Zero stiffness tensegrity structures

Tension members with a zero rest length allow the construction of tensegrity structures that are in equilibrium along a continuous path of configurations, and thus exhibit mechanism-like properties; equivalently, they have zero stiffness. The zero-stiffness modes are not internal mechanisms, as they involve first-order changes in member length, but are a direct result of the use of the special tension members. These modes correspond to an infinitesimal affine transformation of the structure that preserves the length of conventional members, they hold over finite displacements and are present if and only if the directional vectors of those members lie on a projective conic. This geometric interpretation provides several interesting observations regarding zero stiffness tensegrity structures.