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24 M.O. ŞERİFOĞLU, T. HIZARCI and H. AKYILDIZ

There are a lot of direct and indirect benefits of the method. First of all, FMEA is an easy process
to make. It also supplies other failure detection and isolation techniques with meaningful data.
FMEA increases reliability, quality and safety of the finished product. Those improvements
increase customer satisfaction. On the other hand, method requires experts of different disciplines
to work together and their performance will affect the performance of the method directly.

4.2 Fault Tolerant Control Method
Fault tolerant control is the main systematic for most of our modern day autonomous applications.
It is basically a method, used to make a system continue working even if there are errors in the
system. When a fault occurs in a system, ideally it has to be detected fast and then replied with a
fix if possible. Main frame of FTC system consists of a self-repairing, reconfigurable and self-
designing control structure. It consists of two main parts; fault diagnosis and control redesign.
Faults in a system are diagnosed and identified in the fault diagnosis step and then in control
redesign step controllers are adapted to the system with fault so that it can continue working. If
severity of the error is high, power degradation is also an acceptable solution. Fault tolerant
control can be applied to all systems. However, FTC is a complex mixture of three research fields;
FDI (Fault Detection and Isolation), robust control, and reconfigurable control which make it a
complicated and expensive. So it is applied into systems within the condition of critical safety.

4.2.1 Structure

There are several different structure types of FTC and they are grouped under 3 categories.
However, before that terminology should be understood. Here is the basic terminology used in
FTC applications:

 Fault: A deviation of a parameter of the system from acceptable condition
 Failure: A permanent interruption of system’s ability to perform its goal
 Error: A deviation between measured and computed value
 Controlled system: A plant under consideration with sensors and actuators
 Fail-operational: Operational system without a performance change despite failure
 Fail-safe: A system that fails to a state which is considered safe

Fault types are also separated to three categories as seen below:
 Plant faults: Changes dynamics of I/O properties. Physical parameter of system changes
such as coefficients.
 Sensor faults: Plant properties are stable but sensors have errors. These faults represent
incorrect reading from the sensors. They can happen due to wire defects.
 Actuator faults: Plant specifications are not changed but controller effect on the plant is
disturbed. They represent partial or complete loss of control action

GiDB|DERGi Sayı 8, 2017