1. Engineering cycle

2. Basic overview

3. 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

4. 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

5. 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

6. 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

7. 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.

8. 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

9. 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

10. Modelling

11. 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

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

13. 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

14. 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 

15. 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