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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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
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electrotechnical standardization.
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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
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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
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