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CHAPTER - 4
EXPERIMENTAL TEST SETUP
An experimental test setup is required to conduct
environmental vibration tesing on advanced launch vehicle
subsystems. The existing vibration test facility is having capability
to produce a force of 16t by using single shaker system. This
capacity of single shaker system is not sufficient to meet the
advanced launch vehicle reqiurements. Hence, The facility is
augmented to meet the advanced launch vehicle requirement. As
part of the augmentation, two single shakers cofigured as dual
shaker mode with 30t capacity. A dual control system is
commissioned to operate the dual shaker system. A load bearing
platform which acts as a common platform for specimen assembly
is used to make vertical test setup. A large slip table of size 3.0m x
3.4m is commisioned to assemble the test specimen for horizontal
testing. In addition to these modifications two new power
amplifiers are also commissioned to get higher efficiency.
4.1 TEST SETUP CONFIGURATION
The test setup configuration for simulation and control vibration
levels on advanced Launch Vehicle subsystems is shown in the
fig.4.1. It is basically a closed loop feedback control system. It
80
consist sub-systems like Electrodynamic shakers, vibration
controller, power amplifiers, dual mode controller, control
accelerometers and data acquisition.
The required test profile is programmed into PC based
control system. The vibration controller generates and feeds a low
level signal into the power amplifier based on the test
specifications. This signal is amplified by the power amplifier and
drives the armature of the shaker in case of single shaker mode.
The power amplifier is an air-cooled, modular type with very high
efficiency. The power amplifier output is connected through a
matching transformer to the armature of Electrodynamic shaker.
In case of dual mode operation of two Electrodynamic shaker
system in a synchronized mode .i.e. in a same phase, a dual
control system is used. Dual control system consists of Multi
Amplifier Controller (MAC), Difference Monitor Unit (DMU), Phase
Control Unit (PCU) and Current Monitoring Unit (CMU). The MAC
allows operation of maximum of 4 Nos of Electrodynamic shakers
simultaneously. The DMU monitors the difference in the load
currents (armature currents) through CMUs and in conjunction
with the PCU. It corrects the amplitude and phase of the multi
shaker load currents within the specified tolerances. Ultimately,
81
the MAC allows a smooth and synchronized operation of Dual-
amplifiers, in turn the operation of Dual shaker systems.
Finally the feedback signals from the shakers, test specimen,
via a control accelerometer and charge amplifier reaches the
vibration controller. The feed- back signal is measured, digitized
and compared with the specified control spectrum. Then the drive
signal is adjusted, if any corrections are required, to change the
input signal to the shaker through power amplifier to maintain the
required /specified test profile.
Piezoelectric accelerometers are used to sense the vibration
level during the tests. The signal from the accelerometer is
conditioned with the help of a charge amplifier, whose output is fed
to the control system. The location of control and monitor
accelerometers are decided by complexity of the structure. The
vibration channels are electrically calibrated by simulating charge
signal equivalent to 50g into the measurement chain. The
calibration ceiling for the channels was set at 50 g-peak. The
charge amplifiers output is recorded in the magnetic tape recorder
and in the computer system. In case of sine vibration tests, D.C
proportional to „g‟ peak and D.C proportional to log frequency are
acquired in the computer system. In case of random vibration test
A.C signals proportional to grms are recorded in the magnetic tape
82
recorder. Strains at critical locations of the fixture are measured by
bonding strain gauges. The strain measurement is used to verify
the design calculations.
