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Microcomputers in Civil Engineering 11 (1996)289-296

REVIEW ARTICLE

Second Generation of Active Structural Controlin Civil EngineeringG. W. Housner*

Department of Civil Engineering, California Institute of Technology, Pasadena, California 91125, USA

T. T. SoongDepartment of Civ il Engineering, State University of New York at Buffalo, Buffalo, New York 14260, USA

S. F. MasriDepartmentof Civil Engineering, UniversityofSouthern California, Los Angeles, California 90089, USA

Abstract: Research and development in active control ofcivil engineering structures in the United States has approx-imately a 20-year history. Under the leadership of the U.S.National Science Foundation in form ing the U.S. Panel onStructural Control Research and in launching a program in i-tiative in the area of structural contr ol, there has been a surgeof interest in this rese archj eld. Recent progress in active con-trol research in the U nited States is firs t summarized in thispaper. In the sec ondp art, an attempt is made to identify areasof research needs and to recom mendfuture directions fo r thenext generation of active control activities in civil engineer-ing.

1 INTRODUCTIONTo our knowledge, the earliest attemp t to formulate the prob-lem of active control for applications to civil engineeringstructures was made in 1960.17 In the United State s, it appearsthat Yaos conce pt paper in 197244marked the beginning of

* To whom correspondence should be addressed.

active control research when he proposed an error-activatedstructural system whose behavior varies automatically in ac-cordance with unpredictable variations in the loading as wellas environmental conditions and thereby produces desirableresponses under all possible loading conditions. Over 20years have elapsed since that time, and as we take stock, wesee that remarkable progress has been made. Active controlresearch is now at the stage where full-scale systems havebeen installed in actual structures and have performed wellfor the purposes intended.While significant progress has been made, the true po-tential of active vibration control of structures is still to beexploited. For example, most of the current operating sys-tems are not designed for enhancing life safety against largeenvironmental loads. Second, control design in current prac-tice, based primarily on the classic linear quadratic regular(LQR) formulation , is not tailored to civil engineering struc -tural control requirements, thus masking the full potency ofactive control in civil engineering applications.One purpose of this paper is to summ arize recent progressin active control research in the Un ited States. As we embarkon the second g eneration of active control research, another,

0 1996 Microcomputers in Civil Engineering. Published by Blackwell Publishers, 238 Main Street, Cambridge, MA. 02142, USA,and 108 Cowley Road, Oxford OX41JF. UK .


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290 G . W. Housner. T. T . Soong, and S.F . Masri

and more important, purpose is to attempt to identify areasof research needs and to recommend future directions forresearch and development in active control.

2 RECENT PROGRESSSince the NSF initiated its 5-year research program on struc-tural control for safety, performance, and hazard mitigationin 1992,23there has been a surge of interest in this field on thepart of U.S. researchers.As a result, there has been a signif-icant increase in the number of U.S. researchers engaged inthis research field, leading to more diverse research activities.Recent U.S. activities in active and hybrid structural con-trol can be grouped into the following areas:

1. Control algorithms and modeling2. Sensing and actuation3 . Hybrid control4. Implementational issues5. International cooperation

In what follows, several topics are identified in each area, andactivities within each topic are described briefly.2.1 Control algorithms and modelingA predominant number of current control strategieshave cen-tered on the use of linear structural models and linear con-trol laws based on quadratic performance criteria. For spe-cific structural applications and under robustness and stabilityconsiderations, a number of control design alternatives havebeen proposed and developed. These include control designsaccounting for structural nonl ineari tie~~~q~that become im-portant when the structures are subjected to severe seismicloads or in the hybrid control case where the passive elementsundergo large deformation.Nonlinear control laws are also being developed for linearstructures in order to enhance control effectiveness.O Oneof the shortcomings in employing linear control laws is theirinability to produce a significant peak response reductionwhen the peak response occurs during the first few cyclesof the time history, which is usually the case under seismicground excitations. In this work, it is shown that nonlinearcontrol laws can significantly improve peak response reduc-tion under the same constraints imposed on the control forceand other resources.Other alternate control strategies that have been consideredinclude acceleration- and velocity-feedbackcontrol design:Oadaptive control,35fuzzy contr01,~*~~traveling wave-basedcontrol,2frequency-domaincontrol design,33feedback-feed-forward control,34and optimal placement strategies!Stability and robustness concerns have led to critical exam-ination of some currently available control algorithm^."^^^+^^Control robustness in the face of uncertainties arising from

