total
record length. This situation can result in the instrumentation electrical noise and the mechanically
induced background noise having a mean square value that is significant compared with the mean
square value of the input force. Such noise can be reduced by the windowing techniques described in 8.5.
4.2.4 Limited frequency resolution
The frequency increment, in hertz, which results from a discrete Fourier transform (including the
case of a band-limited or “zoom” analysis), is equal to the reciprocal of the record length, in seconds.
Because each record represents a single impact event, the record length is effectively limited to the
time required for the impulse response of the structure to decay to the level of the background noise.
Therefore, the frequency resolution attainable depends on both the response of the structure and the
background noise level. In some cases, it can be impractical (and unnecessary) using impact excitation
to achieve directly the frequency resolution specified in ISO 7626-2; however, accurate mobility values
can be obtained at discrete frequencies with sufficiently fine resolution for most applications. If the test
structure exhibits high modal density (i.e. multiple resonances within a narrow frequency band), it can
be difficult to achieve sufficiently fine resoluti
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