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 2019
© ISO 2019
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Published in Switzerland
ii © ISO 2019 – All rights reserved
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
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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committee has been established has the right to be represented on that committee. International
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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
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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.
iv © ISO 2019 – All rights reserved
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.
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.
vi © ISO 2019 – All rights reserved
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/
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.
2 © ISO 2019 – All rights reserved
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;
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 resolution for an accurate mobility measurement. In those cases,
one of the steady-state excitation methods with “zoom” analysis is preferred.
By its very nature, the spectrum of an impact extends from DC to some upper frequency limit (see
Clause 6). This inability to band limit the excitation spectrum restricts the usefulness of “zoom”
analysis for improving the frequency resolution of impact measurements, and the impact places further
demands on the dynamic range of the measurement system. It also increases the danger of undetected
overloads (clipping) in the measurement system due to high-amplitude out-of-band signals. See 6.3
and 8.4. The time resolution has to be high enough for the impact signal to be recorded at a sufficient
number of sampling points. However, a higher time resolution can decrease the frequency resolution
when the number of data points for the FFT is fixed.
4.2.5 Damping restrictions
Impact excitation has limitations for testing heavily damped structures because the short duration
of the response signal leads to a trade-off between frequency resolution and background noise level,
4 © ISO 2019 – All rights reserved
as discussed in 4.2.4. This limitation can also be understood as a manifestation of the inherently low
average energy level for a given impact force magnitude. Heavily damped structures can require higher
energy excitation in order to balance their high internal energy dissipation characteristics and to
produce sufficient response data for accurate measurement.
A different problem occurs if the structure has extremely light damping. The frequency-response
functions of such a structure exhibit very sharp resonance peaks which requires high-resolution zoom
measurements for accurate definition, as discussed in 4.2.4. The use of an exponential decay window
can help by adding a known amount of artificial decay to the data. If windowing is used, the resulting
mobility data or modal damping values therefrom have to be corrected, as described in 8.5 and Annex A.
4.2.6 Dependence on operator skill
The accuracy of mobility measurements performed using a hand-held impact hammer depends on
the ability of the operator to maintain the correct location and direction of impact. These effects can
normally be held within acceptable limits if the impacts are applied carefully, but they can be significant
if the test structure is small and requires very fine spatial resolution.
Operator skill is also required in order to avoid a double hit of impactor; see 6.4. If there are high
demands on the quality, the shock may be applied by a mechanical pendulum to avoid double hits.
5 Support of the structure under test
5.1 General
Mobility measurements may be performed on structures either in an ungrounded condition (freely
suspended) or in a grounded condition (attached to one or more supports), depending on the objective
of the test.
5.2 Ungrounded measurements
Ungrounded measurements employ a compliant suspension of the test structure. The magnitudes of the
driving-point mobility of the suspension at points of attachments should be at least ten times greater
than the magnitudes of the mobility of the structure at the same attachment points.
5.3 Grounded measurements
Grounded measurements employ a support of the test structure which is representative of its support
in typical applications, unless otherwise specified. A description of the support and attachment should
be included in the test report.
FRFs and modal parameters resulting from grounded measurements can differ significantly from those
measured in ungrounded (free-free) configuration. Due to the clamping, the structure is stiffened in
general and, hence, the natural frequencies increase.
6 Application of the excitation
6.1 Impactor design
A typical impactor consists of a rigid mass with a force transducer attached to one end and an impact
tip attached to the opposite side of the force transducer, as shown schematically in Figure 2. The tip
stiffness and impactor mass shall be selected as described in 6.3, in order to achieve a force pulse of the
desired duration and to avoid double hits.
