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Fault Tolerant Control of the Electric Propulsion 19
for Autonomous Surface Vehicles

5. Fault Scenarios

The faults of interest that can occur in an autonomous surface vessel propulsion system may be
classified in three major categories. Shaft speed faults, Current faults and Location faults.

5.1 FMEA Methodology
In this section, the adopted failure modes and effect analysis (FMEA) methodology is used to
improve design. In other words, a FMEA procedure is used to present the causes and the effects
of the faults in a hierarchical model of a system.

It is far less expensive to prevent problems early than fix problems after launch. Therefore,
FMEA can identify and address safety issues on the system or end users before a potential risk.
Then, it can identify can carry out corrective actions assessing the risk associated with the
identified failure modes, effects and causes. A FMEA is an engineering analysis that performed
finding and correcting weaknesses before the product gets into the hands of the customer.
The first step in any risk assessment is to define the goal of the risk assessment.

Components of the FMEA methodology are:
a) Severity : Severity assesses how serious the effects would be should the potential risk
occur.
b) Probability of Occurrence : The probability of occurrence evaluates the frequency that
potential risks will occur for a given system. The probability score is rated against the
probability that the effect occurs as a result of failure mode.
c) Detectability : Detectability is the probability of the failure being detected before the
impact of failure to the system being evaluated is detected. The detectability score is rated
against the ability to detect the effect of the failure mode or the ability to detect the failure mode
itself.
5.1.1 Risk Score Matrix
The risk score is the product of its three individual component ratings : severity, probability amd
detectability. This composite risk is called a risk priority number (RPN).

RPN = S x P x D
The RPN provides a relative priority for taking action. The bigger one, the more important to
address the corresponding failure being assessed.

5.2 Description of the benchmark
Three kinds of faults have been considered:

Sayı 5, 2016 GiDB|DERGi