Marc Arsenault

Kinematic, Static, and Dynamic Analysis of a Spatial Three-Degree-of-Freedom Tensegrity Mechanism

The use of tensegrity systems as structures has been extensively studied. However, their development for use as mechanisms is quite recent even though they present such advantages as reduced mass and a deployment capability. The object of this paper is to apply analysis methods usually reserved for conventional mechanisms to a planar one-degree-of-freedom tensegrity mechanism. This mechanism is obtained from a three-degree-of-freedom tensegrity system by adding actuation to the latter as well as by making some assumptions of symmetry. Analytical solutions are thus developed for the mechanisms direct and inverse static problems. Furthermore, the working curve, singularities, and stiffness of the mechanism are detailed. Finally, a dynamic model of the mechanism is developed and a preliminary control scheme is proposed.

Static Balancing of Tensegrity Mechanisms

The computation of the equilibrium configurations of tensegrity mechanisms is often a very tedious task even for relatively simple architectures. However, it has been observed that the complexity of this problem is significantly reduced when gravitational loads are compensated with the use of static balancing techniques. In this work, the general static balancing conditions are adapted for the case of tensegrity mechanisms. Afterward, the modified conditions are applied to two new spatial three-degree-of-freedom tensegrity mechanisms.

Optimization of the prestress stable wrench closure workspace of planar parallel three-degree-of-freedom cable-driven mechanisms with four cables

There exist configurations of parallel cable-driven mechanisms (CDM) within their wrench closure workspace (WCW) for which they may become unstable when their cables are subjected to internal forces due to prestress or external loads. With the goal of avoiding such configurations, the prestress stable WCW (PSWCW) is defined as a subset of the WCW where an increase in the prestress level leads to an increase in the overall stiffness of the mechanism. A genetic algorithm is used to optimize the geometry of planar parallel CDMs with four cables with the objective of reaching a desired PSWCW. Results are obtained that may guide the designer in the preliminary selection of mechanism dimensions and cable connectivities.

Kinematic and static analysis of a planar modular 2-DoF tensegrity mechanism

Tensegrity mechanisms have the advantage of being relatively lightweight due to their extensive use of cables and springs. In this work, a novel planar modular 2-DoF tensegrity mechanism that is actuated by cables is introduced. The modular architecture of the mechanism gives it increased flexibility while using cables for the actuation leads to large reachable workspaces. An analysis of the mechanisms statics and kinematics is performed for the case where no external loads are acting

Stiffness Analysis of a Planar 2-DoF Cable-Suspended Mechanism While Considering Cable Mass

The mass of the cables is not considered in most existing research on cable-driven mechanisms (CDM). Moreover, of those papers where cable mass is considered, few have examined its effects on mechanism stiffness. The research presented herein seeks to better understand these effects with regards to a planar two-degree-of-freedom suspended CDM. The mechanism’s stiffness matrix is first developed and then used to generate mappings of intuitive stiffness indices over the workspace. The sagging of the cables under their own weight is found to heavily influence mechanism stiffness. The importance of maintaining a minimum level of cable tension to minimize the effect of cable sagging on the mechanism’s stiffness and workspace is also demonstrated.

Kinematic and Static Analysis of a Three-degree-of-freedom Spatial Modular Tensegrity Mechanism

Tensegrity mechanisms benefit from a reduced inertia due to their extensive use of cables and springs. However, they must be prestressed at all times in order to keep the cables in tension. For a given mechanism architecture, this is only possible in a specific set of configurations such that the analysis of these mechanisms is relatively complex. This paper presents the development and analysis of a new three-degree-of-freedom positional tensegrity mechanism that has a modular architecture. The mechanism, actuated by cables, has a relatively large workspace. For the special case where external and gravitational forces are neglected, solutions are given for the mechanisms equilibrium configuration both for given actuator positions as well as for a specified position of its effector. It is shown that the mechanism can be considered as an assembly of construction elements based on Snelsons X-shape tensegrity system and that these behave independently from one another.

Stiffness Analysis of a 2DOF Planar Tensegrity Mechanism

This paper presents the stiffness analysis of a planar 2DOF tensegrity mechanism. A stiffness model is first derived based on an existing formulation. Several stiffness indices having physical meaning are then extracted from the stiffness matrix for performance evaluation purposes. Stiffness mappings based on these stiffness indices are then plotted over the mechanism’s workspace and observations are made. It is shown, for instance, that in the case of the planar 2DOF tensegrity mechanism, the effect of the prestress on the stiffness is generally not significant when the stiffnesses of the cables and struts are assumed to be linear.

Design of a mechanism to simulate the quasi-static moment–deflection behaviour of the osteoligamentous structure of the C3–C4 cervical spine segment in the flexion–extension and lateral bending directions

The human neck is susceptible to traumatic injuries due to impacts as well as chronic injuries caused by loads such as those attributed to the wearing of heavy headgear. To facilitate the analysis of the loads that cause injuries to the cervical spine, it is possible to replicate the human neck’s behaviour with mechanical devices. The goal of this work is to lay the foundation for the eventual development of a novel mechanism used to simulate the behaviour of the cervical spine during laboratory experiments. The research presented herein focuses on the design of a mechanism capable of reproducing the non-linear relationships between moments applied to the C3 vertebra and its corresponding rotations with respect to the C4 vertebra. The geometrical and mechanical properties of the mechanism are optimized based on the ability of the latter to replicate the load–deflection profile of the osteoligamentous structure of the C3–C4 vertebral pair in the flexion–extension and lateral bending directions. The results show that the proposed design concept is capable of faithfully replicating the non-linear behaviour of the motion segment within acceptable tolerances.

Workspace Computation and Analysis of a Planar 2-DoF Translational Tensegrity Mechanism

Tensegrity mechanisms are interesting candidates for high-acceleration robotic applications since their use of cables allows for a reduction in the weight and inertia of their mobile parts. In this work, a planar two-degree-of-freedom translational tensegrity mechanism that could be used for pick and place applications is introduced. The mechanism uses a strategic actuation scheme to generate the translational motion as well as to ensure that the cables remain taut at all times. Analytical solutions to the direct and inverse kinematic problems are developed and the mechanism’s workspace boundaries are computed in both the actuator and Cartesian spaces. The influence of the mechanism’s geometry on the size and shape of the Cartesian workspace are then studied. Based on workspace size only, it is found that the optimal mechanism geometry corresponds to a relatively large ratio between the length of the struts and the width of the base and end-effector.Copyright

Stiffness Analysis of a 2-DoF Planar Tensegrity Mechanism

This paper presents a general method to perform the stiffness analysis of tensegrity mechanisms. The method is based on an existing stiffness matrix model. Several stiffness indices having physical meaning are introduced. As an example, the method is applied to a planar 2-DoF tensegrity mechanism. Stiffness mappings based on the stiffness indices are generated for the mechanism’s workspace. It is shown for the example mechanism that the effect of the prestress on the stiffness is not significant when linear stiffness models of the components are assumed.Copyright