Integrated structural control design of large space structures

Active robust vibration control of flexible structures

Currently, the design of large space structures attracts engineers due to the need to solve pressing problems in the area of creating promising space systems for longitudinal operation in the outer space. A special class of space systems is formed by transformable structures whose layout can be transformed. Usually, the structures of this class are delivered to the working orbit in the densely-packed position. When the orbit having the required parameters is reached, the structure is deployed. The deployment is done by force actuators, for example, springs, electric motors, and so on. Transformable space structures are multiple-element systems comprised of many interconnected elements. Despite massive progress in the area of designing such structures, the task of ensuring smoothness and reliability of the deployment of structures with guaranteed functioning after deployment is still relevant. The paper proposes using actuators based on shape memory alloys to ensure controlled deployment of large transformable structures. Experimental investigations of a simple actuator based on the Ti-Ni alloy element confirmed the possibility of using it for the deployment of transformable space structures.

An active control system consists of three parts : the vibration system of the controlled object, sensors and actuators, and controllers. Control performance depends on the characteristics of the controllers. Therefore, we discuss control system design strategies and control performance for a typical design method of active structural control in detail. Although there are many control system design methods for active vibration control systems of structural control, we focus attention on a design method using modern and postmodern control theories, which is called robust control theory. We verify the control performances as benchmark problems for LQ control, LQG control, disturbance cancellation control, H2 control, H∞ control, μ control and sliding mode control. In this study, we use two vibration models : the active vibration absorber (AVA) model and the active dynamic vibration absorber (ADVA) model. Finally, we confirm that μ is superior to other control methods for linear structural control, and sliding mode is currently the best method for the nonlinear structural control system compared with other control methods in practical use.

Dynamic aeroelastic response and active control of composite thin-walled beam structures in compressible flow

A q-Markov COVER (q-Markov covariance equivalent realization) algorithm for single-input single-output (SISO) discrete-time systems is proposed. To develop the algorithm, state covariance assignment theory is applied. As a result, procedures for factorizations and pseudo-inversions of matrices are reduced and realizations can be given in simple and explicit forms. The results provide a computational facility and basis for understanding the structure of q-Markov COVERs for SISO systems. It is shown that the q-Markov COVERs are distinguished by the sign of system matrix determinants. >

The main trend in structural technology is to change from conventional earthquake resistant structures, which are designed 'not to collapse even under the strongest earthquake' to structural controlled buildings, which are designed 'to suppress the vibrations itself'. Indications are that control methods will be able to make a genuine contribution to this problem area, which is of great economic and social importance. Current researches in the field of active structural control focus on: system identification, model order reduction, limited control authority, available measurements etc. The article reviews some of the developments in the area of controlled civil structures and highlights a robust control perspective in active vibration suppression control of flexible structures, considering a grey box approach in the system identification process.

Active control of structures is usually performed by application of additional external forces, (tendon or support action, actuator action, [2,5,6]) which vary according to a specified rule depending on the external load variation. In this paper, the concept of active structure adaptation to varying loads will be discussed by assuming that initial distortion fields ∈ 0(x,t) within a structure can be generated and varied. Such distortion fields could, for instance, be generated by temperature fields associated with proper heating or cooling of structure elements or by introducing mechanical dislocations and disclinations in truss or beam structures. For instance, shortening or elongating of bars OC and OD of a hyperstatic truss shown in Fig. 4 induces self-equilibrated stress states σ r and the associated initial strain states ∈r. These states can be used to control the total stress and strain states σ=σ r + σ l and ∈=∈ r + ∈ l of a structure where σ l, and ∈ l are the stress and strain states due to external loading. The concept of active control of civil engineering or space structures by means of distortion fields provides an additional feasible option especially in cases where quasistatic control is required, for instance in counteracting the environmental temperature changes and their effect on structure stress and displacement fields.

The present paper combined the natural environment excitation technology (NExT) with the eigen system realization algorithm (ERA) to identify the modal parameters of a suspension bridge. Firstly, the critical parameters in NExt–ERA, such as record length, window length, and ranks of the Hankel matrix, and their influence on identification accuracy have been discussed. Subsequently, based on the acceleration series obtained from the Xihoumen Bridge Health Monitoring System, 3015 sets of modal frequencies and modal damping ratios of the Xihoumen Bridge were identified under 7 sets of wind speeds by the NExt–ERA. Finally, the probability distribution and confidence interval of the structural modal damping ratios were analyzed. Besides, the dependence of identified frequencies damping ratios on wind speed was also discussed. The results show that at the same wind speed, the mean value, and variance of the torsional and lateral mode damping ratios are larger than those of the vertical mode, but this difference gradually decreases with the increment of wind speed. The modal damping ratios along three directions under different wind speeds all follow the generalized extreme value distribution, but wind speed affects the tailing properties of probability distribution of the vertical and torsional mode damping ratios. The estimated damping ratios could provide references for structural design and vibration control of similar long-span bridges.

