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