BRUEL & KJAER 4824 User manual

Technical
Documentation
HEADQUARTERS: Brüel&Kjær Sound & Vibration Measurement A/S · DK-2850 Nærum · Denmark
Telephone: [phone] · Fax: [phone] · www.bksv.com · [e-mail]
Local representatives and service organisations worldwide
Modal Exciter Type 4824
English BE1666–12
User Manual
ËBE-1666---CÎ

BE166612 October 2011
Modal Exciter Type 4824
User Manual

Trademarks
Neutrik and speakON are registered trademarks of Neutrik AG
Copyright 2001 – 2011, Brüel&Kjær Sound & Vibration Measurement A/S
All rights reserved. No part of this publication may be reproduced or distributed in any form, or
by any means, without prior written consent from Brüel&Kjær Sound & Vibration Measurement
A/S, Nærum, Denmark
Safety Considerations
This apparatus has been designed and tested in accordance with IEC/EN61010–1 and
ANSI/UL61010– 1 Safety Requirements for Electrical Equipment for Measurement,
Control and Laboratory Use. This manual contains information and warnings which must be
followed to ensure safe operation and to retain the apparatus in safe condition. Special note should
be made of the following:
Safety Symbols
The apparatus will be marked with this symbol when it is important that you refer to the
associated warning statements given in the manual.
Protective Earth Terminal Hazardous Voltage
Explosion Hazard
The equipment is not designed to be used in potentially explosive environments. It should not be
operated in the presence of flammable liquids or gases.
Warnings
• Switch off all power to equipment before connecting or disconnecting their digital interface.
Failure to do so could damage the equipment
• Whenever it is likely that the correct function or operating safety of the apparatus has been
impaired, it must be made inoperative and be secured against unintended operation
• Any adjustment, maintenance and repair of the open apparatus under voltage must be
avoided as far as possible and, if unavoidable, must be carried out only by trained service
personnel
• Do not dispose of electronic equipment or batteries as unsorted municipal waste
• It is your responsibility to contribute to a clean and healthy environment by using
the appropriate local return and collection systems
• Hazardous substances in electronic equipment or batteries may have detrimental
effects on the environment and human health
• The symbol shown to the left indicates that separate collection systems must be used
for any discarded equipment or batteries marked with that symbol
• Waste electrical and electronic equipment or batteries may be returned to your
local Brüel&Kjær representative or to Brüel &Kjær Headquarters for disposal

Table of Contents
CHAPTER 1
Introduction......................................................................................................... 1
CHAPTER 2
Operation............................................................................................................. 3
2.1 Preliminary........................................................................................................... 3
2.2 System Checks and Connections........................................................................ 3
2.3 Mounting and Use of Modal Exciter Type 4824................................................... 5
2.4 Connecting Structure, Force Transducer and Modal Exciter............................... 5
2.5 Practical Measurement Considerations............................................................... 7
CHAPTER 3
Operating Characteristics.................................................................................. 9
3.1 Performance Limits.............................................................................................. 9
3.2 Moving Elements ................................................................................................. 9
CHAPTER 4
Specifications................................................................................................... 11
CHAPTER 5
Service and Repair........................................................................................... 13

1
Chapter 1
Introduction
Designed for demanding modal testing applications, Modal Exciter Type4824 provides
precise, reliable, stable and long-lasting operation. Highest quality materials, stringent quality
control and rugged construction assure a versatile means of modal excitation for any modal test
requiring attached excitation.
Based on unique, rare-earth neodymium magnet technology, Modal Exciter Type 4824 features
extremely small physical dimensions relative to the 100N force rating, along with low total
weight and a low-mass, high-rigidity, spring-suspended armature.
Power Amplifier Type2732 has been designed especially to drive Modal Exciter Type 4824
safely to full rating.