errors in the descriptive model, uncertainties in the under-lying parameters, possible degradations, and other factorscontinues to be an active research Efforts are also be-ing made to develop integrated control systems for smartstructures in which vibration control is only one of theirfunctions.29In order to ascertain cost-effectivenessof structural controlas an alternative means for protection of a structure againstearthquakes, an analysis is being carried out that includesthe generation of fragility curves for the structure againstexcessive damage and collapse.2 Together with informationon expected annual and life-cycle damage costs, the structurecan be examined in terms of the potential improvement insafety and damage prevention that can be achieved throughproper application of structural control; the associated cost-effectiveness also can be assessed.At present, several active controi systems have been de-veloped for real structural applications. These are, by andlarge, discrete and localized sensing and control systems.In fact, only a single control mechanism, such as an activebracing system or an active mass damper, is usually incor-porated into a structure. A logical extension of this researcheffort is distributed sensing and control, which is rooted morein aerospace structures and defense systems. Structures ofthis type are adaptive to their external excitations by havingembedded sensors and actuators located about the structureand coordinated through a control network. Such an adaptiveor smart structure is then capable of responding sponta-neously to external loads such as earthquakes to minimizeundesired effects.One such example is research on neural network control-l e r ~ . ~ ~ * ~ ~Owing to the attributes of a neural network, such asadaptivity, robustness, and inherent ability to handle nonlin-earities, and the ability for systems to recover from partialfailure, research is underway to develop the methodology foractive control of structures using neural networks. In one suchneural network structural control system being in~estigated,~~the controller is trained with the aid of an emulator that itselfmust be trained first. The emulator network learns the transferfunction between the actuator signal and the sensor readings.Consequently, the information needed for the training of theactuator can be obtained readily by sending random signalsto theactuators and measuring the response of the structure atthe sensors. After the training of the emulator, the controllernetwork is trained directly on the structure by satisfying theappropriate control criteria. At present, appropriate softwareshave been developed for numerical simulation of the struc-ture, the control system, and the neural network simulators.Laboratory verification will follow in the next phase.2.2 Sensing and actuationAlternativesto conventional sensing and actuation approachesfor control of large-scale flexible structures have been a re-


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Second Generation ofActive Structural Control in Civil Engineering 291

cent focus of research an d developm ent. The use of structuralmembers with em bedded sensors and actuating materials inlarge structures is especially interesting because they canserve as building blocks of a sm art structure at the atomicor molecular level. They can be fabricated in such a mannerthat sensors and controllers are a part of the microstructureof the material itself.Research on neural networks as described above is oneexam ple in this direction. Other activities in this area includethe following:

Electrorheologic (ER) materials. ER materials for struc-tural applications are being deve loped. They have the capacityto adapt or change their material properties almost instanta-neously with cha nges in an app lied electric field. These rapid,reversible change s are well suited for use in vibration controlof civil engineering structures. In ref. 8, the potential use ofER materials in smart structures is explored experimen-tally. A sim ple structural model w as fitted with a con trollableER damper. A control sch eme using force pulses of short du-ration generated by the inertia force of the auxiliary mass wasinvestigated. The se forces are timed to oppose the excitationforce and thus reduce the response. Initial results show thatthese materials have a quick response tim e and can exhibit awide range of effective viscosities that enable them to be in-corporated successfully in practical active structural controlapplications.Active axial force structural members constructed, at leastin part, from ER material are being dev eloped.I2 The dev icecurrently being tested in the laboratory consists of a seriesof closely spaced parallel plates constructed in a rectangularconfiguration. The space betw een the plates is filled with anER material. Alternate plates are connected to an electricalground and the others to a high-voltage, low-amperage powersupply. From in itial tests it has been determin ed that the pro-totype device is capable of even higher forces than expectedand that compac t full-scale configurations are possible.

