|
Home > User Area > CES Selector Documentation > Definitions
Definitions
This is simply a statement of the primary function of the design. A beam carries bending moments; a heat-exchanger tube transmits heat; a bus-bar transmits electric current.
This is the first and most important quantity that you wish to maximise or minimise. Commonly it is weight or cost; but it could be energy stored per unit volume (a spring, a flywheel); or energy dissapated in electrical heating (a bus-bar); or depth of dive (a submarine); - it depends on the application.
They are design requirements which must be met and which therefore limit the optimisation process identified as the objective. Commonly these are: a required value for the stiffness; a required value for the safe load, moment, torque or pressure that can be supported; a limit on the operating temperature; or on resistance to sudden fracture.
It is essential to distinguish between OBJECTIVES and CONSTRAINTS - and that requires a little thought. For example: in the performance-limited design of a bicycle frame, minimising weight might be the OBJECTIVE, with stiffness, strength, toughness and cost as CONSTRAINTS ('as light as possible without costing more than £300'). But in a cost-limited design of a bicycle, minimising cost becomes the OBJECTIVE and weight becomes a CONSTRAINT ('as cheap as possible, without weighing more than 16kg).
- Write down an equation for the OBJECTIVE. This is the 'objective function'.
- Write down equations for the CONSTRAINTS. Eliminate the free variables in this objective function by using the CONSTRAINTS.
- Read off the grouping of material properties which maximise the value of the objective. This is the PERFORMANCE INDEX.
|
Print this article 
