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, pH, biocompatibility etc List of all inputs and the desired outputs

Example Aim: portable device to detect concentration of a H2O2 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 H2O2 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 H2O2 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.

H2O2 sensor Design: Level 1: Level 2: biosensor: standard Sensor for H2O2 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 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