Key
1 force transducer
2 interchangeable impactor tip
3 mass
4 interchangeable mass
Figure 2 — Typical impactor
For small values of impactor mass, the impactor often takes the form of a hand-held hammer with
interchangeable tips and masses. However, the accuracy obtained using a hand-held impactor depends
on the skill of the operator in maintaining the correct location and direction of impact. For small
test structures, it is necessary to provide a suitable mechanical device to guide the impactor to a
repeatable location and direction on the structure. For testing large structures which require higher
energy, the impactor may take the form of a large mass suspended from cables and either dropped or
swung. Alternatively, a smaller mass may be accelerated to a high impact velocity by a spring, solenoid
pneumatic actuator or other means.
The area of the impact surface of the tip should be large enough to withstand the maximum force
employed without permanent deformation of either the tip or the test structure. On the other hand, a
small tip area is necessary if very fine spatial resolution of the location is required. The velocity vector
of the impactor at the moment of impact should be in line with the sensing axis of the force transducer
and should be perpendicular to the surface of the test structure at the point of impact within 10°. It is
generally easier to maintain the proper orientation if the impactor body is relatively long compared
with its cross-sectional dimensions.
6.2 Force spectrum characteristics
A theoretical (Dirac) impulse of infinitesimal duration contains equal energy at all frequencies. However,
the spectrum of any actual force pulse has a finite usable bandwidth which is inversely proportional to
6 © ISO 2019 – All rights reserved
the duration of the pulse. This provides a useful means of concentrating the main excitation energy
into the frequency range below the maximum frequency of interest. In practice, the spectrum of a
single force pulse typically has the form of a main lobe at low frequency followed by higher-frequency
side-lobes whose magnitudes decrease rapidly with frequency. Figure 3 shows a force pulse and the
corresponding energy spectral density. The usable frequency range of this pulse extends up to about
1 000 Hz, depending on the response characteristics of the structure under test.
a) Time history of impact force, unfiltered
b) Energy spectral density corresponding to Figure 3 a)
Key
F force, N
t time, s
G force energy spectral density, N s/Hz
FF
f frequency, Hz
Figure 3 — Typical force pulse and spectrum
There is an inherent trade-off between time-domain resolution and frequency-domain resolution in the
discrete Fourier transform. Due to the sampling relationships of the discrete Fourier transform (see
8.3), the force waveform is represented by only a few discrete samples in the digital record used by the
Fourier analyser. The force waveform is also shaped by the anti-aliasing filter. These factors, which are
necessary for accurate frequency-domain analysis, make the digital records ill-suited for monitoring
the force waveform in time domain during impact measurements (unless the analysis bandwidth is
increased to a frequency well above the usable frequency range of the force pulse). Figure 4 shows the
same force pulse as Figure 3, but low-pass filtered at 625 Hz by the analogue anti-aliasing filter. The
two energy densities show good agreement, although the digitized force waveform in Figure 4 shows a
considerably different shape and peak magnitude.
8 © ISO 2019 – All rights reserved
a) Time history of impact force, low-pass filtered at 625 Hz
b) Energy spectral density corresponding to Figure 4 a)
Key
F force, N
t time, s
G force energy spectral density, N s/Hz
FF
f frequency, Hz
Figure 4 — Effects of low-pass filtering on the force pulse of Figure 3 a) and its spectrum
6.3 Control of the frequency range of excitation
In order to make optimum use of the dynamic range of the measurement system, it is desirable to limit
the frequency range of excitation to the maximum frequency of interest. The excitation bandwidth is
controlled by the tip stiffness and the impactor mass. The frequency range of a given impactor can be
reduced either by decreasing the tip stiffness or by increasing the impactor mass.
The actual frequency range achieved also depends on the effective stiffness and mass of the test
structure at the point of impact. Low structure stiffness sometimes limits the increase in frequency
range achievable by increasing the tip stiffness. In this case, a more effective method of increasing the
frequency range of the excitation is to reduce the mass of the impactor.
The force and response spectra should be checked for excessive energy above the frequency range of
interest by using a force whose actual frequency range becomes equal to the maximum likely to be
employed during the test. The impactor characteristics should then be adjusted, if necessary, to achieve
the desired frequency range.
