Development of economized shaking platforms for seismic testing of scaled models

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976
INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN
– 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME
ENGINEERING AND TECHNOLOGY (IJARET)

ISSN 0976 - 6480 (Print) IJARET
ISSN 0976 - 6499 (Online)
Volume 3, Issue 2, July-December (2012), pp. 60-70
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DEVELOPMENT OF ECONOMIZED SHAKING PLATFORMS
FOR SEISMIC TESTING OF SCALED MODELS

Wani Ahmad, Singh Amarpreet, Iqbal Sana, Lal Nawaf, Bhat Javed

ADDRESS FOR CORRESPONDENCE
Ahmad Wani is B.Tech Civil Engineering, National Institute of Technology, Srinagar, currently with
Structural Erection Dept., National Thermal Power Corp., Ltd., Mouda, Nagpur, India. Email:
wani.ahmed@yahoo.com
Amarpreet Singh is B.Tech Civil Engineering, National Institute of Technology, Srinagar,
currently with Structural Design Dept., RITES, Ltd., Gurgaon, India. E-mail:
amarpreet89@gmail.com
Sana Iqbal is B.Tech Civil Engineering, National Institute of Technology, Srinagar, currently
with Civil Construction Dept., National Thermal Power Corp., Ltd. , Mouda, Nagpur, India.
Email: sanaiqbal9@ymail.com
Nawaf Lal is B. Tech Electrical Engineering, National Institute of
Technology, Srinagar, currently with Bharat Petroleum Corporation Ltd., Faridabad, Haryana. E-mail:
nawaflal@yahoo.com
Dr. Javed Ahmed Bhat is Associate Professor, Civil Engineering Dept., National Institute of
Technology, Srinagar. Email: bhat_javed@yahoo.com

ABSTRACT

Earthquake shake tables have been successfully used in the experimental assessment of
dynamic behavior of structures While six-degree of freedom shake tables reproducing
actual earthquake data employ expensive electro-hydraulic actuators however the uni-axial
servo-motor controlled earthquake simulator, with the incorporation of the cost-effective
lead screw as a linear actuator, and capable of supplying a frequency sweep, while
maintaining a constant acceleration, is an apt arrangement to precisely evaluate the natural
frequency, mode shapes, energy dissipation, etc. of scaled models in an economic
manner.
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This paper describes the development of such a shaking platform at the Dynamics
Laboratory of National Institute of Technology, Srinagar, India, at a very economic
cost, primarily aimed at providing teaching facility or aid of structural dynamics, and
organize local earthquake awareness programs for the persons associated with construction
organizations in the seismically prone Kashmir region, India.

Keywords: earthquake awareness, frequency sweep, lead screw linear actuator, uniaxial shake
table

I. INTRODUCTION
Nearly sixty percent of the people killed by disasters in the last decade have died due to
earthquakes. Asia is the worst hit continent in terms of human losses, where during the last
decade alone, disasters claimed eighty percent of all fatalities [1]. Earthquakes pose a
significant threat to India also because of falling of almost 59% of its geographical area in
earthquake vulnerable zones. The most clearly observable impacts of an earthquake are the
loss of human lives and property, economic & social losses and environmental
degradation. Over the last century, about 75% of fatalities attributed to earthquakes have
been caused by the collapse of buildings [ 2]. The Kashmir region has been struck by
numerous earthquakes in the past, including the 2005 earthquake (October 8, 2005,
magnitude Mw 7.6 which claimed 73,000 lives, left 70,000 injured, 270,000 buildings
were destroyed, and 180,000 buildings damaged). It ranks among the worst natural disasters
in the history of the Indian subcontinent and Pakistan [3].
All these natural disasters highlight the importance of research in the field of earthquake
engineering [4]. There have been numerous organizations who have been working in this area
at national and international level to promote awareness about earthquakes among the youth, so
as to attract research in the field of earthquake engineering. Following the Indian Ocean
tsunami in 2011, and thus realizing the destructive power of earthquakes and continuing need
for scientists, engineers and technologists to help protect society from their effects, international
projects were aimed at raising awareness among young people worldwide of the global
importance of earthquake engineering skills [5]. Such projects involve awareness
programs & earthquake engineering competitions, conducted to act as precursors for young
minds to research in the earthquake engineering field. Similar programs are the need of the
hour in seismically prone areas of developing countries like Kashmir (India), which lack basic
technological knowhow and basic instrumentation facilities for carrying out research work in
this direction. Although, some efforts have been made at NIT Srinagar by performing a
comparative study on different types of construction practices using a Shaker without any record
of natural frequencies, mode shapes or displacements [7].
The problem of the determination of the response of structures to the prescribed exciting
forces in theory can be formulated and solved in very general terms, even for situations
involving plastic deformations. To make any practical use of the analysis, however
requires that quantitative information be available on such basic structural dynamic properties
as natural periods of vibration, mode shapes, energy dissipation, and yield limits. Such
dynamic properties depend in turn on many details of material behaviour and structural