Piezoelectric layers. Piezoelectric actuators are examplesof a new type of actuation device currently receiving atten-tion. For example, the ability of piezoelectric actuators ina composite to cancel vibration is being explored using acantilever beam with base e ~c it a ti o n .~A com plete software-based motion-control system is under development usingobject-oriented programming techniques. A new piezoelec-tric actuator for vibration control of civil engineering struc-tures is also being developed. It consists of a constrainedpiezoelectric layer sandwiched between two layers of a struc-tural member. The most important difference between theproposed actuator and traditional actuators is that the con-strained piezoelectric lay er uses the thickness shear mod e ofthe piezoelectric effect. This enables a much greater controlaction to be applied to the main structure, with the actua-tor layer undergoing minimum bending deformation. It thusneeds much lower voltage to generate a comparable bendingmoment.

Shape memory alloys (SMA). Shape memory alloys are afamily of metal alloys exhibiting solid-state phase transfor-mation and have the cap acity to recover large strains througha temperature-induced or a stress-induced phase transforma-tion. The exploitation of these properties of shape memorymaterials to provide either passive or active control of struc-tures is the basis of recent research in this are a. Several seis-mic simulator tests of model structures with SMA energydissipaters have been carried Results show that theycan provide additional damping and controlled stiffness tothe model structures.

Opticalfiber sensors.The potential of optical fiber sensorinstrumentation for the evaluation of large civil structures isdiscussed in ref. 6 . Although such sensors have been devel-oped during the past 15 years for initial specialized appli-cations in aerospace, hydrospace, and biomedical systems,recent attention has been given to the transitioning of thesemethods to the evaluation of civil structures. The major ad-vantages of optical fiber sensors in this area are their inherentimmunity to electromagnetic interference and instrumenta-tion ground loops, the practicality of multiplexed measure-ments over the long distances typical of civil structural sys-tems, the robustness of fiber sensor elements in harsh envi-ronments, and the resolution and accuracy of measurementspossible for a wide range of physical observables.Sm art materials targeted specifically to civil engineeringapplications are also being developed. For exam ple, a smartcom posite material is being d eveloped that has a self-healingcapability whenever and wherever cracks are generated as aresult of environm ental loads.18 Controlled cracking is usedto activate the release of chemicals into cracks for sealingpurposes. The chem icals are stored in hollow fibers togetherwith crack control reinforcing fibers.

2.3 Hybrid controlThe com bined use of active and passive, or hybrid, systemshas received considerable attention recently. Hybrid controlcan alleviate some of the limitations that exist for either thepassive system or the active system operating singly, thusleading to an effective protective system . The bulk of the workin this area in the United States has been focused on comb in-ing a base isolation system with an active device. Sinc e baseisolation systems exhibit nonlinear behavior, nonlinear andother robust control laws have beenand several small-scale hybrid con trol experiments have beencarried out. In one s t ~ d y , ~ ~ , ~ ~a sliding isolation system wascombined w ith displacement control devices. More recently,a structural model was built and tested with a hybrid con-trol ~ y s t e m . ~ ~ , ~ ~ , ~ The hybrid system consisted of a series oflow-friction sliding bearings using highly p ressurized Tefloninterface sliding against stainless steel. The system was de-veloped to reduce the absolute accelera tion of the foundation


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292 G.W Housner, i? i? Soong, an d S.F . Masri

