BRUEL & KJAER 4912 User manual

Instruction Manual
Portable Stroboscope Type 491 2
033-0587
•. .
...
An ultra-compact unit combining a strobo-
scope
with a tachometer . A 4-digit liquid
crystal display indicates
tacho frequency
(from a
choice of separately available trans-
ducers)
or strobe lamp repetition rate in
hertz
or r /min. The flash rate may be syn-
chronized
externally or driven from an· inter-
nal generator whose frequency range is 5
to 125 Hz (300 to 7500 r/min). Recharge-
able
batteries contained in the handle give
a
minimum of 2,5 hours' operation from
one charge. The xenon lamp provides a con-
centrated beam of
white light whose bright-
ness is independent of frequency above
25 Hz (1500 r/ min).
+ Briiel & Kj~r

PORTABLE STROBOSCOPE
TYPE 4912
From serial number 879442
October 1 980

CONTENTS
1. INTRODUCTION AND SPECIFICATIONS (PRODUCT DATA) ................................................. 1
2. CONTROLS .......................................................................................................................... . 3
3. OPERATION .......................................................................................................................... 6
3.1. ENVIRONMENT AND MOUNTING ...................................... .. .......... .. ........... 6
3.2. POWER SOURCES ..................................................................... .. ........ .. .. 6
Battery Charger
ZG 011 4 .......................................................................... 6
Operation
while Charging .......................................................................... 7
Accelerated Charging ...................................................
.. ........................... 7
External
Power ........................................................................................ 7
3.3. FREE-RUNNING STROBOSCOPE OPERATION .............................................. 7
3.4. OPERATION FROM AN EXTERNAL TRIGGER ............................................... 11
Triggering from another Instrument ............................................................ 11
Triggering from Photoelectric Probe MM 0012 ..... .. .......... ........................... 11
Triggering from Magnetic Transducer MM 0002 ........................................ .. 13
Single-Flash Triggering .................... ... .................. .. ................ .... .............. 14
3.5. PHOTOGRAPHY WITH THE 4912 ............................................................... 14
4. SERVICE AND REPAIR ........................................................................................................ 16

USES:
•
Inspection of rotating or
reciprocating machinery
• Mode study vibration
testing
• Loudspeaker analysis
• Dynamic field balancing of
rotating parts
• Speed/frequency
measurement of rotary,
linear or vibratory motions
• Investigation of high-speed
repetitive industrial
processes on the shop
floor
FEATURES:
•
Combined motion analyzer
and tachometer
• Internal frequency
generator range 5 to
1 25 Hz (300 to
7500 r/min)
The Type 491 2 Portable Strobo-
scope is a compact hand-held instru-
ment which combines the function
of tachometer with that of motion
analyzer . It permits both qualitative
investigation and accurate measure-
ment of various kinds of rapid, repet-
itive motion in trouble-shooting and
design and development situations .
The
Stroboscope utilizes the phen-
omenon of persistance of vision to
"freeze" or slow down motion of vi-
brating, rotating or reciprocating
parts of machinery too rapid for the
Fig.1 . Photoelectric Probe MM 0012
19- 126
• Flash rate obtainable O to
130 Hz (0 to 7800 r/min)
• Tachometer operation 5 to
1500 Hz
•
External trigger range Oto
20000 Hz
•
Constant white light
output independent of
frequency above 25
flashes per second
• 4-digit liquid crystal
display of frequency or
r/min
• Compact self-contained
unit for use in the hand
• Minimum 2,5 hours'
operation off built-in
accelerated-charge
rechargeable NiCd
batteries
• Mains or external DC
operation when required
unaided eye to perceive . When the
machinery is illuminated by the
flashing light from the xenon lamp
in the Stroboscope and the flash
rate is suitably adjusted, the illusion
obtained is of stationary or slowly
moving machinery. This illusion
makes it very easy to assess qualita-
tively how the machinery is behav-
ing, and it also permits comparative
measurements to be made on the
amount of motion taking place .
Since the 491 2 features a 4-digit
liquid crystal display which can be
set
to read either Hz or r /min, it
can also be used to measure accu-
rately the rate or frequency of the
motion (to the nearest r/ min or
0, 1 Hz).
Fig.2. Magnetic Transducer MM 0002
type 4912
Portable
Stroboscope
The 491 2 can be used as a ta-
chometer or a combined tachome-
ter / stroboscope, as well as a free-
running stroboscope. In the free-
running mode the lamp is triggered
from an internal frequency-genera-
tor which may be adjusted between
5 and 1 25 Hz by means of a 1 0-
turn knob . The actual frequency is
indicated on the display .
The 491 2 can be triggered exter-
nally, via a coaxial cable, from a
photoelectric tachometer probe (e.g.
B & K
MM 0012, Fig.1), a magnetic
probe (e.g. B & K MM 0002, Fig.2),
a vibration meter (e.g. B & K Type
2511), or a variety of other motion-
sensing devices or instruments.
When it is used solely as a tachome-
ter, the display will indicate fre-
quency from 5 to 1 500 Hz or the
equivalent r/ min . If the uppe r limit

