BRUEL & KJAER 4809 User guide

- Instructions
and Applications ~
Vibration Exciter
Type 4809
The Vibration Exciter Type 4809 is
a permanent magnet type exciter
for use in accelerometer calibration,
mechanical impedance measurement
or testing of small structures. It
has a force rating of 10 lbf.
(44.5 N) .

VIBRATION EXCITER TYPE 4809
May 1972

CONTENTS
1. INTRODUCTION ...................................... 3
2. DESCRIPTION AND OPERATION ........................ 4
2.1. Principle
of Operation ......................... 4
2.2. Description
.................................. 5
2.3. Replaceable Inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4.
Additional Cooling ............................ 7
2.5. Positioning the Exciter
......................... 7
2.6. Pushrods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3. CHARACTERISTICS .................................. 11
3.1. Frequency Response and Resonance ............. 11
3.2. Rated Force and Limits ....................... 11
3.3. Cross-Motion ............................... 13
4. APPLICATIONS ...................................... 14
4. 1. Accelerometer Calibration . . . . . . . . . . . . . . . . . . . . . 14
4.2. Mechanical Impedance . . . . . . . . . . . . . . . . . . . . . . . . 15
4.3. Force Controlled Testing
...................... 16
5. SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1. INTRODUCTION
The Type 4809 Vibration Exciter is a permanent magnet type exciter
with a force rating of 10 lbf (44.5 N). It has a round table with acentrally
located
mounting hole for connection to the test object. It can be operated
in any position.
The Type 4809
can be used for accelerometer calibration, for mechanical
impedance measurements, for educational demonstrations or for vibration
testing
of small objects or structures.
It has a maximum acceleration capacity of 75 g and a maximum displace-
ment of 0.315 in. (8 mm) peak to peak. Low distortion operation is possible
in the frequency range 20 Hz to 20 kHz.
3

2. DESCRIPTION AND OPERATION
2.1. PRINCIPLE OF OPERATION
The forces generated in an electrodynamic exciter are due to the inter-
action between
an electrical current in a coil and a magnetic field. This is
illustrated in Fig.2.1.
Signal
Source Power
Amplifier
r---;;:;::~
Test
Object
'
I
I F
I
Fig.2. 1. Principle of operation of a 8 & K Exciter
I is the current in the coil, B the flux density of the magnetic field and F
the force generated
by the coil. The magnitude of the force is given by
F
where F
B
I
L
BIL
Force in Newtons
Magnetic
flux, in Gauss
Current in coil, Amperes
Length
of conductor in magnetic field, meters
The current
for the exciter is provided by a power amplifier. A signal
source, such as a sine-wave oscillator or random generator is connected to
the input of the amplifier. As the signal is fed to the coil, a fluctuating force
and hence a
vibratory motion is imparted to the test object. Of course the
4

coil itself is wound about a moving element and this has a certain mass.
Hence the acceleration due to an applied force F will be:
_ (m +me)
- F
a
where m is the mass of the test object and me is the moving element mass.
If the acceleration is given in gravitational units (g) and the weights of
the moving element and test object are known, the maximum acceleration
possible
can be quickly computed. The maximum force available from the
Type 4809 is 10 lbf. (44.5 Newton). The moving element weight is 0.132 lb.
(60 gm.).
If the test object weights 0.37 lb. (.170 kg), the maximum accele-
ration with this load is:
a
2.2. DESCRIPTION
W+We
F 0.13 + 0.37 =
20
10 g
Fig.2.2 shows a sectional drawing
of the Type 4809. The magnetic field,
8, mentioned in section 2.1, is here supplied by a permanent magnet which
projects
up into the space enclosed by the driver coil. A cast iron magnetic
structure surrounds the magnet in order
to co-nfine the magnetic force to
the exciter as much as possible.
Threaded Insert- - ----,
Tangential
Flexure
Magnetic
Structure
Table
.- - ---H HN-- -Permanent
Magnet
Fig.2.2. Sectional drawing of a Vibration Exciter Type 4809
5