CONTROL SYSTEM
AT DARC
DUAL SHAKER CONTROL
SYSTEM IN PA BAY
SH
AK
ER
1
SH
AK
ER
2
DU
AL
SH
AK
ER
SY
ST
EM
WIT
H F
IXT
UR
E F
OR
GS
LV
Mk
3
CO
NT
RO
L
AC
CE
LE
RO
ME
TE
R
O/P
MAU
DMU
PCUF
eed
back
, A
rmat
ure
Cur
rent
Am
plitud
e &
Pha
se
POWER AMPLIFIER 2 (PAII)
CMU 2
CMU 1
POWER AMPLIFIER 1 (PAI)
MAC- Multiple Amplifier Controller
DMU- Differential Monitoring Unit
PCU- Phase Control Unit
CMU- Current Monitoring Unit
Dri
ve S
igna
l
DUAL SHAKER SYSTEM
Fig. 4.1 Dual shaker experimental test setup
4.2 ELECTRODYNAMIC SHAKER
In Principle the electrodynamic shaker operates like a
loudspeaker. The voice coil of a loudspeaker pushes and pulls a
cone in and out causing sound pressure waves. In a shaker it is
83
the armature coil that moves in and out, causing vibration. When
an electric current passes in a coil it produces a magnetic field
around it. This is the basic principle of electromagnetism. English
physicist John Fleming devised the left hand rule to recall the
relative directions of the magnetic field, current, and motion in an
electric generator or motor. The three directions are represented
by the thumb (for thrust or motion), forefinger (for field), and
second finger (for current direction), all held at right angles to each
other. The Armature force in our shaker is directly proportional to
the current in the coil.
F = B I L
F is the force in Newtons
B is the magnetic flux density
I is the current in Amperes
L is the coil length in metres
The magnetic flux density can be thought of as the
concentration of field lines. The force can be increased by
increasing any of the terms within the equation. In a shaker the
Armature coil responds to the output of controller signal which has
been amplified. In a small shaker there is a permanent static
magnet that will attract or repel the magnetic field of the coil and
cause movement by pulling or pushing. If the two magnetic fields
84
are lined up then south to north attracts and north to north repels.
The size of the armature will affect the frequency range of testing.
Fig. 4.2 Electrodynamic shaker constructional details
Newton‟s third law of motion states; “every action has an
equal and opposite reaction” Therefore when vibration occurs
vertically, the amount of thrust to move the test item will react
against the building floor. To prevent damage and vibration to the
surrounding area the Vibrator must have isolation. One method is
to construct a seismic reaction mass below the shaker installation
point. This mass is at least 10 times the force rating of the system.
Many electrodynamic vibration systems already have a form of
isolation. The body is mounted on a spring system, typically air
85
bags that hold the body in a mid position using adjustable air
pressure. As the body reacts to the vibration test there will be
some displacement related to the mass ratios between body and
payload. The amount of vibrator body movement can be calculated
by knowing the displacement of the test, the total moving mass of
the setup and the body mass. The stiffness of the isolation system
tends to give a natural resonant frequency at low frequencies.
Normally this Fn is between 2.5 Hz and 5 Hz.
Fig. 4.3 Suspension system of shaker
86
Table 4.1 Single shaker specifications
Force rating 16,000 kgf
Operating frequency 5 -2000 Hz (Sine)
20 – 2000 Hz (Random)
Maximum acceleration 100 g peak (bare table)
Max. Velocity 1.7 m /sec
Max. Displacement 25 mm p-p
Max. Noise level 0. 1 g
Make LDS, UK
Model V980C
Type Electrodynamic shaker
4.3 DUAL SHAKER SYSTEM
Dual shaker system is capable of producing more force to
drive the specimen. The use of dual shaker system has certain
advantages. It is not practical to provide a fixture for a large
specimen if one single shaker is used. Often, several shakers can
even be arranged to eliminate the fixture, with definite cost
savings. One large shaker would have a reduced frequency
response, and would be inordinately expensive. There is a further
87
option that the several shakers can be used separately and
individually for the testing of smaller specimens, when not needed
to form one large single testing system.
The dual shaker system consists of two electro dynamic
shakers of 16t force rating each. These two shakers are coupled by
a common coupling platform and operated in parallel to get a net
force rating of 30t. The shaker cooling system consists of two
cooling units for armature cooling and for increasing the force
rating. A compressed airline is provided for supplying compressed
air to the dual shaker load support system, shaker isolation
system.