Acontroldesignmethodispresented forlinearplantmodelswith stochasticstatedisturbancesandmeasurement noise such that time domain and frequency domain specie cations are met. A time invariant linear quadratic Gaussian (LQG) controller structure is employed. The LQG controller is parametrized by the diagonal elements ofthestateweighting matrix Qand statenoiseintensitymatrix L.Astandard searchalgorithmisappliediteratively to compute Q and L such that all of the specie cations are satise ed. At each iteration step of the search algorithm, an LQG problem is solved.

This paper compares the Model predictive control (MPC) and Linear quadratic control (LQC) of heavy vehicles via active front steering for rollover prevention in inclement environments. In both control methods, a Disturbance rejection control (DRC) that negates the effects of wind and road bank acting on the vehicle is designed. Load transfer ratio (LTR) is applied to judge rollover by mitigating the absolute value of LTR as much as possible. We tested and compared two different controllers, i) MPC with DRC and ii) LQC with DRC. Two types of environmental conditions were considered, i) typhoon and ii) typhoon on a bumpy road. The simulation results demonstrate that MPC was more successfully implemented than LQC during LTR suppression. This paper also proposes an MPC for coordination of active rear steering and differential braking control maneuvers to prevent rollover in inclement environments. For a feasible comparison, the LQC controller was designed using the same approach adopted for the MPC controller. Results show the proposed coordinated control yields better performance for rollover prevention than LQC.

: The research documented in this report addresses several major technical issues in vibration controller design methodologies that are of fundamental importance to control of large flexible space structures. Fundamentally important analytical and numerical results, as well as useful new insights, have been obtained on: optimality of fixed-form reduced-order compensators; full-order closed-loop stability and robustness with reduced-order controllers; prevention of control and observation spillover; active augmentation of damping, stiffness, and stability, stabilizability by output feedback control; large-angle maneuver with simultaneous vibration suppression; and possibilities for improving current controller design methodologies.

Active vibration control of smart hull structure using Macro Fiber Composite (MFC) actuator is performed. Finite element modeling is used to obtain governing equations of motion and boundary effects of end-capped smart hull structure. Equivalent interdigitated electrode model is developed to obtain piezoelectric couplings of MFC actuator. Modal analysis is conducted to investigate the dynamic characteristics of the hull structure, and compared to the results of experimental investigation. MFC actuators are attached where the maximum control performance can be obtained. Active controller based on Linear Quadratic Gaussian (LQG) theory is designed to suppress vibration of smart hull structure. It is observed that closed loop damping can be improved with suitable weighting factors in the developed LQG controller and structural vibration is controlled effectively.

An adaptive failure compensation control scheme is developed based on linear quadratic (LQ) control for discrete-time possibly nonminimum phase systems with unknown actuator failures with characterizations that some unknown system inputs are stuck at some unknown fixed or varying values at unknown time instants. The failure compensation control problem is formulated as a new LQ control problem. Based on some results on new controller structures and design conditions developed in [9], a stable adaptive law is proposed for the estimation of uncertain parameters from both the plant and actuator-failures. The adaptive failure compensation controller is implemented with adaptive parameter estimates, for achieving closed-loop stability and output regulation. Simulation results are presented to verify the desired adaptive control performance in the presence of uncertain actuator failures.

This paper describes the results of an active structural control experiment performed for the Advanced Control Evaluation for Structures (ACES) testbed at NASA-Marshall as part of the NASA Control-Structure Interaction Guest Investigator Program. The experimental results successfully demonstrate the effectiveness of a 'dipole' concept for line-of-sight control of a pointing system mounted on a flexible structure. The simplicity and effectiveness of a classical 'single-loop-at-a-time' approach for the active structural control design for a complex structure, such as the ACES testbed, are demonstrated.

This chapter presents the results of experiments comparing different computationally efficient model predictive control (MPC) methods applied to a laboratory device, demonstrating the active vibration control (AVC) of lightly damped mechanical structures. Because of the combination of long prediction horizons, short sampling times and large actuator-disturbance asymmetry, the implementation of the predictive control strategy on lightly damped vibrating structures is highly demanding. The vibration damping effect and online timing properties of model predictive control algorithms such as infinite horizon cost dual-mode quadratic programming based MPC (QPMPC), pre-computed explicit multi-parametric programming based MPC (MPMPC), minimum-time MPMPC and the very efficient but suboptimal Newton–Raphson based MPC (NRMPC); all with guaranteed stability and constraint feasibility are analyzed in different disturbance and loading scenarios. All MPC methods along with the baseline linear quadratic (LQ) controller decrease vibration settling to equilibrium by an order of magnitude time. The damping effect of all investigated MPC strategies is comparable with a slight decrease in performance for the suboptimal minimum-time MPMPC and NRMPC controllers. Due to the excessive online computational needs of QPMPC, it is a very unlikely candidate for lightly damped vibrating systems given currently available hardware. The online timing analysis presented here demonstrates that MPMPC provides significantly shorter online execution times, however its suboptimal minimum-time version does not bring a convincing improvement. NRMPC can provide online execution times on par with linear quadratic controllers; however, its suboptimality becomes excessive with increasing prediction model orders.