Planar double chain linkages
Many readers were attracted to deployable structures by a toy known as the Hoberman sphere, Figure 3.1(a). The toy is a one-degree-of-freedom mechanism which resembles a sphere but is capable of expanding up to a
few times of its packaged size. The prime component of the toy is the scissor-like element (also referred to as a pantograph in some references), Figure 3.1(b), which consists of a pair of beams joined together by a pivot (revolute joints) so that free rotation of one beam relative to another about the axis of the pivot is allowed and any other relative motion of the rods is prevented. The scissor-like element is probably the simplest mechanism involving a revolute joint. There are two types of scissor-like elements depending on the locations of the hinges and pivot on the beams that form the element. In the first type, Figure 3.2(a), referred to here as the conventional scissor-like element, the pivot and two end connectors, used for connection with neighbouring elements, are collinear. Therefore, the beams can be made from straight rods. In the second type however, the end hinges and pivot are non-collinear, Figure 3.2(b). Kinked beams or flat plates are therefore used, which gives the name: the angulated scissor-like element.