Large Flexible Structures Research Articles

Control of large angle maneuvers for the flexible solar sail

Solar sail is a new type of spacecraft for deep space exploration, which flies by the pressure of sunlight. The attitude of the sail determines its orbit, so altitude control plays an important role in the mission. However, the large flexible structure leads to some difficulty in attitude control. This paper establishes the reduced dynamic model of a flexible solar sail with foreshortening deformation, and coupling with its attitude and vibration. As usual, large angle maneuvering will lead to the vibration of flexible structure, so the time optimal control of solar sail maneuvering is considered. Bang-Bang control of the solar sail generates large amplitude and sustained vibration, while the combined control based on input shaping can eliminate the vibration efficiently. With the comparison of two reduced models, it is demonstrated that the choice of two models depends on the attention to the stretching deformation.

The Vibration Characteristics of a Large Flexible Vibration Isolation Structure with Finite Element Analysis and Modal Test

A large flexible vibration isolation structure is presented in this thesis, and experimental modal test based on the finite element analysis is carried out in order to find out the vibration characteristics of it. Results show that the natural frequencies and mode shapes calculated by finite element method basically conform to those measured from experimental modal test. Some suggestion to vibration active control in further research is also provided.

Robust Attitude Control Design for the BIOMASS Satellite (Earth Explorer Core Mission Candidate)

AbstractThe purpose of this paper is to describe the current activities, results to date, and future activities of the European Space Agency (ESA) Robust AOCS technology program in support to the Phase A of BIOMASS, candidate as Earth Explorer Core Mission 7. Due to the specificity of the chosen BIOMASS configuration this activity is driven by the expected interaction between the large flexible reflector antenna structure and its attitude control system. Currently ESA has developed a technology program to enable the capabilities of integrating the structural sizing and control system design in order to avoid interactions problems. The objective is the development of an Integrated Modeling, Control and Analysis framework IMCA: it incorporates uncertainty modeling via LFT's, robustness analysis via the Structured Singular Value μ and various robust control synthesis techniques such as H∞ and μ methods. This framework results as natural multivariable extensions of the classical Bode frequency domain techniques. It shall be integrated with a structural design loop into an unified computational framework to exploit control structures interactions in order to increase the spacecraft capabilities, such as better pointing stability, and to improve the overall design when compared to the traditional approach. In the execution of this program two parallel activities will be presented, respectively one by Astrium Limited, UK, and the other by ThalesAlenia Space (TAS), Italy.

Robust H ∞ vibration control for smart solar array structure

The multi-panel deployable solar array structure is the most common system utilized for spacecraft power system. To effectively suppress vibration of the solar array structure is still a challenging task. The present works focus on dynamic modeling and robust H∞ vibration control for a large flexible spacecraft solar array structure using piezoelectric actuators and velocity sensors. The structural dynamic model of the solar array structure with piezoelectric actuators is generated by using finite element techniques. The order of the system is reduced by employing Modal Hankel Singular Value method. Based on Linear Matrix Inequality technique, a robust H∞ dynamic output feedback controller is designed to suppress the vibrations caused by external disturbances. The simulation results show that the vibration can be significantly suppressed with permitted actuator voltages by the controller. The robust H∞ controller can avoid the spillover due to mode truncation.

Vision-based displacement measurement method for high-rise building structures using partitioning approach

The horizontal displacement of a high-rise building structure is usually considered as one of the major indicators to assess the structural safety. It is, however, difficult to directly measure the displacement of such a flexible structure due to the inaccessibility to a reference point usually needed for conventional displacement sensors and the huge size of the structure. In order to resolve this issue, a novel vision-based displacement measurement technique is proposed by employing a partitioning approach (i.e., successive estimation of relative displacements and rotational angles throughout a large flexible structure). A series of the experimental tests using two webcams installed on a flexible steel column structure have been conducted to validate the feasibility of the proposed method. The test results showed that the difference between the displacement measured from the proposed method and the exact value is less than 0.5%. Therefore, the proposed method could be considered as one promising candidate for measuring the displacement of high-rise building structures.

Dielectric electroactive polymer applications in buildings

The use of smart materials in buildings is investigated. In particular, a new category of smart materials, called dielectric electroactive polymers (DEAPs), is discussed. DEAPs are materials whose shape and stiffness can be changed by electric stimulus. These properties led the author to investigate the potential application of DEAPs in buildings. DEAPs are produced by covering a soft polymer with dielectrics on both sides. The properties of DEAPs are described to illustrate the differences between these and other smart materials. A description of the electric behaviour of the material is also presented. Applications of DEAPs in heating, ventilation and air-conditioning systems, to control air or water motion, are additionally considered. Applications in large flexible structures such as building envelopes are described. As DEAPs can be laminated easily and they have great power density, they can be used in the management systems of facades, to control transparency or ventilation of the building envelopes. Finally, vibration isolation possibilities and building monitoring purposes are discussed. The state-of-the-art of DEAP science is reviewed and possible applications in buildings are described.

