Biomimetic adaptive control of a deployable tensegrity structure

Biomimetic behavior includes aspects such as learning from previous experience, self-diagnosis, and adaptation. This thesis describes control methodologies that are essential to development towards biomimetic behavior of a complex deployable structure. Simulations of the structure are improved from previous work to include friction effects of cables sliding over joints. Despite improved simulations, testing shows that significant uncertainties in the behavior of the structure remain. Therefore, control is not effective with predetermined actuation movements. Special methodologies for feedback control using simulations and measurements are required.

A near-full-scale deployable tensegrity structure is used to test methodologies. Active control methods are proposed for deployment, midspan connection of both halves of the structure, and self-stress. This thesis presents a method using feedback to compare measurements and simulations to modify control commands. Since collision of elements is possible in the folded state and produce undesirable bending stresses, a path-planning algorithm is implemented for the first stage of deployment. Error in nodal positions at midspan is successfully reduced through the use of the path-planning method and deployment time is significantly reduced compared with previous work. Lastly, algorithms for self-stress, involving penalty and rejection constraints on element stress, are useful for correcting nodal positions after deployment.

Damage is detected in this thesis using vibration measurements. The method uses dynamic behavior of the structure to determine whether or not the structure is damaged. Using parameters of the structure and a set of candidate locations for the damaged element, candidates are successively excluded until few candidates remain, successfully including the true location of damage.

Adaptation and learning are demonstrated by mitigation methods after damage and in-service loading (such as pedestrians). Active control is useful to manipulate the shape of the tensegrity structure to reduce the member stresses and vertical downwards displacement caused by a damaged element. Though the response improves the condition of the structure to respect the serviceability limit for vertical downwards displacement, the tensegrity structure cannot be fully restored to the design configuration.

Since correction of end-node coordinates can be grouped by the direction of correction and resulting cable-length changes, case-based reasoning is useful to reduce time of execution and to reduce unnecessary cable-length changes. Single pedestrian and crowd loading configuration is applied analytically and experimentally to the tensegrity structure. Application of mitigation techniques is useful beyond serviceability thresholds for a moving load used to simulate in-service loading.

The research question of this thesis: "Is it feasible for a deployable tensegrity structure to improve movement and service performance through behavior biomimetics?". The answer is yes. This work presents methods inspired by those observed in nature for efficient movement that is generalizable for future deployable structures.

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

Micro- and nano-electrostatic force fields

Martin Jenke

Electro-static or electro-dynamic fields generated by micro- or nano-electrodes are today widely used in many different fields, such as micro- and nanogripping, "lab on a chip", and "lab in a cell", for the purpose of manipulation, separation, or analysis of micron-sized particles, cells, or single molecules. Commonly numerical simulations and Electrostatic Force Microscopy (EFM) methods, based on Atomic Force Microscopy (AFM), are used to study and predict the electrostatic force fields above produced structures. However, this measurement methods have several disadvantages. These disadvantages range from simple limitations (e.g. it is not possible to measure in three dimensions or expensive new equipment is needed) to much more serious disadvantages (e.g. missing compensation for known AFM problems, which results in wrong measurements). In this thesis we propose a new EFM method based on simple static force distance curves, which allows measuring accurately, and simultaneously the topography, vertical electrostatic force field and attraction forces. The method is numerical simulated in 3D to study in detail, with simulations and measurements, electrostatic force fields above nanoelectrodes. To do this, we show for the first time the fabrication of interdigitated nanoelectrodes with pitches down to 50 nm with gas assisted focused ion beam milling. We present as well for the first time the fabrication of Pt coated AFM tips with radii between 50 and 600 nm, together with their calibration. These tips not only close the gap between conventional tips with tip radii of about 10 nm and cantilevers with attached spheres. They show as well high mechanical stability, which solves a common problem in EFM, the mechanic stability of the tip. The shape of the tip greatly influences the measurement. We demonstrate this using numerical simulations, and show that already small tip discrepancies can change the measured force up to 50 %. Moreover, we proved that changes in the relative humidity result in electrostatic force changes of up to 45 %. We discovered that the resolution of EFM measurements on nanoelectrodes can be enhanced by using a tip radius of 2 to 2.5 times the pitch of the measured interdigitated nanoelectrode. Based on this and its influence on topography measurements, which are usually made before, in between or at the end of EFM measurements, we give hints for measurements with different tip radii. Beside these important general improvements, discoveries and hints for EFM, we show as well some applications of our new EFM method and the before obtained results. We show that our new method can distinguish the force field caused by trapped charges in SiO2 from the force field caused by nanoelectrodes below it. This enables to study the influence of charge trapping in future semiconductor chips, which use charge trapping to store information. Furthermore, we used our new EFM method and some gripping experiments to study electrostatic micro-and nanogripping. This experiments lead to some propositions for improvements in electrostatic micro-and nanogripping. Such as a new gripper design with gripping object size independent centering effect, which is studied using 3D numerical simulations.

EPFL2008

Deployment of a tensegrity footbridge

Seif Dalil Safaei, Ian Smith, Nicolas Willy Veuve

Deployable structures are structures that transform their shape from a compact state to an extended in-service position. Structures composed of tension elements that surround compression elements in equilibrium are called tensegrity structures. Tensegrities are good candidates for deployable structures since shape transformations occur by changing lengths of elements at low energy costs. Although the tensegrity concept was first introduced in 1948, few full-scale tensegrity-based structures have been built. Previous work has demonstrated that a tensegrity ring topology is potentially a viable system for a deployable footbridge. This paper describes a study of a near-full-scale deployable tensegrity footbridge. The study has been carried out both numerically and experimentally. The deployment of two modules (one half of the footbridge) is achieved through changing the length of five active cables. Deployment is aided by energy stored in low stiffness spring elements. Self-weight significantly influences deployment, and deployment is not reproducible using the same sequence of cable- length changes. Active control is thus required for accurate positioning of front nodes in order to complete deployment through joining both sides at center span. Additionally, testing and numerical analyses have revealed that the deployment behavior of the structure is nonlinear with respect to cable-length changes. Finally, modeling the behavior of the structure cannot be done accurately using friction-free and dimension- less joints. Similar deployable tensegrity structures of class two and higher are expected to require simulation models that include joint dimensions for accurate prediction of nodal positions. DOI: 10.1061/(ASCE)ST.1943-541X.0001260 . © 2015 American Society of Civil Engineers.