"IZIIS" Biaxial Shaking Table Twenty Years of Investigations and Testing

D. Mamuchevski, B. Sc., electronics engineer.

ABSTRACT: The biaxial shaking table has been the most significant part of the experimental investigations and testing capabilities of the Institute of Earthquake Engineering and Engineering Seismology ("IZIIS"), University "Ss. Cyril and Methodius", Skopje, Republic of Macedonia. In this paper, short presentation of the dynamic performances, programming and control facilities and review of the most important types of investigations and testing realized during the period of twenty years are made.

KEYWORDS: Processes control, data acquisition, data processing, acceleration, displacement, dynamic force, programmed motion, earthquake, vibration.

1 INTRODUCTION

The biaxial shaking table in the Institute of Earthquake Engineering and Engineering Seismology ("IZIIS") in Skopje, Republic of Macedonia, was put into operation during February 1983 (officially 26 of July 1983).

The previously knowledge and experience gathered during several years of analytical and experimental investigations based on usage of the single axis shaking table were a good basis to start with a more ambitious project for development of the large testing biaxial system for seismic and vibratory simulations.

The project was fully managed and realized by the "IZIIS" staff with the necessarily technical support from "MTS" company, Mineapolis, USA.

2 DESIGN CONSIDERATIONS AND CRITERIA

Design considerations were based on the main idea to install a system with convenient dynamic performances to provide facilities for experimental laboratory investigations of models of different structures and "full scale" seismic and vibratory testing of electrical equipment, mechanical equipment and assemblages.

The testing system based on such considerations determines special technical requirements for the equipment, mainly the system must have a reasonable power exciting and control performances, wide spectra of transfer characteristics and finally flexible programming, data acquisition and data processing facilities.

The design and acceptance criteria were based on the requirements for accurate simulation of recorded real earthquakes and artificially generated acceleration and displacement time histories. An "accurate simulation" is a term defining a good reproduction of the programmed signal wave form and as much as possible maximal transfer of the command signal spectral power.

Descriptively, the system was considered and designed to be able to simulate real earthquakes with an intensity of about 1.5 of the "El Centro" earthquake intensity (1940, Imperial Valley, USA), and with frequency band compatible with the technical requirements for seismic and vibratory testing,

investigations and qualification of the highest class of equipment such as equipment for nuclear power plants, equipment for high voltage distributive systems, special industrial installations, components for aircraft industry, etc.

In the next item, a more detailed elaboration of the dynamic performances and transfer capabilities of the whole system will be done.

3 INTEGRATED ELECTROHYDRAULIC SYSTEM AND ASSOCIATED SYSTEMS AND DEVICES

The Biaxial Shaking Table is an integrated system formed by the main electrohydraulic system and the associated systems, subsystems and devices. A functional block diagram of the integrated biaxial shaking table system is presented in Figure 1.

The main parts of the integral system are:

Biaxial Shaking Table integrated electrohydraulic system,

Computer System for Processing Control, Data Acquisition and Data Processing,

Data Acquisition Equipment,

Equipment for Calibration and Tuning of the Biaxial Shaking Table and the Associated Systems.

The Biaxial Shaking Table is an integrated electrohydraulic system composed of the following main parts:

Hydraulic Power Supply System,

Hardline Installation,

Hydraulic Actuators driven by electronically controlled Servovalves,

Shaking Table,

Analog Control System.

The Hydraulic Power Supply System supplies the electrohydraulic system by mechanical power necessary for programmed motion of the shaking table.

The system is composed of three hydraulic pumps. Each pump has a working pressure of 3000 psi (about 210 bars), however the flow rate of each pump is 110 gallons/minute. The total flow rate of the system is 330 gallons/minute (about 1200 liters/minute).

The Hardline Installation is composed of three separate lines (pipes): pressure, return and drain line.

The system of Hydraulic Actuators is composed of two identical horizontal units (2x 425KN dynamic load) and four identical vertical units (4x225KN dynamic load). Effectively, the total capacity of the exciting system is about 800KN (80 tones) total mass (shaking table + 40 tones tested specimen mass).

The programmed motion of the actuators is driven by electronically controlled multi-degree servovalves.

The Shaking Table is a slab made of reinforced concrete, with size 5m x 5m, and with total mass of about 40.000 kg.

The Analog Control System provides full integration of the system functions:

programming and control of the shaking table motion,

control and safety of the hydraulic power supply system and associated installations,

proper oil distribution,

over-programming detection and control.

