Bruel & kjaer 4292 User guide

Accelerometer Calibrator and

Preamplifier Type

4292

Bruel

&

Kjcer

Copenhao•n

+

z-0.1mm

-0.01mm

5 6

• •

Amplitude

e

Input.

Accelerometer Calibrator and

Preamplifier Type 4292

A small

compact

unit

containing

a

preamplifier

for

measurements

of

vibration

acceleration,

velocity

and

displacement

in

conjunction

with

8 & K

Accelerometers,

Measuring

Amplifiers,

and

Analyzers. A built-in

shaker

table

operating

at a freq\ilency

79.6 Hz

facilitates

checks

of

accelerometer

sensitivity

and

the

calibration

of

complete

vi.

bration

measurement

systems

at

an

acceleration

level

of

1 g

peak

using

the

rattle

ball method.

,:

ACCELEROMETER

CALIBRATOR

AND

PREAMPLIFIER

TYPE 4292

September 1971

CONTENTS

1.

INTRODUCTION

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.

CONTROLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1.

FrontandTopPanels

..................

.

......

4

2.2. Rear PaneI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.

OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.

1.

Measurement

Arrangement

. . . . . . . . . . . . . . . . . . . . . 7

3.2. Calibration

of

Measur€tnent System . . . . . . . . . . . . . . 9

3.3. Vibration Measurements

.......................

12

4.

DESCRIPTION

.......................................

14

4.1. Calibrator Section

............................

14

Wien Bridge Oscillator

........................

14

Driver Amplifier

.............................

15

Vibration

System

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

15

4.2. Preamplifier Section . . . . . . . . . . . . . . . . . . . . . . . . . .

16

Input

Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

16

0-20

dB

Amplifier

...........................

16

Integration Networks . . . . . . . . . . . . . . . . . . . . . . . . .

16

4.3. Power

Supply

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

20

5.

CHARACTERISTICS

..................................

21

5.1. Calibrator Section

............................

21

Frequency

Characteristics

......................

21

Cal

ibi

·ation Accuracy . . . . . . . . . . . . . . . . . . . . . . . . .

22

5.2. Preamplifier

Section

.

.............

.

....

.

......

22

Input

Characteristics

..........................

22

Low

Frequency

Characteristics

..................

23

High

Frequency

Characteristics

.................

25

Frequency

and Phase Characteristic of Integration

Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

26

Output

Characteristics . . . . . . . . . . . . . . . . . . . . . . . .

27

Distortion

..................................

29

Noise

...........................

.

.........

29

6.

SPECIFICATIONS

....................................

31

1.

INTRODUCTION

The Accelerometer Calibrator and Preamplifier Type 4292

is

designed

as

a small compact

unit

for

vibration measurements.

It

forms

the

link

between

the accelerometer and the Measuring

Amplifier

or

Analyzer.

The preamplifier

has

a field

effect

transistor

input

stage

which

gives

it

an

extremely high

input

impedance (up

to

2 Gr2), whereby a low lower

limit-

ing frequency

can

be

obtained

for

the combination

of

accelerometer

capacitance and preamplifier

input

impedance.

To

permit

easy

reading

of

vibration levels when accelerometers which

do

not

have

simple sensitivity

figures

are

employed

for

measurements, the gain

of

the preamplifier may

be

adjusted

from

0

to

20 dB. For measurements

of

vibration acceleration

an

output

can

be

taken

directly

from

the preamplifier,

whilst

for

measurements

of

velocity

and

displacement the 4292 includes 3 single and 6 double inte-

gration networks. The networks

have

different

low frequency cut

off

limits

and

can

be

selected using the INTEG

RATION

SWITCH,

so

that

the best

possible signal-to-noise

ratio

is

obtained

for

the required measurement fre-

quency range.

The calibrator consists

of

a small

built-in

vibration table which

is

fitted

with

a

10-

32 NF hole for·mounting accelerometers. The table vibrates at a

frequency

of

79.6

Hz

(w

= 500 radian/sec

as

specified

by

the

DIN Standard

45.666) and produces

an

acceleration level

of

1 g peak which may

be

adjusted using

an

AMPLITUDE

control.