Modal Exciter Type 4824 – User Manual2

3
Chapter 2
Operation
2.1 Preliminary
2.1.1 Environment
Modal Exciter Type4824 is designed for operation at ambient temperatures between 5°C and
40°C (41°F to 104°F).
2.2 System Checks and Connections
Before operation, the following system checks and connections should be carried out to ensure
the correct function and safe operation of the apparatus. The connections are shown in Fig.2.1.
Fig.2.1 Typical setup for operation of Type 4824
DC Static Centering Unit 1056
Mains
Power
Mains
Power
Mains
Power
from Controller
010249/5
Blower UH-1035
9-pin sub-D
Female/Female
AQ-0651
Drive Cable Power Amplifier 2732 with 4824
or Power Amplifier 2720 with 4825
Sensor Cable
Air Hose
AF-1103 BNC – BNC
AO-0087
DC Input
4824
or 4825

Modal Exciter Type 4824 – User Manual4
Note:
DC Static Centering Unit Type 1056 is used as an optional unit to apply a pre-tensioning force
to the tension wire (if such a type of stinger is used – traditional push/pull stingers do not
require any pre-tensioning) between the modal exciter and the input transducera attached to the
structure under test.
2.2.1 Connection to Power Amplifier
The exciter's power socket accepts a 4-pin Neutrik®speakON®plug. The pin connections are
shown in Fig.2.2.
Fig.2.2 
Input socket for Type 4824
(Internal view)
Neutral
(coil finish)
Power output
(coil start)
Chassis
-1
-2
+2
+1
010112
For connection to matching Power Amplifier Type2732, use Drive Cable AQ-0649 provided
with the exciter. The pin connections are shown in Fig.2.3
Fig.2.3 
Drive cable AQ-0649.
Soldering side of plug
shown
Brown
Yellow/Green
Blue
Black
-1
-2
+2
+1
-1
-2
+2
+1
010113/2
AQ 0649: Soldering side of Plug shown
2.2.2 Connection of DC Static Centering Unit Type 1056 (Optional)
For use with DC Static Centering Unit Type 1056, the sensory input related to the armature
position is provided directly from the modal exciter via the 9-pin Sub-D interface. For
connection to the modal exciter, use the cable provided with Type 1056.

CHAPTER 2
Operation 5
2.2.3 Grounding Considerations
When using the exciter in complex measurement setups, indiscriminate grounding of the
instrument signal ground lines may introduce mains hum. This can be avoided by ensuring that
the signal ground line of the entire measurement setup is grounded at one point only. To do this
without prejudicing the operating safety of the instruments involved:
1) Connect the signal ground lines of all instruments together. This is done automatically
through the screens of the input and output cables used to interconnect the instruments.
2) If instruments equipped with a mains socket chassis terminal are employed in the
measurement arrangement then check that: a) one and only one of these instruments has its
signal ground line connected via chassis to mains ground, b) the housing of the
measurement transducer is isolated from grounded measurement sources.
3) If the measurement arrangement is free-standing without instrument cabinets touching,
then ensure that each of the other instruments in the measurement arrangement has its
chassis connected to mains ground only or chassis only. Mains ground is preferred for
instruments with a mains socket chassis terminal.
2.2.4 Operating Procedure
After making the system checks and connections described, please read the instructions for the
operation of Signal Generator or Controller, Power Amplifier and DC Static Centering Unit
before commencing operation.
2.3 Mounting and Use of Modal Exciter Type 4824
Modal Exciter Type4824 may be used in any position and angle relative to the structure under
test. Mounted in its trunnion, the exciter may be placed directly on the floor. Some
applications, such as the exciter being suspended at some height from ground – and used
laterally, horizontally or at a skewed angle – may require use of dedicated suspension
techniques. For convenient lateral excitation up to a height of 2 meters, Lateral Modal Exciter
Stand Types UA-1607 and UA-1608 are available.
The “hole-through” design makes it possible to use tension wire stingers or traditional push/
pull stingers with the exciter. Easy and fast attachment of both types of stingers is achieved
with the chuck nut assembly or with an M6 to 10–32 UNF threaded insert.
2.4 Connecting Structure, Force Transducer and Modal
Exciter
Any force transducer/impedance head is – to a certain degree – sensitive to unwanted
environmental effects as well as to bending moments and transverse movements. The purpose
of the stinger, that is, the component that connects the modal exciter to the force transducer/
structure, is to faithfully transmit axial forces, as well as to isolate and decouple the force
transducer/impedance head from side loads and bending moments. For optimum test results,
we recommended using a stinger in every setup.