Two 16t shakers can either be used individually for testing sub-
systems or in dual shaker configuration to enlarge the capacity of
the facility. The shakers are electrically powered with switching
amplifiers and controlled using a digital vibration control system.
The dual shaker facility can efficiently and safely test spacecraft
with a mass of up to 6000 kg in vertical and 20000 kg in
horizontal direction. The dual shaker system is mounted on a 550t
seismic block supported by pneumatic springs so as to minimize
reaction forces to building. In the 30t dual shaker mode, tests can
be performed in both vertical and horizontal configurations, thus
88
making it possible to simulate the effect of launch vibrations in the
three orthogonal axes of the spacecraft.
Table 4.2 Dual shaker system specifications
Force rating 30,000 kgf
Operating frequency 5 -2000 Hz (Sine)
20 – 2000 Hz (Random)
Maximum acceleration 30 g peak (bare table)
Max. Velocity 1.7 m /sec
Max. Displacement 25 mm p-p
Max. Noise level 0. 1 g
Make LDS, UK
Model V980C
Type Electrodynamics
No of shakers 2
4.3.1 Dual shaker system in vertical configuration
The vertical configuration is achieved by coupling the
armatures of the shakers by means of a magnesium dual head
expander with an outside diameter of 2 m. The total mass of the
magnesium dual head expander is 200 kg. Special emphasis is put
on the design of the alignment and guidance system of the Dual
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shaker assembly. It avoids exceeding allowed tolerances on lateral
displacement between stationary and moving parts. The dual head
expander is judiciously ribbed to provide a good rigidity/ mass
ratio and also to provide good transmission of the forces delivered
by the two shakers. It is provided with an additional pneumatic
load-compensation device which gives the assembly a dynamic
load capacity of 6000 kg. The load is mechanically attached to the
dual-head expander by means of M12 inserts arranged in a matrix
pattern.
Fig 4.4 Dual shaker system in vertical configuration
90
4.3.2 Dual shaker system in horizontal configuration
The horizontal configuration is achieved by means of a large
slip table [157] of size 3.4 m by 3 m and having a 40 mm thick
magnesium plate. This plate is guided by means of 39 hydrostatic
bearings [169]. It is fixed to the armatures of the shakers by
magnesium driver bars. In this configuration, the total mass of the
moving element is 2000 kg. As is the case for the dual-head
expander, the load is attached to the large slip table by means of
the M12 insert matrix.
Fig. 4.5 Dual Shaker system in horizontal configuration
91
Table 4.3 Large slip table specifications
Max. Payload Capacity 60000 kg
Max. Over turning moment 1560kNm
No. of hydrostatic bearings 39 (7 fixed, 32 free)
Weight of slip table, drive bar 1646 kg
Type Oil film type
Make M/s LDS, London
Size of the table 3400 x 3000 mm
4.4 INSTRUMENTATION & CONTROL SYSTEM
The instrumentation, control and power amplifier systems at
vibration test facility support the controlled operation of 16t single
shakers or 30t dual shaker systems during the vibration tests, and
also acquire and analyse the vibration data of the specimen during
the tests. The systems can be operated from a remote location for
facilitating vibration testing of explosive specimens.
The vibration test facility is having two 16t shakers of electro
dynamic type. Each shaker is driven by 200kVA power amplifier to
generate the required force. The shakers are operated in closed-
loop mode to control the vibration level at the chosen control
92
location. The types of vibration loads that can be simulated are
Sine, Sine dwell, Random and Shock in all three axes of the test
specimen.
The major subsystems are two power amplifier systems each of
200kVA power output, Cooling/Field Power Supply systems, Dual
shaker operating system, 40 channel control/data acquisition
system and 40 channel signal conditioning system consisting of
vibration sensors and charge amplifiers.
4.4.1 Power Amplifier
The power amplifier is similar to an audio power amplifier and is
meant for amplifying the low-level signal from control system to
high power. It drives the shaker to the required vibration as per
the test requirement. The amplifier is a modular, air-cooled, Class
D type switching amplifier with nominal efficiency of 90%. The
amplifier‟s output is coupled to the shaker through a matching
transformer.