To

facilitate checks

of

accelerome-

ter sensitivities and calibration

of

complete vibration measurement systems

the table contains a hermetically

sealed

chamber in which a small bronze

ball rests. The ball

wi

II

just commence

to

vibrate when

the

acceleration level

of

the table

reaches

1 g peak. This

condition

may

be

heard

and

at

the

same

time

seen

on

an

oscilloscope which may

be

connected

to

the

output

of

the

indicating·instrument

used

for

measurements.

Power

for

the

4292 may

be

provided using

the

B & K Microphone Power

Supply Type 2801,

or

by

any

of

the Measuring

Amplifiers

or Analyzers in

the B & K range.

3

2.

CONTROLS

2.1. FRONT AND

TOP

PANELS

Shaker Table

Sensitivity

Input

Chassis

Ground

271RSI

Fig.2.1.

Front

and top panels

of

the 4292

VIBRATION

TABLE:

4

Vibration

table

of

the 4292 which

is

fitted

with

a 10

-32

NF hole

for

mounting acceler-

ometers

to

be

calibrated. The acceleration

level

of

the

VIBRATION

TABLE

may

be

adjusted using the

AMPLITUDE

control.

INTEGRATION SWITCH:

10

position switch

for

selection

of

the

Accel-

eration,

Velocity and Displacement ranges

of

the

preamplifier

of

the

4292.

An

"Off"

position

is

also provided

on

the

switch

for

dis-

connecting

the

preamplifier

output

from

the

input

of

an external measuring amplifier

which

may

be

connected

for

measurements.

SENSITIVITY:

Potentiometer

control

for

adjustment

of

the

gain

of

the

internal preamplifier

from

0

to

20dB.

INPUT:

Two

input

sockets

for

connection

of

the

out-

put

signal

from

an accelerometer. One

of

these sockets accepts

the

standard B & K

plugs

Type

JP 0101 whilst

the

other

is

fitted

with a 10

-32

NF

thread

and

accepts

the

miniature plugs

Type

JP

0012

or

the

minia-

ture

coaxial cables T'

ype

AO

0037,

AO

0038,

AO

0051,

AO

0052,

or

AO

0500.

GROUND:

Socket

for

grounding

to

the

chassis

of

the

4292.

It

accepts

the

B & K Banana Plug

Type

UB

0002/3.

AMPLITUDE:

Potentiometer

control

for

adjustment

of

the

acceleration level

of

the

VIBRATION

TABLE. When

set

to

its

"Off"

position

the

power

supply

to

the

internal circuitry

of

the

shaker

system

of

the

4292

is

disconnected.

5

2.2. REAR

PANEL

RESONANCE:

PREAMP. PLUG:

6

Preamplifier Plug

I

271£5()

Fig.2.2. Rear panel

of

the 4292

Mechanical control

for

adjusting the mechani-

cal

resonance

of

the

VIBRATION

TABLE

to

79.6 Hz when accelerometers

with

different

masses

are

to

be

calibrated.

7 pin plug

for

direct connection

to

the pream-

plifier

input

socket on B & K Measuring

Amplifiers

and Frequency Analyzers or

to

the

B & K Power Supply

Type

2801. Power

for

the 4292

is

supplied via this plug which also

provides

an

output

for

the internal preampli-

fier. For connections

see

Fig.4.1.

. ;

3.

OPERATION

3.1. MEASUREMENT ARRANGEMENT

For vibration measurements the 4292 should

be

used

with

one

of

the

following

instruments:

a)

Microphone

Amplifier

Type

2603,

or

the Measuring Amplifiers

Type

2606, 2607, or 2608.

b)

Octave and

Third

Octave Spectrometers Type 2113

or

2114, or the

Real

Time

Analyzer

Type

3347.

c)

Frequency Analyzer Type 2107

or

the Heterodyne Analyzer

Type

2010.

d) Microphone Power Supply

Type

2801 plus

an

Electronic

Voltmeter

Type 2409

or

similar.

These instruments

not

only

provide the supply voltages necessary

to

operate the calibrator and preamplifier sections

of

the 4292,

but

they

also

serve

to

provide

an

indication

of

the

acceleration, velocity or displacement

level

of

the vibration

to

be

measured.