Function Generator

SHAKING TABLE CONTROL CONSOLE

Digital Spectrum Analyzer

Input Section Program Selector Magnetic Tape

Recorder

ANALOG CONTROL SYSTEM

Four Channel Oscilloscope

Random Noise Generator

Output Section Read-out Selector

Hydraulic Power Supply Control Section

Digital Voltmeter

TRANSDUCER&SIGNAL CONDITIONING SYSTEM

TESTED SPECIMEN Internal and External Transducers Accelerometers Displacement Transducers Strain Gages Other Transducers (Pressure, Flow, Temperature, ...)

Processes Control and Data Acquisition COMPUTER SYSTEM ------------------- Model: VAXLAB 4000-200 Operating System: Open VMS

BIAXIAL SHAKING TABLE

Programmable Real Time Clock

THREE-VARIABLE SERVO CONTROL SYSTEM Acceleration (measured) Velocity (calculated) Displacement (measured) SERVOVALVE Driven ELECTROHYDRAULIC ACTUATORS

Analog OUTPUT D/AC 8 channels

HYDRAULIC POWER SUPPLY SYSTEM

Analog INPUT A/DC 64 channels

Figure 1.Functional Block Diagram of the " IZIIS-Biaxial Seismic Shaking Table".Integrated Electrohydraulic System and Associated Systems for Programming, Processing Control, Data Acquisition and Data Processing

The system of Hydraulic Actuators is composed of two identical horizontal units (2x 425KN dynamic load) and four identical vertical units (4x225KN dynamic load). Effectively, the total capacity of the exciting system is about 800KN (80 tones) total mass (shaking table + 40 tones tested specimen mass).

The programmed motion of the actuators is driven by electronically controlled multi-degree servovalves.

The Shaking Table is a slab made of reinforced concrete, with size 5m x 5m, and with total mass of about 40.000 kg.

The Analog Control System provides full integration of the system functions:

programming and control of the shaking table motion,

control and safety of the hydraulic power supply system and associated installations,

proper oil distribution,

over-programming detection and control.

The control of the shaking table motion is based on three-variable servo controllers. This concept uses three feedback signals related to the actuator's measured acceleration and displacement feedback signals and actuator's calculated velocity feedback signals (analog velocity computer is used) computed from the integrated acceleration and derived displacement averaged signals.

The control system, integrally, provides control of the five degrees of freedom of the shaking table: horizontal and vertical translation and all three rotations: yaw, pitch and roll.

The sixth degree of freedom (orthogonal horizontal translation) in the existing configuration is prevented and is not controlled. The motion in this direction is prevented by usage of passive hydrostatic bearings.

An active command signals are applied for the translational degrees of freedom, however, the rotations are controlled by the "zero" command signal.

A special part of the control system provides program conditioning, transducer & signal conditioning and over-programming protection.

The electronic and electro-hydraulic equipment is produced by "MTS", company, Minneapolis, USA.

The main dynamic characteristics of the system are: The maximum tested specimen mass: 40.000 kg. The maximal stroke; horizontal: 125 mm, vertical: 60 mm. The maximal acceleration; horizontal: 20 m/s2, vertical: 10 m/s2. The maximal velocity; horizontal: 750 mm/s, vertical: 500 mm/s. The optimal working frequency band for the system is from 0 to 33 Hz.

(with a reduced accuracy, sinus and sinus sweep type of vibrations could be generated in frequency band up to 70 Hz)

The maximal dynamic force, about 800 KN (80 tones), simultaneously in both directions.

The maximal stroke, total dynamic force, total mass (table and test specimen) and hydraulic power supply system capacity (oil pressure and flow rate) determine the transfer properties of the whole system.

The Computer System is used for several tasks: "on line" processes control and data acquisition, "off line" input (command) signal pre-processing, and "off line" acquired experimental data processing.

The existing computer configuration is based on VAXLAB model 4000-200, produced by "Digital Equipment Corporation", USA. The functional block diagram of the computer system configuration is presented in Figure 1.

The important components of the computer configuration are the real time devices, configured in two subsystems:

Digital to Analog Converter (DAC) with 8 channels, and

Analog to Digital Converter (ADC) with 64 channels.

The accuracy of the digital to analog and analog to digital converters is the same for both devices: 12 bits, or the system dynamic range is equal to 66 dB.

The maximal throughput rate of both real time subsystems is 250 KHz.

The Programmable Real Time Clock is hardware option included into the computer system configuration to provide synchronization and programmable sampling rate signal for control of DAC and ADC processes.