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configuration that are not amenable to a fully analytical treatment. Direct
experimental determination of such dynamic characteristics is thus a necessity [6].
Shake tables which reproduce actual earthquakes, consist of rectangular platforms
driven in upto six-degrees of freedom using servo-hydraulic actuators. Such shake tables
are too expensive for construction and operation. Thus, to facilitate teaching aid and
organizing awareness programs using scaled models, smaller-sized unidirectional shake tables
are preferred especially in the developing countries [4].
Sensing the need, a uniaxial servo-motor operated shaking platform was developed for the
Dynamics laboratory at National Institute of Technology, Srinagar at a very economic cost to
facilitate quality teaching of structural dynamics and earthquake engineering, as well as
organize awareness programs associated with the construction organizations in the local vicinity. In
the present study an effort has been made to develop a uniaxial servo-motor operated shaking
platform.
An important aspect about this project which needs to be highlighted is that the shake table
has been fabricated at a cost which is a very economical, as compared to the costs of similar
tables which have been developed earlier [7].

II. SHAKE TABLE COMPONENTS
The shake table developed at NIT Srinagar mainly consist of a wooden board of size 1.5ft x
1.5ft (or 0.5m x 0.5m) as shaking platform over which scaled down models are mounted for
unidirectional seismic assessment. The other features of this uniaxial shake table are as under:
 High precision computer controlled brushless DC (Direct current)motor (Rating:
400 W, 3000 rpm) as vibration inducer (or earthquake like motion simulator).
 Lead screw assembly serving as a linear actuator.
 Amplitude range: + 125 mm.
 Frequency range: 1 Hz to 8 Hz.
 Acceleration range: 0.1g to 0.5g.
 Payload capacity: 25 Kg
 Has the potential to simulate the time history of any previous earthquake.
The whole shake table is mounted over a steel plate of size 1016 mm X 580 mm. Four box
sections (Fig. 1) act as vertical supports for the angles overwhich the table slides. Lead screw,
motor and associated assembly is mounted separately over a channel section to avoid the
problem of misalignment (as the poorly aligned assembly can develop undesired components of
forces in transverse/downward direction causing extra load on the lead screw and the motor
shaft).
The nut of the lead screw transmits power to the center of table through a proper arrangement
which comprises of a box section; square in shape that fits over the nut and a 6mm thick
plate which connects table to the box section. The box section also serves to provide a vertical
clearance of about 10mm, to easily by pass, the unwanted vertical shocks.
The second part of the shake table is the data acquisition system, also
developed at NIT Srinagar, which comprises of a dc power supply (35volt, 25A), a
computer running the MATLAB software, a DC tachogenerator, a 5ᾨ-16A rheostat, a data-
acquisition board (PCL card), an optical isolation circuit, a differential amplifier circuit,
a 15 volt dc supply and the Shake Table itself. The table has been successfully used for
62


observation and measurement of various mode shapes and noting down their corresponding
frequencies.

