Engineering cycle Basic overview Specification A set of

Engineering cycle

Basic overview

Specification • A set of criteria that must be met by the system – it is the driving force of a project • Used to evaluate the end product – must be measurable quantity • Created using the objective behind the project • The output of this stage is a specification document containing the list of requirements of the project

Contents of a specification • • • Dimensions of system components Concentrations Rates of reaction Margin of error – tolerance Conditions – temperature, chemical conditions, p. H, biocompatibility etc • List of all inputs and the desired outputs

Example • Aim: portable device to detect concentration of a H 2 O 2 in breath to warn user of an imminent asthma attack • Specification: • Size of device ~ 10 x 5 cm • Must have sensor able to detect concentration of H 2 O 2 • The sensitivity of the sensor must be between 0. 1μM – 10 μM • Uncertainty in measurements < 0. 05μM • Results must be displayed within 1 minute of measurement • Display a warning message if the concentration of H 2 O 2 too high or in the breath • Batteries must have a life of at least 3 days

Design • How the specification might be met • A detailed plan of each component within the system and how the outputs may be achieved from the inputs • May be in the form of drawings or detailed description • May go back and change the specification if any of the specification is not feasible or unachievable according to the resources

Design • Abstraction is usually used to design a system in a top to bottom manner • First a plan of the overall system may be designed • At the second level of abstraction each of the individual sub-units of the system would be designed in detail • The level of detail in the design document depends on the level of standardisation of parts and their characterisation.

H 2 O 2 sensor • Design: • Level 1: – – – – Sensor for H 2 O 2 detection Processor to process information An LCD to display the results battery slot Casing – strong for protection of components within Place to inhale into the device Power button • Level 2: biosensor: standard – – Size Electrode Reactive coating Enzymes • Level 2: processor: standard – Size – speed

Modelling • Predict system behaviour • Consists of computer simulations or mathematical relations based on the design of the system • May also be a mock-device made with slightly different materials or dimensions, but same basic design • A cost-effective way to verify the design of a system – cheaper and faster • Helps evaluate system completeness as well as improve the understanding of the mechanisms of the processes involved • Assumptions may be made while modelling a system, but these need to be reasonable – must be documented and justified • May need to modify the system design if an error is identified in the system design from the model

Modelling

Example – aerodynamic modelling • Cheaper to simulate air flow around a wing than to actually build the wing and test it • Numerical simulation of the flow around aerofoils performing arbitrary rigid body motions

Implementation • The realisation of the system that has been designed and modelled • The actual end product is constructed by the end of this stage

Testing/validation • Verify whether the constructed system fulfils the requirements set by the specification • Validation – the successful completion of testing • The inputs, outputs and functions of each of the components, as well as the whole system, are compared with the specification values • Test the system under the conditions it might be used in

Testing/validation • Testing methods should be designed such that they can actually validate or invalidate the implementation • Should have a testing protocol - formal documents that typically outline requirements, activities, resources, documentation and schedules to be completed

Glossary • Tolerance: the allowed variation in a physical dimension of the system or a value measured by the system • Transfer function: a mathematical equation representing the relation between the input and output of a system