using a variety of control algorithms from variable friction toacceleration feedback.Other experiments include a hybrid isolation system usingfriction controllable sliding bearing^.^ During earthquakes,this isolation system controIs the friction force on the slidinginterface between the support structure and the ground byadjusting the bearing chamber pressure in order to confinethe sliding displacement within an acceptable range.These experiments were successful in demonstrating theadvantages of a hybrid control system over a passive baseisolation system in motion reduction of the superstructure.Verification of its practical feasibility, however, still awaitsresults using more realistic structural models or full-scalestructures.The cost associated with hybrid systems is anotherimportant consideration.Other hybrid systems that have been considered includecombinations of rubber bearings and variable dampers, rub-ber bearings and actuators, as well as sliding bearings andactuators for bridge application^.^^ The feasibility of addingactive control to further enhance the effectiveness of tunedliquid dampers is being in~estigated.'~Finally, analytical and experimental investigations of hy-brid control systems designed to combine passive dampingprovided by cladding-structure interaction with robust activecontrol systems are being conducted.' Initial research effortshave focused on the incorporation of a simple active controlsystem in a building model already being used for investiga-tions of passive damping through cladding-structure interac-tion. The initial results using an LQR controller have shownthat the roughly 50percent response reduction achieved withpassive elements alone can be reduced by another 50 per-cent using the active control system. At the same time, thepeak force requirements for the active system are reduced bya half compared with the situation without passive dampingaugmentation.2.4 Implementational issuesBased on recent observed performance of full-scale activecontrol systems under actual wind forces and ground excita-t i o n ~ , ~ ~ , ~ ~initial steps have been taken to address severalimplementational issues that have a more direct impact onwider structural applications of active and hybrid control. Oneof these issues being addressed is system integration. Whilemuch progress has been made in the study of components ofpassive and active control systems, more attention needs to bepaid to the overall performance of integrated systems whenapplied to realistic structures. A structural control system(active and hybrid systems in particular) consists of a numberof important components. In addition, some systems operateonly intermittently with long dormant periods.Thus a numberof implementation-related issues must be addressed beforethese systems can be widely accepted.

An integrated control system consists of components such

as sensors, controllers, hydraulic systems, and force gener-ators that are integrated into the active system, and their in-tegrated performance produces a feedback or a feedforwardoperation in a direct active or a hybrid mode. System integra-tion issues currently being addressed include the following:(1) integrated fail-safe operations, ( 2 )integrated safety, reli-ability, and maintenance, (3) softwarehardware integration,and (4) self-identification and diagnostics.2.5 International cooperationAs evidenced by numerous publications and presentations,Japan and the United States have had a particularly closeworking relationship on an international level. Research co-operation between Japanese and U.S. researchers and engi-neers in the area of active and hybrid control of structures forwind and earthquake hazard mitigation amply demonstratesthe synergetic effect of cooperative research addressing im-portant problems of mutual interest. These joint efforts havesignificantly advanced the state of the ar t of active and hybridcontrol technology.

A significant development in recent years is the spreadof this collaborative effort to other countries. A number ofinternational workshops and conferences on structural con-trol have been held in other parts of the world, and severalinternational cooperative programs in structural control havebeen initiated. These include the following: US-China , pas-sive and active control of towers and bridges; U.S.-Taiwan,passive and active control research through full-scale testingprograms;U.S .-Hong Kong, experimental structural controlinvestigations based on a full-scale building; US.-Italy, the-oretical studies on active control; and US.-Spain, controlalgorithm development.The manifestation of these international activities can onlylead to a more rapid advancement of structural control tech-nology and a better potential for a safer and more dynamicfuture for civil engineering construction.

3 SECOND GENERATION OF ACTIVE CONTROLIn order to identify future research directions, it is prudentto consider potentials for payoffs of active control researchin the United States. It is ou r view that, in the short term,substantial benefit can be derived from applying structuralcontrol technology to enhancing integrity and safety of cnti-cal structures and facilities such as hospitals, emergency cen-ters, high-rise buildings and towers, and offshore platforms,whose failure or collapse under extraordinary loads couldlead to disastrous consequences.In the long term, the concept of structural control is notonly attractive but potentially revolutionary, since i t elevatesstructural concepts from a static and passive level to one ofdynamism and adaptability.Its success will have even greater


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Second Gen erationofActive Structural Control in Civil Engineering

Table 1Some distinguishing featuresof civil engineering structural control

StructureSensing and actuation

Control objectivesControl strategies

FeaturesComplex system with few critical modesExhibits nonlinear behaviorFew sensors and actuatorsLimited response variables fo r sensingLarge control forces with high speedReduction of selected maximum responseImprecision in control objectivesSimple but robust and fault tolerantSuboptimal controlImplementable control laws

control

impact on civil infrastructure research when even larger andmore complex structural systems are involved. The construc-tion industry is one of the largest industries in the UnitedStates. At about $500 billion per year, it comprises about9 percent of the national GN P and employs approximately6 million people. Structural control technology can have aprofound impact on this industry as a whole and can helpspearhead technical advances in this critical area.

