12 Nisan 2012 Perşembe

DESKTOP SHAKE TABLE FOR EDUCATIONAL PURPOSE

One of the biggest natural disasters, earthquake causes big destruction. Earthquakes are the movement of the earth's crust, which are characterized by three-dimensional vibrations and caused by tectonic movements. So control of earthquakes is impossible and prediction of them is quite difficult. However we can resist to destructive effects of earthquakes. How can we do this?

by “Designing earthquake-resistant structures”
Behavior of buildings under seismic effects is simulated by computers and test environment using several methods. Shake table testing system is one of these methods .Structure model is built on the shake table and shake table is used to simulate movement of earthquake by producing one, two or three axes seismic movements artificially.
We have developed TESTBOX-SHAKETABLE system to be used for these tests. TESTBOX-SHAKETABLE contains a bottom plate, shake table, servo-electric actuator, servo motor driver, motion controller and power supply unit. Servo motor driver, motion controller and power supply were collected inside a small automation panel.

Figure-1: Solid model view and system components of TESTBOX-SHAKE TABLE

General Specifications Of TESTBOX-SHAKE TABLE
Purpose Of Use: For education in “Earthquake Engineering” and “Structural Dynamics Tests”
Construction Materials of Table: Aluminum Block and Plexiglass
Direction of Vibration: One Axis in Horizontal
Work Capacity: 50 kg
Stroke: 150mm
Max Acceleration: ±2g
Drive Mechanism: Servo-Electric Actuator
Drive Unit: AC type brushless Servo Motor
Motor Control:CONTROLBOX- One Axis Motion Controller
Required Power Servo Motor : 750W 220V AC
Motion Controller : 4.5A 24V DC
Closed-Loop Control:Internal Encoder/Linear Scale/Load Cell

Technical Specifications of TESTBOX-SHAKE TABLE
Dimensions Of Plexi Glass (L x W x H): 70 x 60 x 5 cm
Dimensions Of Aluminum Block (L x W x t): 50x50x2 cm
Work Capacity: 50 kg
Weight Of Table: 20 kg
Max Displacement: ±7,5 cm
Max Force(theoretical): 1000 N
Max Velocity: 40 cm/sn
Max Motor Torque: 1 N.m
Servo Motor Power: 750 W
Infinity Screw Shaft: 5 mm/rev

TESTBOX-SHAKETABLE has a servo motor and internal encoder inside, thus it is generally more accurate than other systems through these features. Sinusoidal motions, real earthquake motions and user controlled movements(arbitrary waveforms) are all possible with sensitive PID parameters and they are software configurable.
So, how we are able to simulate a real earthquake motion?
During the earthquake, vibrations are recorded by data acquisition systems using accelerometers or velocity meters. Recorded acceleration data is double integrated in order to get displacement data. As a result, the displacement data is reached from the acceleration data. Displacement data is loaded into the motion controller using the TESTLAB - Shake Table software.

Figure-2: Acceleration-time graph of the real seismic data


If we take the acceleration signal to sinusoidal form;
a(t)=-A sin(2πft)
A : Amplitude of acceleration signal
f : Frequency of acceleration signal
t : Time the unit

Figure-3: Acceleration-time graph


A=F/(m.g)
F : Maximum Actuator Force (N) m : Maximum weight (kg)
g : Acceleration of gravity (m/s²)
V(t)=∫_0^t a(t) dt


Figure-4: Velocity-time graph


V(t)=A/2πf cos(2πft)
X(t)=∫_0^tV(t)dt
X(t)=A/4π²f²sin(2πft)



Figure-5: Displacement-time graph


Maximum displacement: X_max= A/4π²f²→A= X_max.4.π².f²= F/(m.g) X_(max ).f²= F/(4.π².m.g)
As result of these processes displacement data is reached from the acceleration data.

Figure-6: Displacement-frequency chart of TESTBOX-SHAKETABLE

Figure-7: Displacement-time graph of the real seismic data
The following results are obtained from these tests;
- Building Mode Shapes
- Periods of modes occurred
Buildings according to physical structure and stiffness show different oscillations under earthquake effect. Oscillation forms of the buildings are called "dynamic modes".
Dynamic modes appear different time periods and at different energies, but each dynamic mode contributes at the general behavior of the building. This analysis part is out of the scope of this presentation, which will be presented later in this blog by our colleague engineers.

Figure-8: Building dynamic mode shapes


Şaban AKDERE
TDG Hardware R&D Engineer Electric-Electronic and Mechanical Engineer saban@teknikdestekgrubu.com.tr

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