17.5 Some Practical Information

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Some useful practical information on vibration testing of products is given here.

TABLE 17.3 Random Vibration Tests for a Product Development Application

Vibration Test Root-Mean-Square

Value of

Excitation (g)

Peak Value of the

Excitation

PSD (g2/Hz)

Minimum Times

the Random

Vibration is Applied

Minimum

Duration of

Vibration (min)

Vibration Axes

A 2.7 0.01 1 60 Major horizontal

axis

B 6.0 0.05 2 30 Major horizontal

axis

C 3.2 0.01 1 15 All three

D 5.8 0.02 1 15 All three

E 4.9 0.01 1 15 All three

F 6.3 0.04 2 5 All three

TABLE 17.4 Capabilities of Five Commercial Control Systems for Vibration-Test Shakers

System A B C D E

Random test Yes Yes Yes Yes Yes

Sine test Yes Yes Optional No Yes

Transient and

shock tests

Yes Yes Optional No Yes

Hydraulic shaker OK OK OK OK OK

Preprogrammed

test set-ups

Max. 63 Max. 25 Max. 99 10 per disk Not given

Amplitude

scheduling

32 Levels and

duration

Min. start: 2 25 dB;

min. step: 0.25 dB;

can pick step

durations

10 Levels over

60 dB

0.5 dB steps;

can pick

no. of steps

and rate

No

On-line reference

modification

Yes No Yes No No

Use of measured

spectra as

reference

Yes Yes (measurement –

pass feature)

Yes No No

Transmissibility Yes Measurement

option

Yes No Yes

Coherence Yes Measurement

option

No No Yes

Correlation Yes No No No Yes

Shock response

spectrum

Yes Yes Optional No Yes

Sine on random Yes Sine bursts Optional No No

Random on random Yes No Optional No No

17-52 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

TABLE 17.5 Important Hardware Characteristics of Five Systems for Shaker Control

System A B C D E

Reference spectrum

break points

40 32 50 10 45

Spectrum resolution

(number of

spectral lines)

Can pick 100, 200,

400, 600, 800 lines

Can pick 64, 128, 256, 512 lines

(optional 1024 lines)

Can pick 100, 200, 400,

800 lines

200 Lines 10 Hz spacing Pick any number: 10 to

1000 lines (optional

2048 lines)

Nature or random

drive signal

Not given Gaussian, periodic

pseudo-random

Gaussian Gaussian Pseudo-random

Measured signal

averaging

RMS, peak-hold Arithmetic peak-hold True power Peak pick No

Operator interface Keyboard, menudriven

RS 232

Keyboard, push button,

dialog, set-up

Keyboard, push button,

dialog, menu-driven

Keyboard 10 soft keys,

dialog

Keyboard

Output devices CRT screen, hard copy,

video print, digital plot

Standard or graphics

terminal, X – Y

record printer, digital plot

Graphics terminal, video

hard copier, digital plotter,

X – Y recorder

Like IBM PC, Monochrome-

900 Epson printer

Graphics terminal printer,

hard copy, X – Y plotter

Memory 128K 64K 128K 64K 32K std, 64K option

Mass storage Floppy drive 0.5 MB;

hard drive 10 MB

One floppy drive, 256K Hard þ floppy 8, 20, 30B Two floppy drives 360K

each

Two floppy drives 256K

each

Number of

measurements

(control input)

channels

2 standard; 16 optional 2 standard; 4 optional,

multiplexer optional

1 standard; 16, 31 optional 1 standard; 4 optional Not given

Number of controller

output channels

One One One One One

Vibration Testing 17-53

© 2005 by Taylor & Francis Group, LLC

TABLE 17.6 Specifications of Five Shaker Control Systems

System A B C D E

Accelerometer signal

(controller input)

^ 125 mV to ^8 V

full scale

10 mV RMSa to ^8 V max.

typically . 500 mV RMS

Max. 10 V peak, 3.5 V, RMS 1 to 1000 mV/g user

picked

Not given

Controller output

signal

2.4 V RMS (random)

20 V peak to peak

(sine and random)

20 V P – P max. 50 mA max. 20 V P – P max. 10 V peak 3 V RMS Not given

Input frequency ranges Random: DC to 200,

500 Hz, 1, 2, 3, 4, 8 kHz;.

Sine: 1 – 8 Hz; shock:

10 – 125 Hz,

312 Hz,…, 5kHz

Seven ranges max. freq.:

500 Hz – 5 kHz, min.

freq ¼ 1 line

100, 500 Hz, 1, 2, 4, 5, 10 kHz 10 – 2000 Hz 10 – 5000 Hz

Control loop time 2.1 sec (2 kHz, 200 lines) 0.3 sec, 64 lines; 0.9 sec, 256 lines,

3 sec, 1024 lines (2kHz)

4 sec, 100 lines, 8 sec,

200 lines (2 kHz)

2 sec 2.5 sec for 256 lines

at 2 kHz

Equalization time

for 10 dB range

Within ^ 3 dB in two loops 2 or 3 loops Within ^ 1 dB in one loop Within ^ 1 dB in 6 sec Not given

Resolution 12 bit 12 bit 12 bit

Dynamic range 65 dB 65 dB 72 dB 60 dB —

Control accuracy ^ 1 dB at Q ¼ 30; ^ 2 dB

at Q ¼ 50 (100 Hz

Resonance

at 1 octave/min

^1 dB (at 90% confidence) ^ 1dB over 72 dB ^ 1 dB ^ 1 dB (at 95% confidence)

Sine sweep rate OK 0.1 – 100 oct/min (log)

1 Hz – 100 kHz/min (linear)

0.1 – 100 oct/min max.;

0.1 Hz – 6 kHz/min

N/A 0.001 – 10 oct/min;

1 – 6000 Hz/min

a RMS, root-mean-square.

17-54 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

17.5.1 Random Vibration Test Example

Table 17.3 lists several random vibration tests in the frequency range of 0 to 500 Hz in an application

related to product development.

17.5.2 Vibration Shakers and Control Systems

Table 17.4 lists capabilities of five commercial control systems that may be used for shaker control in

random vibration testing of products.

Table 17.5 summarizes important hardware characteristics of the five control systems.

Table 17.6 gives some important specifications of the five control systems.

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