If

a recording

of

the measurements

is

required they may

be

connected

to

the 2305 Level Recorder, which in the

case

of

the Spectrometers

or

Analyzers mentioned

will

be

found especially

useful in

that

an

analysis

of

the vibration investigated

can

be

carried

out

automatically on preprinted frequency calibrated paper. In the

case

of

the

Real

Time

Analyzer

Type

3347 a

third

octave analysis

is

carried

out

and

instantaneously displayed on a cathode ray tube. Therefore a graphic

re-

cording

will

not

always

be

found

strictly

necessary.

For measurements

the

instruments should

be

connected

to

the 4292

as

shown in Fig.3.1.

Two

possible methods

of

connecting the instruments are:

1.

The measuring arrangement

is

placed in the immediate

vicinity

of

the

measuring

point.

The PREAMP. PLUG

of

the 4292 should'

be

connected

to

the

seven

input

socket on the indicating instrument (PREAMP. INPUT on

7

8

Fig.3.1.

Instruments

for

vibration measurements with the 4292

instruments Type

2606,2607,2608,2113,2114,2010

and

3347

or

CONDENSER MICROPHONE on instruments

Type

2603, 2107

or

INPUT on the instrument

Type

2801),

and

the appropriate

input

mode selected on the instrument concerned.

2.

The measuring arrangement

is

placed some distance away

from

the

measuring

point.

The 7 core cable connecting the PREAMP. plug

to

the 4292 may

be

extended using the Extension

Cables

Type

AO

0027

(length

3m),

Type

AO

0028

(10m)

or Type AO 0029

(30m).

If

a

greater cable length

is

required then several extension

cables

may

be

connected together. The influence

of

capacitive loading on the high

frequency response

of

the 4292 when

different

lengths

of

extension

cable

are

employed

is

shown in Fig.5.6.

3.2.

CALIBRAtiON

OF

MEASURING

SYSTEM

To

calibrate the measuring arrangement

for

a vibration acceleration level

of

1 g peak (9.81 m/sec2) the

built

in vibration table

of

the

4292 may be

used

as

follows:

1.

Set the 4292 on a

flat,

rigid horizontal surface and connect its PRE-

AMP. PLUG

to

the

seven

input

socket

of

the indicating instru-

ment

to

be

used

for

measurements.

See

section

3.

1.

2.

Using a

10-

32 NF

mounting

stud, screw the accelerometer hand-

tight

(mounting torque 18 kgcm or 15 lb in)

onto

the shaker table.

If

miniature accelerometers

are

used

they should

be

fitted

using a 3 mm

mounting stud

with

the 3

mm

to

10-32

NF Adapter Type

DB

1425.

In this

case

the max. torque

is

10 kg.cm

or

8.51b in.

3.

Set the controls on the indicating instrument

for

linear operation

using its PREAMP. (or CONDENSER MICROPHONE) input,

with

METER FUNCTIONS

of

"Peak

Slow",

or

"RMS

Fast" selected. Set

the

INPUT

and

OUTPUT SECTION

ATTENUATORS

or

the

METER

RANGE and RANGE

MULTIPLIER

switches

to

"1

00

mV"

and

"x

1"

respectively

for

calibration using accelerometers

with

sensitivities

greater than

15mV/g,

or

to

"10mV"

and

"x

1"

respectively

for

accelerometers

with

sensitivities

less

than 15

mV

/g.

4.

On the 4292 set the

INTEG

RATION

SWITCH

to

its

"Accel.

-10

m/sec2" mode

and

adjust the

AMPLITUDE

control

to

obtain a

small meter deflection on the indicating instrument.

If

necessary

adjust the

SENSITIVITY

control.

5.

Using the RESONANCE

control

on

the

rear panel

of

the 4292 adjust

the mechanical resonance

of

the shaker ·system

so

that

the meter

indication peaks

at

a

maximum

value.

If

the ball mounted in the table

9

10

begins

to

rattle

turn

the

AMPLITUDE

control

ant

.iclockwise until

the

rattling ceases and readjust

the

RESONANCE

control

until

the

meter

indication again peaks

at

maximum

value. Since

the

RESONANCE

control

is

mechanically coupled

to

the

table's

spring suspension, it

is

best whilst monitoring

the

meter

indication

to

remove

one's

hand

from

the

control

after

making each

adjustment.

6.

Turn

the

AMPLITUDE

control

clockwise until

the

ball in

the

table

just

commences

to

rattle.

At

this

point

the

acceleration level

of

the

table

is

1 g

peak

(i.e. 9.81

m/sec

2 at sea level).

