| Period |
The smallest interval of time in which a cyclic vibration repeats itself. |
| Periodic vibration (same as deterministic vibration) |
An oscillation whose waveform regularly repeats. Compare with probabilistic vibration. |
| Phase |
(Of periodic quantity), the fractional part of a period between a reference time (such as when x=0) and a particular time of interest or between two
motions of electrical signals having the same fundamental frequency. |
| Piezoelectric transducer |
One which depends upon deformation of its sensitive crystal or ceramic element to generate electrical charge and voltage. Many present day accelerometers
are PE. |
| Platform |
Per MIL-STD-810D, any vehicle, surface or medium that carries an equipment. For example, an aircraft is the carrying platform for internally installed
avionics equipment and externally mounted stores. |
| Power spectral density or PSD |
Describes the power of random vibration intensity, in mean-square x per frequency units, as g2/Hz or m2/s3. Accelerated spectral density or ASD is
preferred abroad. |
| Probabilistic vibration |
One whose magnitude at any future time can only be predicted on a statistical basis. |
| Quadrature motion |
(or side or lateral motion or cross tald) any motion perpendicular to the reference axis. Shakers are supposed to have zero quadrature motion. |
| Random vibration (same as probablistic vibration) |
One whose instantaneous magnitudes cannot be predicted. Adjective Gaussian applies if they follow the Gaussian distribution. May be broad band,
covering a wide continuous frequency range, or narrow band covering a relatively narrow frequency range. No periodic or deterministic components. |
| Redundancy, active |
Redundancy, wherein all redundant items are operating simultaneously rather than being switched on when needed. |
| Redundancy, standby |
Redundancy, wherein the alternative means of performing the function is inoperative until needed and is switched on upon failure of the primary means of
performing the function. |
| Reliability |
1. The ability of an item to perform a required function under stated conditions for a stated period of time. 2. The characteristic of an item expressed
by the probability that it will perform a required function under stated conditions for a stated period of time. Definition (2) is most commonly used in
engineering applications. |
| Repeatability |
1. The maximum deviation from the mean of corresponding data points taken under identical conditions. 2. the maximum difference in output for identically
repeated stimuli (no change in other test condition). |
| Resolution |
The smallest change in input that will produce a detectable change in an instrument’s output. Differs from precision in that human capabilities are
involved. |
| Resonance |
Forced vibration of a true SD of system causes resonance when f=fn, when any freq change decreases system response. Therefore, resonance represents
maximum sprung mass response, if freq. is varied while input F is held constant. |
| Ringing |
Continued oscillation after an external force or excitation is removed, as after a guitar string is plucked. |
| Screening Test |
A test or combination of tests intended to remove unsatisfactory items or those likely to exhibit early failures |
| Sensitivity |
Of a mechanical to electrical sensor or pickup, the ratio between electrical signal (output) and mechanical quantity (input). |
| Shock machine |
Or shock test machine, a device for subjecting a system to controlled and reproducible mechanical shock pulses. |
| Shock pulse |
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