Sridhar Sana

Microsensors for health monitoring of smart structures

Health monitoring of structural systems has gained a lot of interest in recent times. In this paper, we consider the wireless data acquisition for health monitoring of smart structures. Some of the work done towards development of micro sensors for wireless health monitoring of smart structures is presented. The concept of smart sensors is demonstrated with the help of commercially available micro controller and wireless Rx/Tx modules. Application of these smart sensors in health monitoring is also demonstrated on a laboratory set up. A subspace system identification method known as N4SID is used for getting the state space matrices of the nominal and the damaged systems. The concepts are demonstrated on simple test article. Finally, the future goals in the development of micro sensors are given.

Overview of control design methods for smart structural system

Smart structures are a result of effective integration of control system design and signal processing with the structural systems to maximally utilize the new advances in materials for structures, actuation and sensing to obtain the best performance for the application at hand. The research in smart structures is constantly driving towards attaining self adaptive and diagnostic capabilities that biological systems possess. This has been manifested in the number of successful applications in many areas of engineering such as aerospace, civil and automotive systems. Instrumental in the development of such systems are smart materials such as piezo-electric, shape memory alloys, electrostrictive, magnetostrictive and fiber-optic materials and various composite materials for use as actuators, sensors and structural members. The need for development of control systems that maximally utilize the smart actuators and sensing materials to design highly distributed and highly adaptable controllers has spurred research in the area of smart structural modeling, identification, actuator/sensor design and placement, control systems design such as adaptive and robust controllers with new tools such a neural networks, fuzzy logic, genetic algorithms, linear matrix inequalities and electronics for controller implementation such as analog electronics, micro controllers, digital signal processors (DSPs) and application specific integrated circuits (ASICs) such field programmable gate arrays (FPGAs) and Multichip modules (MCMs) etc. In this paper, we give a brief overview of the state of control in smart structures. Different aspects of the development of smart structures such as applications, technology and theoretical advances especially in the area of control systems design and implementation will be covered.

Distributed computing and sensing for structural health monitoring systems

Structural health monitoring involves automated evaluation of the condition of the structural system based on measurements acquired from the structure during natural or controlled excitation. The data acquisition and the ensuring computations involved in the health monitoring process can quickly become prohibitively expensive with the increase in size of the structure under investigation. In this paper, we propose a distributed sensing and computation architecture for health monitoring of large structures. This architecture involves a central processing unit that communicates with several data communication and processing clusters paced on the structure by wireless means. With this architecture the computation and acquisition requirements on the central processing unit can be reduced. Two different hardware implementation of this architecture one involving RF communication links and the other utilizing commercial wireless cellular phone network are developed. A simple health monitoring experiment that uses neural network based pattern classification is carried out to show effectiveness of the architecture.

Design of Robust Controllers for Smart Structural Systems with Actuator Saturation

The design and implementation of robust controllers on smart structural systems is often constrained by available control force of the actuators. The Lead Zirconite Titanate (PZT) actuators which are used for the control of flexible structures have limited control authority. The performance of the smart structural systems is often limited by the control effort constraint instead of the closed loop stability and performance. Due to the limited availability of control effort, it is desirable to utilize all of the control force in order to obtain the best performance. The research results described in this paper integrate robust control design methodologies with constrained actuator techniques for designing controllers for smart structural systems. In order to implement the proposed controllers, a two-dimensional lattice structure is designed and fabricated. The robust controllers in this paper have been designed for this test article. We have also developed the uncertainty modeling for this test article. An H2/H. controller has been designed and implemented on the lattice structure. The experimental results of the closed-loop system and the verification of the robustness properties are also presented.

Application of Linear Matrix Inequalities in the Control of Smart Structural Systems

Natural frequency variations, unmodeled dynamics and control input constraints are some of the major problems in the design of controllers in smart structural systems. The performance and robustness of the closed-loop system are often constrained by the limited actuation force available from Lead Zirconate Titanate (PZT) actuators. Hence, it is desirable to design controllers to maximize the performance and robustness without violating the control input constraints. In this paper, we give a methodology for designing output feedback robust controllers for smart structural systems using Linear Matrix Inequalities (LMIs). A procedure is developed to incorporate the real parameter uncertainty resulting from natural frequency variations and complex uncertainty due to unmodeled dynamics in a Linear Fractional Representation (LFR) to be utilized in the design. The control input constraints are satisfied by finding an invariant ellipsoid of closed loop trajectories for a given set of initial condition disturbances. An iterative procedure is given to solve the optimization problem involving Bilinear Matrix Inequalities (BLMIs). The design procedure is applied on a smart structural test article and the effectiveness of the proposed method is verified by robustness analyses and experimental tests.