The

condition

where

the

ball

just

commences

to

rattle

can rapidly

be

determined

by

listening. However, for a more accurate

determination

of

this

condition,

or

for self

education

on

the

first calibration

attempt,

one

of

the

following

methods

may

be

used:

a) Monitor

the

output

signal from

the

accelerometer

using a pair

of

headphones

or

a loudspeaker. These should

be

connected

to

the

out-

put

of

the

indicating

instrument

using a

power

amplifier. When

the

ball begins

to

rattle

the

level

of

background noise heard will sharply

increase.

b)

If

the

indicating

instrument

is

provided

with

frequency

weighting

networks, select A-weighting and

turn

the

AMPLITUDE

control

clockwise.

At

first

the

meter

needle

of

the

instrument

will rise slowly,

as

the

network

will

only

pass

the

harmonics

of

the

vibration

table's

resonance and

not

the

fundamental.

However, as

the

AMPLITUDE

control

is

turned

further,

the

meter

needle will reach a

point

on

the

meter

scale where

the

rate

at

which

it

rises will sharply increase,

due

to

the

increase

in

harmonics produced

when

the

ball begins

to

rattle.

Therefore

the

meter needle should be

set

to

this

point

on

the

scale.

After

this has been achieved reset

the

instrument

for

operation

using

its linear

frequency

mode.

c)

If a

2107

is

used as

the

indicating

instrument

then

a similar proce-

dure

to

that

described

in

item

b)

may be followed,

but

with

the

2107

set

to

its rejection mode and its

centre

freq~ency

tuned

to

79.6

Hz.

d)

By

far

the

most

accurate

method

of

determining

when

the

ball

begins

to

rattle

is

to

monitor

the

output

signal

from

the

accelerometer

using an oscilloscope

connected

to

the

output

of

the

indicating instru-

ment.

The

moment

the

ball begins

to

rattle

a small

amount

of

distor-

tion

wi

II

occur

on

the

negative slope

of

the

monitored

signal as shown

in Fig.3.2.

Acceleration

below 1 g

~·

.

Acceleration

1 g Acceleration

above 1 g peak

271344

Fig.3.2. Oscillogram showing the

output

waveforms from an accelerometer

for

various vibration acceleration levels

of

the vibration table.

An

acceleration level

of

1g peak is indicated when distortion

first

appears

on

the negative slope

of

the waveform

7.

Using

the

SENSITIVITY

control

of

the

4292

adjust

the

meter

pointer

of

the

indicating

instrument

as follows:

Peak Indication

For

instruments

with

a

meter

function

set

to

"Peak

Slow",

adjust

the

meter

pointer

to

indicate

98

mV

for

accelerometers with a sensitivity

greater

than

15 mV/g

or

to

9.8

mV for accelerometers with a sensitiv-

ity less

than

15 mV/g. For

measurements

using accelerometers with a

sensitivity greater

than

15 mV/g, 100 mV

on

the

meter

scale will cor-

respond

to

a

peak

acceleration level

of

, 10 m/sec2 (i.e. 1.02 g Peak)

similarly

for

measurements using accelerometers

with

a sensitivity less

than

15 mV/g, 10 mV

on

the

meter

scale will also correspond

to

a

peak

acceleration level

of

10

m/sec

2 (i.e. 1.02 g Peak).

RMS Indication

For

instruments

with

a

meter

function

set

to

"RMS

Fast",

adjust

the

meter

pointer

to

indicate

69

mV

for

accelerometers with a sensitivity

greater

than

15 mV/g,

or

to

6.9

mV

for

accelerometers with a sensitiv-

ity less

than

15

mV

/g.

For

measurements using accelerometers with a

sensitivity greater

than

15 mV/g, 100 mV

on

the

meter

scale will cor-

respond

to

an RMS acceleration level

of

10 m/sec2 (i.e. 1.02 g RMS).

Similarly

for

accelerometers

with

sensitivity less

than

15 mV/g,

10 mV

on

the

meter

scale will correspond

to

aRMS

acceleration level

of

10 m/sec2 (i.e. 1.02 g RMS).

11

Once the vibration measurement system

has

been calibrated

turn

the

AMPLITUDE

control

to

its

"Off"

position

and

remove the accelerometer

from the vibration table,

first

disconnecting the

lead

connected

to

its

output

socket before unscrewing

it

from

the table. This prevents the accelerometer

and

its connection cable

from

being damaged.

