GENERAL NOTES ON RANDOM RATINGS
Random Ratings have been calculated in accordance with International Standards Organisation document I.S.O. 5344. The force ratings are based on a control spectrum in the range 20-2000 Hz and the ratio of the peak to rms acceleration levels are at least 3:1.
To achieve these high peak acceleration levels, all LDS power amplifiers are designed to provide peak to rms current levels of at least 3:1. For example the DPAK and SPAK range of amplifiers are rated at 50A rms random for each 5 kW module and the peak current is rated at 150A peak.
When comparing LDS vibration systems with competitors it is important to confirm that the ratings are calculated in accordance with I.S.O. 5344 and that the power amplifiers are capable of providing a peak current of at least three times the normal sine rms level. If the peak current level cannot be achieved or if excessive voltage clipping takes place in the amplifier then the power spectral density of the acceleration waveform may contain unwanted harmonics and energy which could damage a test load.
I.S.O. 5344 states that broad band rated random force shall be calculated with a flat spectrum 100-2000 Hz and with 20 dB/decade roll-off from 100 Hz to 20 Hz. The same force rating is available for the more familiar flat spectrum in the range 20-2000 Hz which was used before the introduction of the I.S.O. standard.
Variation of random ratings with frequency range, spectrum shape and load. The available force delivered by the vibration generator is a function of both the shape of the acceleration spectral density (spectrum shape) and of the load dynamic response (which may not correspond to the case of a pure mass load). Since most real loads are only totally coupled to the vibration generator in the lower part of the frequency range, the available rms acceleration may be greater than the acceleration as calculated from pure mass loads.
Under other conditions, if the actual shape of the test acceleration spectral density is different from the manufacturers specified spectrum shape, the rms current of the vibration generator or the random rms acceleration limit may reduce the available acceleration (or force output) to a lower level than that calculated using pure mass loads.
Coming to a practical level, LDS random ratings have been specified using mass loads of about twice the armature mass - this approximates to test load M1 of I.S.O. 5344. The test spectrum shape is flat in the frequency range 20-2000 Hz and the acceleration (force) waveform has a peak to rms ratio of at least 3:1.
For frequency ranges other than 20-2000 Hz some de-ratings may be necessary.
In general random thrust rating for a spectral shape with a frequency range of less than 1000 Hz must be de-rated. The exact de-rating will depend on the characteristics of the vibration generator and the frequency range. If a test is considered to be narrow band we can provide a system performance prediction for any defined random profile.
In some cases when a sine/random transformer coupling is used, the de-rating for random tests in the frequency range can be minimised by operating the vibration system with the voltage tap normally used for sine operation. Care must be taken to ensure that the maximum vibration generator current is not exceeded.