III. THE LEAD SCREW ASSEMBLY
A lead screw, also known as a power screw or translation screw, is a screw designed to
translate turning motion into linear motion. Lead screws are manufactured in the same way as
other thread forms.
There are a number of advantages in using lead screw; some of them may be
summarized as-Large load carrying capability, Compact, Simple to design, Easy to
manufacture; no specialized machinery is required, Large mechanical advantage, Precise and
accurate linear motion, Smooth, quiet, and low maintenance, Minimal number of parts, Most
are self-locking. The disadvantages are that most lead screws are not very efficient. Due to the
low efficiency they cannot be used in continuous power transmission applications. They
also have a high degree for friction on the threads, which can wear the threads out quickly.
The lead screw assembly is an alternative approach used in the project in place of the ball
screw arrangement which is practically a friction less and an apt arrangement as a linear
actuator. Various minor shortcomings in lead screw over ball screw arrangement could
possibly come through by keen maintenance of the equipment.
In the present study the pitch of the lead screw has been adopted as per the total run of the nut
over threaded length in one second for the maximum amplitude. The motor has a capacity
of 50rps while running on full speed. Maximum amplitude of the motion is 12.4425cm for a
frequency of 1 Hz which means the motor has to complete one cycle of total run = 49.69cm ≈
50cm in one second. For the desired amplitude pitch required comes out to be; 1 rotation =
50cm / (50rps) = 1cm. As the motor would not be running at its full speed always since for
simulating simple harmonic motion it’ll have to change its direction of rotation every time and
for each extreme position the velocity of motion will be zero, thereby assuming an average
rotation speed of 35rps and with factor of safety as 1.05 the pitch was adopted as 15mm.
Figure 1 shows the picture of the fabricated lead screw with variable pitches.
This arrangement is equally suitable for the shaking assembly when used with the high
precision motor.

Motor
Lead Screw

Supports Alignment assembly

Fig. 1. Photograph shows the shake table foundation assembly along with the servo-motor fitted
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to the variable pitch lead- screw.

IV. INSTRUMENT SETUP AND WORKING
A Brush less DC (BLDC) motor controlled by means of PI controller is used to drive the entire
assembly. Limitations of brushed DC motors overcome by BLDC motors include lower
efficiency and susceptibility of the commutator assembly to mechanical wear and
consequent need for servicing, at the cost of potentially less rugged and more complex
and expensive control electronics, thus facilitating the cost effectiveness. The speed of the
motor can be easily controlled by the application of a DC voltage across its control
terminals, whereas for changing the direction of rotation a relay is provided which when
actuated causes reversal in the motor’s direction of rotation. By continuously varying the
control voltage in tandem with the signal to the relay, any desired motion of the motor can be
achieved. The control of these signals is achieved by means of a PI controller which issues
control signals to the motor based on feedback mechanism and guides the motion of the motor
towards the desired performance. Various instrumentation components are:

A. DC Tachogenerator (Feedback mechanism)
A small dc motor is used as a dc generator to measure the speed of the shake table assembly.
The field circuit of the dc generator is fed from the same variable dc supply as the BLDC
motor. A 5ᾨ rheostat is used to feed a constant voltage of 1 volt to the field circuit. As the shaft
of the generator rotates, it results in the generation of a dc armature voltage which is
proportional to the speed of rotation, if the field voltage is kept constant. This DC armature
voltage if then filtered and fed to the computer in a feedback loop. The dc tachogenerator has an
advantage over ac tachogenerator that the direction of the armature voltage reverses when the
direction of rotation of the shaft reverses. Hence the DC tachogenerator gives both the
magnitude and the direction of speed, which is desired for our application.

B. Differential amplifier
The armature voltage from the dc tachogenerator is full of common mode noise and cannot
be directly fed to the PCL card for measurement of speed. The common mode noise is first
filtered using a differential amplifier. In addition to the differential amplifier a 25uF
capacitor is also connected across the input of the differential amplifier to further reduce the
noise.

C. Optical isolator circuit
If the relay circuit and the speed control circuit of the BLDC motor are not isolated
from each other, it leads to unnecessary loading of the motor. Isolating the ground of the
control circuit from that of the relay circuit is achieved by means of an opto-isolator circuit.
This simple circuit isolates the two grounds by means of optical isolation through the MCT2E
chip. By using this chip the input and the output to the relay circuit are isolated.

