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DESIGN OF A TWO-AXIS SHAKING TABLE TO SIMULATE EARTHQUAKE MOTION

Introduction

Earthquake strikes are a destructive, costly, and deadly force. High magnitude earthquakes in heavily populated areas historically cause thousands of deaths that could be prevented if buildings were better made to resist such strong forces. The cost of repair and clean up cost often extends into the tens of millions1. The design of an earthquake simulation table that can be used in educational settings has been assigned as an undergraduate senior capstone project. The table will be used to analyze the effect of earthquake forces on smart structures to help develop an understanding of the effect of earthquake forces and design earthquake-resistant designs in architecture. Our client has requested that the simulation table be able to emulate real-time earthquake data in two horizontal directions (X,Y) in a strict budget. We have defined the problem, gathered information, generated alternatives, evaluated potential solutions, and have a proposed design for the machine.

Methodology

Multiple earthquake tables have been designed for the same need by competitors. These designs consist of state of the art motors and materials and are sold at high prices in comparison with our own budget. We studied these designs to determine four potential solutions for the primary component of the earthquake table, the motor. These are hydraulic, pneumatic, linear, servo motor and ball screw, and servo motor with belt. We evaluated the solutions with four evaluation metrics in mind: cost (40%), emulation accuracy (30%), lifespan (20%), and ease of maintenance (10%). We determined that using a servo motor with a ball screw was the best option for the given requirements.

The servo motor contains an internal drive, which receives input signals from the controller, and converts it into the AC power required by the motor. A ball screw is used to translate the servo motor’s rotational motion into linear motion, causing the table to move horizontally as needed. Using the ball screw has the additional benefit of simultaneously increasing the force that is applied to the object in motion, allowing for higher accelerations with less strain on the motor. It is also moderately priced, especially compared to linear and hydraulic motors. We have developed a 3D model of the entire 2-axis table with exact specifications in SolidWorks. We believe that this is a more economical, easy, and inexpensive to maintain than competitive products and will satisfy the emulation and lifespan requirements of potential customers.

To control the table, an arduino mega was selected, due to its high memory and large number of I/O pins. This memory was necessary due to the high amount of information required to be transmitted between the arduino and the controlling computer. This data will come from an online database of past earthquake acceleration data, which can be easily converted in a software that we will develop, and consequently input through a controller into the two motors. This database, the Ground Motion Database, is produced by the Pacific Earthquake Engineering Research Center. This database has searchable acceleration data for thousands of actual earthquakes, in a wide range of magnitudes2.

References [1] Facts and Statistics: Earthquakes and Tsunamis. Insurance Information Institute. 2017. Accessed Sep 2017. <https://www.iii.org/fact-statistic/facts-statistics-earthquakes-and-tsunamis>.

[2] Peer Ground Motion Database. Pacific Earthquake Engineering Research Center. 2013. Accessed Sep. 2017. <https://ngawest2.berkeley.edu/>.