Distributed arithmetic implementation of multivariable controllers for smart structural systems

Smart structural systems require the electronic control systems which are integrated into the structures to be small, light weight and power-efficient. The field programmable gate array (FPGA) is a good platform to implement such controllers. In our previous work, FPGA-based digital controllers were built and tested on a simple structural system. In order to implement multivariable controllers, the hardware resources for FPGA-based architecture need to be further reduced. Distributed arithmetic (DA) has long been proven to be a very efficient means to mechanize computations that are dominated by inner products involving constant multiplicand. The computational requirements of the smart structural controllers match this type very well. In this paper various DA structure controllers are designed and results are compared with multiply-and-accumulate structure controllers. Single- and multi-variable controllers are implemented and tested on a cantilevered beam.

Vibration isolation/suppression: research experience for undergraduates in mechatronics and smart structures

This paper provides an account of a student research project conducted under the sponsoring of the National Science Foundation (NSF) program on Research Experience for Undergraduates (REU) in Mechatronics and Smart Strictures in the summer of 2000. The objective of the research is to design and test a stand-alone controller for a vibration isolation/suppression system. The design specification for the control system is to suppress the vibrations induced by the external disturbances by at least fiver times and hence to achieve vibration isolation. Piezo-electric sensors and actuators are utilized for suppression of unwanted vibrations. Various steps such as modeling of the system, controller design, simulation, closed-loop testing using d- Space rapid prototyping system, and analog control implementation are discussed in the paper. Procedures for data collection, the trade-offs carried out in the design, and analog controller implementation issues are also presented in the paper. The performances of various controllers are compared. The experiences of an undergraduate student are summarized in the conclusion of the paper.

Multi-Objective Controller Design for Smart Structures Using Linear Matrix Inequalities

Smart structures have emerged as a result of integration of research in the areas of sensors, actuators, micro electronics, signal processing and controller design. In this paper, we develop an integrated controller design procedure for disturbance rejection and performance optimization in smart structures. There are two main challenges in such an integration. The first problem known as spill over problem (see [1], is the degradation of the performance due to the effect of unmodeled dynamics on the closed-loop system. The second challenge is the constraint on the available actuation force which can also limit the achievable performance. Hence to design controllers for effective integration with structural systems, it is necessary that these constraints are incorporated in the controller design process, necessitating a multi-objective design approach. In recent times, Linear matrix inequalities(LMIs) (see [2]) have emerged as a powerful tool for formulating and solving such multi-objective design problems. We formulated the integration problem in smart structures as a problem of designing an output feedback robust controller in the presence of uncertainties due to unmodeled dynamics and control input limits to achieve maximum possible attenuation for a given set of finite energy disturbances. The proposed method is employed to design a controller for a smart structural test article. The controller is then implemented using dSpace system and experimental results are included.

Robust control of input-limited smart structural systems

Integration of controllers with smart structural system require the controllers to consume less power and to be small in hardware size. These requirements pose as limits on the control input and the order of the controllers. Use of reduced order model of the plant in the controller design can cause spill over problems in the closed loop system due to possible excitation of the unmodeled dynamics. In this paper we present the design of output feedback robust controllers for smart structures in the presence of control input limits considering unmodeled dynamics as additive uncertainty in the design. The performance requirements for the design are specified as regional pole placement constraints on the closed loop poles. Formulation of this multi-objective design problem in terms of matrix inequalities resulted in a feasibility problem involving bilinear matrix inequalities (BLMIs) in the unknown variables. To facilitate the solution of this feasibility problem, a change of variables is used to convert these BLMIs into linear matrix inequalities (LMIs) which can be readily solved by the use of available tools. Finally, this design procedure is applied on an experimental smart structure and the results are presented.

Design of robust controllers for smart structural systems with structured uncertainties

Effective integration of sensors, actuators and controllers with the structures is key to the success of smart structures. This concept has been manifested in numerous applications of smart structures in the areas such as civil, aerospace and automotive engineering. Control systems to be integrated with the structure is of paramount importance for ensuring the performance requirements in the presence of modal parameter variations, modeling errors and control effort constraints. The primary uncertainty associated with smart structural systems use the natural frequency variations. Linear Matrix Inequalities (LMIs) can be utilized to incorporate the real parameter uncertainty due to parameter variations and control input limits in the controller design. One of the challenges in the design of such controllers is the conservatism due to over bounding effect from the multiple constraints. Additional conservatism can also come from the approximation of the real parametric uncertainty due to modal parameter variations as sector bounded nonlinear, time varying or complex valued uncertainty. Using the traditional robustness analysis methods such as small gain theorem in the controller design will result in conservative designs leading to poor performance. In this paper, we present a controller synthesis procedure based on Popov stability results for reducing the conservatism in the design. Robust controllers are designed for real- parametric uncertainty arising from natural frequency variations in the presence of control input limits. Maximum possible attenuation in the structural response due to finite energy disturbances is also achieved. Trade-off between the robustness versus the control input limit is discussed. The design procedure is applied on a smart structural test article and the results are presented.