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Map Fault Conditions to Actions by Using Truth Tables

This example uses:

Open Model

This example shows how to use truth tables to map fault conditions of a system directly to their consequent actions.

Truth tables are only supported in Simulink®. For more information, see Use Truth Tables to Model Combinatorial Logic.

Detect Faults in a System

This model maps the fault conditions and actions of an aircraft elevator control system by using truth tables.

This list describes the requirements for the fault detection system in the model.

  • Hydraulic pressure 1 failure — While there are no other failures, turn off the left outer actuator.

  • Hydraulic pressure 2 failure — While there are no other failures, turn off the left inner actuator and the right inner actuator.

  • Hydraulic pressure 3 failure — While there are no other failures, turn off the right outer actuator.

  • Actuator position failure — While there are no other failures, isolate that specific actuator.

  • Hydraulic pressure 1 and left outer actuator failures — While there are no other failures, turn off the left outer actuator.

  • Hydraulic pressure 2 and left inner actuator failures — While there are no other failures, turn off the left inner actuator.

  • Hydraulic pressure 3 and right outer actuator failures — While there are no other failures, turn off the right outer actuator.

  • Multiple failures on left hydraulics and actuators — Isolate the left outer actuator and the left inner actuator.

  • Multiple failures on right hydraulics and actuators — Isolate the right outer actuator and the right inner actuator.

  • Intermittent actuator failures — If an actuator has been switched on and off five times during operation, isolate that specific actuator.

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 truth table functions are called at entry and during actions for the chart so that fault checks execute at each time step.

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