TIIOho ht.T"9 .
• • • tnt . (Mn .
Portable
s~
Type 4,912
Fig.3. Control panel of Type 4912
is exceeded, the display flashes to
indicate out-of-range . When it is
used as a stroboscope
synchronized
Frequency Ranges:
Flash rate: 0
to 130 Hz (with external
triggering)
Internal generator: 5 to 1 25 Hz
Display: 0 Hz to 1500 Hz and Or / min to
90000 r/ min . Display flashes if upper li-
mit is exceeded
External trigger: 0 to 20000 Hz
Tachometer Display:
Type:
4-digit liquid crystal display, 7-seg-
ment numerals 12 mm (0,5 in) high
Configuration: Display reads Hz to near -
est 0, 1 or 1 Hz, and r/ min to nearest 1
or 10 r/ min
Accuracy: ± 1 part per thousand,
± 0, 1 Hz or± 1 r/ min in lower ranges
± 1 Hz or± 10 r/ min in higher ranges
Light Source:
Type:
Xenon white-light discharge tube
Average Light output: Greater than
2500 lux up to 250 mm ( 10 in) range
and greater than 1000 lux up to
500 mm (20 in) range, for flash rates
above
25 Hz (1500 r/ min)
with the machinery or system being
studied, the flash rate range is O to
130 Hz.
Specifications 491 2
Flash duration: 5 µs approx.
Beam diameter: Typically 60 mm (2.4 in)
at
500 mm range
External Trigger:
Connector type: 3-pole coaxial BNC, ac-
cepting plug JP 0315
Trigger voltage: ± 100 mV, adjustable
(rising to ± 200 mV at 20000 Hz), on in-
nermost pole of socket, referred to shield
Supply for trigger source: + 5 V DC to
+ 7 V DC (internal batteries), or + 6 V DC
to + 10 V DC (ext. power source), on pole
no . 2 (ring) of socket, referred to shield
(can be used to power Photoelectric
Probe MM 0012)
Power Input:
Connector type:
standard 7-pin DIN
socket accepting
plug JP 0703
Charging: + 6 to + 15 V DC, 0 to
600 mA, 7 hours at max . rate; or from
battery charger ZG 0113, 10 hours
External power: + 6 to + 15 V DC, max .
7 W
to 15 W respectively with strobo-
Fig.4. Battery Charger ZG 0113
In this operating mode the exter -
nal
trigger will lock to triggering fre-
quencies up to 20000 Hz, and if
they are greater than 1 30 Hz the
flash rate will be a· submultiple of
the triggering frequency.
The 491 2 is a completely self-
contained unit, powered from re-
chargeable batteries built into its
carrying handle . The integration of
the light source, power supply and
generator into a hand-held instru-
ment weighing only 1 ,3 kg makes
the Portable Stroboscope ideal for
studies of moving machinery in
cramped, confined spaces and rem-
ote locations where transportability
is essential. It may be recharged
from the mains using battery
charger ZG 0113 (Fig.4, supplied),
or recharged and operated from the
mains using Mains Power Supply
Type 2808 and control cable AO
0035 (both obtainable to order).
scope operation, 1 W to 5,5 W respec-
tively with tacho . only
Battery condition: moving-coil indicator
Weight:
1,3 kg (2 ,8 lb approx .)
Dimensions:
Height:
265 mm (10.4 in)
Width: 135 mm (5,3 in)
Depth: 93 mm (3,7 in)
Accessories Included:
Battery charger
3-pole BNC plug
7-pin DIN plug
2 spare fuses
Accessories Available:
Photoelectric tachometer
probe
Magnetic probe
Mains Power Supply
Control cable
Tripod bush adaptor
ZG 0113
JP 0315
JP 0703
VF 0010
MM 0012
MM 0002
Type 2808
AO 0035
UA 0588