Structural Characterization of Protein‐Imprinted Gels Using Lattice Monte Carlo Simulation

AbstractMolecular imprinting is a technique for the creation of artificial recognition sites in synthetic organic or inorganic polymers, formed by cross‐linking in the presence of a template molecule. Removal of the templates leaves cavities that fit the template molecules in size, shape and functionality. Although this technique has been shown to be effective when targeting small rigid molecules, attempts to extend it to larger templates, such as proteins, have failed to show similar success. As opposed to small molecules, proteins are characterized by large size, flexible structure, and large number of functional groups available for recognition. These characteristics, among others, make it impossible to use imprinting procedures of small molecules for protein imprinting. In order to plan new imprinting strategies for proteins, one must reveal the problematic points in the polymerization process of the current methods, which result in low imprinting efficiency. In our research we focus on a bulk imprinting method using radical polymerization of hydrogels, applied to small globular proteins (e.g., lysosyme and cytochrome c). We model the system using lattice Monte Carlo (MC) simulation, using a modified kinetic gelation model (KGM) to simulate the polymerization process, which results in an inflexible protein‐imprinted gel. The effect of initiator molar ratio, polymer volume fraction (VF), cross‐linker density, and protein charge density and distribution on gelation, gel structure, and on the imprinted pore structure and functionality is investigated in the dry state of the gel. It is found that the imprinted pores wrap the template protein more fully and with larger number of linking sites than the non‐imprinted pores. The insertion of non‐template proteins with various charge densities and distributions into the imprinted pores indicates that charge density of the adsorbing protein is the dominant factor influencing the number of linked recognition sites.

Selection of tested component to predict dynamic properties of large structures by component modal tests

Large flexible structures cause the full-scale ground tests to be impossible sometimes because they are not strong enough to withstand the force of gravity. The focus of the research into this problem is to confirm the dynamic characteristics of large structures without testing the whole structure. Admire et al. used the mass additive method in a modal test to avoid problems associated with design and development of test fixtures for Space Shuttle payloads [1]. Karpel et al. proposed a method to test components with boundary coordinates loaded with rigid, heavy dummy masses supported by soft springs [2]. Dummy masses generate local displacements near the boundaries to predict the dynamic properties of the whole structure. Misawa and Funamoto [3] proposed a method to predict the dynamic characteristics of large structures with component modal tests. In this method, to simulate the dynamic behavior of structures, reduced mass and stiffness matrices of untested components are replaced with a mass and a spring attached to a tested component. Misawa et al. showed a procedure to select a location with additional mass and stiffness for measuring accurate dynamic characteristics of structures by component modal tests [4]. This method needs no CMS.

New Sensor and Actuator for Space Tether Control Technology

Tether technology is highly significant for the realization of large space flexible structures. It is possible to decrease the weight of a structure by employing tether on the system. In addition, a tether system can be employed to increase the stiffness of space structure systems and also to suppress the vibration of flexible space structures. When tether is used in a mission, the measurement of the tether's dynamic behavior, as both tension and speed, is necessary, in order to control the vibration of flexible space structures. The range of tension should approach micro level at 10-2 N in micro power, to control the structure in space. In this paper, the effectiveness of vibration control is examined by employing a tether on a flexible space structures such as a SSPS. The vibration control is analyzed numerically to simulate the behavior of a flexible space structure in a flexible panel-tether system and the effectiveness is shown for the tether tension control. A contactless measurement method is also studied by measuring the tether tension and the speed without contacting tether. This method allows the tether tension to be measured without any disturbance.

Coordinated Control of Space Robot Teams for the On-Orbit Construction of Large Flexible Space Structures

The construction of future space structures in Earth orbit such as solar power stations and space telescopes will require coordinated teams of autonomous space robots. These robot teams can excite undesirable vibrations in the structures while manipulating or assembling them. Controlling robot teams physically interacting with structural elements in space is challenging. The combined system dynamics are described by sets of nonlinear partial differential equations. Here, these dynamic equations are transformed into a set of linear time-varying ordinary differential equations. The control of the high-frequency robots is then decoupled from the control of the much lower frequency structures. This allows linear optimal control theory to be used to control the robots and minimize structural vibrations. Simulation and experimental studies shown here demonstrate the validity of the approach.

