ISO 07626-2:2015 pdf download – Mechanical vibration and shock —Experimental determination ofmechanical mobility — Part 2: Measurements using single-point translation excitation with an attached vibration exciter.
6.2.3 Slowly swept sinusoidal excitation
The excitation for a given measurement is a sinusoidal signal continuously swept in frequency from the lower to the upper limit of the frequency range of interest. The rate at which the frequency is swept shall be slow enough to achieve quasi-steady-state response olthe structure: requirements for selecting the sweep rate are given in 22.i. Over a small interval of time, the energy of excitation is concentrated in the small frequency band swept during that interval.
6.2.4 Stationary random excitation
The waveform of stationary random excitation has no explicit mathematical representation, but does have certain statistical properties. The spectrum of the excitation signal shall be specified by the spectral density of the exciting force. Recommendations for shaping the spectral density to concentrate the excitation in the frequency range of Interest are given in 9A. All vibration modes having frequencies within this frequency range are excited simultaneously.
6.2.5 Other excitation waveforms General
Additional types of waveforms, described in 6.23.2 to also simultaneously excite all vibration modes within a frequency band of interest. The methods of signal processing and excitation control used rn conunctlon with these waveforms are similar to those used with stationary-random excitation. These waveforms are repetitive and are recommended when synchronous time-domain averaging of the response waveform is necessary to measure properly the motion response of the structure. Pseudo-random excitation
The excitation signal is synthesized digitally in the frequency domain to attain a desired spectrum shape. An inverse Fourier transformation of the spectrum may be performed to generate repetitive digital signals which are then converted to analogue electrical signals to drive the vibration exciter. Periodic-chirp excitation
A periodic chirp is a rapid repetitive sweep of a sinusoidal signal in which the frequency is swept up or down between selected frequency hmits, The signal may be generated either digitally or by a sweep oscillator and should be synchronized with the signal processor for waveform averaging to improve the signal-to-noise ratio. Periodic-impulse excitation
A suitably shaped impulse function, usually generated digitally, is periodically repeated. The signal processor should be synchronized with the signal generator. The Impulse function shape (typically hail- sine or decaying step functions) shall be chosen to meet the excitation frequency requirements.
62.5.5 PerIodic-random excitation
A periodic-random excitation combines the features ofstationary random and pseudo-random excitation in that it satisfies the conditions for a periodic signal and yet changes with time so that it excites the structure In a random manner; this is done by using different pseudo-random excitation for each average.
6.3 Vibration exciters
Devices commonly attached to the structure under test to apply input forces having desired waveforms include electrodynamic. electrohydraulic, piezoelectric, and rotating eccentric mass vibration exciters. The frequency ranges of general applicability for each type of exciter are shown in Figure 2.
The basic requirement of a vibration exciter is that it shall provide a sufficient force and displacement capability so that mobility measurement can be made over the entire frequency range of interest with an adequate signal-to-noise ratio. A vibration exciter with higher force output might be required to apply adequate broad-band random excitation to a given structure than is needed for sinusoidal excitation. Exciters with lower force output may be used if a band limiting of the random noise is selected or if time- domain averaging of the excitation and response signal wave-forms is used (see G2.).
NOTE The coherence function can be used as a measure olthe adequacy of the vibration exciter In relation to background and electronic noise,
The excitation-force Input to a structure gives rise to a reaction force which Is provided either by the exciter support or by the inertia of the exciter itself; these approaches are illustrated in Figures 3 a) arid J b). If necessary, an additional mass should be attached to the exciter. An incorrect set-up which would allow transmission of exciter reaction forces to the structure via a path other than through the force transducer. i.e. through a common base on which both the exciter and the structure are mounted. is illustrated in Figure 3c).