BATTERY LEVEL:
2. CONTROLS
Frequency
Battery Level ---- •
Power Input
Ext. Trig . Input
Internal Generator
800511
Fig. 2. 1. Rear panel of the 491 2
Moving-coil indicator of the battery condition. Also indicates whether
voltage is sufficient when external power is used. The pointer leans to
the left when the instrument is switched off or the battery or external
voltage insufficient to power the 491 2 within specification. The poin-
ter leans to the right when the voltage is sufficient. If the indicator
3

EXT. TRIG. INPUT:
FREQUENCY:
FREQUENCY SELECTOR:
FUNCTION SELECTOR:
INTERNAL GENERATOR:
4
shows that the battery voltage is insufficient, the battery should be re-
charged (section
3.2 .1 ).
Three-pole coaxial socket accepting standard TNC plug JP 031 5 (sup-
plied)
or standard BNC plug JP 0035 to trigger the stroboscope lamp
externally or feed the tachometer. The sheath (outermost contact) is
ground, and is connected to pin 2 of POWER INPUT. The middle con-
tact (ring) carries a positive DC supply for an active tachometer probe
such as
MM 0012. This supply is + 5 V to + 7 V when the 4912 is
powered from its internal batteries, and + 6 V to + 15 V when it is pow-
ered from an external source. The innermost contact (tip) is the trigger
input. The 4912 is triggered by a positive-going voltage at a threshold
set by TRIGGER LEVEL between -0, 1 V and + 0, 1 V (rising to -0,2 V
to + 0,2 V at 20000 Hz). Input impedance is 50 kO approx., in series
with 680 nF. Max . input 30 V Pk-Pk.
4-digit liquid crystal display indicating either EXT. TRIG. INPUT fre-
quency or INTERNAL GENERATOR frequency as selected by FUNCTION
SELECTOR. Displays hertz
or revolutions per minute (r/min) in four
ranges selected by FREQUENCY SELECTOR.
4-position switch for selecting FREQUENCY display indication.
"x 10 r/min" Display range 10 to 90000 r/min.
"r/min" Display range 1 to 9000 r/min.
"Hz" Display range 1 to 1 500 Hz.
"Hz" Display range 0, 1 to 1 50,0 Hz.
Four-position switch.
"Power Off" Power is removed from the 491 2. The FREQUENCY dis-
play
may continue to indicate for a few seconds after power is
switched off.
"Tacho" The 491 2 behaves solely as a tachometer, displaying the fre-
quency of the EXT. TRIG. INPUT signal. The lamp does not light. Upper
limit 1 500 Hz (signified by flashing of FREQUENCY display).
"Ext. Trigger" The 491 2 behaves as a combined stroboscope and ta-
chometer. The lamp flashes in synchronism with the EXT. TRIG. INPUT
signal, and the FREQUENCY display indicates the frequency of this sig-
nal. If this frequency exceeds 1 30 Hz, the stroboscope lamp will flash
at the highest submultiple of the trigger frequency less than 1 30 Hz,
and
will continue to synchronize correctly with EXT. TRIG. INPUT fre-
quencies up to 20 000 Hz. The display indicates the signal frequency
provided this lies within the range selected with FREQUENCY SELEC-
TOR.
"Int. Gen." The 491 2 behaves as a free-running stroboscope. The
stroboscope
lamp flashes at the frequency set by INTERNAL GENERA-
TOR
and indicated on the FREQUENCY display in Hz or r/min as se-
lected.
The frequency range available is 5 to 125 Hz.
10-turn thumb-wheel for setting the frequency of the internal genera-
tor between 5 Hz (maximum anticlockwise) and 1 25 Hz (maximum
clockwise). Operative only in "Int. Gen." position of FUNCTION SELEC-
TOR.