The moving element is a cylinder machined out of aluminum with a table
at one end and a driver coil wound around the open end. The table has a
diameter
of 1.16 inches (29.5 mm). In the center is a threaded hole.
This hole is NOT to be used for direct mounting to a test object or
connecting rod. It is intended to receive one of the special inserts supplied
with the Exciter.
The moving element
is suspended by two sets of radial flexures. These
are in turn mounted on tangential flexures. Each flexure is a sandwich of
metal and rubber bonded together. The design is such that a high degree of
damping is provided for the resonant modes of the flexure. In this way
continuous operation
is possible at any frequency.
The
leads to the driver coil are epoxied onto the top surfaces of the
flexures and led out to the two terminals. The terminals can accept either a
standard B & K connector
(JP 0101) or banana plugs (JB 0002/3).
Rubber overtravel stops (not shown in Fig.2.2) are provided to limit the
peak-to-peak travel to 0.315 inches (8 mm). These are intended for emer-
gency
use, to protect the Exciter and the test specimen. If the moving
element hits these stops, there is a clearly audible bumping sound and the
signal waveform from any transducer mounted on the Exciter will be distor-
ted. The level
of acceleration should be reduced at once.
~3.REPLACEABLEINSERTS
The Exciter is provided with 5 inserts, a bottle of thread lock<ing cement,
and an insert mounting tool. The inserts have an internal thread for a stud
with 32 threads per inch and a no. 10 screw.
The inserts
are made of aluminum and are intended to act as mechanical
"fuses", protecting the moving element
from damage. The inner threads of
the inserts fail before the moving element is damaged, for most types of
abusive treatment. In order to minimize insert replacement and, since me-
chanical fuses are not 100% safe, to avoid possible damage to the moving
element, the user is warned to be careful to avoid the two most common
causes of abuse:
The
first is the use of screws that are too long, and which therefore hit
the bottom of the insert hole before the attachment to the test object is
tight. A suitable choice of length will avoid this. The internal thread on the
6

inserts is 0.16 inches (4.0 mm) long, measured down from the table surface.
The hole
is 0.27 inches (6.8 mm) deep.
The second
cause of abuse is the application of excessive mounting to-
rque. A
safe limit for the mounting torque of the Type 4809 is 30 lb.in.
(0.35 kgm).
To install an insert, the insert is placed on the tool, with the pin of the
tool inside of the insert internal thread and with the blade of the tool in the
slot of the insert. Both the insert and the hole are thoroughly cleaned with a
solvent to remove all traces of oil or contaminants from fingerprints. A drop
of cement is placed on the insert external threads, and the insert is screwed
into the hole. If properly installed, the upper surface of the insert should be
about 0.1 millimeters or 0.005 inches below the table surface.
If an insert is damaged, it can be unscrewed with the same tool. It is
advisable to heat the insert with a soldering iron to weaken the cement
before unscrewing.
2.4.
ADDITIONAL COOLING
Under normal operation, up to its rated current of 5 Amperes, the 4809
can operate continuously without overheating. However,since the table is an
integral part of the moving element skeleton, its temperature can increase
considerably under full load conditions. If the specimen is temperature-
sensitive, this could have some effect on the test. For instance, with many
accelerometers, an increase in temperature of 10°C can mean a change in
sensitivity
of 2%.
For this reason, openings in the casing of the Exciter are provided so that
additional cooling may be provided. If the two steel plugs on either side of
the body are removed, compressed air fittings may be screwed in. The
openings
are threaded with 12 mm threads with 1.75 mm pitch.
2.5. POSITIONING THE EXCITER
For accelerometer calibration or testing of small objects up to 1 kg. the
Exciter
would normally be used in its upright position, standing on the
rubber feet provided.
For many uses, though, it is more convenient to take
the Exciter
to the test object and operate it in a position other than the
upright one.
7

