BS EN 60068-2-64:1995:2006 Edition
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Environmental testing. Test methods – Test Fh. Vibration, broad-band random (digital control) and guidance
| Published By | Publication Date | Number of Pages |
| BSI | 2006 | 40 |
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| 1 | BRITISH STANDARD BS EN 60068-2-64: 1995 IEC 68-2-64: 1993 Environmental testing – Part 2: Test methods – Test Fh: Vibration, broad-band random (digital control) and guidance |
| 2 | This British Standard, having been prepared under the direction of the Electrotechnical Sector Board (L/1), was published under the authority of the Standards Board and comes into effect on 15 April 1995 The following BSI references relate to the work on this standard: Committee reference GEL/50/1 Committees responsible for this British Standard The United Kingdom participation in the preparation of this British Standard was entrusted by Technical Committee GEL/50, Enviro… Federation of the Electronics Industry GAMBICA (BEAMA Ltd.) Society of Environmental Engineers Society of Motor Manufacturers and Traders Limited The following bodies were also represented in the drafting of the standard, through subcommittees and panels: AEA Technology Association of Manufacturers of Domestic Electrical Appliances BEAMA Ltd. |
| 3 | Contents |
| 4 | National foreword This British Standard has been prepared by Subcommittee GEL/15/1 and is the English language version of EN 60068-2-64:1994 Envir… IEC 50 (302):9183 IEC 50 (503):1983 (IEC 68-1:1988) (IEC 68-2-6:1982) (IEC 68-2-47:1982) BS EN 60068-2 Test methods BS EN 60068-2-47:1993 Tests. Mounting of components equipment and other articles for dynamic tests and guidance (IEC 721 series) (IEC 72 series) a HD 323.2.6 S2 includes A1:1983 + A2:1985 to IEC 68-2-6. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, the EN title page, pages 2 to 34, 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. |
| 5 | EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM EN 60068-2-64 UDC 621.3:620.193:534.1 Descriptors: Environmental testing, electricity, component, equipment, mechanical test, vibration, broad-band random, digital control, procedures, components specifications writing, equipment specifications writing English version Environmental testing Part 2: Test methods Test Fh: Vibration, broad-band random (digital control) and guidance (IEC 68-2-64:1993 + corrigendum 1993) This European Standard was approved by CENELEC on 1994-03-08. CENELEC members are bound to comply with the CEN/CENELEC Internal … Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the Central Secretariat or to any CENELEC member. This European Standard exists in three official versions (English, French, German). A version in any other language made by tran… CENELEC members are the national electrotechnical committees of Austria, Belgium, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom. |
| 6 | At the request of the CENELEC Reporting Secretariat SR 50A, the International Standard IEC 68-2-64:1993 and its corrigendum October 1993, were submitted to the CENELEC Unique Acceptance Procedure (UAP) in July 1993 for acceptance as a European Standard. The text of the International Standard was approved by CENELEC as EN 60068-2-64 on 8 March 1994. The following dates were fixed: Annexes designated “normative” are part of the body of the standard. Annexes designated “informative” are given only for information. In this standard, Annex A and Annex ZA are normative and Annex B and Annex C are informative. |
| 7 | Introduction This standard for broad-band random vibration testing is intended for general application to specimens of electrotechnical produ… It should be noted that random vibration testing is a complex subject requiring both a good basic understanding of the philosophy of the test and the exercise of considerable engineering judgement. Compared with most other tests, test Fh is not based on deterministic but on statistical techniques. Broad-band random vibration testing is therefore described in terms of probability and statistical averages. Annex A is a normative annex giving the requirements for the vibration response investigation. Specification writers will find in clause 11 a list of details to be considered for inclusion in specifications, and in Annex B (informative), the guidance. Annex C is an informative annex, cross-referenced to the relevant clauses, giving the conversion between the quoted values (in dB or percentages) and the values with the alternative magnitudes. 1 Object The object of this International Standard is to provide two standard test methods (method 1 and method 2) for determining the ab… It is also to reveal the accumulated effects of stress induced by random vibration, and the resulting mechanical weakness and de… This standard is applicable to specimens which may be subjected to vibration of a stochastic nature resulting from transportatio… Although primarily intended for electrotechnical products, this standard is not restricted to them and may be used in other fields where desired. 