POWER INPUT:
TRIGGER LEVEL:
Standard 7-pin DIN socket accepting plug JP 0703, for recharging the
battery and/or powering the instrument from an external source. The
connections are shown in Fig.2.2. Pin 2 is connected to the outermost
contact of EXT. TRIG. INPUT. When a plug is inserted, the 4912 can
be
powered only from an external source, unless the plug has been
modified as described in section 3.2.
+ 6 V to+ 15 V + 6 V
to+ 12 V
----+ill----- DC, max. 6 W, to
DC, 0 to 600 mA, ___ __...,t--1•
to charge the
batteries
ov
power the instrument
externally
790542
Fig.2.2. POWER INPUT connections (external view)
Single-turn level-setting control for adjusting the voltage at which the
EXT. TRIG. INPUT operates. Adjustable between -0, 1 V and + 0, 1 V
(rising
to -0,2 V and + 0,2 Vat 20000 Hz) .
5

3. OPERATION
3.1. ENVIRONMENT AND MOUNTING
The Portable Stroboscope Type 491 2 may be used over a wide temperature range,
-1 0° to + 55° ( 14° to 131 °F). The permissible upper limit of relative humidity is 90%,
non-condensing.
The carrying handle of the 491 2 incorporates a standard tapped hole for tripod moun-
ting (3/8-in Whitworth). It may be mounted on the portable tripod UA 0587 or any other
similar support. It may also be stood upright on its base provided the supporting surface
is reasonably level.
3.2. POWER SOURCES
The Type 491 2 may be powered from its internal rechargeable batteries, or from a sui-
table external source . The pointer of the BATTERY LEVEL indicator leans to the right
(thus : /) when the battery voltage is sufficient to power the 491 2. This condition will oc-
cur after a relatively short period of charging. Consequently the indicator cannot be used
as a
guide to whether or not the battery is fully charged. The only way to be sure that
the battery is fully charged is to keep the 491 2 on charge for the period of time speci-
fied below. The indicator will show whether or not the battery has received some
charge;
it will also show eventually whether the battery needs to be replaced by indicat-
ing that the battery voltage falls to below the usable level after a shorter operating time
than when the Stroboscope was new. The battery can undergo approximately a thou-
sand charge/discharge cycles before its performance deteriorates, however. Replace-
ment is a service operation. Note that the true battery condition is indicated by the BAT-
TERY LEVEL
indicator only when the charging source is switched off or disconnected.
3.2.1. Battery Charger ZG 0113
6
The
simplest method of recharging the batteries is to use the battery charger ZG 0113
supplied (Fig.4, Chapter 1 ). Before this can be done, the battery charger must be fitted
with a suitable plug for connection to a single-phase AC mains supply . For mains vol-
tages between 1 00 V and 1 30 V, connect the supply between the blue and black conduc-
tors in the unterminated cable attached to the battery charger . For voltages between
200V and 240V, connect it between the blue and brown conductors. The unused con-
ductor should be cut back or insulated.The 7-pin DIN plug attached to the other cable on
the battery charger is then plugged into the POWER INPUT socket on the 4912 . Recharg-
ing
completely exhausted batteries takes about 10 hours with the ZG 0113 . No harm
will result from prolonging the charging time. However, if the instrument is used infre-
quently, it should not be left on permanent charge as this practice tends to shorten bat-
tery life.