The amplifier consists of one control bay and three power module
bays. The control bay houses Preamplifier, Control, interlock and
interface PCBs, and each power module bay houses HT DC Power
supply, auxiliary power supplies and eight power amplifier
modules each of 8 kVA power output. The amplifiers input signal is
conditioned and then amplified by the power modules to provide
93
the high power necessary to drive the vibrator‟s armature. The
amplifier‟s output can be manually controlled by the master gain
located on Local Control Panel or Remote Control Panel.
Complete system monitoring is provided by the amplifiers interlock
control circuitry. If any parameter of the vibrator, cooling/field
power supply, power amplifier fails the amplifier will shutdowns in
a controlled manner and an interlock is displayed. The important
internal safety interlocks of the power amplifier include output
over voltage, output over current, module fault, HT level, Auxiliary
supply etc.
As shown in the overall block diagram (Fig.5.5) the amplifier is
interfaced with shaker and field power supply/cooling system for
auto switch on and interlocks monitoring. It is connected to
Remote Control Panel (RCP) through fiber optic link for ensuring
remote operation. It is also interfaced with Dual Shaker Operating
(DSO) system for ensuring switch on and interlocks monitoring
during dual shaker system operation.
Principle of operation:
Class D Power amplifier is basically a switching amplifier or PWM
amplifier and it is intended to achieve highest possible efficiency
and reliability while retaining the other best qualities of
conventional linear power amplifier. In this amplifier the signal to
94
be amplified modulates a PWM carrier, and the PWM carrier drives
the power output devices to either completely ON or OFF condition,
thereby reducing the power losses in the output devices. Power
MOSFETs are the normally used power devices and IGBT are
gaining wider application since they are offering the advantages of
MOSFETs and BJT. The output of the power devices is passed
through a low pass filter which removes the high frequency carrier
and outputs amplified input signal.
Table 4.4 Specifications of Power amplifier
Amplifier type Class D switching
Power output 200 kVA (100Vrms, 2000Arms)
Frequency range 5 Hz to 3 kHz
Input impedance 10k Ohms
CMMR 100 dB
SNR > 65 dB
Harmonic distortion < 2%
No. of power modules 25, each one of 8 kVA power
output
Make LDS,UK
95
Table 4.5 Specifications of matching transformer
Type auto-wound
Power output 192 kVA
Input voltage 100 V rms
Output voltage
Tap -1(Sine)
Tap -2(Random)
134 V rms, 1433 A rms
173 V rms, 1112 A rms
Make LDS,UK
4.4.2 Dual shaker operating system
The dual shaker operating system ensures the operation of the
two independent shakers in in-phase and at same amplitude by
compensating the character differences of individual shakers, and
also monitors drive currents of both shakers for finding phase and
amplitude difference, and shuts down the system in case of
exceeding the set threshold levels. The use of DSO enables both the
shakers operating in tandem and force output equals the sum of
both the shakers, thus making it possible to test larger specimens.
The dual shaker operating system as shown in Fig.4.6
consists of MAC, DMU and PCU4. Using DSO the two shakers can
be set to be used in individual mode (perform test on shaker 1, or on
96
shaker 2) or in dual mode where both shakers are coupled to a
common payload in the push-push configuration.
4.4.2.1 Multiple Amplifier Control Unit (MAC)
The MAC is used to control the two power amplifiers and their
corresponding shakers from the vibration test facility. MAC is
interfaced to power amplifiers and provides the overall system
control and display functions required to operate safely and
conveniently both the shakers. The vibration control system‟s
output is connected to power amplifiers through MAC. The output
Gain of each power amplifier is controlled with the Master Gain
located on MAC. MAC provides the following basic functions.
o Interlock cross-coupling
o Emergency stop cross coupling
o Input signal gain, splitting/inversion
o Synchronised switch on /switch off and status display
o Selectable low-pass filter
o Interlock inputs from DMU, PCU4 and external sources
4.4.2.2 Phase Control Unit (PCU)
PCU allows adjustment of the magnitude and phase of the current
supplied to each vibrator so that the force generated by the vibrators
is same. The adjustment of PCU is carried out in four stages i.e.,
setting of loop gain, fine adjustment to ensure current to the
97
vibrators is identical in amplitude and phase, fine adjustments to
compensate for transformer ratio etc., and acceleration
measurements.