The Time Scaling Factor is a programmable parameter, especially used in model testing applications to provide compatibility between model scale ratio and excitation parameters. The value of the time scaling factor, is normally 1.00, but the software provides possibilities to change the values from 0.10 to 10.0.

In case of real time applications, with time scaling factor of 1.00, the controlled and acquired sequences are 20.48 seconds long, because the size of working buffers determines the maximal number of commanding and acquired data.

The "on line" processes control and data acquisition are real time processes programmed, controlled and synchronized by computer hardware and application software facilities.

The application software controls simultaneously two analog output signals with equivalent sampling rate of 1000 samples/second/channel and 64 analog input signals with sampling rate of 100 samples/second/channel, with the time scaling factor 1.00. The sampling intervals, in real time are changed proportionally with the time scaling factor values.

The software provides a very accurate and smooth control signal and synchronization of both processes (DAC and ADC), without data sky and other types of delay effects and numerical noises in data conversion processes.

Additionally, the applicative software provides data processing of two type of signals: input (command) signals and acquired signals.

The signal processing could be performed in time and frequency domain.

The processed data could be presented in form of: tables, time histories, amplitude or response spectra and hysteresis curves, presenting relevant data related to the characteristic values and amplitude and frequency content of signals.

An input (command) signal pre-processing covers the calculation of the commanding and reference signals, normalization, quantitation and coding of the signals selected as seismic or vibratory excitations.

The input (command) signals must be pre-processed to define the parameters (amplitude span and time scaling factor) for proper programming of shaking table motion to prevent some non compatibilities, nonlinear distortion and attenuation of command signals and over-programming.

The acquired data could be preliminary and finally processed.

The preliminary data processing of the acquired signals provides proper test conduction, including optimal development of test procedures and adequate recording, monitoring, analyzing and prediction of the tested specimen behavior.

The final data processing consists of additional data reduction, experimental and instrumental errors correction. This is part of the experimental data analyses, data interpretation and comparative analyses.

The software package for control, data acquisition and data processing is developed by the "IZIIS" staff.

The Data Acquisition Equipment is composed of three main parts:

Electromechanical Transducers and

Sensors for non-mechanical quantities

Accelerometers, Displacement Transducers, Strain Gages and other types of sensing devices (transmitters for pressure, flow, temperature, ...).

Transducer&Signal Conditionners

DC and AC type of excitation, Selectable Gain, Low-pass Filters with selectable high corner (cut off) frequency,

Subsystem for analog to digital conversion (ADC).

The ADC subsystem, (hardware and software) is part of the processing control and data acquisition computer system.

The Equipment for Calibration and Tuning of the Biaxial Shaking Table and the Associated Systems is composed of the following devices:

Signal, Wave Form and Random Noise Generators,

DC Calibrators,

Digital Multimeters,

Four trace Oscilloscope,

Two channel Spectrum Analyzer.

4 TESTING AND PROGRAMMING FACILITIES

The analog control system of the shaking table is configured to accept different type of programming devices (see Figure 1), such as:

Digital to Analog Converter (subsystem into computer system),

Signal Generator (Function Generator, Wave Form Generator, Random Noise Generator),

Analog Tape Recorder.

This kind of devices can provide a different type of programming command (input) signals, such as: sinusoidal signals, sinus sweep signals, sine beat signals, random noise signals, multi-frequency composed signals, real earthquake and artificial earthquake signals.

The most important feature of the system is the software capability for calculation and generation of artificial earthquake type of signals based on design acceleration spectra, floor response spectra and test acceleration spectra, especially used in seismic qualification and prove testing of equipment.

With this type of programming devices and with the associated systems for data acquisition and signal analyses and processing, the shaking table provides two very important investigation and testing properties:

Accurate experimental determination of the dynamic characteristics (natural frequencies, damping ratios, mode shapes of vibration, modal parameters) of the tested specimen by usage of different types of exciting signals and investigation & test methods, and,

Investigation and Testing of the dynamic behavior, dynamic stability, seismic and vibratory withstanding of the tested specimen and especially simulation of vibratory aging.