3.2.2. Operation while Charging
The 4912 cannot be operated while connected to the battery charger unless the 7-pin
DIN plug is modified as follows. Slide back the plastic sheath round the plug and remove
the two parts of the metal screen. To free the sheath it may be necessary to depress the
metal key visible through a window in the side of the sheath. The 491 2 may then be op-
erated
whether or not the modified plug is pushed into the POWER INPUT socket for re-
charging, provided the batteries are not completely exhausted to begin with. When the
Stroboscope is operating at more than about 20 flashes per second, it drains more cur-
rent from the battery than can be supplied from the ZG 0113 (which is intended only for
charging, not for powering). However, the use of the ZG 0113 during operating can
greatly extend the operating-time of the 491 2 between charges.
3.2.3. Accelerated Charging
The rapid-charge batteries of the 491 2 may be completely recharged from exhaustion in
7
hours if a charging source of 600 mA is provided. The required voltage at this current
is 15 V. Neither figure should be exceeded. The charge source positive should be con-
nected
to pin 3, and negative to pin 2, at the POWER INPUT connector (see Fig.2.2).
3.2.4. External Power
The 491 2 may be powered from a 6 to 15 V supply connected to POWER INPUT with
positive to pin 1 and negative to pin 2, whether or not the batteries are being charged at
the same time. The maximum power consumption varies from 1 W at 6 V DC to 5,5 W
at 15 V DC with tacho-only operation, and from 7 W at 6 V DC to 15 W at 15 V DC with
stroboscope operation. The 491 2 is protected against accidental reverse connection of
the external power source.
Type
2808 Power Supply, available separately, may be used to operate the 491 2 while
at the same time recharging the batteries from exhaustion in about 1O hours to full
charge. To do this, connect the Type 2808 to a suitable mains supply as described in its
instruction manual, connect its OUTPUT socket to POWER INPUT on the 491 2 using
cable
AO 0035 and set the CHARGE switch to "On".
3.3. FREE-RUNNING STROBOSCOPE OPERATION
The Stroboscope lamp will begin flashing at a rate determined by the setting of INTER-
NAL GENERATOR as soon as FUNCTION SELECTOR is set to "Int. Gen.", unless BAT-
TERY LEVEL
indicates that there is insufficient supply voltage for proper operation. The
pointer leans to the right when the voltage is sufficient. If it does not, recharge the bat-
teries or provide a suitable external power source as described in section 3.2.
\
To obtain a stationary or slowly moving image of a rapidly moving object, first ensure
that the object is illuminated mainly by the light from the Stroboscope. Fig.3.1 and Table
3.1
show the characteristics of the beam from the lamp. Set INTERNAL GENERATOR to
its maximum clockwise position, and turn it gradually anticlockwise while observing the
object. This procedure will initially result in multiple images (unless the frequency of mo-
tion is very high) as the flash rate of the Stroboscope passes through synchronism with
the harmonics of the motion frequency of the object. The first stationary imagine which
is not in multiple occurs when the flash rate and the motion frequency are identical;
they may be read off the FREQUENCY indicator. To obtain the desired form of indication
(Hz or r/min), set FREQUENCY SELECTOR as preferred.
7

8
Fig.3. 1. Polar diagram showing typical beam geometry for the
491 2 light source
llluminance
Amount
Distance from lamp (m)
off axis
0, 1 0,25 0,5 1,0
00
3400 3000 1000 330
10
2950 980 300
20
3300 2800 900 220
30
2300 700 150
40
2000 450 100
50
3000 1500 300 70
60
1000 200 45
70
2400 500 100 31
100
1500 170 30 8
15° 800
20°
250
25°
150
30° 100
800158
Table 3. 1. Typical beam geometry characteristics for the 4912 light
source
Sometimes it can be difficult to distinguish multiple and single images of a rotating ob-
ject which is rotationally symmetrical, such as a fan or gear wheel. In such cases it is
helpful to stop the machine and make a distinctive mark, with chalk or ink, on the rotat-
ing part. Start the machine again and obtain a stationary imagine. Check that the mark
occurs at only one point in its orbit, and if it occurs at several, reduce the Stroboscope
frequency. If exclusion of ambient light proves to be difficult, the contrast of such a mark
can be improved by the use of a patch of retroreflective adhesive tape. This material has
the property of reflecting light back along its incident direction. A patch of this tape stuck
to the rotor should be viewed from a position immediately adjacent to the Stroboscope in

