Portfolio for Safety-Directed Design of a Brake-By-Wire System for Car

Resit Coursework for 661985 - Safety Critical Systems

Part 1 - Safety Analysis

The Portfolio explores the iterative design of a Brake-By-Wire (BBW) system for cars. This is Part 1 of the assignment and is worth 40% of the Portfolio.

Note that theresit coursework is based on the same BBW system of the original coursework with a modified architecture.

You will analyse this architecture using fault tree analysis and you will be asked to reflect on results. If

you have failed the original coursework please study my feedback and model solutions - they will help you understand how to analyse this system.

The proposed architecture for the system is given in Figure 1 below:

Figure 1: The proposed architecture of the BBW system

System Specification

•    The BBW features separate braking on each wheel.

•    All components of the system are powered by a common power supply (PS).

•    An electromechanical pedal (PL) receives the braking demand from the driver and sends this as message (PLm) to a pedal node PN.

•    Two replicas of the message PNm are sent by the pedal node to busses B1 and B2.

•    Four Wheel nodes (WN1 … WN4) each read both identical messages (B1m, B2m) from the two busses.

•    As long as one of the messages is received a wheel node can create the braking output applied to the corresponding wheel (WN1b ... WN4b).

Failures

Each component in this system has only one failure mode that shares the name of the component.  For example:

•    The failure mode of component PS is PS

•    The failure mode of component B1 is B1

The failure mode of each component leads to omission of all outputs. For example:

•    If PS fails, you get O-p (Omission of p)

•    If PN fails, you get O-PNm on both busses

In the absence of component failures, all four wheels apply the braking output and the car brakes correctly.

When components fail, the system may fail to brake on one or more wheels. The effects vary depending on the number of wheel failures. For example:

•    If one wheel fails to brake, or three wheels fail to brake, the car is likely to skid offits course.

In this case, to correct the skidding failure, an electronic stability program could release the wheel that is diagonal to the wheel that fails to brake. The car then brakes slowly, and the stopping distance is increased.

•    If all wheels fail, then the car experiences catastrophic loss of braking.

Assignment Tasks

Based on this design:

1.   Draw, or alternatively specify clearly using a set of logical expressions, a fault tree for the event “Omission of braking output by WN1”, i.e. for the event O-WN1b (30 marks).

2.   Calculate the minimal cut-sets of the fault tree (20 marks)

3.   Identify components that are single point of failure (5 marks)

4.   Based on the cut-sets, describe weaknesses and strengths of the system (10 marks)

5.   Draw, or alternatively specify clearly using a set of logical expressions, a fault tree for the “Loss of braking in all three wheels W1, W2 and W3” that will cause skidding. Name the top event “O-W123” (20 marks)

6.   Calculate the minimal cut-sets for this tree (15 marks) Notes:

• Explain your solutions in [1-6] above with a short paragraph of text to show your understanding. Avoid verbosity. Up to 30% of marks will be deducted for lack of explanation.

• Fault trees should be constructed systematically by traversing the model of the system

architecture and applying the algorithm taught in the course. Unsystematic, simplified, fault trees that somehow capture the failure logic correctly will be awarded less marks.  If the calculation of cut-sets that follows from such simplified fault trees is trivial, it will be awarded less marks.

•    For clarity, in your fault trees, use the names of components, messages and component failure modes as displayed in Figure 1. Marks will be deducted if you use names that don’t correspond to the figure.

•    To avoid repetition of branches expand the branch that is repeated only once and use references to the top event of this branch elsewhere. Marks will be deducted if you unnecessarily expand repeated branches.