5 FIELD OF APPLICATIONS

During the twenty years of active usage, the existing capabilities of the biaxial shaking table and the associated systems, have provided realization of various types of experiments, such as:

testing and analyses of steel structures and scaled structure models, especially investigation of the nonlinear behavior of structures,

testing and analyses of scaled models and segments of dams,

testing and analyses of scaled models of high buildings,

testing and analyses of models of special buildings and historical monuments,

"full scale" testing of elements and joints or small scale ratio models prepared from natural materials,

experimental and analytical investigations of different types of base isolation systems and elements,

development of new models and systems for base isolation,

"full scale" seismic withstand testing of high voltage equipment (breakers, current transformers, isolators, metal enclosed assemblages, etc),

comparative experimental and analytical investigations of the seismic withstanding and dynamic stability of high voltage equipment,

simulation of traffic conditions, withstand testing and functional prove testing of military communication equipment,

testing of segments of piping systems,

"full scale" testing of large scale valves, electromechanical driving assemblages and other devices for control of the piping flow and systems,

seismic and vibratory testing of measuring, processing and breaking equipment for nuclear power plants,

seismic qualification of equipment for nuclear power plants,

testing and analyses of spatial damper systems for attenuation of ambient vibrations of conductors in high voltage distributive networks,

testing of hydraulic equipment,

testing of carrier structures and oscillatory characteristics of vehicles,

testing of vehicle models and subsystems, etc.

6 RESUME

The "IZIIS" management, professional staff and other staff members made significant efforts to build a large scale investigation system and relatively expensive biaxial seismic and vibratory simulation system, in very complicated local conditions (limited founds for fundamental scientific investigations, relatively small market for such type of system, period of high inflation and unstable economy, low capacity existing economy, complicated environmental conditions, etc).

The final price of the system was significantly reduced due to the participation of "IZIIS" staff in the processes of design, installation, testing, integration of the electrohydraulic system and associated systems and subsystems, especially including of commercial computer equipment in the full system configuration and development of software for processing control and data acquisition.

The enthusiasm for scientific and applicative investigations, activities and efforts of the professional staff in the Institute, provide a wide spectra of application of the system, satisfying scientific and financial effects and significant participation of the existing biaxial simulation system in the "IZIIS" development and affirmation.

REFERENCES

D. Jurukovski, D. Mamuchevski, "Biaxial System for Earthquake Simulation with Three-Variable Controllers", Proceedings of the Eight European Conference on Earthquake Engineering, Volume 5, Lisbon, Portugal, 1986

D. Jurukovski, D. Mamuchevski, "Usage of a Biaxial Shaking Table for Testing of Mechanical and Electrical Equipment for NPP", Specialist Meeting on Earthquake Ground Motion and Antiseismic Evaluation of Nuclear Power Plants", Moscow, USSR (Russian),1986

G.M. Dimirovski, D.J. Mamuchevski, D.V. Jurukovski, M.M. Carevski, "A Two-level Computer Control of Biaxial Shaking Table Using Three-Variable Local Controllers", IFAC Seventh World Congress, Munich, West Germany, 1987

D.Jurukovski, D.Mamuchevski, "Seismic Behavior of Mechanical and Electrical Equipment", International Atomic Energy Agency Specialist Meeting, Wien, Austria, 1989

G.M.Dimirovski, D.J.Mamuchevski, D.V.Jurukovski, N.E.Cough, "Supervisory Time-Compressed Control of an Earthquake Simulator Plant", IFAC Word Congress, Sydney, Australia, 1993

D.Jurukovski, D.Petrovski, Lj.Taskov, D.Mamuchesvki, S.Stamatovska, M.Bojadjiev, "Seismic Functional Qualification of Active Mechanical and Electrical Components Based on Shaking Table Testing; Shaking Table Testing of the NPP 'Kozloduy' Control Panel YKTC", International Atomic Energy Agency Coordination Meeting, St. Petersburg, Russian Federation, 1995

D.Jurukovski, Lj.Taskov, D.Mamuchevski, D.Petrovski, "Shaking Table Testing of Mechanical Components", SMiRT Post Conference Seminar No. 16, Iguazu, Argentina, 1995

D.Jurukovski, D.Mamuchevski, "An Approach to an Optimal Procedure for Experimental Seismic Qualification of Control Panels", International Atomic Energy Agency Specialist Meeting, Bergamo, Italy, 1996

D.Jurukovski, D.Mamuchevski, S.Stamatovska, "Seismic Testing of Realys Used on NPP 'Kozloduy', Unit 5", International Atomic Energy Agency Specialist Meeting, San Francisco, USA.

INTRODUCTIONDESIGN CONSIDERATIONS AND CRITERIAINTEGRATED ELECTROHYDRAULIC SYSTEM AND ASSOCIATED SYSTEMS AND DEVICESTESTING AND PROGRAMMING FACILITIESFIELD OF APPLICATIONSRESUME