2 Normative references The following normative documents contain provisions which, through reference in this text, constitute provisions of this part o… IEC 50(301, 302, 303):1983, International Electrotechnical Vocabulary (IEV) – Chapter 301:General terms on measurements in electricity – Chapter 302:Electrical measuring instruments – Chapter 303:Electronic measuring instruments. (Advance edition) IEC 68, Environmental testing. IEC 68-1:1988, Environmental testing – Part 1: General and guidance. IEC 68-2, Environmental testing – Part 2: Tests. IEC 68-2-6:1982, Environmental testing – Part 2: Tests – Test Fc and guidance: Vibration (sinusoidal). IEC 68-2-47:1982, Environmental testing – Part 2: Tests – Mounting of components, equipment and other articles for dynamic tests including shock (Ea), bump (Eb), vibration (Fc and Fd) and steady-state acceleration (Ga) and guidance. |
| 8 | IEC 721, Classification of environmental conditions. ISO 2041:1990, Vibration and shock – Vocabulary. 3 Definitions The terms used are generally defined in ISO 2041 or IEC 50 (301, 302, 303) and in IEC 68-1 or IEC 68-2-6. Where, for the conveni… The additional terms and definitions that follow are also applicable for the purposes of this standard. 3.1 – 3 dB bandwidth, Br (ISO 2041 modified) frequency bandwidth between two points in a frequency response function which is 0,707 of the maximum response when associated with a single resonance peak (see 4.3.6.2) 3.2 acceleration spectral density (ISO 2041 modified) mean-square value of that part of an acceleration signal passed by a narrow-band filter of a centre frequency, per unit bandwidth, in the limit as the bandwidth approaches zero and the averaging time approaches infinity (see 4.3.4) 3.3 bias error systematic error in the estimate of the acceleration spectral density due to the finite frequency resolution used in practice (see 4.3.6.2) 3.4 check-point point, located on the fixture, on the vibration table or on the specimen, as close as possible to one of its fixing points and in any case rigidly connected to it (see A.2.4.1) NOTE 1 A number of check-points are used as a means of ensuring that the test requirements are satisfied. NOTE 2 If four or fewer fixing points exist, each is used as a check-point. If more than four fixing points exist, four representative fixing points will be defined in the relevant specification to be used as check-points. NOTE 3 In special cases, for example for large or complex specimens, the check-points will be prescribed by the relevant specification if not close to the fixing points. NOTE 4 Where a large number of small specimens are mounted on one fixture, or in the case of a small specimen where there are se… 3.5 control acceleration spectral density acceleration spectral density measured at the reference point (see 4.3.3) 3.6 control system loop sum of the following actions 3.7 crest factor (ISO 2041) ratio of the peak value to the r.m.s. value (see 4.3.3) 3.8 damping ratio ratio of actual damping to critical damping in a system with viscous damping |
| 9 | 3.9 distortion (definition identical to that in clause 3 of IEC 68-2-6, not identical with ISO 2041 definition) 3.10 drive signal clipping limitation of the instantaneous value of the drive signal (see 4.3.3) 3.11 effective frequency range range from the actual frequency below f1 to the actual frequency above f2 due to initial and final slopes (see Figure 2) 3.12 error acceleration spectral density difference between the specified acceleration spectral density and the control acceleration spectral density 3.13 equalization minimization of the error acceleration spectral density 3.14 final slope part of the specified acceleration spectral density above f2 (see B.2.4) 3.15 frequency resolution width of the frequency intervals in the acceleration spectral density in Hertz. It is equal to the reciprocal of the record leng… 3.16 “gn” standard acceleration due to the earth’s gravity, which itself varies with altitude and geographical latitude (see 5.3) NOTE For the purposes of this standard, the value of gn is rounded up to the nearest whole number, that is 10 m/s2. 3.17 indicated acceleration spectral density estimate of the true acceleration spectral density read from the analyser presentation corrupted by the instrument error, the random error and the bias error (see 4.3.6) 3.18 initial slope part of the specified acceleration spectral density below f1 (see B.2.4) 3.19 instrument error error associated with each analogue item of the input to the control system and control system analogue items (see B.2.3.2) |