4.4.2.3Difference Monitoring Unit (DMU)
The DMU compares the armature drive currents of both the
vibrators and shuts down the system if they are not equal within
defined limits.
The DMU performs the following functions
o Accepts and acknowledges system operating mode from MAC
o Calibration of armature currents
o Measures the armature current for each vibrator and
compares the currents from both vibrators
Interlock cross-coupling and system safety is provided by the MAC
and will cause a system shutdown when an interlock occurs in any
one of the systems. Similarly, the Emergency Stop Hub unit will
allow simultaneous shutdown when an E-stop button is pressed. For
increased system/operator safety, an emergency stop button pressed
on an un-powered system will also cause a shutdown on all systems
4.4.3 Vibration controller
The vibration test control system is of m+p make consists of data
acquisition and signal conditioning modules interfaced to the PC
98
through IEEE 1394 FireWire, and the test control software supports
the four basic modes of vibration testing ie Sine, Sine dwell, Random
and Shock with provision for notching. The system was initially
procured with 16 control/data channel capacity and later upgraded
to 40 channels for meeting GSLVM3 test requirements. The system
facilitates the entry of different test profiles as per the test
requirement, control parameters and the setting of channel
sensitivities. The safety features of the system include Control loop
check, alarm /abort limits on the test profile and grms abort.
4.4.4 Instrumentation system
The process of maintaining the specified vibration levels at
the fixture top flange by adjusting the system input vibration
energy with in abort limits is called controllability.
The systems are configured to measure the vibration levels occurring
at various locations on the specimen during the vibration test. The
vibration levels are sensed by the piezoelectric accelerometers of
B&K make. The accelerometers output is connected to charge
amplifiers for amplification and conditioning. The charge amplifiers
output is connected to control/data acquisition system for recording
and processing. The typical specifications of the sensors and
amplifiers used are given below. The accelerometers and the charge
amplifiers are periodically calibrated. A mini shaker and reference
99
accelerometer are used for calibrating the accelerometers in back-to-
back comparison method. The charge amplifiers are calibrated using
the charge simulator.
4.5. Data acquisition:
The required test profile is programmed into PC based
control system. The vibration controller generates and feeds a low
level signal into the power amplifier based on the test
specifications. This signal is amplified by the power amplifier and
drives the armature of the shaker. The power amplifier output is
connected through a matching transformer to the armature of
Electrodynamic shaker. The vibration is sensed by control
accelerometers.
Piezoelectric accelerometers are used to sense the vibration
level during the tests. The signal from the accelerometer is
conditioned with the help of a charge amplifier, whose output is fed
to the control system. The location of control and monitor
accelerometers are decided by complexity of the structure. The
vibration channels are electrically calibrated by simulating charge
signal equivalent to 50g into the measurement chain. The charge
amplifiers output is recorded in the magnetic tape recorder and in
100
the computer system. In case of sine vibration tests, D.C
proportional to „g‟ peak and D.C proportional to log frequency are
acquired in the computer system. In case of random vibration test
A.C signals proportional to grms are recorded in the magnetic tape
recorder. This way data acquisition is taking place.
Finally the feedback signals from the shakers, test specimen,
via a control accelerometer and charge amplifier reaches the
vibration controller. The feed- back signal is measured, digitized
and compared with the specified control spectrum. Then the drive
signal is adjusted, if any corrections are required, to change the
input signal to the shaker through power amplifier to maintain the
required /specified test profile.
101
F
ig.4
.6 B
lock
Dia
gra
m o
f D
ual
Contr
ol
Syst
em