order to maximise the strobed patch brightness. Light falling on the patch from other di-
rections will not be reflected into the eye.
The large
diameter of the INTERNAL GENERATOR wheel enables the 4912 to be oper-
ated with one hand, when it is used in this mode. The wheel may be set with the thumb
of the hand holding the handle.
The direction of rotation or linear motion of objects may be determined by offsetting the
INTERNAL GENERATOR frequency slightly from a position of synchronism. If the wheel
is off-set anticlockwise (lower flash rate), the direction of apparent motion is the same
as the direction of actual motion.
The frequency range of the internal generator is 5 to 1 25 Hz (flashes per second), corre-
sponding to 300 to 7500 revolutions per minute. Objects having higher motion frequen-
cies may be observed readily; more than one cycle of motion will then take place be-
tween flashes.
If the highest stationary single image is obtained with a flash frequency of more than
62,5 Hz (3750 r/min), it is possible that the motion frequency is higher than this. It
could be two, three or more times the flash frequency. To find out whether this is so,
and if it is, determine the true motion frequency, proceed as follows:
1. Record the highest flash frequency (in Hz) at which a single stationary image is ob-
tained.
2. Continue turning the INTERNAL GENERATOR wheel anti-clockwise, reducing the fre-
quency until another single stationary imagine is obtained (if none is obtained when
the flash frequency has fallen to half the frequency recorded in step 1, then the true
motion frequency is the same as the flash frequency recorded in step 1 ).
3. If another single stationary image is obtained, record the flash frequency (in Hz) at
which it occurs.
4. Locate the highest flash frequency (from step 1) on the left-hand scale in Fig.3.2.
5. Locate the second-highest flash frequency (from step 3) on the centre scale in
Fig.3.2.
6. Lay a straight edge across Fig.3.2 so that it passes through the two points located
in steps 4 and
5.
7. The true motion frequency may be read off the right-hand scale, in Hz or r /min, at
the point where the straight edge intercepts the scale.
Example
Highest flash frequency = 75 Hz (left-hand scale)
Second-highest frequency = 50 Hz (centre scale)
Follow broken line to right-hand scale
True motion frequency = 1 50 Hz = 9000 r /min.
Fig.3.2 is obtained from the equation:
9

65
70
75
i
>
80
(.)
C
Q)
::,
I
~
;;::
...
~
85
.r::.
0,
::i:
90
95
100
105
110
115
120
125
Fig.3 . 2.
10
125
130
8000
135
45
140
145
yo 9000
-
--
160
-
-
_ 50
-
10000
170
-
-
-
-
180
11 000
190
g
55
~
200
12 000
C c
Cl>
I
::,
:i:
I
13000
~
>
~
(.)
60
~
~
;;::
::,
14000
::,
I
C"
Cl>
t
.r::.
C
250
15000
C
0,
i:
0 0
-6
-~ ·g
C
65
~
16000
~
0
u
Cl>
Cl) 17
000
300
18 000
70
19 000
2 0000
75
400
80
500
85
90
600
700
95
800
100
1,0k
105
110
2,0k
115
120
125
800459
Nomogram
for determining motion
frequencies
higher
than
125Hz with
free - running
strobo-
scope operation
where:
fm = true motion frequency;
f
h-1 = second-highest flash frequency;
fh = highest flash frequency.
When FUNCTION SELECTOR is set to "Power Off", the indication on the FREQUENCY
DISPLAY
may linger for a few seconds. This is a normal condition.

