BRUEL & KJAER 4809 Guía del usuario

Instructions

and

Applications

Vibration Exciter

Type 4809

The Vibration Exciter Type

4809

a permanent magnet type exciter

for

use in accelerometer calibration,

mechanical impedance measurement

testing

small structures. It

has a force rating of 10 lbf.

(44.5

VIBRATION

EXCITER TYPE 4809

May 1972

CONTENTS

INTRODUCTION

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

DESCRIPTION

AND

OPERATION

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

2.1. Principle

Operation

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

2.2. Description

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

2.3. Replaceable Inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.4.

Additional

Cooling

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

2.5. Positioning the Exciter

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

2.6. Pushrods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

CHARACTERISTICS

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

3.1. Frequency

Response

and

Resonance

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

3.2. Rated Force and

Limits

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

3.3. Cross-Motion

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

APPLICATIONS

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

Accelerometer Calibration . . . . . . . . . . . . . . . . . . . . . 14

4.2. Mechanical Impedance . . . . . . . . . . . . . . . . . . . . . . . . 15

4.3. Force Controlled Testing

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

SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

INTRODUCTION

The

Type

4809

Vibration

Exciter

a permanent magnet type exciter

with

a force rating

lbf

(44.5 N).

has

a round table

with

acentrally

located

mounting

hole

for

connection

the test object.

can

operated

in any position.

The Type 4809

can

used

for

accelerometer calibration,

for

mechanical

impedance measurements,

for

educational demonstrations or

for

vibration

testing

small objects

structures.

has

maximum

acceleration capacity

75 g and a maximum displace-

ment

0.315

in. (8 mm) peak

peak.

Low

distortion

operation

possible

in the frequency range

20 kHz.

DESCRIPTION

AND

OPERATION

2.1. PRINCIPLE OF OPERATION

The forces generated in

electrodynamic exciter are due

the inter-

action between

electrical current

a coil and a magnetic field. This

illustrated in Fig.2.1.

Signal

Source Power

Amplifier

r---;;:;::~

Test

Object

I F

Fig.2.

Principle

operation

a 8 & K

Exciter

the

current in the coil, B the

flux

density

the magnetic field and F

the force generated

the coil. The magnitude

the force

given

where F

BIL

Force in Newtons

Magnetic

flux,

Gauss

Current in coil, Amperes

Length

conductor

in magnetic field, meters

The current

for

the exciter

provided by a power amplifier. A signal

source, such

a sine-wave oscillator

random generator

connected

the

input

the amplifier. As the signal

fed

the coil, a

fluctuating

force

and hence a

vibratory

motion

imparted

the test object.

course

the

coil itself

wound about a moving element and this

has

a certain

mass.

Hence the acceleration due

applied force F

will

be:

_ (m

+me)

- F

where m

the

mass

the test object and

the moving element

mass.

the acceleration

given in gravitational units

(g)

and the weights

the moving element and test object

are

known,

the

maximum acceleration

possible

can

quickly

computed. The

maximum

force available

from

the

Type

4809

10 lbf. (44.5 Newton). The moving element weight

0.132 lb.

(60 gm.).

the test object weights 0.37 lb. (.170 kg),

the

maximum

accele-

ration

with

this load

is:

2.2. DESCRIPTION

W+We

F 0.13 + 0.37 =

10 g

Fig.2.2 shows a sectional drawing

the

Type

4809. The magnetic field,

8, mentioned in section 2.1,

here supplied

a permanent magnet which

projects

into

the

space

enclosed by the driver coil. A cast

iron

magnetic

structure surrounds the magnet in order

co-nfine the magnetic force

the exciter

much

possible.

Threaded Insert- -

----,

Tangential

Flexure

Magnetic

Structure

Table

---H

HN--

-Permanent

Magnet

Fig.2.2. Sectional drawing

a Vibration

Exciter

Type

4809

The moving element

a cylinder machined

out

aluminum

with

a table

at one end and a driver coil wound around the open end. The table

has

diameter

1.16 inches (29.5 mm).

the center

a threaded hole.

This hole

NOT

used

for

direct

mounting

a test object

connecting rod.

intended

receive one

the special inserts supplied

with

the Exciter.