Modal Exciter Type 4824 – User Manual6
Two types of stinger technology exist; tension (piano) wire-based stingers and push/pull
stingers. Modal Exciter Type 4824 accommodates both these types of stingers.
2.4.1 Tension Wire
A thin metal (piano) wire will have zero compression and negligible bending stiffness. A
tension wire test setup is therefore utilised in such a way that there is a constant tension in the
wire, upon which the oscillatory force is superimposed. A piezoelectric-based force transducer/
impedance head is by definition an active transducer that requires a dynamic force input in
order to give an output. Hence, the constant tension provides no output from the input
transducer and only the oscillatory part of the signal will be measured.
Accomodating a tension wire with Modal Exciter Type4824 is achieved by using a “hole
through” design of the exciter and trunnion, whereby the modal exciter can be “slid” along the
wire. A chuck assembly that clamps the exciter to the wire at the desired position replaces the
traditional exciter table.
Inconnectionwithoneofthe(optional)LateralModalExciterStandTypesUA-1607orUA-1608,
thetensionwireconceptbecomesespeciallysimpleandelegant.Thetensionwiregoesthroughthe
exciter, then goes around the pulley and connects to the spring fastened to the base of the exciter
stand. The turnbuckle, positioned in between the spring coil and the base, achieves adjustable
pretensioning.
In a practical setup, first connect the force transducer/impedance head to be used in the test on
the chosen input location on the structure.
2.4.2 DC Static Centering
Pre-tensioning of a tension (piano) wire stinger can also be achieved using the (optional) DC
Static Centering Unit Type 1056. This allows for use of piano wire stinger technology without
spring-based mechanical pretensioning found on dedicated horizontal exciter stands such as
Lateral Modal Exciter Stand Types UA-1607 or UA-1608.
The use of DC Static Centering Unit Type 1056 becomes especially useful when limited space or
skewed angle mounting is necessary. In both cases, the size and form of UA-1607 and UA-1608
may prevent it from being successfully installed. Using the DC Static Centering Unit means that
ModalExciter Type4824 – mountedinitstrunnion –canbe placed inanypositionandangle, and
useofatension(piano)wire can stillbeaccomplished.
2.4.3 Push/Pull Stingers
Traditional push/pull stingers are normally connected to Modal Exciter Type4824 using one of
the UA-1612 threaded adaptors that come as a standard accessory with the exciter. One end of
this adaptor screws into the female M6 threaded end of the exciter rod while the other end
accepts a 10–32 UNF threaded stud.
One of the most important considerations when choosing a push/pull stinger is its operating
frequency range. This is determined by its length, stiffness, velocity of sound in the particular
stinger material and the compliance of the structure under test.

CHAPTER 2
Operation 7
2.5 Practical Measurement Considerations
2.5.1 Distributing and Minimising Dynamic Force Levels
There are several reasons why exciting the structure under test with the smallest possible force
level, at more than one input location, is desirable. Firstly, too high force levels may drive the
structure into non-linear behaviour. Secondly, higher force levels require larger exciters, which,
inherently, have more massive armatures leading to increased force drop-offs at resonance
frequencies. Thirdly, large and complex structures that exhibit local modes generally require
force inputs at several distributed locations in order to excite all modes sufficiently well.
Finally, “repeated roots”, i.e., two modes at the same frequency may occur in certain, mostly
bisymmetrical, structures. The only way to obtain a valid modal model in such a situation is by
employing a modal test setup with multiple inputs, that is, a Multiple Input Single Output
(MISO) or Multiple Input Multiple Output (MIMO) test strategy1.
2.5.2 Minimising Exciter/Test Structure Interaction
Effective mechanical decoupling of the structure under test and the modal exciter(s) is required
to minimise the mechanical impedance “disturbance” caused by the exciter(s). At resonance
frequencies, where modal parameters are subsequently being extracted, force drop-offs are
inevitable2, thereby decreasing the accuracy of the mode shape displacement determination.
Force drop-offs are caused by the fact that, at resonance, the structure becomes highly
compliant. The exciter may then use all the available energy to accelerate its own mechanical
components, leaving no force with which to drive the structure under test. Very little force is
therefore being put into the structure and the signal level of the force may then drop towards
the normal noise level in the instrumentation.
The lighter the mass of the moving element in the exciter, the lesser this problem will be.
Modal Exciter Type4824 has been fitted with low weight, and precision machined magnesium
armatures that ensure minimal problems with force drop-offs.
1Note that any multiple input test strategy will require a dedicated MIMO Frequency Response Function
estimator. The FFT analyzer that will be utilised as part of the test setup will need this special FRF
estimator in order to work.
2In addition to the weight of the armature, other parameters will influence the degree of force drop-off
experienced. In general, lightly damped structures will be more susceptible to force drop-offs than heavily
damped structures. Also, driving the exciter with the power amplifier in current feedback mode will
significantly minimise force drop-offs.