In line with these observations and in order to achievethese short- and long-term goals, research needs in the areaof active control can be grouped into two broad areas: (1) civilengineering-targeted research and ( 2 ) the more global view.3.1 Civil engineering-targeted research3.1.1 Unique requirements and challengesMuch of the theoretical basis in the development of activestructural control over the last 20 years is rooted in modemcontrol theory. For example, most of the control algorithmsused in the current operating control systems for tall struc-tures are based on the principles of linear quadratic regu-lator (LQR). However, it needs to be recognized that con-trol applications to civil engineering structures are unique inmany ways and present a different set of challenges. Table 1summarizes some of what we believe are the key featuresassociated with civil engineering structural control. Some ofthese features suggest a departure from conventional LQ Rformulation. For example, in comparison with conventionalcontrol design as practiced during the first generation, Ta-ble 2 shows possible differences as we tackle the controldesign problem during the second generation.3.1.2 Experiments, experiments, experimen tsWhile experimental verification constitutes an important el-ement of the development of any technologic innovation, itis particularly crucial in this area because software and hard-

Table 2Possible differences in control design

293

~ - _ _First generation Second generationState variablesObservationsPerformance

indexConstraintsControl Laws

Relative displacementsRelative velocitiesState variables

S (x T Q x+uTRu)diExternally imposedLinear

Relative displacementsAbsolute accelerationsAbsolute velocitiesAbsolute accelerations

~ ~ ~ ~~Note: x,state variables; u,control force; g [ x ( t ) ] ,some function o fx;Q and R,weighting matrices.

ware requirements for implementation of a feasible activecontrol system for structural applications are in many waysunique. Testing of possible control devices that can deliverthe required control force, for example, is necessary in orderto assess the implementability of theoretical results. Practi-cal issues such as time delay and spillover effects can onlybe addressed after one learns of their magnitude and effectsthrough experiments.As stated in the recommendations of a working group onexperimental methods,I3 there are several issues involvedwhen planning experiments for evaluation of structural con-trol systems. Performance objectives must be clearly under-stood and delineated. The types of excitations that are avail-able and that can accurately duplicate nature need to be in-vestigated. Control components must be tested so that theirbehavior may be accounted for in the system. Finally, issuesrelated to system integration must be well understood.3.1.3 Standardized benchmark testsWhile a large variety of structural control systems exists andhas found applications, there is a lack of a common basis onwhich the performances of these systems can be evaluated andcompared to arrive at a recommendation under certain spec-ified conditions such as control objectives, structural type,loading conditions, and system configuration.

Research efforts are needed to develop a set of standard-ized performance evaluation procedures and a standardizedtest program under which various control systems (passive,active, hybrid) can be realistically evaluated and their per-formances compared. Modifications, redesign, and furtherdevelopment of these systems can be made, if necessary, toimprove their performance and applicability.The standardized test program will involve the develop-ment of analysis and simulation packages, laboratory testsof scaled-down model structures, and field tests and obser-vations of full-scale structures as outlined in Fig. 1. The


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294 G.W.Housner, T . T . Soong, and S . E Masri

Excilation1 Package 1 Objectives SpecificationsComponent

I+Analysis

andSimulation

*.............__...______.):........__._______.____

1Control

TestComponent -

ComponentImprovement

I SnctureTest

ITestPerformance Evaluationand Design Guidelines

Fig. 1. Standardized test program.

success of carrying out this program will permit the devel-opment of comprehensive performance evaluation and de-sign guidelines for control/structure systems. These guide-lines need to be simple but comprehensive to include specificissues of structural performance along with resources, main-tenance procedures, and control operations. These guidelinesare needed to allow realistic design and performance evalua-tion of control systems.