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Fig. 2 (a). Line drawing shows details of the box section used for force transfer between lead
screw and table

Fig. 2(b). Line drawing shows the lead screw connection with the table

Fig. 2(c). Line drawing shows the variable pitch lead screw

D.PCL Card
Dynalog Micro Systems Pvt. Ltd. Provides AD-DA card – PCL 207, a high performance
analog interface card for IBM/PC/XT/AT and other compatible computers. PCL 207 uses the
hardware-based successive approximations method and provides 40,000 samples per second.
The true 12-bit conversion gives an overall accuracy of 0.015 per cent reading +/- 1 bit.
The output channel provides fast settling time at high accuracy. A multiplexer in the
input stage provides eight single-ended analog inputs. Standard output voltage ranges are user
selectable whereas the input range is fixed. The PCL-207 data acquisition card uses the
successive approximation method for the analog to digital data conversion. The card
has one channel of DAC conversion and 8 channels of ADC conversion. The card has 16
registers for data acquisition and data output, the 16 registers have address in sequence to the
base address. The ADC card has 20 pins connector. The pins 1, 3, 5, 7, 9, 11, 13, 15 are for
analog to digital conversion. Pin number 17 is for digital to analog data conversion. The pins 2,
4, 6, 8, 10, 12, 14, 16, 18, 20 are ground connections.
The entire system is comprised of a dc power supply (35volt, 25 A), a PC running the
SIMULINK software, a DC tachogenerator, a data-acquisition board (PCL card), an optical
isolation circuit, a differential amplifier circuit and the Shake Table itself.
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Using SIMULINK, the user specifies the amplitude and frequency of the sine wave. The
sine wave is the user- specified or desired command position of the stage. The voltage
needed to move the stage at the desired sine wave position is calculated in SIMULINK through
the PI control mechanism and sent through the analog output channel of the PCL card. The
voltage appearing at the output channel of the PCL card is sent to the control terminals of the
motor. The table moves back and forth at the position and frequency of the commanded sine
wave. The resulting speeds of the motor are measured by the dc tachogenerator. The
DC tachogenerator is connected to the PCL card and the signal can be displayed and
processed further in SIMULINK.
The BLDC motor has a feature that it doesn’t instantaneously start to
follow a particular control voltage after starting; rather it takes some time to respond to
the control signals. A ramp signal is provided to the motor for certain duration to determine its
steady state characteristics. This simulation checks the response (speed) of the motor as the
control voltage is increased. The ramp is kept within the limits of 0-4 volts with a saturation
block. The analog input from the tachogenerator has a lot of noise, which is first filtered
out by a digital filter after using a differential amplifier for filtering out the common
mode signal and is then displayed on the scope. The stored values of voltage and speed are
then plotted. In this simulation the relay is fed a constant signal keeping the motion
unidirectional.
After obtaining the speed v/s voltage characteristics for one direction, the relay is operated
and the characteristics of the motor are now obtained in the other direction. From the two plots
[Fig. 3(a). and Fig. 3(b)] it is evident that the two characteristics (forward and reverse are
almost identical). Also the characteristics of the motor are linear in the range from 1.6 to 3
volts. Before 1.6 volts, the motor has a dead band and after 3 volts the motor enters a saturation
region and the characteristics become non-linear.

Fig. 3(a). Graph of motor characteristics (forward)

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Fig. 3(b). Graph of motor characteristics (reverse)

Fig. 4. Final control scheme using PI controller

Fig. 5. Logic subsystem of the final scheme

VI. FINAL CONTROL SCHEME

In this final control scheme using PI controller, the control signal generated by the PI
controller and the output signal sensed by the speed sensor are both fed to a control logic
subsystem. This control subsystem based on these two signals generates the final control signal
which is fed to the control terminals of the motor and also the signal which triggers the relay.