| 10 | 3.20 multipoint control, averaging control acceleration spectral density formed from the arithmetic average of the acceleration spectral densities at more than one check-point (see B.2.1.2) 3.21 multipoint control, extremal control acceleration spectral density formed from the maximum of the acceleration spectral density of each frequency line at more than one check-point (see B.2.1.2) 3.22 preferred testing axes three orthogonal axes which correspond, as far as is practicable, to the most vulnerable axes of the specimen (see 8.1) 3.23 random error error changing from one estimate to another of the acceleration spectral density because of the limitation of averaging time and filter bandwidth in practice (see B.2.3.3) 3.24 record collection of equally spaced data points in the time domain that are used in the calculation of the Fast Fourier Transform (see B.1) 3.25 reference point point, chosen from the check-points, whose signal is used to control the test, so that the requirements of this standard are satisfied NOTE The point may be a fictitious reference point (see A.2.4.2). 3.26 reproducibility [IEC 50 (301, 302, 303)] the closeness of the agreement between the results of measurements of the same value of the same quantity, where the individual measurements are made NOTE The term “reproducible” also applies to the case where only certain of the preceding conditions are taken into account. 3.27 response points specific locations on the specimen at which data are gathered for the purpose of the vibration response investigation. These points are not check or reference points (see A.3.1) 3.28 root-mean-square value (ISO 2041 modified) the root-mean-square value (r.m.s. value) of a single-valued function over an interval between f1 and f2 is the square root of the average of the squared values of the function over the interval (see 4.3.4) NOTE In this test method, the r.m.s. values of acceleration, velocity and displacement can be calculated according to B.2.5. 3.29 standard deviation, B (ISO 2041 modified) in vibration theory, the mean value of vibration is equal to zero. Therefore, the standard deviation is equal to the r.m.s. value |
| 11 | 3.30 statistical accuracy ratio of true acceleration spectral density to indicated spectral density (see 4.3.5) 3.31 statistical degrees of freedom (ISO 2041 modified) for estimation of acceleration spectral density of random data with a time-averaging technique, the effective number of statistical degrees of freedom is derived from the frequency resolution and the effective averaging time (see 4.3.5) 3.32 sweep cycle (IEC 68-2-6) traverse of the specified frequency range once in each direction, for example 5 Hz to 500 Hz to 5 Hz (see A.2.5) 3.33 true acceleration spectral density acceleration spectral density of the random waveform acting on the specimen (see 4.3.6) 3.34 window function (ISO 2041) truncated function that is used for reducing the errors in processing weighted data points (see 4.3.6.2) 4 Requirements for testing 4.1 General The required characteristics apply to the complete vibration system, which includes the power amplifier, vibrator, test fixture, specimen and control system when loaded for testing. This standard provides for two test methods. Method 1 normally uses only random vibration. However, the relevant specification may require a vibration response investigation, with either sinusoidal or random vibration excitation, before, or before and after, the random vibration (see 8.2 and 8.6). Method 2 uses a vibration response investigation, with either sinusoidal or random vibration excitation, prior to the random vib… 4.2 Vibration response investigation The requirements for sinusoidal excitation, which are derived from those of test Fc (IEC 68-2-6), are prescribed in the normativ… 4.3 Testing with random excitation 4.3.1 Basic motion The basic motion of the fixing points of the specimen, which shall be prescribed by the relevant specification and have substant… 4.3.2 Transverse motion The acceleration spectral density at the check-points in any axis perpendicular to the specified axis shall not exceed the speci… |
| 12 | At some frequencies, or with large-size or high-mass specimens, it may be difficult to achieve these values. In such cases, the relevant specification shall state which of the following applies: a) any transverse motion in excess of that given above shall be stated in the test report; or b) transverse motion which is known to offer no hazard to the specimen need not be monitored. 4.3.3 Distribution The instantaneous acceleration values at the reference point shall have a normal (Gaussian) distribution within the tolerance ba… If a fictitious reference point is used for control, the requirement for the distribution applies to all the check-points used to form the control acceleration spectral density. The crest factor or drive signal clipping shall be prescribed by the relevant specification and shall have a value of at least 2… 4.3.4 Vibration tolerances The indicated acceleration spectral density in the required axis at the check and reference points between f1 and f2 in Figure 2… The r.m.s. value of acceleration, computed or measured, between f1 and f2, shall be within ± 10 % of the r.m.s. value associated with the specified acceleration spectral density. These values are valid for both single-point and multipoint control. At some frequencies or with large-size or high-mass specimens, it may be difficult to achieve these values. In such cases it is expected that the relevant specification will prescribe a wider tolerance. The initial and final slope shall be not less than + 6 dB/octave and not more than – 24 dB/octave respectively. (See B.2.4.) 4.3.5 Statistical accuracy The statistical accuracy is determined from the statistical degrees of freedom (Nd) as follows: 4.3.6 Frequency resolution The frequency resolution necessary to minimize the difference between the true and the indicated acceleration spectral density shall be within the limits given in 4.3.6.1 for method 1, or in 4.3.6.2 for the calculation in the case of method 2. 4.3.6.1 Method 1 The maximum value of the frequency resolution shall be selected from Table 1. Table 1 – Frequency resolution, method 1 |