Modal Exciter Type 4824 – User Manual8

9
Chapter 3
Operating Characteristics
3.1 Performance Limits
Fig.3.1 Sine performance curves for Modal Exciter Type 4824
1000
100
10
1110 100 1000 10000
Frequency (Hz)
Accelleration
Type 4824
44g
30g
14g
8g
1.5m/s
25.4mm
m= 0Kg 44g
m= 0.1Kg 30g
m= 0.5Kg 14g
m= 1Kg 8g
010241
3.2 Moving Elements
The low armature weight helps to ensure high-quality force measurements by minimising force
drop-offs at the test specimen’s resonance frequencies. Four upper radial flexures and four
lower radial flexures, the latter providing an additional guide for best stabilisation, form a
strong rectilinear guidance system which keeps the driver coil perfectly centered in the
magnetic assembly’s air gap. In transverse directions and in torsion, the flexure system
provides very high stiffness to counteract rotational movement of the test specimen. Also,
through this configuration, the modal exciter can absorb high lateral forces without damage to
the exciter’s internal construction.

Modal Exciter Type 4824 – User Manual10

11
Chapter 4
Specifications
Compliance with Standards
,
CE-mark indicates compliance with: EMC Directive and Low Voltage Directive.
C-Tick mark indicates compliance with the EMC requirements of Australia and
New Zealand
Safety EN610101 and IEC610101: Safety requirements for electrical equipment for measurement, control and laboratory use.
UL31111: Standard for Safety Electrical measuring and test equipment
EMC Emission
EN 500811: Generic emission standard. Part 1: Residential, commercial and
light industry.
EN 500812: Generic emission standard. Part 2: Industrial environment.
CISPR22: Radio disturbance characteristics of information technology
equipment. Class B Limits.
FCC Rules, Part 15: Complies with the limits for a Class B digital device.
EMC Immunity
EN 500821: Generic immunity standard. Part 1: Residential, commercial and
light industry.
EN 500822: Generic immunity standard. Part 2: Industrial environment.
Note 1: The above is guaranteed using accessories listed in this Manual only.
Temperature IEC60068 21 & IEC6006822: Environmental Testing. Cold and Dry Heat. Operating Temperature: +5to+40C (+41to+104F)
Storage Temperature: 25to+70C (–13to+158F)
Humidity IEC6006823: Damp Heat: 90% RH (non-condensing at 40C (104 F))
Mechanical
Non-operating:
IEC6006826: Vibration: 0.3mm, 20m/s2, 10500Hz
IEC60068227: Shock: 1000m/s2
IEC60068229: Bump: 1000 bumps at 250m/s2
Enclosure IEC60529 (1989): Protection provided by enclosures: IP20

Modal Exciter Type 4824 – User Manual12
Specifications
MATCHING POWER AMPLIFIER
Type 2732
RATED PEAK FORCE (SIN/RANDOM):
100/70N
USEFUL FREQUENCY RANGE
2–5000Hz
OPERATING FREQUENCY RANGE
DC – 5000Hz
MAX. RATED TRAVEL
25.4mm (1 inch)
MAX. VELOCITY (SINE/RANDOM)
1.5/1.5m/s
MAX. ACCELERATION (SINE/RANDOM)
471/333m/s2(48/34g)
RATED CURRENT
5.5A
SUSPENSION STIFFNESS
4N/mm
EFFECTIVE MOVING MASS
0.23kg (0.51lb)
MAIN RESONANCE FREQUENCY
>6000Hz
WEIGHT WITH TRUNNION
21kg (46.3lb)
DIMENSIONS
See Fig.4.1
Fig.4.1 Dimensions of Modal Exciter Type 4824
220
180
M6
10
10
M6
63
63
100
68.9
M8
306
ø200
ø10
76
142.4 99.1
ø7
226
010123

13
Chapter 5
Service and Repair
Modal Exciter Type 4824 is designed and constructed to provide many years of safe, reliable
operation. However, if the correct function or operating safety of the instrument is impaired,
disconnect it immediately from the mains source and secure it against unintended operation.
For repair, contact your local Brüel& Kjær representative. Under no circumstances should
repair be attempted by persons not qualified in electronic instrumentation service.

Modal Exciter Type 4824 – User Manual14