Elevator Control System

A typical aircraft has two elevators, one on each side of the fuselage, attached on the horizontal tails. To enhance the safety of the aircraft, the elevator control system contains these redundant parts:

If the aircraft is flying perfectly level, then the actuator position should maintain a constant value. The fault detection system registers a failure in an actuator if:

The fault detection system also registers a fault in one of the hydraulic circuits if the pressure is out of bounds or if the pressure changes rapidly. In this example, the fault detection system checks that:

Fault Detection Control Logic

The Stateflow® chart Mode Logic defines the fault detection logic for the elevator control system. The chart contains a parallel substate for each actuator in the system. Each actuator can be in one of five modes: Passive, Standby, Active, Off, and Isolated. These operating modes are represented as substates of the parallel states.

By default, the outer actuators start in Active mode and the inner actuators start in Standby mode. If a failure is detected in the outer actuators or in the hydraulic circuits that are connected to them, the fault detection system responds by disabling the outer actuators and activating the inner actuators.

Inject Failures Into Fault Detection System

To experiment with the model, during simulation, you can introduce hydraulic circuit and actuator position failures into the fault detection system through the Failure Injection UI.

For example, to inject a failure in Hydraulic Circuit 1, select the H1 check box and click Update. The UI runs this MATLAB® code to communicate with the Simulink® model:

function Inject_failure_Callback(hObject,eventdata,handles)

mname = gcs;

...

blockname = mname+ ...
    "/Signal conditioning and failures /Hydraulic Pressures/Measured "+ ...
    newline+"Hydraulic system 1 pressures/Hydraulic pressure/H1_fail";
val = get(handles.H1,"Value");

if val
    set_param(blockname,value="1");
else
    set_param(blockname,value="0");
end

...

end

This code turns on a switch in the Signal conditioning subsystem that causes the fault detection system to register a fault in the hydraulic circuit.

The chart Mode Logic responds to failures in the hydraulic circuits and actuators by using truth table functions and event broadcasting. For example, if the fault detection system registers an isolated failure in Hydraulic Circuit 1, then:

After the fault detection systems registers a failure in Hydraulic Circuit 1, the left outer actuator is turned off, the right outer actuator is placed on standby, and the inner actuators are activated.

Map Fault Conditions to Actions

This example uses two truth tables to map the fault conditions in the aircraft elevator system to their consequent actions. This list describes the requirements for the fault detection system in the model.

In the Mode Logic chart, a pair of truth table functions define the logic to satisfy these requirements. L_switch controls the left elevator and R_switch controls the right elevator. This truth table is for the left elevator.

The first requirement indicates that if a failure is only detected in the hydraulic pressure 1 system, turn off the left outer actuator. In the truth table, this requirement is represented by the decision D1. If there is low pressure in the hydraulic system 1, then D1 specifies that action 2 is performed. Action 2 sends an event go_off to the left actuator, Actuators.LO.

Similarly, the other requirements are mapped to the appropriate actions in the truth table. For example, if the left outer actuator fails, D3 causes action 3. Action 3 sends the event go_isolated to Actuators.LO to isolate the left actuator.

The entry and during actions of the chart call the truth table functions so the fault checks execute at each time step.

Recover from Hydraulic Failures

The fault detection control logic enables the system to recover from a hydraulic circuit failure. For example, to bring the Hydraulic Circuit 1 back online, in the Failure Injection UI, clear the H1 check box and click Update. In the chart, the condition !u.low_press[0] becomes true, so the substate LO transitions from the Off mode to the Standby mode. As a result, the left outer actuator can then be activated in the event that the fault detection system registers another failure later in the simulation.

Isolate Actuators After Failures

When the fault detection system registers a failure in one of the actuators, that actuator can no longer be activated. In the chart Mode Logic, the failure of an actuator is represented by the substate Isolated. This substate has no outgoing transitions so once an actuator enters the Isolated state, it remains in that state for the rest of the simulation.

References

Pieter J. Mosterman and Jason Ghidella, "Model Reuse for the Training of Fault Scenarios in Aerospace," in Proceedings of the AIAA® Modeling and Simulation Technologies Conference, CD-ROM, paper 2004-4931, August 16 - 19, 2004, Rhode Island Convention Center, Providence, RI.

Jason R. Ghidella and Pieter J. Mosterman, "Applying Model-Based Design to a Fault Detection, Isolation, and Recovery System," in Military Embedded Systems, Summer, 2006.