| 13 | 4.3.6.2 Method 2 The frequency resolution Be shall be derived from the frequency response investigation by selecting the resonance with the narrowest – 3 dB bandwidth Br (see A.2.6 and A.3.1). The frequency resolution is then calculated from: The factor a takes account of bias error Eb and shall be derived from Table 2 (see B.2.3.4). Table 2 – Factor a and bias error for rectangular window function Eb in dB NOTE For other types of window function the factor a should be divided by the factor W taken from Table B.3 (see B.2.3.4). 4.4 Mounting Unless otherwise prescribed by the relevant specification, the specimen shall be mounted in accordance with IEC 68-2-6, in which there is reference to IEC 68-2-47. 5 Severities 5.1 General The test severity is determined by the combination of all the following parameters: Each parameter shall be prescribed by the relevant specification. They shall be: a) chosen from the values given in 5.2 to 5.5; or b) derived from the known environment if this gives significantly different values; or c) derived from other known sources of relevant data (for example IEC 721). 5.2 Test frequency range If option a) of 5.1 is chosen, then the test frequency range shall be selected from Table 3. Table 3 – Test frequency range Frequencies f1 and f2 and their relation to the acceleration spectral density are shown in Figure 2. 5.3 Acceleration spectral density If option a) of 5.1 is chosen, then the acceleration spectral density (0 dB in Figure 2) between f1 and f2 shall be selected from the following values in (m/s2)2/Hz: 0,05; 0,1; 0,5; 1,0; 5,0; 10,0; 50,0; 100,0. NOTE For those wishing to continue giving values in gn, the value of 10 m/s2 is ascribed to gn for the purposes of this standard. |
| 14 | 5.4 Shape of acceleration spectral density curve This test specifies an acceleration spectral density curve with a flat horizontal portion (see Figure 2). In special cases, it m… 5.5 Duration of testing The duration in each axis shall be selected from the following values, in minutes, with a tolerance of : 1; 3; 10; 30; 100; 300. 6 Pre-conditioning The relevant specification may call for pre-conditioning, and shall then prescribe the conditions. 7 Initial measurements The specimen shall be submitted to the visual, dimensional and functional checks prescribed by the relevant specification (see B.7). 8 Testing 8.1 General Testing follows different paths according to whether method 1 or method 2 is prescribed by the relevant specification (see Figure 8). The steps are: Method 1 Method 2 The specimen shall be vibrated excited in each of the preferred testing axes in turn, unless otherwise prescribed by the relevant specification. The order of the testing along these axes is not important, unless prescribed by the relevant specification. The control acceleration spectral density at the reference point shall be derived from one check-point if single-point control i… Special action is necessary when a specimen normally intended for use with vibration isolators needs to be tested without them (see B.4). 8.2 Vibration response investigation When method 1 is prescribed by the relevant specification, a vibration response investigation is not required as part of the met… |
| 15 | When method 2 is prescribed by the relevant specification, the test frequency range shall be investigated in order to study the … When sinusoidal excitation is used, at least one sweep cycle over the test frequency range prescribed by the relevant specificat… The vibration amplitude may also be reduced in order to prevent a higher stress on the specimen than during random vibration testing, for example when the specimen is very sensitive to sinusoidal vibration. When random excitation is used, the r.m.s value of acceleration shall be not more than 25 % of the value specified to be used du… 8.3 Low-level excitation for equalization prior to testing Prior to random vibration testing at the specified level, a preliminary random excitation at lower levels with the real specimen… The permitted durations for preliminary random excitation are the following: The duration of the preliminary random excitation shall not be subtracted from the specified duration of exposure for random vibration testing. 8.4 Random vibration testing The relevant specification shall select the appropriate test frequency range (f1 to f2), the acceleration spectral density, the … 8.5 Intermediate measurements When prescribed by the relevant specification, the specimen shall be functioning during a prescribed time interval during the testing, and its performance shall be checked (see B.6). 8.6 Final vibration response investigation When method 2 is applied, and in the case of method 1 if the relevant specification has prescribed an initial vibration response… 9 Recovery It is sometimes necessary to provide a period of time after testing and before final measurements to allow the specimen to attai… 10 Final measurements The specimen shall be submitted to the visual, dimensional and functional checks prescribed by the relevant specification. |