3.4. OPERATION FROM AN EXTERNAL TRIGGER
In both the "Tacho" and "Ext. Trig." settings of FUNCTION SELECTOR, the 4912 will op-
erate only if a suitable changing voltage is applied to the EXT. TRIG. INPUT, as described
below.
In the "Tacho" setting, the Stroboscope lamp does not flash at all - this position enables
the 491 2 to function solely as a tachometer, indicating the frequency of the EXT. TRIG.
INPUT
voltage on its FREQUENCY display, in a form selected by means of FREQUENCY
SELECTOR. The
indication limits are 5 to 1500 Hz (corresponding to 300 to 90000 revo-
lutions per minute). If the external trigger frequency exceeds the upper limit, the FRE-
QUENCY
display will flash to indicate overload. The 491 2 will operate for approximately
15 hours from the internal batteries between charges in the "Tacho" mode.
In the "Ext. Trig." setting of FUNCTION SELECTOR, the lamp is triggered by the voltage
at EXT. TRIG. INPUT, and although the maximum flash rate is limited to 130 flashes per
second, the flashes can be synchronized with trigger frequencies up to 20000 Hz. The
FREQUENCY display indicates the trigger frequency. In this mode the lamp can also be
triggered by events occurring at irregular or sequentially varying intervals.
The 491 2 is triggered externally by a rising voltage passing a threshold set between
-0, 1 V and + 0, 1 V by means of TRIGGER LEVEL, which should be adjusted as neces-
sary to obtain satisfactory triggering. At the highest trigger frequencies the trigger range
rises to -0,2 V to+ 0,2 V.
Specific considerations concerning external triggering by other instruments and photo-
electric and magnetic probes are dealt with in the following sections.
3.4.1. Triggering from another Instrument
The 491 2 may be triggered from the AC recorder output from a measuring instrument
such as the B & K Portable Vibration Meter Type 2511, for use in dynamic balancing, for
example. It may be triggered from an excitation source such as the output of a sine gen-
erator or exciter control (B & K Types 1023, 1027 or 1047 for example) for loudspeaker
analysis or mode study vibration testing. For these applications the appropriate output
from the instrument should be connected to EXT. TRIG. INPUT on the 491 2 by means of
a standard BNC plug (JP 0035). Various ready-wired cables are available from B & K.
The EXT. TRIG. INPUT on the 4912 has an input impedance of 50 kO and is AC-coupled
by a 0,68 µF capacitor. The alternating component of the voltage applied to it should not
exceed 30 V Pk-Pk. The DC component should be kept within ± 63 V. The trigger level
may be adjusted as required between -0, 1 V and + 0, 1 V (-0,2 V to + 0,2 Vat 20 kHz)
as
described above.
3.4.2. Triggering from Photoelectric Probe MM 0012
The Photoelectric Tachometer Probe MM 0012 (Fig.1, Chapter 1) provides a very conve-
nient transducer for triggering the 491 2 where the range of motion is great enough,
i.e., more than a few millimetres. The MM 0012 is supplied complete with a 3 m con-
necting cable AO 0158 which should be used to connect it to EXT. TRIG. INPUT on the
4912. This cable also carries the supply to the MM 0012, which is an active device. It
should not be connected if the 491 2 is powered from an external supply exceeding 11 V
DC as this may damage the Probe.
11

770521
Fig.3.3. Dimensions of Photoelectric Tachometer Probe MM 0012
Fig. 3.4. MM 0012 Photoelectric Tachometer Probe in use
Fig. 3.5.
MM 0012 Photoelectric Tachometer Probe in use
12

Detailed information on how to apply the Probe is given in the Product Data· sheet de-
scribing the MM 0012 . It should be fixed at a distance of about 1 mm from the moving
surface for best results, though it can be used at a greater distance, especially if it is as-
sisted by an
auxiliary light source such as a hand torch powered by a battery (not
mains). The surface should have fixed to it a contrasting mark (light on dark, dark on
light) whose passage through the beam causes triggering. The lamp will flash in syn-
chronism with the dark-to-light transition. The dimensions of the MM 0012 are shown
in Fig.3.3, and examples of fixing arrangements in Figs.3.4 and 3.5.
3.4.3. Triggering from Magnetic Transducer MM 0002
The Magnetic Transducer MM 0002 (Fig. 2, Chapter 1) is a variable-reluctance device
with a built-in permanent magnet . Changes in the reluctance of its magnetic circuit, part
of which is external to the Transducer, cause a voltage at its terminals . This voltage can
be made
sufficient to trigger the 491 2.
If the moving object to be studied with the Stroboscope is ferromagnetic, it is sufficient
to mount the Transducer so that the object passes as closely as possible- 1 or 2 millime-
tres to the Transducer at its active end. The dimensions of the MM 0002 are given in
Fig.3.6. If the moving object is not ferromagnetic, or the motion is continuous rather
---- - - do
0.3
262197
Fig.3. 6. Dimensions of Magnetic Transducer MM 0002
Fig . 3. 7. MM 0002 Magnetic Transducer in use
13

