BRUEL & KJAER 4915 - Page 18

BRUEL & KJAER 4915

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12

INT. GEN:

extreme

clockwise

setting

POWER:

"On"

The

Lamp

Unit

should

start

to

flash

at

the

frequency

indicated

on

the

DISPLAY

of

the

4913.

If

not,

press

the

"On"

/"Off"

switch

on the

hand-grip

of

the

Lamp

Unit.

3.

Keeping

at

a

safe

distance

which

will

allow

close-up

observation,

aim

the

light

source

at

the

moving

object

to

be

investigated.

4.

Slowly

turn

the INT. GEN.

knob

anticlockwise,

whilst

observing

the

moving

object.

This

will

initially

result

in

multiple

images

being

observed

as

the

flash

rate

is

de-

creased

and

passes

through

synchronism

with

the

multiples

or

sub-multiples

of

the

motion

frequency

of

the

object.

5.

Ignoring

the

multiple

images,

continue

to

turn

the INT. GEN.

knob

anticlockwise

and

stop

the

moment

the

first

stationary

single

image

is

obtained.

When

this

occurs

the

flash

rate

will

coincide

with

the

fundamental

or

one

of

the

sub-multiples

of

the

motion

frequency

of

the

object.

6.

If

it

is

difficult

to

see

whether

the

image

obtained

is a

single

or

multiple

one, then

stop

the

machine

and using

chalk

or

reflective

tape

make

a

contrasting

mark

on the

surface

of

the

object

to

be

examined.

When

the

machine

is

restarted,

commence

turning

the INT. GEN.

knob

from

its

extreme

clockwise

position

until a

stationary

image

with

only

one

contrasting

mark

is

visible.

If

it

is

not

possible

to

obtain

a

stationary

image,

then

the

4913

should

be

triggered

from

an

external

signal

source

as

described

in

section

3.3. The 4913 will then

function

as a

tachometer,

enabling

a

direct

indication

of

the

motion

frequency

up

to

10

kHz

(ie

600 k r

/min)

to

be

obtained.

7.

Having

obtained

a

stationary

image,

note

the

frequency

indication

given

by

the

4913.

To

check

whether

the

indicated

frequency

corresponds

with

the

true

motion

frequen-

cy

and

not

a

sub-multiple,

set

the DISPLAY SELECTOR

to

"Delay"

and the DELAY

knob

to

its

extreme

anticlockwise

position.

Slowly

turn

the

DELAY

knob

to

move

the

image

through

exactly

half

or

one

or

more

whole

motion

cycles

and

note

the

phase

indication

on

the

4913. The

true

motion

frequency

may

then be

calculated

using

the

relation:

where

fm

is

the

true

motion

frequency

in Hz

or

r/min

n is

the

number

of

motion

cycles

f

1

is

the

frequency

indication

in Hz

or

r/min

P

1

is

the

phase

delay

indication

in

degrees

for

n

motion

cycles

Example:

Frequency

indication

f

1

= 75 Hz

Number

of

motion

cycles

n = 5

Phase

indication

P

1

= 18°

True

motion

frequency

f m = 5 x 75 x

360°

18

fm

= 7500 Hz

or

450 k

r/min

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