| 16 | The relevant specification shall provide the criteria upon which the acceptance or rejection of the specimen is to be based (see B.7). 11 Information to be given in the relevant specification When this test is included in a relevant specification, the following details shall be given in so far as they are applicable, paying particular attention to the items marked with an asterisk (*), as this information is always required. (sinusoidal or random excitation) method 2* |
| 17 | A.1 Introduction Vibration response investigations are most useful for examining the dynamic behaviour of a specimen, and may be necessary for method 1 if mechanical or structural effects need to be determined. For a random vibration test according to method 2, the response investigation is essential in order to select the resonance with the narrowest – 3 dB bandwidth Br to enable calculation of the limiting frequency resolution Be given in equation (2). It is necessary to obtain the frequency response characteristics of the test specimen from several locations to avoid missing a … It is very important that any measuring arrangements made to detect the effect upon the selected part of the specimen shall not substantially change the dynamic behaviour of that part and of the specimen as a whole. It should be remembered that, in the case of non-linear resonances, the resonance frequencies will change depending on the direc… A vibration response investigation carried out after the random vibration testing can be used to identify changes in the frequen… A.2 Sinusoidal vibration The requirements to be satisfied are the following, which are generally in accord with those of test Fc (IEC 68-2-6). A.2.1 Basic motion The basic motion shall be a sinusoidal function of time such that the fixing points of the specimen, which shall be prescribed b… A.2.2 Transverse motion The maximum vibration amplitude of the fundamental motion at the check points in any axis perpendicular to the specified axis sh… At some frequencies, or with large-size or high-mass specimens, it may be difficult to achieve these values. In such cases, the relevant specification shall state which of the following applies: a) any transverse motion in excess of that given above shall be stated in the test report; b) transverse motion which is known to offer no hazard to the specimens need not be monitored. A.2.3 Distortion The acceleration distortion measurement shall be carried out on the signal from the reference point, and shall cover the frequen… The distortion, as defined in 3.9, shall not exceed 25 % of the basic motion. |
| 18 | When sinusoidal excitation is used for the vibration response investigation (see 8.2) and the distortion is high, the measuring … In the case of large or complex specimens, where the specified values of distortion cannot be satisfied at some parts of the fre… The relevant specification may require that the distortion together with the frequency range affected is noted, whether or not a tracking filter has been used. A.2.4 Vibration tolerances The basic motion along the required axis at the check and reference points shall be equal to the specified values within the fol… A.2.4.1 Check-points The vibration tolerance on the amplitude at each check-point with single-point control is: A.2.4.2 Reference point Usually the test is performed with single point control (see B.2.1) when the vibration tolerance on the amplitude at the reference point is: If it is difficult to achieve the required value, multipoint control may be used by controlling the averaged or extremal value of the signals at the check-points. In either of these cases of multipoint control the point is a fictitious reference point. A.2.4.3 Frequency The tolerances are: If both initial and final vibration response investigations are performed, the following tolerances shall apply: A.2.5 Sweeping Generally, sweeping shall be continuous with the frequency changing exponentially with time at a rate of not greater than 1 octave per minute over the whole test frequency range (see 3.32). NOTE With a digital control system it is not strictly correct to refer to the sweeping being “continuous”, but the difference is of no practical significance. A.2.6 Calculation of Be, Br During the sweep with sinusoidal excitation, the response at the prescribed response points shall be divided by the input vibrat… |
