1、BRITISH STANDARD BS 6897-5: 1995 ISO 7626-5: 1994 Experimental determination of mechanical mobility Part 5: Measurement using impact excitation with an exciter which is not attached to the structureBS6897-5:1995 This British Standard, having been prepared under the directionof the Engineering Sector
2、Board (E/-), was publishedunder the authority ofthe Standards Board and comesinto effect on 15 March 1995 BSI 09-1999 The following BSI references relate to the work on this standard: Committee reference GME/21 Draft for comment 91/70504 DC ISBN 0 580 23924 1 Committees responsible for this British
3、Standard The preparation of this British Standard was entrusted to Technical Committee GME/21, Mechanical vibration and shock, upon which the following bodies were represented: Federation of Civil Engineering Contractors Imperial College of Science and Technology Institute of Sound and Vibration Res
4、earch Institution of Mechanical Engineers Lloyds Register of Shipping Ministry of Defence Open University Power Generation Contractors Association PGCA (BEAMA Ltd.) Railway Industry Association of Great Britain Sira Test and Certification Ltd. Society of British Aerospace Companies Limited Society o
5、f Environmental Engineers The following bodies were also represented in the drafting of the standard, through subcommittees and panels: British Coal Corporation Electricity Association Health and Safety Executive University of Cranfield Amendments issued since publication Amd. No. Date CommentsBS689
6、7-5:1995 BSI 09-1999 i Contents Page Committees responsible Inside front cover National foreword ii Introduction 1 1 Scope 2 2 Normative references 2 3 Definitions 3 4 General characteristics of impact measurements 3 5 Support of the structure under test 5 6 Application of the excitation 6 7 Transdu
7、cer system 11 8 Processing of the transducer signals 11 9 Tests for validity of the measurements 21 Annex A (informative) Correction of mobility measurements for the effects of exponential windowing 22 Annex B (informative) Bibliography 22 Figure 1 Instrumentation block diagram for impact excitation
8、 4 Figure 2 Typical impactor 6 Figure 3 Typical force pulse and spectrum 8 Figure 4 Effects of low-pass filtering on the force pulse ofFigure 3a) and its spectrum 9 Figure 5 Spectrum effects of two impacts in the force record 10 Figure 6 Transient captured force-time history with pre-trigger 12 Figu
9、re 7 The effect of “force windowing” 14 Figure 8 Leakage errors caused by a force window 15 Figure 9 Unwindowed response and frequency-response function 17 Figure 10 Exponentially windowed response and frequency-response function 19 List of references Inside back coverBS6897-5:1995 ii BSI 09-1999 Na
10、tional foreword This British Standard has been prepared by Technical Committee GME/21. It is identical with ISO7626-5:1994 Vibration and shock Experimental determination of mechanical mobility Part 5: Measurements using impact excitation with an exciter which is not attached to the structure, publis
11、hed by the International Organization for Standardization (ISO). ISO 7626-5 was prepared by Technical Committee ISO/TC108, Mechanical vibration and shock, in which the UK played an active part. A British Standard does not purport to include all the necessary provisions of a contract. Users of Britis
12、h Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Cross-references International Standard Corresponding British Standard ISO 2041:1990 BS 3015:1991 Glossary of terms relating to mechanical vibratio
13、n and shock (Identical) ISO 7626-1:1986 BS 6897 Experimental determination of mechanical mobility Part 1:1987 Specification for transducers (Identical) ISO 7626-2:1990 Part 2:1990 Measurements using single-point translation excitation with an attached vibration exciter (Identical) Summary of pages T
14、his document comprises a front cover, an inside front cover, pagesi andii, pages1 to24, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.BS6897-
15、5:1995 BSI 09-1999 1 Introduction General introduction to ISO7626 on mobility measurement Dynamic characteristics of structures can be determined as a function of frequency from mobilitymeasurements or measurements of the related frequency-response functions, known as accelerance and dynamic complia
16、nce. 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. The magnitude and the phase of these functions are frequency dependent. Accelerance and dynamic compliance differ from mobility only in that the
17、motion response is expressed in terms of acceleration or displacement, respectively, instead of in terms of velocity. In order to simplify the various parts of ISO7626, only the term “mobility” will be used. It is understood that all test procedures and requirements described are also applicable to
18、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, mode shapes and damping rat
19、ios); c) predicting the dynamic interaction of interconnected structures; d) checking the validity and improving the accuracy of mathematical models of structures; e) determining dynamic properties (i.e. the complex modulus of elasticity) of materials in pure or composite forms. For some application
20、s, a complete description of the dynamic characteristics may be required using measurements of translational forces and motions along three mutually perpendicular axes as well as measurements of moments and rotational motions about these three axes. This set of measurements results in a6 6 mobility
21、matrix for each location of interest. For N locations on a structure, the system thus has an overall mobility matrix of size 6N 6N. 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 mo
22、bilities by exciting with a force at a single point in a single direction and measuring the translational response motions at key points on the structure. In other applications, only rotational mobilities may be of interest. In order to simplify the use of the various parts of ISO7626 in the various
23、 mobility measurement tasks encountered in practice, ISO7626 will be published as a set of five separate parts: ISO 7626-1 covers basic definitions and transducers. The information in ISO 7626-1 is common to most mobility measurement tasks. ISO 7626-2 covers mobility measurements using single-point