'---
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) to facilitate the suspension of the Exciter
by cables or other hangers. The plugs are located along a line passing
through the center of gravity of the instrument.
272125
Fig.2.4. Stand for using Type 4809 in horizontal position
8

For use in a horizontal or near-horizontal position, a stand can be easily
fabricated from sheet steel, as shown in Fig.2.4.
2.6. PUSHRODS
Since the Type 4809 has a central point through which all its force is
directed and since it is usually more convenient to have the driving point a
short distance from
the Exciter, a pushrod is generally used to transfer the
force. This can also be used to provide external protection for the Exciter.
The dimensions and stiffness of
the rod can be designed in such a way that
side loads and moments result in bending of the rod rather than rubbing of
the driver coil.
Fig.2.5 shows a sketch of a pushrod for use with the Type 4809 Exciter.
The
two halves are screwed tight to the Exciter table and the object, then
locked together by means of four center screws.
5 mm Steel Rod Silver Solder
10- 32 UNF I /
~ ~ -----'---__j]J----,(]Rt.29 . 5 mm
M3 Screws c_l0 -32 UNF
Fig.2.5. Pushrod for use with the Type 4809
Fig.2.6 shows a pushrod in use transmitting force to a gas pressure meas-
uring device
in the laboratory of a soft drink manufacturer. The gas measu-
ring
apparatus and bottle-holding jig are free to move horizontally on
rollers and the pushrod is attached to the entire fixture with a locking pin
which permits
frequent removal of the fixture.
9

272124
Fig.2. 6. Horizontal vibration fixture with Type 4809 built in
10

t
3. CHARACTERISTICS
3.1. FREQUENCY RESPONSE AND RESONANCE
In Fig.3. 1 is shown a plot of the frequency response of a Type 4809
Exciter. The acceleration in g is plotted with respect to frequency for a
constant voltage.
~~~ ~~~ ~~~
ODOOOODCDDDDCDDOOOOOOOOOOOOODDOOOODDODOOOOOOOODOOD
BrUel & Kjcer
50 25
Potentiometer Ranv- :____50___d8 Rectifier :~ LOIIINI' lim. Freq.: _20___Hz 'Wr. Speed : _____lLmmfs-c. Paper SpMd :__L_ ~Mc .
1 0 75
Copenhagen :
dB dB
.W~2C 316
Moasu•l"ll Obj.:
Vibration
Exciter Ty ~
..1§QL__3C 15 100
Constant
Voltage = 5 V
~~~ = Typ_L 20 10 31 .6 g
4343 accelero-
meter
~m
- ---~51 m
~
~
Slgn.: __ o o
Qp 1124 10 20 Hz 50 100 200
*Multi ply Frequency Scale by
' dB dB
860
2 15
0 0
500 1000 2000 5000 10000 20000 40000 D A B C Un.
Zero level: 1612/2112 A B C Lin.
Fig.3. 1. Frequency response of the Type 4809
The acceleration level increases with frequency to a damped peak at
about 150 Hz. This is the suspension resonance of the Exciter. The acceler-
ation level then remains constant from 700 to 5000 Hz, after which it
climbs sharply to a peak at 16 kHz. This peak is the major axial resonance
of the moving element itself. It can be seen that the Exciter could be
operated in the range 700 to 5000 Hz without using a compressor loop to
control the oscillator. However, the use of a compressor loop is generally
recommended, since resonances from the test object also enter in.
3.2. RATED FORCE AND LIMITS
Fig.3.2 shows a diagram of loading limits with respect to frequency for
11

