2 Introduction
Third, the motion structures generally use fewer but more robust types of
joints because of the environments in which they typically operate. Moreover, when synthesising the motion structures, the positions and orientations of the parts during the motion are far more important than other
physical properties such as velocity and acceleration, as the cycle time of a
motion structure is generally in a matter of minutes rather than seconds or
less for conventional mechanisms.
There are two ways to synthesise motion structures. Structural engineers
and architects, responsible for most of the motion structures for civil
engineering applications such as retractable roofs and façades, are inclined
to adopt concepts involving a small number of larger moving bodies whose
motions are then synchronised by electronic means (Ishii, 2000). Each
moving body may consist of many members but the internal relative
motions among the members are prohibited. The reason for taking this
approach is because of the difficulty in assembling a large number of
members together while retaining the internal mobility. This approach
becomes less effective when the purpose of motion structures is to achieve
small and compact packaging size. An alternative is to select known conventional mechanisms as basic building blocks and then to assemble such
blocks together in such a way that the degrees of freedom of each mechanism are retained. For example, the design of an umbrella frame can be
regarded as a combination of a number of identical linkages, each of which
is a supporting frame with three hinges and one slider. The sliders are then
merged at the central pole so that the entire frame has one degree of
freedom.
In the past few years, the authors have created a number of novel
motion structures by the latter approach. Here we are to share with readers
our experience. Almost all of the motion structures in the book are one
degreeoffreedom assembly created by basic mechanisms such as the
planar and spatial linkages with only hinge joints.
In the following pages, the reader will first encounter in Chapter 2 the
terms and definitions in commonly used mechanism theory, mobility criteria and analytical methods for synthesising mechanisms, some basic
mechanisms and a link between mechanisms and structures. The basic
mechanisms are then used to construct planar motion structures in Chapter
3, followed by rings and domes in Chapter 4. Chapters 5 and 6 are about
truly three dimensional motion structures using spatial linkages as building
blocks. A method that uses tilings for designing the layouts of three dimensional motion structures is given in Chapter 7, which ends the book.
The book is limited to the geometrical aspects of motion structures. In
analysis and design synthesis, it is always assumed that the motion structures are made of rigid bodies. It should be clear that the rigid body is only
an approximate reality in practice. This assumption reduces the motion of
a motion structure to geometry. This is true if the bearing clearance and
elastic deformation of members are sufficiently small. The motions of a
Third, the motion structures generally use fewer but more robust types of
joints because of the environments in which they typically operate. Moreover, when synthesising the motion structures, the positions and orientations of the parts during the motion are far more important than other
physical properties such as velocity and acceleration, as the cycle time of a
motion structure is generally in a matter of minutes rather than seconds or
less for conventional mechanisms.
There are two ways to synthesise motion structures. Structural engineers
and architects, responsible for most of the motion structures for civil
engineering applications such as retractable roofs and façades, are inclined
to adopt concepts involving a small number of larger moving bodies whose
motions are then synchronised by electronic means (Ishii, 2000). Each
moving body may consist of many members but the internal relative
motions among the members are prohibited. The reason for taking this
approach is because of the difficulty in assembling a large number of
members together while retaining the internal mobility. This approach
becomes less effective when the purpose of motion structures is to achieve
small and compact packaging size. An alternative is to select known conventional mechanisms as basic building blocks and then to assemble such
blocks together in such a way that the degrees of freedom of each mechanism are retained. For example, the design of an umbrella frame can be
regarded as a combination of a number of identical linkages, each of which
is a supporting frame with three hinges and one slider. The sliders are then
merged at the central pole so that the entire frame has one degree of
freedom.
In the past few years, the authors have created a number of novel
motion structures by the latter approach. Here we are to share with readers
our experience. Almost all of the motion structures in the book are one
degreeoffreedom assembly created by basic mechanisms such as the
planar and spatial linkages with only hinge joints.
In the following pages, the reader will first encounter in Chapter 2 the
terms and definitions in commonly used mechanism theory, mobility criteria and analytical methods for synthesising mechanisms, some basic
mechanisms and a link between mechanisms and structures. The basic
mechanisms are then used to construct planar motion structures in Chapter
3, followed by rings and domes in Chapter 4. Chapters 5 and 6 are about
truly three dimensional motion structures using spatial linkages as building
blocks. A method that uses tilings for designing the layouts of three dimensional motion structures is given in Chapter 7, which ends the book.
The book is limited to the geometrical aspects of motion structures. In
analysis and design synthesis, it is always assumed that the motion structures are made of rigid bodies. It should be clear that the rigid body is only
an approximate reality in practice. This assumption reduces the motion of
a motion structure to geometry. This is true if the bearing clearance and
elastic deformation of members are sufficiently small. The motions of a