Note:

After

the system

has

been calibrated, avoid touching the SENSI-

TIVITY

control

of

the 4292

as

otherwise the calibration

will

be

lost.

3.3.

VIBRATION

MEASUREMENTS

12

1.

Carry

out

the calibration procedure described in section 3.2.

2.

Mount

the accelerometer

as

rigidly

as

possible

onto

the measurement

object. This can

be

done using one

of

the methods described in the

instruction manual

for

the accelerometer.

3.

Taking

care

to

avoid cable

whip,

connect the

output

of

the

acceler-

ometer

to

the

10-

32 NF preamplifier

INPUT

socket

of

the 4292

using one

of

the Miniature Accelerometer cables Type

AO

0037,

AO

0038,

AO

0051,

AO

0052,

or

AO 0500. The standard

input

socket

to

the preamplifier may also

be

used

when

the

Coaxial Adapter

Type

JP

0028

is employed.

4.

Keep the 4292

as

far away

as

possible

from

the vibration environment

and any other unrequired influences.

5.

Set the

INTEG

RATION

SWITCH

for

the required

type

of

measure-

ments i.e. acceleration velocity,

or

displacement.

In

the

"Velocity"

or:

"Displacement"

modes

of

this switch a lower

limiting

frequency (1 Hz

for

"3

m/sec", 10

Hz

for

"0.3

m/sec" and 100 Hz

for

"0.03

m/sec"

velocity

ranges,

or

1Hz

for

"1000

mm",

3Hz

for

"100

mm",

10Hz

for

"10

mm",

30Hz

for

"1

mm",

100Hz

for

"0.1

mm"

and

300Hz

for

"0.01

mm"

displacement

ranges)

which

is

equal

to

or

less

than the

lowest measurement frequency

of

interest should

be

selected. This

ensures

that

signals

with

frequency components equal

to,

or greater

than the lowest frequency

of

interest

are

correctly integrated.

6.

Keeping the

OUTPUT

SECTION

ATTENUATOR

(or RANGE

MULTIPLIER)

switch in its

"x1"

position

as

far

as

possible, adjust

the

INPUT

SECTION

ATTENUATOR

(or

METER

RANGE) switch

to

obtain a suitable meter indication

without

overload on the indicat-

ing instrument.

7.

From the meter indication the measured vibration acceleration

(m/sec2), velocity (m/sec)

or

displacement (mm)

can

be

derived

from:

Meter indication (Volts) x

INTEGRATION

SWITCH setting (Accel.

m/sec2,

Velocity

m/sec or Displacement m) X 100

for

measurements

using

an

accerometer

with

a sensitivity

less

than 15 mV/g, or x 10

for

accelerometers

with

a sensitivity greater than 15 mV/g.

Example:

For vibration measurements using

an

accelerometer

with

a sensitivity

of

9.9

mV/g

and

an

INTEGRATION

SWITCH setting

of

"Accel.

10 m/sec2} the measured acceleration level

for

a meter indication

of

0.78 V peak

can

be

calculated

as

follows:

Acceleration

level=

0.78 x 10 x 100

Acceleration level = 780 m/sec2 · peak

Example:

For vibration measurements using

an

accelerometer

with

a sensitivity

of

20

mV/g

and

an

INTEGRATION

SWITCH setting

"Velocity

0.3 m/sec" the measured velocity level

for

a meter indication

of

1.5 V

RMS

can

be

calculated

as

follows:

Velocity

level=

1.5 x 0.3 x 10

Velocity

level=

4.5 m/sec RMS.

Example:

For vibration measurements using

an

accelerometer

with

a sensitivity

of

9.9

mV/g

and

an

INTEGRATION

SWITCH setting

of

"Displace-

ment

0.1

mm",

the measured displacement level

for

a meter indica-

tion

of

1.

7 V peak

can

be

calcu

Ia

ted

as

follows:

Displacement

level=

1.7 x

0.1

x 100

Displacement level = 17 mm peak.

13

4. DESCRIPTION

The

principle

of

operation

of

the

Accelerometer Calibrator

Type

4292

will be described

with

reference

to

the

schematic diagram given

in

Fig.4.1.

