ABSTRACT
This chapter introduces the basic concepts that underpin most precision mechanical design and that are much used throughout this book. An accurate device must be repeatable and stable. Central to this requirement, and given pride of place here, is the geometric notion of the kinematic constraint of ideal rigid bodies. It is applied to the determination of the relative motion of two elements, to the efficacy of positioning devices and to the behaviour of mechanisms. All real structures are elastic, which causes deviations from true kinematic behaviour, but which may also be exploited directly as pseudo-kinematic constraint. The various themes of kinematic design identify design approaches that are potentially precise, but they say nothing about the individual stability of single elements in a system. All controlled displacements depend on measurements relative to a datum, but forces that must be carried through a structure distort it and may move that datum. Hence the concepts of measurement and force loops are examined in some detail, as are the principles of alignment. Then some methods of improving loop performance are introduced. Nulling techniques may give highly precise control with few side-effects from non-linearity. Mechanical or computer-based compensation methods can reduce loop sensitivity to thermal disturbances and be used to separate predictable systematic errors. Finally, there are some observations on the potential benefits of symmetry and of scaling devices to smaller dimensions. Taken together, the topics covered provide a means of assessing proposals. A design that does not comply with the suggested practices is not necessarily wrong, but it is reasonable to require an explicit justification for the non-compliance.
