APPENDIX 3
ANSWERS TO EXERCISES
Ian Alexander
This appendix offers some possible answers to the Exercises of Appendix 2. Given the very brief sketch of the system context, these can be no more than plausible suggestions—they are certainly not definitive. If you study the problems carefully you should be able to refine these answers and add more detail.
1. Mission
To maintain safe visibility through the windscreen(s).
More specifically: to clear rain, drizzle, snow, hail, or mud from the windscreen, quickly and reliably enough to maintain visibility, for all the car versions in all the expected climates.
2. Stakeholders
Beneficiary
Functional Beneficiary
| Stakeholder: | Viewpoint: |
| Driver | wants comfortable, safe, impressive car |
| Other | Road User wants not to be injured (by drivers who can't see clearly) |
Financial Beneficiary
| Stakeholder: | Viewpoint: |
| Board, Shareholder | want to minimise risk; want good Return on Investment (RoI). |
Political Beneficiary
| Stakeholder: | Viewpoint: |
| Global Marketing Director | wants to show other board members that Global Marketing understands the market and deserves high position on board |
Purchasing
| Stakeholder: | Viewpoint: |
| Product Manager | wants new car versions rolled out on time, to budget |
Negative
| Stakeholder: | Viewpoint: |
| Other Directors | doubtful about Global Marketing |
| Environmental Activist | wants to reduce pollution, off-road driving |
Regulatory
Voluntary (Standardising)
| Stakeholder: | Viewpoint: |
| Auto Industry Standards body | wants to promote high standards of manufacture |
Enforcing
| Stakeholder: | Viewpoint: |
| EU / other safety regulators | want cars to comply with national laws |
Operational Roles
Human Operators
Normal
| Stakeholder: | Viewpoint: |
| Driver | wants operation to be intuitive, similar to other cars (of the same make), and effective |
Maintenance
| Stakeholder: | Viewpoint: |
| Mechanic | wants system to be quick to diagnose and to fix |
Neighbouring Systems
| Stakeholder: | Viewpoint: |
| Door/Window Management | not soak passengers |
| Power Management | conserve battery power for critical functions |
3. Use Case Sketches
Clear Windscreen of Rain
Primary Scenario
It starts to rain. The windscreen is wiped regularly until the rain stops.
Clear Windscreen of Drizzle
Primary Scenario
It starts to rain lightly. The windscreen is wiped intermittently until the rain stops.
Alternative Path
It starts to rain very lightly. The driver commands the windscreen to be cleared of drizzle.
The windscreen is wiped intermittently.
The drizzle stops. The driver cancels the command to clear the windscreen of drizzle.
Clear Windscreen of Mud Splash
Primary Scenario
The car drives through a puddle or goes past a dirty truck. Muddy water splashes on to the windscreen. The driver commands the windscreen to be cleared of mud splash.
The windscreen is simultaneously washed and wiped five times.
Maintain Windscreen Cleaning Subsystem
Primary Scenario
The mechanic refills the WCS with correctly diluted windscreen washing fluid.
The mechanic connects the WCS test equipment and commands a test.
The WCS test equipment commands a sequence of tests (Clear Mud Splash, Clear Drizzle, Clear Rain). The mechanic observes the test operation.
4. Exceptions
Clear Windscreen of Rain
Exceptions:
Wiping fails to start in rain: The driver commands the windscreen to be cleared of rain.
Clear Windscreen of Drizzle
Exceptions:
Wiping fails to start in drizzle: the driver commands the windscreen to be cleared of drizzle. The windscreen is wiped intermittently. The drizzle stops. The driver cancels the command to clear the windscreen of drizzle.
Maintain Windscreen Cleaning Subsystem
Exceptions:
WCS test equipment reports a fault: Mechanic reads the fault report and cleans, replaces, or fastens the affected components as necessary. Mechanic reruns the automated test sequence.
Mechanic observes a fault: Mechanic cleans, replaces, or fastens the affected components as necessary. Mechanic reruns the automated test sequence.
Did you think of other exceptions?—good. There are many other possibilities.
5. NFRs
Interfaces
Incoming
NFR-1: 12 Volt nominal DC power supply
NFR-2: Driver commands on CAN bus
Outgoing
NFR-3: Fault Message to Fault Handler on CAN bus
Physical Connectors
NFR-4: 12 V DC 2-pin power connector
NFR-5: CAN bus connector
Usage Qualities
Reliability
NFR-6: The WCS shall fail less than once per 10,000 hours of operation.
Maintainability
NFR-7: The WCS shall be composed entirely of Line Replaceable Units (LRUs).
NFR-8: A trained mechanic shall be able to replace any WCS LRU within 5 minutes.
Supportability & Integrated Logistic Support
NFR-9: The WCS test equipment shall enable 1st-level diagnosis to the affected LRU.
NFR-10: The packaging of each WCS LRU shall be bar-coded with its manufacturer's ID and manufacturer's part number.
Safety
Note: windscreen visibility is safety-related but must remain the responsibility of the driver. A caution to this effect is needed in the Driver's Manual.
Self-Diagnosability
NFR-11: The WCS shall be able to report pump failure.
NFR-12: The WCS shall be able to report washing fluid pipe blockage.
Commonality / Interoperability
NFR-13: The same WCS shall be used on the saloon, cabriolet, and estate versions.
Did you think of other qualities?—well done. There are many other possibilities.
Upgradability
NFR-14: The WCS control functionality shall be upgradable by replacement of a single Control Unit.(Note: this will be in an LRU)
Toxicity & Disposability
NFR-15: The WCS shall comply with Standard 123-456-789 Use of Toxic Materials.
Storage & Shelf Life
NFR-16: WCS parts, stored without washing fluid, shall have a shelf life of at least 5 years.
6. Test Approach Sketch
Rain Test
Stress (continuous heavy rain) Test
Drizzle Test
Drizzle (manual start) Test
The rain and drizzle tests call for a Rain/Drizzle generator capable of working continuously for 24 hours.
Mud Splash Test
The Splash Test requires a test area that can tolerate hundreds of litres of muddy water.
Fault Diagnosis (standard Test Equipment) Test
The Diagnosis test requires a test car and a standard garage-issue WCS Test Equipment.
Rough Track (vibration, mud splash) Test
The Rough Track Test requires use of a test car on a vibration / mud splash test track.
Icing Test
The Icing Test requires a refrigerated test room.