Resonance

adjustment

Input

socket

FET

Input

stage

10-32

NF threaded hole

Driver

amplifier

40 V supply

Amplitude

adjustment Wien

bridge

oscillator

+ 12 V supply

Voltage + 150 V

regulator

0- 20 dB Integration

Output

a

mplifi

er

.,_

__

____.---.---1

networks

.,._

_______

....,

Sensitivi

ty

adjustment

Fig.4.

1.

Schematic diagram

of

the 4292

(Front

view

of

plug)

Output

and

Power Supply

plug

2

71000

4.1.

CALIBRATOR

SECTtON

4.1.1: Wien Bridge Oscillator

The

Wien Bridge Oscillator consists

of

an

integrated circuit differential

amplifier with positive feedback from its

output

to

its

primary

input

via

an

R-C

network.

It

produces an

amplitude

stabilized, sinusoidal voltage

at

a

frequency

of

79.6

Hz

to

excite

the

vibration system

of

the

4292. Its

operat-

ing frequency corresponds

to

an angular

frequency

of

500

rad/sec as speci-

fied by

the

DIN

Standard

45666.

14

4.1.2. Driver

Amplifier

This

is

a simple

two

stage,

transistorized push-pull power

amplifier

which provides

an

impedance match between the

output

of

the oscillator

and driver coil

of

the vibration system. The amplitude

of

the

output

signal

from

the Driver

Amplifier

determines the peak acceleration level

of

the

vibration table and may

be

adjusted using

the

AMPLITUDE

potentiometer

control

to

regulate the

input

signal

to

the amplifier.

4.1.3.

Vibration

'System

A schematic diagram

of

the vibration system

is

shown in Fig.4.2.

Its

principle

of

operation

is

extremely simple. The vibration table

is

excited

by

a sinusoidal force produced

by

an

electromagnet operated

from

the 79.6 Hz

voltage

from

the

output

of

the Driver

Amplifier.

Shaker table

10-32

NF

threaded hole

Resonance

adjustment

27f23(

Fig.4.2. Schematic diagram

of

the vibration system

of

the 4292

The vibration table

has

a

10-

32 NF hole

for

mounting accelerometers

and

is

supported

by

a

taut

bronze band.

To

correct

for

different

accelerome-

ter

masses

which may

be

placed on the vibration table, the mechanical

resonance

of

the table may

be

adjusted

to

79.6

Hz

by

altering the tension

of

the bronze band using the RESONANCE control at the rear

of

the instru-

ment. The acceleration level at which the table vibrates may

be

adjusted

using the

AMPLITUDE

control

to

regulate the i'nput

to

the

Driver

Ampli-

fier.

To

facilitate calibration

of

the table's acceleration level, a small bronze

15

ball resting

on

three

points

in

a hermetically sealed chamber

in

the

vibration

table

is

used. When

the

ball

just

commences

to

vibrate

the

acceleration level

of

the

table

will be 1 g peak.

To

isolate

the

vibration

system

from

the

chassis

of

the

instrument

and

thus

minimise interference

from

external

vibrations,

the

system

is

hung

from a spring suspension which has a

resonant

frequency

of

approximately

15Hz.

4.2. PREAMPLIFIER SECTION

4.2.1.

Input

Stage

There

are

two

alternative

input

sockets

to

the

preamplifier section

of

the

4292. One accepts

the

standard B & K coaxial plugs

Type

JP

0101 whilst

the

other

is

fitted

with

a

10-

32

NF

thread

and

accepts

the

miniature

coaxial plugs

Type

JP

0012.

The

sockets are

connected

in parallel

and

lead

to

the

input

stage

which

uses a

FET

(field

effect

transistor)

to

give it an

extremely

high

input

impedance

(450

Mn

to

2

GQ,

depending on setting

of

the

SENSITIVITY

control).

This

is

especially

important

as it permits a

low-lower limiting frequency

to

be

obtained

for a given

combination

of

accelerometer capacitance and preamplifier

input

impedance.

The

FET also

helps give

the

preamplifier a high signal-to-noise ratio.

4.2.2. 0 -

20

dB Amplifier

The

output

of

the

input

stage

is

coupled

to

a

two

stage DC amplifier.

To

give

the

amplifier a good

temperature

stability

the

input

stage consists

of

two

transistors

connected

to

form

a differential pair.