6.4 Avoidance of impactor double hits
If more than a single impact occurs within the data record, the Fourier transforms of the pulses tend
to cancel at certain frequencies, creating sharp notches in the force spectrum (see Figure 5). This can
cause significant errors in the mobility measurement at these frequencies, due to a low signal-to-noise
ratio in the force spectrum. Even if the impact is applied very carefully, it can be impossible to avoid
double hits when exciting at a very responsive point on the test structure with a relatively massive
impactor. The solution is to reduce the mass of the impactor; the tip stiffness should then be adjusted to
maintain the desired frequency range of excitation.
10 © ISO 2019 – All rights reserved
a) Time history of double-impact force
b) Energy spectral density corresponding to Figure 5 a)
Key
F force, N
t time, s
G force energy spectral density, N s/Hz
FF
f frequency, Hz
Figure 5 — Spectrum effects of double impacts in the force record
If the second impact is small compared with the primary impact, the force spectrum can exhibit a
slight ripple rather than deep notches. Moderate dips in the force spectrum may normally be tolerated.
Multiple impacts are most easily detected in the frequency domain by checking the force spectrum
at each impact location. It is also desirable to monitor the time-domain waveform; a storage-type
oscilloscope may be used to observe the unfiltered force signal, so that the anti-aliasing filter does not
obscure secondary impacts “ringing” from the primary impact.
Never use a “force window” (see 8.5) to eliminate secondary impacts from the force record prior to
Fourier processing. When using a force window to reduce noise in the force signal, make sure that it
does not mask multiple impacts which actually occur. The response still includes the effects of the
multiple impacts, thus resulting in an erroneous estimate of the frequency response function.
7 Transducer system
7.1 General
Transducers and their associated signal conditioning should be selected in accordance with the criteria
given in ISO 7626-1 and ISO 7626-2. For use in impact measurements, it is especially important that the
transducer system has low noise and a large linear dynamic range.
7.2 Impactor calibration
Although the effective mass between the force transducer and the structure under test does not affect
the motion-response signal, it can have a significant effect on the force calibration of the impactor. The
following operational calibration test should be performed at the beginning and end of each series of
measurements, and it should be repeated whenever the mass or tip of the impactor is changed.
This operational calibration is essentially the same as the procedure described in ISO 7626-2:2015, 7.5.2;
it is performed by measuring either the mobility or the accelerance of a freely suspended rigid calibration
block of known mass. The measured frequency response for the calibration block should agree with its
known value (for example, an accelerance magnitude of 1/m, where m is the total mass of the calibration
block, including any attached transducers) within ±5 % over the frequency range of interest. This should
be accomplished by using a known calibration value for the motion-response transducer and adjusting
the calibration constant for the signal to achieve the correct mobility or accelerance magnitude. The
operational calibration test should also be performed on any additional response transducers which
are used in the measurements (for example multi-axis response transducers); these transducers should
be assigned calibration values consistent with the force calibration obtained above. The measurement
system and the impactor shall be in the same configuration as used in the mobility measurements which
employ this calibration value. The mass of the calibration block shall be chosen so that its mobility is
representative of the range of mobilities to be measured.
If the calibration measurement does not yield an accelerance value which is essentially constant over
the frequency range of interest, the cause of this discrepancy should be investigated and resolved
before proceeding with the test.
12 © ISO 2019 – All rights reserved
8 Processing of the transducer signals
8.1 Filtering
The transducer signals shall be low-pass filtered in an analogue way before the data are sampled
and digitized by the Fourier analyser, in order to prevent out-of-band signals from being improperly
interpreted within the analysis range, a phenomenon known as “aliasing”. Most commercially available
analysers include built-in filters with cut-off frequencies matched to the available frequency ranges of
the analyser. The adequacy of alias protection can be checked by using a good-quality signal generator
to produce a full-scale sine wave input at a variet
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