| 19 | A.3 Random excitation The requirements of 4.3 shall be followed when conducting a response investigation with random excitation, and it is recommended that a flat acceleration spectral density input be used. A.3.1 Calculation of Be, Br The acceleration spectral density at the prescribed response points shall be divided by the acceleration spectral density input … It should be noted that the frequency resolution for this method of response investigation will need to be sufficiently high to … B.1 General introduction To achieve reproducibility is not easy. Because of the statistical nature of the random signal, the complex response of the spec… The performance of most digital vibration control equipment likely to be employed for random vibration testing can be expected t… Equalization of the specified acceleration spectral density requires several repetitions of the control loop, the duration depen… The control algorithm of the random vibration involves a compromise between control accuracy and control loop time, which is aff… Method 1 gives a lower probability of achieving reproducibility as compared with method 2 (see 1). In the case of method 2, the … A very large bias error can occur for low damping ratio and low resonance frequency of a specimen with the small number of frequ… |
| 20 | Table B.1 – Lower resonance frequency limits for a given bias error for 200 frequency lines for bias errors of 3 dB and 6 dB B.2 Requirements for testing B.2.1 Single-point and multipoint control The test requirements are confirmed by the acceleration spectral density computed from the random signal measured at the reference point. For stiff or small-size specimens, for example in component testing, there need only be one check-point, which then becomes the reference point. In the case of large or complex specimens, for example equipments with well-spaced fixing points, either one of the check-points… B.2.1.1 Single-point control Measurements are made at one reference point and the indicated acceleration spectral density is directly compared with the specified acceleration spectral density. B.2.1.2 Multipoint control When multipoint control is specified or necessary, a choice of frequency domain control strategies is available. B.2.1.2.1 Averaging strategy In this method, the acceleration spectral density is computed from the signal from each check-point. A composite acceleration spectral density is formed by arithmetically averaging the acceleration spectral density from the check-points. This arithmetically averaged acceleration spectral density is then compared with the specified acceleration spectral density. B.2.1.2.2 Extremal strategy In this method, a composite acceleration spectral density is computed from the maximum extreme values of each frequency line of the acceleration spectral density measured at each check-point. This strategy will produce an acceleration spectral density that represents the envelope of the acceleration spectral density from each check-point. B.2.2 Distribution B.2.2.1 Distribution of the instantaneous values The distribution of the instantaneous values of the drive signal employed during the testing is known as the normal or Gaussian distribution, and is defined by the equation: |
| 21 | The mean value of the drive signal time history is assumed to be zero. The normal probability density function is shown in Figure 3. The concept of the normal distribution is theoretical; in practice it is usually not possible to have truly normal data. Most signals have a finite range of values but normally distributed signals necessarily have an infinite range. Among the reasons for assuming a normal distribution are the following. B.2.2.2 Crest factor The crest factor, or the signal clipping level, limits the instantaneous value of a broad-band random process (see Figure 4). The crest factor is required by this standard to be not less than 2,5 (see 4.3.3). For normally distributed random amplitudes th… However, where the relevant specification prescribes a higher acceleration spectral density in the low-frequency region, say bel… If a shaped acceleration density curve is employed which has a considerable low-frequency content compared with the upper-freque… The crest factor can only be applied to the digital vibration control system output drive signal, since non-linearities in the s… B.2.3 Tolerances B.2.3.1 Vibration tolerances When establishing the tolerance limits for the acceleration spectral density at the reference point, all errors should be consid… A compromise should be established for the three different types of errors. In B.2.3.2, B.2.3.3 and B.2.3.4, some indications are given of how to assign the contribution of each individual source to the total error. B.2.3.2 Instrument error This consists of the errors associated with the transducer, cable, amplifier, anti-aliasing filter and analogue to digital converter chain. It should be subtracted from the total tolerance limits specified for the test. B.2.3.3 Random error When dealing with the analysis of random signals, special attention should be given to the random error due to the finite length of the effective averaging time (Ta). Looking at the procedure for estimating the acceleration spectral density G(f), the mean square error E of the estimate (f) is given by: |
