Environmental Engineering Reference
In-Depth Information
Fault information
Control
re-design
Diagnosis
Disturbance
Fault
Controller
Plant
Fig. 7.2
Schematic of the fault tolerant system [ 6 ]
• Make use of the information obtained from the supervision level and adapt the
controller parameters and/or reconfigure the structure of the controller so that
the required remedial activity can be achieved (Execution level).
Hence, the FTC loop extends the usual feedback controller by a supervision
level. In the absence of a fault, the system matches its nominal response so that the
nominal controller attenuates the disturbance and ensures good reference follow-
ing and other requirements on the closed-loop system. In this situation, the
diagnostic block recognises that the closed-loop system is fault-free and no change
of the control law is necessary.
If a fault occurs, the supervision level makes the control loop fault-tolerant. The
diagnosis block identifies the fault and the control re-design block adjusts the
controller to the new controller parameter set. Following this, the reconfigured
system continues to satisfy the control target.
Thus, FTC is the control loop that has the ability to fulfil the required system
performances even if faults occur through utilising the help provided by the
supervision level. Approaches for synthesizing the FTC loop are classified as
either a passive FTC (PFTC) or active FTC (AFTC). In the PFTC approach, the
control loop is designed to tolerate some anticipated types of faults. The effec-
tiveness of this strategy, that usually handles anticipated faults scenarios, depends
upon the robustness of the nominal closed-loop system. Additionally, since the
robustness of the closed-loop system to the fault is considered during the design
cycle of the nominal controller, this may lead to post-fault degraded performance
of the closed-loop system. However, it is interesting to note that the PFTC system
does not require the FDD and controller reconfiguration and hence it has the ability
 
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