3.2 More global views3.2.1 Integration with other protection technologiesIn incorporating active control into a structure either as a newdesign or as a retrofit, it is important to consider active controlas a member of a family of innovative protection technolo-gies that include, among others, base isolation and passiveenergy dissipation. For a specific application, technical mer-its and cost-effectiveness of active control systems can thenbe evaluated more realistically in this context.The standardized benchmark tests proposed in the pre-ceding section also will contribute to achieving a better un-derstanding of relative merits of these innovative protectivesystems.

3.2.2 Smart materials and sm art StructuresAt present, most of the operating control systems installed incivil engineering structures are discrete with localized sens-ing and control systems. A logical extension of this researcheffort is distributed sensing and control, which is rooted morein aerospace structures and defense systems. Structures ofthis type are adaptive to their external excitations by havingembedded sensors and actuators located about the structureand coordinated through a control network. Such an adaptiveor smart structure is then capable of responding sponta-neously to external loads such as earthquakes to minimizeundesired effects.Structural members with embedded sensors and actuatingmaterials are inherently composite materials. The use of com-posite materials in smart structures is especially interestingbecause they can serve as building blocks of a smart struc-ture at the atomic or molecular level. They can be fabricatedin such a manner that sensors and controllers are a part of themicrostructure of the material itself.In another direction, active control can be broadened toserve a number of additional intelligent functions. With itssensing and control capabilities, the integration of active con-trol into structural design, construction, health monitoring,maintenance, and repair, as well as response control, can leadto substantially better utilization of materials and lower cost.3.2.3 Infrastructure researchActive control has a close tie with the Civil Infrastructure Re-search (CIS) Program being developed by the U.S. NationalScience F~undation.~~CIS is envisioned to comprise threeimportant elements common to most of the infrastructuresystems: deterioration science, assessment technologies, andrenewal engineering. Someof the research topics included inthese research elements are indicated in Table 3. It is clearlyseen that active control technology can play an importantrole particularly in assessment technology related to sensing,monitoring, and nondestructiveevaluation and in renewal en-gineering related to repair, strengthening, and renewal.In this context, a more global view of active control re-search is again required. An infrastructure system can beconsidered to have nodes (structures, towers, dams, storagetanks) and interconnections (highways, bridges, pipeline^).'^Much effort has been devoted to the use of control for indi-vidual nodes such as buildings, but less effort has gone intothe development of integrated system monitoring and controlstrategies for infrastructures.

4 CONCLUDING REMARKSAs the preceding sections show, much progress has beenmade in research and developmentof active control technol-ogy in the United States. However, the true potential of activecontrol has not been exploited fully. While recent progress in


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Second Generation of Active Structural Control in Civil Engineering 29 5

Table 3. Infrastructure research program24Deterioration Assessment Renewal

science technologies engineeringFailure processes Loads and natural System mode linghazardsCorrosion Health monitoring Performanc e criteriaFatigue and Sensing and New and smart

environmental hazards instrumentation materialsStrength and Nondestructive Strengthen ing anddurability evaluation repairStability and integrity Dam age assessment Structural control

several key areas has been sum marized , it is our intention topropose at this time new thrust areas to which mo re focusedresearch efforts need to be d irected.It is gratifying to note that the U.S. National Science Foun-dation, from which much of U.S. support for structural con-trol research has come, has recognized the im portance of thisresearch area for im provements to existing and future civilinfrastructure systems. Beginning in 1989, a focused effortto foster coordinated multidisciplinary research and develop-ment in the United States was made. Funding was providedfor creating a U.S. Panel on Structural Control Research.In 1991, the National Science Foundation established a re-search initiative for structural control for safety, performance,and hazard mitigation. A 5-year program was launched witha budget of $1 million per year. The goal of the programwas to fund research for developing control system s, robots,actuators, sensors, and energy absorbers for structures andto investigate practical designs, fabrication, and installationtechniques for field applications.

ACKNOWLEDGMENTSIt is a pleasure to acknowledge the support of the U.S. Na-tional Scienc e Foundation in forming the U.S.Panel on Struc-tural Control Research, in developing the 5-year ResearchInitiative on Structural Control, and in promoting researchefforts in this area. In particular, we wish to thank Dr. S. C.Liu and Dr. M. P. Singh of the National S cience Foundationfor their guidance, encourag emen t, and active interest in thisresearch area.

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