24、translational excitation with an attached exciter. ISO 7626-3 covers mobility measurements using single-point rotational excitation with an attached exciter. This information is primarily intended for rotor system rotational resonance predictions. ISO 7626-4 covers measurements of the entire mobilit
25、y matrix using attached exciters. This includes the translational, rotational and combination terms required for the 6 6 matrix for each location on the structure. ISO 7626-5 (this part of ISO 7626) covers mobility measurements using impact excitation with an exciter which is not attached to the str
26、ucture. Mechanical mobility is defined as the frequency-response function formed by the ratio of the phasor of the translational or rotational response velocity to the phasor of the applied force or moment excitation. If the response is measured with an accelerometer, conversion to velocity is requi
27、red to obtain the mobility. Alternatively, the ratio of acceleration to force, known as accelerance, may be used to characterize a structure. In other cases, dynamic compliance, the ratio of displacement to force, may be used. NOTE 1Historically, frequency-response functions of structures have often
28、 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. It should be noted, however, that this is misleading because the arithmetic reciprocal of mobility does not, in g
29、eneral, 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 mat
30、rix, orviceversa. This point is elaborated upon in Annex A of ISO7626-1:1986.BS6897-5:1995 2 BSI 09-1999 Introduction to this part of ISO 7626 Impact excitation has become a popular method for measuring the frequency response of structures because of its inherent speed and relatively low cost to imp
31、lement. However, the accuracy of mobility measurements made by using impact excitation is highly dependent upon both the characteristics of the test structure and on the experimental techniques employed. With impact excitation, it may be difficult or impossible in certain cases to obtain the accurac
32、y which is attainable using continuous excitation with an attached exciter, and the impact method carries an increased danger of gross measurement errors. (See ref. 7). In spite of these limitations, impact testing can be an extremely useful excitation technique when applied properly. This part of I
33、SO 7626 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
34、 characteristics of the test structure, especially its degree of nonlinearity, will limit the accuracy which can be achieved. Subclause4.2 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 se
35、ries 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 lin
36、earity, 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 may be difficult to impact the structure in all desired directions at certain locations, and in such cases it
37、may 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 2The use of a multi-axis response transducer at a fixed location does not provide information about the multi-axis response at othe
38、r locations. For example, if a fixed response transducer is used in performing measurements for a modal test, and if the impact is applied in only a single direction at each point, then only the mode shape components in that direction are obtained. 1 Scope This part of ISO7626 specifies procedures f
39、or measuring mechanical mobility and other frequency-response functions of structures excited by means of a translational 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, eit
40、her 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 part of ISO762
41、6 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. This restriction in scope is based solely on the wide availability of equipment which implements these methods and on the large base o
42、f 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 mod
43、e shapes can be quite helpful in planning a random or sinusoidal test for accurate mobility measurements. However, these uses of impact excitation to obtain qualitative results should not be confused with its use for mobility measurements. This part of ISO 7626 is limited to the use of impact excita
44、tion techniques for making accurate mobility measurements. 2 Normative references The following standards contain provisions which, through reference in this text, constitute provisions of this part of ISO7626. At the time of publication, the editions indicated were valid. All standards are subject
45、to revision, and parties to agreements based on this part of ISO7626 are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of currently valid International Standards. ISO 2041:1990, Vibration and
46、 shock Vocabulary. ISO 7626-1:1986, Vibration and shock Experimental determination of mechanical mobility Part 1: Basic definitions and transducers. BS6897-5:1995 BSI 09-1999 3 ISO 7626-2:1990, Vibration and shock Experimental determination of mechanical mobility Part 2: Measurements using single-po
47、int translation excitation with an attached vibration exciter. 3 Definitions For the purposes of this part of ISO7626, the definitions given in ISO2041 apply. For convenience, the most important definitions used in this part of ISO7626 are given below. 3.1 frequency-response function the frequency-d
48、ependent ratio of the motion-response phasor to the phasor of the excitation force NOTE 3Frequency-response functions are properties of linear dynamic systems which do not depend on the type of excitation function. Excitation can be harmonic, random or transient functions of time. The test results o
49、btained with one type of excitation can thus be used for predicting the response of the system to any other type of excitation. NOTE 4Linearity of the system is a condition which, in practice, will 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 non-linear (for example, certain riveted structures) should not be tested with impulse excitation and great care is req
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