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

A transmission of kinetic energy into a system in a relatively short interval compared with the system’s natural period. A natural decay of oscillatory motion follows. Usually displayed as time history, as on an oscilloscope.

Simple harmonic motion

Periodic vibration that is a sinusoidal function of time.