| 22 | Referring now to the random error it holds: and with reference to the normalized random error: Should the acceleration spectral density be estimated by the Fast Fourier Transform calculation, as in most digital vibration co… Thus, via Er it is possible to find a relationship between the number of statistical degrees of freedom [Nd] and the analysing parameters: When exponential averaging is used for a very high number of iterations, Nd becomes: If the estimate of the acceleration spectral density is performed with n linear averages per iteration the relation becomes: Table B.2 and Figure 6 give the estimated accuracy of acceleration spectral density versus the number of degrees of freedom for different confidence levels. |
| 23 | Table B.2 – Accuracy of acceleration spectral density versus degrees of freedom for different confidence levels of freedom B.2.3.4 Bias error Looking back at the expression (4) of the mean square error, not only the random error but also the bias error is directly related to the frequency resolution Be. Certain features of the data processing procedure require to be known since they directly affect the frequency resolution Be and consequently the bias error Eb. As a first order approximation, the normalized bias error is given by Equation (11) leads to The windowing process is one of the principal data processing features affecting the effective frequency resolution. When an acceleration spectral density is calculated, the averaging process is performed on windowed records. This will lead to an effective frequency resolution which depends on the type of window. In Table B.3, the value of the window function factor, W, is given for some typical window functions. |
| 24 | Table B.3 – Type of window function and corresponding factor W B.2.4 Initial and final slope This standard calls for a flat acceleration spectral density that is specified between f1 and f2 (see Figure 2). However, a prac… Normally the initial slope should be 6 dB/octave or steeper. In circumstances where the acceleration spectral density level at f… In general, digital vibration control equipment has a dynamic range for the acceleration spectral density of the order of 8 dB b… These problems do not apply to the final slope which is defined as that part of the specified acceleration spectral density above f2. This slope should be equal to or steeper than – 24 dB/octave. B.2.5 Calculation of r.m.s. values of acceleration, velocity and displacement The total r.m.s. value of acceleration, velocity and displacement for the effective frequency range is the square root of the su… These mean square values can be calculated from the following equations (see Figure 7 for reference to subscripts n and n + 1) with S in (m/s2)2/Hz. a) Mean square value of acceleration in (m/s2)2 |
| 25 | b) Mean square value of velocity in (m/s)2 c) Mean square value of displacement in mm2 NOTE In equations 14, 17 and 19, In is the natural logarithm. These equations are based on straight lines on log-log plots. The slope M for this application is defined as: B.3 Testing procedures A flow chart showing the testing procedures is given in Figure 8. Details for conducting vibration response investigations by sine or random excitation are given in Annex A. Where the test is simply to demonstrate the ability of a specimen to survive and operate at the appropriate excitation levels, t… For endurance testing of an equipment normally mounted on isolators, the isolators are usually fitted. If it is not possible to … The relevant specification may require an additional test on a specimen with the external isolators removed or blocked in order … B.4 Equipment normally used with vibration isolators B.4.1 Transmissibility factors for isolators When a specimen would normally be mounted on isolators, but they are not available and their characteristics are unknown, it is … |
| 26 | a) Curve A relates to a type of loaded isolator of high resilience having a natural frequency, when considering a single degree of freedom, not exceeding 10 Hz. b) Curve B relates to a type of loaded isolator of medium resilience having a natural frequency, as qualified above, in the range 10 Hz to 20 Hz. c) Curve C relates to a type of loaded isolator of low resilience having a natural frequency, as qualified above, in the range of 20 Hz to 35 Hz. Curve B is derived from vibration measurements made on typical aircraft equipment fitted with highly damped all-metal mountings, having a natural frequency of approximately 15 Hz considering a single degree of freedom. Very little data was available for isolators represented by Curves A and C. These were derived by extrapolation from Curve B, considering natural frequencies of 8 Hz and 25 Hz respectively. The transmissibility curves have been estimated to envelop the transmissibility characteristics likely to arise in an installati… The most appropriate transmissibility curve should be selected from 9. In the case where random excitation is prescribed, the sp… The resulting acceleration spectral density may lead to test levels which might be impossible to reproduce in the laboratory. In… B.4.2 Temperature effect It is important to note that many isolators contain material which is temperature dependent. If the fundamental resonance freque… B.5 Test severities B.5.1 Selection of test severities The frequency range and acceleration spectral density levels given have been selected to cover a wide range