The moving element

suspended

two

sets

radial flexures. These

are

turn

mounted on tangential flexures.

Each

flexure

a sandwich

metal and rubber bonded together. The design

such

that

a high degree

damping

provided

for

the resonant modes

the flexure.

this way

continuous operation

possible at any frequency.

The

leads

the driver coil

are

epoxied

onto

the

top

surfaces

the

flexures

and

led

out

the

two

terminals. The terminals

can

accept either a

standard B & K connector

(JP

0101)

banana plugs (JB 0002/3).

Rubber overtravel stops

(not

shown in Fig.2.2)

are

provided

limit

the

peak-to-peak travel

0.315 inches (8 mm). These

are

intended

for

emer-

gency

use,

protect the Exciter and the test specimen.

the moving

element hits these stops, there

a clearly audible bumping sound

and

the

signal

waveform

from

any transducer mounted on the Exciter

will

distor-

ted. The level

acceleration should

reduced at once.

~3.REPLACEABLEINSERTS

The Exciter

provided

with

5 inserts, a

bottle

thread

lock<ing

cement,

and

insert mounting

tool.

The inserts

have

internal thread

for

a stud

with

32 threads per inch and a no. 10 screw.

The inserts

are

made

aluminum and

are

intended

act

mechanical

"fuses", protecting the moving element

from

damage. The inner threads

the inserts fail before the moving element

damaged,

for

most types

abusive treatment. In order

minimize insert replacement and, since me-

chanical

fuses

are

not

100%

safe,

avoid possible damage

the moving

element, the user

warned

careful

avoid the

two

most common

causes

abuse:

The

first

the

use

screws

that

are

too

long, and which therefore

hit

the

bottom

the insert hole before the attachment

the test object

tight. A suitable choice

length

will

avoid this. The internal thread on

the

inserts

0.16 inches (4.0 mm) long, measured

down

from

the table surface.

The hole

0.27 inches (6.8 mm) deep.

The second

cause

abuse

the application

excessive mounting to-

rque. A

safe

limit

for

the

mounting

torque

the

Type

4809

lb.in.

(0.35 kgm).

install

insert, the insert

placed on the

tool,

with

the pin

the

tool

inside

the insert internal thread and

with

the blade

the

tool

in the

slot

the insert. Both the insert and the hole

are

thoroughly

cleaned

with

solvent

remove all traces

oil

contaminants

from

fingerprints. A

drop

cement

placed on the insert external threads,

and

the insert

screwed

into

the hole.

properly installed, the upper surface

the insert should

about

0.1

millimeters

0.005 inches below the table surface.

insert

damaged,

can

unscrewed

with

the

same

tool.

advisable

heat the insert

with

a soldering iron

weaken the cement

before unscrewing.

2.4.

ADDITIONAL

COOLING

Under normal operation,

its rated current

5 Amperes, the 4809

can

operate continuously

without

overheating. However,since the table

integral part

the

moving element skeleton, its temperature

can

increase

considerably under

full

load conditions.

the specimen

temperature-

sensitive, this could

have

some effect on the test. For instance,

with

many

accelerometers,

increase in temperature

10°C

can mean a change in

sensitivity

2%.

For

this reason, openings in the casing

the Exciter

are

provided

that

additional cooling may

provided.

the

two

steel plugs on either side

the

body

are removed, compressed air

fittings

may

screwed in. The

openings

are

threaded

with

12 mm threads

with

1.75 mm

pitch.

2.5. POSITIONING THE

EXCITER

For

accelerometer calibration

testing

small objects up

kg.

the

Exciter

would

normally

used in its

upright

position, standing on the

rubber feet provided.

For

many

uses,

though,

more convenient

take

the Exciter

the test object and operate

in a position other than the

upright

one.

'---

M 12 x 1 Thread z.,z,s

Fig.2.3. Plug

for

forced

air

openings

For

this

reason

the

threaded

plugs

for

the

forced air

openings

have a

groove

cut

into

them

(see Fig.2.3)

facilitate

the

suspension

the

Exciter

by cables

other

hangers.

The

plugs are located along a line passing

through

the

center

gravity

the

instrument.

272125

Fig.2.4. Stand

for

using Type

4809

horizontal

position

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