Ph.D. Research Proposal: Biomimetic Characteristics of an Active Deployable Structure

Biomimetic structures are structures that demonstrate increased functionality through mimicking qualities of biological organisms. Self-repair and adaptation mechanisms are examples of biological qualities that can be adapted in structural engineering. Over the last decades, great strides have been made in advancing theory and practice of active structural control. However, little scientific progress has been made on biomimetic structures. Advances in sensor, actuator, and microprocessor technologies provide increasing possibilities for implementing active control systems in the built environment. Intelligent control methodologies such as self-diagnosis, self-repair and learning could be integrated into structural systems to provide innovative solutions. The general goal of this thesis is to study biomimetic characteristics of an active and deployable tensegrity bridge. Building on previous research carried out at EPFL, this thesis proposal includes the following objectives: 1) design an active control system in order to ensure damage tolerance of a deployable tensegrity pedestrian bridge; 2) extend existing strategies for self-diagnosis of the deployable tensegrity bridge to avoid ambiguous results; 3) extend existing strategies in order to achieve a more robust self-repair scheme; 4) develop algorithms that allow the active control system to learn efficiently using case-based reasoning; 5) validate the methodologies developed with experiments on a near full-scale (1/3) model. A literature survey of biomimetics, structural control, tensegrity structures, deployable structures, deployable tensegrity structures, active tensegrity structures, case-based reasoning, system identification, and multi-objective search has identified that these objectives are original. Results obtained from the preliminary studies demonstrate the potential of this research strategy. A research plan containing 19 subtasks that will be completed by the end of April 2012 leaves sufficient buffer time before the official end of this Ph.D. research on September 30, 2012.

Towards biomimetic behavior of an active deployable tensegrity structure

Nicolas Willy Veuve

Tensegrity structures have non-conventional structural properties since they require self-stress in order to be stable. When a tensegrity structure integrates sensors, actuators, (structural members able to change their length) and a control system, deployment and modification of structural behavior becomes feasible. Control of tensegrity structures is a special type of control since multiple-degrees of freedom depend on a lower number of coupled actuators. In addition, there is no direct closed-form solution for achieving a given control task, and this complicates the search for control commands. This thesis investigates the behavior of a near full-scale deployable tensegrity footbridge that deploys from both sides and connects at mid-span. Tests on the structure demonstrate that deployment behavior is not reproducible and therefore, pre-defined control commands are unsuccessful for achieving desired positions. Active control is thus, required for mid-span connection of the two bridge halves. Available simulation models involve assumptions that are not satisfied on the near full-scale structure. In this case non-dimensional, friction free joints are poor assumptions. Using measurements of the response in real-time integrates variable behavior. This advantage shows potential for controlling full-scale structures where changing environment conditions are likely. The active-control methodology for the mid-span connection of the two bridge halves, combining simplified simulations and real-time measurements, accommodates lack of accuracy due to modelling simplifications and non-repeatable behavior. A strategy to partially re-use previous control command reduces the time required for the mid-span connection. In this way, the structure learns from its experience. A damage detection and location method that includes target reliability is tested for single cable damage situations. Despite important modeling simplifications, it is possible to focus on possible damage locations through combination of actuation, measurements, simulations and data interpretation techniques. The structure thus performs self-diagnosis. Active control for structural behavior enhancement and adaptation to damage (self-adaptation) is successfully tested. Simulations are used in order to avoid testing control command that lead to excessive internal forces. Real-time measurements provide a direct evaluation of the actuation effect. Stochastic search is the component that guides the interaction with measurements. Combination of these components results in adaptation and learning since the control system gradually improves performance of the structure through the interaction with the environment during the stochastic search process. Self-diagnosis, self-adaptation and learning are features that are usually associated with living organisms. Thus, this research contributes towards behavior-based biomimetics in structural engineering.

EPFL2016

Toward development of a biomimetic tensegrity footbridge

Seif Dalil Safaei, Ian Smith, Nicolas Willy Veuve

Biomimetic structures interact with their environment, change their properties, learn and self-repair, thereby providing properties that are similar to living organisms. Interactions with the environment involve unique challenges in the field of computational control, algorithms, damage tolerance, and structural analysis. Tensegrity structures are pin-jointed structures of cables and struts in a self-stress state. Tensegrity structures are suitable for active control since the shape of the structure can be changed by changing the length of the elements. Consequently, they are good candidates for biomimetic structures. This paper describes research that is moving toward a case study of biomimetic behaviour of a deployable tensegrity footbridge. This footbridge is made of four modules. Each module is composed of pentagonal circuit-pattern including interconnected struts in a ring configuration that can be folded if cable lengths are changed. Various actuator combinations can be selected for deployment. This property is particularly interesting for biomimetic structures since a single shape change can be achieved many ways. Methodologies for deployment and folding of tensegrity footbridge via combinations of spring and cable clustered actuation are described. Analytical predictions are compared with test results of a near-full-scale tensegrity footbridge. Strategies for folding and deployment are different. A continuous cable and spring configuration is feasible for deployment of tensegrity footbridge. Since the deployment behaviour is non-linear and since deformed geometry as well as joint friction influences the deployment pattern, pre-defined control commands cannot provide the desired deployed position. Active deployment control is thus justified.

International Center for Numerical Methods in Engineering (CIMNE)2014

Adaptive civil engineering structures

Bernard Adam

Most civil structures are passive and static. They are designed according to serviceability and safety requirements. When subjected to actions, they generally deflect in a passive manner. For example, they do not adapt actively to external loading. A more challenging functionality of civil structures is active adaptation to changing requirements, such as load modification, temperature variation, support settlements and eventually damage. This thesis aims to make progress in the field of adaptive civil structures. Intelligent control methodologies such as self diagnosis, multi-objective shape control, self repair and reinforcement learning are proposed and verified experimentally. Although these methodologies are validated experimentally only on a five module active tensegrity structure, there is potential for extension to more complex civil structures. The most important conclusions of this thesis are synthesized below: Self diagnosis can extend active structural control of tensegrity structures to situations where there may be partially defined loading events and damage. Multi-objective search is attractive for computing commands that control shape of an active tensegrity structure while maintaining robustness of both the structure and the active control system. These commands are particularly useful to control structures over scenarios of multiple loading events. Self repair of a damaged active tensegrity structure is possible. The active control system can be used to apply control commands that increase stiffness and decrease stresses within an active tensegrity structure that is damaged. Reinforcement learning is attractive for improving the control of an active tensegrity structure using previous loading events. Improvements involve lower command computation times and better control quality. Such interactions between learning algorithms and active control systems are attractive for control tasks. The integration of intelligent control methodologies such as self diagnosis, multi-objective shape control, self repair and reinforcement learning to an active tensegrity structure create an example of an adaptive civil structure that can be applied to a range of practical situations in the future.