of applications. Whe… Wherever possible, the test severity applied to the specimen should be related to the environment to which the specimen will be … When determining the test severity, consideration should be given to the possible need to allow an adequate safety margin between it and the conditions of the real environment. In general, the shorter duration tests will give low confidence level results. Therefore, special consideration should be given to the selection of the frequency resolution Be and effective averaging time Ta to minimize the random and bias errors. B.5.2 Examples of test severities typically employed for various applications IEC 721 lists environmental conditions for various applications. It should be remembered that there will be instances where the real severities differ from those listed in IEC 721. B.6 Equipment performance When appropriate, specimens should be operated either throughout the test or at appropriate phases of the test, in a manner representative of their functioning conditions. |
| 27 | For specimens in which vibration may influence the switch-on and switch-off function, for example interfering with the operation… If the test is to demonstrate survival only, the functional performance of specimens should be assessed after the completion of the vibration test. B.7 Initial and final measurements The purpose of the initial and final measurements is to compare particular parameters in order to assess the effect of vibration on the specimen. The measurements may include, as well as visual requirements, electrical and mechanical operational and structural characteristics (see 7 and 10). |
| 28 | The values of magnitude in this standard are given in percentages or in decibels. This follows the common practice established o… Table C.1 – Conversion 50 % + 3 dB – 6 dB 200 % – 3 dB + 10 % – 10 % 50 % + 0,8 dB – 1,0 dB Table 2 + 3 dB – 3 dB + 2 dB – 2 dB + 1 dB – 1 dB + 0,5 dB – 0,5 dB 200 % 50 % 158 % 63 % 126 % 79 % 112 % 89 % – 6 dB 0 dB 50 % 100 % 100 % 25 % 0 dB – 12 dB – 25 % + 50 % – 50 % – 2,5 dB + 3,5 dB – 6 dB |
| 29 | Table C.1 – Conversion – 15 % – 1,4 dB + 3 dB 200 % + 6 dB 200 % 400 % 90 % 95 % 99 % – 0,9 dB – 0,5 dB – 0,1 dB 0 dB – 3 dB + 200 % 100 % 50 % |
| 30 | Figure 1 – Tolerance band for distribution of instantaneous acceleration values |
| 31 | Figure 2 – Tolerance boundaries for acceleration spectral density Figure 3 – Gaussian (normal) probability density function |
| 32 | Figure 4 – Representation of signal clipping |
| 33 | Figure 5 – Non-Gaussian probability density function |
| 34 | Figure 6 – Statistical accuracy of acceleration spectral density versus degrees of freedom for different confidence levels |
| 35 | Figure 7 – Relationship between acceleration spectral density and frequency |
| 36 | Figure 8 – Flow chart for vibration, broad-band random test |
| 37 | Figure 9 – Generalized transmissibility factors for isolators |
| 38 | This European Standard incorporates by dated or undated reference, provisions from other publications. These normative reference… NOTE When the international publication has been modified by CENELEC common modifications, indicated by (mod), the relevant EN/HD applies. EN 60721 series a HD 323.2.6 S2 includes A1:1983 + A2:1985 to IEC 68-2-6 |
| 39 | List of references See national foreword. |
| 40 | BS EN 60068-2-64: 1995 IEC 68-2-64: 1993 BSI 389 Chiswick High Road London W4 4AL BSI – British Standards Institution BSI is the independent national body responsible for preparing British Standards. It presents the UK view on standards in Europe and at the international level. It is incorporated by Royal Charter. Revisions British Standards are updated by amendment or revision. Users of British Standards should make sure that they possess the latest amendments or editions. It is the constant aim of BSI to improve the quality of our products and services. We would be grateful if anyone finding an ina… BSI offers members an individual updating service called PLUS which ensures that subscribers automatically receive the latest editions of standards. Buying standards Orders for all BSI, international and foreign standards publications should be addressed to Customer Services. Tel: 020 8996 9001. Fax: 020 8996 7001. In response to orders for international standards, it is BSI policy to supply the BSI implementation of those that have been published as British Standards, unless otherwise requested. Information on standards BSI provides a wide range of information on national, European and international standards through its Library and its Technical… Subscribing members of BSI are kept up to date with standards developments and receive substantial discounts on the purchase pri… Copyright Copyright subsists in all BSI publications. BSI also holds the copyright, in the UK, of the publications of the internationalsta… This does not preclude the free use, in the course of implementing the standard, of necessary details such as symbols, and size,… If permission is granted, the terms may include royalty payments or a licensing agreement. Details and advice can be obtained from the Copyright Manager. Tel: 020 8996 7070. |