the Type 4809 Vibration Exciter powered by a Type 2706 Power Amplifier.
Above 70 Hz, it can be seen that the limits are based solely on the amount
of force available, as mentioned in section 2.1. However, for a given acceler-
ation level, displacement increases as frequency decreases. Therefore, at the
lower frequencies the Exciter must be held to the 0.315 inch (8 mm) limit
in order to avoid hitting the end stops.
100 I I I I
75 g, 736 m/s
2- - Bare Table
50
Cl
q>
c::
20
0
·;:;
~
Q>
10
Qi
u
u
<(
5
2
t/ 50 g, 490 m/s2 Payload 30.8 gram, 0.068 lb
.9'} J _,
I l I .l
- ~ '* 30 g, 294 m/s2 Payload 91.6 gram, 0.20 lb
:.:::-~<ff
r-11?
20 g, 196 m/s
2 Payload 170 gram, 0.37 lb
~~
I I I I I I
t--.~ . to
'R .•
10 g, 98.1 m/s
2 Payload 394 gram, 0.87 lb
· ~ · S
J'
I l l l l l
CY
5 g, 49 m/s2 Payload 850 gram, 1.87 lb
v
1
10 Hz 20 50 100 200 500 1 kHz 2 5 10 20
Frequency
271061
Fig.3.2. Load rating curves for the Type 4809
._. & K,lc.r BrUel & Kjcar
·.u.l & KJao<
~ij-~ & Kfao~ a -Potontl om••• • ••
0 0
50
Copenhagen
aooaccoccooooooooaoccaaoaoooooaoco
M.asurinQ Obj .:
Cross-Motion
~~·,;vpe 3Cl~15 '3:1 i%
SOg AM S
-1( 0. 1%
~
0...0· 24 2 72
Sign ,,
"'R Rectifi er : A~~ lCIIIIlW lift\. Freq .:~ z Wr. S~ : 25 mmJ~ . Paper Spwd : 1 mmJwc. ...,c
: 8-160
' 16-145
Qp 1124
~lt i ply :~ • ..::: 50 by 100 200 5000
oiOOOO ~ ~ ! .c Li n. o.l
0
I
Fig.3.3. Cross-motion of the Type 4809
12

3.3. CROSS-MOTION
If an Exciter is to be used for accelerometer calibration, it is important
that its motion be as rectilinear as possible, to avoid adding any error to the
calibration. The cross-motion of the Type 4809 is very low, as can be seen
in Fig.3.3, where cross-motion, expressed as a percentage of the axial
motion,
is plotted against frequency for an acceleration level of 50 g RMS.
Except for a few isolated peaks, the cross-motion is well below 1%
throughout the frequency range of the Type 4809.
13

4. APPLICATIONS
4.1. ACCELEROMETER CALIBRATION
By attaching a Type 8305 Standard Accelerometer to the table of the
Type 4809 and then mounting the accelerometer to be tested on top of the
Type 8305, back-to-back comparison calibration may be performed. Fig.4.1
shows a typical calibration setup using
the Type 2970 Sensitivity Com-
parator.
Conditioning
Amplifier
2626
Sine- Random Generator
Type 1024
:_·~· - ;
; - ~
· · - -~ ~ - ! -•-
Standard
Accelerometer
Signal
Control
Power Amplifier 2706
Conditioning
Amplifier
2626
;.=-,;;__---f:lo:l.l:i--.
Conditioned
Unknown Signal
Level Recorder
2305
---.a.J~, ...
----0 I _=
I I
Vibration Exciter 4809
272//i'
Fig.4. 1. Accelerometer Calibration
The signal from the Standard Accelerometer is used to control the com-
pressor loop
in the Generator, which means that it determines the vibration
14

