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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

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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,

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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

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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

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POWER AMPLIFIER 2 (PAII)

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POWER AMPLIFIER 1 (PAI)

MAC- Multiple Amplifier Controller

DMU- Differential Monitoring Unit

PCU- Phase Control Unit

CMU- Current Monitoring Unit

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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.

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