than reciprocating, one of the high-µ discs supplied with the Transducer may be cem-
ented to the moving object or secured with a double-sided self-adhesive disc YO 0073.
The Transducer is supplied with a 1,2 m connecting cable AO 0038, requiring adaptor
JP 0145 (not supplied) to connect the 10-32 UNF miniature screw-locking coaxial plug
on the cable to the EXT. TRIG. INPUT socket on the 491 2.
The Magnetic Transducer is especially to be preferred for triggering in dirty environ-
ments or in general where the optical technique of the MM 0012 is not suitable. For
more detailed information, refer to the Product Data for the MM 0002.
3.4.4. Single-Flash Triggering
A single flash can be obtained when FUNCTION SELECTOR is set to "Ext . Trigger" by
connecting together the tip and the ring of the EXT. TRIG. INPUT socket, momentarily.
TRIGGER LEVEL should be set with the white mark on the knob slightly to the left of the
central position. This facility may be used to trigger the flash from the flash contacts of a
camera.
For correct synchronization of the shutter, the "X" contacts should be used on the cam-
era. Select the shutter speed in accordance with the recommendations in the camera's
instruction manual for its use with electronic flash. In general, any shutter speed may
be used with a between-the-lens shutter. With a camera having a focal-plane shutter,
the highest speed which may be used is usually limited to some setting between 1 /30
and 1 /125 s, depending on the characteristics of the shutter. This setting will be stated
in the instruction manual. If in doubt, use a setting of 1/ 30 s or longer.
Photography with the 4912 is discussed in section 3.5.
3.5. PHOTOGRAPHY WITH THE 4912
14
The recommended exposures for photographing objects illuminated by the flash of the
4912 are given in Table 3.2. The conditions in which they will be found optimum are:
subject-to-lamp distance, 250 mm (10 in); subject elements illuminated, up to 2,5° off
axis.
The 491 2 may be set to give a single flash in synchronism with the opening of the cam-
era shutter, as explained in section 3 .4.4. In this case the exposures given in the first
column of the Table should be used.
If the camera is not synchronized with the 491 2, then a shutter speed must be selected
which is slow enough to ensure that at least one flash occurs while the shutter is open.
It is preferable to ensure that at least three occur, to minimize the effect of the uncer-
tainty of the timing on the actual number of flashes which do occur. If the flash fre-
quency F is less than 25 Hz, then the number of flashes which occur during shutter-
open time should be calculated, and the exposure read from the corresponding column
of the Table. F should be read from the FREQUENCY display provided this indicates less
than 25 Hz (1500 r/ min). Set FREQUENCY SELECTOR to one of the "Hz" positions. The
number of flashes which occur can be calculated from the formula F x T, where T is the
exposure-time in seconds.
If the flash rate F exceeds 25 Hz (1 500 r /min), then the exposure received by the film
will be independent of F provided at least three flashes occur while the shutter is open.
This is because the averaged ii luminance generated by the 491 2 is kept constant aboVe
25 Hz. In this situation the exposure should be taken from the column corresponding to

the actual shutter speed setting in use. It is preferable that F is greater than or equal to
3 x 1/ T.
The recommendations made in Table 3.2 are for black and white film. Reliance should
not be placed on their application to colour film. Instead, the user should experiment
with several exposures to determine the most suitable. Acceptable pictures on colour
films having the same speed ratings as the black and white films given may be obtained
from the use of the table, or an increase in exposure may be found necessary .
Number
of flashes occuring while
the shutter
is open: 1 2 4 8
(Flash rate less than 25 Hz)
Exposure-time in seconds : 1/30 1/15 1/8
1/4
(Flash rate greater than 25 Hz)
Aperture settings for Kodak Tri-X f/4 f/5,6 f/8 f/11
Aperture settings for Kodak
Plus-X
f/5,6 f/8
f/11
f/16
800 1 57
Table 3.2. Recommended exposures for photography with the 4912
If the purpose of using the 491 2 for photography is to obtain a clear picture of a moving
object, it is important to ensure that the illumination of the object by the 491 2 predon:,i-
nates over its illumination by other light sources. This can be checked in cases of uncer-
tainty by measuring the exposure required in the absence of the 491 2, using an ordi-
nary photographic exposure meter . The measured exposure should significantly exceed
that recommended in the Table, if an acceptable picture is to be obtained.
15

16
4. SERVICE AND REPAIR
The 491 2 is designed and constructed to provide the user with many years of reliable op-
eration. However, should a fault occur which impairs its correct function, then all power
should be disconnected to prevent the risk of further damage. For repair consult the se-
parate Service Instruction provided or consult your local B & K service representative. Un-
der no circumstances should repair be attempted by persons not qualified in the service
of electronic instrumentation.