level of the Exciter itself. The output from both accelerometers goes
through the Sensitivity Comparator, which compares the two outputs and
indicates
the difference on a meter scale. The gain of the Conditioning
Amplifier on the unknown line is adjusted until this meter reading falls to a
minimum. A careful balancing
and adjusting procedure is necessary to
ensure the highest possible accuracy. This procedure is thoroughly covered
in
the Type 2970 manual and will not be gone into here.
4.2. MECHANICAL IMPEDANCE
Mechanical impedance can be defined as the resistance of an object to
being set in motion. A dynamic force applied to an object will result in a
certain velocity,
in accordance with the equation:
F
z v
where F is the force, V the velocity and Z the mechanical impedance
vectors.
In the equation, the quantities are stated as vectors, as in the majority of
cases there will be a phase difference between the applied force and the
resulting velocity. On the other hand, it is not always necessary to consider
this phase difference when making measurements. The numerical value of
mechanical impedance is often all that is required, and, since Z is pro-
portional to 0, a plot of mechanical impedance can be obtained by holding
the velocity constant and varying the frequency.
Fig.4.2 shows the basic setup for measuring the numerical value of
mechanical impedance. The signal from the Beat Frequency Oscillator
Type 1022 is amplified in the Power Amplifier Type 2706 and fed to the
Vibration Exciter. Between the Exciter and the measuring point on the test
structure is a Type 8001 Impedance Head which has a force gauge and an
accelerometer built in. The acceleration output is integrated in the
Type 2625 and the resulting velocity signal is passed on to the Type 2608
Measuring Amplifier. Here the velocity level can be monitored as the signal
is amplified and fed back to the Generator. The compressor loop thus
formed holds the vibration level at a constant velocity.
With the velocity held constant in this way, the force signal can then be
amplified and fed into the Type 2307 Level Recorder. A plot of the
mechanical impedance versus frequency can thus be obtained.
15

~ 0
• 1 ~ 1 :: e :;
i i ~
Level Recorder 2307 ;
(W •f!!1-+ . ~ --+-----'v'---1
••• ;.e.;;,·!-
- ~ _ ; _ •
Power Amplifier 2706
Impedance Head 8001
Vibration Exciter 4809
~===========P=re=am~p=lif~ier-2-62~3--~-----4~
Fig.4.2. Mechanical Impedance measurement
4.3. FORCE CONTROLLED TESTING
27.Zt'1'6
For structural testing it is the general rule to control the Exciter with a
force-controlled compressor loop such as that shown in Fig.4.3.
16
Exciter 4809 /
Force Transduc er
8200
Fig.4.3. Force-controlled structural test
Z721Z/

The Power Amplifier Type 2706 amplifies the signal from the Beat
Frequency Oscillator
Type 1022 and feeds it to the Exciter. The signal from
the Force Transducer is boosted in the Conditioning Amplifier Type 2626
and fed to the compressor circuit of the Oscillator. In this way, the force
level of the shaker system can be held constant throughout a frequency
sweep regardless
of table and specimen resonances, provided suitable com-
pressor speeds and sweep speeds are selected.
The Force Transducer Type 8200 has a nominal charge sensitivity of
17.8 pC/Ibf. (4 pC/N). While there are no direct reading force scales for the
Measuring Amplifier, a proper choice of acceleration scales will accomplish
the same purpose. The SA 0071 scale is for accelerometers with a sensitivity
of 6-17 mV/g. Since the output of the Type 2626 is 10 mV/g once the
sensitivity of the transducer is dialed in, the SA 0071 could be used to read
Newtons
or pounds instead of g.
17

5. SPECIFICATIONS
The following specifications apply when the Exciter Type 4809 is
powered with the Power Amplifier Type 2706.
RATED FORCE:
Dynamic:
Acceleration Limit:
Current Limit:
Displacement Lim it:
Velocity Limit:
10 lb. (44.5 N), except as limited by dis-
placement limits, as shown on Fig.3.2.
75 g (736 m/s2)
5 Amp.
RMS
0.315 in. (8 mm), peak-to-peak
65 in/s (1.65 m/s), peak
Moving Element
Dynamic Weight: 0.132 lb. (60 gm.)
Resonance Frequency Unloaded
Table: 20 kHz
Stray Magnetic Field: 20 X 1o-3 Weber/m2 at table surface
MechanicaiFuselnserts:
10-32 UNF center insert in 1.16 in.
(29.5 mm) diameter table
Cross Motion: less than 3% of the axial motion to
15kHz
Dynamic Flexure Stiffness: 112.4 lb/in. (20 N/mm)
Dimensions: Diameter 5.87 in. (149 mm)
Height 5.63 in. (143 mm)
18