Pure versus Hybrid Tensegrity Structures

When tensegral and non-tensegral elements are combined to form a hybrid structure, the advantages of the tensegrity approach are not fully realized. The concentration of forces at the connections between tensegral and rigid components makes the hybrid structure likely to fail at the interface. Anchoring a tensegrity to a fixed surface causes bending moments and torques. Pure tensegrity structures are needed to properly model biomechanics.

Context: Tom Flemons Archive

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Hybrid structures do not fully realize the advantages of the tensegrity approach

New Approaches to Mechanizing Tensegrity Structures (2018) Tensegral and non-tensegral elements can be combined to form a hybrid structure but the advantages of the tensegrity approach may not be fully realized. For example, tensegrity masts can replace rigid components in a traditional articulated-rigid-body design that employs revolute and prismatic joints. This offers the advantages of substantially reducing weight while increasing the resilience of the structure. However, the concentration of forces at the connections between tensegral and rigid components makes the hybrid structure likely to fail at the interface. Such concentration of forces does not occur in a pure tensegrity structure, instead every component is integrated into the whole such that a local stress is passed globally through the system. A tensegrity structure is compliant throughout and deforms in response to forces exerted on it.

Anchoring a tensegrity to a fixed surface causes bending moments and torques

(April 28, 2016) A different problem arises when we try to imagine a hybrid system where by a tensegrity is anchored to a fixed surface. At the interface between a tensegrity and a solid, bending moments and torque manifest. The general rule here is, if a tensegrity is fixed to the ground, or attached to a solid object in any way it loses some of it’s tensegral qualities and is prone to failure at the point of interface. For example imagine a tensegrity mast anchored to the ground and a tree falling onto it. It’s going to fail in the same way a tree does – the base will shear and the struts that were fixed to the base will break. In robotics the standard way to build a quadruped or biped is to attach articulating limbs to a fixed chassis or armature. The unyielding aspect of a chassis like torso allows the robot leverage to move it’s limbs by means of hydraulics or motors acting against the immoveable base. This doesn’t work in a tensegrity because there is no fixed parts to lever against so different strategies have to be employed.

Pure tensegrity structures are needed to properly model biomechanics

(Aug 8, 2015) The vertebrate skeleton can only be analyzed tensegrally if the fascial matrix is wrapping it. By itself two bones meeting at a joint can’t be considered a tensegrity. A revolute joint (knee, finger, elbow), prismatic joint (scapula), or a universal joint (pelvis, thumb) is not a tensegrity in and of itself. I can build a highly compliant version of each of these mechanical joints which in Robert Skelton’s terms may be referred to as a tensegrity system but I would argue that except in rare circumstances they are not necessarily a tensegrity itself. Whether a hinge joint between two rigid bodies is formed with a cylinder and a rod (door hinge) or a saddle sling composed of tensioned cables doesn’t matter as much as what the hinge is connecting. The action is the same and compliancy is just a measure of ‘slop’ in the system. i.e. an old door hinge that has been damaged may show signs of being too loose in the axial plane which is undesirable. A saddle sling may allow a greater range of movement because the tensioned materials are never as rigid as solid compression materials, but if what is being connected are solid objects (door to a door frame, bone to a bone) there is really no difference in the actual mechanism. Thus my models of the spine based on ‘solid’ tetrahedrons i.e.  each built from 4 struts extending from a fixed centre [images below], can’t be really considered as a tensegrity structure by itself. Only if the tetrahedrons are modelled as tensegrities themselves can we have the conditions needed to describe it as a tensegrity structure and not simply a tensegrity like system. A tensegrity system describes the necessary conditions for a tensegrity but without pure tensegrity bodies involved it is not a sufficient definition.

Thoroughly investigate pure tensegrity mechanisms before venturing into hybrid structures

(Dec 21, 2017) I think it’s important to stick as closely as possible to pure tensegrity mechanisms to thoroughly investigate the possibilities and parameters of the field before venturing into hybrid structures. I’ve always thought that the end result of my research would result in structures which borrowed heavily from tensegrity but maybe didn’t look like a tensegrity. An example of this would be a spiral tensegrity mast made from braided tubing. It isn’t technically tensegrity but it has the same geometry and operates In a manner that’s very similar. cf Snelson’s work on three-dimensional weaving: Tensegrity, Weaving and the Binary World.

I think it’s important to work from first principles. I try to remember I am modelling force vectors passing through compliant structures to obtain optimal strength to weight ratios and preserve multiple redundancy in critical failure paths. By that I mean one of the chief strengths of tensegrities are their ability to continue to be relatively integral even if somewhat damaged. This is a unique characteristic that is hard to replicate in standard mechanisms… If the joint is damaged that means everything below that joint is now compromised. In theory it is possible to design tensegrity systems whereby there are workarounds to localized injury.