# In-the-Loop Simulation Experiment of Aero-Engine Fault-Tolerant Control Technology
## 3 Results
### Fault Tolerant Control
#### 3.1.2 Fault-Tolerant Control Based on Switching Control Rate  
As shown in Figure 4, when a sensor fails, the control rate participated by the failed sensor can be replaced by the control rate not participated by the failed sensor, and fault tolerance is carried out by switching the control rate. Limited to space, the fault tolerance effect based on the switching control rate is verified by taking Nh sensor fault as an example.  
Figure 3: Control effect of Nh sensor fault on Nh and Nl without analytical redundancy and using analytical redundancy, the control effect of Nh sensor fault on Nh and Nl. (**a**) Control effect of Nh sensor fault on Nh (no analytical margin). (**b**) Control effect of Nh sensor fault on Nl (no analytical margin). (**c**) Control effect of Nh sensor fault on Nh (using analytical margin). (**d**) Control effect of Nh sensor fault on Nl (using analytical margin).  
Figure 5 is the schematic diagram of the changes of the three related values of the high pressure speed when Nh sensor fails and Nh is switched to Nl or Nh is switched to EPR (ratio of total pressure at LPT outlet to total pressure at LPC inlet) at the design point and the non-design point.  
The simulation process is as follows: at the design point and the non-design point (height is 10 km and Mach is 1.4), the closed-loop control is switched at 2 s, the filtering is turned on at 5 s, the injection performance degrades at 10 s, the injection sensor fails at 75 s and the control law is switched (Nh is switched to Nl or EPR), the PLA is uniformly increased from 85 s to 89 s, and the PLA is uniformly decreased from 89 s to 93 s. PLA is increased evenly from 93 s to 97 s, and the operation stops at 100 s.  
The simulation results show that the control system switches the closed-loop Nh control rate to closed-loop Nl or EPR control after the failure of the Nh sensor and excludes the Nh fault sensor from the control system. The normal Nl or EPR sensor is used as the feedback quantity of the control system, which realizes the fault tolerance of the Nh fault. According to the simulation results, it can be verified that the fault-tolerant effect at both the design point and the non-design point is good.  
The method of switching control rate can be used for active fault tolerance. After experimental verification, the solution in Table 6 can be selected. However, the method of switching control rate only studies a few sensors which are closely related to the controlled quantity. When other sensors fail, the switching control rate cannot be used for fault tolerance; in addition, the switching control rate method in this paper is basically one-to-one, but in engineering applications, faults are likely to occur at the same time, which brings challenges to fault tolerance. Switching to other control laws for control is a suboptimal control, because the control difference of the control rate itself on the engine will lead to the deviation or even sudden change of the engine performance when switching the control rate, which may bring unpredictable consequences. Therefore, relatively speaking, fault tolerance based on analytical redundancy can effectively avoid the above problems, and it is simpler, which is a better choice for fault-tolerant control.  
Figure 4: Schematic diagram of fault tolerance based on switching control.