67


Since there is noise present in the sensed speed signal, it cannot alone be used for
triggering the relay. Also the control signal alone cannot be used for triggering the relay
since even when the control signal reaches to zero speed, its not necessary that the motor is
also stationary. Triggering the relay when the motor is still in motion causes jerks in its
movements. The control logic subsystem takes care of these constraints. It accepts both the
control signal generated by the PI controller and the signal from the speed sensor and based
on their values devises a scheme for the smooth functioning of the motor.
This control scheme is now used for running the shake table assembly in a sinusoidal
manner from 1 Hz to 4 Hz. The reference signal and the output of the motor are compared on
a scope. Also the various control signals generated at various stages of the control logic are
also displayed.

Fig. 6(a). Response of the control scheme at various frequencies

Fig. 6(b). Response of system at frequency of 1 Hz

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Fig. 7(a). Photograph showing shake table during the model testing (response being recorded
by 1g accelerometer)
{view by overhead camera}

MDF Model

Wooden Table

Tachogenerator

Fig. 7(b). Photograph showing model testing in progress

V. ECONOMIC ANALYSIS
A cost analysis of the project indicates that the total cost of the project, which included
establishing the shaking platform, purchase and installation of the servo motor, a computer and
control items, as well as response recording instrumentation sums to a total of 1,00,000 INR,
which is quite less as compared to the costs incurred by earlier educational shake tables [7].

VI. RESULTS AND DISCUSSION
The cutback in costs has been achieved by incorporating lead screw as a linear actuator to
convert translation into rotation, instead of the ball screw. Moreover, in contrast to the
earlier projects [7], the idea of having two separate shake tables for two sets of frequencies has
been done away with, by the innovative incorporation of two different sets of pitch on the
same lead screw, thus amplifying its scope of simulating ground motion. Also, the shake table
is capable of providing a frequency sweep while maintaining a constant acceleration, for all
69


accelerations ranging from 0.1g to 0.5g, which is not a feature in the earlier attempts [7]. The
shake table is capable enough of simulating random earthquake accelerograms, within an
acceleration of 0.5g.
The shake table has proved to be very effective in finding various dynamic parameters of
scaled down models made by the use of MDF (Medium Density Fibre) as model material. Few
results of the tested models are presented herewith; figure showing mode shapes
corresponding to a three storey model representing a nine storey RCC building. (Calculation of
mode shapes has been done analytically and found in correlation with the experimentally
observed values).

Fig 8. Mode shapes (experimentally obtained) [found in correlation with the analytical
results].

REFERENCES

[1] United Nations International Strategy for Disaster Reduction (UNISDR), Press Release,
UNISDR 2010/01; 28 January 2010
[2] National Disaster Management Authority (NDMA), Government of India,
http://ndma.gov.in/ndma/earthquake.html
[3] Sung Jig Kim, Amr S. Elnashai, “Characterization of shaking intensity distribution and
seismic assessment of RC buildings for the Kashmir (Pakistan) earthquake of
October 2005”, In Engineering Structures 31(12):2998-3015
[4] Baran, T., Tanrikulu, A.K., Dundar, C. and Tanrikulu, A.H. (2011), CONSTRUCTION
AND PERFORMANCE TEST OF A LOW-COST SHAKE TABLE. Experimental
Techniques, 35: 8–16. doi: 10.1111/j.1747-1567.2010.00631.x
[5] Selling earthquake engineering to young people, Wendy Daniell 1 ; Adam Crewe 2;
Proceedings of the ICE – Civil Engineering, Volume 158, Issue 2, 01 May 2005 , pages 73 –
79 , ISSN: 0965-089X, E-ISSN: 1751-7672
[6] Robert L. Wiegel, Bruce A. Bolt; Earthquake Engineering, Prentice-Hall (1970) Nature, pg.
127.
[7] C. S. Sanghvi, H S Patil and B J Shah, Development of Low cost shake table and
instrumentation setup for earthquake engineering laboratory, International Journal od
Advanced Engineering Technology, E-ISSN
0976-3945, IJAET/Vol.III/ Issue I/January-March, 2012/46-49

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Development of economized shaking platforms for seismic testing of scaled models

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