The

overall gain

of

the

amplifier may be

adjusted

from 0

to

20 dB

by

adjustment

of

the

SENSITIV-

ITY

control

which

is

a

potentiometer

in

the

main feedback loop of

the

amplifier.

4.2.3. Integration Networks

For

vibration

measurements

using an accelerometer

with

the

4292,

the

output

from

the

0

to

20

dB amplifier will be

proportional

to

the

accelera-

tion

of

the

vibration measured. However, it will

often

be required

to

obtain

an

output

which

is

proportional

to

the

velocity,

or

displacement

of

the

vibration. Since

the

quantities

acceleration, velocity and displacement of a

16

•.

vibration are interrelated,

then

if

one

of

these

quantities

is

kept

constant,

the

magnitude of

the

remaining

two

can be

determined.

For example, if

we

consider

the

general expression for

the

acceleration

of

a periodic vibration

which

is

given by:

a = A0 sin (wt)

then

the

velocity will be given by:

v =

fa

dt

and

the

displacement

by:

-Ao cos (wt)

w

d = JJa

dt

·

dt

= f v

dt

-A

=~sin

(wt)

w

4.1.

4.2.

4.3.

It

can be seen therefore,

that

by integrating.

the

acceleration signal

once

with respect

to

time

we divide it

by

w

to

obtain

the

velocity·, whilst

by

integrating twice with respect

to

time

we

divide by w2

to

obtain

the

dis-

placement. This

is

shown

in

Fig.4.3

in

which

the

velocity and

displacement

spectra are derived from integrating an acceleration spectrum

of

constant

magnitude. Both

the

velocity and

the

displacement spectra decrease

in

pro-

portion

with

the

frequency and have a slope

of

20

dB/decade

and

40

dB/decade respectively (6 dB/Octave and 12

dB/Octave

respectively).

ii:l

+5

~

- 5

c:

0 -

10

a.

~

-

15

';

-

20

>

·

~

-

25

Ill

ai

-:ll

c:::

-

35

-40

-

45

-50

-

55

-

60

-65

-

70

1--

)I

I

"""

I v "

lfi/

r'\.

vv

"

1/

-

Acceleration = a

"r--.

..........

i\.

t.....~locity

=~

' '

"""!'-..

\

'r--

r--.

'r--.

1

\.Displacement

= a 2

'I'-..

'\.1

I I I I I I I

IW

'r--

1\.

I I

~".....

'\

I I

-

75

1

10

2

10

3 2

10

4

Hz

2

Frequency (Hz)

nu~·

Fig.4.3. Frequency dependence

of

velocity

and

displacement

upon

acceler-

a~n

·

17

...

~·

·

..

Although

for

periodic

vibrations

both

velocity and

displacement

can

be

calculated

mathematically

from

the

acceleratiQn, this

is

not

possible

for

complex

vibrations

and

therefore

integration has

to

be

performed

electrical-

ly.

For

this

reason

the

4292

employs

nine passive

R-C

integration

networks

which

may

be

selected using

the

INTEGRATION SWITCH.

In

the

velocity

modes

of

this

switch

the

acceleration signal

from

the

output

of

the

0

to

20

dB

amplifier

is

integrated

using

the

type

of

network

shown

in

Fig.4.4, in

which

the

voltage across

the

capacitor

is

given

by:

e =-1 J i

dt

c c

and

the

total

input

voltage is:

e = eR + ec = Ri +

i-f

i

dt

4.4.

4.5.

To

obtain

the

correct

integration.

the

values

of

R and

Care

chosen

such

t~at

eR

~ec,

and

consequently

i = Wwhich,

substituted

into

equation

4.4

g1ves

1

ec = RC f e

dt

4.6.

A specific

combination

of

resistor and capacitor can

only

be

used

for

integration

over

a limited

frequency

range.

At

high

frequencies

the

signal

level across

the

capacitor

will decrease

to

a

too

low value

and

consequently

the

signal-to-noise

ratio

will

be

too

small.

At

frequencies less

than

the

lower

limiting

frequency

(-7

dB

point

on

frequency

characteristics

for

velocity

~eR-.f

I

...

CJ

CI

r

R

ec

eoutput

1 I l

2U25S

Fig.4.4. Schematic

of

an R-C integration

network

used

for

velocity meas-

urement

18

BRUEL & KJAER 4292 User guide

Popular Test Equipment manuals by other brands