ISO 20283-4:2012 機械的振動 — 船の振動の測定 — Part 4: 船の推進機械の振動の測定と評価 | ページ 6

3.1

free route

condition achieved when the ship is proceeding at a constant speed and course with helm adjustment of ±2° or less and no throttle adjustment

[SOURCE:ISO 20283-2:2008, 3.3]

3.2

vibration severity

value, or set of value, such as maximum value, average or r.m.s. value, or other parameters that are descriptive of the vibration, referring to instantaneous values or to average values

[SOURCE:ISO 2041:2009, 2.51]

Note 1 to entry: The vibration severity is a generic term, which in the past has been used in relation to vibration velocity. However, it is now more generally used as descriptive of other measurement units, such as displacement and acceleration.

3.3

peak value

maximum value of a vibration during a specified time interval

[SOURCE:ISO 2041:2009, 2.44]

Note 1 to entry: A peak value is usually taken as the maximum deviation of that vibration from the mean value. A positive peak value is the maximum positive deviation and a negative peak value is the maximum negative deviation, see Figure 1.

Note 2 to entry: In vibration, usually the peak value is understood as half of the peak-to-peak value (of a vibration) since positive and negative peak values can be different, see also 3.4.

3.4

peak-to-peak value

difference between the maximum positive and maximum negative values of a vibration during a specified interval

[SOURCE:ISO 2041:2009, 2.45]

3.5

r.m.s. value

root mean square value (computed by the square root of the sum of the squares of the magnitude) of a fast Fourier transform spectrum with a defined bandwidth or of a time signal during a specified time interval (e.g. a period of the fundamental frequency)

EXAMPLE 1:

An r.m.s. value within a time interval t₁tot₂

B	is the window noise bandwidth factor depending on the anti-leakage window:
	B = 1,5 for a Hanning window,
	B = 3,77 for a flat top window,
	B = 1 without an anti-leakage window;
Xn	is the narrow band magnitude obtained by FFT.

Figure 1 — Values of a vibration u ( t ) (simplified for a sine wave)

Key

1	peak value, û
2	peak-to-peak value, from −ûtoû
3	r.m.s. value
4	period duration