Create and Evaluate Fault Trees Programmatically

R2026b

Since R2026b

In this example, you programmatically create and evaluate a fault tree in the Safety Analysis Manager. You create a fault tree, define its gates and events, and evaluate the tree. You then repeat the evaluation by sweeping the event probability, and plot the top-level gate failure probability against the swept event probability.

For information about interactively creating and evaluating fault trees by using the Safety Analysis Manager, see:

• Create Fault Tree Events

• Create Fault Tree Gates

• Evaluate Fault Tree Documents

Create Fault Tree Document and Tree Structure

Create a new fault tree document by using the safetyAnalysisMgr.newDocument function. New fault tree documents contain one fault tree with a single OR top-level gate. Access the fault tree and its gate from the FaultTreeDocument object.

myTreeDoc = safetyAnalysisMgr.newDocument("fault-tree");
myFaultTree = myTreeDoc.FaultTrees(1);
topGate = myFaultTree.Gates(1);

You can add gates or events to another gate in the fault tree document. In this example, create an AND gate as an input to the top-level gate by using the createGate function. Then, create three basic events by using the createEvent function. Assign one event to the top-level gate and two events to the AND gate.

andGate = createGate(topGate,Type="and",Label="RedundancyGate");
sensorFailure = createEvent(topGate,Label="SensorFailure");
powerLoss = createEvent(andGate,Label="PowerLoss");
backupFailure = createEvent(andGate,Label="BackupFailure");

The events represent a sensor, a power system, and a backup power system. In this example, the top-level failure occurs if the sensor fails or both the power and backup systems fail simultaneously. Open the Safety Analysis Manager to view the structure.

safetyAnalysisManager

Define Failure Probabilities

By default, basic events use the constant failure model. The constant failure model assumes that the unavailability and failure frequency of the event do not change with time. Events can use the constant, rate, time at risk, mean time to failure (MTTF), dormant, or logical failure model types. See Define Event Properties. In this example, set the Q and W values for each event.

sensorFailure.FailureModel.Parameters.Q.Value = 0.01;
sensorFailure.FailureModel.Parameters.W.Value = 0.001;

powerLoss.FailureModel.Parameters.Q.Value = 0.05;
powerLoss.FailureModel.Parameters.W.Value = 0.005;

backupFailure.FailureModel.Parameters.Q.Value = 0.03;
backupFailure.FailureModel.Parameters.W.Value = 0.003;

Evaluate the Fault Tree Document

By default, fault trees evaluate using exact computation. For larger fault tree documents, the rare-event approximation can improve evaluation speed. You can configure the evaluation by setting the analysis type of the EvalConfig object.

In this example, evaluate the fault tree document by using the evaluate function.

evaluate(myTreeDoc)

After the evaluation, access the failure probability of the top-level gate from the FailureProperties object. For more information on evaluating fault trees, see Evaluate Fault Tree Documents.

topGateQ = topGate.FailureProperties.Q

topGateQ =

    0.0115

Sweep Event Probability and Collect Results

To investigate how the powerLoss event failure probability affects the top-level gate failure probability, sweep the q value for the powerLoss event from 0 to 0.05. For each value, update the failure model parameter, re-evaluate the fault tree, and store the top-level gate failure probability.

qSweep = linspace(0,0.05,20);
topGateResults = zeros(size(qSweep));

for i = 1:length(qSweep)
    powerLoss.FailureModel.Parameters.Q.Value = qSweep(i);
    evaluate(myTreeDoc);
    topGateResults(i) = topGate.FailureProperties.Q;
end

Plot Results

Plot the top-level gate failure probability against the swept powerLoss event probability.

figure
plot(qSweep,topGateResults,LineWidth=2)
xlabel("Power Loss Probability (q)")
ylabel("Top-Level Gate Failure Probability (q)")
title("Effect of Power Loss Probability on System Failure")
grid on