Joint Friction during Deployment of a Near-Full-Scale Tensegrity Footbridge

Most deployable structures, such as operable roofs and masts, move over one degree of freedom. This paper describes a structure that involves loosely coupled movement over several degrees of freedom. Analysis models of these structures are typically inaccurate. A source of inaccuracy is joint friction. Static and kinetic friction are studied experimentally and analytically. Simulations have been modified to account for these effects, and two methods are used to quantify friction effects. Friction has a significant effect on the movement of the tensegrity structure. Of two candidate parameters, cable tension and interior cable angle, cable angle is the factor that best characterizes friction effects. Values of static and kinetic friction coefficients are not significantly different in this context, and this leads to a reduction in the complexity of the friction model for simulation. Including friction effects in analysis decreases the difference between simulations and tests. Lastly, strut elements of the tensegrity structure are most critically affected by friction. (C) 2017 American Society of Civil Engineers.

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.

Asce-Amer Soc Civil Engineers2015

A dynamic-relaxation formulation for analysis of cable structures with sliding-induced friction

Nizar Bel Hadj Ali, Ian Smith, Ann Christine Sychterz

In many cable structures, special types of joints are designed to allow cables to slide freely along the joints. Thus, a number of discrete cable elements are substituted by a single continuous cable that slides over multiple nodes. Continuous cables are employed in engineering applications such as cranes, domes, tensioned membranes and tensegrity structures. Most studies involve the assumption of friction-less movement of cable elements through pulleys. Nevertheless, frictionless sliding is usually an unrealistic hypothesis. In this paper, the dynamic relaxation (DR) method is extended to accommodate friction effects in tensioned structures that include continuous cables. The applicability of this DR formulation is demonstrated through analysis of three case studies that involve both numerical and experimental investigation. Results show potential to efficiently accommodate sliding-induced friction. (C) 2017 Elsevier Ltd. All rights reserved.

Elsevier2017

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

Towards biomimetic behavior of an active deployable tensegrity structure

Nicolas Willy Veuve

EPFL2016