Page 31 - Sayı-08

RISK ASSESSMENT OF UNDERWATER GLIDERS USING 29
UNCERTAINTY ANALYSIS AND FAULT TOLERANT CONTROL

Parent Item / Potential Potential Sev Potential Causes Prob Detection Mode D RPN
Failure Effect of of Failure et
Mechanism Function Modes Failure

Unable to Cannot transfer or
connect to
Crack/ satellite or low retrieve data from
Break/
Antenna Underwater rates of 4 Collusion 4 satellite or very 8 128
connection slow rates- transfer

Comm. taking a lot more
System
than usual

No Cannot transfer or

Satellite connection No satellite 4 Human error- 3 retrieve data from 5 60
/offline connection random satellite for long

satellite time

Breakdown- Unable to A different reading
Connection - detect water
Pressure depth/ adjust 5 Random- 3 than expected set- 8 120
sensor false systematic cannot get any
readings pitch of
vehicle depth value

Breakdown- Unable to A different reading
Connection - detect sea floor
Altimeter distance/adjust 4 Random- 4 than expected set- 9 144
false systematic cannot get any
readings pitch of
Sensory Unit vehicle value

Breakdown- Unable to A different reading
Connection - determine
Gyro pitch and roll 5 Random- 3 than expected set- 7 105
false angles of systematic cannot get any
readings
vehicle value

Collision Breakdown- Hit an object 6 Random- A different reading
sensor Connection - or stop systematic 2 than expected set- 8 96

false movement sensor is offline
readings

Loss of Unable to achieve
forward
Hull Side Crack/Break velocity 7 Collusion- 4 necessary distance 8 224
Wings Material fracture between 2 satellite

connections

4.4 Application of FTC Method

Most of the errors in a glider are the type that can bring system in the edge of loss of vehicle.
However, there are also some measurements that can be taken to increase mission time if errors
can be identified beforehand. First we need to determine how each sub-system work and what to
do in a total failure situation. It can be done by making basic flow charts for all subsystems. In
the Figure 5 the main control logic can be seen. For underwater vehicles, thruster related faults
are a significant source as they have direct effect in performance of UV and in recent years some
FTC studies emerged for UV splitting the system into thruster and sensor subsystems (Liu & Zhu,
n.d.). For underwater gliders there are no external thruster, but internal motors are used a lot and

Sayı 8, 2017 GiDB|DERGi