Simulation of Protein-Imprinted Polymers. 1. Imprinted Pore Properties

Molecular imprinting is an established method for the creation of artificial recognition sites in synthetic materials through polymerization and cross-linking in the presence of template molecules. Removal of the templates leaves cavities that are complementary to the template molecules in size, shape, and functionality. Although this technique is effective when targeting small molecules, attempts to extend it to larger templates, such as proteins, have failed to show similar success. As opposed to small molecules, proteins are characterized by large size, flexible structure, and large number of functional groups available for recognition, which make it impossible to use imprinting protocols of small molecules for protein imprinting. In this research we use lattice Monte Carlo simulations of an imprinting process using radical polymerization of hydrogels as a simple model for protein-imprinted polymers (PIPs). We investigate the properties of the resulting polymer gel by studying the effects of initiator, cross-linker, and monomer concentrations and the presence of protein on gel structure and porosity. The structure and functionality of the imprinted pore is studied through diffusion of the protein inside the pore immediately following polymerization. The imprinting effect is evaluated by comparing the interaction energy of the protein in the imprinted gel with the energy of a random process.

Modelling of dynamic interaction between structure and trolley for mega container cranes

This article deals with the analysis of trolley impact on the dynamic behaviour of the flexible structure of the mega quayside container crane (QCC) boom, identified as the most relevant structural part. It develops a modelling method for the dynamic response of the large flexible structure of the QCC boom under a moving trolley. By using FEM the original structure of the whole crane structure is reduced to an equivalent model of the boom. The boom is in this way modelled as a system with distributed parameters, comprising reduced stiffnesses and lumped masses from other parts of the upper structure. The article looks at the moving mass approach to achieve the desired performance of the QCC. Differential equations of the mathematical model are obtained by using Lagrange's equations and the assumed mode method. The continuum is discretized by a finite number of admissible functions. Deterministic simulation gives the dynamic response of the boom for quay-to-ship container transfer. Results are obtained for the boom deflection and bending moment values, as well as for the dynamic amplification factor of deflection.

System Identification and Robust Controller Design Using Genetic Algorithms for Flexible Space Structures

This paper is concerned with the problem of identifying and controlling flexible structures. The structures used exhibit some of the characteristics found in large flexible space structures (LFSSs). Identifying LFSS are problematic in the sense that the modes are of low frequency, lightly damped, and often closely spaced. The proposed identification algorithm utilizes modal contribution coefficients to monitor the data collection. The algorithm is composed of a two-step process, where the input signal for the second step is recomputed based on knowledge gained about the system to be identified. In addition, two different intelligent robust controllers are proposed. In the first controller, optimization is concerned with performance criteria such as rise time, overshoot, control energy, and a robustness measure among others. Optimization is achieved by using an elitism based genetic algorithm (GA). The second controller uses a nested GA resulting in an intelligent linear quadratic regulator/linear quadratic Gaussian (LQR/LQG) controller design. The GAs in this controller are used to find the minimum distance to uncontrollability of a given system and to maximize that minimum distance by finding the optimal coefficients in the weighting matrices of the LQR/LQG controller. The proposed algorithms and controllers are tested numerically and experimentally on a model structure. The results show the effectiveness of the proposed two-step identification algorithm as well as the utilization of GAs applied to the problem of designing optimal robust controllers.

Deployment Control of Spinning Space Webs

Future solar sail and solar power satellite missions require deployment of large and lightweight flexible structures in space. One option is to spin the assembly and use the centrifugal force for deployment, stiffening and stabilization. Some of the main advantages with spin deployment are that the significant forces are in the plane of rotation, a relatively simple control can be used and the tension in the membrane or web can be adjusted by the spin rate to meet the mission requirements. However, a successful deployment requires careful development of new control schemes. The deployment rate can be controlled by a torque, applied either to a satellite in the center or by thrusters in the corners, or by deployment rate control, obtained by tether, spool braking or folding properties. Analytical models with only three degrees of freedom were here used to model the deployment of webs and membranes for various folding patterns and control schemes, with focus on space webs folded in star-like arms coiled around a center hub. The model was used to investigate control requirements and folding patterns and to obtain optimal control laws for centrifugal deployment. New control laws were derived from the optimal control results and previously presented control strategies. Analytical and finite element simulations indicate that the here developed control laws yield less oscillations, and most likely more robustness, than existing controls. Rotation-free (RF) shell elements can be used to model inflation or centrifugal deployment of flexible memebrane structures by the finite element method. RF elements approximate the rotational degrees of freedom from the out-of-plane displacements of a patch of elements, and thus avoid common singularity problems for very thin shells. The performance of RF shell elements on unstructured grids is investigated in the last article of this thesis, and it is shown that a combination of existing RF elements performs well even for unstructured grids.

