BS ISO 5348:2021 pdf download – Mechanical vibration and shock — Mechanical mounting of accelerometers
6.3 Determination of the mounted fundamental resonance frequency
6.3.1 General It is very useful, although difficult at times in practice, to accurately determine the mounted fundamental resonance frequency of the accelerometer mounted on a structure. The resonance frequency in the nominal measuring direction can vary widely from that in the lateral direction (which is usually lower). For multi-axial accelerometers, the resonance frequencies of the axes can vary considerably. The following methods can be of use in determining the approximate resonance frequency, thus ensuring that an adequate margin exists between the resonance frequency and test frequency.
6.3.2 Vibration excitation method
A suitable electrodynamic vibration exciter with reference transducer can be used to assess the influence of the quality of mounting surfaces and materials. For this purpose, the materials under test are mounted between the armature of the vibration exciter and the transducer and its output signal as a function of the vibration frequency is measured. For the method of determining the fundamental (resonance) frequency, see ISO 5347-22 and ISO 16063-32.
6.3.3 Shock excitation methods
For the method of determining the mounted fundamental resonance frequency by shock excitation, see ISO 16063-32. Beside ISO 16063-32, the following measurement technologies are also in use: the pendulum impact test, the drop test, a simple hammer blow and breakage of a pencil lead. In the first case, the accelerometer is attached to a counterweight suspended from a pendulum while a similarly suspended weight acts as a hammer providing the blow.
In the drop test, the accelerometer is mounted onto a hammer which is guided in its vertical fall onto a stationary anvil to provide the shock. The mounting of the accelerometer to the weight should be similar to the test body (actual structure under test) mounting. When it is impossible to represent the test body by the mass of the hammer or anvil in a realistic way, the weight should be made of the same material and of sufficient size to be a reasonable representation of the test body in terms of stiffness.
One hammer blow applied near the mounted accelerometer on the actual structure can provide the necessary information, if structural resonances in the test body can be disregarded. The accelerometer output signal produced by the shock under suitable conditions has the resonance frequency superimposed (see Figure 4) in cases where the shock duration, t S , is shorter than 5/f Res , where f Res is the lowest mounted fundamental resonance frequency of the accelerometer. Some experimentation is required with the energy of shock (i.e. the height from which the weight is released) and the stiffness of the impact surface (e.g. steel or lead lined) to obtain a suitable period of impact for displaying the resonance effects. The lowest resonance should be excited during the shock.
The use of a suitable single-event recorder, for example, a storage oscilloscope, enables the frequency of the resonance ripple (i.e. of the oscillations) to be determined. These methods are particularly suited for high frequencies. When repeated, consistently well‑defined shocks can give additional information about the stability of the mounting.
A broadband shock response spectrum can be generated by breaking a pencil lead (preferably with a diameter of 0,5 mm and a hardness of 2H) in the direction of the sensitive axis of the accelerometer under test in the vicinity of the mounting area. For this purpose, a commercially available mechanical pencil is equipped with a plastic moulded part which specifies the breaking angle and prevents bouncing (see ASTM E976). Here the shock duration, t S , is shorter than 1/f Res and the accelerometer under test is excited in its lowest fundamental mounted resonance frequency [16] , as shown in Figure 5.