Shake Table Test

Shake table test is a setup devised to simulate the actual conditions which an Architectural body gets exposed to when Earthquake hits it. There are many methods to check the strength of the architectural building, one of which is Shake table test, where in the model of the building is subjected to seismic waves same as that of an earthquake. This device shakes structural models or building components with a wide range of simulated ground motions, including reproductions of recorded earthquakes time-histories. This setup is thus created to simulate conditions of an earthquake and further make analysis of the data provided by sensors present in the setup. The behavior of the building is then recorded and accordingly plotted on the graph so as to understand how the model reacts to a specific range of vibrations.

Here an accelerometer (IMU Sensor) is used to get the instantaneous acceleration values at different platforms, hence, according to the readings the further motion of the building can be damped by using different types of material or by using a spring, hence, dampening the further motion. The whole setup is designed on the embedded platform along with a vibrating base to simulate the earthquake conditions.

Using video records and data from transducers, it is possible to interpret the dynamic behavior of the specimen. Earthquake shaking tables are used extensively in seismic research, as they provide the means to construct structures in such a way that they are subjected to conditions representative of true earthquake ground motions.

They are also used in other fields of engineering to test and qualify vehicles and components of vehicles that must respect heavy vibration requirements and standards.

• The setup:

1. In this setup the building that was excited was of two floors, made up of the Acryllic sheet.
2. The embedded platform used is Arduino Uno interfaced with LabVIEW, via LINX. LINX is a software that interfaces arduino with LabVIEW and it holds communication between the two without the use of C Programming. We will talk about the LabVIEW part in detail.
3. The next most important thing was to create vibrations, to make the project cost effective, the idea that vibrates the phone was used. An unbalanced DC Motor was used which made the motor very jerky hence, it created vibrations.
4. IMU Sensor was used to check the acceleration of the moving building, where IMU stands for Inertial Measurement Unit.

• LABVIEW:

The simulations is created through random signals generated through LabView and consecutively sends the PWM signals generated through the random signal to a Vibrating source on the test bed, which in our case is an unbalanced motor load.

Earthquake is caused to due to sudden release of energy in the earth's crust which causes the rise of seismic waves. The seismic waves are nothing but unpredictable random waves with changing amplitude and frequency. Hence, in this device the random signals were generated through a random number function in LabView. It is equivalent to change in amplitude with time and that change can not be determined, also it was calibrated from a scale of 0 to 100% change. This signal generation was fed into PWM signal hence, changing the speed of the motor hence, causing more jerky motion. Also, 2 LEDs were also interfaced with this PWM signal, indicating whether the quake is safe or dangerous. When the scale was below 6, the LED glowing was green where as above it was red along with the increase in the noise intensity of the buzzer again indicating danger.

The PWM pins on the Arduino were used (~ as symbol), and the pin number could be easily be set from the labview front panel.

• Unbalanced DC Motor:

There are numerous techniques available to provide damped vibrations at the base of setup to create an earthquake simulation. These include hydraulic actuators, high torque servos, induction motor with cam-follower mechanism. Since the platform we have built contains smaller scale dimensions, Method of  Unbalanced Motor Load was used to provide vibrations to the setup.

The principle behind an unbalanced motor setup is that when the an unequal distribution of mass occurs through the axis of rotation of a genric motor, the motor now tries to rotate about an axis which is 'away' from the previous or geometric centre of mass of the previous setup. Thus due to shifting away of COM and the rotation of motor about this axis creates a regular Oscillations and proportional to the PWM generated through random functions from the LabView.

        Figure 1: Actual setup of Off Balanced Motor

• Acceleration data Acquisition:

The sensor used was a MPU-6050, is a motion tracking device (Gyroscope + accelerometer), which is a 6DOF IMU sensor and accelerometer was the main sensor used for data acquisition purpose.

The MPU-6050 devices combines a 3-axis gyroscope and a 3-axis accelerometer on the same silicon die, together with an onboard Digital Motion Processor (DMP), which processes complex 6-axis Motion Fusion algorithms. The device can access external magnetometers or other sensors through an auxiliary master I²C bus, allowing the devices to gather a full set of sensor data without intervention from the system processor. For primary process which is the vibrating source, simulation uses the LINX setup for LabView and VISA Serial Read Setup for data acquisition which is the secondary setup.

Since both processes are to be run in real time and that both primary and secondary processes run on altogether different platforms , two different laptops were used.

As at a the same time both LINX and VISA can not be used, they work individually.

The acceleration data was plotted on to the graph and with change in random signal the graph changed it's amplitude and frequency depicting of the seismic wave.

Figure 2: MPU 6050

• The Embedded Setup:

  

  Figure 3: The embedded setup with specifications.

  Figure 3: The embedded setup with specifications.

Figure 4: The embedded platform having the L293d circuit to drive the motor.

The link of the video of shake table test:

https://www.youtube.com/watch?v=jyDMH1Q3ulQ

The code will be available to blog followers upon request. :)