Simulating Simulink Models

Simulating Simulink Models with the sim Command

Use this skill when you need to configure a simulation non-destructively — parameter overrides, custom inputs, batch execution, or structured output access. For persistent, reusable pass/fail behavioral testing (especially of individual subsystems), use testing-simulink-models instead. For trivial one-shot simulations without configuration, a direct sim() call suffices without this skill.

When to Use

• Overriding model or block parameters non-destructively (setVariable, setBlockParameter, setModelParameter) — without modifying the .slx file
• Passing custom input signals to root-level Inport blocks via setExternalInput with a Dataset
• Running parameter sweeps or batch simulations (SimulationInput arrays, parsim, Fast Restart)
• Accessing logged signal data (logsout) for analysis after simulation

When NOT to Use

• Trivial one-shot simulations without parameter overrides or custom inputs — a direct sim('ModelName') call works without this skill
• Writing declarative Gherkin-based tests → use testing-simulink-models
• Testing an individual subsystem or component → use testing-simulink-models (requires Simulink Test; auto-creates a harness, compiles only the subsystem — much faster than sim() which always compiles the entire model)
• Adding, connecting, or deleting blocks → use building-simulink-models
• Checking model structure for unconnected ports → use model_check tool directly
• Generating requirements from model behavior → use generate-requirement-drafts

Minimal working pattern

Always simulate using Simulink.SimulationInput and Simulink.SimulationOutput:

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in = Simulink.SimulationInput('MyModel');
in = in.setModelParameter('StopTime', '10');
out = sim(in);

Setting parameters

Use SimulationInput methods to configure the simulation:

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% Model-level parameters (StopTime, SolverType, SimulationMode, etc.)
in = in.setModelParameter('StopTime', '10', 'SolverType', 'Fixed-step');

% Block parameters — resolve path from blk_X ID (never type block names manually)
blkPath = Simulink.ID.getFullName('MyModel:5');
in = in.setBlockParameter(blkPath, 'Gain', '5');

% MATLAB workspace variables used by the model
in = in.setVariable('Kp', 1.2);

Input signals

Pass input signals through Inport blocks using a Simulink.SimulationData.Dataset. Elements are matched to Inport blocks by index position — the first element maps to the Inport with port number 1, the second to port number 2, and so on.

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dt = 0.01;
N = 1000;
t = dt*(0:N)';
u = sin(2*pi*t);

ts = timeseries(u, t);

ds = Simulink.SimulationData.Dataset;
ds{1} = ts;

in = in.setExternalInput(ds);
out = sim(in);

You can also use timetable as an input format:

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secs = seconds(t);
tt = timetable(secs, u);

ds = Simulink.SimulationData.Dataset;
ds{1} = tt;

in = in.setExternalInput(ds);

Discovering logged data

First, discover what kinds of logged data the model produces using who, then inspect signal names within logsout:

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in = Simulink.SimulationInput('MyModel');
out = sim(in);

% See what logging properties exist (logsout, yout, tout, etc.)
who(out)

% List individual signal names within logsout
disp(out.logsout.getElementNames);

Accessing logged data

Logged signals are available through out.logsout. Access them directly by name:

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% Plot a logged signal
plot(out.logsout.get('signalName').Values)

% Get time and data separately
sig = out.logsout.get('signalName').Values;
plot(sig.Time, sig.Data)

Multiple simulations

When running many simulations, create an array of Simulink.SimulationInput objects:

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in = repmat(Simulink.SimulationInput('MyModel'),N,1);
for k = 1:N
    in(k) = Simulink.SimulationInput('MyModel');
    in(k) = in(k).setVariable('gain', gains(k));
end
out = sim(in);

To enable fast restart for iterative sweeps (compiles the model only once):

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out = sim(in, 'UseFastRestart', 'on');

Parallel simulation (parsim)

To run multiple simulations in parallel, use parsim instead of looping over sim:

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for k = 1:N
    in(k) = Simulink.SimulationInput('MyModel');
    in(k) = in(k).setVariable('gain', gains(k));
end
out = parsim(in);

parsim also supports 'UseFastRestart','on' for faster batch runs.

Guardrails

• Never use set_param, load_system, or open_system to drive simulation — SimulationInput replaces all of these.
• Never wrap SimulationOutput access in try-catch or isfield — sim either returns a valid object or throws. SimulationOutput has no isfield method.
• Never create unnecessary intermediate variables for logged data — access directly via out.logsout.get('name').Values.
• Always use in/out as variable names for SimulationInput/SimulationOutput.
• Always use setExternalInput with a Dataset — don't pass comma-separated lists of variables.

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