PI force feedback control for large flexible structure vibration with active tendons

Active tendon, consisting of a displacement actuator and a collocated force sensor, was first presented by Preumont and his co-workers to attenuate the vibration of large flexible space structures, and the control algorithm adopted by them was integral force feedback. This paper presents a new proportional-integral (PI) force feedback algorithm to achieve larger damping ratios for the structure without the requirement of structure model. Stability of the control system is shown, and simulations of a structure similar to JPL-MPI demonstrate the effectiveness of the proposed algorithm for vibration control of space structures.

Wind turbines: New challenges and advanced control solutions

Wind turbines: New challenges and advanced control solutions

A finite element based stability test for equilibria of flexible structures in circular orbits

Large flexible structures could be one of the most challenging space technologies of the future. For example, the well-known space elevator may one day guide astronauts along a 36,000 km long tether from the earth's surface to a geostationary orbit. In this paper, we imagine an arbitrary space structure in a stationary circular motion around a celestial body. Usually such systems require two problems to be solved: the strength of the materials and an overall stability analysis. The latter task is a non-trivial one since in orbiting systems not the angular rate but the angular momentum must be kept constant when applying, for example, Dirichlet's criterion. In particular, this fact becomes important if the system's dimension is in the magnitude of the orbital radius. Therefore, this paper will focus on a general applicable method to determine the stability of flexible earth-orbiting structures. The analysis process is based on the reduced energy momentum method presented by J.C. Simo, T.A. Posbergh and J.E. Marsden, which has been customized for use in systems with cyclic coordinates by the author. In the presented approach the second variation of an amended potential is derived from a standard finite element analysis. An efficient stability test is introduced, which limits the computational effort to the evaluation of a few eigenvalues and eigenvectors of the system's tangent stiffness matrix. Finally, two examples are discussed to demonstrate the application of the method.

4029 大面積柔軟構造物を有するスピン衛星の姿勢運動解析(S61-2 宇宙システムの誘導・制御(2),21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

This paper describes the attitude dynamics of spacecraft with large flexible structure, such as huge antenna or membrane of solar sail. The coupled motion of the rigid spacecrafts with the flexible structure is complicated, and it is important to predict the motion for the design of configuration or operation planning. It requires a lot of time to calculate the motion of the flexible structure by numerical simulation, and the analysis using simple model is important for exhaustive validation. In this study, a simplified model of the attitude dynamics considering the first vibration mode of flexible structure is introduced. Using this model, the vibration mode of the attitude motion of spacecraft is analyzed. The result of the analysis is confirmed with numerical simulation and compared with the result obtained by use of proven model.

Vibration Control of Cantilever Using SMA Tendons

In recent years tendon control systems for large flexible structures have been investigated, and in most of researches on vibration control with tendon mechanism, electric motors are used as actuators. In this paper, shape memory alloys (SMAs) formed like wires are used as tendons. Because SMA tendons are considered as actuators which shrink with heat produced by applied current, space saved tendon control systems can be realized without lubrication. An actuator unit which consists of an SMA tendon and a spring is set up, and transfer functions of the actuator unit relating an applied voltage to a generated force are measured. Characteristics of the actuator unit are determined experimentally by curve fitting technique for measured transfer functions. H-inf controllers considering uncertainty of the actuator unit performance are designed and installed in the vibration control system for a flexible cantilever with SMA tendons. It is validated that free and forced vibrations of the flexible cantilever can be well restrained by the designed controller with robust stability.

Sensor architecture for the robotic control of large flexible space structures

AbstractThe future construction and maintenance of very large space structures such as orbital solar power stations and telescopes will require teams of free‐flying space robots. For robots to effectively perform these tasks, they will require knowledge of the structure's vibrations. Here, a robotic based sensor architecture for the vibration estimation of very large structures is presented. It is shown that this information can be effectively estimated by combining data provided by free‐flying remote robot “observers” equipped with range sensors with structure‐mounted acceleration sensors. A modified Kalman filter fuses low‐bandwidth vision data from the remote sensing robots with the high bandwidth, but spatially sparse structure‐mounted acceleration sensors. Results from experimental studies are presented that confirm the effectiveness of this approach. 2007 Wiley Periodicals, Inc.